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Large-Area, Highly Uniform Evaporated Formamidinium Lead Triiodide Thin Films for Solar Cells
 J Borchert, RL Milot, JB Patel, CL Davies, AD Wright, L Martínez Maestro, HJ Snaith, LM Herz, MB Johnston ACS Energy Lett., 2:2799-2804 (Nov 2017) [ pdf ][ ref ]Perovskite thin-film solar cells are one of the most promising emerging renewable energy technologies because of their potential for low-cost, large-area fabrication combined with high energy conversion efficiencies. Recently, formamidinium lead triiodide ($\rm {FAPbI_3}$) and other formamidinium (CH(NH$_2$)$_2$) based perovskites have been explored as interesting alternatives to methylammonium lead triiodide ($\rm {MAPbI_3}$), because they exhibit better thermal stability. However at present a major challenge is up-scaling of perovskite solar cells from small test-cells to full solar modules. We show that co-evaporation is a scalable method for the deposition of homogeneous $\rm FAPbI_3$ thin-films over large areas. The method allows precise control over film thickness and results in highly uniform, pin-hole free layers. Our films exhibited a high charge-carrier mobility of 26\,$\rm cm^2 V^{-1}s^{-1}$, excellent optical properties and a bimolecular recombination constant of $7\times10^{-11}$\,cm$^3$s$^{-1}$. Solar cells fabricated using these vapor-deposited layers within a regular device architecture produced stabilized power conversion efficiencies of up to 14.2\,$\rm\%$. Thus we demonstrate that efficient $\rm FAPbI_3$ solar cells can be vapor-deposited, which opens up a pathway towards large-area stable perovskite photovoltaics.
Near-infrared and short-wavelength infrared photodiodes based on dye-perovskite composites
 QQ Lin, ZP Wang, M Young, JB Patel, RL Milot, LM Maestro, RR Lunt, HJ Snaith, MB Johnston, LM Herz Adv. Funct. Mater., 27:1702485 (Oct 2017) [ pdf ][ ref ]Organohalide perovskites have emerged as promising light-sensing materials because of their superior optoelectronic properties and low-cost processing methods. Recently, perovskite-based photodetectors have successfully been demonstrated as both broadband and narrowband varieties. However, the photodetection bandwidth in perovskite-based photodetectors has so far been limited to the near-infrared regime owing to the relatively wide band gap of hybrid organohalide perovskites. In particular, short-wavelength infrared photodiodes operating beyond 1 mu m have not yet been realized with organohalide perovskites. In this study, narrow band gap organic dyes are combined with hybrid perovskites to form composite films as active photoresponsive layers. Tuning the dye loading allows for optimization of the spectral response characteristics and excellent charge-carrier mobilities near 11 cm(2) V-1 s(-1), suggesting that these composites combine the light-absorbing properties or IR dyes with the outstanding charge-extraction characteristics of the perovskite. This study demonstrates the first perovskite photodiodes with deep near-infrared and short-wavelength infrared response that extends as far as 1.6 mu m. All devices are solution-processed and exhibit relatively high responsivity, low dark current, and fast response at room temperature, making this approach highly attractive for next-generation light-detection techniques.
Photon reabsorption masks intrinsic bimolecular charge-carrier recombination in {CH$_3$NH$_3$PbI$_3$} perovskite
 TW Crothers, RL Milot, JB Patel, ES Parrott, J Schlipf, P Muller-buschbaum, MB Johnston, LM Herz Nano Lett., 17:5782-5789 (Sep 2017) [ pdf ][ ref ]An understanding of charge-carrier recombination processes is essential for the development of hybrid metal halide perovskites for photovoltaic applications. We show that typical measurements of the radiative bimolecular recombination constant in CH3NH3PbI3 are strongly affected by photon reabsorption that masks a much larger intrinsic bimolecular recombination rate constant. By investigating a set of films whose thickness varies between 50 and 533 nm, we find that the bimolecular charge recombination rate appears to slow by an order of magnitude as the film thickness increases. However, by using a dynamical model that accounts for photon reabsorption and charge-carrier diffusion we determine that a single intrinsic bimolecular recombination coefficient of value 6.8 X 10(-10) cm(3)s(-1) is common to all samples irrespective of film thickness. Hence, we postulate that the wide range of literature values reported for such coefficients is partly to blame on differences in photon out-coupling between samples with crystal grains or mesoporous scaffolds of different sizes influencing light scattering, whereas thinner films or index-matched surrounding layers can reduce the possibility for photon reabsorption. We discuss the critical role of photon confinement on free charge-carrier retention in thin photovoltaic layers and highlight an approach to assess the success of such schemes from transient spectroscopic measurement.
Crystallization kinetics and morphology control of formamidinium-cesium mixed-cation lead mixed-halide perovskite via tunability of the colloidal precursor solution
 DP McMeekin, ZP Wang, W Rehman, F Pulvirenti, JB Patel, NK Noel, MB Johnston, SR Marder, LM Herz, HJ Snaith Adv. Mater., 29:1607039 (Aug 2017) [ pdf ][ ref ]The meteoric rise of the field of perovskite solar cells has been fueled by the ease with which a wide range of high-quality materials can be fabricated via simple solution processing methods. However, to date, little effort has been devoted to understanding the precursor solutions, and the role of additives such as hydrohalic acids upon film crystallization and final optoelectronic quality. Here, a direct link between the colloids concentration present in the [HC(NH2)(2)](0.83)Cs0.17Pb(Br0.2I0.8)(3) precursor solution and the nucleation and growth stages of the thin film formation is established. Using dynamic light scattering analysis, the dissolution of colloids over a time span triggered by the addition of hydrohalic acids is monitored. These colloids appear to provide nucleation sites for the perovskite crystallization, which critically impacts morphology, crystal quality, and optoelectronic properties. Via 2D X-ray diffraction, highly ordered and textured crystals for films prepared from solutions with lower colloidal concentrations are observed. This increase in material quality allows for a reduction in microstrain along with a twofold increase in charge-carrier mobilities leading to values exceeding 20 cm(2) V-1 s(-1). Using a solution with an optimized colloidal concentration, devices that reach current-voltage measured power conversion efficiency of 18.8% and stabilized efficiency of 17.9% are fabricated.
Band-tail recombination in hybrid lead iodide perovskite
 AD Wright, RL Milot, GE Eperon, HJ Snaith, MB Johnston, LM Herz Adv. Funct. Mater., 27:1700860 (Aug 2017) [ pdf ][ ref ]Traps limit the photovoltaic efficiency and affect the charge transport of optoelectronic devices based on hybrid lead halide perovskites. Understanding the nature and energy scale of these trap states is therefore crucial for the development and optimization of solar cell and laser technology based on these materials. Here, the low-temperature photoluminescence of formamidinium lead triiodide (HC(NH2)(2)PbI3) is investigated. A power-law time dependence in the emission intensity and an additional low-energy emission peak that exhibits an anomalous relative Stokes shift are observed. Using a rate-equation model and a Monte Carlo simulation, it is revealed that both phenomena arise from an exponential trap-density tail with characteristic energy scale of approximate to 3 meV. Charge-carrier recombination from sites deep within the tail is found to cause emission with energy downshifted by up to several tens of meV. Hence, such phenomena may in part be responsible for open-circuit voltage losses commonly observed in these materials. In this high-quality hybrid perovskite, trap states thus predominantly comprise a continuum of energetic levels (associated with disorder) rather than discrete trap energy levels (associated, e.g., with elemental vacancies). Hybrid perovskites may therefore be viewed as classic semiconductors whose band-structure picture is moderated by a modest degree of energetic disorder.
Towards higher electron mobility in modulation doped gaas/algaas core shell nanowires
 JL Boland, G Tutuncuoglu, JQ Gong, S Conesa-boj, CL Davies, LM Herz, AFI Morral, MB Johnston Nanoscale, 9:7839-7846 (Jun 2017) [ pdf ][ ref ]Precise control over the electrical conductivity of semiconductor nanowires is a crucial prerequisite for implementation of these nanostructures into novel electronic and optoelectronic devices. Advances in our understanding of doping mechanisms in nanowires and their influence on electron mobility and radiative efficiency are urgently required. Here, we investigate the electronic properties of n-type modulation doped GaAs/AlGaAs nanowires via optical pump terahertz (THz) probe spectroscopy and photoluminescence spectroscopy over the temperature range 5 K-300 K. We directly determine an ionization energy of 6.7 +/- 0.5 meV (T = 52 K) for the Si donors within the AlGaAs shell that create the modulation doping structure. We further elucidate the temperature dependence of the electron mobility, photoconductivity lifetime and radiative efficiency, and determine the charge-carrier scattering mechanisms that limit electron mobility. We show that below the donor ionization temperature, charge scattering is limited by interactions with interfaces, leading to an excellent electron mobility of 4360 +/- 380 cm(2) V-1 s(-1) at 5 K. Above the ionization temperature, polar scattering via longitudinal optical (LO) phonons dominates, leading to a room temperature mobility of 2220 +/- 130 cm(2) V-1 s(-1). In addition, we show that the Si donors effectively passivate interfacial trap states in the nanowires, leading to prolonged photoconductivity lifetimes with increasing temperature, accompanied by an enhanced radiative efficiency that exceeds 10% at room temperature.
The influence of surfaces on the transient terahertz conductivity and electron mobility of gaas nanowires
 HJ Joyce, SA Baig, P Parkinson, CL Davies, JL Boland, HH Tan, C Jagadish, LM Herz, MB Johnston J. Phys. D-Appl. Phys., 50:224001 (Jun 2017) [ pdf ][ ref ]Bare unpassivated GaAs nanowires feature relatively high electron mobilities (400-2100 cm(2) V-1 s(-1)) and ultrashort charge carrier lifetimes (1-5 ps) at room temperature. These two properties are highly desirable for high speed optoelectronic devices, including photoreceivers, modulators and switches operating at microwave and terahertz frequencies. When engineering these GaAs nanowire-based devices, it is important to have a quantitative understanding of how the charge carrier mobility and lifetime can be tuned. Here we use optical-pump-terahertzprobe spectroscopy to quantify how mobility and lifetime depend on the nanowire surfaces and on carrier density in unpassivated GaAs nanowires. We also present two alternative frameworks for the analysis of nanowire photoconductivity: one based on plasmon resonance and the other based on Maxwell-Garnett effective medium theory with the nanowires modelled as prolate ellipsoids. We find the electron mobility decreases significantly with decreasing nanowire diameter, as charge carriers experience increased scattering at nanowire surfaces. Reducing the diameter from 50 nm to 30 nm degrades the electron mobility by up to 47%. Photoconductivity dynamics were dominated by trapping at saturable states existing at the nanowire surface, and the trapping rate was highest for the nanowires of narrowest diameter. The maximum surface recombination velocity, which occurs in the limit of all traps being empty, was calculated as 1.3 x 10(6) cm s(-1). We note that when selecting the optimum nanowire diameter for an ultrafast device, there is a trade-off between achieving a short lifetime and a high carrier mobility. To achieve high speed GaAs nanowire devices featuring the highest charge carrier mobilities and shortest lifetimes, we recommend operating the devices at low charge carrier densities.
Optoelectronics fast silicon photodiodes
 MB Johnston Nat. Photonics, 11:268-269 (May 2017) [ pdf ][ ref ]There is typically a compromise between speed and efficiency when designing silicon photodiodes. Now, researchers have exploited microstructuring to achieve fast and thin devices that are also efficient.
An ultrafast switchable terahertz polarization modulator based on iii-v semiconductor nanowires
 SA Baig, JL Boland, DA Damry, HH Tan, C Jagadish, HJ Joyce, MB Johnston Nano Lett., 17:2603-2610 (Apr 2017) [ pdf ][ ref ]Progress in the terahertz (THz) region of the electromagnetic spectrum is undergoing major advances, with advanced THz sources and detectors being developed at a rapid pace. Yet, ultrafast THz communication is still to be realized, owing to the lack of practical and effective THz modulators. Here, we present a novel ultrafast active THz polarization modulator based on GaAs semiconductor nanowires arranged in a wire-grid configuration. We utilize an optical pump-terahertz probe spectroscopy system and vary the polarization of the optical pump beam to demonstrate ultrafast THz modulation with a switching time of less than 5 ps and a modulation depth of -8 dB. We achieve an extinction of over 13% and a dynamic range of -9 dB, comparable to microsecond-switchable graphene- and metamaterial-based THz modulators, and surpassing the performance of optically switchable carbon nanotube THz polarizers. We show a broad bandwidth for THz modulation between 0.1 and 4 THz. Thus, this work presents the first THz modulator which combines not only a large modulation depth but also a broad bandwidth and picosecond time resolution for THz intensity and phase modulation, making it an ideal candidate for ultrafast THz communication.
Single n(+)-i-n(+) inp nanowires for highly sensitive terahertz detection
 K Peng, P Parkinson, Q Gao, JL Boland, ZY Li, F Wang, S Mokkapati, L Fu, MB Johnston, HH Tan, C Jagadish Nanotechnology, 28:125202 (Mar 2017) [ pdf ][ ref ]Developing single-nanowire terahertz (THz) electronics and employing them as sub-wavelength components for highly-integrated THz time-domain spectroscopy (THz-TDS) applications is a promising approach to achieve future low-cost, highly integrable and high-resolution THz tools, which are desirable in many areas spanning from security, industry, environmental monitoring and medical diagnostics to fundamental science. In this work, we present the design and growth of n(+)-i-n(+) InP nanowires. The axial doping profile of the n+-i-n+ InP nanowires has been calibrated and characterized using combined optical and electrical approaches to achieve nanowire devices with low contact resistances, on which the highly-sensitive InP singlenanowire photoconductive THz detectors have been demonstrated. While the n+-i-n+ InP nanowire detector has a only pA-level response current, it has a 2.5 times improved signal-tonoise ratio compared with the undoped InP nanowire detector and is comparable to traditional bulk THz detectors. This performance indicates a promising path to nanowire-based THz electronics for future commercial applications.
The 2017 terahertz science and technology roadmap
 SS Dhillon, MS Vitiello, EH Linfield, AG Davies, MC Hoffmann, MB Johnston J. Phys. D-Appl. Phys., 50:043001 (Feb 2017) [ pdf ][ ref ]Science and technologies based on terahertz frequency electromagnetic radiation (100 GHz-30 THz) have developed rapidly over the last 30 years. For most of the 20th Century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to \'real world\' applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2017, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 18 sections that cover most of the key areas of THz science and technology. We hope that The 2017 Roadmap on THz science and technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies.
Influence of Interface Morphology on Hysteresis in Vapor-Deposited Perovskite Solar Cells
 JB Patel, J Wong-Leung, S Van Reenen, N Sakai, JTW Wang, ES Parrott, M Liu, HJ Snaith, LM Herz, MB Johnston , 3:1600470 (Feb 2017) [ pdf ][ ref ]Hysteresis in the current–voltage characteristics of vapor-deposited perovskite solar cells is shown to originate from an amorphous region of CH3NH3PbI3 at the interface with the device's electron transport layer. Interface engineering is used to produce highly crystalline perovskite material at this interface which results in hysteresis-free evaporated planar heterojunction solar cells.
Cs$_2$InAgCl$_6$: a new lead-free halide double perovskite with direct band gap
 G Volonakis, AA Haghighirad, RL Milot, WH Sio, MR Filip, B Wenger, MB Johnston, LM Herz, HJ Snaith, F Giustino J. Phys. Chem. Lett., 8:772- (Feb 2017) [ pdf ][ ref ]A(2)BB\'X-6 halide double perovskites based on bismuth and silver have recently been proposed as potential environmentally friendly alternatives to lead-based hybrid halide perovskites. In particular, Cs2BiAgX6 (X = Cl, Br) have been synthesized and found to exhibit band gaps in the visible range. However, the band gaps of these compounds are indirect, which is not ideal for applications in thin film photovoltaics. Here, we propose a new class of halide double perovskites, where the B3+ and B+ cations are In3+ and Ag+, respectively. Our first-principles calculations indicate that the hypothetical compounds Cs2InAgX6 (X = Cl, Br, I) should exhibit direct band gaps between the visible (I) and the ultraviolet (Cl). Based on these predictions, we attempt to synthesize Cs2InAgCl6 and Cs2InAgBr6, and we succeed to form the hitherto unknown double perovskite Cs2InAgCl6. X-ray diffraction yields a double perovskite structure with space group Fm3m. The measured band gap is 3.3 eV, and the compound is found to be photosensitive and turns reversibly from white to orange under ultraviolet illumination. We also perform an empirical analysis of the stability of Cs2InAgX6 and their mixed halides based on Goldschmidts rules, and we find that it should also be possible to form Cs2InAg(Cl1xBrx)(6) for x < 1. The synthesis of mixed halides will open the way to the development of lead-free double perovskites with direct and tunable band gaps.
