Complete Self-Separation of GaN Epitaxial Layer from Sapphire Substrate Induced by Trace Lithium Metal

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

Abstract

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

CLC Number:

O782

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.

Figures/Tables 3

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）

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

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

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