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Semiconductor nanowire growth and spectroscopy

Publications

1. The 2023 terahertz science and technology roadmap,
   A Leitenstorfer, AS Moskalenko, T Kampfrath, J Kono, E Castro-camus, K Peng, N Qureshi, D Turchinovich, K Tanaka, AG Markelz, M Havenith, C Hough, HJ Joyce, MB Johnston, J Cunningham
   J. Phys. D-Appl. Phys., 56:223001 (2023) [ abstract | pdf | doi:10.1088/1361-6463/acbe4c ]
   Terahertz (THz) radiation encompasses a wide spectral range within the electromagnetic spectrum that extends from microwaves to the far infrared (100 GHz-similar to 30 THz). Within its frequency boundaries exist a broad variety of scientific disciplines that have presented, and continue to present, technical challenges to researchers. During the past 50 years, for instance, the demands of the scientific community have substantially evolved and with a need for advanced instrumentation to support radio astronomy, Earth observation, weather forecasting, security imaging, telecommunications, non-destructive device testing and much more. Furthermore, applications have required an emergence of technology from the laboratory environment to production-scale supply and in-the-field deployments ranging from harsh ground-based locations to deep space. In addressing these requirements, the research and development community has advanced related technology and bridged the transition between electronics and photonics that high frequency operation demands. The multidisciplinary nature of THz work was our stimulus for creating the 2017 THz Science and Technology Roadmap (Dhillon et al 2017 J. Phys. D: Appl. Phys. 50 043001). As one might envisage, though, there remains much to explore both scientifically and technically and the field has continued to develop and expand rapidly. It is timely, therefore, to revise our previous roadmap and in this 2023 version we both provide an update on key developments in established technical areas that have important scientific and public benefit, and highlight new and emerging areas that show particular promise. The developments that we describe thus span from fundamental scientific research, such as THz astronomy and the emergent area of THz quantum optics, to highly applied and commercially and societally impactful subjects that include 6G THz communications, medical imaging, and climate monitoring and prediction. Our Roadmap vision draws upon the expertise and perspective of multiple international specialists that together provide an overview of past developments and the likely challenges facing the field of THz science and technology in future decades. The document is written in a form that is accessible to policy makers who wish to gain an overview of the current state of the THz art, and for the non-specialist and curious who wish to understand available technology and challenges. A such, our experts deliver a 'snapshot' introduction to the current status of the field and provide suggestions for exciting future technical development directions. Ultimately, we intend the Roadmap to portray the advantages and benefits of the THz domain and to stimulate further exploration of the field in support of scientific research and commercial realisation.
2. Polarization anisotropy in nanowires: fundamental concepts and progress towards terahertz-band polarization devices,
   MB Johnston, HJ Joyce
   Prog. Quantum Electron., 85:100417 (2022) [ abstract | pdf | doi:10.1016/j.pquantelec.2022.100417 ]
   Pronounced polarization anisotropy in semiconductor nanowires has been exploited to achieve polarization-sensitive devices operating across the electromagnetic spectrum, from the ultraviolet to the terahertz band. This contribution describes the physical origins of optical and electrical anisotropy in nanowires. Polarization anisotropy arising from dielectric contrast, and the behaviour of (nano)wire grid polarizers, are derived from first principles. This review discusses experimental observations of polarization-sensitive light-matter interactions in nanowires. It then describes how these phenomena are employed in devices that detect or modulate polarized ter-ahertz radiation on ultrafast timescales. Such novel terahertz device concepts are expected to find use in a wide variety of applications including high-speed terahertz-band communications and molecular fingerprinting.
3. 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 (2017) [ abstract | pdf | doi:10.1088/1361-6463/aa6a8f ]
   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.
4. 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 (2017) [ abstract | pdf | doi:10.1021/acs.nanolett.7b00401 ]
   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.
5. 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 (2016) [ abstract | pdf | doi:10.1088/0268-1242/31/10/103003 ]
   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.
6. 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 (2016) [ abstract | pdf | doi:10.1021/acsnano.5b07579 ]
   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.
7. 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 (2015) [ abstract | pdf | doi:10.1021/nl504566t ]
   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.
8. 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 (2015) [ abstract | pdf | doi:10.1021/nl5033843 ]
   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.
9. 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 (2014) [ abstract | pdf | doi:10.1021/nn5034746 ]
   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.
10. 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 (2014) [ abstract | pdf | doi:10.1021/nl503043p ]
    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.
11. 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 (2014) [ abstract | pdf | doi:10.1063/1.4879895 ]
    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.
12. 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 (2014) [ abstract | pdf | doi:10.1002/adfm.201301328 ]
    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.
13. 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) [ abstract | pdf | doi:10.1109/COMMAD.2014.7038695 ]
    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.
14. Electron-Beam Patterning of Polymer Electrolyte Films To Make Multiple Nanoscale Gates for Nanowire Transistors,
    DJ Carrad, AM Burke, RW Lyttleton, HJ Joyce, HH Tan, C Jagadish, K Storm, H Linke, L Samuelson, AP Micolich
    Nano Lett., 14:94-100 (2014) [ abstract | pdf | doi:10.1021/nl403299u ]
    We report an electron-beam based method for the nanoscale patterning of the poly(ethylene oxide)/LiClO4 polymer electrolyte. We use the patterned polymer electrolyte as a high capacitance gate dielectric in single nanowire transistors and obtain subthreshold swings comparable to conventional metal/oxide wrap-gated nanowire transistors. Patterning eliminates gate/contact overlap, which reduces parasitic effects and enables multiple, independently controllable gates. The method’s simplicity broadens the scope for using polymer electrolyte gating in studies of nanowires and other nanoscale devices.
