HgCdTe Versus Hgznte: Electronic Properties and Vacancy Formation Energies
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HgCdTe VERSUS HgZnTe: ELECTRONIC PROPERTIES AND VACANCY FORMATION ENERGIES M.A. BERDING*, A.-B. CHEN**, AND A. SHER*
* SRI International, Menlo Park, CA 94025 **Auburn University, Auburn, AL 36849 ABSTRACT
The alloy variation of the band gap and the electron and hole effective masses have been calculated for HgCdTe and HgZnTe. Band-gap bowing is larger in HgZnTe than in HgCdTe because of the larger bond length mismatch of HgTe and ZnTe; electron and hole effective masses are found to be comparable for the two alloys for a given band gap. We have calculated the electron mobility in both alloys with contributions from phonon, impurity, and alloy scattering. Contributions to the E1 line width due to alloy and impurity scattering in Hg0.7Cd 0.3Te have been calculated. Results of calculations of the vacancy formation energies in HgTe, ZnTe, and CdTe are discussed. INTRODUCTION HgCdTe, the most popular material for current narrow-gap device applications, is plagued by poor structural properties; it is subject to Hg loss at surfaces, contains large asgrown dislocation densities, and is generally fragile. HgZnTe has recently been proposed [1] as an alternative to HgCdTe based on extended bond-orbital calculations. Alloying HgTe with ZnTe instead of CdTe should produce a structurally superior material because the shorter bond length of ZnTe should result in solid solution (alloy) hardening of the HgTe. Alloy hardening in the nearly lattice-matched HgCdTe system would be expected to be less. Recent Knoop hardness measurements on Hg 0.89 Zn 0.11Te and Hg0 .80Cd 0.2 0Te, both corresponding to near-zero band gaps, found the HgZnTe to be the harder material [2]. Preliminary results of bulk and epitaxially grown HgZnTe showed dislocation densities comparable to, or less than, those of HgCdTe for compositions of technological interest [2,3]. HgZnTe has yet to prove itself as a replacement for HgCdTe, although indications to date are that it is fulfilling its promise as an electronically equivalent, structurally superior replacement for HgCdTe in narrow-gap device applications. This paper addresses both electronic and structural issues which are important in comparing the relative properties and merits of the two alloys under consideration here. We present recent results on the electronic properties of HgZnTe and compare them to those of HgCdTe, using the coherent potential approximation (CPA) [4] to compute alloy band structures. The alloy variation of the band gap and the electron and hole effective masses for HgZnTe and HgCdTe are calculated and compared. Electron mobilities for HgCdTe and HgZnTe are calculated and the contributions to the E1 electroreflectance line width from impurity and alloy scattering are estimated for HgCdTe. We also discuss the results of our calculation of the vacancy formation energies in the II-VI compounds HgTe, CdTe, and ZnTe, and their alloys. The vacancy formation energy is related to diffusion activation energy and provides one estimate of the relative stability of the two alloys, HgZnTe and HgC
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