Comprehensive surface analysis of GaN-capped AlGaN/GaN high electron mobility transistors

AlGaN/GaN-based high electron mobility transistor heterostructures with different GaN cap layer thicknesses were grown by metalorganic vapor phase epitaxy (MOVPE) and molecular beam epitaxy. The influence of the different growth methods on the structure of the GaN caps was examined by x-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry (TOF-SIMS). Due to the high surface sensitivity of these methods, several effects, such as residual Al incorporation into the GaN cap during growth, etching of the GaN cap during cool-down, and preferential etching of Ga out of the AlGaN barrier could be evaluated. It is demonstrated that these effects play a major role for MOVPE-grown structures.