Influence of the V/III molar ratio on the structural and electronic properties of MOVPE grown GaN

Low pressure MOVPE has been used to grow a series of GaN epilayers, deposited onto (0001) sapphire substrates. All the growth parameters have been kept constant, except for the V/III molar ratio, which has been changed from sample to sample by changing the ammonia flow rate. The growth rate has been measured vs the V/III molar ratio at both low temperature (550 degree C, growth of the buffer layer) and normal process temperature. A strong dependence of the growth rate vs the V/III molar ratio is found at both temperatures. Systematic photoluminescence and reflectivity experiments have been performed at 2 K on all samples. The photoluminescence is dominated by the donor bound exciton and donor acceptor pair are visible on samples grown under the lowest V/III molar ratio, indicating higher impurity incorporation. An optimum V/III molar ratio of 5000 is proposed, which corresponds to the highest growth rate observed and to the lowest full width at half maximum for the photoluminescence line.