Achieving an ultra-thin GaN channel layer in AlGaN/GaN/AlN high electron mobility transistors

Abstract

AlGaN/GaN high electron mobility transistors (HEMTs) with an AlN back barrier have shown significant potential in high-voltage and high-frequency applications. However, achieving an ultra-thin and smooth GaN channel layer on AlN templates is challenging due to large lattice mismatch, as the mismatch-induced compressive strain inevitably results in surface roughening during GaN growth. Herein, a temperature-variable GaN growth strategy featuring continuous source supply is proposed to balance the strain and surface morphology. Specifically, intentionally rough low-temperature GaN is first employed to suppress uncontrolled compressive strain relaxation in pseudomorphic growth, and then ultra-thin GaN grown at high temperature can recover the surface morphology. It is crucial that the GaN growth be uninterrupted during the temperature increase process, which protects the surface from GaN decomposition. As such, a root mean square roughness of 0.74 nm in a 10 × 10 μm2 area is realized at a GaN thickness of 160 nm, laying a solid foundation for the stacking of an AlGaN/GaN/AlN heterostructure featuring an ultra-thin GaN channel layer. Eventually, the fabricated HEMTs exhibit a high breakdown voltage of 2300 V and an ON/OFF current ratio of 109. This study provides a feasible solution for stacking AlGaN/GaN/AlN HEMTs featuring an ultra-thin and smooth GaN channel layer, and accelerates their practical application.