The optical properties of cubic and hexagonal GaN thin films, grown by electron-cyclotron resonance microwave plasma-assisted molecular-beam epitaxy on silicon and sapphire substrates, respectively, have been studied at photon energies up to 25 eV with conventional and synchrotron-radiation spectroscopic ellipsometry. The fundamental gaps of the two polytypes are located at different energies, namely at 3.25 and 3.43 eV for cubic and hexagonal GaN. Analysis of the dielectric function of the two phases in the region 4.5-9.5 eV with appropriate models yields the energy location and broadening of the observed critical points. These critical points are assigned to specific points in the zinc-blende and wurtzite Brillouin zones, respectively, making use of the latest published band-structure studies and a comparison is made between the corresponding results for GaN, GaAs, and GaP. Measurements in the temperature SiC(0001}, s with which the lattice and thermal expansion mismatch is very low. An A1N {Refs. 12, 39 -41) or GaN (Refs. 7, 33, 41}buffer layer is reported to improve the surface morphology and electrical properties of the material. a-GaN is prepared mainly by meta1-organic vapor-phase epitaxy and metal-orIIanic chemical-vapor deposition, as well as gas-source (GS) and electroncyclotron resonance (ECR) molecular-beam epitaxy (M BE}. Less common growth techniques include atomic-layer epitaxy at low temperatures, hydride

The optical properties of cubic and hexagonal GaN films in the region of the fundamental gap are studied with spectroscopic ellipsometry at temperatures between 110 and 630 K. It is verified that the gap of hexagonal GaN is higher than that of the cubic polytype. The parameters of the gaps are determined against temperature and the temperature shifts are found to be lower than and close to those of GaAs and GaP in the cases of cubic and hexagonal GaN, respectively. Additional theoretical calculations of the electronic structure of both polytypes using the full-potential linear-mu5n-tin-orbital method reveal a significant contribution to the Eo gap from the 8~10 transitions. The resulting gap energies are compared with the literature and the difference between the two GaN polytypes is discussed. The dielectric function c& (co) is directly calculated from the band structure and its features at energies up to 9.5 eV are discussed and compared to experiment.

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The optical properties of amorphous hydrogenated carbon films prepared with various techniques are studied with conventional and synchrotron-radiation spectroscopic ellipsometry (SE) and the pseudodielectric function 〈ε(ω)〉 of diamondlike and graphitelike films is presented in the energy region 1.5–10 eV. Characteristic features of the measured 〈ε(ω)〉 and the calculated electron-energy-loss (EEL) function are found to serve as useful criteria for the classification of such materials. The results and information obtained by SE are compared to those obtained by EEL and Raman spectroscopy techniques, which are the most widely used for this purpose. Thermal annealing experiments up to 675 °C with in situ monitoring of the 〈ε(ω)〉 reveal the undergoing structural changes in the material character from diamondlike into graphitelike during the annealing. The major modifications which turn the material into sp2-like are found to take place around and above 550 °C. The fundamental gap along with other optical parameters of the materials are compared to those of diamond and graphite and their shift with temperature is discussed and used to illustrate further the prevalence of the graphitic character during and after the annealing. Finally, the optimum growth parameters for the production of diamondlike material are discussed in the case of the glow-discharge and ion-beam deposited films.

The optical properties of commercially available 3C- and 6H-SiC single crystals were studied in the energy region 1.5–9.5 eV with conventional and synchrotron-radiation spectroscopic ellipsometry. The surface perfection of the materials was investigated by transmission electron and atomic force microscopies. The calculated values for the effective and static dielectric functions were higher than those found in the literature. This is consistent with the fine structure and absolute values of the corresponding reflectance spectra, which were higher than those reported in other studies. The dispersion of the refractive indices in the energy region below and above the indirect gap is determined and can be used as reference and for the design and analysis of semiconductor structures and devices.