X-ray induced effects in the optical and thermal properties of a-Se 1− x As x ( x = 0, 0.005, 0.06) doped with 0–220 ppm Cs

Optical properties of vacuum deposited films of (a) pure a-Se (b) a-Se:0.5%As, (c) a-Se:6%As, (d) a-Se:6%As doped with Cs at the ppm level (140–220 ppm) have been measured from the transmission spectra over the range from the visible to the IR, covering 450–2500 nm. There is an increase in the refractive index of pure a-Se upon alloying with 6%As and a further increase with ppm-level Cs doping. There is a slight reduction in the bandgap with 6%As alloying. All four samples were subjected to x-irradiation at 30 and 70 kVp (21.56 and 37.6 keV average photon energies) with corresponding incident doses of 15.4 and 44.8 Gy in air. As a result of X-ray irradiation, there is an increase in the refractive index and a decrease in the film thickness (increase in the density), which confirms that the changes in n stem primarily from changes in the film density as expected from the Clasius-Mossotti equation. Upon the cessation of X-ray irradiation, n and d were observed to change, that is recover, towards their pre-exposure equilibrium values over a time scale of hours. After about 13 h of resting in the dark, n and d were found to have recovered to their equilibrium values within experimental errors. We also examined the time evolution of n, d, E U (Urbach bandgap) and δE (Urbach width) when a-Se:6%As doped with ppm-level Cs is brought to 75 °C, annealed there for 10 min and then brought back down to room temperature (23.5 °C). It was found that n and d follow opposite trends as expected from the Clausius–Mossotti equation, that is, the changes in n stem primarily from changes in the density. There is an increase in d and a drop in n when the sample is annealed at 75 °C. As soon as the sample is brought back to room temperature n and d slowly recover towards their equilibrium values and after a few hours, n and d values are indistinguishable from the original equilibrium values within experimental errors. Thus, n and d (or density) exhibit typical glass relaxation behavior. At 75 °C, δE is larger and E U is smaller. Upon cooling, both E U and δE show relaxation towards their equilibrium values similarly to n and d . The Wemple—DiDomenico model was also used to model the refractive index dispersion data and the parameters E o and E d were determined for all the samples before and after 70 kVp irradiation. It was observed that there is a slight increase in E d upon X-ray irradiation for all the samples. The examination of the thermal stability of a-Se and a-Se:0.5%As films shows that even after very high doses, several thousands of Grays deposited into a-Se, the Hruby thermal stability coefficient is unaffected and there is no permanent deterioration in the stability.