Photoluminescence, photoconductivity, and optical absorption spectroscopic studies were made of crystals which had been grown from the melt by gradient freezing or from the vapor phase by high-pressure physical vapor transport. A model for donor- and acceptor-related sub-bands in the energy gap was proposed which explained the experimental data. An emission with a peak at 1.2eV was attributed to residual disorder on the cation sub-lattice. A lower absorption after annealing was explained in terms of a reduction in the disorder on the cation sub-lattice and of changes in the Fermi-level position. The n-type conductivity of crystals which had been grown under Ge-deficient conditions by using the high-pressure physical vapor transport technique was related to the presence of additional donor states.

N.Dietz, I.Tsveybak, W.Ruderman, G.Wood, K.J.Bachmann: Applied Physics Letters, 1994, 65[22], 2759-61