No abstract

No abstract

The structure of evaporated As2S3, As, Se"and GeSe, 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 sub~irates 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 As, S3, As, Se"and GeSe, as-deposited films, respectively. The measurements also indicate that the As-As bonds of the As, S, film are incorporated into S,As-AsS, units, suggesting the presence of As4S4 molecules. Calculations using this model are in good agreement with x-ray diffraction data in the literature. The data from As, Se3 evaporated films also indicate that molecular structures may be present. There is no evidence, however, for molecular structures in GeSe, films.

No abstract

Data are presented which show that a major part of the localized electronic state distribution in hydrogenated amorphous silicon is in thermal equilibrium at elevated temperatures. Measurements of electronic transport are reported, with particular emphasis on the effects of annealing and cooling the samples. Two regimes of behavior are observed. When samples are cooled below a temperature TF, the electronic and atomic structures slowly relax with a temperature-dependent time constant.In n-type samples the relaxation time is several weeks at room temperature, and T& is -130'C. In p-type samples the time constant is a few hours and T& is -80'C. The second regime above TF corresponds to a relaxation time short compared to experimental times, and the structure attains a metastable thermal equilibrium.We show that the defect-compensation model of doping provides an accurate phenomenological description of the results. Furthermore, a quantitative fit to the data is obtained using the known density-of-states distribution. The bonding rearrangements that enable changes in the localized-state structure are discussed. We propose that the motion of bonded hydrogen is important, and that it can be considered to form a separate substructure that has properties similar to a glass. In this model the equilibration temperature TE is identified with the glass transition temperature. New measurements of hydrogen diffusion are presented to support the model.