Photoinduced dichroism and its low-temperature characteristics in obliquely deposited amorphous As–Ge–Se–S thin films

Abstract

The photoinduced dichroism and its low-temperature characteristics in obliquely deposited amorphous As–Ge–Se–S thin films have been studied using a linearly polarized 632.8 nm He–Ne laser light and a control system capable of increasing the film temperature from 77 K. Our experimental results have been discussed on the basis of native valence-alternation pairs depended on the film fabrication. Although the absorption slopes in the extended region (hν>Eop) decrease with increasing deposition angle, the optical energy gap Eop remains the same as for 0° films, i.e., Eop=2.04 eV. A columnar structure with an inclination angle of approximately 70° is formed in 80° deposited films. While the value of saturated dichroism Dsat itself remains constant, independent of the intensity, it depends strongly on the deposition angle θ and the film temperature T, i.e., Dsat(θ,T). For example, Dsat(80°,77 K) is more than four times Dsat(80°,300 K), and the values of Dsat(0°,300 K) and Dsat(80°,300 K) are about 4.2% and 6.0%, respectively. We believe that an enhanced D in a columnar film is attributed to the increase of disorder (boids and dangling bonds). The photoinduced dichroism relaxes during a dark relaxation after its saturation and eventually sustains any value of nonzero, which depends on the inducing light energy and especially the value obtained after a prolonged exposure (∼3.3 h) is evaluated to be about 4.5 for 80° deposited film. However, it is eliminated completely by the exposure of nonpolarized light only for about 3 min.