Studies of GaN layers grown on sapphire using an RF-source

We have studied the relation between MBE growth parameters and him quality after growth of GaN on the c-plane of sapphire using a conventional Ga-source and an RF-source for the excitation of N-2. The growth rate was varied using Ga-flux, determined by RHEED oscillations on GaAs, r(GaAs) = 0.1-0.8 mu m/h. The N-2-flux was 0.5-1.5 sccm. Most layers of GaN were grown at 760 degrees C as measured by a pyrometer, while higher temperatures were utilised to determine the dismissal of growth. The RF-power and AlN buffer layer parameters were kept constant. The GaN-thicknesses were 0.2-1 mu m/h. For low growth rates of similar to 50 nm/h, there was a preferential growth of microcrystals in the growth direction which therefore overemphasised the measured film thickness. No film could be grown above 800 degrees C as the desorption rate was too high. For a GaN growth rate of 300 nm/h the sticking coefficient was similar to 85%. By varying the Ga- and N-2-fluxes it was evident that the film quality, as provided by the photoluminescence spectra, strongly depended on the growth parameters. Photoluminescence peak intensities generally improved with film thickness below 1 mu m. In 1 mu m thick films, we observed excitonic related peaks close to the band gap as well as peaks 50-60 meV below due to the presence of defects or impurities. We found a strong correlation between growth parameters and optimum growth and therefore the highest layer quality could be obtained only when all parameters were carefully optimised.