In random solid solutions the atomic-scale structure, i.e., the nature of the near-neighbor (nn) environment, is not well understood because of the fact that standard diffraction techniques average the structure over distances which are large on the scale of a lattice constant. One consequence of this lack of microscopic information is that calculations of the properties of solid solutions have often relied on simple approximations. One of the most used of these models is the virtual-crystal approximation (VCA) 1 which assumes that all atoms occupy the average lattice positions defined by the x-ray lattice constants. With use of the VCA, properties of the alloy, such as the electronic band structure, can be calculated whether or not the alloy lattice constant varies linearly with composition between those of the end members, i.e., follows Vegard's Law. 2 Similarly for dilute alloys, the assumption that the impurity-host distance is equal to the host-host distance is often used to calculate alloy properties, even those which may depend very sensitive-(to be published), Paper No. IAEA-CN-41/A-3. 6 G. Becker et al., to be publishedo ly on distance, e.g., the magnetic properties and the NMR and ESR spectra. However, the validity of this assumption, namely, an average distance or equal impurity and host distances, has never been systematically addressed with experimental measurements.We have used extended x-ray-absorption fine structure (EXAFS) to address these issues in random solid solutions since this technique is well suited to the study of local bonding, especially the determination of nn distances relative to a well-defined standard. As a result EXAFS has been used successfully to study other issues in alloys. These include studies of dilute binary metal alloy systems 3 where the main issues addressed were local clustering or chemical order, such as Guinier-Preston zones, and deviations from the continuum elastic theory. Other EXAFS studies of ternary alloys 4 " 6 have indicated that the nn distances do differ from the average, but the main emphasis was on other issues and so these studies were not performed over a wide In random solid solutions of Gaix In x As, the Ga-As and In-As near-neighbor distances change by only 0.04 Aasi varies from 0.01 to 0.99, despite the fact that this alloy accurately follows Vegard's law, with a change in average near-neighbor spacing of 0.17 A. This result contradicts the underlying assumption of the virtual-crystal approximation. Nonetheless, the cation sublattice approaches a virtual crystal with a broadened single distribution of second-neighbor distances, whereas the anion sublattice exhibits a bimodal anion-anion second-neighbor distribution.1412
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In this work we used Raman spectroscopy to investigate the structural characteristics of as-deposited amorphous and micro-crystalline silicon films. For amorphous silicon films, the order (or disorder) of the silicon network was quantified using properties of the Raman spectra that were related to key deposition conditions. We found that a strong relationship exists between the structural order of the silicon matrix and the deposition temperature and deposition rate. A quantitative model was proposed relating the intensity ratio of transverse optical phonon peak to longitudinal optical phonon peak to the surface diffusion length, a parameter that was calculated from available data. It was found that optimization of the as-deposited silicon microstructure is possible by selecting deposition conditions yielding peak–ratio values in the vicinity of 0.53. For as-deposited micro-crystalline silicon films, Raman spectroscopy was used to estimate the initial crystalline fraction of the film and monitor the crystallization process during annealing. These data were used to confirm the crystallization mechanism in mixed-phase silicon films and identify the effect of different process parameters on the crystallization time of the annealed films.
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