Silicon Nanocrystal Photodetectors by One-Dimensional Nickel Silicidation

Abstract

The sensitive detection of photons is of central importance in imaging, environmental sensing, and spectroscopy. Thus, the studies of photocarrier dynamics are indeed to understand the operation of photodetectors which typically include photogeneration of electron-hole pairs, their efficient transport, and collection as an electrical signal. In particular, Si-based photodetectors which are routinely used for imaging applications at visible and near-infrared wavelengths (< 1.1 um) are important because the compatibility of Si photodetectors with mature Si electronics is capable of fabricating fast and stable applications with wider bandwidth than electronic devices. However, some severe disadvantages of silicon such as an inefficient light absorption lower the performance of Si-based photodetetor. In order to overcome these limitations, many people have attempted to assemble nanostructures into photodetectors using their unique properties arising from the finite size. In fact, the representative figure-of-merit to characterize the photodetector is photoconductive gain as the ratio of lifetime to transit time. From the expression, two factors contributing to the strongly enhanced photosensitivity are recognized

prolonged lifetime and shorten transit time. While abundant researches are verified the contributions of lifetime to gain, the influence of transit time has yet to be fully studied. Here, we report a strongly size-dependent photoconductive gain in Si nanocrystals with different lengths which are formed by controllable Ni diffusion into Si nanowires. Rapid Thermal Annealing (RTA) enables to systemically control the channel length of Si NCs in nanoscale without an obvious Ni contamination in Si segments. By employing a spatially resolved photocurrent imaging technique, we find that the photocurrent is strongly localized in Si segments not NiSi2 parts under biased conditions. It is also noticed that photoconductive gain is strongly enhanced more than 2 orders of magnitude at the same bias voltage with decreasing channel length from 671 nm to 30 nm. In addition, high internal gain as ~ 104 is detected in Si NC of 30 nm in length due to enhanced electric field and reduced transit time of photocarrier. Our findings represent inherent size effects of internal gain in Si NCs because the semiconductor channel is free of extrinsic dopants under uniform electric field without any contact barriers. Thereby our study provides a new insight into nanooptoelectronics especially detection applications.