We describe time-integrated and spectrally resolved degenerate four-wave mixing ͑DFWM͒ on hexagonal GaN on sapphire for different laser energies and intensities. Our measured DFWM signal decays exhibit no contributions originating from polariton propagation effects in the 1-to 3-m-thick bulk samples or from exciton/free-carrier scattering. At low temperatures and low excitation intensities the dephasing time of the A exciton is as long as 3 ps and is most likely due to scattering by defects. At excitation densities above N ex ϭ5ϫ10 15 cm Ϫ3 exciton-exciton scattering becomes the dominant dephasing mechanism and a fifth-order diffraction signal is observed above N ex ϭ3ϫ10 16 cm Ϫ3 . ͓S0163-1829͑97͒50744-2͔
RAPID COMMUNICATIONSR12 720 56 PAU, KUHL, SCHOLZ, HAERLE, KHAN, AND SUN
Two-inch free-standing GaN wafers were implanted by 100 keV H + 2 ions with a dose of 1.3 × 10 17 cm −2 at room temperature. The hydrogen implantation induced damage in GaN extends between 230 to 500 nm from the surface as measured by cross-sectional transmission electron microscopy (XTEM). The wafer bow of the free-standing GaN wafers was measured using a Tencor long range profilometer on a scan length of 48 mm before and after the hydrogen implantation. Before implantation the bow of two different free-standing GaN wafers (named A and B) with different thicknesses was 1.5 µm and 6 µm, respectively. Initially, both wafers were concave in shape. After implantation the bow changed to convex with a value of 36 µm for wafer A and a value of 32 µm for wafer B. High dose hydrogen implantation leads to an in-plane compressive stress in the top damaged layer of the GaN, which is responsible for the enhancement of wafer bow and change of bow direction. The high value of bow after implantation hinders the direct wafer bonding of the free-standing GaN wafers to sapphire or any other handle wafers. Tight bonding between hydrogen implanted GaN wafers and the handle wafers is a necessary requirement for the successful layer transfer of thin GaN layers onto other substrates based on wafer bonding and layer splitting (Smart-cut).
The density and temperature dependence of the exciton dephasing time of two hexagonal GaN films on sapphire is measured using degenerate four-wave mixing (DFWM). The residual 4 ps dephasing time at low temperature and density is caused by exciton-impurity scattering. We present a theory of DFWM for various amount of inhomogeneous broadening. Good agreement of the temperature dependence of the dephasing time is found between theory and experiment.
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