THz emission spectroscopy of narrow bandgap semiconductors

This dissertation presents a model for emission of electromagnetic transients in the terahertz (THz) region from optically excited narrow bandgap semiconductors. This model explains the THz emission from the surface field acceleration mechanism and from the photo-Dember effect independently. It relates intrinsic parameters of the semiconductor, namely the majority and minority carrier concentrations and the mobilities, to the radiated THz field. The conditions that enhance the THz emission process in the case of surface field acceleration and in the case of the photo-Dember effect have been clearly identified.; In this work three types of narrow bandgap semiconductors were investigated as sources of THz radiation. First the THz emission from a set of Te doped GaSb samples was studied. GaSb:Te is an interesting material because samples can be grown with a broad range of carrier concentrations. A Ga xIn1-xSb ingot was also studied. In this material system the electron mobility and the bandgap range from the GaSb values (Eg = 0.73eV) to the InSb values (Eg = 0.17eV). This is important in order to understand the extent to which the reduced bandgap of InSb is favorable for the THz emission process. This ingot was also used as a demonstration of how the THz time-domain spectroscopy technique can reveal the native defect density distributions in this ingot. Additionally the THz emission from InN was studied. The recently revised bandgap value of InN make it a good material for optically excited THz emission. In order to investigate the ultrafast scattering mechanisms in InN, an ultrafast photo-reflection setup was used. Subpicosecond non-radiative recombination was observed in silicon doped InN films.