RESIDUAL IMPURITIES IN GAN AL2O3 GROWN BY METALORGANIC VAPOR-PHASE EPITAXY/

Residual impurities in GaN films on sapphire (Al2O3) substrates grown by two-step metalorganic vapor phase epitaxy (MOVPE) have been investi gated. We have mainly investigated the incorporation of carbon into th e GaN films with GaN buffer layers on Al2O3 during MOVPE growth, compa ring trimethygallium (TMGa) and triethygallium (TEGa) as the typical g allium precursors. The films were characterized by secondary ion mass spectroscopy analysis, photoluminescence, and Hall measurements. The c arbon, hydrogen, and oxygen concentrations increase with decreasing gr owth temperature in using TMGa. Especially the carbon concentration in creases with decreasing a V/III ratio, for both TMGa and TEGa. There i s about two times more carbon in the GaN films grown using TEGa than t hose using TMGa. The carbon from TMGa mainly enhances the D-A pair emi ssion (similar to 378 nm), which shows the carbon makes an acceptor le vel at nitrogen sites in GaN. On the other hand, the carbon from TEGa enhances a deep emission (similar to 550 nm), which shows the carbon m akes not only an acceptor level but deep levels at interstitial sites in GaN. The carbon impurities originate from methyl radicals for TMGa, or ethyl radicals for TEGa. It is supposed that, in the case of TEGa, the carbon impurities are not always located at nitrogen sites, but a re also located at interstitial sites because of the C-C bonding in et hyl radicals.