Van der Waals Epitaxy of GaSe on Sapphire—Insights into Phase Control and Morphological Transitions

The wafer-scale epitaxy of two-dimensional (2D) layered materials is a key enabler for the integration of 2D materials in next-generation optoelectronic devices. Among 2D materials, post-transition metal chalcogenides (PTMCs: III-VI with III=Ga, in and VI=Se, Te) have recently sparked great interest among 2D materials beyond graphene due to their unique electronics and optoelectronics properties [1]. However, challenges such as interlayer contamination during van der Waals heterostructure stacking and the high surface reactivity of PTMCs, which can alter device performance, pose significant challenges to scalable integration for industrial applications.
In this work, we explore the wafer-scale growth of PTMC GaSe on sapphire substrates using molecular beam epitaxy (MBE). We employ in-situ Raman spectroscopy wafer mapping in ultra-high vacuum (UHV) to precisely identify the GaSe crystal phase and quality while simultaneously providing comprehensive, statistical analysis of the growth behavior. To systematically investigate the effects of temperature and Se/Ga flux ratio on the GaSe growth, we use spatial gradients in these parameters across the 2” substrate during a single growth run. By combining in-situ reflection high-energy electron diffraction (RHEED) measurements and ex-situ AFM topographies of different growth regimes, we identify the optimal growth conditions for the growth of strain-free 2D planar 1:1 GaSe.
Furthermore, we translate the optimized growth conditions to a van der Waals substrate, specifically hexagonal boron nitride (hBN) exfoliated on Si/SiO₂. This extension highlights the versatility of our approach for GaSe growth on various substrates, opening the door to advanced heterostructure fabrication for future optoelectronic applications.
[1] Cai, H., Gu, Y., Lin, Y. C., Yu, Y., Geohegan, D. B., & Xiao, K. (2019). Synthesis and emerging properties of 2D layered III–VI metal chalcogenides. Applied Physics Reviews, 6(4).