15. Understanding the True Shape of Au-Catalyzed GaAs Nanowires,
    N Jiang, J Wong-Leung, HJ Joyce, Q Gao, HH Tan, C Jagadish
    Nano Lett., 14:5865-5872 (2014) [ abstract | pdf | doi:10.1021/nl5027937 ]
    With increasing interest in nanowire-based devices, a thorough understanding of the nanowire shape is required to gain tight control of the quality of nanowire heterostructures and improve the performance of related devices. We present a systematic study of the sidewalls of Au-catalyzed GaAs nanowires by investigating the faceting process from the beginning with vapor–liquid–solid (VLS) nucleation, followed by the simultaneous radial growth on the sidewalls, and to the end with sidewall transformation during annealing. The VLS nucleation interface of our GaAs nanowires is revealed by examining cross sections of the nanowire, where the nanowire exhibits a Reuleaux triangular shape with three curved surfaces along {112}A. These curved surfaces are not thermodynamically stable and adopt {112}A facets during radial growth. We observe clear differences in radial growth rate between the ⟨112⟩A and ⟨112⟩B directions with {112}B facets forming due to the slower radial growth rate along ⟨112⟩B directions. These sidewalls transform to {110} facets after high temperature (>500 °C) annealing. A nucleation model is proposed to explain the origin of the Reuleaux triangular shape of the nanowires, and the sidewall evolution is explained by surface kinetic and thermodynamic limitations.
16. Electronic comparison of InAs wurtzite and zincblende phases using nanowire transistors,
    AR Ullah, HJ Joyce, AM Burke, J Wong-Leung, HH Tan, C Jagadish, AP Micolich
    Phys. Status Solidi-Rapid Res. Lett., 7:911–-914 (2013) [ abstract | pdf | doi:10.1002/pssr.201308014 ]
    We compare the electronic characteristics of nanowire field-effect transistors made using single pure wurtzite and pure zincblende InAs nanowires grown from identical catalyst particles. We compare the transfer characteristics and field-effect mobility versus temperature for these devices to better understand how differences in InAs phase govern the electronic properties of nanowire transistors.
17. Direct Observation of Charge-Carrier Heating at WZ--ZB {InP} Nanowire Heterojunctions,
    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 (2013) [ abstract | pdf | doi:10.1021/nl402050q ]
    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.
18. 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 (2013) [ abstract | pdf | doi:10.1088/0957-4484/24/21/214006 ]
    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.
19. Phase Separation Induced by {Au} Catalysts in Ternary {InGaAs} Nanowires,
    Y Guo, H Xu, GJ Auchterlonie, T Burgess, HJ Joyce, Q Gao, HH Tan, C Jagadish, H Shu, X Chen, W Lu, Y Kim, J Zou
    Nano Lett., 13:643–650 (2013) [ abstract | pdf | doi:10.1021/nl304237b ]
    We report a novel phase separation phenomenon observed in the growth of ternary InxGa1-xAs nanowires by metalorganic chemical vapor deposition. A spontaneous formation of core–shell nanowires is investigated by cross-sectional transmission electron microscopy, revealing the compositional complexity within the ternary nanowires. It has been found that for InxGa1-xAs nanowires high precursor flow rates generate ternary InxGa1-xAs cores with In-rich shells, while low precursor flow rates produce binary GaAs cores with ternary InxGa1-xAs shells. First-principle calculations combined with thermodynamic considerations suggest that this phenomenon is due to competitive alloying of different group-III elements with Au catalysts, and variations in elemental concentrations of group-III materials in the catalyst under different precursor flow rates. This study shows that precursor flow rates are critical factors for manipulating Au catalysts to produce nanowires of desired composition.
20. 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) [ abstract | pdf | doi:10.1021/jp405734f ]
    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.
21. 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 (2012) [ abstract | pdf | doi:10.1021/nl3034027 ]
    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.
22. 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 (2012) [ abstract | pdf | doi:10.1038/ncomms2235 ]
    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.
23. Precursor flow rate manipulation for the controlled fabrication of twin-free {GaAs} nanowires on silicon substrates.,
    J Kang, Q Gao, P Parkinson, HJ Joyce, H Tan, Y Kim, Y Guo, H Xu, J Zou, C Jagadish
    Nanotechnology, 23:415702 (2012) [ abstract | pdf | doi:10.1088/0957-4484/23/41/415702 ]
    Vertically oriented GaAs nanowires (NWs) are grown on Si(111) substrates using metal-organic chemical vapor deposition. Controlled epitaxial growth along the 111 direction is demonstrated following the deposition of thin GaAs buffer layers and the elimination of structural defects, such as twin defects and stacking faults, is found for high growth rates. By systematically manipulating the AsH(3) (group-V) and TMGa (group-III) precursor flow rates, it is found that the TMGa flow rate has the most significant effect on the nanowire quality. After capping the minimal tapering and twin-free GaAs NWs with an AlGaAs shell, long exciton lifetimes (over 700ps) are obtained for high TMGa flow rate samples. It is observed that the Ga adatom concentration significantly affects the growth of GaAs NWs,
24. 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 (2012) [ abstract | pdf | doi:10.1021/nl3026828 ]
    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.
25. 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 (2012) [ abstract | pdf | doi:10.1021/nl301898m ]
    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.
