| Summary: | 博士 === 國立成功大學 === 物理學系 === 89 === Modulation spectroscopy of photoreflectance was employed to investigate electro—optical properties of GaAs and InAlAs surface-intrinsic-n+ (SIN+) structures. The photovoltage induced by the light incident on the sample and thus the surface barrier height can be expressed as a function of temperature or the power of the incident light, according to thermionic theory and current-transport theory. Room temperature photoreflectance (PR) was measured for a range of values of incident power. The built-in electric field and the surface barrier heights were determined from the Franz-Keldysh oscillation (FKOs) in the PR spectra. The surface Fermi level and surface state density were obtained as fitting parameters when the experimentally measured barrier heights were least squares fitted as a function of the impinging power, to the theoretically derived equations mentioned above. Similarly, surface barrier heights were obtained as a function of temperature, from PR spectra measured at various temperatures with the probe and pump beams both maintained at 1mW/cm2. The surface Fermi level and surface state density were also obtained as fitting parameters when the experimentally measured surface barrier height was least squares fitted as a function of temperature to the theoretical equation. The results obtained from both approaches are similar.
The fast Fourier transformation (FFT) was applied first to the FKO specta of the GaAs SIN+ structure. The FFT spectra shows two peaks corresponding to light-hole and heavy-hole transitions. The peak positions depend on the electric field in the sample. Assume that light- and heavy-holes experience the same electric field. The band gaps of light- and heavy-holes change until the electric fields determined from the peak positions are identical. The difference between band gaps is associated with the splitting of light- and heavy-hole energy levels in the valence band. According to deformation potential theory and model-solid theory, valence band splitting can be theoretically estimated from the strain induced by electric field. The results agree closely with those obtained from FFT analysis. The fast Fourier transformation was also, for the first time, applied to the PR spectra of a semiconductor other than material like GaAs. The FFT of the PR spectra of InAlAs appears as a broad peak, produced by the overlap of the two peaks corresponding to light- and heavy-hole transitions. The FFT spectra were deconvoluted to resolve these two peaks. The same procedure as used to determine the valence band splitting in GaAs can be used to find the valence band splitting experimentally from the FFT analysis and theoretically from the strain induced by an electric field. The results yielded are mutually consistent. Furthermore, surface state densities of light- and heavy-holes, and other electro-optic properties, were derived from the FFT peak positions, amplitudes and surface barrier heights as functions of pump beam intensity.
This work also measured PR spectra under high magnetic fields B. The period of oscillation of the surface Fermi level, expressed as a function of B or (1/B), elucidates many electro-optical properties of the semiconductors in the magnetic field, such as Landau level splitting, carrier behavior, density of surface and interface states, and the 3-dimensional density of electron states. This experiment is difficult to perform and demands complicated theoretical analysis; however, results break new ground in optical measurements under high magnetic fields.
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