Detailed photoluminescence intensity and lineshape studies of doped a-as₂se₃films

Careful investigations at 77 K and 22 K of the photoluminescence intensity and lineshape of a-As₂Se₃doped with In, Ga, Na and T1 are reported. The samples were the same co-evaporated (at T_g) films for which two orders of magnitude changes in hole transit time with doping have previously been measured. In contrast only gradual, but systematic, decreases in the photoluminescence intensity are observed for concentrations up to 10²¹atoms per cm³. Small, systematic, and impurity specific line broadening also is found of the approximate magnitude 6x 10^-3eV/(decade concentration). However, no additional structure appears. These observations are discussed in light of current ideas concerning charged defects, photoluminescence, transport, and metallic impurities in chalcogenide glasses. The intensity decreases measured here provide evidence that the photoluminescence and transittime results, and by inference their associated defect centres, are not completely uncorrelated as was previously thought. In the context of a dipole-centre model for radiative recombination, a distribution of charged-pair separations, which includes a small fraction of unbound pairs still close enough to experience Coulomb attraction, is indicated. However, alternative impurity-induced non-radiative processes cannot be ruled out. The observed lineshape variations are thought to be a higher-order effect, outside the scope of current models. Whatever their cause, they seem to indicate a small change in the energy distribution of intergap states with doping.