Basic studies of gallium Nitride growth on Sapphire by metalorganuc Chemical Vapor Deposition and optical properties of deposited layers

We have studied the growth of gallium nitride on c-plane sapphire substrates. The layers were grown in a horizontal metalorganic chemical vapor deposition reactor at atmospheric pressure using trimethylgallium (TMG) and ammonia (NH3). Variation of the V/III ratio (150-2500) shows a distinct effect on the growth rate. With decreasing V/Ill ratio, we find an increasing growth rate. Variation of the growth temperature (700-1000 deg. Cel.) shows a weak increase in growth rate with temperature. Furthermore, we performed secondary ion mass spectroscopy measurements and find an increasing carbon incorporation in the GaN films with decreasing ammonia partial pressure and a growing accumulation of carbon at the substrate interface. Photoluminescence measurements show that samples with high carbon content show a strong yellow luminescence peaking at 2.2 eV and a near band gap emission at 3.31 eV. With increasing carbon content, the intensity of the 3.31 eV line increases suggesting that a carbon related center is involved. In conclusion, we have presented the following results. The MOCVD growth rate of GaN increases with growth temperature in the region from 700 to 1000 deg. Cel.. It decreases with increasing V/III ratio. This effect is partly caused by a parasitic homogeneous gas phase nucleation which leads to a depletion of the gas phase from TMG. With decreasing V/III ratio, we observed an increasing carbon incorporation which was measured by SIMS. The PL measurements showed that the luminescence efficiency increases at higher growth temperatures. Furthermore samples with high carbon content show a strong yellow luminescence at 2.2 eV and a sharp near band edge emission at 3.31 eV.