CPPP 43 P Steady-state photocurrent spectra of GexAsxSe1-2x amorphous films: compositional tendency
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VASILIEV, Ion, YOVU, M., KOLOMEYKO, Eduard. CPPP 43 P Steady-state photocurrent spectra of GexAsxSe1-2x amorphous films: compositional tendency. In: Materials Science and Condensed Matter Physics, 13-17 septembrie 2010, Chișinău. Chișinău, Republica Moldova: Institutul de Fizică Aplicată, 2010, Editia 5, p. 168.
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Materials Science and Condensed Matter Physics
Editia 5, 2010
Conferința "Materials Science and Condensed Matter Physics"
Chișinău, Moldova, 13-17 septembrie 2010

CPPP 43 P Steady-state photocurrent spectra of GexAsxSe1-2x amorphous films: compositional tendency


Pag. 168-168

Vasiliev Ion, Yovu M., Kolomeyko Eduard
 
Institute of Applied Physics
 
Disponibil în IBN: 19 aprilie 2021


Rezumat

It is known, that in amorphous semiconductors the steady-state photoconductivity spectra give the information about the recombination centers and the localized states in a mobility gap. In amorphous chalcogenides these centers are the defects with negative effective correlation energy (U-centers) [1]. On the one hand, these defects create quasi- discrete energy levels in the gap and determine the recombination processes of non-equilibrium charge carriers, on the other hand - take part in processes of dielectric polarization of a sample. Hence, measurement of steady-state photoconductivity spectra can confirm the picture of a dielectric relaxation which is based on the electron jumps between the defects [2]. Fig. 1 shows the spectral distribution of a steady-state photocurrent of GexAsxSe1-2x amorphous films for x=0.07, 0.14, 0.16 and 0.20. It seems that spectra show features which are typical for chalcogenide films. Namely, with increasing the photon energy a photocurrent (normalized to light intensity F), gradually increases and achieves a maximum value at hn~2.2 eV; that is, near the edge of fundamental absorption.figureFig. 1. Spectral distributions of a steady-state photocurrent of GexAsxSe1-2x amorphous films for x=0.07 (1), 0.14 (5), 0.16 (6) and 0.20 (8). Bands of optical transitions nearby 1.6 and 1.9 eV, are shown by arrows.For x=0.07 this increase of photoconductivity in infra-red region of a spectrum follows the exponential law that reflects well-known Urbach rule for sub-band gap of optical absorption, and specifies an exponential distributions of the localized states with characteristic parameter EU~0.074 eV. In the sample with x=0.014, value of a photocurrent both in infra-red region of a spectrum and in the region of band-to-band absorption is essentially more. The spectrum looks like a wavy line and allow to allocate the two bands of optical transitions: OT1 and OT2. Their positions near photon energies hn~1.6 and 1.9 eV is probably associated with energy levels, located in the top (OT1) and of the bottom (OT2) of the band gap portions, accordingly. With transition to composition x=0.16 and x=0.20 (curves 6, 8) there is an appreciable reduction of a photocurrent in all region of the investigated wavelengths. It is possible to assume, that at x=0.14 the glass-forming network is formed by the "As-Se" and "GeSe" chemical bonds [4], and that the prevalent structural units of GeSe2 determine the optical band gap Eg0 about 2.2 eV. In this case the optical energies, which were identified on a Fig. 1 as hn~1.6 and 1.9 eV correspond to thermal energies Eth~1.33 (1.43) and 0.66 eV, which were found in [3] for D+ and D- centers with negative U in GeSe2, because the energy difference: 1.6-1.33~0.27 eV and 0.66-(2.2-1.9) ~0.36 eV is almost the same and it is not far from polaron energy.