Device Characteristics of VLWIR MCT Photodiodes

Abstract : We have characterized the current-voltage characteristics, spectral response and quantum efficiencies of Hg(1-x)Cd(x)Te photodiodes with x values equal to .209 and .194. The devices studied are double layer planar heterojunctions (DLPHJ) photodiodes fabricated by MBE. Arsenic ion implantation is used to form the heavily doped p-side. The base is indium-doped at carrier density levels of ^5x10(exp 14)/cu cm. For x - 0.194 at T =30K, the bandgap wavelength, lamba(g) is 22 micrometers. The reported IV's characteristics mostly deal with the behavior of small diodes, whereas the spectral response and quantity efficiencies are obtained from measurements on large diodes. Model calculations of the spectral response and quantity efficiencies are found to be in good agreement. For the device architecture with x=.209, a good fit to the zero-bias impedance area product R(O)A is obtained using current expressions for diodes of small geometries. From this analysis, we obtained for the hole minority carrier mobility a value of 610 sq cm/v-sec at T= 78K. This result is considered to be an independent measurement of micro(h). For the case with x =.194, we observe significant deviation in the current-density-temperature characteristics from these expected from ideal diode model calculations. Specifically, the optical bandgap obtained from the spectral response is significantly different from the electrical bandgap obtained from the temperature dependence of dark currents. This is in contrast with prior reported work for material with x=.194, where good agreement was observed between measured and calculated characteristics. Some of the I-V features of the recent devices support that the p-n junction is situated in the graded region and this could account for some of the observed electrical behavior. For devices of excellent quality, we observe temperature-assisted tunneling current mechanisms at T=5K, i.e., bumps in the relatively low forward bias region.