Efficient and air-stable mixed-cation lead mixed-halide perovskite solar cells with n-doped organic electron extraction layers
 ZP Wang, DP McMeekin, N Sakai, K Wojciechowski, JB Patel, MB Johnston, HJ Snaith Adv. Mater., 29:UNSP 1604186 (Feb 2017) [ pdf ][ ref ]Air-stable doping of the n-type fullerene layer in an n-i-p planar heterojunction perovskite device is capable of enhancing device efficiency and improving device stability. Employing a (HC(NH2)(2))(0.83)Cs0.17Pb(I0.6Br0.4)(3) perovskite as the photoactive layer, glass-glass laminated devices are reported, which sustain 80% of their \"post burn-in\" efficiency over 3400 h under full sun illumination in ambient conditions.
A review of the electrical properties of semiconductor nanowires: insights gained from terahertz conductivity spectroscopy
 HJ Joyce, JL Boland, CL Davies, SA Baig, MB Johnston Semicond. Sci. Technol., 31:103003 (Oct 2016) [ pdf ][ ref ]Accurately measuring and controlling the electrical properties of semiconductor nanowires is of paramount importance in the development of novel nanowire-based devices. In light of this, terahertz. (THz) conductivity spectroscopy has emerged as an ideal non-contact technique for probing nanowire electrical conductivity and is showing tremendous value in the targeted development of nanowire devices. THz spectroscopic measurements of nanowires enable charge carrier lifetimes, mobilities, dopant concentrations and surface recombination velocities to be measured with high accuracy and high throughput in a contact-free fashion. This review spans seminal and recent studies of the electronic properties of nanowires using THz spectroscopy. A didactic description of THz time-domain spectroscopy, optical pump-THz probe spectroscopy, and their application to nanowires is included. We review a variety of technologically important nanowire materials, including GaAs, InAs, InP, GaN and InN nanowires, Si and Ge nanowires, ZnO nanowires, nanowire heterostructures, doped nanowires and modulation-doped nanowires. Finally, we discuss how THz measurements are guiding the development of nanowire-based devices, with the example of single-nanowire photoconductive THz receivers.
Radiative monomolecular recombination boosts amplified spontaneous emission in hc(nh2)(2)sni3 perovskite films
 RL Milot, GE Eperon, T Green, HJ Snaith, MB Johnston, LM Herz J. Phys. Chem. Lett., 7:4178-4184 (Oct 2016) [ pdf ][ ref ]Hybrid metal-halide perovskites have potential as cost-effective gain media for laser technology because of their superior optoelectronic properties. Although lead-halide perovskites have been most widely studied to date, tin-based perovskites have been proposed as a less toxic alternative. In this Letter, we show that amplified spontaneous emission (ASE) in formamidinium tin triiodide (FASnI(3)) thin films is supported by an observed radiative monomolecular charge recombination pathway deriving from its unintentional doping. Such a radiative component will be active even at the lowest charge-carrier densities, opening a pathway for ultralow light-emission thresholds. Using time-resolved THz photoconductivity analysis, we further show that the material has an unprecedentedly high charge-carrier mobility of 22 cm(2) V-1 s(-1) favoring efficient transport. In addition, FASnI(3) exhibits strong radiative bimolecular recombination and Auger rates that are over an order of magnitude lower than for lead halide perovskites. In combination, these properties reveal that tin-halide perovskites are highly suited to light-emitting devices.
Broadband phase-sensitive single {InP} nanowire photoconductive terahertz detectors
 K Peng, P Parkinson, JL Boland, Q Gao, YC Wenas, CL Davies, ZY Li, L Fu, MB Johnston, HH Tan, C Jagadish Nano Lett., 16:4925-4931 (Aug 2016) [ pdf ][ ref ]Terahertz time-domain spectroscopy (THz-TDS) has emerged as a powerful tool for materials characterization and imaging. A trend toward size reduction, higher component integration, and performance improvement for advanced THz-TDS systems is of increasing interest. The use of single semiconducting nanowires for terahertz (THz) detection is a nascent field that has great potential to realize future highly integrated THz systems. In order to develop such components, optimized material optoelectronic properties and careful device design are necessary. Here, we present antenna-optimized photoconductive detectors based on single InP nanowires with superior properties of high carrier mobility (similar to 1260 cm(2) V-1 s(-1)) and low dark current (similar to 10 pA), which exhibit excellent sensitivity and broadband performance. We demonstrate that these nanowire THz detectors can provide high quality time-domain spectra for materials characterization in a THz-TDS system, a critical step toward future application in advanced THz-TDS system with high spectral and spatial resolution.
Electron-phonon coupling in hybrid lead halide perovskites
 AD Wright, C Verdi, RL Milot, GE Eperon, MA Pérez-Osorio, HJ Snaith, F Giustino, MB Johnston, LM Herz Nat. Commun., 7:11755 (May 2016) [ pdf ][ ref ]Phonon scattering limits charge-carrier mobilities and governs emission line broadening in hybrid metal halide perovskites. Establishing how charge carriers interact with phonons in these materials is therefore essential for the development of high-efficiency perovskite photovoltaics and low-cost lasers. Here we investigate the temperature dependence of emission line broadening in the four commonly studied formamidinium and methylammonium perovskites, HC(NH2)(2)PbI3, HC(NH2)(2)PbBr3, CH3NH3PbI3 and CH3NH3PbBr3, and discover that scattering from longitudinal optical phonons via the Frohlich interaction is the dominant source of electron-phonon coupling near room temperature, with scattering off acoustic phonons negligible. We determine energies for the interacting longitudinal optical phonon modes to be 11.5 and 15.3 meV, and Frohlich coupling constants of similar to 40 and 60 meV for the lead iodide and bromide perovskites, respectively. Our findings correlate well with first-principles calculations based on many-body perturbation theory, which underlines the suitability of an electronic band-structure picture for describing charge carriers in hybrid perovskites.
Bandgap-tunable cesium lead halide perovskites with high thermal stability for efficient solar cells
 RJ Sutton, GE Eperon, L Miranda, ES Parrott, BA Kamino, JB Patel, MT Horantner, MB Johnston, AA Haghighirad, DT Moore, HJ Snaith Adv. Energy Mater., 6:1502458 (Apr 2016) [ pdf ][ ref ]Highest reported efficiency cesium lead halide perovskite solar cells are realized by tuning the bandgap and stabilizing the black perovskite phase at lower temperatures. CsPbI2Br is employed in a planar architecture device resulting in 9.8% power conversion efficiency and over 5% stabilized power output. Offering substantially enhanced thermal stability over their organic based counterparts, these results show that all-inorganic perovskites can represent a promising next step for photovoltaic materials.
Effect of structural phase transition on charge-carrier lifetimes and defects in ch3nh3sni3 perovskite
 ES Parrott, RL Milot, T Stergiopoulos, HJ Snaith, MB Johnston, LM Herz J. Phys. Chem. Lett., 7:1321-1326 (Apr 2016) [ pdf ][ ref ]Methylammonium tin triiodide (MASnI(3)) has been successfully employed in lead-free perovskite solar cells, but overall power-conversion efficiencies are still significantly lower than for lead-based perovskites. Here we present photoluminescence (PL) spectra and time resolved PL from 8 to 295 K and find a marked improvement in carrier lifetime and a substantial reduction in PL line width below similar to 110 K, indicating that the cause of the hindered performance is activated at the orthorhombic to tetragonal phase transition. Our measurements therefore suggest that targeted structural change may be capable of tailoring the relative energy level alignment of defects (e.g., tin vacancies) to reduce the background dopant density and improve charge extraction. In addition, we observe for the first time an above-gap emission feature that may arise from higher-lying interband transitions, raising the prospect of excess energy harvesting.
Increased Photoconductivity Lifetime in GaAs Nanowires by Controlled n-Type and p-Type Doping
 JL Boland, A Casadei, G Tutuncuoglu, F Matteini, CL Davies, F Jabeen, HJ Joyce, LM Herz, A Fontcuberta i Morral, MB Johnston ACS Nano, 10:4219-4227 (Mar 2016) [ pdf ][ ref ] Controlled doping of GaAs nanowires is crucial for the development of nanowire-based electronic and optoelectronic devices. Here, we present a noncontact method based on time-resolved terahertz photoconductivity for assessing n- and p-type doping efficiency in nanowires. Using this technique, we measure extrinsic electron and hole concentrations in excess of 1018 cm–3 for GaAs nanowires with n-type and p-type doped shells. Furthermore, we show that controlled doping can significantly increase the photoconductivity lifetime of GaAs nanowires by over an order of magnitude: from 0.13 ns in undoped nanowires to 3.8 and 2.5 ns in n-doped and p-doped nanowires, respectively. Thus, controlled doping can be used to reduce the effects of parasitic surface recombination in optoelectronic nanowire devices, which is promising for nanowire devices, such as solar cells and nanowire lasers.
Structured organic-inorganic perovskite toward a distributed feedback laser
 M Saliba, SM Wood, JB Patel, PK Nayak, J Huang, JA Alexander-webber, B Wenger, SD Stranks, MT Horantner, JTW Wang, RJ Nicholas, LM Herz, MB Johnston, SM Morris, HJ Snaith, MK Riede Adv. Mater., 28:923-929 (Feb 2016) [ pdf ][ ref ]A general strategy for the in-plane structuring of organic–inorganic perovskite films is presented. The method is used to fabricate an industrially relevant distributed feedback (DFB) cavity, which is a critical step toward all-electrially pumped injection laser diodes. This approach opens the prospects of perovskite materials for much improved optical control in LEDs, solar cells, and also toward applications as optical devices.
Hybrid Perovskites for Photovoltaics: Charge-Carrier Recombination, Diffusion, and Radiative Efficiencies
 MB Johnston, LM Herz Accounts Chem. Res., 49:146--154 (Jan 2016) [ pdf ][ ref ]Photovoltaic (PV) devices that harvest the energy provided by the sun have great potential as renewable energy sources, yet uptake has been hampered by the increased cost of solar electricity compared with fossil fuels. Hybrid metal halide perovskites have recently emerged as low-cost active materials in PV cells with power conversion efficiencies now exceeding 20%. Rapid progress has been achieved over only a few years through improvements in materials processing and device design. In addition, hybrid perovskites appear to be good light emitters under certain conditions, raising the prospect of applications in low-cost light-emitting diodes and lasers.
Formation Dynamics of CH$_3$NH$_3$PbI$_3$ Perovskite Following Two-Step Layer Deposition
 JB Patel, RL Milot, AD Wright, LM Herz, MB Johnston J. Phys. Chem. Lett., 7:96-102 (Jan 2016) [ pdf ][ ref ]Hybrid metal-halide perovskites have emerged as a leading class of semiconductors for optoelectronic devices because of their desirable material properties and versatile fabrication methods. However, little is known about the chemical transformations that occur in the initial stages of perovskite crystal formation. Here we follow the real-time formation dynamics of MAPbI3 from a bilayer of lead iodide (PbI2) and methylammonium iodide (MAI) deposited through a two-step thermal evaporation process. By lowering the substrate temperature during deposition, we are able to initially inhibit intermixing of the two layers. We subsequently use infrared and visible light transmission, X-ray diffraction, and photoluminescence lifetime measurements to reveal the room-temperature transformations that occur in vacuum and ambient air, as MAI diffuses into the PbI2 lattice to form MAPbI3. In vacuum, the transformation to MAPbI3 is incomplete as unreacted MAI is retained in the film. However, exposure to moist air allows for conversion of the unreacted MAI to MAPbI3, demonstrating that moisture is essential in making MAI more mobile and thus aiding perovskite crystallization. These dynamic processes are reflected in the observed charge-carrier lifetimes, which strongly fluctuate during periods of large ion migration but steadily increase with improving crystallinity.
A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells
 DP McMeekin, G Sadoughi, W Rehman, GE Eperon, M Saliba, MT Hörantner, A Haghighirad, N Sakai, L Korte, B Rech, MB Johnston, LM Herz, HJ Snaith Science, 351:151-155 (Jan 2016) [ pdf ][ ref ]Metal halide perovskite photovoltaic cells could potentially boost the efficiency of commercial silicon photovoltaic modules from ∼20 toward 30% when used in tandem architectures. An optimum perovskite cell optical band gap of ~1.75 electron volts (eV) can be achieved by varying halide composition, but to date, such materials have had poor photostability and thermal stability. Here we present a highly crystalline and compositionally photostable material, [HC(NH2)2]0.83Cs0.17Pb(I0.6Br0.4)3, with an optical band gap of ~1.74 eV, and we fabricated perovskite cells that reached open-circuit voltages of 1.2 volts and power conversion efficiency of over 17% on small areas and 14.7% on 0.715 cm2 cells. By combining these perovskite cells with a 19%-efficient silicon cell, we demonstrated the feasibility of achieving >25%-efficient four-terminal tandem cells.
Efficient perovskite solar cells by metal ion doping
 JTW Wang, ZP Wang, S Pathak, W Zhang, DW Dequilettes, F Wisnivesky-rocca-rivarola, J Huang, PK Nayak, JB Patel, HAM Yusof, Y Vaynzof, R Zhu, I Ramirez, J Zhang, C Ducati, C Grovenor, MB Johnston, DS Ginger, RJ Nicholas, HJ Snaith Energy Environ. Sci., 9:2892-2901 ( 2016) [ pdf ][ ref ]Realizing the theoretical limiting power conversion efficiency (PCE) in perovskite solar cells requires a better understanding and control over the fundamental loss processes occurring in the bulk of the perovskite layer and at the internal semiconductor interfaces in devices. One of the main challenges is to eliminate the presence of charge recombination centres throughout the film which have been observed to be most densely located at regions near the grain boundaries. Here, we introduce aluminium acetylacetonate to the perovskite precursor solution, which improves the crystal quality by reducing the microstrain in the polycrystalline film. At the same time, we achieve a reduction in the non-radiative recombination rate, a remarkable improvement in the photoluminescence quantum efficiency (PLQE) and a reduction in the electronic disorder deduced from an Urbach energy of only 12.6 meV in complete devices. As a result, we demonstrate a PCE of 19.1% with negligible hysteresis in planar heterojunction solar cells comprising all organic p and n-type charge collection layers. Our work shows that an additional level of control of perovskite thin film quality is possible via impurity cation doping, and further demonstrates the continuing importance of improving the electronic quality of the perovskite absorber and the nature of the heterojunctions to further improve the solar cell performance.
Enhanced uv-light stability of planar heterojunction perovskite solar cells with caesium bromide interface modification
 WZ Li, W Zhang, RJ Sutton, JD Fan, AA Haghighirad, MB Johnston, LD Wang, HJ Snaith Energy Environ. Sci., 9:490-498 ( 2016) [ pdf ][ ref ]Interfacial engineering has been shown to play a vital role in boosting the performance of perovskite solar cells in the past few years. Here we demonstrate that caesium bromide (CsBr), as an interfacial modifier between the electron collection layer and the CH3NH3PbI3-xClx absorber layer, can effectively enhance the stability of planar heterojunction devices under ultra violet (UV) light soaking. Additionally, the device performance is improved due to the alleviated defects at the perovskite-titania heterojunction and enhanced electron extraction.
