Sub-Band-Gap Absorption in As2se3 Films From Photocapacitance Spectroscopy

Abstract

The rier photocapacitance spectra were measured in Al/a-As2Se3 thin (1.2 μm) film diodes in the photon energy range from 0.8 to 1.8 eV at the temperature 297.,.322 K. With these data the values of optical absorption coefficient α were determined from 10-3 to 10 4 cm-1 using the initial rate of photocapacitance relaxation [1]. The measurements were performed by quasi-static C-V method under continuous optical excitation with sub-band-gap light. It was found that the spectral dependence of the absorption coefficient could be divided into three regions. In the region I within 1.5 to 1.8 eV the absorption coefficient is described by a typical for the Urbach edge exponential function with the slope of 20 eV-1. The photon energy 1.76 eV corresponding to the value of α=104 cm-1 coincides with the optical gap of amorphous As2Se3 at 300 K. As the temperature rised the strong parallel shift of the Urbach edge to lower energy occurred. Below the band edge in the region II between 1.1 and 1.3 eV the tail of weak absorption was observed. The origin of this tail is often associated with impurities or charged defects. The model calculation, which incorporates the density of conducting states and a Gaussian-form distribution of the gap states yields the value of peak energy 1.36 eV and a standard deviation 0.095 eV. Finally in the spectral region III from 0.8 to 1.1 eV a broad shoulder (up to the region II) with flat dependence on photon energy and extremely low (10-2 cm-1) absorption coefficient was revealed. In this region photoinduced optical absorption of about 10-1 cm-1 stimulated by illumination with photon energy 1.8 eV during 30 min has been observed at 300 K. If one assumes that the upper limit for the optical cross section of an induced state is 10-17 cm2 then the estimated lower limit of 1016 cm-3 is obtained for the density of states in the mid-gap from the observed induced absorption coefficient. This value is in good agreement with the estimation of photoinduced spin concentration in As2Se3 [2].