26. 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 (2012) [ abstract | pdf | doi:10.1002/smll.201200156 ]
    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.
27. Taper-free and vertically oriented ge nanowires on ge/si substrates grown by a two-temperature process,
    JH Kim, SR Moon, HS Yoon, JH Jung, Y Kim, ZG Chen, J Zou, DY Choi, HJ Joyce, Q Gao, HH Tan, C Jagadish
    Cryst. Growth Des., 12:135-141 (2012) [ abstract | pdf | doi:10.1021/cg2008914 ]
    Taper-free and vertically oriented Ge nanowires were grown on Si (111) substrates by chemical vapor deposition with Au nanoparticle catalysts. To achieve vertical nanowire growth on the highly lattice mismatched Si substrate, a thin Ge buffer layer was first deposited, and to achieve taper-free nanowire growth, a two-temperature process was employed. The two-temperature process consisted of a brief initial base growth step at high temperature followed by prolonged growth at lower temperature. Taper-free and defect-free Ge nanowires grew successfully even at 270 degrees C, which is 90 degrees C lower than the bulk eutectic temperature. The yield of vertical and taper-free nanowires is over 90%, comparable to that of vertical but tapered nanowires grown by the conventional one-temperature process. This method is of practical importance and can be reliably used to develop novel nanowire-based devices on relatively cheap Si substrates. Additionally, we observed that the activation energy of Ge nanowire growth by the two-temperature process is dependent on Au nanoparticle size. The low activation energy (similar to 5 kcal/mol) for 30 and SO nm diameter Au nanoparticles suggests that the decomposition of gaseous species on the catalytic Au surface is a rate-limiting step. A higher activation energy (similar to 14 kcal/mol) was determined for 100 nm diameter Au nanoparticles which suggests that larger Au nanoparticles are partially solidified and that growth kinetics become the rate-limiting step.
28. Taper-free and kinked germanium nanowires grown on silicon via purging and the two-temperature process,
    JH Kim, SR Moon, Y Kim, ZG Chen, J Zou, DY Choi, HJ Joyce, Q Gao, HH Tan, C Jagadish
    Nanotechnology, 23:115603 (2012) [ abstract | pdf | doi:10.1088/0957-4484/23/11/115603 ]
    We investigate the growth procedures for achieving taper-free and kinked germanium nanowires epitaxially grown on silicon substrates by chemical vapor deposition. Singly and multiply kinked germanium nanowires consisting of 〈111〉 segments were formed by employing a reactant gas purging process. Unlike non-epitaxial kinked nanowires, a two-temperature process is necessary to maintain the taper-free nature of segments in our kinked germanium nanowires on silicon. As an application, nanobridges formed between (111) side walls of V-grooved (100) silicon substrates have been demonstrated.
29. Removal of Surface States and Recovery of Band-Edge Emission in {InAs} Nanowires through Surface Passivation,
    MH Sun, HJ Joyce, Q Gao, HH Tan, C Jagadish, CZ Ning
    Nano Lett., 12:3378-3384 (2012) [ abstract | pdf | doi:10.1021/nl300015w ]
    Surface states in semiconductor nanowires (NWs) are detrimental to the NW optical and electronic properties and to their light emission-based applications, due to the large surface-to-volume ratio of NWs and the congregation of defects states near surfaces. In this paper, we demonstrated an effective approach to eliminate surface states in InAs NWs of zinc-blende (ZB) and wurtzite (WZ) structures and a dramatic recovery of band edge emission through surface passivation with organic sulfide octadecylthiol (ODT). Microphotoluminescence (PL) measurements were carried out before and after passivation to study the dominant recombination mechanisms and surface state densities of the NWs. For WZ-NWs, we show that the passivation removed the surface states and recovered the band-edge emission, leading to a factor of ∼19 reduction of PL linewidth. For ZB-NWs, the deep surface states were removed and the PL peaks width became as narrow as ∼250 nm with some remaining emission of near band-edge surface states. The passivated NWs showed excellent stability in atmosphere, water, and heat environments. In particular, no observable changes occurred in the PL features from the passivated NWs exposed in air for more than five months.
30. Defect-free GaAs/AlGaAs core-shell nanowires on Si substrates,
    JH Kang, Q Gao, HJ Joyce, HH Tan, C Jagadish, Y Kim, YA Guo, HY Xu, J Zou, MA Fickenscher, LM Smith, HE Jackson, JM Yarrison-rice
    Cryst. Growth Des., 11:3109-3114 (2011) [ abstract | pdf | doi:10.1021/cg2003657 ]
    We report straight and vertically aligned defect-free GaAs nanowires grown on Si(111) substrates by metal-organic chemical vapor deposition. By deposition of thin GaAs buffer layers on Si substrates, these nanowires could be grown on the buffer layers with much less stringent conditions as otherwise imposed by epitaxy of III-V compounds on Si. Also, crystal-defect-free GaAs nanowires were grown by using either a two-temperature growth mode consisting of a short initial nucleation step under higher temperature followed by subsequent growth under lower temperature or a rapid growth rate mode with high source,flow rate. These two growth modes not only eliminated planar crystallographic defects but also significantly reduced tapering: Core-shell GaAs-AlGaAs nanowires grown by the two-temperature growth mode showed improved optical properties with strong photoluminescence and long carrier life times.
31. {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 (2011) [ abstract | pdf | doi:10.1016/j.pquantelec.2011.03.002 ]
    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.