Low Ensemble Disorder in Quantum Well Tube Nanowires
 CL Davies, P Parkinson, N Jiang, JL Boland, S Conesa-Boj, HH Tan, C Jagadish, LM Herz, MB Johnston Nanoscale, 7:20531--20538 (Dec 2015) [ pdf ][ ref ]We have observed very low disorder in high quality quantum well tubes (QWT) in GaAs-Al$_{0.44}$Ga$_{0.56}$As core-multishell nanowires. Room-temperature photoluminescence spectra were measured from 150 single nanowires enabling a full statistical analysis of both intra- and inter-nanowire disorder. By modelling individual nanowire spectra{,} we assigned a quantum well tube thickness{,} a core disorder parameter and a QWT disorder parameter to each nanowire. A strong correlation was observed between disorder in the GaAs cores and disorder in the GaAs QWTs{,} which indicates that variations in core morphology effectively propagate to the shell layers. This highlights the importance of high quality core growth prior to shell deposition. Furthermore{,} variations in QWT thicknesses for different facet directions was found to be a likely cause of intra-wire disorder{,} highlighting the need for accurate shell growth
Charge-Carrier Dynamics and Mobilities in Formamidinium Lead Mixed-Halide Perovskites
 W Rehman, RL Milot, GE Eperon, C Wehrenfennig, JL Boland, HJ Snaith, MB Johnston, LM Herz Adv. Mater., 27:7938--7944 (Dec 2015) [ pdf ][ ref ]The mixed-halide perovskite FAPb(BryI1–y)3 is attractive for color-tunable and tandem solar cells. Bimolecular and Auger charge-carrier recombination rate constants strongly correlate with the Br content, y, suggesting a link with electronic structure. FAPbBr3 and FAPbI3 exhibit charge-carrier mobilities of 14 and 27 cm2 V−1 s−1 and diffusion lengths exceeding 1 μm, while mobilities across the mixed Br/I system depend on crystalline phase disorder.
Colour-selective photodiodes
 MB Johnston Nat. Photonics, 9:633-636 (Oct 2015) [ pdf ][ ref ]Perovskite semiconductors have altered the landscape of solar cell research. Now researchers show that these materials may also offer a flexible platform for colour imaging and wavelength-selective sensing.
Temperature-dependent charge-carrier dynamics in {CH$_3$NH$_3$PbI$_3$} perovskite thin films
 RL Milot, GE Eperon, HJ Snaith, MB Johnston, LM Herz Adv. Funct. Mater., 25:6218-6227 (Oct 2015) [ pdf ][ ref ]The photoluminescence, transmittance, charge-carrier recombination dynamics, mobility, and diffusion length of CH3NH3PbI3 are investigated in the temperature range from 8 to 370 K. Profound changes in the optoelectronic properties of this prototypical photovoltaic material are observed across the two structural phase transitions occurring at 160 and 310 K. Drude-like terahertz photoconductivity spectra at all temperatures above 80 K suggest that charge localization effects are absent in this range. The monomolecular charge-carrier recombination rate generally increases with rising temperature, indicating a mechanism dominated by ionized impurity mediated recombination. Deduced activation energies E-a associated with ionization are found to increase markedly from the room-temperature tetragonal (E-a approximate to 20 meV) to the higher-temperature cubic (E-a approximate to 200 meV) phase adopted above 310 K. Conversely, the bimolecular rate constant decreases with rising temperature as charge-carrier mobility declines, while the Auger rate constant is highly phase specific, suggesting a strong dependence on electronic band structure. The charge-carrier diffusion length gradually decreases with rising temperature from about 3 m at -93 degrees C to 1.2 m at 67 degrees C but remains well above the optical absorption depth in the visible spectrum. These results demonstrate that there are no fundamental obstacles to the operation of cells based on CH3NH3PbI3 under typical field conditions.
 MA Perez-Osorio, RL Milot, MR Filip, JB Patel, LM Herz, MB Johnston, F Giustino J. Phys. Chem. C, 119:25703--25718 (Oct 2015) [ pdf ][ ref ] In this work, we investigate the vibrational properties of the hybrid organic/inorganic halide perovskite MAPbI3 (MA = CH3NH3) in the range 6–3500 cm–1 by combining first-principles density-functional perturbation theory calculations and low-temperature infrared (IR) absorption measurements on evaporated perovskite films. By using a group factor analysis, we establish the symmetry of the normal modes of vibration and predict their IR and Raman activity. We validate our analysis via explicit calculation of the IR intensities. Our calculated spectrum is in good agreement with our measurements. By comparing theory and experiment, we are able to assign most of the features in the IR spectrum. Our analysis shows that the IR spectrum of MAPbI3 can be partitioned into three distinct regions: the internal vibrations of the MA cations (800–3100 cm–1), the cation librations (140–180 cm–1), and the internal vibrations of the PbI3 network (<100 cm–1). The low-frequency region of the IR spectrum is dominated by Pb–I stretching modes of the PbI3 network with Bu symmetry and librational modes of the MA cations. In addition, we find that the largest contributions to the static dielectric constant arise from Pb–I stretching and Pb–I–Pb rocking modes, and that one low-frequency B2u Pb–I stretching mode exhibits a large LO–TO splitting of 50 cm–1.
Plasmonic-induced photon recycling in metal halide perovskite solar cells
 M Saliba, W Zhang, VM Burlakov, SD Stranks, Y Sun, JM Ball, MB Johnston, A Goriely, U Wiesner, HJ Snaith Adv. Funct. Mater., 25:5038-5046 (Aug 2015) [ pdf ][ ref ]Organic-inorganic metal halide perovskite solar cells have emerged in the past few years to promise highly efficient photovoltaic devices at low costs. Here, temperature-sensitive core-shell Ag@TiO2 nanoparticles are successfully incorporated into perovskite solar cells through a low-temperature processing route, boosting the measured device efficiencies up to 16.3%. Experimental evidence is shown and a theoretical model is developed which predicts that the presence of highly polarizable nanoparticles enhances the radiative decay of excitons and increases the reabsorption of emitted radiation, representing a novel photon recycling scheme. The work elucidates the complicated subtle interactions between light and matter in plasmonic photovoltaic composites. Photonic and plasmonic schemes such as this may help to move highly efficient perovskite solar cells closer to the theoretical limiting efficiencies.
Enhanced amplified spontaneous emission in perovskites using a flexible cholesteric liquid crystal reflector
 SD Stranks, SM Wood, K Wojciechowski, F Deschler, M Saliba, H Khandelwal, JB Patel, SJ Elston, LM Herz, MB Johnston, APHJ Schenning, MG Debije, MK Riede, SM Morris, HJ Snaith Nano Lett., 15:4935–4941 (May 2015) [ pdf ][ ref ]Organic-inorganic perovskites are highly promising solar cell materials with laboratory-based power conversion efficiencies already matching those of established thin film technologies. Their exceptional photovoltaic performance is in part attributed to the presence of efficient radiative recombination pathways, thereby opening up the possibility of efficient light-emitting devices. Here, we demonstrate optically pumped amplified spontaneous emission (ASE) at 780 urn from a 50 nm-thick film of CH3NH3PbI3 perovskite that is sandwiched within a cavity composed of a thin-film (similar to 7 mu m) cholesteric liquid crystal (CLC) reflector and a metal back-reflector. The threshold fluence for ASE in the perovskite film is reduced by at least two orders of magnitude in the presence of the CLC reflector, which results in a factor of two reduction in threshold fluence compared to previous reports. We consider this to be due to improved coupling of the oblique and out-of-plane modes that are reflected into the bulk in addition to any contributions from cavity modes. Furthermore, we also demonstrate enhanced ASE on flexible reflectors and discuss how improvements in the quality factor and reflectivity of the CLC layers could lead to single-mode lasing using CLC reflectors. Our work opens up the possibility of fabricating widely wavelength-tunable \"mirror-less\" single-mode lasers on flexible substrates, which could find use in applications such as flexible displays and friend or foe identification.
{In$_x$Ga$_{1-x}$As} nanowires with uniform composition, pure wurtzite crystal phase and taper-free morphology
 AS Ameruddin, HA Fonseka, P Caroff, J Wong-leung, RLMOH Veld, JL Boland, MB Johnston, HH Tan, C Jagadish Nanotechnology, 26:205604 (May 2015) [ pdf ][ ref ]Obtaining compositional homogeneity without compromising morphological or structural quality is one of the biggest challenges in growing ternary alloy compound semiconductor nanowires. Here we report growth of Au-seeded InxGa1-xAs nanowires via metal-organic vapour phase epitaxy with uniform composition, morphology and pure wurtzite (WZ) crystal phase by carefully optimizing growth temperature and V/III ratio. We find that high growth temperatures allow the InxGa1-xAs composition to be more uniform by suppressing the formation of typically observed spontaneous In-rich shells. A low V/III ratio results in the growth of pure WZ phase InxGa1-xAs nanowires with uniform composition and morphology while a high V/III ratio allows pure zinc-blende (ZB) phase to form. Ga incorporation is found to be dependent on the crystal phase favouring higher Ga concentration in ZB phase compared to the WZ phase. Tapering is also found to be more prominent in defective nanowires hence it is critical to maintain the highest crystal structure purity in order to minimize tapering and inhomogeneity. The InP capped pure WZ In0.65Ga0.35As core-shell nanowire heterostructures show 1.54 mu m photoluminescence, close to the technologically important optical fibre telecommunication wavelength, which is promising for application in photodetectors and nanoscale lasers.
Charge selective contacts, mobile ions and anomalous hysteresis in organic-inorganic perovskite solar cells
 Y Zhang, MZ Liu, GE Eperon, TC Leijtens, D McMeekin, M Saliba, W Zhang, A Petrozza, LM Herz, MB Johnston, H Lin, HJ Snaith Mater. Horizons, 2:315-322 (May 2015) [ pdf ][ ref ]High-efficiency perovskite solar cells typically employ an organic-inorganic metal halide perovskite material as light absorber and charge transporter, sandwiched between a p-type electron-blocking organic hole-transporting layer and an n-type hole-blocking electron collection titania compact layer. Some device configurations also include a thin mesoporous layer of TiO2 or Al2O3 which is infiltrated and capped with the perovskite absorber. Herein, we demonstrate that it is possible to fabricate planar and mesoporous perovskite solar cells devoid of an electron selective hole-blocking titania compact layer, which momentarily exhibit power conversion efficiencies (PCEs) of over 13%. This performance is however not sustained and is related to the previously observed anomalous hysteresis in perovskite solar cells. The \"compact layer-free\" meso-superstructured perovskite devices yield a stabilised PCE of only 2.7% while the compact layer-free planar heterojunction devices display no measurable steady state power output when devoid of an electron selective contact. In contrast, devices including the titania compact layer exhibit stabilised efficiency close to that derived from the current voltage measurements. We propose that under forward bias the perovskite diode becomes polarised, providing a beneficial field, allowing accumulation of positive and negative space charge near the contacts, which enables more efficient charge extraction. This provides the required built-in potential and selective charge extraction at each contact to temporarily enable efficient operation of the perovskite solar cells even in the absence of charge selective n-and p-type contact layers. The polarisation of the material is consistent with long range migration and accumulation of ionic species within the perovskite to the regions near the contacts. When the external field is reduced under working conditions, the ions can slowly diffuse away from the contacts redistributing throughout the film, reducing the field asymmetry and the effectiveness of the operation of the solar cells. We note that in light of recent publications showing high efficiency in devices devoid of charge selective contacts, this work reaffirms the absolute necessity to measure and report the stabilised power output under load when characterizing perovskite solar cells.
Fast charge-carrier trapping in {TiO$_2$} nanotubes
 C Wehrenfennig, CM Palumbiny, HJ Snaith, MB Johnston, L Schmidt-mende, LM Herz J. Phys. Chem. C, 119:9159-9168 (Apr 2015) [ pdf ][ ref ]One-dimensional semiconductors such as nanowires and nanotubes are attractive materials for incorporation in photovoltaic devices as they potentially offer short percolation pathways to charge-collecting contacts. We report the observation of free-electron lifetimes in TiO2 nanotubes of the order of tens of picoseconds. These lifetimes are surprisingly short compared to those determined in films of TiO2 nanoparticles. Samples of ordered nanotube arrays with several different tube wall thicknesses were fabricated by anodization and have been investigated by means of optical-pump-terahertz-probe (OPTP) spectroscopy, which allows measurement of transient photoinduced conductivity with picosecond resolution. Our results indicate a two-stage decay of the photoexcited electron population. We attribute the faster component to temporary immobilization of charge in shallow trap states, from which electrons can detrap again by thermal excitation. The slower component most likely reflects irreversible trapping in states deeper below the conduction band edge. Free-electron lifetimes associated with shallow trapping appear to be independent of the tube wall thickness and have very similar values for electrons directly photoexcited in the material and for those injected from an attached photoexcited dye. These results suggest that trap states are not predominantly located at the surface of the tubes. The effective THz charge-carrier mobility in the TiO2 nanotubes is determined (0.1-0.4 cm(2)/(Vs)) and found to be within the same range as carrier mobilities reported for TiO2 nanoparticles. Implications for the relative performance of these nanostructures in dye-sensitized solar cells are discussed.
Highly efficient perovskite solar cells with tunable structural color
 W Zhang, M Anaya, G Lozano, ME Calvo, MB Johnston, H Miguez, HJ Snaith Nano Lett., 15:1698-1702 (Mar 2015) [ pdf ][ ref ]The performance of perovskite solar cells has been progressing over the past few years and efficiency is likely to continue to increase. However, a negative aspect for the integration of perovskite solar cells in the built environment is that the color gamut available in these materials is very limited and does not cover the green-to-blue region of the visible spectrum, which has been a big selling point for organic photovoltaics. Here, we integrate a porous photonic crystal (PC) scaffold within the photoactive layer of an opaque perovskite solar cell following a bottom-up approach employing inexpensive and scalable liquid processing techniques. The photovoltaic devices presented herein show high efficiency with tunable color across the visible spectrum. This now imbues the perovskite solar cells with highly desirable properties for cladding in the built environment and encourages design of sustainable colorful buildings and iridescent electric vehicles as future power generation sources.
Single Nanowire Photoconductive Terahertz Detectors
 K Peng, P Parkinson, L Fu, Q Gao, N Jiang, Y Guo, F Wang, HJ Joyce, JL Boland, HH Tan, C Jagadish, MB Johnston Nano Lett., 15:206-210 (Jan 2015) [ pdf ][ ref ]Spectroscopy and imaging in the terahertz (THz) region of the electromagnetic spectrum has proven to provide important insights in fields as diverse as chemical analysis, materials characterization, security screening, and nondestructive testing. However, compact optoelectronics suited to the most powerful terahertz technique, time-domain spectroscopy, are lacking. Here, we implement single GaAs nanowires as microscopic coherent THz sensors and for the first time incorporated them into the pulsed time-domain technique. We also demonstrate the functionality of the single nanowire THz detector as a spectrometer by using it to measure the transmission spectrum of a 290 GHz low pass filter. Thus, nanowires are shown to be well suited for THz device applications and hold particular promise as near-field THz sensors.
Optical description of mesostructured organic-inorganic halide perovskite solar cells
 M Anaya, G Lozano, ME Calvo, W Zhang, MB Johnston, HJ Snaith, H Miguez J. Phys. Chem. Lett., 6:48-53 (Jan 2015) [ pdf ][ ref ]Herein we describe both theoretically and experimentally the optical response of solution-processed organic inorganic halide perovskite solar cells based on mesostructured scaffolds. We develop a rigorous theoretical model using a method based on the propagation of waves in layered media, which allows visualizing the way in which light is spatially distributed across the device and serves to quantify the fraction of light absorbed by each medium comprising the cell. The discrimination between productive and parasitic absorption yields an accurate determination of the internal quantum efficiency. State-of-the-art devices integrating mesoporous scaffolds infiltrated with perovskite are manufactured and characterized to support the calculations. This combined experimental and theoretical analysis provides a rational understanding of the optical behavior of perovskite cells and can be beneficial for the judicious design of devices with improved performance. Notably, our model justifies the presence of a solid perovskite capping layer in all of the highest efficiency perovskite solar cells based on thinner mesoporous scaffolds.
