Growth mechanisms in semipolar (202¯1) and nonpolar m -plane (101¯0) AlGaN/GaN structures grown by PAMBE under N-rich conditions

• First report on the AlGaN/GaN growth on semipolar (20–21) surface by plasma assisted MBE.
• Uniform semipolar (20–21) AlGaN grown under N-excess in contrast to m-plane.
• On the (20–21) semipolar surface the {10–11} and {10–10} nano-facets were found.
• Ga desorption possible from m-plane and semipolar-plane under N-rich conditions.
• Model explaining the Ga losses from m-plane is presented.

The growth of multiquantum well (MQW) AlGaN/GaN structures was carried out by plasma assisted molecular beam epitaxy simultaneously on semipolar (202¯1), m  -plane (101¯0) and c-plane (0 0 0 1) GaN substrates under nitrogen-rich (N-rich) conditions in order to investigate the growth mechanisms on different crystal surfaces. A smooth surface with atomic steps was found only for the semipolar (202¯1) structure as opposite to m-plane and c-plane surfaces which were rough and covered by islands. Transmission electron microscopy (TEM) studies of the semipolar MQW structure showed uniform AlGaN layers with sharp interfaces and no extended defect formation, which proves good structural quality. At higher resolution, on the surface and at the interfaces of the semipolar MQWs, {10-11} and {10-10} nano-facets were found. In contrast to the semipolar case, the TEM studies of the m-plane structure showed no clear periodicity and very uneven distribution of Al. The AlN was formed on island top surfaces and AlGaN grew only on island slopes that were tilted from the m-plane. The growth rate was slightly smaller for the semipolar structure and much smaller for the m-plane one, as compared to the c-plane MQW structure. This can be attributed to Ga losses from semipolar and m-plane surfaces, in contrast to c-plane where no Ga losses were observed. The mechanisms for the Ga losses under N-rich conditions are discussed.