In a previous study, we successfully grew flat N-polar AlN layers on a c-plane sapphire substrate with a misorientation angle of 2.0°by metalorganic vapor phase epitaxy. However, its surface had undulations due to step bunching, and therefore further improvement of the surface flatness is required. In this study, we employed pulsed H 2 etching during the growth of N-polar AlN layers to improve the surface flatness. Atomic force microscopy results indicated that the surface flatness was significantly improved, exhibiting a root mean square value of 0.4 nm. Further, the deep ultraviolet emission from AlGaN-based multiple quantum wells (MQWs) on the N-polar AlN layers was characterized, and the effect of the surface flatness on the optical characteristics was investigated. The surface flatness was found to play a crucial role in improving the optical characteristics of MQWs on N-polar AlN layers.

This study proposes a nitrogen-polar (N-polar) AlxGa1-xN/Al0.9Ga0.1N/AlN structure that can generate a large amount of two-dimensional electron gas (2DEG) to enhance the device development of samples. Additionally, we have analyzed the critical thickness of N-polar AlGaN/AlN based on the theoretical calculations of three different values of film thickness. The metalorganic vapor-phase epitaxy method is used to grow N-polar AlxGa1-xN/Al0.9Ga0.1N/AlN on sapphire substrates. The substrates with a misorientation angle of 2° along the m-axis and a-axis directions are selected to determine the effect of the off-cut angle on sample flatness and current-voltage characteristics. Furthermore, we determine the effect of Al contents on N-polar AlxGa1-xN/Al0.9Ga0.1N/AlN under the optimum growth conditions of the growth thickness of the top layer of AlGaN and sapphire substrate. The results indicate that the current throughput increases with a decrease in Al content. Lastly, we have fabricated the N-polar AlGaN/AlN heterostructure field effect transistor (FET) to demonstrated the static FET characteristics.