Optical properties, photoconductivity, and energy levels in crystalline and amorphous arsenic triselenide

Abstract

The optical and electrical properties of arsenic triselenide single crystals and amorphous films have been studied. For crystals there are indirect gaps at 2.1 eV for E ⊥ c, 2.03 eV and 2.1 eV E c; the first direct gaps are at 2.19 eV and 2.28 eV for E ⊥ c and 2.26 eV for E c. The absorption curves of evaporated films show no features corresponding to those of the crystals. There is, however, evidence for an Urbach type threshold in the region of low absorption, followed by either a direct forbidden transition with Eg = 2.03 eV or an indirect allowed transition with Eg = 1.80 eV at higher energies. Similarities exist between the absorption spectra of crystalline arsenic triselenide and trigonal selenium, suggesting that the band structures may be similar.The photoconductivity responses of both crystals and evaporated films are in general consistent with the absorption curves. A maximum photocurrent occurs at energies near the indirect gap in the crystals; while a general increase of photocurrent with decreasing wavelength was found in the film.The electrical properties are mainly governed by the indirect gap near 1.8 eV, and this explains the dc conductivity of the crystals and thick glass films; the conductivity of thin evaporated films is complicated by a varying activation energy up to the melting temperature. The ac conductivity is complex with components: (i) increases linearly with frequency, and (ii) increases as the square of frequency with an activation energy ca. 5 × 10−3 eV. The mechanism for (i) is usually associated with a hopping type conduction and dominates the conductivity of the crystals at high temperature, while the second process is dominant at low temperatures for the crystals and for the films at all temperatures. No adequate explanation has yet been found for the second process.