Temperature dependence of Hg0.68Cd0.32Te infrared photoconductor performance

An experimental and theoretical study has been carried out of the temperature dependent noise and responsivity performance of n-type x=0.32 Hg_1-xCd_xTe (corresponding to a 4.6 μm cutoff wavelength at 80 K) photoconductors. The fundamental noise sources that ultimately limit the specific detectivity, D*_λ, at the three main temperatures of interest (i.e., 80 K, 200 K, and 300 K) are identified and correlated with the experimental material parameters of the device. A device model is presented for the responsivity and noise voltage which takes into account surface effects such as surface recombination and accumulation layer shunting. For a given set of device and material parameters this model is well able to account for the observed experimental values of responsivity and noise voltage over the full temperature range from 80-300 K. Using a theoretical model, it is shown that under ideal conditions it is possible to achieve background limited performance at temperatures up to 210 K. Experimental results are presented for responsivity, noise voltage, semiconductor surface charge density and D*_λ for a frontside-illuminated Hg_1-xCd_x Te photoconductive detector, as a function of temperature in the range 80-300 K. The devices were fabricated on Liquid Phase Epitaxially (LPE) grown n-type Hg_0.68Cd_0.32Te, and were passivated with anodic oxide/ZnS on the front side. The detector area is 250×250 μm² and has a cut-off wavelength of 4.6 μm at 80 K. For a signal wavelength of 4 μm and a 40° field of view, a background limited D*_λ of 3.8×10¹¹ cm Hz¹2 / W^-1 was obtained for temperatures up to 180 K, while D*_λ of 1.4×10¹¹ and 2×10⁹ cm Hz¹2 / W^-1 were measured at 200 K and 300 K, respectively. These figures are comparable to the highest reported D*_λ for n-type x=0.32 Hg_1-xCd_xTe mid-wavelength infrared photoconductive detectors operating at these temperatures.