Effect of Al incorporation in nonpolar m-plane GaN/AlGaN multi-quantum-wells using plasma-assisted molecular-beam epitaxy

This paper assesses the difficulties associated with Al incorporation in nonpolar m-plane GaN/AlGaN multi-quantum-wells grown on free-standing m-plane GaN. Structures with average Al mole fraction below 6% show atomically flat surfaces and no extended structural defects, in spite of alloy fluctuations up to 30% of the average concentration. Increasing the average Al composition of the alloy above 23% induces anisotropic degradation of the surface morphology, with appearance of elongated features which increase the surface roughness, along with formation of stacking faults, dislocations, and nm-sized Al-rich clusters. The effect of all these structural features on the MQWs optical performance is discussed.