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Broad Band Single Germanium Nanowire Photodetectors with Surface Oxide Controlled High Optical Gain

We have investigated photoconductive properties of single Germanium Nanowires(NWs)of diameter less than 100 nm in the spectral range of 300 to 1100 nm showing ultra large peak Responsivity in excess of 10^{7}AW^{-1}.The NWs were grown by Vapor Liquid Solid method using Au nanoparticle as catalyst. In this report we discuss the likely origin of the ultra large responsivity that may arise from a combination of various physical effects which are a): Ge and GeO_{x} interface states which act as scavengers of electrons from the photo-generated pairs,leaving the holes free to reach the electrodes,b) Schottky barrier at the metal and NW interface which gets lowered substantially due to carrier diffusion in contact region and (c) photodetector length being small (approximately few μm), negligible loss of photogenerated carriers due to recombination at defect sites. We have observed from power dependence of the optical gain that the gain is controlled by trap states. We find that the surface of the nanowire has presence of a thin layer of GeO_{x} (as evidenced from HRTEM study) which provide interface states. It is observed that these state play a crucial role to provide a radial field for separation of photogenerated electron and hole pair which in turn leads to very high effective photoconductive gain that reaches a very high at low illumination density.