32. Growth of Straight InAs-on-GaAs Nanowire Heterostructures,
    ME Messing, J Wong-Leung, Z Zanolli, HJ Joyce, HH Tan, Q Gao, LR Wallenberg, J Johansson, C Jagadish
    Nano Lett., 11:3899-3905 (2011) [ abstract | pdf | doi:10.1021/nl202051w ]
    One of the main motivations for the great interest in semiconductor nanowires is the possibility of easily growing advanced heterostructures that might be difficult or even impossible to achieve in thin films. For III–V semiconductor nanowires, axial heterostructures with an interchange of the group III element typically grow straight in only one interface direction. In the case of InAs–GaAs heterostructures, straight nanowire growth has been demonstrated for growth of GaAs on top of InAs, but so far never in the other direction. In this article, we demonstrate the growth of straight axial heterostructures of InAs on top of GaAs. The heterostructure interface is sharp and we observe a dependence on growth parameters closely related to crystal structure as well as a diameter dependence on straight nanowire growth. The results are discussed by means of accurate first principles calculations of the interfacial energies. In addition, the role of the gold seed particle, the effect of its composition at different stages during growth, and its size are discussed in relation to the results observed.
33. Tailoring GaAs, InAs, and InGaAs nanowires for optoelectronic device applications,
    HJ Joyce, Q Gao, J Wong-leung, Y Kim, HH Tan, C Jagadish
    IEEE J. Sel. Top. Quantum Electron., 17:766 -778 (2011) [ abstract | pdf | doi:10.1109/JSTQE.2010.2077621 ]
    GaAs, InAs, and InGaAs nanowires each exhibit significant potential to drive new applications in electronic and optoelectronic devices. Nevertheless, the development of these devices depends on our ability to fabricate these nanowires with tight control over critical properties, such as nanowire morphology, orientation, crystal structure, and chemical composition. Although GaAs and InAs are related material systems, GaAs and InAs nanowires exhibit very different growth behaviors. An understanding of these growth behaviors is imperative if high-quality ternary InGaAs nanowires are to be realized. This report examines GaAs, InAs, and InGaAs nanowires, and how their growth may be tailored to achieve desirable material properties. GaAs and InAs nanowire growth are compared, with a view toward the growth of high-quality InGaAs nanowires with device-accessible properties.
34. Self-healing of fractured GaAs nanowires,
    Y Wang, HJ Joyce, Q Gao, X Liao, HH Tan, J Zou, SP Ringer, Z Shan, C Jagadish
    Nano Lett., 11:1546-1549 (2011) [ abstract | pdf | doi:10.1021/nl104330h ]
    In-situ deformation experiments were carried out in a transmission electron microscope to investigate the structural response of single crystal GaAs nanowires (NWs) under compression. A repeatable self-healing process was discovered in which a partially fractured GaAs NW restored its original single crystal structure immediately after an external compressive force was removed. Possible mechanisms of the self-healing process are discussed.
35. Super deformability and Young’s modulus of GaAs nanowires,
    Y Wang, L Wang, HJ Joyce, Q Gao, X Liao, Y Mai, HH Tan, J Zou, SP Ringer, H Gao, C Jagadish
    Adv. Mater., 23:1356--1360 (2011) [ abstract | pdf | doi:10.1002/adma.201004122 ]
    A significant size effect on the mechanical properties of GaAs nanowires (NWs) is reported. A remarkable elastic strain of ≈11% for NWs with diameters of 50–150 nm and obvious plastic deformation in NWs with diameters ≤25 nm are revealed. The Young’s modulus of the NWs can be more than double that of bulk GaAs.
36. CdS/CdSe lateral heterostructure nanobelts by a two-step physical vapor transport method,
    YL Kim, JH Jung, HS Yoon, MS Song, SH Bae, Y Kim, ZG Chen, J Zou, HJ Joyce, Q Gao, HH Tan, C Jagadish
    Nanotechnology, 21:145602 (2010) [ abstract | pdf | doi:10.1088/0957-4484/21/14/145602 ]
    The two-dimensional heterostructure nanobelts with a central CdSe region and lateral CdS structures are synthesized by a two-step physical vapor transport method. The large growth rate difference between lateral CdS structures on both +/-(0001) sides of the CdSe region is found. The growth anisotropy is discussed in terms of the polar nature of the side +/-(0001) surfaces of CdSe. High-resolution transmission electron microscopy reveals the CdSe central region covered with non-uniform CdS layer/islands. From micro-photoluminescence measurements, a systematic blueshift of emission energy from the central CdSe region in accordance with the increase of lateral CdS growth temperature is observed. This result indicates that the intermixing rate in the CdSe region with CdS increases with the increase of lateral CdS growth temperature. In conventional CdSSe ternary nanostructures, morphology and emission wavelength were correlated parameters. However, the morphology and emission wavelength are independently controllable in the CdS/CdSe lateral heterostructure nanobelts. This structure is attractive for applications in visible optoelectronic devices.
