Strain relief by In-doping and its effect on the surface and on the interface structures in (Al)GaN on sapphire grown by metalorganic vapor-phase epitaxy

We have performed a growth technique that involves isoelectronic In-doping into GaN and AlGaN in order to reduce defects. The films were grown by atmospheric metalorganic vapor-phase epitaxy (MOVPE) at 950°C in a H 2 or N 2 carrier gas (denoted below by H 2 -(Al)GaN and N 2 -(Al)GaN, respectively) with a low-temperature-deposited AlN buffer layer. By using this technique, we were able to control the strain in the films; with increasing trimethylindium (TMIn) flow, the strain in GaN was decreased; accordingly, the tilting and twisting components of crystalline mosaicity (tilt and twist, respectively) were also decreased. The Raman shift and photoluminescence (PL) emission peak energy were shifted in accordance with the strain in GaN. The PL linewidth decreased with the decrease in the strain in GaN. Strong enhancement of the excitonic PL intensity of In-doped GaN and AlGaN was observed. The surface morphology was also dramatically improved on In-doping, and the growth pits on the films were distinguished. These results are not due to the surfactant effect of In because we confirmed from the secondary-ion mass spectroscopy (SIMS) results that In was incorporated in the films. The above results could be explained by considering the solid-solution hardening effect.