Growth of high-quality N-polar GaN on bulk GaN by plasma-assisted molecular beam epitaxy

There is an interest in growing N-polar GaN on bulk GaN for high electron mobility transistors (HEMTs). Current N-polar HEMT technology is dominated by devices grown on non-native substrates which possess a high density of dislocations due to lattice mismatch. N-polar GaN films grown directly on non-miscut GaN substrates have displayed a high density of pits and depressions on the surface. This work demonstrates that surface impurities play a major role in the formation of surface-pits in epitaxially grown film. By subjecting the GaN substrate to an ultra-violet (UV) O-3 (ozone) clean prior to growth and initiating growth with a 2 nm coherently strained AlN layer, grown under metal-rich conditions, we have demonstrated N-polar GaN films with a nearly pit-free surface. This AlN initiation layer (AIL) likely captures impurities on the substrate surface thus decoupling the substrate surface from the epitaxially grown film. It is demonstrated that utilizing thicker AILs, up to 8 nm, further improves film quality and surface morphology. The methods employed in this study to produce high-quality N-polar GaN grown on bulk GaN will pave the way for future GaN devices with an order of magnitude or more lower threading dislocation density (TDD).