High-quality a-plane GaN grown with flow-rate modulation epitaxy on r-plane sapphire substrate

We use the flow-rate modulation epitaxy (FME) technique to improve the crystal quality of a pit-free a-plane GaN (1 1 2¯ 0) film grown on r-plane sapphire (1 1¯ 0 2) substrate. With the FME technique, the width of the rocking curve in X-ray diffraction measurement is significantly reduced. Also, the surface roughness in either atomic-force-microscopy scanning or -step profiling is decreased. Here, the FME technique means to alternatively turn on and off the supply of Ga atoms, while N atoms are continuously supplied without changing the flow rate. Under the used growth conditions, the optimized FME parameters include the on/off periods at 10/10 s. During the period of closing the flow of trimethylgallium (TMGa), the continuous supply of nitrogen can lead to the formation of stoichiometry structure under the high-Ga growth condition, which is required for pit-free growth. Also, during this period, Ga atoms can further migrate to result in a flatter surface. Therefore, the crystal quality of the a-plane GaN sample can be improved.