SUBBAND OPTICAL-EXCITATION AND RECOMBINATION IN A-SI-H

We present a comprehensive investigation of the excitation and recombination below the gap in a-Si:H using several optical techniques. These techniques include optical absorption spectroscopy, photoluminescence (PL) spectroscopy and time resolved PL as a function of excitation energy. The excitation energies have been extended well below the optical gap down to about 1.1 eV. The PLE spectra for PL measured at 1.0 eV and 0.9 eV follow the Urbach tail of the absorption. The below gap absorption contributes little to these PLE spectra. This fact means that the contribution to the PL at high energies may be primarily due to an excitation from a bandtail to a band edge followed by recombination between the band tails. On the other hand, the below-gap absorption does contribute to the PLE at 0.8 eV. This behavior is consistent with the a recombination of carriers through the defect states. In addition, we found that the lifetime of the PL decay becomes shorter when the excitation energy is below about 1.66 eV.