The effect of film tensile stress on crystallite nucleation and growth in thermally annealed a-Si:H

Abstract

The influence of stress in thin films appears to be a widespread issue when such films are thermally annealed to facilitate crystallite nucleation and growth. It is therefore of interest to examine on a fundamental level how stress influences crystallite nucleation and growth in a thermally annealed thin film system that has been extensively studied and well characterized. This article reports crystallite nucleation rates, growth rates and activation energies for nucleation and growth in different spatial regions of a thermally annealed a-Si:H film. The rates far from a cleaved film edge are representative of a film region that is under high tensile stress, while rates near a cleaved film edge are representative of a film region that has undergone stress relief. The existence of or reduction in film stress is supported by μ-Raman measurements. It is shown that film stress increases the film nucleation rate and decreases the crystallite growth rate, resulting in significantly smaller crystallite sizes in the fully crystallized stressed film areas compared to those observed in the stress relieved areas. By combining the activation energy data for nucleation and growth, it is shown how film stress affects two fundamental structural parameters that control crystallization, the height of the amorphous-crystalline energy barrier and the critical crystallite size.