Direct growth of wafer-scale self-separated GaN on reusable two-dimensional material substrate

Gallium nitride (GaN) is extensively employed in light-emitting diodes, lasers, and high-power electronic devices due to its wide direct band gap and chemical stability.¹ Given the difficulty of obtaining high-quality GaN substrates, most current GaN-based devices are heteroepitaxially grown on foreign substrates, such as sapphire, Si, and silicon carbide.² However, the conventional growth of GaN films on these substrates generates a high defect density and considerable biaxial strain, attributed to the large lattice and thermal mismatch between the GaN films and their substrates. Thus, the lack of GaN native substrates is the main obstacle for the extending development of GaN-based devices. Although several conventional methods exist for obtaining free-standing GaN, such as laser lift-off, void-assisted separation, and chemical etching of the intermediate layer between GaN and the original substrate, the process of removing the substrate from a thick GaN film is complicated, expensive, and time-consuming. Furthermore, the original substrate can be damaged or contaminated during this process.³<br/>Recently, the van der Waals epitaxial growth of III-V films on two-dimensional (2D) materials has been proposed to effectively mitigate the lattice mismatch effect caused by the weak bonds between III-V films and 2D materials.⁴ Fluorphlogopite mica is a thermally stable material with a flexible atomic flat surface and low commercial cost, making it an ideal 2D material substrate. However, the dangling bond-free surface of 2D materials suppresses the nucleation of the III-V film, limiting the growth of large-area single crystals. Although some studies have proposed graphene as an intermediate layer in bridging techniques for growing and lifting off III-V films,⁵ the integration of GaN growth with mica for self-separation is a relatively unexplored area.<br/>In this research, we successfully grew a 2-inch thick GaN film with high uniformity on a fluorphlogopite mica substrate via hydride vapor phase epitaxy. The 2-inch thick GaN film easily self-separated from the mica substrate during rapid cooling. After the film growth process, the residual GaN on the mica substrate was effectively eliminated through deionized water, owing to the hydrophilic characteristic of mica and the weak bonding between GaN and mica. Moreover, we demonstrated the capability of reusing the mica substrates by repeatedly growing self-separated GaN films on the same substrate. Additionally, to verify that the thick GaN film was of device quality, we demonstrated a fully functional ultraviolet light-emitting diode with a wavelength of 378 nm. In summary, our proposed approach may facilitate the epitaxy of large-area single-crystalline GaN on 2D materials, offering a new substrate option in the self-separation technology of III-V materials.<br/><br/><b>References</b><br/>1. W. Han et al. Synthesis of gallium nitride nanorods through a carbon nanotube-confined reaction. Science, 1997, 277, 1287-1289.<br/>2. S. J. Pearton et al. GaN: Processing, defects, and devices. Journal of Applied Physics, 1999, 86, 1-78.<br/>3. H. Kum et al. Epitaxial growth and layer-transfer techniques for heterogeneous integration of materials for electronic and photonic devices. Nature Electronics, 2019, 2, 439-450.<br/>4. J. Yu et al. Van der Waals epitaxy of III-nitride semiconductors based on 2D materials for flexible applications. Advanced Materials 2020, 32, 1903407.<br/>5. H. Kim et al. Graphene nanopattern as a universal epitaxy platform for single-crystal membrane production and defect reduction. Nature Nanotechnology, 2022, 17, 1054-1059.