Efficient, semitransparent neutral-colored solar cells based on microstructured formamidinium lead trihalide perovskite
 GE Eperon, D Bryant, J Troughton, SD Stranks, MB Johnston, T Watson, DA Worsley, HJ Snaith J. Phys. Chem. Lett., 6:129-138 (Jan 2015) [ pdf ][ ref ]Efficient, neutral-colored semitransparent solar cells are of commercial interest for incorporation into the windows and surfaces of buildings and automobiles. Here, we report on semitransparent perovskite solar cells that are both efficient and neutral-colored, even in full working devices. Using the microstructured architecture previously developed, we achieve higher efficiencies by replacing methylammonium lead iodide perovskite with formamidinium lead iodide. Current voltage hysteresis is also much reduced. Furthermore, we apply a novel transparent cathode to the devices, enabling us to fabricate neutral-colored semitransparent full solar cells for the first time. Such devices demonstrate over 5% power conversion efficiency for average visible transparencies of almost 30%, retaining impressive color-neutrality. This makes these devices the best-performing single-junction neutral-colored semitransparent solar cells to date. These microstructured perovskite solar cells are shown to have a significant advantage over silicon solar cells in terms of performance at high incident angles of sunlight, making them ideal for building integration.
Modulation Doping of GaAs/AlGaAs Core--Shell Nanowires With Effective Defect Passivation and High Electron Mobility
 JL Boland, S Conesa-Boj, P Parkinson, G Tutuncuoglu, F Matteini, D Ruffer, A Casadei, F Amaduzzi, F Jabeen, CL Davies, HJ Joyce, LM Herz, A Fontcuberta i Morral, MB Johnston Nano Lett., 15:1336-1342 (Jan 2015) [ pdf ][ ref ] Reliable doping is required to realize many devices based on semiconductor nanowires. Group III–V nanowires show great promise as elements of high-speed optoelectronic devices, but for such applications it is important that the electron mobility is not compromised by the inclusion of dopants. Here we show that GaAs nanowires can be n-type doped with negligible loss of electron mobility. Molecular beam epitaxy was used to fabricate modulation-doped GaAs nanowires with Al0.33Ga0.67As shells that contained a layer of Si dopants. We identify the presence of the doped layer from a high-angle annular dark field scanning electron microscopy cross-section image. The doping density, carrier mobility, and charge carrier lifetimes of these n-type nanowires and nominally undoped reference samples were determined using the noncontact method of optical pump terahertz probe spectroscopy. An n-type extrinsic carrier concentration of 1.10 +- 0.06 × 1016 cm–3 was extracted, demonstrating the effectiveness of modulation doping in GaAs nanowires. The room-temperature electron mobility was also found to be high at 2200 +- 300 cm2 V–1 s–1 and importantly minimal degradation was observed compared with undoped reference nanowires at similar electron densities. In addition, modulation doping significantly enhanced the room-temperature photoconductivity and photoluminescence lifetimes to 3.9 +- 0.3 and 2.4 +- 0.1 ns respectively, revealing that modulation doping can passivate interfacial trap states.
Optical properties and limiting photocurrent of thin-film perovskite solar cells
 JM Ball, SD Stranks, MT Horantner, S Huttner, W Zhang, EJW Crossland, I Ramirez, M Riede, MB Johnston, RH Friend, HJ Snaith Energy Environ. Sci., 8:602-609 ( 2015) [ pdf ][ ref ]Metal-halide perovskite light-absorbers have risen to the forefront of photovoltaics research offering the potential to combine low-cost fabrication with high power-conversion efficiency. Much of the development has been driven by empirical optimisation strategies to fully exploit the favourable electronic properties of the absorber layer. To build on this progress a full understanding of the device operation requires a thorough optical analysis of the device stack providing a platform for maximising the power conversion efficiency through a precise determination of parasitic losses caused by coherence and absorption in the non-photoactive layers. Here we use an optical model based on the transfer-matrix formalism for analysis of perovskite-based planar heterojunction solar cells using experimentally determined complex refractive index data. We compare the modelled properties to experimentally determined data and obtain good agreement revealing that the internal quantum efficiency in the solar cells approaches 100percent. The modelled and experimental dependence of the photocurrent on incidence angle exhibits only a weak variation{,} with very low reflectivity losses at all angles highlighting the potential for useful power generation over a full daylight cycle.
Ultrafast Transient Terahertz Conductivity of Monolayer {MoS$_2$} and {WSe$_2$} Grown by Chemical Vapor Deposition
 CJ Docherty, P Parkinson, HJ Joyce, M Chiu, C Chen, M Lee, L Li, LM Herz, MB Johnston ACS Nano, 8:11147-11153 (Nov 2014) [ pdf ][ ref ] We have measured ultrafast charge carrier dynamics in monolayers and trilayers of the transition metal dichalcogenides MoS2 and WSe2 using a combination of time-resolved photoluminescence and terahertz spectroscopy. We recorded a photoconductivity and photoluminescence response time of just 350 fs from CVD-grown monolayer MoS2, and 1 ps from trilayer MoS2 and monolayer WSe2. Our results indicate the potential of these materials as high-speed optoelectronic materials.
Solution deposition-conversion for planar heterojunction mixed halide perovskite solar cells
 P Docampo, FC Hanusch, SD Stranks, M Doblinger, JM Feckl, M Ehrensperger, NK Minar, MB Johnston, HJ Snaith, T Bein Adv. Energy Mater., 4:1400355 (Oct 2014) [ pdf ][ ref ]The alkylammonium metal trihalide perovskite absorbers first used in working photovoltaic devices were based on liquid elec- trolyte sensitized solar cells. Introduced by Kojima et al., the devices exhibited a starting point power conversion efficiency of 3.8% and, with further work, they were quickly improved to reach over 6%.[1] It was not until a solid-state configuration was employed, however, that high device efficiencies were achieved.[2] Initial results were reported at 9% for perovskite sensitized titania-based devices[2b] and further improvements were simultaneously achieved in a “meso-superstructured” configuration by replacing the mesoporous TiO2 scaffold with an electronically inactive mesoporous Al2O3 layer, exhibiting device efficiencies of over 12%.[2c,3] Some of the key advantages for this material system over other competing device concepts are that they are compatible with solution-processing tech- niques and can be fully processed at low temperatures, thus enabling their use in flexible device applications.[4]
Electron Mobilities Approaching Bulk Limits in Surface-Free'' {GaAs} Nanowires
 HJ Joyce, P Parkinson, N Jiang, CJ Docherty, Q Gao, HH Tan, C Jagadish, LM Herz, MB Johnston Nano Lett., 14:5989-5994 (Sep 2014) [ pdf ][ ref ]Achieving bulk-like charge carrier mobilities in semiconductor nanowires is a major challenge facing the development of nanowire-based electronic devices. Here we demonstrate that engineering the GaAs nanowire surface by overcoating with optimized AlGaAs shells is an effective means of obtaining exceptionally high carrier mobilities and lifetimes. We performed measurements of GaAs/AlGaAs core–shell nanowires using optical pump-terahertz probe spectroscopy: a noncontact and accurate probe of carrier transport on ultrafast time scales. The carrier lifetimes and mobilities both improved significantly with increasing AlGaAs shell thickness. Remarkably, optimized GaAs/AlGaAs core–shell nanowires exhibited electron mobilities up to 3000 cm2 V–1 s–1, reaching over 65% of the electron mobility typical of high quality undoped bulk GaAs at equivalent photoexcited carrier densities. This points to the high interface quality and the very low levels of ionized impurities and lattice defects in these nanowires. The improvements in mobility were concomitant with drastic improvements in photoconductivity lifetime, reaching 1.6 ns. Comparison of photoconductivity and photoluminescence dynamics indicates that midgap GaAs surface states, and consequently surface band-bending and depletion, are effectively eliminated in these high quality heterostructures.
Charge carrier recombination channels in the low-temperature phase of organic-inorganic lead halide perovskite thin films
 C Wehrenfennig, M. Liu, HJ Snaith, MB Johnston, LM Herz APL Mater., 2:081513 (Aug 2014) [ pdf ][ ref ]The optoelectronic properties of the mixed hybrid lead halide perovskite CH3NH3PbI3−xClx have been subject to numerous recent studies related to its extraordinary capabilities as an absorber material in thin film solar cells. While the greatest part of the current research concentrates on the behavior of the perovskite at room temperature, the observed influence of phonon-coupling and excitonic effects on charge carrier dynamics suggests that low-temperature phenomena can give valuable additional insights into the underlying physics. Here, we present a temperature-dependent study of optical absorption and photoluminescence (PL) emission of vapor-deposited CH3NH3PbI3−xClx exploring the nature of recombination channels in the room- and the low-temperature phase of the material. On cooling, we identify an up-shift of the absorption onset by about 0.1 eV at about 100 K, which is likely to correspond to the known tetragonal-to-orthorhombic transition of the pure halide CH3NH3PbI3. With further decreasing temperature, a second PL emission peak emerges in addition to the peak from the room-temperature phase. The transition on heating is found to occur at about 140 K, i.e., revealing significant hysteresis in the system. While PL decay lifetimes are found to be independent of temperature above the transition, significantly accelerated recombination is observed in the low-temperature phase. Our data suggest that small inclusions of domains adopting the room-temperature phase are responsible for this behavior rather than a spontaneous increase in the intrinsic rate constants. These observations show that even sparse lower-energy sites can have a strong impact on material performance, acting as charge recombination centres that may detrimentally affect photovoltaic performance but that may also prove useful for optoelectronic applications such as lasing by enhancing population inversion.
Charge-carrier dynamics in vapour-deposited films of the organolead halide perovskite {CH$_3$NH$_3$PbI$_{3-x}$Cl$_x$}
 C Wehrenfennig, M. Liu, HJ Snaith, MB Johnston, LM Herz Energy Environ. Sci., 7:2269--2275 (Jun 2014) [ pdf ][ ref ]We determine high charge-carrier mobilities [greater-than-or-equal] 33 cm2 V-1 s-1 and bi-molecular recombination rates about five orders of magnitude below the prediction of Langevin{'}s model for vapour-deposited CH3NH3PbI3-xClx using ultrafast THz spectroscopy. At charge-carrier densities below [similar]1017 cm-3 intrinsic diffusion lengths are shown to approach 3 microns{,} limited by slow mono-molecular decay processes.}
An ultrafast carbon nanotube terahertz polarisation modulator
 CJ Docherty, SD Stranks, SN Habisreutinger, HJ Joyce, LM Herz, RJ Nicholas, MB Johnston J. Appl. Phys., 115:203108 (May 2014) [ pdf ][ ref ]We demonstrate ultrafast modulation of terahertz radiation by unaligned optically pumped single-walled carbon nanotubes. Photoexcitation by an ultrafast optical pump pulse induces transient terahertz absorption in nanowires aligned parallel to the optical pump. By controlling the polarisation of the optical pump, we show that terahertz polarisation and modulation can be tuned, allowing sub-picosecond modulation of terahertz radiation. Such speeds suggest potential for semiconductor nanowire devices in terahertz communication technologies.
 C Wehrenfennig, M. Liu, HJ Snaith, MB Johnston, LM Herz J. Phys. Chem. Lett., 5:1300-1306 (Apr 2014) [ pdf ][ ref ] The organic–inorganic hybrid perovskites methylammonium lead iodide (CH3NH3PbI3) and the partially chlorine-substituted mixed halide CH3NH3PbI3–xClx emit strong and broad photoluminescence (PL) around their band gap energy of 1.6 eV. However, the nature of the radiative decay channels behind the observed emission and, in particular, the spectral broadening mechanisms are still unclear. Here we investigate these processes for high-quality vapor-deposited films of CH3NH3PbI3–xClx using time- and excitation-energy dependent photoluminescence spectroscopy. We show that the PL spectrum is homogenously broadened with a line width of 103 meV most likely as a consequence of phonon coupling effects. Further analysis reveals that defects or trap states play a minor role in radiative decay channels. In terms of possible lasing applications, the emission spectrum of the perovskite is sufficiently broad to have potential for amplification of light pulses below 100 fs pulse duration.
 C Wehrenfennig, GE Eperon, MB Johnston, HJ Snaith, LM Herz Adv. Mater., 26:1584-1589 (Mar 2014) [ pdf ][ ref ]Organolead trihalide perovskites are shown to exhibit the best of both worlds: charge carrier mobilities around 10 cm2/V/s and low bi-molecular charge recombination constants. The ratio of the two is found to defy the Langevin limit of kinetic charge capture by over four orders of magnitude. This mechanism causes long (micron) charge-pair diffusion lengths crucial for flat-heterojunction photovoltaics.
Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells
 GE Eperon, SD Stranks, C Menelaou, MB Johnston, LM Herz, HJ Snaith Energy Environ. Sci., 7:982-988 (Mar 2014) [ pdf ][ ref ]Perovskite-based solar cells have attracted significant recent interest, with power conversion efficiencies in excess of 15% already superceding a number of established thin-film solar cell technologies. Most work has focused on a methylammonium lead trihalide perovskites, with a bandgaps of ~1.55 eV and greater. Here, we explore the effect of replacing the methylammonium cation in this perovskite, and show that with the slightly larger formamidinium cation, we can synthesise formamidinium lead trihalide perovskites with a bandgap tunable between 1.48 and 2.23 eV. We take the 1.48 eV-bandgap perovskite as most suited for single junction solar cells, and demonstrate long-range electron and hole diffusion lengths in this material, making it suitable for planar heterojunction solar cells. We fabricate such devices, and due to the reduced bandgap we achieve high short-circuit currents of >23 mA cm-2, resulting in power conversion efficiencies of up to 14.2%, the highest efficiency yet for solution processed planar heterojunction perovskite solar cells. Formamidinium lead triiodide is hence promising as a new candidate for this class of solar cell.
Dependence of Dye Regeneration and Charge Collection on the Pore-Filling Fraction in Solid-State Dye-Sensitized Solar Cells
 CT Weisspfennig, DJ Hollman, C Menelaou, SD Stranks, HJ Joyce, MB Johnston, HJ Snaith, LM Herz Adv. Funct. Mater., 24:668--677 (Feb 2014) [ pdf ][ ref ]Solid-state dye-sensitized solar cells rely on effective infiltration of a solid-state hole-transporting material into the pores of a nanoporous TiO2 network to allow for dye regeneration and hole extraction. Using microsecond transient absorption spectroscopy and femtosecond photoluminescence upconversion spectroscopy, the hole-transfer yield from the dye to the hole-transporting material 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) is shown to rise rapidly with higher pore-filling fractions as the dye-coated pore surface is increasingly covered with hole-transporting material. Once a pore-filling fraction of ≈30% is reached, further increases do not significantly change the hole-transfer yield. Using simple models of infiltration of spiro-OMeTAD into the TiO2 porous network, it is shown that this pore-filling fraction is less than the amount required to cover the dye surface with at least a single layer of hole-transporting material, suggesting that charge diffusion through the dye monolayer network precedes transfer to the hole-transporting material. Comparison of these results with device parameters shows that improvements of the power-conversion efficiency beyond ≈30% pore filling are not caused by a higher hole-transfer yield, but by a higher charge-collection efficiency, which is found to occur in steps. The observed sharp onsets in photocurrent and power-conversion efficiencies with increasing pore-filling fraction correlate well with percolation theory, predicting the points of cohesive pathway formation in successive spiro-OMeTAD layers adhered to the pore walls. From percolation theory it is predicted that, for standard mesoporous TiO2 with 20 nm pore size, the photocurrent should show no further improvement beyond an ≈83% pore-filling fraction.
Single GaAs/AlGaAs Nanowire Photoconductive Terahertz Detectors
 K Peng, P Parkinson, L Fu, Q Gao, N Jiang, Y Guo, F Wang, HJ Joyce, JL Boland, MB Johnston, HH Tan, C Jagadish , 2014:221-222 ( 2014) [ pdf ][ ref ]Photoconductive terahertz detectors based on single GaAs/AlGaAs core-shell nanowire have been designed and fabricated. The devices were characterised in a terahertz time-domain spectroscopy system, showing excellent sensitivity comparable to the standard bulk ion-implanted InP receiver, with a detection bandwidth of 0.1 ~ 0.6 THz. Finite-difference time-domain simulations were performed to understand the origin of the narrow bandwidth of current detectors as well as optimize antenna designs to improve detector performance.
