Intermediate crystalline states produced by isothermal annealing of sputter-deposited a-Si films

The crystalline states produced by isothermally annealing sputter-deposited hydrogen-free amorphous-Si (a-Si) films greatly differ from those of a-Si:H films. Strained network and numerous vacancies in the a-Si film are indicated by the <ε₂> spectrum. Annealing the sample at temperatures between 300 and 550 °C relaxes the strained network due to local exchanges of Si–Si bond while maintaining the amorphous state. Self-assembly of hemispherical grains of microcrystalline Si on the film surface occurs during the onset of crystallization (600 °C). At a slightly elevated temperature of 690 °C, the cohesion of Si atoms in the film creates nanocrystalline Si (2-nm diameter), which directly corresponds to the emergence of the absorption peak feature below 3 eV in the <ε₂> spectrum. Dense voids at the interfacial region severely limit the range of solid phase epitaxy, thus facilitating preferential nucleation in the film.