Uncooled photovoltaic Hg_(1-x)Cd_xTe LWIR detectors

摘要

We report an advanced Hg_(1-x)Cd_xTe photovoltaic detector based on monolithic Hg_(1-x)Cd_xTe heterostructure with 3-dimensional architecture. It consists of a narrow gap, p-type Hg_(1-x)Cd_xTe small size (≈10x10x7 μm) absorber of infrared radiation buried in a graded gap Hg_(1-x)Cd_xTe layer surrounding absorber and heterojunction contacts obtained by selective doping of the graded gap Hg_(1-x)Cd_xTe layer surrounding the absorber region. The heterostructure is passivated with a ZnS layer and coated with contact metallization to n~+ and p-type regions. The device is supplied with 50x50 μm immersion microlens formed directly in the CdZnTe substrate. These two layers also play a role of a mirror that improves quantum efficiency for weakly absorbed infrared radiation. In addition, the mirror eliminates backside incidence of thermal radiation, which prevents generation of dark current. The design of the device is optimized to achieve the best compromise between requirements of good absorption and collection efficiency, low thermal generation; and low parasitic impedance. Test devices have been prepared using the modified isothermal vapor phase epitaxy of Hg_(1-x)Cd_xTe on profiled CdZnTe substrates, negative epitaxy of Hg_(1-x)Cd_xTe to widen band gap of surface regions, selective doping, multiple chemical etching and ion milling, vacuum deposition of dielectric and metal layers. The test structures of photovoltaic detectors were optimized for wavelengths varying between 4 and 12 μm. The examples are 10.6 μm laser radiation receivers and 8-9.5 μm detectors of thermal radiation. Since the devices operate at zero bias mode, they do not exhibit low frequency noise. The measurements show the possibility to achieve detectivity of ≈1·10~9 cmHz~(1/2)/W at the 8-9 m range. Potentially, the devices can be assembled in large focal plane arrays. This will enable obtaining a NETD of less than 0.1 K for staring thermal imagers operating with F#=1 optics and a 30/sec frame rate.