Synchrotron x-ray diffraction was employed to measure the lattice constants a and c of GaN films grown with an AlN buffer layer on sapphire (0001) over a thickness range of 50 Å to 1 μm. We used multiple reflections and a least-squares fit method for high reliability. As the thickness increased, the lattice constant a increased from 3.133 Å to 3.196 Å and c decreased from 5.226 Å to 5.183 Å. The expected trend was fitted to an equilibrium theory, allowing the critical thickness of GaN on AlN to be estimated at 29 Å ± 4 Å in good agreement with a theoretical prediction.
We report a low-resistant and high-transparent Ru-based ohmic contact on p-type GaN through oxidation annealing under O2 atmosphere. A low resistivity of 4.5×10−5 Ω cm2 and high light transmittance (84.6%) were simultaneously obtained from oxidized Ru (50 Å)/Ni (50 Å) contact. The RuO2 formed on p-type GaN played a role in reducing the effective Schottky barrier height for injection of holes and NiO layer produced on the RuO2 acted as a diffusion barrier for the outdiffusion of released Ga and N atoms, resulting in the reduction of the contact resistivity as well as the enhancement of light transmission.
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