人工晶体学报2026，Vol.55Issue(4)：603-608,6.DOI:10.16553/j.cnki.issn1000-985x.2025.0236
微量锂金属诱导氮化镓外延层与蓝宝石衬底完整自分离研究
Complete Self-Separation of GaN Epitaxial Layer from Sapphire Substrate Induced by Trace Lithium Metal
摘要
Abstract
Commercial gallium nitride(GaN)devices are typically fabricated on heterogeneous GaN epi-wafers.However,cutting-edge research continues to focus on the development of homoepitaxial devices based on free-standing GaN single crystal substrates,aiming to leverage the superior material properties of homoepitaxy to further enhance device performance.Among the emerging technologies for manufacturing such GaN substrates,the sodium flux method is promising due to its capability to produce large,stress-free,high-quality crystals.A significant challenge is the separation of the thick GaN layer from the original sapphire substrate.Previous work by Japan Osaka University introduced a lithium-doped flux method involving a two-step process with a Li concentration of～10％(mole fraction,the same below),requiring a complex reactor design.This study presents a significant simplification of this approach. The primary goal of this study is to demonstrate and investigate a simplified,single-step sodium flux method for the epitaxial growth of a thick,crack-free GaN single crystal on a sapphire-based template,achieving complete self-separation of the grown layer from the sapphire substrate.A GaN template(～30 μm thick)was first prepared on a sapphire substrate using metalorganic chemical vapor deposition and hydride vapor phase epitaxy(HVPE).This template served as the seed.Epitaxial growth was conducted in a high-pressure autoclave using a sodium flux with a molar composition of n(Na)∶n(Ga)∶n(C)∶n(Li)=73.0∶27.0∶0.5∶0.3,corresponding to a very low Li concentration of approximately 0.3％.The growth proceeded at 850℃ and 3.5 MPa under a nitrogen atmosphere for 120 h.The morphology,crystallinity,surface roughness and structural changes of the separated GaN layer and sapphire substrate were analyzed by optical microscopy,X-ray diffraction,atomic force microscopy and thickness measurement.The curved wafer was measured before and after growth to evaluate stress evolution. A crack-free,mirror-like GaN single crystal layer with an average thickness of 1 053 μm is successfully grown.Crucially,the GaN epilayer completely and spontaneously separates from the sapphire substrate upon cooling.The separation interface is located precisely at the original GaN/sapphire boundary.The sapphire substrate shows clear etching features,with its thickness reduces by～10 μm and sidewall/edge etching observed.In contrast,the separated surface of the GaN layer is smooth(root mean square roughness is 3.04 nm)with no signs of etching,indicating highly selective etching of sapphire.The GaN surface exhibits millimeter-scale hexagonal hillocks.Bowing measurements reveals that the initial convex bow of the template(due to thermal stress from HVPE growth)transformed into a concave bow of the free-standing GaN layer after growth and separation,indicating significant stress relaxation. This work successfully demonstrates a novel,simplified sodium flux process using a trace amount of lithium(0.3％)to achieve simultaneous epitaxial growth and complete self-separation of a millimeter-thick GaN single crystal from its sapphire substrate in a single step.The self-separation mechanism is attributed to the synergistic effect of the slow,selective etching of the sapphire substrate by the Li-doped flux and the progressive accumulation and release of interfacial stresses during crystal growth.The key innovation of this paper is to significantly reduce the required Li concentration by an order of magnitude(from～10％to 0.3％)while maintaining the self-separation functionality,eliminating the need for a complex two-step process or high-pressure mechanical additions.This study provides fundamental insights into the stress-mediated,flux-assisted liftoff mechanism and proposes a markedly simplified and more cost-effective technical route for the sodium-flux-based fabrication of large,free-standing GaN substrates,potentially accelerating their commercialization for next-generation high-power and high-frequency electronic devices.关键词
GaN单晶/钠助熔剂法/晶体生长/自分离/界面应力Key words
gallium nitride single crystal/sodium flux method/crystal growth/self-separation/interface stress分类
数理科学引用本文复制引用
张敏,刘强,姜永京,肖继宗,谢胜杰,刘南柳,王琦,童玉珍,张国义,王新强..微量锂金属诱导氮化镓外延层与蓝宝石衬底完整自分离研究[J].人工晶体学报,2026,55(4):603-608,6.基金项目
国家重点研发计划(2022YFB3605202) （2022YFB3605202）