37. Phase perfection in zinc blende and wurtzite III-V nanowires using basic growth parameters,
    HJ Joyce, J Wong-leung, Q Gao, HH Tan, C Jagadish
    Nano Lett., 10:908-915 (2010) [ abstract | pdf | doi:10.1021/nl903688v ]
    Controlling the crystallographic phase purity of III-V nanowires is notoriously difficult, yet this is essential for future nanowire devices. Reported methods for controlling nanowire phase require dopant addition, or a restricted choice of nanowire diameter, and only rarely yield a pure phase. Here we demonstrate that phase-perfect nanowires, of arbitrary diameter, can be achieved simply by tailoring basic growth parameters: temperature and V/III ratio. Phase purity is achieved without sacrificing important specifications of diameter and dopant levels. Pure zinc blende nanowires, free of twin defects, were achieved using a low growth temperature coupled with a high Will ratio, Conversely, a high growth temperature coupled with a low Will ratio produced pure wurtzite nanowires free of stacking faults. We present a comprehensive nucleation model to explain the formation of these markedly different crystal phases under these growth conditions. Critical to achieving phase purity are changes in Surface energy of the nanowire side facets, which in turn are controlled by the basic growth parameters of temperature and V/III ratio. This ability to tune crystal structure between twin-free zinc blende and stacking-fault-free,wurtzite not only will enhance the performance of nanowire devices but also opens new possibilities for engineering nanowire devices, without restrictions on nanowire diameters or doping.
38. Novel growth and properties of GaAs nanowires on Si substrates,
    JH Kang, Q Gao, HJ Joyce, HH Tan, C Jagadish, Y Kim, DY Choi, Y Guo, H Xu, J Zou, MA Fickenscher, LM Smith, HE Jackson, JM Yarrison-rice
    Nanotechnology, 21:035604 (2010) [ abstract | pdf | doi:10.1088/0957-4484/21/3/035604 ]
    Straight, vertically aligned GaAs nanowires were grown on Si(111) substrates coated with thin GaAs buffer layers. We find that the V/III precursor ratio and growth temperature are crucial factors influencing the morphology and quality of buffer layers. A double layer structure, consisting of a thin initial layer grown at low V/III ratio and low temperature followed by a layer grown at high V/III ratio and high temperature, is crucial for achieving straight, vertically aligned GaAs nanowires on Si(111) substrates. An in situ annealing step at high temperature after buffer layer growth improves the surface and structural properties of the buffer layer, which further improves the morphology of the GaAs nanowire growth. Through such optimizations we show that vertically aligned GaAs nanowires can be fabricated on Si(111) substrates and achieve the same structural and optical properties as GaAs nanowires grown directly on GaAs(111) B substrates.
39. Vertically oriented epitaxial germanium nanowires on silicon substrates using thin germanium buffer layers.,
    Jae Jung, Hyun Yoon, Yu Kim, Man Song, Yong Kim, Zhi Chen, Jin Zou, Duk Choi, Jung Kang, HJ Joyce, Qiang Gao, HH Tan, C Jagadish
    Nanotechnology, 21:295602 (2010) [ abstract | pdf | doi:10.1088/0957-4484/21/29/295602 ]
    We demonstrate a method to realize vertically oriented Ge nanowires on Si(111) substrates. Ge nanowires were grown by chemical vapor deposition using Au nanoparticles to seed nanowire growth via a vapor-liquid-solid growth mechanism. Rapid oxidation of Si during Au nanoparticle application inhibits the growth of vertically oriented Ge nanowires directly on Si. The present method employs thin Ge buffer layers grown at low temperature less than 600 degrees C to circumvent the oxidation problem. By using a thin Ge buffer layer with root-mean-square roughness of approximately 2 nm, the yield of vertically oriented Ge nanowires is as high as 96.3%. This yield is comparable to that of homoepitaxial Ge nanowires. Furthermore, branched Ge nanowires could be successfully grown on these vertically oriented Ge nanowires by a secondary seeding technique. Since the buffer layers are grown under moderate conditions without any high temperature processing steps, this method has a wide process window highly suitable for Si-based microelectronics.
40. Growth temperature and {V/III} ratio effects on morphology and crystal structure of {InP} nanowires,
    S Paiman, Q Gao, HJ Joyce, Y Kim, HH Tan, C Jagadish, X Zhang, Y Guo, J Zou
    J. Phys. D-Appl. Phys., 43:445402 (2010) [ abstract | pdf | doi:10.1088/0022-3727/43/44/445402 ]
    The effects of growth temperature and V/III ratio on the morphology and crystallographic phases of InP nanowires that are grown by metal organic chemical vapour deposition have been studied. We show that higher growth temperatures or higher V/III ratios promote the formation of wurtzite nanowires while zinc-blende nanowires are favourable at lower growth temperatures and lower V/III ratios. A schematic map of distribution of zinc-blende and wurtzite structures has been developed in the range of growth temperatures (400–510 ◦C) and V/III ratios (44 to 700) investigated in this study.
41. 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 (2009) [ abstract | pdf | doi:10.1021/nl9016336 ]
    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.
42. Evolution of wurtzite structured GaAs shells around InAs nanowire cores,
    M Paladugu, J Zou, YN Guo, X Zhang, HJ Joyce, Q Gao, HH Tan, C Jagadish, Y Kim
    Nanoscale Res. Lett., 4:846-849 (2009) [ abstract | pdf | doi:10.1007/s11671-009-9326-6 ]
    GaAs was radially deposited on InAs nanowires by metal-organic chemical vapor deposition and resultant nanowire heterostructures were characterized by detailed electron microscopy investigations. The GaAs shells have been grown in wurtzite structure, epitaxially on the wurtzite structured InAs nanowire cores. The fundamental reason of structural evolution in terms of material nucleation and interfacial structure is given.
