In this thesis, we use optical techniques, especially the photoluminescence spectroscopy, to study the properties of nano-structured and crystalline ZnO.
We have measured the optical reflectance, photoreflectance, and photoluminescence (PL) spectra of ZnO thin films. By using the Kramers-Kronig transformation, the absorption resonance energies for the free excitons (FX) was obtained from the OR spectra. The PL and PR spectra provide additional evidences for the peak assignment. Bound exciton (BX) transitions were dominant in the PL spectrum at 12 K. The temperature dependence of the transition energies of the FXs and BXs follows the empirical Varshni formula.
We observed a characteristic of the quantum confinement effect, i.e., obviously blueshifted FX emission peaks, in the ZnO nanorods by successfully suppressing the defect-related visible emissions. For the defect emissions, we found that the green and yellow emission could be excited below the band edge energy, while the orange-red emission could only be excited above the band edge. The green, yellow, and orange-red emissions likely originate from different defect-related transitions, while the chemical origins were still unclear.
The valence band symmetry ordering problem of wurtzite ZnO was solved by investigating the magneto-PL spectra of the free A-exciton 1S state. The results clearly indicate that the top valence band has Γ7
symmetry. The out-of-plane component B c
of the magnetic field, which is parallel to the sample's c axis, leads to linear Zeeman splittings of both the dipole-allowed Γ5
exciton state and the weakly allowed Γ1
exciton states. The in-plane field B⊥c
, which is perpendicular to the c axis, enhances the oscillator strength of the weak Γ1
states by forming a mixed exciton state.
Furthermore, we measured the magneto-PL spectra of the BX lines to identify the type of the defect centers. Neutral and ionized BX transitions are characterized, while the chemical origins are unknown.
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