Structural changes ina−Si:Hfilm crystallinity with high H dilution

Abstract

Using infrared absorption (ir) spectroscopy, H evolution, and x-ray diffraction (XRD), the structure of high-H-dilution, plasma-enhanced chemical vapor deposition $a-\mathrm{S}\mathrm{i}:\mathrm{H}$ films “on the edge of crystallinity” is examined. From the ir Si-H wag mode peak frequency and the XRD results, we postulate the existence of very small Si crystallites contained within the as-grown amorphous matrix with the majority of the bonded H located on these crystallite surfaces. Upon annealing, a low-temperature H-evolution peak appears, and film crystallization is observed at temperatures as low as 500 °C, which is far below that observed for $a-\mathrm{S}\mathrm{i}:\mathrm{H}$ films grown without H dilution. While the crystallite sizes and volume fraction are too small to be detected by XRD in the as-grown films, these crystallites catalyze the crystallization of the remainder of the amorphous matrix upon annealing, enabling the evolution of H at low temperatures. The large spatial inhomogeneity in the H bonding thus produced throughout the film is suggested to be one of the reasons for the reduced Staebler-Wronski effect observed in solar cells utilizing these films.