Ultrahigh broad band photoresponse in single Germanium Nanowire photodetectors*

We have investigated photoconductive properties of single Germanium Nanowires (NWs) in the broad spectral range of 300-1100 nm and in the broadband Near Infra-red spectrum. The peak Responsivity (R) reaches ~10⁷A/W at a minimal bias of 2V at λ=850 nm. In NWs with diameter ranging from 30 nm to 90 nm, R lies in the range 10⁵-10⁷A/W. The NWs grown by Vapor-Liquid-Solid mechanism can detect upto ~ 10^-15 W, with Detectivity ~10¹³ cmHz^1/2/W. In this report we discuss the likely origin of the ultra large R that may arise from a combination of various physical effects which are (a) Ge/GeO₂ interface states which act as “scavengers” of electrons from the photo-generated pairs, leaving the holes free to reach the electrodes, (b) Schottky barrier (~0.2 -0.3 eV) at the metal/NW interface which gets lowered substantially due to carrier diffusion in contact region and (c) photodetector length being small (~few μm), negligible loss of photogenerated carriers due to recombination at defect sites. We have confirmed, from power dependence of the optical gain that there is presence of trap states at the surface region and have estimated the surface trap density ~10¹³cm^-2 s^-1 that predominantly lie at the Ge/GeO₂ interface.