Thermally induced effects in evaporated chalcogenide films. I. Structure

The structure of evaporated ${\mathrm{As}}_2$ ${\mathrm{S}}_3$, ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$, and Ge${\mathrm{Se}}_2$ films, and the influence of annealing at the glasstransition temperature, are studied by extended x-ray-absorption fine structure (EXAFS) and by Raman spectroscopy. In addition, the topology of each film is analyzed by calculating the x-ray diffraction for several models. The films were prepared by evaporation onto substrates held near room temperature. All the as-deposited films exhibited significant homopolar bonding in contrast to the almost totally heteropolar bonding of the corresponding bulk quenched glasses. Upon annealing of the films, the measurements indicate that the density of homopolar bonds decreases, and the films more closely resemble the bulk quenched glasses. The Raman spectra are quantitatively analyzed with two models to characterize the disorder, and then compared to the EXAFS results. The analysis indicates an As-As coordination of ∼ 0.6 and ∼ 0.4 and a Ge-Ge coordination of ∼ 0.3 for the ${\mathrm{As}}_2$ ${\mathrm{S}}_3$, ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$, and Ge${\mathrm{Se}}_2$ as-deposited films, respectively. The measurements also indicate that the As-As bonds of the ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ film are incorporated into ${\mathrm{S}}_2$As-As${\mathrm{S}}_2$ units, suggesting the presence of ${\mathrm{As}}_4$ ${\mathrm{S}}_4$ molecules. Calculations using this model are in good agreement with x-ray diffraction data in the literature. The data from ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ evaporated films also indicate that molecular structures may be present. There is no evidence, however, for molecular structures in Ge${\mathrm{Se}}_2$ films.