X-ray induced effects in the optical and thermal properties of a-Se1-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 delta E (Urbach width) when a-Se:6%As doped with ppm-level Cs is brought to 75 A degrees C, annealed there for 10 min and then brought back down to room temperature (23.5 A degrees 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 A degrees 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 A degrees C, delta E is larger and E (U) is smaller. Upon cooling, both E (U) and delta E 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.