Advancing the Growth of GaN on AlScN and AlYN by Metal–Organic Chemical Vapor Deposition

Abstract

High electron mobility transistors (HEMT) based on Al_1-xSc_xN/GaN and Al_1-xY_xN/GaN heterostructures promise increased device performance and reliability due to the high sheet charge carrier density and the possibility to grow strain-free layers on GaN. Metal–organic chemical vapor deposition (MOCVD) offers high throughput, high structural quality, and good electrical characteristics. The growth of GaN layers on Al_1-xSc_xN and Al_1-xY_xN is challenging, but at the same time crucial as passivation or for multichannel structures. GaN is observed to grow three-dimensionally on these nitrides, exposing not-passivated areas to surface oxidation. In this work, growth of 2–20 nm-thick, two-dimensional GaN layers is demonstrated. Optimization of growth conditions is enabled by understanding island formation on the atomic scale by aberration corrected scanning transmission electron microscopy (STEM) and hard X-ray photoelectron spectroscopy (HAXPES). Increased growth temperature, an AlN interlayer, low supersaturation conditions and the carrier gas are found to be key to enhance Ga adatom mobility. Growth of single crystalline GaN layers on Al_1-xSc_xN and Al_1-xY_xN is unlocked and prevents oxidation of the underlying layers. Few nanometer thick GaN caps allow for depositing the gate metallization directly on the cap, whereas thicker ones allow for the growth of heterostructures for normally-off devices and multichannel structures.