Improvement of electrical properties by insertion of AlGaN interlayer for N-polar AlGaN/AlN structures on sapphire substrates

In this study, we propose nitrogen-polar (N-polar) Al0.1Ga0.9N/Al0.9Ga0.1N/aluminum nitride (AlN) structures. N-polar Al0.1Ga0.9N/Al0.9Ga0.1N/AlN was grown on a sapphire substrate with a misorientation of 2 degrees with respect to the m-axis using metal-organic vapor deposition. The effects of varying the Al0.9Ga0.1N interlayer thickness from 30 nm to 1 mu m using pulsed H2 etching on the planarity and current-voltage characteristics of the samples were investigated. The current first improved upon increasing the interlayer thickness from 30 to 300 nm, owing to the reduction in interfacial impurities between (aluminum) gallium nitride (Al)GaN) and Al0.9Ga0.1N, but subsequently decreased upon further increasing the thickness because of the relaxation growth of the interlayer. Furthermore, pulsed H2 etching of the Al0.9Ga0.1N interlayer suppressed step bunching and improved planarity. Subsequently, the proposed method was employed to fabricate N-polar Al0.1Ga0.9N/Al0.9Ga0.1N/AlN heterostructure FETs, which demonstrated five times higher source-drain current (I DS) than that of conventional structures without an interlayer.