ULTRAFAST CARRIER DYNAMICS IN THIN FILM HYDROGENATED AMORPHOUS SILICON AND SILICON-GERMANIUM ALLOYS
Felver, Josef J.
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This dissertation presents an investigation of the ultrafast carrier dynamics in thin film hydrogenated amorphous silicon (a-Si:H) and silicon germanium alloys (a-Si_(1-x) Ge_x:H). These materials have attracted considerable interest for photovoltaic and optoelectronic applications. However, a detailed understanding of photoconductivity mechanisms is crucial for optimizing their photovoltaic efficiency. Three sets of experiments were conducted using femtosecond transient absorption methods. (1) An investigation of carrier-lattice thermalization times in a-Si:H as a function of probe wavelength. Following excitation, the carriers will lose energy via phonon emission while relaxing toward the band edge. The near edge states were probed using a series of above gap energies. An induced transmittance response, caused by Pauli blocking of the near edge states, reflected a carrier-lattice thermalization time of ~150 fs, faster than the 240 fs time scale previously determined for crystalline silicon. (2) The lattice equilibration time was measured using a series of below gap probe energies. The short-time response was dominated by a fast exponential rise in the induced absorbance signal with a formation time of ~260 fs. The exponential rise was assigned to heating of the lattice since the band gap decreases with increasing temperature. Since the temperature dependence of the band gap is, in general, mediated by acoustic phonons, the associated change in optical absorbance reflects the time scale for redistribution of energy into these phonon modes, and the response measured in a-Si:H indicates that this process is substantially faster than the picosecond time scales characteristic of crystalline semiconductors. The exponential rise time was uniform for the range of probe wavelengths (870 – 950 nm). (3) The bimolecular recombination dynamics of a-Si:H and a-Si_(1-x) Ge_x:H were measured as a function of carrier density, sample temperature, and germanium content over the course of ~300 ps. The carrier dynamics were successfully modeled in terms of diffusion-limited bimolecular recombination, with an effective time-dependent mobility reflecting dispersive transport. The dispersion parameter showed a linear temperature dependence that agreed with the multiple trapping model of dispersive transport. The bimolecular recombination dynamics did not show a significant dependence on alloy composition for the range x = 0 to 0.3.