Departamento de Física, Facultad de Ciencias, Universidad de Chile.
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Simulation of intrinsic defects in CdTe: interplay of size effects and gap error
The quantum states and thermodynamical properties of intrinsic defects in CdTe are studied by means of density functional theory calculations. It is shown that supercell size effects are particularly strong for some defects.? For example, a Te vacancy inside a cubic 64-atoms supercell modifies drastically the CdTe band structure, as shown in the Figure, opening the bandgap and disturbing the dispersion of the valence and conduction bands.? Therefore, the 64-atoms supercell cannot be used as a model to calculate some isolated defects.
Figure 1. Band diagram computed with 64- and 63-atoms supercell for perfect crystal (a) and for Te vacancy in the charge states 2+ to 2- (b) to (f).
The electronic structures of isolated defects are recovered using a cubic 216-atoms supercell. Upon defect creation in this supercell, the band structure resembles a perturbation of the band structure of the perfect crystal, as expected for a diluted defect. However, it is shown that? other size effects, together with the gap underestimation typical of the LDA/GGA fuctionals,?can modify the defects symmetries and formation energies.? In this work, corrected values are reported for the formation energies, including the supercell size and quasiparticle corrections for the valence and conduction band edges. It is shown that the interplay of different size effects and the use of different correction schemes is the probable cause of the scattering in formation energies reported in the literature.
Support by FONDECYT Grant 1130437 is acknowledged.