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Publications

1. 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 (2008) [ abstract | pdf | doi:10.1088/0957-4484/19/39/395704 ]
   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.
2. 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 (2008) [ abstract | pdf | doi:10.1103/PhysRevB.78.085320 ]
   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.
3. 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 (2006) [ abstract | pdf | doi:10.1063/1.2398915 ]
   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.