Infrared absorption behavior in CdZnTe substrates

Infrared (IR) optical transmission measurements of polished CdZnTe wafers c an provide useful information about excess impurities, stoichiometry, and i nhomogeneities (precipitates and inclusions). We have investigated the IR t ransmission behavior of Cd0.96Zn0.04Te between 8 mum and 20 mum at room tem perature. The measurements were made before and after thermal treatments in volving control of the Cd and Zn overpressures, which served to minimize th e Cd (cation) vacancy population. Our results support the polar optical pho non scattering theory of Jensen, according to which the absorption in donor dominated CdZnTe varies as lambda (m) with m = 3. For material dominated b y accept;ors, we show that the theoretical absorption by inter-valence band transitions can be approximated by a similar power law with exponent m = 1 , and that Cd-vacancy dominated wafers are in reasonable agreement with thi s. We find some wafers in which the as-grown condition exhibits partial com pensation of impurity-donors by Cd vacancy accepters, and demonstrate remov al of the compensation by annealing to fill the vacancies. In a separate gr oup of wafers, we find that an observed increase in absorption occurring du ring growth of a HgCdTe layer by liquid phase epitaxy can be explained in t erms of an increase in Cd vacancies caused by diffusion of Cd to Te precipi tates. This effect can be reversed by annealing in Cd-Zn vapor, which fills vacancies and eliminates some precipitates. Impurity concentrations were m easured by glow discharge mass spectrometry (GDMS).