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. Low growth rates in AlScN/GaN heterostructures grown by metal-organic chemical vapor cause the formation of linearly graded interlayers and degradation of the electrical characteristics. Growth rates are enhanced with novel Sc precursors, and high interface abruptness and homogeneous layers are achieved. Structural quality strongly affects the performance of the 2D electron gas, which is the core of high-electron-mobility transistors. image