Structural changes in a-Si : H film crystallinity with high H dilution

Using infrared absorption (ir) spectroscopy, H evolution, and x-ray diffrac tion (XRD), the structure of high-H-dilution, plasma-enhanced chemical vapo r deposition a-Si: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 e xistence of very small Si crystallites contained within the as-grown amorph ous matrix with the majority of the bonded H located on these crystallite s urfaces. Upon annealing, a low-temperature H-evolution peak appears, and fi lm crystallization is observed at temperatures as low as 500 degrees C, whi ch is far below that observed for a-Si:H films grown without H dilution. Wh ile the crystallite sizes and volume fraction are too small to be detected by XRD in the as-grown films, these crystallites catalyze the crystallizati on of the remainder of the amorphous matrix upon annealing, enabling the ev olution 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 rea sons for the reduced Staebler-Wronski effect observed in solar cells utiliz ing these films.