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

Abstract

We have investigated photoconductive properties of single germanium nanowires (NWs) of diameter <100 nm in the spectral range of 300–1100 nm and in the broad band near-infrared spectrum showing peak responsivity at a minimal bias of 2 V. The NWs were grown by the vapor–liquid–solid method using Au nanoparticles as the catalyst. In this report, we discuss the likely origin of the ultra large that may arise from a combination of various physical effects which are (a) Ge/GeOx interface states which act as “scavengers” of electrons from the photogenerated 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 because of carrier diffusion in the contact region, and (c) photodetector length which is small (∼few μm), and there is negligible loss of photogenerated carriers because of 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 the presence of a thin layer of GeOx (as evidenced from the high-resolution transmission electron microscopy study) which provides interface states. It is observed that these states play a crucial role to provide a radial field for separation of the photogenerated electron–hole pair which in turn leads to highly effective photoconductive gain that reaches a value >107 at an illumination intensity of 10 μW/cm2.