Characterization of impurities in p-type HgCdTe by photo-Hall techniques

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Abstract
Due to mixed conduction effects, conventional temperature-dependent Hall measurements do not yield an accurate characterization of doping levels in high-purity p-type HgCdTe (NA≲5×1015 cm−3). In the present work, we report a new and highly promising technique for determining compensation densities in p-type material by measuring the mobility of photoexcited electrons at low temperatures where most of the holes are frozen out and the total impurity concentration is equal to 2ND. As long as the photoexcitation is sufficiently strong that the electrons dominate the conductivity yet sufficiently weak that scattering by photoexcited holes is negligible, the compensation may be determined directly by fitting a comprehensive theory for transport in a photoexcited narrow gap semiconductor to the measured mobilities. Experimental photo-Hall results are reported for p-type Hg1−xCdxTe samples (0.215<x<0.23) with acceptor densities in the 1015 to 1016 cm−3 range. Photoexcitation is provided by a pulsed CO2 laser. At temperatures between 10 and 30 K, theory and experiment agree well over an extremely large range of photoexcited electron densities (1012–1016 cm−3). Donor and acceptor concentrations and acceptor binding energies have been determined for samples with both single (probably Cu) and double (probably Hg vacancies) acceptors. Since the method has its greatest sensitivity to ND at low excitation levels (n<1013 cm−3), it should be possible to perform routine characterizations using only a blackbody source. The model predicts that donor densities as low as 1014 cm−3 may be measurable.

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