43. Crystallographically driven Au catalyst movement during growth of InAs/GaAs axial nanowire heterostructures,
    M Paladugu, J Zou, YN Guo, X Zhang, HJ Joyce, Q Gao, HH Tan, C Jagadish, Y Kim
    J. Appl. Phys., 105:073503 (2009) [ abstract | pdf | doi:10.1063/1.3103265 ]
    The movement of Au catalysts during growth of InAs on GaAs nanowires has been carefully investigated by transmission electron microscopy. It has been found that Au catalysts preferentially stay on (112)(B) GaAs sidewalls. Since a {112} surface is composed of a {111} facet and a {002} facet and since {111} facets are polar facets for the zinc-blende structure, this crystallographic preference is attributed to the different interface energies caused by the different polar facets. We anticipate that these observations will be useful for the design of nanowire heterostructure based devices. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3103265]
44. Unexpected benefits of rapid growth rate for III-V nanowires,
    HJ Joyce, Q Gao, HH Tan, C Jagadish, Y Kim, MA Fickenscher, S Perera, TB Hoang, LM Smith, HE Jackson, JM Yarrison-Rice, X Zhang, J Zou
    Nano Lett., 9:695-701 (2009) [ abstract | pdf | doi:10.1021/nl803182c ]
    In conventional planar growth of bulk III-V materials, a slow growth rate favors high crystallographic quality, optical quality, and purity of the resulting material. Surprisingly, we observe exactly the opposite effect for Au-assisted GaAs nanowire growth. By employing a rapid growth rate, the! resulting nanowires are markedly less tapered, are free of planar crystallographic defects, and have very high purity with minimal intrinsic dopant incorporation. Importantly, carrier lifetimes are not adversely affected. These results reveal intriguing behavior in the growth of nanoscale materials, and represent a significant advance toward the rational growth of nanowires for device applications.
45. Evolution of epitaxial InAs nanowires on GaAs (111)B,
    X Zhang, J Zou, M Paladugu, YA Guo, Y Wang, Y Kim, HJ Joyce, Q Gao, HH Tan, C Jagadish
    Small, 5:366-369 (2009) [ abstract | pdf | doi:10.1002/smll.200800690 ]
46. Nanowires for optoelectronic device applications,
    Q Gao, HJ Joyce, S Paiman, JH Kang, HH Tan, Y Kim, LM Smith, HE Jackson, JM Yarrison-rice, X Zhang, J Zou, C Jagadish
    Phys. Status Solidi C, 6:2678-2682 (2009) [ abstract | pdf | doi:10.1002/pssc.200982528 ]
    GaAs and InP based nanowires were grown epitaxially on GaAs or InP (111)B substrates by metalorganic chemical vapor deposition using Au nanoparticles as catalyst. In this paper, we will give an overview of nanowire research activities in our group. In particular, the effects of growth parameters for GaAs and InP nanowires on the crystal quality were studied in detail. We demonstrated the ability to obtain defect-free GaAs nanowires via either two-temperature procedure, or by controlling V/III ratio or growth rate. The crystal structure of InP nanowires, ie, WZ or ZB, can also be engineered by just controlling the V/III ratio. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
47. Formation of hierarchical InAs nanoring/GaAs nanowire heterostructures,
    M Paladugu, J Zou, YN Guo, X Zhang, HJ Joyce, Q Gao, HH Tan, C Jagadish, Y Kim
    Angew. Chem.-Int. Edit., 48:780-783 (2009) [ abstract | pdf | doi:10.1002/anie.200804630 ]
48. High purity {GaAs} nanowires free of planar defects: growth and characterization,
    HJ Joyce, Q Gao, HH Tan, C Jagadish, Y Kim, MA Fickenscher, S Perera, TB Hoang, LM Smith, HE Jackson, JM Yarrison-{R}ice, X Zhang, J Zou
    Adv. Funct. Mater., 18:3794-3800 (2008) [ abstract | pdf | doi:10.1002/adfm.200800625 ]
    We investigate how to tailor the structural, crystallographic and optical properties of GaAs nanowires. Nanowires were grown by Au nanoparticle-catalyzed metalorganic chemical vapor deposition. A high arsine flow rate, that is, a high ratio of group V to group I I I precursors, imparts significant advantages. It dramatically reduces planar crystallographic defects and reduces intrinsic carbon dopant incorporation. Increasing V/III ratio further, however, instigates nanowire kinking and increases nanowire tapering. By choosing an intermediate V/III ratio we achieve uniform, vertically aligned GaAs nanowires, free of planar crystallographic defects, with excellent optical properties and high purity. These findings will greatly assist the development of future GaAs nanowire-based electronic and optoelectronic devices, and are expected to be more broadly relevant to the rational synthesis of other III-V nanowires.
49. Polarity driven formation of InAs/GaAs hierarchical nanowire heterostructures,
    M Paladugu, J Zou, YN Guo, X Zhang, HJ Joyce, Q Gao, HH Tan, C Jagadish, Y Kim
    Appl. Phys. Lett., 93:201908 (2008) [ abstract | pdf | doi:10.1063/1.3033551 ]
    The structural and morphological characteristics of InAs/GaAs radial nanowire heterostructures were investigated using transmission electron microscopy. It has been found that the radial growth of InAs was preferentially initiated on the {112}(A) sidewalls of GaAs nanowires. This preferential deposition leads to extraordinarily asymmetric InAs/GaAs radial nanowire heterostructures. Such formation of radial nanowire heterostructures provides an opportunity to engineer hierarchical nanostructures, which further widens the potential applications of semiconductor nanostructures.
