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

doi:10.16553/j.cnki.issn1000-985x.2025.0236

摘要/Abstract

摘要： 基于异质衬底制备的氮化镓器件已广泛应用于通信和消费电子行业。然而，尖端研究仍聚焦于采用自支撑氮化镓单晶衬底的同质外延器件开发，旨在利用同质外延技术更优越的材料特性来进一步提升器件性能。在众多氮化镓衬底制备技术中，钠助熔剂法被视为下一代大尺寸商用氮化镓单晶衬底的主流候选制造技术之一。本文报道的基于钠助熔剂法的工艺技术，在生长熔体中加入的微量锂金属可在生长过程中缓慢溶解蓝宝石衬底，此过程与在生长边缘不断积累的应力协同作用，最终实现了蓝宝石衬底与氮化镓外延层完整的自分离。该技术成功实现了毫米级无裂纹氮化镓单晶的制备，生长所得氮化镓单晶表面呈现镜面特征，并可观察到毫米尺度的六方形小丘显微形貌。本文报道的自分离工艺有助于开发工艺步骤更为简化的基于钠助熔剂法制备氮化镓单晶衬底的技术路线。

关键词: GaN单晶; 钠助熔剂法; 晶体生长; 自分离; 界面应力

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.

Key words: gallium nitride single crystal; sodium flux method; crystal growth; self-separation; interface stress

中图分类号:

O782

张敏, 姜永京, 肖继宗, 谢胜杰, 刘南柳, 王琦, 童玉珍, 张国义, 王新强, 刘强. 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.

ZHANG Min, JIANG Yongjing, XIAO Jizong, XIE Shengjie, LIU Nanliu, WANG Qi, TONG Yuzhen, ZHANG Guoyi, WANG Xinqiang, LIU Qiang. Complete Self-Separation of GaN Epitaxial Layer from Sapphire Substrate Induced by Trace Lithium Metal[J]. Journal of Synthetic Crystals, 2026, 55(4): 603-608.

图/表 3

图1 GaN膜层表面显微形貌。（a）生长后GaN单晶样品的反光照片；（b）~（d）分别为图1（a）中样品表面上A、B、C三点的DIC模式显微形貌照片

Fig.1 Microscopic morphology of GaN film surface. （a） Reflect photo of as-grown GaN single crystal sample；（b）~（d） are DIC mode microscopic morphology images of position A， B and C on sample surface in Fig.1（a）

图2 蓝宝石衬底和GaN外延层显微形貌。（a）生长后蓝宝石衬底与GaN单晶外延层自分离实物照片；（b）蓝宝石分离面边缘区域显微形貌；（c）蓝宝石分离面中心区域显微形貌；（d）蓝宝石背面中心区域显微形貌；（e）GaN分离面中心区域显微形貌

Fig.2 Microscopic morphology of sapphire substrate and GaN epitaxial layer. （a） Physical photo of self-separation of sapphire substrate and GaN single crystal epitaxial layer after growth；（b） microscopic morphology of sapphire separation surface edge region；（c） microscopic morphology of sapphire separation surface central region；（d） microscopic morphology of sapphire back surface central region；（e） microscopic morphology of GaN separation surface central region

图3 GaN单晶外延层与蓝宝石衬底实现完整自分离的机理

Fig.3 Mechanism of complete self-separation between GaN single crystal epitaxial layer and sapphire substrate

