Raman Spectroscopy and Positron Lifetime Studies of Structural Relaxation and Defect Evolution in Amorphous Silicon

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Abstract
Correlations between structural relaxation and defect evolution in amorphous silicon (a-Si) and hydrogenated amorphous silicon (a-Si:H) have been investigated by Raman scattering spectroscopy and positron lifetime measurements. Various a-Si films were prepared by ion implantation, evaporation, and electron-cyclotron resonance plasma-enhanced chemical vapor deposition (ECR CVD). The bond angle deviation, Δθ, of nonhydrogenated a-Si decreased during 450° C isothermal annealing, while Δθ of a-Si:H increased due to dehydrogenation. Ion implanted a-Si contained only small vacancies of which size was the same as that of the divacancy in crystalline Si, which were dissociated and annihilated during annealing. On the other hand, both evaporated a-Si and ECR CVD a-Si:H contained larger vacancies, and nanometer-size vacancy clusters were formed during the annealing processes, more significantly in ECR CVD a-Si:H. Based on these experimental results, we concluded that large vacancies tend to form vacancy clusters or microvoids which suppress structural relaxation in a-Si.