Trapping time in processed polycrystalline silicon measured by picosecond time-resolved reflectivity

We have used the methods of picosecond time-resolved reflectivity to measure the carrier lifetime in fine grain polycrystalline silicon films grown by low pressure chemical vapor deposition at 625 °C. After monatomic hydrogen diffusion or implantation with phosphorus ions followed by high temperature annealing (1150 °C), the trapping time τ increased from 40 to 150 ps, consistent with passivation of the grain boundaries or an increase in grain size, respectively. If implantation was not followed by annealing, τ decreased to less than 10 ps, while if it was followed by low temperature annealing (900 °C), which approximately restored the original grain size, τ recovered to 50 ps, very close to the trapping time measured in the as-grown samples. In all cases, we found indications that trapping of carriers was much faster than their subsequent recombination.