Effect of buffer layers and stacking faults on the reduction of threading dislocation density in GaN overlayers grown by metalorganic chemical vapor deposition

We have studied the effect of the trimethylgallium (TMGa) flow rate in the GaN buffer layer on the optical and structural quality. From low temperature photoluminescence measurements, a GaN overlayer grown on a buffer layer with the TMGa flow rate of 80 μmol/min shows the intense donor-acceptor pair transition peak at 3.27 eV and the weak yellow band emission at 2.2 eV, which are related with stacking faults and threading dislocations from transmission electron microscopy images, respectively. As the TMGa flow rate of the GaN buffer increases, the threading dislocation density rapidly decreased and stacking faults increased in the GaN overlayers. Also, a total threading dislocation density at the optimum condition of the buffer layer is the very low 1 × 10⁸ cm^-2, which is due to the interaction of stacking faults with the vertical threading dislocations and the bending of threading dislocations near the stacking faults. High-resolution x-ray diffraction results show that a high density of stacking faults is correlated with the compressive strain of a GaN overlayer at the growth temperature.