Micro-Raman scattering from single crystal GaN films, both free-standing and attached to Al2O3 substrates, was performed over the temperature range from 78 to 800 K. These measurements reveal that the Raman phonon frequency decreases and the linewidth broadens with increasing temperature. This temperature dependence is well described by an empirical relationship which has proved to be effective for other semiconductors. The experiments also demonstrate that the strain from Al2O3 substrates compresses the epitaxial GaN in the c-axis direction.
High-resolution x-ray diffraction has been used to analyze the type and density of threading dislocations in a series (0001)-oriented GaN epitaxial film. Photoluminescence (PL) and carrier mobility of the films are measured at room temperature. The intensities of both the band edge (3.42 eV) peak and yellow luminescence (YL) are strongly related to the threading dislocation density of the GaN films. But different types of dislocations show different relationship with the intensities of PL and YL. The fundamental correlation is found not only between the interaction of edge- and screw-type dislocations and the carrier mobility but also between the interaction and the intensities of both the band edge peak and the YL.
The structural influences of the laser lift-off (LLO) techniques on the created (0001) GaN surface region are characterized by cross-sectional high-resolution transmission electron microscopy and fitted using the model of stress waves caused by a longitudinal impact at the end of a cylindrical bar extending to infinity. The authors study reveals that, in addition to the superficial damage caused by laser absorption, the stress saltation in GaN crystal where the shock waves come into being induces deformation of the lattices and generates a cluster of half loops above the LLO interface. After that, the lattice deformation will be induced every time the partial dissipation of the steady-state shock waves takes place until the shock wave is dissipated to elastic mode.
The yellow luminescence (YL) in as-grown high-resistance (HR) and unintentional-doped (UID) GaN films grown by metal organic chemical vapor deposition has been investigated by means of photoluminescence and monoenergetic positron annihilation spectroscopy. It is found there is stronger YL in UID-GaN with higher concentration of gallium vacancy (VGa), suggesting that VGa-involved defects are the origin responsible for the YL in UID-GaN. Contrastly, there is much stronger YL in HR-GaN that is nearly free from VGa, suggesting that there is another origin for the YL in HR-GaN, which is thought as the carbon-involved defects. Furthermore, it is found that the HR-GaN film with shorter positron diffusion length Ld exhibits stronger YL. It is suggested that the increased wave function overlap of electrons and holes induced by the extremely strong space localization effect of holes deduced from the short Ld is the vital factor to enhance the YL efficiency in HR-GaN.
In this paper, we reported a Raman scattering study of epitaxial graphene on different doped 6H-SiC ͑0001͒ substrates and investigated the substrate induced charge-transfer doping to the epitaxial graphene. We found that the charge carrier type and concentration of epitaxial graphene can be altered by SiC substrates with different doping level and doping type. This effect is comparable to that obtained by electrochemical doping. As Raman scattering is very sensitive to the doping level, the charge carrier concentration of epitaxial graphene can be estimated by the Raman G-peak shift. Our results are fundamental and may have implications for future epitaxial-graphene-based micro/ nanoelectronic devices.
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