Point defects, localized vibrational modes, and free-carrier absorption of Al-doped CdTe

Abstract

Infrared absorption bands attributable to the localized vibrational modes for several Al‐related point defects have been observed for CdTe. The proposed defects, their point‐group symmetries, and the vibrational frequencies at 80 °K in cm⁻¹ are as follows: Al_Cd, T_d, 299; Al_Cd‐V_Cd, C_s, 282,287,326; Al_Cd‐Sb_Te, C_3v, 279,334; Al_Cd‐Cu_Cd, C_s, 291,294, 310; Al_Cd‐Ag_Te or Al_Cd‐Ag_i, C_3v, 293,314; Al_Cd‐Au_Te or Al_Cd‐Au_i, C_3v, 292,313; and possibly Al_Cd‐Au_Cd, C_s, 288 (two unresolved bands), 326. The Al and Sb were introduced during crystal gwoth and the Cu, Ag, and Au by saturation diffusions. Annealing of CdTe:Al with [Al]∼1×10¹⁹ cm⁻³ in unsaturated Cd‐rich atmospheres produced electron concentrations of 2×10¹⁸ and 2×10¹⁷ cm⁻³. Free‐carrier absorption measurements at 298 and 80 °K give a nearly temperature‐independent absorption coefficient with a frequency dependence of ω⁻³ after ther higher carrier density anneal with ω⁻⁴ for the lower carrier concentration. The free‐carrier absorption results are compared with previously reported data and also recent theoretical calculations for CdTe which predict, under appropriate conditions, α∼ω⁻³ for the polar optical mode scattering and α∼ω⁻⁴ for ionized impurity scattering.