Effect of chlorine impurities on the long-wavelength absorption edge of CdTe single crystals

The optical absorption spectra of the nominally undoped and Cl-doped CdTe single crystals in the region of the fundamental long-wavelength absorption edge are studied at room temperature. The crystals are grown by the modified physical vapor transport method. It is shown that the exponential dependence of the absorption coefficient of the undoped and lightly doped samples can be described by the Urbach rule. In the heavily doped crystals, the absorption edge is due to optical transitions involving density-of-state tails formed by fluctuations of the dopant concentration. The typical energy range of fluctuations of the extrinsic potential profile and the concentrations of charged centers are calculated for the heavily doped material.