Grain Nucleation and Grain Growth During Crystallization of HWCVD a-Si:H Films
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0910-A07-05
Grain Nucleation and Grain Growth During Crystallization of HWCVD a-Si:H Films S. P. Ahrenkiel1, B. Roy2, A. H. Mahan1, and D. S. Ginley1 1 National Center for Photovoltaics, National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO, 80401 2 Metallurgical & Materials Engineering, Colorado School of Mines, Golden, CO, 80401
ABSTRACT Hydrogenated amorphous silicon (a-Si:H) films of high and low hydrogen content were deposited directly on molybdenum, carbon-coated TEM grids by hot-wire chemical vapor deposition. The material was annealed at 600°C and 630°C for variable times to achieve various degrees of crystallinity. The films thickness of 100 nm allowed characterization by TEM without additional thinning. The grain growth in such thin films is nearly two-dimensional, allowing clear identification of crystalline and amorphous regions. Thus, the crystalline volume fraction can be tracked by simple image-processing methods. The evolution of crystallization by grain nucleation and growth for these films is accurately described by classical phase-change kinetics. Analysis of the randomly distributed grains at early stages of crystallization also provides the average areal grain number density and grain size. From the image analysis, we determine the grain nucleation rate and the grain growth velocity. The final grain size is then estimated by extrapolation to the fully crystallized state, assuming the kinetic parameters remain constant after the onset of crystallization. INTRODUCTION Recrystallization of a-Si:H is being investigated as a viable approach for the production of inexpensive, device-quality, polycrystalline thin films. Some investigators are focusing on metalcatalyzed growth, or even solid-phase epitaxy, with seeded or interfacial nucleation. However, crystallization on amorphous substrates, such as glass, via homogenous nucleation, is highly desirable because of its inherent simplicity, and its contribution for understanding the fundamental properties of the material. Crystallization on amorphous substrates is comprised of two stages: an initial incubation period, during which the majority of the bonded hydrogen is released, and a period of simultaneous grain nucleation and grain growth. The initial H content of a-Si:H films increases as the growth temperature is decreased [1]. Whereas incorporated H is beneficial for the electronic properties of as-deposited a-Si:H, the continued presence of H during recrystallization may be less desirable. For example, increasing the intital hydrogen content has been shown to extend the total crystallization time [2-4]. After crystallization, material with initially high H levels also shows increased broadening of x-ray
diffraction peaks, which may result from a combination of grain size, defect density, or strain effects. Therefore, microscopic determinations of grain size and grain morphology are needed to determine the structural manifestations of incorporated H. EXPERIMENT Samples were grown by HWCVD on 1737 corning glass, and also directly on C
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