Structural and electronic signatures of stacking and intralayer transitions in epitaxial bilayer WSe₂ on GaP(111)B

Two-dimensional materials from the transition metal dichalcogenides family host multiple crystal phases and stacking configurations that strongly affect their electronic properties. Here, we investigate the structural and electronic phase evolution of epitaxial bilayer WSe₂ grown by molecular beam epitaxy on Se-terminated GaP(111)B substrates. Films grown at low temperature (∼250 °C) exhibit a coexistence of rhombohedral 3R-stacked bilayers together with a minor crystal-phase heterostructure involving a 1T' layer. In this configuration, the 1T' phase is stabilized only as a monolayer stacked on top of a 1H layer (1T'/1H), while a fully stacked 1T' bilayer is not observed. The GaP(111)B substrate likely plays an important role in stabilizing these metastable configurations through interface interactions and lattice registry effects. Post-growth annealing at 650 °C under selenium flux drives a complete transition toward the thermodynamically stable 2H stacking through a rearrangement of the interlayer registry. This transformation is evidenced by the disappearance of the electronic signatures of the 1T' phase together with structural changes observed by diffraction and microscopy. Our results demonstrate that annealing provides an effective route to control stacking order and crystal phases in epitaxial WSe₂ bilayers, opening new opportunities for phase engineering in two-dimensional materials.