Lattice Polarity Manipulation of AlN Films on SiC Substrates for N-Polar GaN HEMTs

Realization of nitrogen-polar (N-polar) AlN on SiC is important for the development of high-performance GaN high-electron mobility transistors (HEMTs). However, AlN films grown on SiC substrates are mostly metal-polar, and it is difficult to achieve an N-polar AlN on them. In this work, we manipulated the lattice polarity of AlN grown on SiC by varying the V/III ratio. Our results show that AlN films grown at a low V/III ratio undergo lattice polarity reversal from N-polarity to metal-polarity near the AlN/SiC interface. This occurs because oxygen enrichment occurs in AlN, forming a thin AlON layer close to the interface. Importantly, we suppress the oxygen enrichment and thus the formation of AlON in AlN under a high V/III ratio, i.e., an N-rich growth condition, and finally achieve an N-polar AlN film on SiC. We also find that the threshold V/III ratio that realizes N-polar AlN on SiC without lattice polarity reversal is ∼6000. Furthermore, we prepared a GaN/AlGaN HEMT structure based on the obtained N-polar AlN, and the 2-dimensional electron gas density and mobility at the heterostructure interface are 1.5 × 10¹³ cm^–2 and 923 cm²/V·s, respectively. This work is expected to promote the development of N-polar GaN HEMTs on SiC.