  GaSe is an important member of the post-transition metal chalcogenide family
and is an emerging two-dimensional (2D) semiconductor material. Because it is a
van der Waals material, it can be fabricated into atomic-scale ultrathin films,
making it suitable for the preparation of compact, heterostructure devices. In
addition, GaSe possesses unusual optical and electronic properties, such as a
shift from an indirect-bandgap single-layer film to a direct-bandgap bulk
material, rare intrinsic p-type conduction, and nonlinear optical behaviors.
These properties make GaSe an appealing candidate for the fabrication of
field-effect transistors, photodetectors, and photovoltaics. However, the
wafer-scale production of pure GaSe single crystal thin films remains
challenging. This study develops an approach for the direct growth of
nanometer-thick GaSe films on GaAs substrates using molecular beam epitaxy. It
yields smooth thin GaSe films with the rare {\gamma}'-polymorph. We analyze the
formation mechanism of {\gamma}'-GaSe using density functional theory and
speculate that it is stabilized by Ga vacancies since the formation enthalpy of
{\gamma}'-GaSe tends to become lower than that of other polymorphs when the Ga
vacancy concentration increases. Finally, we investigate the growth conditions
of GaSe, providing valuable insights for exploring 2D/3D quasi-van der Waals
epitaxial growth.
