Understanding Interfaces in AlScN/GaN Heterostructures

Aluminum scandium nitride barrier layers increase the available sheet charge carrier density in gallium nitride-based high-electron-mobility transistors and boost the output power of high-frequency amplifiers and high voltage switches. Growth of AlScN by metal-organic chemical vapor deposition is challenging due to the low vapor pressure of the conventional Sc precursor Cp3Sc, which induces low growth rates of AlScN and leads to thermally-induced AlScN/GaN-interface degradation. In this work, novel Sc precursors are employed to reduce the thermal budget by increasing the growth rate of the AlScN layer. The AlScN/GaN interfaces are investigated by high-resolution X-ray diffraction, high-resolution transmission electron microscopy, time-of-flight secondary ion mass spectrometry, capacitance–voltage, current–voltage and temperature-dependent Hall measurements. Linearly graded interlayers with strain-induced stacking faults, edge, and screw dislocations form at the AlScN/GaN interface at growth rates of 0.015 nms−1. Growth rates of 0.034 nms−1 and higher allow for abrupt interfaces, but a compositional grading in the barrier remains. Homogeneous barrier layers can be achieved at growth rates of 0.067 nms−1 or by growing an AlN interlayer. The electrical properties of the heterostructures are sensitive to Sc accumulations at the cap/barrier interface, residual impurities from precursor synthesis, and surface roughness. This study paves the way for high-performing devices.