Formation energies and energy levels of deep defects in narrow-gap semiconductors
Patterson, James D.
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We use a Green's function technique for deep defect energy level calculations in mercury cadmium telluride, mercury zinc telluride, and mercury zinc selenide. The formation energy is calculated from the difference between the total binding energy with an impurity cluster and with a perfect cluster. These alloys are among those that have been experimentally grown in microgravity aboard the Space Shuttle. To evaluate the quality of these crystals, it is necessary to characterize them, and one important aspect of this characterization is the study of deep defects which can limit carrier lifetime. Relaxation effects are calculated with molecular dynamics. The resulting energy shift can be greater for the interstitial case than the substitutional one. Relaxation in vacancies is also considered. The charged state energy shift (as computed by a modified Haldane- Anderson model) can be twice that caused by relaxation. However, different charged states for vacancies had little effect on the formation energy. For all cases we considered the concentration of Cd or Zn in the range appropriate for a band gap of 0.1 eV. The emphasis of our calculation is on chemical trends. Only limited comparison to experiment and other calculations is possible, but what there is supports the statement that our results are at least of the right order of magnitude.