Molecular Beam Epitaxy of 2D-layered Gallium Selenide on GaN substrates
  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. 
Choong Hee Lee, Sriram Krishnamoorthy, Dante J. O'Hara, Jared M.   Johnson, John Jamison, Roberto C. Myers, Roland K. Kawaka...
cond-mat.mtrl-sci cond-mat.mes-hall