| dc.description.abstract | The characteristics of p-type Mg-doped GaN films diffused with Si are studied. N-type conductivity is achieved, and the carrier mobility of diffused GaN is 90-150 cm2V-1s-1, higher than of p-GaN but less than that of epitaxially grown n-GaN. The Mg acceptor states could become deep compensating defects, and the compensation ratio NA/ND is 0.3, 0.45, 0.6, and 0.75 for 800, 900, 1000, and 1100°C diffused GaN, respectively. The carrier transport may be dominated by electron hopping through these deep compensating centers or through diffusion. The results of temperature-dependent carrier concentration indicate that thermal annealing may induce defects at the surface, leading to an additional activation energy Ed ~ 10 meV in the 200-500 K region in diffused GaN.
Photoluminescence (PL) of homoepitaxial and heteroepitaxial ZnO films grown by plasma-assisted molecular beam epitaxy is studied. Homoepitaxial ZnO layers were grown on an O-face melt-grown ZnO (0001) substrate. Heteroepitaxial ZnO layers were grown on an epitaxial GaN template predeposited by metalorganic chemical vapor deposition on a c-plane sapphire substrate. The low-excitation PL spectra of ZnO epilayers excited by a He-Cd laser exhibit only bound-exciton emission with phonon replicas. There are green luminescence from the ZnO substrate but not from the ZnO epilayers. However, under high-excitation by a N2 pulse laser, the emission due to exciton-exciton scattering dominates the PL spectrum from the heteroepitaxial ZnO layer but is not observed from the homoepitaxial ZnO layer. The difference is probably due to the different quality of the ZnO substrate and GaN template.
We have also investigated heteroepitaxial ZnO films grown under various O/Zn flux ratios. PL spectra of ZnO epilayers excited by a He-Cd laser exhibit exciton emission at 376 nm with a full width at half maximum (FWHM) of 10 nm (90 meV) at room temperature. The exciton emission intensity of stoichiometric condition is 2 times greater than that of O-rich and Zn-rich conditions. Samples grown under stoichiometric and Zn-rich conditions do not exhibit defect-related green luminescence, but samples grown under O-rich condition do. In these heteroepitaxial ZnO layers there exists interstitial Zn and Zn vacancies. X-ray diffraction measurements revealed that there exists a residual compressive strain, ε ~ -0.2%, in the [0002] direction of the ZnO epilayer. The residual strain might be attributed to grain boundaries of ZnO. | en_US |