 CK Yong, J Wong-Leung, HJ Joyce, J Lloyd-Hughes, Q Gao, HH Tan, C Jagadish, MB Johnston, LM Herz Nano Lett., 13:4280–4287 (Sep 2013) [ pdf ][ ref ]We have investigated the dynamics of hot charge carriers in InP nanowire ensembles containing a range of densities of zinc-blende inclusions along the otherwise wurtzite nanowires. From time-dependent photoluminescence spectra, we extract the temperature of the charge carriers as a function of time after nonresonant excitation. We find that charge-carrier temperature initially decreases rapidly with time in accordance with efficient heat transfer to lattice vibrations. However, cooling rates are subsequently slowed and are significantly lower for nanowires containing a higher density of stacking faults. We conclude that the transfer of charges across the type II interface is followed by release of additional energy to the lattice, which raises the phonon bath temperature above equilibrium and impedes the carrier cooling occurring through interaction with such phonons. These results demonstrate that type II heterointerfaces in semiconductor nanowires can sustain a hot charge-carrier distribution over an extended time period. In photovoltaic applications, such heterointerfaces may hence both reduce recombination rates and limit energy losses by allowing hot-carrier harvesting.
Efficient planar heterojunction perovskite solar cells by vapour deposition
 M. Liu, MB Johnston, HJ Snaith Nature, 501:395--398 (Sep 2013) [ pdf ][ ref ]Many different photovoltaic technologies are being developed for large-scale solar energy conversion1, 2, 3, 4. The wafer-based first-generation photovoltaic devices1 have been followed by thin-film solid semiconductor absorber layers sandwiched between two charge-selective contacts3 and nanostructured (or mesostructured) solar cells that rely on a distributed heterojunction to generate charge and to transport positive and negative charges in spatially separated phases4, 5, 6. Although many materials have been used in nanostructured devices, the goal of attaining high-efficiency thin-film solar cells in such a way has yet to be achieved7. Organometal halide perovskites have recently emerged as a promising material for high-efficiency nanostructured devices8, 9, 10, 11. Here we show that nanostructuring is not necessary to achieve high efficiencies with this material: a simple planar heterojunction solar cell incorporating vapour-deposited perovskite as the absorbing layer can have solar-to-electrical power conversion efficiencies of over 15 per cent (as measured under simulated full sunlight). This demonstrates that perovskite absorbers can function at the highest efficiencies in simplified device architectures, without the need for complex nanostructures.
Electronic properties of {GaAs}, {InAs} and {InP} nanowires studied by terahertz spectroscopy
 HJ Joyce, CJ Docherty, Q Gao, HH Tan, C Jagadish, J Lloyd-Hughes, LM Herz, MB Johnston Nanotechnology, 24:214006 (May 2013) [ pdf ][ ref ]We have performed a comparative study of ultrafast charge carrier dynamics in a range of III–V nanowires using optical pump–terahertz probe spectroscopy. This versatile technique allows measurement of important parameters for device applications, including carrier lifetimes, surface recombination velocities, carrier mobilities and donor doping levels. GaAs, InAs and InP nanowires of varying diameters were measured. For all samples, the electronic response was dominated by a pronounced surface plasmon mode. Of the three nanowire materials, InAs nanowires exhibited the highest electron mobilities of 6000 cm^2/V/s , which highlights their potential for high mobility applications, such as field effect transistors. InP nanowires exhibited the longest carrier lifetimes and the lowest surface recombination velocity of 170 cm/s. This very low surface recombination velocity makes InP nanowires suitable for applications where carrier lifetime is crucial, such as in photovoltaics. In contrast, the carrier lifetimes in GaAs nanowires were extremely short, of the order of picoseconds, due to the high surface recombination velocity, which was measured as 5.4 × 10^5 cm/s. These findings will assist in the choice of nanowires for different applications, and identify the challenges in producing nanowires suitable for future electronic and optoelectronic devices.
Optimizing the Energy Offset between Dye and Hole-Transporting Material in Solid-State Dye-Sensitized Solar Cells
 CT Weisspfennig, MM Lee, J Teuscher, P Docampo, SD Stranks, HJ Joyce, H Bergmann, I Bruder, DV Kondratuk, MB Johnston, HJ Snaith, LM Herz J. Phys. Chem. C, 117:19850-19858 ( 2013) [ pdf ][ ref ] The power-conversion efficiency of solid-state dye-sensitized solar cells can be optimized by reducing the energy offset between the highest occupied molecular orbital (HOMO) levels of dye and hole-transporting material (HTM) to minimize the loss-in-potential. Here, we report a study of three novel HTMs with HOMO levels slightly above and below the one of the commonly used HTM 2,2′,7,7′- tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD) to systematically explore this possibility. Using transient absorption spectroscopy and employing the ruthenium based dye Z907 as sensitizer, it is shown that, despite one new HTM showing a 100% hole-transfer yield, all devices based on the new HTMs performed worse than those incorporating spiro-OMeTAD. We further demonstrate that the design of the HTM has an additional impact on the electronic density of states present at the TiO2 electrode surface and hence influences not only hole- but also electron-transfer from the sensitizer. These results provide insight into the complex influence of the HTM on charge transfer and provide guidance for the molecular design of new materials.
Strong Carrier Lifetime Enhancement in {GaAs} Nanowires Coated with Semiconducting Polymer
 CK Yong, K Noori, Q Gao, HJ Joyce, HH Tan, C Jagadish, F Giustino, MB Johnston, LM Herz Nano Lett., 12:6293-6301 (Dec 2012) [ pdf ][ ref ] The ultrafast charge carrier dynamics in GaAs/conjugated polymer type II heterojunctions are investigated using time-resolved photoluminescence spectroscopy at 10 K. By probing the photoluminescence at the band edge of GaAs, we observe strong carrier lifetime enhancement for nanowires blended with semiconducting polymers. The enhancement is found to depend crucially on the ionization potential of the polymers with respect to the Fermi energy level at the surface of the GaAs nanowires. We attribute these effects to electron doping by the polymer which reduces the unsaturated surface-state density in GaAs. We find that when the surface of nanowires is terminated by native oxide, the electron injection across the interface is greatly reduced and such surface doping is absent. Our results suggest that surface engineering via π-conjugated polymers can substantially improve the carrier lifetime in nanowire hybrid heterojunctions with applications in photovoltaics and nanoscale photodetectors.
Extreme sensitivity of graphene photoconductivity to environmental gases
 CJ Docherty, C Lin, HJ Joyce, RJ Nicholas, LM Herz, L Li, MB Johnston Nat. Commun., 3:1228 (Nov 2012) [ pdf ][ ref ]Graphene is a single layer of covalently bonded carbon atoms, which was discovered only 8 years ago and yet has already attracted intense research and commercial interest. Initial research focused on its remarkable electronic properties, such as the observation of massless Dirac fermions and the half-integer quantum Hall effect. Now graphene is finding application in touch-screen displays, as channels in high-frequency transistors and in graphene-based integrated circuits. The potential for using the unique properties of graphene in terahertz-frequency electronics is particularly exciting; however, initial experiments probing the terahertz-frequency response of graphene are only just emerging. Here we show that the photoconductivity of graphene at terahertz frequencies is dramatically altered by the adsorption of atmospheric gases, such as nitrogen and oxygen. Furthermore, we observe the signature of terahertz stimulated emission from gas-adsorbed graphene. Our findings highlight the importance of environmental conditions on the design and fabrication of high-speed, graphene-based devices.
Ultra-low Surface Recombination Velocity in InP Nanowires Probed by Terahertz Spectroscopy
 HJ Joyce, J Wong-Leung, C Yong, CJ Docherty, S Paiman, Q Gao, HH Tan, C Jagadish, J Lloyd-Hughes, LM Herz, MB Johnston Nano Lett., 12:5325-5330 (Oct 2012) [ pdf ][ ref ]Using transient terahertz photoconductivity measurements, we have made non-contact, room temperature measurements of the ultrafast charge carrier dynamics in InP nanowires. InP nanowires exhibited a very long photoconductivity lifetime of over 1ns, and carrier lifetimes were remarkably insensitive to surface states despite the large nanowire surface area-to-volume ratio. An exceptionally low surface recombination velocity (170cm/s) was recorded at room temperature. These results suggest that InP nanowires are prime candidates for optoelectronic devices, particularly photovoltaic devices, without the need for surface passivation. We found that the carrier mobility is not limited by nanowire diameter, but is strongly limited by the presence of planar crystallographic defects such as stacking faults in these predominantly wurtzite nanowires. These findings show the great potential of very narrow InP nanowires for electronic devices, but indicate that improvements in the crystallographic uniformity of InP nanowires will be critical for future nanowire device engineering.
Noncontact Measurement of Charge Carrier Lifetime and Mobility in {GaN} Nanowires
 P Parkinson, C Dodson, HJ Joyce, KA Bertness, NA Sanford, LM Herz, MB Johnston Nano Lett., 12:4600--4604 (Sep 2012) [ pdf ][ ref ] The first noncontact photoconductivity measurements of gallium nitride nanowires (NWs) are presented, revealing a high crystallographic and optoelectronic quality achieved by use of catalyst-free molecular beam epitaxy. In comparison with bulk material, the NWs exhibit a long conductivity lifetime (>2 ns) and a high mobility (820 ± 120 cm2/(V s)). This is due to the weak influence of surface traps with respect to other III–V semiconducting NWs and to the favorable crystalline structure of the NWs achieved via strain-relieved growth.
Simulation of fluence-dependent photocurrent in terahertz photoconductive receivers
 E Castro-Camus, MB Johnston, J Lloyd-Hughes Semicond. Sci. Technol., 27:115011 (Sep 2012) [ pdf ][ ref ]A semi-classical Monte Carlo simulation of carrier dynamics in photoconductive detectors of terahertz (THz) radiation is presented. We have used this simulation to elucidate the importance of carrier trapping in the operation of photoconductive detectors. Simulations of the detection of single-cycle THz pulses by photoconductive antennas based on GaAs with trap densities between 2 × 10 17 and 2 × 10 18 cm −3 are presented. We show that the high frequency (>1 THz) spectral response of photoconductive devices decreases with increasing excitation fluence. Our simulations reveal that this effect is a direct consequence of the saturation of trapping centres.
 SD Stranks, C Yong, JA Alexander-Webber, C Weisspfennig, MB Johnston, LM Herz, RJ Nicholas ACS Nano, 6:6058--6066 (Jul 2012) [ pdf ][ ref ]We describe studies of new nanostructured materials consisting of carbon nanotubes wrapped in sequential coatings of two different semiconducting polymers, namely poly(3-hexylthiophene) (P3HT) and poly(9,9’-dioctylfluorene-co-benzothiadiazole) (F8BT). Using absorption spectroscopy, steady state and ultrafast photoluminescence measurements, we demonstrate the role of the different layer structures in controlling energy levels and charge transfer in both solution and film samples. By varying the simple solution processing steps, we can control the ordering and proportions of the wrapping polymers in the solid-state. The resulting novel coaxial structures open up a variety of new applications for nanotube blends and are particularly promising for implementation into organic photovoltaic devices. The carbon nanotube template can also be used to optimise both the electronic properties and morphology of polymer composites in a much more controlled fashion than achieved previously, offering a route to producing a new generation of polymer nanostructures.
Terahertz properties of graphene
 CJ Docherty, MB Johnston J Infrared Milli Terahz Waves, 33:797 (Jul 2012) [ pdf ][ ref ]Graphene has proved itself as being unique in terms of fundamental physics, and of particular importance for post–silicon electronics. Research into graphene has divided into two branches, one probing the remarkable electronic and optical properties of graphene, and the other pursuing technologically viable forms of the material. Terahertz time domain spectroscopy (THz TDS) is a powerful tool for both, able to characterise the free carrier response of graphene and probe the inter and intraband response of excited carriers with sub-ps time resolution. We review THz TDS and related THz measurements of graphene.
Ultrafast Dynamics of Exciton Formation in Semiconductor Nanowires
 CK Yong, HJ Joyce, J Lloyd-Hughes, Q Gao, HH Tan, C Jagadish, MB Johnston, LM Herz Small, 8:1725--1731 (Jun 2012) [ pdf ][ ref ]The dynamics of free electron–hole pairs and excitons in GaAs–AlGaAs–GaAs core–shell–skin nanowires is investigated using femtosecond transient photoluminescence spectroscopy at 10 K. Following nonresonant excitation, a bimolecular interconversion of the initially generated electron–hole plasma into an exciton population is observed. This conducting-to-insulating transition appears to occur gradually over electron–hole charge pair densities of 2–4 × 10^16 cm-3. The smoothness of the Mott transition is attributed to the slow carrier-cooling during the bimolecular interconversion of free charge carriers into excitons and to the presence of chemical-potential fluctuations leading to inhomogeneous spectral characteristics. These results demonstrate that high-quality nanowires are model systems for investigating fundamental scientific effects in 1D heterostructures.
 MB Johnston J Infrared Milli Terahz Waves, 33:391-392 (Apr 2012) [ pdf ][ ref ]Terahertz science and technology is a research area that has expanded rapidly in the past decade, with terahertz spectroscopy and imaging now becoming important tools in applications as diverse as nanotechnology and industrial quality control. The power of terahertz spectroscopy is based on the ability to generate coherent terahertz radiation and to detect its electric field as a function of time. Photoconductive devices were the key technologies that first enabled such coherent emission and detection, and thus seeded modern terahertz spectroscopy. Intense research into photoconductive device technology is currently leading to devices that are highly efficient, robust and industrially compatible. Furthermore, novel photoconductive devices are stimulating the development of yet more novel forms of spectroscopy, and new applications. This Special Issue is devoted to assessing the present status of photoconductive terahertz technologies and to discussing future challenges facing the field.
The Origin of an Efficiency Improving Light Soaking Effect in SnO$_2$ Based Solid-State Dye-Sensitized Solar Cells
 P Tiwana, P Docampo, MB Johnston, LM Herz, HJ Snaith Energy Environ. Sci., 5:9566-9573 ( 2012) [ pdf ][ ref ]We observe a strong light-soaking effect in SnO2 based solid-state dye-sensitized solar cells (SDSCs). Both with and without the presence of UV light, the devices short-circuit photocurrent and efficiency increase significantly over 20-30 minutes, until steady-state is achieved. We demonstrate that this is not due to improved charge collection and investigate the charge generation dynamics employing optical-pump terahertz-probe spectroscopy. We observe a monotonic speeding-up of the generation of free-electrons in the SnO2 conduction band as a function of the light-soaking time. This improved charge generation can be explained by a positive shift in the conduction band edge or, alternatively, an increase in the density of states (DoS) at the energy at which photoinduced electron transfer o
Electron Mobility and Injection Dynamics in Mesoporous {ZnO, SnO$_2$, and TiO$_2$} Films Used in Dye-Sensitized Solar Cells
 P Tiwana, P Docampo, MB Johnston, HJ Snaith, LM Herz ACS Nano, 5:5158-5166 (Jun 2011) [ pdf ][ ref ]High-performance dye-sensitized solar cells are usually fabricated using nanostructured TiO2 as a thin-film electron-collecting material. However, alternative metal-oxides are currently being explored that may offer advantages through ease of processing, higher electron mobility, or interface band energetics. We present here a comparative study of electron mobility and injection dynamics in thin films of TiO2, ZnO, and SnO2 nanoparticles sensitized with Z907 ruthenium dye. Using time-resolved terahertz photoconductivity measurements, we show that, for ZnO and SnO2 nanoporous films, electron injection from the sensitizer has substantial slow components lasting over tens to hundreds of picoseconds, while for TiO2, the process is predominantly concluded within a few picoseconds. These results correlate well with the overall electron injection efficiencies we determine from photovoltaic cells fabricated from identical nanoporous films, suggesting that such slow components limit the overall photocurrent generated by the solar cell. We conclude that these injection dynamics are not substantially influenced by bulk energy level offsets but rather by the local environment of the dye-nanoparticle interface that is governed by dye binding modes and densities of states available for injection, both of which may vary from site to site. In addition, we have extracted the electron mobility in the three nanoporous metal-oxide films at early time after excitation from terahertz conductivity measurements and compared these with the time-averaged, long-range mobility determined for devices based on identical films. Comparison with established values for single-crystal Hall mobilities of the three materials shows that, while electron mobility values for nanoporous TiO2 films are approaching theoretical maximum values, both early time, short distance and interparticle electron mobility in nanoporous ZnO or SnO2 films offer considerable scope for improvement.
