Improvement in the optical absorption of PECVD microcrystalline Si thin film through modification of the crystalline fraction through an annealing process

Intrinsic microcrystalline silicon (μc-Si) thin films with varying crystalline fractions (XC) were deposited using plasma-enhanced chemical vapor deposition. Effort was made to modify the XC and optical properties by annealing at different temperatures, pressures, and ambient gases. After annealing, the amount of Si-H in the μc-Si thin film was decreased for an initial XC of 7%, while that of Si-H2 mainly decreased to an initial XC of 60-70%. The XC of μc-Si with an initial XC of 7% increased by annealing. However, initial XC of 60% and 70% resulted in a decrease of XC upon annealing. It is suggested that an increase in crystal size of μc-Si films by annealing decreases XC, while a decrease in crystal size increases XC. Depending on the annealing conditions, the optical absorption property of the μc-Si thin film changed. When the initial XC was 7%, the absorption coefficient increased as XC increased to 16%, but slightly decreased with XC values from 51% to 70%. Therefore, we suggest that there is an optimum XC that exhibits the highest optical absorption.