| The electronic and optical properties of semiconductors are closely related in the THz (or sub-millimeter) regime, where quasi-optical measurements are difficult due to lack of powerful, coherent sources. This is especially true for thin film semconductors of the type studied for photovoltaics, because of their thickness being so much less than the wavelength of the probe. For that reason, the quasi-optical methods of dielectric spectroscopy, tends to break down also for low loss samples (loss tangents below 10 -5). In such a case it is useful to put the sample in a high-Q cavity and measure the perturbation of the cavity modes. Provided that the average mode frequency divided by the shift in mode frequency is less than the Q (quality factor) of the mode, then the perturbation should be resolvable. Cavity perturbation techniques are not new, but there are technological difficulties in working in the millimeter/submillimeter wave region. In the first part of this dissertation, the applications of cavity perturbation to the dielectric characterization of semi-conductor thin films of the type used in the manufacture of photovoltaics in the 100 and 350 GHz range are shown and explained. The complex optical constants of hot-wire chemical deposition grown-1mum thick amorphous silicon (a-Si:H) film on borosillicate glass substrate are measured. The real part of the refractive index and dielectric constant of the glass-substrate varies from frequency-independent to linearly frequency-dependent. I found a power-law behavior of the frequency-dependent optical conductivity from 316 GHz (9.48 cm-1 ) down to 104 GHz (3.12 cm -1). This sub-millimeter optical conductivity in the dark represents the phonon- damping. In the second part, light-enhanced photoconductivity measurement with the use of an open cavity is performed to measure the electronic conductivity. The data are well fit by a multiple trapping model, which is consistent with our underlying knowledge of the physics. The dispersive nature of electronic transport is illustrated in some detail. |
