Optimization of AlGaN-based spacer layer for InAlN/GaN interfaces

AlGaN‐based spacer layers for lattice‐matched and nearly lattice matched InAlN/GaN interfaces were examined in Al2O3/InAlN/AlGaN/AlN/GaN structures. An Al2O3 overlayer was deposited to investigate the characteristics under positive bias by capacitance‐voltage (C‐V) measurement. The C‐V characteristic for a sample with an Al0.38Ga0.62N/AlN double spacer layer indicated unfavorable electron accumulation at the InAlN/AlGaN interface inside the barrier under positive bias. To suppress the unfavorable accumulation, attempts were made to increase the Al molar fraction of the AlGaN layer to reduce the conduction band discontinuity and interface charge at InAlN/AlGaN interface. An Al0.44Ga0.56N/AlN double spacer layer and an Al0.44Ga0.56N single spacer layer of almost the same total thickness were investigated. Although both spacer layers result in normal C‐V characteristics without the indication of unfavorable electron accumulation, the InAlN layer on a 1.5‐nm‐thick Al0.44Ga0.56N single spacer layer exhibited superior surface morphology without deteriorating the mobility of the two‐dimensional electron gas despite the absence of the AlN layer. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)