Electroabsorption on the Indirect Gap of As₂Se₃

Abstract

The elcctroabsorpt,ion spectra of As,Se, single crystals are studied near 2 eV for polarization of light E 1) a and E 11 c. The low temperature spectra show fine structure related to indirect transitions with creation of phonons of an energy of 5, 11.5, 24.5, and 26.5 meV. This fine structure broadens rapidly above 50 K and disappears at' 77 K. With rising temperature a low-energy tail of the spectra develops which is attributed t o indirect transitions with annihilation of phonons.The energy of the indirect gap is determined to 1.989 eV a t 10 K. Above 100 K it' decreases with a temperature coefficient of -6.25 A I m Berejch urn 2 eV wird das Elektroabsorptionsspektrum von As,Se,-Einkristallen fur die Polarisation E j j a und E j j e gemessen. Bei tiefen Teniperaturen zeigen die Spektren Feinstruktur, die indirekten Ubergangen unter Erzeugang von Phononen der Energien 5; 11,5; 24,5 und 26,5 meV entspricht. Diese Strukturen verbreitern sich rasch ab etwa 50 K und sind bei 77 K bereits verschwunden. Auf der niederenergetischen Seite des Spektrums entwickelt sich mit wachsendcr Temperatur ein Auslaufer, der indirekten Ubergangen unter Vernichtung von Phononen zugeordnet wird. Die Breite des indirekten Gaps, die bei 1 0 K 1,989eV betragt, nimmt) obcrhalb 100 K ab mit einem Temperaturkoeffizienten von --6,25 :I lo-* eV/K. eV/K.

Deep electron traps in As2Se3 single crystals studied by photoconductivity and TSC measurements

“…Ionization of the excited state at higher temperatures results in free carrier generation and in a higher quantum efficiency for photoconductivity. The maximum of the photoconductivity spectrum indeed shifts faster than would be expected from the temperature coefficient of the gap (Althaus and Weiser 1980). Differences of the photoconductivity below and above 2.15 eV are apparent also in its temperature dependence, shown in fig.…”

“…A similar value has been proposed from an interpretation of the fine structure of electro-absorption spectra near 2.0eV (Sussmann et al 1981). The fine structure of the electro-absorption spectra about 50 meV above the first transition, however, may equally well correspond to an exciton related to a lower lying valence band (Althaus and Weiser 1980).…”

“…Photons below 2-15 eV have a reduced efficiency for the creation of free carriers. The absorption in this range may be due to the creation of excitons (Althaus andWeiser 1980, Sussmann, Searle andAustin 1981) and below 1.9eV to an excitation of a defect state close to the band edge as concluded from the excitation spectrum of the luminescence (Street, Searle and Austin 1975). Ionization of the excited state at higher temperatures results in free carrier generation and in a higher quantum efficiency for photoconductivity.…”

“…The absence of paramagnetic centres (Bishop, Strom and Taylor 1975) and of variable-range hopping, even at low temperatures, indicates that pinning of E , is not due to a high density of states at the Fermi level. Anderson therefore proposed the existence of centres with a negative electron correlation energy which favours spin pairing (Anderson 1975). The energy necessary to overcome the Coulomb repulsion of paired electrons should be gained by charge transfer between dangling bonds and by the formation of extra bonds (Mott, Davis and Street 1975).…”

Computational prediction of high thermoelectric performance in As2Se3 by engineering out-of-equilibrium defects

“…As 2 Se 3 is a semiconductor with a band gap of 2.0 eV. 12 The measurements of electrical conductivity [7][8][9] exhibit an Arrhenius behavior, indicating that carrier transport is a thermally activated process. The activation energies obtained from the experimental results are similar, however, the measured values of the electrical conductivity showed large variation among the experiments.…”