  Broadening the variety of two-dimensional (2D) materials and improving the
synthesis of ultrathin films are crucial to the development of the
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 the
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
substrate temperature, Se:Ga flux ratio, and growth rate. Then we used a 3-step
mode to grow Ga2Se2 films with a thickness as low as 3 tetralayers and a
surface roughness as low as 0.61 nm, far exceeding the performance of direct
growth. Finally, we found that the 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.
