Epitaxial growth of atomically thin Ga₂Se₂ films on c-plane sapphire substrates

Broadening the variety of two-dimensional (2D) materials and improving the synthesis of ultrathin films are crucial to the development of semiconductor industry. As a state-of-the-art 2D material, Ga₂Se₂ has attractive optoelectronic properties when it reaches the atomically thin regime. However, its van der Waals epitaxial growth, especially for atomically thin films, has seldom been studied. In this paper, we used molecular beam epitaxy to synthesize Ga₂Se₂ single-crystal films with a surface roughness down to 1.82 nm on c-plane sapphire substrates by optimizing the substrate temperature, Se:Ga flux ratio, and growth rate. Then, we used a three-step mode to grow Ga₂Se₂ films with a thickness as low as three tetralayers and a surface roughness as low as 0.61 nm, far exceeding the performance of direct growth. Finally, we found that surface morphology strongly depends on the Se:Ga flux ratio, and higher growth rates widened the suitable flux ratio window for growing Ga₂Se₂. Overall, this work advances the understanding of the vdW epitaxy growth mechanism for post-transition metal monochalcogenides on sapphire substrates.