Fluence- and temperature-dependent studies of carrier dynamics in radiation-damaged silicon-on-sapphire and amorphous silicon

Abstract

We study the effects of lattice damage level, pump fluence, and temperature on carrier dynamics in thin silicon films. Two samples of radiation-damaged silicon-on-sapphire (RD-SOS) and one amorphous silicon thin film on sapphire were investigated. The first RD-SOS sample was O+ implanted with doses of 1×1013 cm−2 at 100 and 200 keV; the second RD-SOS sample was O+ implanted with a dose of 1×1015 cm−2 at 100 and a second dose of 2×1015 cm−2 at 200 keV; the third sample was a nonhydrogenated amorphous-silicon thin film grown by electron-beam evaporation. Carrier concentrations up to 7.4×1020 cm−3 were injected into the samples with 100 fs, 400 nm pump pulses, while the transient optical properties were probed with subpicosecond-wide terahertz (THz) pulses. Using a thin film Drude model, we derived the carrier relaxation time and effective carrier mobility for the three samples. The increase of lattice damage decreased both the relaxation time constant and the carrier mobility. A slight increase in relaxation time was observed for increasing pump fluence, but mobility values were not affected. No change in relaxation time or mobility was found for temperatures from 5 to 300 K. We find average relaxation time constants of 5.5 ps in the first sample, 1.4 ps in the second sample, and 0.58 ps in the third sample, and average carrier mobility values of 383, 44, and 4.4 cm2/V s, respectively. The presence of a single relaxation time constant is consistent with a trap-influenced relaxation mechanism and not of Auger recombination for carrier concentrations <1021 cm−3, indicative of the absence of thermally activated processes in the relaxation mechanism.