A room temperature photovoltaic detector for the mid-infrared (1.8-3.4 m) wavelength region

Abstract

In this paper, we report on an (pin) photovoltaic detector operating in the mid-infrared spectral region. Basic detector characteristics have been measured and compared with other detectors in this wavelength range. The typical detectivity of the photodiodes is cm at room temperature, which compares very favourably with that of TE cooled HgCdTe and is at least three times that of cooled PbSe photoconductors over the same wavelength range. The heterojunction photodiodes offer the advantage of increased sensitivity and extended wavelength response at room temperature compared with that of currently available commercial photodetectors, making them an attractive alternative for a number of mid-infrared applications including optical gas sensors and infrared spectrometers. The heterojunction photodiodes were grown on p type (100) InAs substrates by liquid phase epitaxy (LPE). Several different device structures were fabricated and compared in order to investigate the dependence of dark current on doping concentration and layer structure, with a view towards device optimization. It was found that at room temperature the dark current is dominated by bulk leakage (minority carrier diffusion) rather than other mechanisms, such as mismatch dislocation or surface leakage. We established that the main reasons why the properties of these heterojunction photodiodes are for superior to those of more conventional detectors including homojunction InAs photodiodes are (a) that the wider-bandgap layer has a lower equilibrium population of thermionic minority carriers which contribute to the diffusion current, resulting in lower leakage, and (b) the structure has a transparent window layer allowing better capture of photons in the active region.