  Large area epitaxy of two-dimensional (2D) layered materials with high
material quality is a crucial step in realizing novel device applications based
on 2D materials. In this work, we report high-quality, crystalline, large-area
gallium selenide (GaSe) films grown on bulk substrates such as c-plane sapphire
and gallium nitride (GaN) using a valved cracker source for Se. (002)-oriented
GaSe with random in-plane orientation of domains was grown on sapphire and GaN
substrates at a substrate temperature of 350-450 C with complete surface
coverage and smooth surface morphology. Higher growth temperature (575 C)
resulted in the formation of single-crystalline {\epsilon}-GaSe triangular
domains with six-fold symmetry confirmed by in-situ reflection high electron
energy diffraction (RHEED) and off-axis x-ray diffraction (XRD). A two-step
growth method involving high temperature nucleation of single crystalline
domains and low temperature growth to enhance coalescence was adopted to obtain
continuous (002)-oriented GaSe with an epitaxial relationship with the
substrate. While six-fold symmetry was maintained in the two step growth,
{\beta}-GaSe phase was observed in addition to the dominant {\epsilon}-GaSe in
cross-sectional scanning transmission electron microscopy images. This work
demonstrates the potential of growing high quality 2D-layered materials using
molecular beam epitaxy and can be extended to the growth of other transition
metal chalcogenides.
