Effect of doping with transition and rare-earth metals on the electrical and optical properties of AgGaGe3Se8 single crystals

We have studied the electrical, optical, and thermoelectric properties of AgGaGe3 Se-8 single crystals grown by the Bridgman - Stockbarger method and doped with transition and rare- earth metals during growth. AgGaGe3Se8 is an anisotropic p - type semiconductor with a band gap of similar to 2.25 eV ( T similar to 290 K), which decreases slightly, to similar to 2.20 eV, on doping with Nd, Gd, and Er ( N similar to 5 x 10(18) cm(-3)). The high concentration of stoichiometric silver vacancies ( N similar to 1.8 x 10(20) cm(-3)) and random distribution of the Ga and Ge atoms over the cation sites give rise to static disorder, so that the structure of AgGaGe3Se8 approaches that of disordered systems. This leads to the formation of electronic states in the band gap, intrinsic edge broadening, and, as a consequence, a drop in transmission. Depending on the concentration and nature of the dopant, doping of AgGaGe3Se8 may lead to structural ordering and bleaching or may increase the destructive factor at high doping levels ( N > 5 x 10(18) cm(-3)). 60 Co gamma irradiation to a dose of 1.2 - 1.3 kGy reduces the absorption coefficient of AgGaGe3Se8 crystals doped with high concentrations of Mn and Cu ( N > 5 x 10(18) cm(-3)). A model is proposed which explains the observed effects.