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

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Abstract
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 110 k bar and for a-As2SeS2 to 17 k bar. 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 me V k bar−1, while the band edge E g and the PL excitation peak (which remains tied to the edge) red-shift by - 14me V k bar−1. Hence, the Stokes shift decreases drastically, and by 110 k bar 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, E PL ≈ 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.