50. Nature of heterointerfaces in GaAs/InAs and InAs/GaAs axial nanowire heterostructures,
    M Paladugu, J Zou, YN Guo, X Zhang, Y Kim, HJ Joyce, Q Gao, HH Tan, C Jagadish
    Appl. Phys. Lett., 93:101911 (2008) [ abstract | pdf | doi:10.1063/1.2978959 ]
    The structural and compositional characteristics of heterointerfaces of Au-catalyzed GaAs/InAs and InAs/GaAs axial nanowire heterostructures were comprehensively investigated by transmission electron microscopy. It has been found that the GaAs/InAs interface is not sharp and contains an InGaAs transition segment, and in contrast, the InAs/GaAs interface is atomically sharp. This difference in the nature of heterointerfaces can be attributed to the difference in the affinity of the group III elements with the catalyst material. (C) 2008 American Institute of Physics.
51. Nearly intrinsic exciton lifetimes in single twin-free GaAs/AlGaAs core-shell nanowire heterostructures,
    S Perera, MA Fickenscher, HE Jackson, LM Smith, JM Yarrison-rice, HJ Joyce, Q Gao, HH Tan, C Jagadish, X Zhang, J Zou
    Appl. Phys. Lett., 93:053110 (2008) [ abstract | pdf | doi:10.1063/1.2967877 ]
    CW and time-resolved photoluminescence measurements are used to investigate exciton recombination dynamics in GaAs/AlGaAs heterostructure nanowires grown with a recently developed technique which minimizes twinning. A thin capping layer is deposited to eliminate the possibility of oxidation of the AlGaAs shell as a source of oxygen defects in the GaAs core. We observe exciton lifetimes of similar to 1 ns, comparable to high quality two-dimensional double heterostructures. These GaAs nanowires allow one to observe state filling and many-body effects resulting from the increased carrier densities accessible with pulsed laser excitation.
52. Vertically standing Ge nanowires on GaAs(110) substrates,
    MS Song, JH Jung, Y Kim, Y Wang, J Zou, HJ Joyce, Q Gao, HH Tan, C Jagadish
    Nanotechnology, 19:125602 (2008) [ abstract | pdf | doi:10.1088/0957-4484/19/12/125602 ]
    The growth of epitaxial Ge nanowires is investigated on (100), (111) B and (110) GaAs substrates in the growth temperature range from 300 to 380 degrees C. Unlike epitaxial Ge nanowires on Ge or Si substrates, Ge nanowires on GaAs substrates grow predominantly along the < 110 > direction. Using this unique property, vertical < 110 > Ge nanowires epitaxially grown on GaAs(110) surface are realized. In addition, these Ge nanowires exhibit minimal tapering and uniform diameters, regardless of growth temperatures, which is an advantageous property for device applications. Ge nanowires growing along the < 110 > directions are particularly attractive candidates for forming nanobridge devices on conventional (100) surfaces.
53. Polarization and temperature dependence of photoluminescence from zincblende and wurtzite {InP} nanowires,
    A Mishra, LV Titova, TB Hoang, HE Jackson, LM Smith, JM Yarrison-rice, Y Kim, HJ Joyce, Q Gao, HH Tan, C Jagadish
    Appl. Phys. Lett., 91:263104 (2007) [ abstract | pdf | doi:10.1063/1.2828034 ]
    We use polarization-resolved and temperature-dependent photoluminescence of single zincblende (ZB) (cubic) and wurtzite (WZ) (hexagonal) InP nanowires to probe differences in selection rules and bandgaps between these two semiconductor nanostructures. The WZ nanowires exhibit a bandgap 80 meV higher in energy than the ZB nanowires. The temperature dependence of the PL is similar but not identical for the WZ and ZB nanowires. We find that ZB nanowires exhibit strong polarization parallel to the nanowire axis, while the WZ nanowires exhibit polarized emission perpendicular to the nanowire axis. This behavior is interpreted in terms of the different selection rules for WZ and ZB crystal structures. (c) 2007 American Institute of Physics.
54. Novel growth phenomena observed in axial {InAs}/{GaAs} nanowire heterostructures,
    M Paladugu, J Zou, YN Guo, GJ Auchterlonie, HJ Joyce, Q Gao, HH Tan, C Jagadish, Y Kim
    Small, 3:1873-1877 (2007) [ abstract | pdf | doi:10.1002/smll.200700222 ]
55. Dynamics of strongly degenerate electron-hole plasmas and excitons in single InP nanowires,
    LV Titova, TB Hoang, JM Yarrison-rice, HE Jackson, Y Kim, HJ Joyce, Q Gao, HH Tan, C Jagadish, X Zhang, J Zou, LM Smith
    Nano Lett., 7:3383-3387 (2007) [ abstract | pdf | doi:10.1021/nl071733l ]
    Low-temperature time-resolved photoluminescence spectroscopy is used to probe the dynamics of photoexcited carriers in single InP nanowires. At early times after pulsed excitation, the photoluminescence line shape displays a characteristic broadening, consistent with emission from a degenerate, high-density electron-hole plasma. As the electron-hole plasma cools and the carrier density decreases, the emission rapidly converges toward a relatively narrow band consistent with free exciton emission from the InP nanowire. The free excitons in these single InP nanowires exhibit recombination lifetimes closely approaching that measured in a high-quality epilayer, suggesting that in these InP nanowires, electrons and holes are relatively insensitive to surface states. This results in higher quantum efficiencies than other single-nanowire systems as well as significant state-filling and band gap renormalization, which is observed at high electron-hole carrier densities.
