An experimental investigation of selenium and amorphous compositions of selenium containing either arsenic or thallium has been undertaken to ascertain the relationship between molecular transformations characteristic of these systems and electrical resistivity. Annealing amorphous selenium below 217 °C produces a complete transformation to the hexagonal polycrystalline phase with elimination of the Se8ring species. A sharp lowering in resistivity of five orders of magnitude accompanies such a reversible phase change. This purely thermally generated phenomenon is considered analogous to the memory effect observed in certain amorphous semiconductors. The addition of arsenic, a selenium molecular chain branching element, and thallium a chain cleaver was found to significantly modify these characteristics, including both amorphous and crystalline resistivities as well as crystallization rates. A figure of merit defined as the ratio of these resistivity values is proposed to evaluate amorphous selenides for device applications. © 1972.
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