参考文献 19

[1]	LIU Q， FUJIMOTO N， SHEN J， et al. Lattice bow in thick， homoepitaxial GaN layers for vertical power devices［J］. Journal of Crystal Growth， 2020， 539： 125643.
[2]	WANG X， ZHANG Y M， WANG M Y， et al. Research on the epitaxial growth of power/RF HEMT structures on n-GaN and Fe-doped SI-GaN free-standing substrates by MOCVD［J］. Vacuum， 2025， 235： 114135.
[3]	LUO C T， YANG C， ZHAO Z J， et al. Novel high-voltage GaN CAVET with high threshold voltage and low reverse conduction loss［J］. Microelectronics Journal， 2024， 148： 106195.
[4]	IMANISHI M， USAMI S， MURAKAMI K， et al. Characteristics of vertical transistors on a GaN substrate fabricated via Na-flux method and enlargement of the substrate surpassing 6 inches［J］. Physica Status Solidi （RRL）-Rapid Research Letters， 2024， 18（11）： 2400106.
[5]	WOSTATEK T， CHIRALA V Y M R， STODDARD N， et al. Ammonothermal crystal growth of functional nitrides for semiconductor devices： status and potential［J］. Materials， 2024， 17（13）： 3104.
[6]	MORI Y， IMANISHI M， MURAKAMI K， et al. Recent progress of Na-flux method for GaN crystal growth［J］. Japanese Journal of Applied Physics， 2019， 58： SC0803.
[7]	KIM J， SEO O， KUMARA L S R， et al. Highly-crystalline 6 inch free-standing GaN observed using X-ray diffraction topography［J］. CrystEngComm， 2021， 23（7）： 1628-1633.
[8]	VON DOLLEN P， PIMPUTKAR S， ALREESH M A， et al. A new system for sodium flux growth of bulk GaN. Part II： in situ investigation of growth processes［J］. Journal of Crystal Growth， 2016， 456： 67-72.
[9]	VON DOLLEN P， PIMPUTKAR S， ALREESH M A， et al. A new system for sodium flux growth of bulk GaN. Part I： system development［J］. Journal of Crystal Growth， 2016， 456： 58-66.
[10]	SI Z W， LIU Z L， HU Y Q， et al. Growth behavior and stress distribution of bulk GaN grown by Na-flux with HVPE GaN seed under near-thermodynamic equilibrium［J］. Applied Surface Science， 2022， 578： 152073.
[11]	WU W X， YAO J N， SUN Z H， et al. The role of growth conditions in modulating nitrogen diffusion during gallium nitride crystal growth by sodium flux method［J］. Crystal Growth & Design， 2025， 25（3）： 554-562.
[12]	YAMADA T， IMANISHI M， MURAKAMI K， et al. Influence of GaN/sapphire contact area on bowing of GaN wafer grown by the Na-flux method with a sapphire dissolution process［J］. Japanese Journal of Applied Physics， 2020， 59（2）： 025505.
[13]	YAMADA T， IMANISHI M， NAKAMURA K， et al. Crack-free GaN substrates grown by the Na-flux method with a sapphire dissolution technique［J］. Applied Physics Express， 2016， 9（7）： 071002.
[14]	FORONDA H M， ROMANOV A E， YOUNG E C， et al. Curvature and bow of bulk GaN substrates［J］. Journal of Applied Physics， 2016， 120（3）： 035104.
[15]	FUJIKURA H， KONNO T. Roughening of GaN homoepitaxial surfaces due to step meandering and bunching instabilities and their suppression in hydride vapor phase epitaxy［J］. Applied Physics Letters， 2018， 113（15）： 152101.
[16]	SOCHACKI T， AMILUSIK M， LUCZNIK B， et al. HVPE-GaN growth on misoriented ammonothermal GaN seeds［J］. Journal of Crystal Growth， 2014， 403： 32-37.
[17]	KONYS J， BORGSTEDT H U. The product of the reaction of alumina with lithium metal［J］. Journal of Nuclear Materials， 1985， 131（2/3）： 158-161.
[18]	SHIM J H， PARK J S， PARK J G. A bow-free freestanding GaN wafer［J］. RSC Advances， 2020， 10（37）： 21860-21866.
[19]	LI M D， WU J J， HE J M， et al. An extended Stoney’s formula including nonlinear deformation for large size wafer of multilayers with arbitrary thicknesses［J］. Scripta Materialia， 2020， 186： 29-32.