Optical properties and residual stress in group III-V nitride films

We report spectroscopic ellipsometry (SE), reflectance difference/anisotropy (RD/RA) and low-temperature reflectance data on epitaxial GaN thin-film samples covering the widest range of tensile and compressive stress ({dollar}-{dollar}3.8 to 3.5 kbar) thus far. SE allows us to assess the preparation of smooth and abrupt GaN surfaces by chemical treatments in real time above the bandedge, and, coupled with the reflectance data, the Edn/dE contribution to dispersion below the bandedge, which is important for laser action. The reflectance data in the vicinity of the fundamental absorption edge explicitly show the nonlinear behavior of the B-A and C-A splittings vs. the energy of the A exciton. Lineshape ambiguities that hindered previous interpretations have been resolved with reciprocal space analysis, allowing us to obtain band parameters such as {dollar}Deltasb{lcub}rm so{rcub}{dollar} = 17.0 {dollar}pm{dollar} 1 meV and {dollar}Deltasb{lcub}rm CF{rcub}{dollar} = 9.8 {dollar}pm{dollar} 1 meV with increased confidence. Deviations from the observed nonlinear behavior are interpreted as originating from the anisotropic relaxation of in-plane residual stress, as supported by preliminary RD/RA data.; To make preliminary correlations between these fundamental optical, physical and electronic properties we also report trends in residual stress as a function of film thickness, growth temperature and substrate orientation for GaN/ AlN/ 6H-SiC heterostructures. In an effort to control such processes, we have developed a method to modulate the strain state (normally {dollar}>{dollar}2 kbar, tensile) of moderately thick ({dollar}sim{dollar}2{dollar}mu{dollar}m) GaN based structures grown on 6H-SiC to a range of compressive stresses (0 to {dollar}-{dollar}2kbar) by the introduction of a strain mediating layer above the standard high temperature AlN buffer layer. The strain characteristics of subsequently deposited nitride layers can be modulated by changing the growth parameters of this layer. This is achieved by use of in-situ techniques during crystal growth without degrading the structural and optical properties of the deposited layers. Analogous measurements, where possible, for related III-V Nitrides were performed as well.