56. Evolution of InAs branches in InAs/GaAs nanowire heterostructures,
    M Paladugu, J Zou, GJ Auchterlonie, YN Guo, Y Kim, HJ Joyce, Q Gao, HH Tan, C Jagadish
    Appl. Phys. Lett., 91:133115 (2007) [ abstract | pdf | doi:10.1063/1.2790486 ]
    Branched nanowire heterostructures of InAs/GaAs were observed during Au-assisted growth of InAs on GaAs nanowires. The evolution of these branches has been determined through detailed electron microscopy characterization with the following sequence: (1) in the initial stage of InAs growth, the Au droplet is observed to slide down the side of the GaAs nanowire, (2) the downward movement of Au nanoparticle later terminates when the nanoparticle encounters InAs growing radially on the GaAs nanowire sidewalls, and (3) with further supply of In and As vapor reactants, the Au nanoparticles assist the formation of InAs branches with a well-defined orientation relationship with GaAs/InAs core/shell stems. We anticipate that these observations advance the understanding of the kink formation in axial nanowire heterostructures. (C) 2007 American Institute of Physics.
57. Twin-free uniform epitaxial {GaAs} nanowires grown by a two-temperature process,
    HJ Joyce, Q Gao, HH Tan, C Jagadish, Y Kim, X Zhang, YN Guo, J Zou
    Nano Lett., 7:921-926 (2007) [ abstract | pdf | doi:10.1021/nl062755v ]
    We demonstrate vertically aligned epitaxial GaAs nanowires of excellent crystallographic quality and optimal shape, grown by Au nanoparticle-catalyzed metalorganic chemical vapor deposition. This is achieved by a two-temperature growth procedure, consisting of a brief initial high-temperature growth step followed by prolonged growth at a lower temperature. The initial high-temperature step is essential for obtaining straight, vertically aligned epitaxial nanowires on the (111)B GaAs substrate. The lower temperature employed for subsequent growth imparts superior nanowire morphology and crystallographic quality by minimizing radial growth and eliminating twinning defects. Photoluminescence measurements confirm the excellent optical quality of these two-temperature grown nanowires. Two mechanisms are proposed to explain the success of this two-temperature growth process, one involving Au nanoparticle-GaAs interface conditions and the other involving melting-solidification temperature hysteresis of the Au-Ga nanoparticle alloy.
58. Growth mechanism of truncated triangular {III}--{V} nanowires,
    J Zou, M Paladugu, H Wang, GJ Auchterlonie, YN Guo, Y Kim, Q Gao, HJ Joyce, HH Tan, C Jagadish
    Small, 3:389-393 (2007) [ abstract | pdf | doi:10.1002/smll.200600503 ]
59. Resonant excitation and imaging of nonequilibrium exciton spins in single core-shell {GaAs}-{AlGaAs} nanowires,
    TB Hoang, LV Titova, JM Yarrison-Rice, HE Jackson, AO Govorov, Y Kim, HJ Joyce, HH Tan, C Jagadish, LM Smith
    Nano Lett., 7:588-595 (2007) [ abstract | pdf | doi:10.1021/nl062383q ]
    Nonequilibrium spin distributions in single GaAs/AlGaAs core-shell nanowires are excited using resonant polarized excitation at 10 K. At all excitation energies, we observe strong photoluminescence polarization due to suppressed radiative recombination of excitons with dipoles aligned perpendicular to the nanowire. Excitation resonances are observed at 1- or 2-LO phonon energies above the exciton ground states. Using rate equation modeling, we show that, at the lowest energies, strongly nonequilibrium spin distributions are present and we estimate their spin relaxation rate.
60. Temperature dependence of photoluminescence from single core-shell {GaAs}-{AlGaAs} nanowires,
    LV Titova, TB Hoang, HE Jackson, LM Smith, JM Yarrison-Rice, Y Kim, HJ Joyce, HH Tan, C Jagadish
    Appl. Phys. Lett., 89:173126 (2006) [ abstract | pdf | doi:10.1063/1.2402234 ]
    Temperature-dependent polarized microphotoluminescence measurements of single GaAs/AlGaAs core-shell nanowires are used to probe their electronic states. The low-temperature emission from these wires is strongly enhanced compared with that observed in bare GaAs nanowires and is strongly polarized, reflecting the dielectric mismatch between the nanowire and the surrounding air. The temperature-dependent band gap of the nanowires is seen to be somewhat different from that observed in bulk GaAs, and the PL rapidly quenches above 120 K, with an activation energy of 17 meV reflecting the presence of nonradiative defects. (c) 2006 American Institute of Physics.
61. Influence of nanowire density on the shape and optical properties of ternary {InGaAs} nanowires,
    Y Kim, HJ Joyce, O Gao, HH Tan, C Jagadish, M Paladugu, J Zou, AA Suvorova
    Nano Lett., 6:599 (2006) [ abstract | pdf | doi:10.1021/nl052189o ]
    We have synthesized ternary InGaAs nanowires on (111)B GaAs surfaces by metal-organic chemical vapor deposition. Au colloidal nanoparticles were employed to catalyze nanowire growth. We observed the strong influence of nanowire density on nanowire height, tapering, and base shape specific to the nanowires with high In composition. This dependency was attributed to the large difference of diffusion length on (111)B surfaces between In and Ga reaction species, with In being the more mobile species. Energy dispersive X-ray spectroscopy analysis together with high-resolution electron microscopy study of individual InGaAs nanowires shows large In/Ga compositional variation along the nanowire supporting the present diffusion model. Photoluminescence spectra exhibit a red shift with decreasing nanowire density due to the higher degree of In incorporation in more sparsely distributed InGaAs nanowires.