Characterization of Polycrystalline GaN Layers Grown on Alkali-Metal-Free Glass Substrates by Molecular-Beam Epitaxy Assisted by Electron Cyclotron Resonance Plasma

摘要

We have investigated the optical and crystallographic properties of GaN layers grown on alkali metal-free glass substrates by molecular-beam epitaxy assisted by electron cyclotron resonance plasma. The as-grown layers exhibited dominant wurtzite-GaN (α-GaN) crystal structure. The relatively strong near band-edge photoluminescence (PL) emissions, probably assigned as emissions associated with residual acceptor-like alkali metals and/or oxygen donor impurities out-diffused from the substrates and with donor-like N vacancy in α-GaN crystal grains, were observed at 3.312-3.317 eV with broad full-widths at half-maximum (FWHMs) of 275-350 meV at 8.5 K. The PL intensities abruptly increased up to a Ga beam flux pressure of 2.5x10~(-7) Torr and decreased at 3.0x10~(-7) Torr, reflecting the generation of N vacancies under the Ga-rich condition. The FWHM decreased from 350 to 275 meV with an increase of the Ga beam flux pressure from 1.5x10~(-7) to 2.5x10~(-7) Torr. These results indicate that the crystalline quality of the layer is improved and impurities incorporation suppressed with an increase of the Ga beam flux pressure, suggesting the substitution of the impurities on Ga atom sites. We also have compared these results with characteristics of GaN layers grown on Si(111) and SiO_2 substrates under the same growth conditions and discussed the origin of the near band-edge PL emissions from both, the excitation intensity and temperature dependences.