Anomalous pressure response of luminescence in c-As₂S₃ and a-As₂SeS₂ consequences for defect structure in chalcogenides

The effect of compression on the native-defect photoluminescence (PL) common to crystalline and amorphous As chalcogenide solids is studied for the first time. Hydrostatic pressure measurements at 13 K are reported for c-As₂S₃ to llOkbar and for a-As₂Ses₂ to 17 kbar. The experiments require special cryogenic diamond-anvil cell techniques, in which solid argon is the pressure transmitting medium. For c-As₂S₃ it is found that the PL peak E_PL blue-shifts by +1·2 meV kbar⁻¹ while the band edge E_g and the PL excitation peak (which remains tied to the edge) red-shift by-14meV kbar⁻¹. Hence, the Stokes shift decreases drastically, and by 110kbar the PL peak is partially reabsorbed in the approaching band edge. The limited results for As₂Ses₂ glass indicate a similar dependence. This behaviour is quite anomalous since it violates the well-established (at P=0) mid-bandgap rule, Epl ≈ E_g/2, that holds for c-and a-chalcogenides of widely varying composition. The pressure dependence of the Street-Mott defect energy-level scheme is deduced from the measurements on c-As₂S₃. A realistic configuration-coordinate model based on layer inter-linking defects is developed. This model succeeds in explaining the pressure results because it explicitly accounts for the layered molecular properties of c-As₂S₃ by invoking both soft inter-layer and stiff intra-layer phonons. It is concluded for c-As₂S₃ that the PL defects exist on the surface of the layers, and that they couple adjacent layers loosely in the ground state but tightly in the excited state. This picture can be adopted to explain the similar PL pressure response in As₂Ses₂ glass, if locally layer-like clusters now play the role of the crystalline layers. A model of this type was recently proposed by Phillips. An evaluation of the present results in terms of this and other current models suggests that the specific defect structure could be obtained from a detailed calculation guided by the PL pressure response.