The excitonic gaps and Urbach tail in crystalline As2Se3

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Abstract
Absorption and electroabsorption measurements have been made on crystalline As2Se3 over a wide range of electric fields and temperatures. These confirm that the lowest electronic transitions are indirect, and show the importance of excitonic effects. A detailed comparison of experiment and theory shows good agreement, and an exciton binding energy of 57 meV is deduced. Phonon and exciton structure are seen at low temperatures, which broaden into an Urbach tail as the temperature is raised. An exponential tail can also be produced in electroabsorption, either by the application of high fields at low temperatures, or by raising the temperature. We show that an electric field broadens the edge at low temperatures. At high temperatures, it narrows the existing Urbach tail, as reported earlier by us for chalcogenide glasses. This behaviour has not previously been described for crystals, and is tentatively explained by a field-induced decrease in the electron–phonon coupling. At about 200 meV above the indirect edge lies the first direct edge, which also shows excitonic effects. Both edges move rapidly to lower energies, with a large linear temperature coefficient of -8 k.