Solid-phase crystallization kinetics and grain structure during thermal annealing of a-Si:H grown by chemical vapor deposition

Solid-phase crystallization kinetics are examined during thermal annealing of as-deposited hydrogenated amorphous silicon (a-Si:H) thin films deposited by hot-wire chemical vapor deposition (HWCVD) and plasma-enhanced chemical vapor deposition (PECVD). The influence of deposition temperature of HWCVD material on crystallization is also considered. Real-time observation of the crystallization process using in situ transmission electron microscope heating allowed tracking of the crystalline volume fraction and grain number density by image-processing methods. Beyond an initial incubation period, roughly constant grain nucleation rate and growth velocity are observed. Extrapolation from early stages of crystallization allows estimation of the final average grain sizes. PECVD material shows a much lower nucleation rate than does HWCVD material under the same annealing conditions, whereas the grain growth velocities are comparable, leading to dramatically larger grain sizes in PECVD material. X-ray diffraction line widths from PECVD material are broader compared to HWCVD material. The diffraction line broadening is primarily determined by intragranular defect structure, rather than grain size. Low-temperature preannealing reduced the final XRD line widths of HWCVD material, indicating an influence on defect structure or density. Lattice contraction during crystallization of HWCVD material is observed to be independent of the initial hydrogen content.