{III}--{V} semiconductor nanowires for optoelectronic device applications
 HJ Joyce, Q Gao, HH Tan, C Jagadish, Y Kim, J Zou, LM Smith, HE Jackson, JM Yarrison-Rice, P Parkinson, MB Johnston Prog. Quantum Electron., 35:23-75 (Mar 2011) [ pdf ][ ref ]Semiconductor nanowires have recently emerged as a new class of materials with significant potential to reveal new fundamental physics and to propel new applications in quantum electronic and optoelectronic devices. Semiconductor nanowires show exceptional promise as nanostructured materials for exploring physics in reduced dimensions and in complex geometries, as well as in one-dimensional nanowire devices. They are compatible with existing semiconductor technologies and can be tailored into unique axial and radial heterostructures. In this contribution we review the recent efforts of our international collaboration which have resulted in significant advances in the growth of exceptionally high quality III–V nanowires and nanowire heterostructures, and major developments in understanding the electronic energy landscapes of these nanowires and the dynamics of carriers in these nanowires using photoluminescence, time-resolved photoluminescence and terahertz conductivity spectroscopy.
All-optical full-color displays using polymer nanofibers
 HQ Yu, DW Liao, MB Johnston, BJ Li ACS Nano, 5:2020-2025 (Mar 2011) [ pdf ][ ref ]We report a number of crossed nanofiber structures for full-color micro/nanodisplays, which were formed by assembling flexible poly(trimethylene terephthalate) (PTT) nanofibers under an optical microscope with the assistance of micromanipulators. The color pixels of the displays consist of micro/nanometer sized color spots in a radius of 300-1500 nm, which were realized through crossed junctions of the PTT nanofibers. The colors of the spots were tuned by changing the power ratios of the launched red, green, and blue lights. We further present a new way to develop white light illumination by combination of red, green, and blue lights with assembly techniques and low production costs.
Ultrafast Charge Separation at a Polymer-Single-Walled Carbon Nanotube Molecular Junction
 SD Stranks, C Weisspfennig, P Parkinson, MB Johnston, LM Herz, RJ Nicholas Nano Lett., 11:66-72 (Jan 2011) [ pdf ][ ref ]We have investigated the charge photogeneration dynamics at the interface formed between single-walled carbon nanotubes (SWNTs) and poly(3-hexylthiophene) (P3HT) using a combination of femtosecond spectroscopic techniques. We demonstrate that photoexcitation of P3HT forming a single molecular layer around a SWNT leads to an ultrafast (~430 fs) charge transfer between the materials. The addition of excess P3HT leads to long-term charge separation in which free polarons remain separated at room temperature. Our results suggest that SWNT-P3HT blends incorporating only small fractions (1%) of SWNTs allow photon-to-charge conversion with efficiencies comparable to those for conventional (60:40) P3HT−fullerene blends, provided that small-diameter tubes are individually embedded in the P3HT matrix.
Improved performance of {GaAs}-based terahertz emitters via surface passivation and silicon nitride encapsulation
 C Headley, L Fu, P Parkinson, XL Xu, J Lloyd-Hughes, C Jagadish, MB Johnston IEEE J. Sel. Top. Quantum Electron., 17:17-21 (Jan 2011) [ pdf ][ ref ]We have improved the stability and performance of terahertz (THz) photoconductive (Auston) switches using a combination of (NH4)(2)S surface passivation (SP) and silicon nitride (Si3N4) encapsulation. The influences of SP and encapsulation on the ultrafast electron dynamics in GaAs were examined using THz emission spectroscopy and optical pump-THz probe spectroscopy. The power of THz radiation from the surface of photoexcited GaAs increased by a factor of 5 after passivation and encapsulation, while the process lengthened the trapping time for photoexcited charge carriers. By fabricating and assessing the performance of photoconductive switches, we found that passivation and encapsulation increased the average THz power generated fourfold.
Role of Ultrafast Torsional Relaxation in the Emission from Polythiophene Aggregates
 P Parkinson, C Muller, N Stingelin, MB Johnston, LM Herz J. Phys. Chem. Lett., 1:2788-2792 (Sep 2010) [ pdf ][ ref ]An understanding of aggregation effects in semiconducting polymers is essential for their use in optoelectronic devices; however, the dynamic evolution of such interchain states is not well understood. Here, we have investigated a blend of semiconducting poly(3-hexylthiophene) (P3HT) with an electronically inert ultrahigh-molecular-weight polyethylene (UHMW-PE) matrix that is shown to allow precise control over the extent to which the P3HT chains aggregate. We determined the singlet exciton population within isolated and aggregated P3HT regions using femtosecond time-resolved photoluminescence measurements and found a strong ultrafast decay pathway in the aggregated case only. Comparison of the emission from the two lowest vibronic bands demonstrates a changeover from an initial vibrationally “hot” photoexcited state to a geometrically relaxed aggregate state within 13 ps, corresponding to time scales for torsional relaxation in these materials. We conclude that formation of an aggregate excited state in conjugated polymers is mediated by vibrational relaxation from a low-symmetry to a high-symmetry ordered state for the ensemble.
Dynamic terahertz polarization in single-walled carbon nanotubes
 XL Xu, P Parkinson, K-C Chuang, MB Johnston, RJ Nicholas, LM Herz Phys. Rev. B, 82:085441 (Aug 2010) [ pdf ][ ref ]We have investigated the anisotropic dynamic dielectric response of aligned and well-isolated single-walled carbon nanotubes using optical-pump terahertz (THz)-probe techniques. The polarization anisotropy measurements demonstrate that the THz radiation interacts only with radiation polarized parallel to the nanotubes which have been selectively excited by a polarized pump pulse thus allowing controlled THz polarization to be achieved from unaligned nanotubes.
Terahertz emission from lateral photo-{Dember} currents
 G Klatt, F Hilser, W Qiao, M Beck, R Gebs, A Bartels, K Huska, U Lemmer, G Bastian, MB Johnston, M Fischer, J Faist, T Dekorsy Opt. Express, 18:4939-4947 (Mar 2010) [ pdf ][ ref ]The photo-Dember effect is a source of impulsive THz emission following femtosecond pulsed optical excitation. This emission results from the ultrafast spatial separation of electron-hole pairs in strong carrier gradients due to their different diffusion coefficients. The associated time dependent polarization is oriented perpendicular to the excited surface which is inaptly for efficient out coupling of THz radiation. We propose a scheme for generating strong carrier gradients parallel to the excited surface. The resulting photo-Dember currents are oriented in the same direction and emit THz radiation into the favorable direction perpendicular to the surface. This effect is demonstrated for GaAs and In0.53Ga0.47As. Surprisingly the photo-Dember THz emitters provide higher bandwidth than photoconductive emitters. Multiplexing of phase coherent photo-Dember currents by periodically tailoring the photoexcited spatial carrier distribution gives rise to a strongly enhanced THz emission, which reaches electric field amplitudes comparable to a high-efficiency externally biased photoconductive emitter. (C) 2010 Optical Society of America
Ultrafast Terahertz Conductivity Dynamics in Mesoporous {TiO}$_2$: {Influence} of Dye Sensitization and Surface Treatment in Solid-State Dye-Sensitized Solar Cells
 P Tiwana, P Parkinson, MB Johnston, HJ Snaith, LM Herz J. Phys. Chem. C, 114:1365-1371 (Jan 2010) [ pdf ][ ref ]We have used optical-pump terahertz-probe spectroscopy to explore the photoinduced conductivity dynamics in mesoporous anatase TiO2 films, commonly employed as the electron-transporting electrode in dye-sensitized solar cells. We find an intrinsic mobility value of 0.1 cm(2)/(V s) and diffusion length of similar to 20 nm for electron motion through the TiO2 matrix. The photoconductivity dynamics in TiO2 films, both before and after sensitization with a ruthenium bypyridyl complex termed Z907, were examined in order to study the charge injection, trapping, and recombination time scales. We observe a biphasic charge injection from Z907, with a fast sub-500 fs component, followed by a slower 70-200 ps component. This is followed by photoconductivity decay over the first few nanoseconds, predominantly reflecting charge carrier trapping. In addition, we have utilized terahertz spectroscopy to investigate the influence of treating the titania surface with TiCl4 on early-time charge dynamics, In the solar Cells, Surface treatment of the mesoporous TiO2 with TiCl4 is critical to enable efficient operation. Here, we find that neither early-time charge mobility, nor charge injection rate or decay times are significantly affected by the treatment, which suggests that it may instead have an impact on phenomena occurring on longer time scales.
Extraction of the anisotropic dielectric properties of materials from polarization-resolved terahertz time-domain spectra
 E Castro-Camus, MB Johnston J. Opt. A-Pure Appl. Opt., 11:105206 (Oct 2009) [ pdf ][ ref ]The anisotropic complex dielectric properties of materials in the terahertz band is a topic that has attracted considerable attention recently in the fields of physics, chemistry and biochemistry. The mathematical formalism for analysing polarization-resolved terahertz time-domain data is presented, and particular cases including birefringence, optical activity and circular dichroism are discussed.
Carrier lifetime and mobility enhancement in nearly defect-free core--shell nanowires measured using time-resolved terahertz spectroscopy
 P Parkinson, HJ Joyce, Q Gao, HH Tan, X Zhang, J Zou, C Jagadish, LM Herz, MB Johnston Nano Lett., 9:3349-3353 (Sep 2009) [ pdf ][ ref ]We have used transient terahertz photoconductivity measurements to assess the efficacy of two-temperature growth and core--shell encapsulation techniques on the electronic properties of GaAs nanowires. We demonstrate that two-temperature growth of the GaAs core leads to an almost doubling in charge-carrier mobility and a tripling of carrier lifetime. In addition, overcoating the GaAs core with a larger-bandgap material is shown to reduce the density of surface traps by 82%, thereby enhancing the charge conductivity.
Terahertz Excitonic Response of Isolated Single-Walled Carbon Nanotubes
 X Xu, K Chuang, RJ Nicholas, MB Johnston, LM Herz J. Phys. Chem. C, 113:18106--18109 (Sep 2009) [ pdf ][ ref ]We have investigated the ultrafast far-infrared transmission of isolated single-walled carbon nanotubes using optical-pump THz-probe spectroscopy. The THz dielectric response is dominated by excitons with an initial, rapid decay due to Auger recombination followed by a slow decay of isolated single excitons. Frequency-dependent analysis of the photoinduced dielectric function suggest an internal excitonic excitation at 11 meV with further low-frequency (0.6 and 1.4 THz) absorption features at high densities ascribed to exciton complexes. A featureless conductivity bleaching is attributed to an exciton-induced reduction in the mobility of free carriers caused by phase-space filling.
Photoconductive response correction for detectors of terahertz radiation
 E Castro-Camus, L Fu, J Lloyd-Hughes, HH Tan, C Jagadish, MB Johnston J. Appl. Phys., 104:053113 (Sep 2008) [ pdf ][ ref ]Photoconductive detectors are convenient devices for detecting pulsed terahertz radiation. We have optimized Fe+ ion-damaged InP materials for photoconductive detector signal to noise performance using dual-energy doses in the range from 2.5×1012 to 1.0×1016 cm−2. Ion implantation allowed the production of semiconducting materials with free-carrier lifetimes between 0.5 and 2.1 ps, which were measured by optical pump terahertz probe spectroscopy. The time resolved photoconductivity of the detector substrates was acquired as a function of time after excitation by 2 nJ pulses from a laser oscillator. These data, when combined with a deconvolution algorithm, provide an excellent spectral response correction to the raw photocurrent signal recorded by the photoconductive detectors.
Efficient generation of charges via below-gap photoexcitation of polymer-fullerene blend films investigated by terahertz spectroscopy
 P Parkinson, J Lloyd-Hughes, MB Johnston, LM Herz Phys. Rev. B, 78:115321 (Sep 2008) [ pdf ][ ref ]Using optical-pump terahertz-probe spectroscopy, we have investigated the time-resolved conductivity dynamics of photoexcited polymer-fullerene bulk heterojunction blends for two model polymers: poly[3-hexylthiophene] (P3HT) and poly[2-methoxy-5-(-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) blended with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). The observed terahertz-frequency conductivity is characteristic of dispersive charge transport for photoexcitation both at the −* absorption peak (560 nm for P3HT) and significantly below it (800 nm). The photoconductivity at 800 nm is unexpectedly high, which we attribute to the presence of a charge-transfer complex. We report the excitation-fluence dependence of the photoconductivity over more than four orders of magnitude, obtained by utilizing a terahertz spectrometer based upon on either a laser oscillator or an amplifier source. The time-averaged photoconductivity of the P3HT:PCBM blend is over 20 times larger than that of P3HT, indicating that long-lived hole polarons are responsible for the high photovoltaic efficiency of polymer:fullerene blends. At early times (~ps) the linear dependence of photoconductivity upon fluence indicates that interfacial charge transfer dominates as an exciton decay pathway, generating charges with mobility of at least ~0.1 cm2 V−1 s−1. At later times, a sublinear relationship shows that carrier-carrier recombination effects influence the conductivity on a longer time scale (>1 µs) with a bimolecular charge annihilation constant for the blends that is approximately two to three orders of magnitude smaller than that typical for neat polymer films.
Conductivity of nanoporous {InP} membranes investigated using terahertz spectroscopy
 SKE Merchant, J Lloyd-Hughes, L Sirbu, IM Tiginyanu, P Parkinson, LM Herz, MB Johnston Nanotechnology, 19:395704 (Aug 2008) [ pdf ][ ref ]We have investigated the terahertz conductivity of extrinsic and photoexcited electrons in nanoporous indium phosphide (InP) at different pore densities and orientations. The form of electronic transport in the film was found to differ significantly from that for bulk InP. While photo-generated electrons showed Drude-like transport, the behaviour for extrinsic electrons deviated significantly from the Drude model. Time-resolved photoconductivity measurements found that carrier recombination was slow, with lifetimes exceeding 1~ns for all porosities and orientations. When considered together, these findings suggest that the surfaces created by the nanopores strongly alter the dynamics of both extrinsic and photoexcited electrons.
Terahertz photoconductivity of mobile electrons in nanoporous {InP} honeycombs
 J Lloyd-Hughes, SKE Merchant, L Sirbu, IM Tiginyanu, MB Johnston Phys. Rev. B, 78:085320 (Aug 2008) [ pdf ][ ref ]Nanostructured semiconductors with favorable optoelectronic properties can be created by electrochemical etching, a fabrication process that is scalable for mass market applications. Using terahertz photoconductivity measurements, we demonstrate that nanoporous InP has an unusually long carrier recombination lifetime that exceeds 100 ns at low temperatures and low carrier density, and an electron mobility half that of bulk InP. Modeling confirms that these observations result from band bending with holes confined to the surface and electrons away from the pores.
Low-energy collective dynamics of charge stripes in the doped nickelate {La}$_{2-x}${Sr}$_{x}${NiO}$_{4+\delta}$ observed with optical conductivity measurements
 J Lloyd-Hughes, D Prabhakaran, AT Boothroyd, MB Johnston Phys. Rev. B, 77:195114 (May 2008) [ pdf ][ ref ]We have investigated charge dynamics in the static stripe ordered phase of La_{2-x}Sr_{x}NiO_{4+\\delta} at lattice temperatures below the charge ordering transition, via optical conductivity measurements at low energies (1 – 10 meV). The thermally activated dynamic response of the charge stripes is found to be characteristic of a collective mode such as a pinned charge density wave. At incommensurate doping levels, the pinning energy is reduced, owing to the presence of real-space defects in the stripe order, and a pronounced increase in the oscillator strength is seen. The results provide compelling evidence for the existence of low-energy collective modes of the charge stripes.
Conformational changes of photoactive yellow protein monitored by terahertz spectroscopy
 E Castro-Camus, MB Johnston Chem. Phys. Lett., 455:289-292 (Apr 2008) [ pdf ][ ref ]Observing the structural dynamics of proteins under conditions as close as possible to those in a living organism is essential for understanding the biological functions of proteins accurately. Here we demonstrate that terahertz spectroscopy is a convenient probe of conformational changes in proteins suspended in physiological buffer solution. We have observed that the partial unfolding of photoactive yellow protein leads to a clear increase in absorption at terahertz frequencies. Using normal mode and molecular dynamics simulations we show that this increase in absorption is related to an increase in the density of delocalised vibrational modes in the more flexible partially unfolded state.
