Evaluation for photo-induced changes of photoluminescence and optical energy gap in amorphous Se100−xGex (x=5, 25 and 33) thin films

Abstract We have investigated photo-induced changes of photoluminescence (PL) and optical energy gap (EOP) in amorphous Se100−xGex (x=5, 25 and 33) thin films from illumination by HeCd laser light (hν=3.81 eV). The EOP is obtained by a linear extrapolation of (αhν)1/2 vs. hν to the energy axis. The optical absorption coefficient (α) is calculated from the extinction coefficient k measured in the wavelength range of 290–900 nm. Although the values of ΔEOP are very different, all films exhibit a photo-induced photodarkening (PD) effect that is a red shift of EOP. In particular, ΔEOP in a-Se75Ge25 thin film exhibits the largest value (i.e., ΔEOP∼40 meV for a-Se95Ge5, ΔEOP∼200 meV for a-Se75Ge25, ΔEOP∼130 meV for a-Se67Ge33). PL spectra in a-SeGe by hνHeCd have no Stokes shift (SS). Although the PL intensity in the Se100−xGex system is very small, the PL spectra (three-trial average) show a dependence on both composition and illumination time. The intensities of five band peaks (I1–I5) observed in the range of 1–2 eV increase with an increase of Ge content. The intensity of I2 peak (the strongest intensity) increases with increasing illumination time, while the peak-energy position decreases. This is evidence of PD. A decrease in PL peak-energy position with Ge contents shows a similar tendency to that in EOP. That is, the ΔE(I2) is 25.2, 45.0 and 44.5 meV for a-Se95Ge5, a-Se75Ge25 and a-Se67Ge33, respectively. We propose in this paper a triple-well potential model that can universally explain the energy-induced phenomena in the a-SeGe, such as the PD and thermal bleaching, the native charged-defect generation and the no-SS PL, etc.