Determination via luminescence spectroscopy and x-ray diffraction of the strain and composition of GaN and Al(x)Ga(1-x)N thin films grown on 6H-SiC(0001) substrates

This dissertation describes the luminescence and x-ray diffraction characterization of GaN and {dollar}rm Alsb{lcub}x{rcub}Gasb{lcub}1-x{rcub}N{dollar} thin films that were deposited on 6H-SiC(0001) substrates. These materials have applications for optoelectronic devices that are operational in the UV to green regions of the spectrum and for high-temperature, high-frequency and high-power microelectronic devices. The primary tools used in this research were photoluminescence and cathodoluminescence spectroscopies and high-resolution x-ray diffraction.; Biaxial strains resulting from the mismatches in thermal expansion coefficients and lattice parameters in GaN films grown on AlN buffer layers previously deposited on vicinal and on-axis 6H-SiC(0001) substrates were measured using photoluminescence. A linear relationship between the bound exciton energy (E{dollar}rmsb{lcub}BX{rcub}{dollar}) and the biaxial strain along the c-axis direction was observed. A marked variation in the biaxial strain in GaN films deposited on off- and on-axis SiC was determined. It was attributed to the difference in the density and nature of the microstructural defects that originate at the steps on the SiC surface. The strain in the GaN films was either in tension or compression; whereas, only tensile strains were reported in all previous studies using SiC wafers. This indicated that the lattice mismatch strain in the former films was not fully relieved by defect formation. This result was confirmed by the observation via HRTEM of a 0.9% residual compressive strain at the GaN/AlN interface.; Cathodoluminescence was used to determine the optical spectra in {dollar}rm Alsb{lcub}x{rcub}Gasb{lcub}1-x{rcub}N{dollar} films over the entire composition range of x. A bowing parameter of b = 1.65 eV for the bound exciton peak was observed. This bound exciton peak became more localized as the Al mole fraction increased. This was attributed to the increase in the ionization energy (E{dollar}rmsb{lcub}D{rcub}{dollar}) of the donor to which the exciton was bound. The donor-acceptor pair (DAP) band and the so-called "yellow" emission band that are commonly observed in GaN were also observed in the {dollar}rm Alsb{lcub}x{rcub}Gasb{lcub}1-x{rcub}N{dollar} films. Both bands exhibited a strong sublinear dependence on the Al mole fraction. A strong, broad emission band at 3.2 eV in AlN that has commonly been attributed to oxygen impurities was shown to also be closely related to the yellow band of GaN.; {dollar}rm Alsb{lcub}x{rcub}Gasb{lcub}1-x{rcub}N{dollar} layers with values of x {dollar}leq{dollar} 0.12 and having a thickness range of 0.35-0.5 {dollar}mu{dollar}m grown on GaN films that were deposited on 6H-SiC(0001) substrate/AlN(0001) buffer layer combinations had the same a-axis lattice constants (a). A comparison of the a values revealed that the {dollar}rm Alsb{lcub}x{rcub}Gasb{lcub}1-x{rcub}N{dollar} layers were strained to the GaN in-plane lattice constants, and thus were in tension. This was observed from both absolute lattice parameter measurements (2{dollar}theta{dollar}-{dollar}omega{dollar} scans) and high-resolution reciprocal space maps of asymmetric planes obtained by x-ray diffraction. These results were surprising, given that the critical thickness of these AlGaN layers on GaN should be less than 300 A. Higher Al mole fractions (x {dollar}>{dollar} 0.12) {dollar}rm Alsb{lcub}x{rcub}Gasb{lcub}1-x{rcub}N{dollar} layers of comparable thicknesses were severely cracked.