C. S. Nichols

In situ studies of grain growth in thin metal films

Grain growth in thin films of aluminum has been studied using in situ transmission electron microscopy and a heating stage. Videotapes taken during grain growth were analyzed with the intent of searching for the predominant local rearrangement processes responsible for growth. Evolution of a soap froth can be decomposed into only two elementary local topology rearranging events. We have found numerous exceptions to prevailing theories that compare grain growth in thin films to the evolution of such froths. These observations suggest that a more complete picture of grain growth is necessary and that such a theory must include more complex local rearrangement processes.

Toward Realizing the Structure-Property Link in Polycrystalline Thin Films

Many materials for engineering applications are used in polycrystalline form and contain grain boundaries with a range of structures and properties. However, most research on grain boundaries to date has focussed exclusively on symmetric coincidence site lattice interfaces. To go beyond descriptions for these simple interfaces and thence to an aggregate of grains and grain boundaries in a polycrystal will require a new approach. Here we discuss two models for properties of polycrystalline materials, including their advantages and drawbacks, and indicate the microstructural variables available to optimize properties.

Aggregates of grain boundaries: structure and properties

Abstract The properties of polycrystalline materials are usually described in terms of constitutive laws which relate a microscopic property to an average property of a grain boundary and a length scale such as the average grain diameter. It is known however that the properties of individual grain boundaries vary widely. The boundary to boundary differentiation in properties can be incorporated into models and related to experimental data if the individual grain orientations are determined and the rule connecting misorientation to properties is known. Excess volume at grain boundaries can be related to energy and diffusivity. It may be possible to measure excess volume experimentally using phase contrast imaging in the electron microscope and thus determine a quantity which is directly related to the properties of an individual boundary. In turn this information can in principle be exploited to relate the orientation distribution to property distribution and the behavior of a polycrystalline body.

Impurity-defect complexes in hydrogenated amorphous silicon

The two most outstanding features observed for dopants in hydrogenated amorphous silicon (a-Si:H) -- a shift in the Fermi level accompanied by an increase in the defect density and an absence of degenerate doping -- have previously been postulated to stem from the formation of substitutional dopant-dangling bond complexes. Using first-principles self-consistent pseudopotential calculation in conjunction with a supercell model for the amorphous network and the ability of network relaxation from the first-principles results, we have studied the electronic and structural properties of substitutional fourfold-coordinated phosphorus and boron at the second neighbor position to a dangling bond defect. We demonstrate that such impurity-defect complexes can account for the general features observed experimentally in doped a-Si:H. 16 refs., 2 figs., 1 tab.