Electronic structure, structural and optical properties of thermally evaporated CdTe thin films

Thin films of CdTe were deposited on glass substrates by thermal evaporation. From the XRD measurements it is found that the films are of zinc-blende-type structure. The lattice parameter was determined as a=6.529Å, which is larger than 6.48 Å of the powder sample, because the recrystallized lattice of the grown films is subjected to a compressive stress aroused as a result of the lattice mismatch and/or differences in thermal expansion coefficient between the CdTe and the underlying substrate. Transmittance, absorption, extinction and refractive coefficients are measured. Electronic structure, band parameters and optical spectra of CdTe were calculated from ab initio studies within the LDA and LDA+U approximations. It is shown that LDA underestimates the band gap, energy levels of the Cd-4d states, s–d coupling and band dispersion. However, it calculates the spin–orbit coupling correctly. LDA+U did not increase much the band gap value, but it corrected the s–d coupling by shifting the Cd-4d levels towards the experimentally determined location and by splitting the LDA-derived single s peak into two peaks, which originates from admixture of s and d states. It is shown that the s–d coupling plays an important role in absorption and reflectivity constants. The calculated optical spectra fairly agree with experimental data. Independent of wave vector scissors operator is found to be a good first approximation to shift rigidly the band gap of CdTe underestimated by LDA.