Optical characterization of wide band gap semiconductors and nanostructures

The optical properties of ZnO-based wide band semiconductors and their nanostructures deposited by pulsed laser deposition are investigated. The ordinary and extraordinary refractive indices of wurtzite Mg_xZn _1−xO (x = 0–0.36) single crystalline films in the wavelength range of 457–968 nm are measured for the first time by prism coupling techniques. The excitonic absorption features are clearly visible at room temperature despite the alloy broadening effect. The band gaps and exciton binding energies are obtained by modeling the absorption spectra near the band gap. Intense band-edge photoluminescence (PL) from Mg_xZn _1−xO films is demonstrated.; The luminescence properties of MgZnO/ZnO multiple quantum wells (well width ∼35–80Å in a series of samples) grown on c-plane sapphire substrates via a ZnO buffer layer are demonstrated. Despite poorly-defined well/barrier interfaces and high defect densities in the material, the PL peaks shift toward shorter wavelengths relative to the band edge of bulk ZnO when the well width decreases. The mechanism is qualitatively identified as confined-carrier transitions in partially intermixed quantum wells as a result of Mg atom interdiffusion during the deposition.; A broad absorption peak around 314 nm and a bright ultraviolet PL peak at 328.6 nm are measured on ZnO quantum dots (average dot size ∼2.5 nm, smaller than the exciton Bohr diameter 3.6 nm) embedded in a polycrystalline AlN matrix. The three-dimensional carrier confinement and recombination centers in Al₂O₃ are likely to be responsible for the distinct optical characteristics. In contrast with most existing reports on ZnO nanoparticles, the defect luminescence in the visible range is largely absent in our studies. The same technique is also used to deposit crystalline Ge quantum dots (size 73, 130, 160 and 260 Å ± 5%) embedded in Al₂O₃ or AlN matrices. The spectral positions of E ₁/E₁ + Δ_l and E₂ transitions shift to higher energies (ΔE₁ = 1.19 eV, ΔE₂ = 0.57 eV) in the absorption spectra as dot size decreases (73 Å) due to the quantum size effect, regardless of the surrounding matrix. The results of this study provide useful information and guidelines on the design of ZnO-based quantum heterostructure optical devices.