Exciton dissociation in polymer field-effect transistors studied using terahertz spectroscopy
 J Lloyd-Hughes, T Richards, H Sirringhaus, MB Johnston, LM Herz Phys. Rev. B, 77:125203 (Mar 2008) [ pdf ][ ref ]We have used terahertz time-domain spectroscopy to investigate photoinduced charge generation in conjugated polymer field-effect transistors. Our measurements show that excitons dissociate in the accumulation layer under the application of a gate voltage, with a quantum efficiency of ~0.1 for an average gate field of ~1×10^8 Vm-1. The transistor history is found to affect the exciton dissociation efficiency, which decreases as holes are increasingly trapped in the accumulation layer. The quantum efficiency of charge formation from excitons is compared with the two contrasting models proposed by Onsager and Arkhipov based on the assumption that field-induced exciton dissociation is assisted by the Brownian diffusive motion or an initial excess energy supplied by excited vibrational modes, respectively.
Terahertz magnetoconductivity of excitons and electrons in quantum cascade structures
 J Lloyd-Hughes, HE Beere, DA Ritchie, MB Johnston Phys. Rev. B, 77:125322 (Mar 2008) [ pdf ][ ref ]We examined the quasiparticles formed by the photoexcitation of GaAs/AlGaAs terahertz quantum cascade structures using terahertz time-domain spectroscopy. At low temperature and excitation density the measured conductivity was excitonic, with a 1s-2p transition energy indicative of three-dimensional excitons correlated across the quantum well barriers. Free electrons increasingly dominated the conductive response at higher lattice temperatures and excitation densities. Under an external magnetic field transitions from the 1s level into 2p states with different magnetic quantum number were observed, while at high excitation densities the electron cyclotron resonance became more prominent.
Excitation-density-dependent generation of broadband terahertz radiation in an asymmetrically excited photoconductive antenna
 PC Upadhya, W Fan, A Burnett, J Cunningham, AG Davies, EH Linfield, J Lloyd-Hughes, E Castro-Camus, MB Johnston, H Beere Opt. Lett., 32:2297 (Aug 2007) [ pdf ][ ref ]The generation of terahertz (THz) transients in photoconductive emitters has been studied by varying the spatial extent and density of the optically excited photocarriers in asymmetrically excited, biased low-temperature-grown GaAs antenna structures. We find a pronounced dependence of the THz pulse intensity and broadband (>6.0 THz) spectral distribution on the pump excitation density and simulate this with a three-dimensional carrier dynamics model. We attribute the observed variation in THz emission to changes in the strength of the screening field.
Transient terahertz conductivity of {GaAs} nanowires
 P Parkinson, J Lloyd-Hughes, Q Gao, HH Tan, C Jagadish, MB Johnston, LM Herz Nano Lett., 7:2162-2165 (Jul 2007) [ pdf ][ ref ]The time-resolved conductivity of isolated GaAs nanowires is investigated by optical-pump terahertz-probe time-domain spectroscopy. The electronic response exhibits a pronounced surface plasmon mode that forms within 300 fs before decaying within 10 ps as a result of charge trapping at the nanowire surface. The mobility is extracted using the Drude model for a plasmon and found to be remarkably high, being roughly one-third of that typical for bulk GaAs at room temperature.
An ion-implanted InP receiver for polarization resolved terahertz spectroscopy
 E Castro-Camus, J Lloyd-Hughes, L Fu, HH Tan, C Jagadish, MB Johnston Opt. Express, 15:7047-7057 (May 2007) [ pdf ][ ref ]We report on the construction, optical alignment and performance of a receiver which is capable of recording the full polarization state of coherent terahertz radiation. The photoconductive detector was fabricated on InP which had been implanted with Fe+ ions. The device operated successfully when it was gated with near infrared femtosecond pulses from either a Ti:sapphire laser oscillator or a 1 kHz regenerative laser amplifier. When illuminated with terahertz radiation from a typical photoconductive source, the optimized device had a signal to noise figure of 100:1 with a usable spectral bandwidth of up to 4 THz. The device was shown to be very sensitive to terahertz polarization, being able to resolve changes in polarization of 0.34 degrees. Additionally, we have demonstrated the usefulness of this device for (i) polarization sensitive terahertz spectroscopy, by measuring the birefringence of quartz and (ii) terahertz emission experiments, by measuring the polarization dependence of radiation generated by optical rectification in (110)-ZnTe.
Plasmonics: Superfocusing of terahertz waves
 MB Johnston Nat. Photon., 1:14-15 (Jan 2007) [ pdf ][ ref ]The promising field of terahertz imaging has long been limited by poor resolution. Researchers now believe that the intriguing properties of surface-plasmon polaritons on corrugated wires could help beat the diffraction limit and inspire a new generation of terahertz photonic devices.
Influence of surface passivation on ultrafast carrier dynamics and terahertz radiation generation in GaAs
 J Lloyd-Hughes, SKE Merchant, L Fu, HH Tan, C Jagadish, E Castro-Camus, MB Johnston Appl. Phys. Lett., 89:232102 (Dec 2006) [ pdf ][ ref ]The carrier dynamics of photoexcited electrons in the vicinity of the surface of (NH4)(2)S-passivated GaAs were studied via terahertz emission spectroscopy and optical-pump terahertz-probe spectroscopy. Terahertz emission spectroscopy measurements, coupled with Monte Carlo simulations of terahertz emission, revealed that the surface electric field of GaAs reverses after passivation. The conductivity of photoexcited electrons was determined via optical-pump terahertz-probe spectroscopy and was found to double after passivation. These experiments demonstrate that passivation significantly reduces the surface state density and surface recombination velocity of GaAs. Finally, it was demonstrated that passivation leads to an enhancement in the power radiated by photoconductive switch terahertz emitters, thereby showing the important influence of surface chemistry on the performance of ultrafast terahertz photonic devices. (c) 2006 American Institute of Physics.
Charge trapping in polymer transistors probed by terahertz spectroscopy and scanning probe potentiometry
 J Lloyd-Hughes, T Richards, H Sirringhaus, E Castro-Camus, LM Herz, MB Johnston Appl. Phys. Lett., 89:112101 (Sep 2006) [ pdf ][ ref ]Terahertz time-domain spectroscopy and scanning probe potentiometry were used to investigate charge trapping in polymer field-effect transistors fabricated on a silicon gate. The hole density in the transistor channel was determined from the reduction in the transmitted terahertz radiation under an applied gate voltage. Prolonged device operation creates an exponential decay in the differential terahertz transmission, compatible with an increase in the density of trapped holes in the polymer channel. Taken in combination with scanning probe potentiometry measurements, these results indicate that device degradation is largely a consequence of hole trapping, rather than of changes to the mobility of free holes in the polymer.
Longitudinal electron bunch profile diagnostics at 45MeV using coherent Smith-Purcell radiation
 G Doucas, V Blackmore, B Ottewell, C Perry, PG Huggard, E Castro-Camus, MB Johnston, J Lloyd-Hughes, MF Kimmitt, B Redlich, A van der Meer Phys. Rev. Spec. Top.-Accel. Beams, 9:092801 (Sep 2006) [ pdf ][ ref ]We have used coherent Smith-Purcell radiation in order to investigate the longitudinal ( temporal) profile of the electron bunch at the FELIX facility. Detection of the far-infrared radiation was achieved by a simple and compact experimental arrangement, consisting of an array of 11 room-temperature pyroelectric detectors. Accurate determination of the background radiation, use of high quality optical filters, and an efficient light collection system are essential for this type of experiment. The radiated power is in good agreement with the predictions of the surface current description of this process. It is concluded that 90% of the bunch particles are contained within 5.5 ps, with a temporal profile that could be approximately triangular in shape.
Simulation and optimisation of terahertz emission from InGaAs and InP photoconductive switches
 J Lloyd-Hughes, E Castro-Camus, MB Johnston Solid State Commun., 136:595-600 (Dec 2005) [ pdf ][ ref ]We simulate the terahertz emission from laterally biased InGaAs and InP using a three-dimensional carrier dynamics model in order to optimise the semiconductor material. Incident pump-pulse parameters of current Ti:Sapphire and Er:fibre lasers are chosen, and the simulation models the semiconductor\'s bandstructure using parabolic T, L and X valleys, and heavy holes. The emitted terahertz radiation is propagated within the semiconductor and into free space using a model based on the Drude-Lorentz dielectric function. As the InGaAs alloy approaches InAs an increase in the emitted power is observed, and this is attributed to a greater electron mobility. Additionally, low-temperature grown and ion-implanted InGaAs are modelled using a finite carrier trapping time. At sub-picosecond trapping times the terahertz bandwidth is found to increase significantly at the cost of a reduced emission power.
Polarization-sensitive terahertz detection by multicontact photoconductive receivers
 E Castro-Camus, J Lloyd-Hughes, MB Johnston, MD Fraser, HH Tan, C Jagadish Appl. Phys. Lett., 86:254102 (Jun 2005) [ pdf ][ ref ]We have developed a terahertz radiation detector that measures both the amplitude and polarization of the electric field as a function of time. The device is a three-contact photoconductive receiver designed so that two orthogonal electric-field components of an arbitrary polarized electromagnetic wave may be detected simultaneously. The detector was fabricated on Fe+ ion-implanted InP. Polarization-sensitive detection is demonstrated with an extinction ratio better than 100:1. This type of device will have immediate application in studies of birefringent and optically active materials in the far-infrared region of the spectrum. (c) 2005 American Institute of Physics.
Three-dimensional carrier-dynamics simulation of terahertz emission from photoconductive switches
 E Castro-Camus, J Lloyd-Hughes, MB Johnston Phys. Rev. B, 71:195301 (May 2005) [ pdf ][ ref ]A semi-classical Monte Carlo model for studying three-dimensional carrier dynamics in photoconductive switches is presented. The model was used to simulate the process of photoexcitation in GaAs-based photoconductive antennas illuminated with pulses typical of mode-locked Ti:Sapphire lasers, We analyzed the power and frequency bandwidth of THz radiation emitted from these devices as a function of bias voltage. pump pulse duration and pump pulse location. We show that the mechanisms limiting the THz power emitted from photoconductive switches fall into two regimes: when illuminated with short duration (< 40 fs) laser pulses the energy distribution of the Gaussian pulses constrains the emitted power. while for long (> 40 fs) pulses, screening is the primary power-limiting mechanism. A discussion of the dynamics of bias field screening in the gap region is presented, The emitted terahertz power was found to be enhanced when the exciting laser pulse was in close proximity to the anode of the photoconductive emitter, in agreement with experimental results, We show that this enhancement arises from the electric field distribution within the emitter combined with a difference in the mobilities of electrons and holes.
Carrier dynamics in ion-implanted GaAs studied by simulation and observation of terahertz emission
 J Lloyd-Hughes, E Castro-Camus, MD Fraser, C Jagadish, MB Johnston Phys. Rev. B, 70:235330 (Dec 2004) [ pdf ][ ref ]We have studied terahertz (THz) emission from arsenic-ion implanted GaAs both experimentally and using a three-dimensional carrier dynamics simulation. A uniform density of vacancies was formed over the optical absorption depth of bulk GaAs samples by performing multienergy implantations of arsenic ions (1 and 2.4 MeV) and subsequent thermal annealing. In a series of THz emission experiments the frequency of peak THz power was found to increase significantly from 1.4 to 2.2 THz when the ion implantation dose was increased from 10(13) to 10(16) cm(-3). We used a semiclassical Monte Carlo simulation of ultrafast carrier dynamics to reproduce and explain these results. The effect of the ion-induced damage was included in the simulation by considering carrier scattering at neutral and charged impurities, as well as carrier trapping at defect sites. Higher vacancy concentrations and shorter carrier trapping times both contributed to shorter simulated THz pulses, the latter being more important over experimentally realistic parameter ranges.
Emission of collimated THz pulses from photo-excited semiconductors
 MB Johnston, A Dowd, R Driver, EH Linfield, AG Davies, DM Whittaker Semicond. Sci. Technol., 19:S449-S451 (Apr 2004) [ pdf ][ ref ]It is shown experimentally that surface-field terahertz (THz) emitters can produce well-collimated beams of THz radiation, making them useful devices for time-domain spectroscopy applications. Simulations of the carrier-dynamics are used to explain the mechanism of THz generation in InAs and GaAs, and it is shown that inter-valley scattering of electrons must be considered in order to fully describe THz emission from InAs.
Selective dielectrophoretic manipulation of surface-immobilized DNA molecules
 WA Germishuizen, C Walti, R Wirtz, MB Johnston, M Pepper, AG Davies, APJ Middelberg Nanotechnology, 14:896-902 (Aug 2003) [ pdf ][ ref ]The fabrication of nanoscale molecular devices is becoming increasingly important and research into their fabrication has intensified over the last few years. In particular, the attachment of molecular objects onto various surfaces has attracted considerable attention. Here, we report a multistep surface immobilization procedure, which allows the specific and controlled attachment of very long DNA molecules onto gold electrodes. Further, we report the effect of dielectrophoresis on these surface-bound DNA molecules with respect to amplitude and frequency, and we show that selected surface-immobilized DNA molecules can be manipulated by dielectrophoresis. Finally, we investigated the use of dielectrophoresis in conjunction with the multistep surface immobilization of fluorescently labelled, surface-bound lambda-DNA in a basic data-storage device.
Low-energy vibrational modes in phenylene oligomers studied by THz time-domain spectroscopy
 MB Johnston, LM Herz, ALT Khan, A Kohler, AG Davies, EH Linfield Chem. Phys. Lett., 377:256-262 (Aug 2003) [ pdf ][ ref ]Low-energy vibrational modes have been investigated in polycrystalline biphenyl, para-terphenyl, para-quaterphenyl and para-sexiphenyl using THz time-domain spectroscopy (THz-TDS). A number of both internal and external infrared-active modes were observed for wavenumbers ranging between 20 and 80 cm(-1). The temperature dependence of these modes is consistent with structural phase transitions occurring in the molecular crystal, indicating that THz-TDS is a sensitive probe of the conformation of conjugated molecular systems. (C) 2003 Elsevier B.V. All rights reserved.
The development of terahertz sources and their applications
 AG Davies, EH Linfield, MB Johnston Phys. Med. Biol., 47:3679-3689 (Nov 2002) [ pdf ][ ref ]The terahertz region of the electromagnetic spectrum spans the frequency range between the mid-infrared and the millimetre/microwave. This region has not been exploited fully to date owing to the limited number of suitable (in particular. coherent) radiation sources and detectors. Recent demonstrations,, using pulsed near-infrared femtosecond laser systems, of the viability of THz medical imaging and spectroscopy have sparked international interests yet much research still needs to be undertaken to optimize both the power and bandwidth in such THz systems. In this paper, we review how femtosecond near-infrared laser pulses can be converted into broad band THz radiation using semiconductor crystals. and discuss in depth the optimization of one specific generation mechanism based on ultra-fast transport of electrons and holes at a semiconductor surface. We also outline a few of the opportunities for a technology that can address a diverse range of challenges spanning the physical and biological sciences, and note the continuing need for the development of solid state, continuous wave. THz sources which operate at room temperature.
Generation of high-power terahertz pulses in a prism
 MB Johnston, DM Whittaker, A Dowd, AG Davies, EH Linfield, X Li, DA Ritchie Opt. Lett., 27:1935-1937 (Nov 2002) [ pdf ][ ref ]A compact, high-power emitter of half-cycle terahertz (THz) radiation is demonstrated. The device consists of an epitaxial InAs emitter upon a GaAs prism and produces THz pulses that are 20 times more powerful than those from conventional planar InAs emitters. This improvement is a direct result of reorienting the transient THz dipole such that its axis is not perpendicular to the emitting surface. (C) 2002 Optical Society of America.
