Relation between Ga Vacancies, Photoluminescence, and Growth Conditions of MOVPE-Prepared GaN Layers

A set of GaN layers prepared by metalorganic vapor phase epitaxy under different technological conditions (growth temperature carrier gas type and Ga precursor) were investigated using variable energy positron annihilation spectroscopy (VEPAS) to find a link between technological conditions, GaN layer properties, and the concentration of gallium vacancies (V(Ga)). Different correlations between technological parameters and V(Ga) concentration were observed for layers grown from triethyl gallium (TEGa) and trimethyl gallium (TMGa) precursors. In case of TEGa, the formation of V(Ga) was significantly influenced by the type of reactor atmosphere (N(2) or H(2)), while no similar behaviour was observed for growth from TMGa. V(Ga) formation was suppressed with increasing temperature for growth from TEGa. On the contrary, enhancement of V(Ga) concentration was observed for growth from TMGa, with cluster formation for the highest temperature of 1100 °C. From the correlation of photoluminescence results with V(Ga) concentration determined by VEPAS, it can be concluded that yellow band luminescence in GaN is likely not connected with V(Ga); additionally, increased V(Ga) concentration enhances excitonic luminescence. The probable explanation is that V(Ga) prevent the formation of some other highly efficient nonradiative defects. Possible types of such defects are suggested.