Effects of Substrate Miscut on Growth Instabilities in Homoepitaxially Grown GaN/AlGaN System

We study a series of GaN/AlxGa1−xN quantum wells grown by NH3 molecular beam epitaxy on GaN substrates with a very low dislocation density of ∼104 cm−2. The (0001)-oriented substrates with varying offcut angles are used, exhibiting misorientations ranging from 0.1° to 2.0° toward the m-direction. The impact of the miscut angle on the surface morphology, structural properties, and optical emission is investigated by atomic force microscopy (AFM), cathodoluminescence (CL), and photoluminescence (PL), respectively. In terms of surface quality, the optimum miscut angle is found to be 1.0°. Under selected growth conditions, it yields a step-flow growth mode with the lowest surface roughness. Step meandering, observed at a 0.5° offcut, leads to a negligibly small increase in surface roughness and is correlated with the brightest and narrowest optical emission. Stronger surface roughness is observed due to the formation of hillocks (0.1° offcut), step bunches (1.5° offcut), and more complex surface morphologies (2° offcut). Step bunching is shown to be particularly detrimental to the homogeneity of the AlGaN layers: CL measurements show that the Al content varies dramatically in strongly bunched areas compared to flatter regions. PL spectra indicate that the Al concentration in the AlGaN layers tends to decrease when they are grown on substrates with stronger misorientations. These observations can be understood qualitatively by assuming that the ratio between Ga and Al incorporation rates is enhanced on steeper surfaces.