Simulation of terahertz generation at semiconductor surfaces
 MB Johnston, DM Whittaker, A Corchia, AG Davies, EH Linfield Phys. Rev. B, 65:165301 (Apr 2002) [ pdf ][ ref ]A three-dimensional semiclassical Monte Carlo model is presented to describe fast carrier dynamics in semiconductors after photoexcitation. Far-field terahertz (THz) radiation patterns are calculated for both InAs and GaAs with, and without, application of external magnetic fields. This analysis distinguishes between surface depletion field and photo-Dember mechanisms for generating THz radiation. The theoretical model reproduces experimental data from GaAs and InAs, and demonstrates that a magnetic field enhances THz emission by rotating the emitting dipole with respect to the sample surface, leading to an increased coupling of radiation through the surface.
Magnetic-field-induced enhancement of terahertz emission from III-V semiconductor surfaces
 MB Johnston, A Corchia, A Dowd, EH Linfield, AG Davies, R McLaughlin, DD Arnone, M Pepper Physica E, 13:896-899 (Mar 2002) [ pdf ][ ref ]We discuss the origins of the magnetic - field-induced enhancement of terahertz (THz) emission from bulk semiconductor surfaces. The principal effect of the magnetic field is to rotate the THz dipole and hence dramatically increase the THz power radiated through the semiconductor surface. It also significantly affects the ability of the photo-created carriers to screen surface electric fields. The sensitivity of THz emission to the motion of photo-created carriers makes this an ideal probe of hot carrier dynamics both in bulk semiconductors and sophisticated heterostructures. (C) 2002 Elsevier Science B.V. All rights reserved.
Thermally stimulated luminescence in ion-implanted GaAs
 M Gal, LV Dao, E Kraft, MB Johnston, C Carmody, HH Tan, C Jagadish J. Lumines., 96:287-293 (Mar 2002) [ pdf ][ ref ]We have studied the temperature dependence of the luminescence of ion implanted GaAs between 10 and 300 K. We found that at certain temperatures the luminescence increases with increasing temperature. We attribute these localised increases in the luminescence intensity to the thermal excitation of carriers out of traps, or in other words, to thermally stimulated luminescence or thermoluminescence. Model calculations which include thermoluminescence produce excellent agreement with the experimental data and allow us to determine the trap parameters. (C) 2002 Elsevier Science B.V. All rights reserved.
Theory of magnetic-field enhancement of surface-field terahertz emission
 MB Johnston, DM Whittaker, A Corchia, AG Davies, EH Linfield J. Appl. Phys., 91:2104-2106 (Feb 2002) [ pdf ][ ref ]We present a theoretical treatment of surface-field THz generation in semiconductors, which explains the power enhancement observed when a magnetic field is applied. Our model consists of two parts: a Monte Carlo simulation of the dynamics of carriers generated by a subpicosecond optical pulse, and a calculation of the resulting THz radiation emitted through the semiconductor surface. The magnetic field deflects the motion of the carriers, producing a component of the THz dipole parallel to the surface. This causes the power transmitted through the surface to be increased by more than one order of magnitude. (C) 2002 American Institute of Physics.
Effects of magnetic field and optical fluence on terahertz emission in gallium arsenide
 A Corchia, R McLaughlin, MB Johnston, DM Whittaker, DD Arnone, EH Linfield, AG Davies, M Pepper Phys. Rev. B, 6420:205204 (Nov 2001) [ pdf ][ ref ]The excitation density dependence of magnetic-field-enhanced terahertz (THz=10(12) Hz) emission from (100) GaAs is studied. It is found that THz power saturates at a higher optical-excitation density, when a magnetic field is applied. This observation explains the different magnetic field enhancements that have been reported recently. At low excitation densities the results are shown to be consistent with a simple model of carrier-carrier scattering, whilst at higher densities surface field screening becomes important.
Carrier capture and relaxation in Stranski-Krastanow InxGa1-xAs/GaAs(311)B quantum dots
 C Lobo, N Perret, D Morris, J Zou, DJH Cockayne, MB Johnston, M Gal, R Leon Phys. Rev. B, 62:2737-2742 (Jul 2000) [ pdf ][ ref ]We have investigated the structure and optical properties of In0.6Ga0.4As/GaAs(311)B quantum dots (QD'''s) formed by the Stranski-Krastanow growth mode during metal-organic chemical-vapor deposition. We find that (311)B QD structures display a higher energy QD luminescence emission and a stronger wetting-layer emission than (100) QD'''s of similar diameter and density. Temperature-dependent photoluminescence (PL) measurements reveal shallow QD confinement energies and strong interaction between neighboring quantum dots. Longer PL rise times of the ground-state emission of (311)B QD'''s compared to (100) QD'''s are ascribed to the effect of differing numbers, energies, and level spacings of QD confined states on intersublevel relaxation mechanisms at low-carrier excitation densities.
Enhanced coherent terahertz emission from indium arsenide in the presence of a magnetic field
 R McLaughlin, A Corchia, MB Johnston, Q Chen, CM Ciesla, DD Arnone, GAC Jones, EH Linfield, AG Davies, M Pepper Appl. Phys. Lett., 76:2038-2040 (Apr 2000) [ pdf ][ ref ]We demonstrate enhancement of terahertz (THz) emission from indium arsenide at 170 K in magnetic fields (B) up to 8 T. An order of magnitude increase in visible to terahertz conversion efficiency was observed, with no suggestion of saturation of the TE polarization at higher magnetic fields. Free-space electro-optic sampling measurements confirmed the coherent nature of this radiation over the field range investigated, and gave an insight into the carrier motion subsequent to photoexcitation, which may be responsible for the observed THz power enhancement. (C) 2000 American Institute of Physics. [S0003-6951(00)03115-6].
Wavelength shifting of adjacent quantum wells in V-groove quantum wire structure by selective implantation and annealing
 XQ Liu, W Lu, XS Chen, SC Shen, HH Tan, S Yuan, C Jagadish, MB Johnston, LV Dao, M Gal, J Zou, DJH Cockayne J. Appl. Phys., 87:1566-1568 (Feb 2000) [ pdf ][ ref ]Intermixing induced by selective implantation was used to modify the two-dimensional (2D) quantum wells in the V-grooved quantum wire structure. Photoluminescence measurement of the implanted samples shows the obvious blueshift of the interband transition energy while quantum wire is not influenced by implantation. So the selective implantation method has been demonstrated in this article as a useful technique to isolate the energy levels of quantum wire structure from its neighbor 2D structures, which is preferred for the optoelectronic device application of quantum wire. (C) 2000 American Institute of Physics. [S0021-8979(00)02703-1].
Proton implantation and rapid thermal annealing effects on GaAs/AlGaAs quantum well infrared photodetectors
 N Li, N Li, W Lu, XQ Liu, XZ Yuan, ZF Li, HF Dou, SC Shen, Y Fu, M Willander, L Fu, HH Tan, C Jagadish, MB Johnston, M Gal Superlattices Microstruct., 26:317-324 (Nov 1999) [ pdf ][ ref ]We report the use of intermixing techniques to modify GaAs/AlGaAs multiple quantum wells (MQWs). A large shift in the response wavelength of the GaAs/AlGaAs MQW-based infrared photodetector is obtained by proton implantation and then a standard annealing procedure (950 degrees C for 30 s). The photoluminescence (PL) and photoresponse spectra were measured as functions of ion dose in the range from 5 x 10(14) to 2.5 x 10(15) cm(-3). The peak photoresponse wavelength was tunable between 8.2 and 9.8 mu m for the infrared radiation and the energy position of the PL peak from the MQW material changed from 1.62 to 1.645 eV. The effects of the ion implantation and thermal annealing on the device performance have been well characterized theoretically by the inter-diffusion of Al atoms across the GaAs/AlGaAs heterointerfaces and the relaxation energy of free carriers. (C) 1999 Academic Press.
Interdiffused quantum-well infrared photodetectors for color sensitive arrays
 MB Johnston, M Gal, N Li, ZH Chen, XQ Liu, N Li, W Lu, SC Shen, L Fu, HH Tan, C Jagadish Appl. Phys. Lett., 75:923-925 (Aug 1999) [ pdf ][ ref ]Proton implantation and rapid thermal annealing were used to tune the infrared spectral response of quantum-well infrared photodetectors (QWIP) by up to 1.4 mu m. Multiple proton implants at energies between 200 and 420 keV were used to create homogeneous quantum-well intermixing throughout the device'''s multiple-quantum-well structure. Photoluminescence and spectral response measurements were used to study the effect of proton implantation on QWIPs for a series of doses up to 3.5x10(15) protons cm(-2). By using a mask during implantation, a method of constructing a color sensitive array is proposed. (C) 1999 American Institute of Physics. [S0003-6951(99)02333-5].
Proton irradiation-induced intermixing in InGaAs/(Al)GaAs quantum wells and quantum-well lasers
 L Fu, HH Tan, MB Johnston, M Gal, C Jagadish J. Appl. Phys., 85:6786-6789 (May 1999) [ pdf ][ ref ]Proton irradiation with subsequent rapid thermal annealing was used to investigate intermixing of InGaAs/GaAs and InGaAs/AlGaAs quantum wells. Large photoluminescence (PL) energy shifts were observed in both materials. Comparatively, InGaAs/AlGaAs samples showed larger PL energy shifts than InGaAs/GaAs samples because of the presence of Al in the barriers and also better recovery of PL intensities, which is mainly due to dynamic annealing effects in AlGaAs during irradiation. Based on this, InGaAs/AlGaAs quantum-well lasers were fabricated and up to 49.3-nm-emission wavelength shift was observed in the proton-irradiated laser with no significant degradation in device characteristics. (C) 1999 American Institute of Physics. [S0021-8979(99)09609-7].
Si and C delta-doping for device applications
 G Li, MB Johnston, A Babinski, S Yuan, M Gal, SJ Chua, C Jagadish J. Cryst. Growth, 195:54-57 (Dec 1998) [ pdf ][ ref ]Growth conditions have been optimised for Si and C delta-doped AlGaAs at 630 degrees C. Very high free carrier densities up to 6 x 10(18) and 3 x 10(19) cm(-3), respectively, for Si and C delta-doped AlGaAs, were obtained. The key parameters to precisely control delta-doping concentrations were discussed. Growth of high quality Si and C delta-doped nipi structures, Si delta-modulation doped In0.2Ca0.8As/GaAs quantum wells, and high performance Zn-free C delta-doped In0.2Ga0.8As/GaAs GRINSCH lasers were also reported. (C) 1998 Elsevier Science B.V. All rights reserved.
Improved carrier collection in intermixed InGaAs/GaAs quantum wells
 LV Dao, MB Johnston, M Gal, L Fu, HH Tan, C Jagadish Appl. Phys. Lett., 73:3408-3410 (Dec 1998) [ pdf ][ ref ]We have used photoluminescence up conversion to study the carrier capture times into intermixed InGaAs/GaAs quantum wells. We have found that the capture into the intermixed wells is markedly faster than capture into the reference (unintermixed) quantum wells. The reasons for the significant reduction in the capture time is related to the shape of the intermixed quantum well. Such a reduction in the capture time is beneficial both in terms of the quantum efficiency and the frequency response of intermixed optoelectronic devices. (C) 1998 American Institute of Physics. [S0003-6951(98)01849-X].
Si and C delta-doping of GaAs grown by metal organic vapour phase epitaxy for fabrication of nipi doping superlattices
 C Jagadish, G Li, MB Johnston, M Gal Mater. Sci. Eng. B-Solid State Mater. Adv. Technol., 51:103-105 (Feb 1998) [ pdf ][ ref ]The growth conditions for Si and C delta-doped nipi doping superlattices in GaAs have been optimised at the growth temperature of 630 degrees C. We found that the Si delta-doping concentration can be significantly changed by delta-doping time over the range of 10(12)-10(13) cm(-2) at the optimised gas flow velocity. The similar range of the free hole density in C delta-doped GaAs has also been obtained simply by varying the TMAl flow rate during the delta-doping step. The full compensation of free electron and hole density in the Si and C delta-doped nipis can be achieved by choosing proper Si delta-doping time and TMAl flow rate. Growth of one Si and C delta-doped nipi in GaAs was demonstrated. Apart from the well-known effect of photo-excitation intensity on the effective band gap energy, the time-resolved photoluminescence reveals that the photoluminescence peak wavelength significantly increases in the relaxation process of the photo-excited nipi doping superlattice. (C) 1998 Elsevier Science S.A. All rights reserved.
Photoluminescence study of the dynamical properties of GaAs sawtooth superlattices
 MB Johnston, M Gal, G Li, C Jagadish J. Appl. Phys., 82:5748-5752 (Dec 1997) [ pdf ][ ref ]The dynamic properties of a sawtooth superlattice (delta-doped nipi) were examined by photoluminescence (PL) spectroscopic techniques, The structure was grown on a semi-insulating GaAs substrate by metalorganic vapor phase epitaxy, using C and Si delta-doping. The excitation intensity dependence of the sample'''s PL was measured over six decades which produced a shift of 200 meV in the peak of the PL photon energy, The dynamic properties of the sawtooth superlattice were probed using time resolved PL and carrier lifetime measurements. Time resolved PL was measured over 6 orders of magnitude in delay time. The luminescence wavelength from the sawtooth superlattice sample was found to shift to low energies over time after pulsed excitation, indicating the temporal evolution of the band edges. A new and sensitive technique for measuring radiative recombination lifetimes at low excitation intensities was developed. Therefore delta-doped sawtooth superlattices are shown to have a tunable band gap as well as an intensity tunable carrier lifetime. (C) 1997 American Institute of Physics.
Growth of Si and C delta-doped nipi doping superlattices in GaAs by metal organic vapor phase epitaxy
 G Li, C Jagadish, MB Johnston, M Gal Appl. Phys. Lett., 69:4218-4220 (Dec 1996) [ pdf ][ ref ]We have optimized growth conditions of metal organic vapor phase epitaxy (MOVPE) in order to grow Si and C delta-doped nipi doping superlattices in GaAs. Trimethylaluminium (TMAl) and silane (SiH4) were used as p-type and n-type doping precursors, respectively. We report that at 630 degrees C, full compensation of free electrons and holes can be obtained in the MOVPE-grown Si and C delta-doped nipi doping superlattices over a very wide range of the sheet carrier densities (10(12)-10(13) cm(-2)) by choosing proper TMAl flow rate and Si delta-doping time or SiH4 flow rate. The experimental results on electrical and optical characterization of Si and C delta-doped nipi doping superlattices in GaAs with 150 Angstrom thick undoped spacer layers are presented. (C) 1996 American Institute of Physics.
Effects of anodic oxide induced intermixing on the structural and optical properties of quantum wire structure grown on nonplanar GaAs substrate
 Y Kim, S Yuan, R Leon, C Jagadish, M Gal, MB Johnston, MR Phillips, MAS Kalceff, J Zou, DJH Cockayne J. Appl. Phys., 80:5014-5020 (Nov 1996) [ pdf ][ ref ]Effects of anodic oxide induced intermixing on the structural and optical properties of stacked GaAs quantum wire (QWR) structures grown on a sawtooth-type nonplanar GaAs substrate are investigated. Cross-sectional transmission electron microscope (XTEM) observation, temperature dependent photoluminescence (PL) and cathodoluminescence (CL) imaging were used. Intermixing was achieved by pulsed anodic oxidation of the GaAs cap layer and subsequent rapid thermal annealing, was verified by XTEM analysis. A significant enhancement of QWR PL is observed accompanied by a notable blueshift of the sidewall quantum well (SQWL) PL due to the intermixing. Furthermore, an extended necking region is observed after the intermixing by spatially resolved CL. The temperature dependence of the PL intensities of both SQWL and QWR show maxima at approximately T similar to 110 K indicating the role of the extended necking region in feeding carriers to SQWL and QWR. (C) 1996 American Institute of Physics.
Static Electrification by Nonwetting Liquids. Contact Charging and Contact Angles
 VV Yaminsky, MB Johnston Langmuir, 11:4153-4158 (Oct 1995) [ pdf ][ ref ]A glass slide hydrophobized with dimethyldichlorosilane appears to be electrostatically charged when a nonwetting liquid recedes from its surface. The surface charge has negative polarity. The density of the charge which arises on retraction from water is 10(-4) C/m(2). The half-life of the charge on the surface in air is 0.5 h. The charging effect is observed with different liquids and solutions for which contact angle and surface tension are high. A correlation of the value of the charge with the conductivity and double layer parameters is not pronounced. The electrification leads to a long range attraction between the plate and the liquid and influences the wetting of the substrate.