CONTRIBUTION OF DEFECTS TO ELECTRONIC, STRUCTURAL, AND THERMODYNAMIC PROPERTIES OF AMORPHOUS-SILICON

The structure of pure, nonhydrogenated amorphous silicon (a-Si) was modified by means of ion implantation, furnace annealing, and pulsed laser annealing. Defects in a-Si were probed by measuring the photocarrier lifetime tau at low carrier densities (10(18)/cm3) with subpicosecond resolution using pump-probe reflectivity measurements. The average cross section of defect-related midgap states for free-carrier capture is found to be 6 x 10(-16) cm2. In addition, the average bond-angle distortion DELTA(theta) in a-Si was derived from Raman spectroscopy. Annealing as-implanted a-Si for 1 h at T less-than-or-equal-to 500-degrees-C induces defect annihilation as well as network relaxation. In contrast, 32 ns pulsed laser heating of a-Si just below the melting threshold leads to relaxation of DELTAtheta without significant defect annihilation. This annealing behavior can be understood on the basis of defect diffusion kinetics. Implanting fully relaxed a-Si with 1 MeV B+, Si+, and Xe+ up to damage levels of 0.004 displacements per atom raises the defect density without affecting DELTAtheta. Only after the defect density has saturated at higher damage levels is DELTAtheta returned to the as-implanted level. The electronic density of states of a-Si is determined using optical-absorption spectroscopy, yielding N(sat) almost-equal-to 0.5 at. % for the saturation defect density in a-Si at room temperature. Electron paramagnetic resonance shows that a minor fraction (0.02 at. %) of these defects is spin active. The response of c-Si and relaxed a-Si to implantation damage is comparable, suggesting that the defect populations in both materials are similar. Comparing carrier lifetime measurements and Raman spectroscopy for the various experimental treatments demonstrates that there is no unique correlation between the defect density and DELTAtheta in a-Si. Assuming that defects and DELTAtheta have independent enthalpic contributions, the Gibbs free energy of various structural states of a-Si is calculated. These calculations indicate that the melting temperature of a-Si may vary from 1010 to 1490 K.