Minority carrier lifetimes in HgCdTe alloys

Abstract

We present the results of full band structure calculations of Fermi levels, intrinsic carrier densities, and one-photon absorption coefficients in undoped HgCdTe alloys. The full band structure is used in the calculation of majority and minority carrier densities and the minority carrier lifetimes limited by radiative, Auger-1, and Auger-7 mechanisms in both n- and p-doped alloys. The lifetimes we predicted differ substantially from those calculated with widely used analytical expressions, except in cases where the hole density is small. This difference originates from the significantly non-parabolic and anisotropic valence bands, which are of increasing significance as the hole density increases. From a comparison of the calculated and measured lifetimes, we deduce that the lifetimes at low temperatures are limited by the Shockley-Read-Hall (SRH) recombination. We have generalized the SRH expression to include Fermi-Dirac statistics, but we still treat the density, energy level, and cross section as adjustable parameters. We find that the calculated radiative and Auger recombination lifetimes, as well as SRH lifetimes, can fit the measured lifetimes using traps located (a) near the conduction band edge in n-HgCdTe and (b) near the valence band edge for p-HgCdTe. In addition, the movement of Fermi level with respect to the trap level explains the observed temperature-dependence of the lifetimes. We conclude that there is considerable room for improvement in HgCdTe material quality.