| Abstract [eng] |
When the surface of the semiconductor is illuminated by femtosecond laser pulse electromagnetic oscillation of several picoseconds duration is generated at reflection angle. Spectrum of this electromagnetic pulse is in terahertz (THz) range. The nature of this radiation is associated with the movement of the charge carriers and driving forces of this movement is diverse: surface electric field, different carrier diffusion coefficients as well as non-linear optical effects, phonon oscillations, optical carrier orientation. Obviously, the THz pulse contains information about these mechanisms, while at the same time allows to investigate carrier dynamics after photo excitation. So the main goal of this dissertation was to create a THz excitation and picosecond photoconductivity spectroscopy techniques, which allows investigating band structure of different materials and to study various processes immediately after photo excitation. In order to show capabilities of THz excitation spectroscopy 4 material (InAs, InSb, InN, GaAs) were investigated in this work. Subsidiary valley positions of conduction band were determined from these spectra, which coincided very well with the known values. THz pulse generation from catalyst free InAs nanowires (NW) array was studied for the first time in this work by THz excitation spectroscopy method. Electron mobility in these nanowires depends on their diameter due to scattering by surface defects. Due to same reason momentum relaxation time become much shorter. Therefore THz generation mechanism in InAs nanowires is different from bulk InAs and moreover its efficiency depends on NW diameter. It was determined that the main mechanism responsible for THz pulse generation in InAs NW is photo-Dember effect. Tellurium (Te) is one of the few semiconductors, which frequency of phonon oscillations is easily recorded by ordinary time domain spectroscopy system. Three crystallographic orientations crystals were investigated in this work. It was found that three generation mechanisms are present. When the semiconductor is illuminated with photon energy smaller than 0.9 eV, THz is generated due to anisotropic photoconductivity. When the photon energy exceeds 0.9 eV, two other mechanism sets in: lateral photo-Dember effect and ordinary photo-Dember effect. At larger photon energies the latter two begins to dominate. Picosecond photoconductivity spectroscopy technique is presented in this dissertation. Unlike conventional photoconductivity, this technique is able to investigate electron movement and processes that happens several hundreds femtoseconds after photo excitation. At the same time, this method allows to measure energetic position and width of deep level defect band. Picosecond photoconductivity spectra of three different materials were measured in this work: low temperatures grown GaAs, InGaAs and GaAsBi. The first two materials has defect band in the middle of the band gap. Position and width of this defect band was assessed by latter method. Subsidiary valley position of conduction band in GaAsBi was determined too. |
