Native Defect Formation and Ionization Energies in Cadmium Telluride
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Native Defect Formation and Ionization Energies in Cadmium Telluride John E. Jaffe and Mary Bliss Pacific Northwest National Laboratory
ABSTRACT Deep intrinsic energy levels near the middle of the band gap in CdTe have been reported in a number of experiments. Based on earlier defect-supercell electronic structure calculations, at least some of these features have been attributed to the second ionization level of the Cd vacancy, while the TeCd antisite, possibly complexed with a Cd vacancy, has also been suggested to account for some midgap levels. Using high-accuracy LDA calculations with full lattice relaxation out to third neighbors, we find that (i) both acceptor states of the Cd vacancy are shallow, (ii) the donor states of an isolated TeCd are both more than 1 eV above the valence band maximum, (iii) the TeCd-VCd complex does indeed have acceptor states near midgap in CdTe and probably accounts for the native defect states in that energy range.
INTRODUCTION Gamma-ray spectrometers are an important application of semiconductor alloys based on CdTe, and require an intrinsic or compensated semi-insulating state. The doping behavior of CdTe is affected by intrinsic defects (including vacancies, interstitials, antisites and complexes of more than one point defect) which can produce self-compensation of dopants, carrier traps and Fermi-level pinning. Understanding the effects of these native defects is essential, since they are always present. Experiments have found four particularly prominent native defect levels in CdTe and Cd1-xZnxTe (CZT): (1) shallow donor states around EVBM + 0.14 eV (2) deep acceptors near EVBM + 0.43 eV and (3) EVBM + 0.74 eV, and (4) a deep donor around ECBM – 0.60 eV A consensus exists that (1) is the first ionization level of the Cd vacancy, which we will denote VCd(0|-1). However, the second ionization level VCd(–1|–2) has been identified with (2) by some workers and (3) by others. An apparently distinct feature near EVBM + 0.74 eV was identified with the TeCd antisite defect, possibly in a complex with one or more other point defects. Finally, the deep donor (4) has been variously identified as the Cd interstitial (+2|0) or the TeCd (+2|+1) level. Thus, there is uncertainty in the identification of these defect levels in CdTe. THEORY We have calculated defect formation and ionization energies for the Cd vacancy VCd and the Te antisite TeCd, which are the dominant point defects in Te-rich CdTe. (We focus on this composition limit since it is typical in detector applications.) We also consider a complex of VCd and TeCd on next-nearest-neighbor atomic sites. Our calculations employ periodic supercell F8.31.1 Downloaded from https://www.cambridge.org/core. UNSW Library, on 10 May 2020 at 14:10:15, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-719-F8.31
density function theory (DFT) within the local density approximation (LDA) using plane-wave basis functions, ultrasoft pseudopotentials1, and geometry optimizat
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