Optical Spectroscopy of Wide-band-gap Semiconductors: Raman and Photoluminescence of Gallium Nitride, Zinc Oxide and Their Nanostructures
Type of DegreeDissertation
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Micro-Raman and photoluminescence (PL) were performed to study GaN and ZnO and their nanostructures. Various ZnO nanostructures were successfully synthesized at low temperatures using thermal chemical vapor deposition method. The ultraviolet lasing in optically pumped ZnO nanonails was observed for the first time. The lasing threshold was found to be 17 MW cm–2. Very sharp emission peaks (full width at half maximum of 0.08 nm) were observed in the emission spectrum, indicating a high Q factor of the cavity formed by the hexagonally shaped nanonail head. The analysis of the lasing spectra strongly suggests the whispering gallery mode lasing from a hexagonally shaped head of the single ZnO nanonails. The as-grown ZnO nanorods were thermal annealed under different ambient conditions and their photoluminescence were collected after each annealing processes. The observed change in photoluminescence strongly suggests that positively charged impurity ions or interstitial Zn ions are the recombination centers for green luminescence observed in the present sample. A model based on the interplay between the band bending at the surface and the migration of positively charged impurity ions or Zn ions was proposed, which satisfactorily explains the observed photoluminescence. Raman scattering was performed to study the aligned GaN nanorods grown by plasma-assisted molecular-beam epitaxy. It was determined from the E2 peak that the GaN nanorods are relatively strain free. The free carrier concentration, as well as electron mobility was obtained by the line shape analysis of the coupled A1(LO) phonon plasmon mode for the first time for semiconductor nanostructures. Since the position of the LO phonon peak was found to be dependent on both the temperature and the LO phonon-plasmon coupling, special attention has been paid to exclude the temperature-induced peak shift. The local temperature of the nanorod sample was estimated based on the ratio of Stokes to anti-Stokes Raman peak intensity. GaN MISFET with re-grown by selected area MOVPE n+ layer has been analyzed by micro-Raman and micro-PL. The material properties were extracted by using both spectroscopies.