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 investigated. We have mainly investigated the incorporation of carbon into the GaN films with GaN buffer layers on Al2O3 during MOVPE growth, comparing trimethygallium (TMGa) and triethygallium (TEGa) as the typical gallium precursors. The films were characterized by secondary ion mass spectroscopy analysis, photoluminescence, and Hall measurements. The carbon, hydrogen, and oxygen concentrations increase with decreasing growth temperature in using TMGa. Especially the carbon concentration increases with decreasing a V/III ratio, for both TMGa and TEGa. There is about two times more carbon in the GaN films grown using TEGa than those using TMGa. The carbon from TMGa mainly enhances the D-A pair emission (similar to 378 nm), which shows the carbon makes an acceptor level 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 makes 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 are also located at interstitial sites because of the C-C bonding in ethyl radicals.