Stacking order and electronic band structure in MBE-grown trilayer WSe2 films

Few-layer quantum materials, such as transition-metal dichalcogenides (TMDs), are paving the path to the design of high-efficiency devices in the field of microelectronics and optoelectronics. However, heterostructures of quantum materials coming from different families, while they would immensely broaden the range of possible applications, remain challenging. Here, we demonstrate the large-scale integration of compounds from two highly multifunctional families: the three-dimensional conventional semiconductor GaP and the two-dimensional TMD semiconductor ${\mathrm{WSe}}_{2}$ which is particularly interesting in terms of its potential for electronic, spintronic, and photonics applications. We show that a 2H-2H (or AA\ensuremath{'}A) trilayer of ${\mathrm{WSe}}_{2}$ can be grown by molecular-beam epitaxy (MBE) onto gallium phosphide (GaP) substrate. A sharp, high-quality ${\mathrm{WSe}}_{2}$-GaP interface was confirmed by scanning high-resolution transmission electron microscopy and x-ray photoemission spectroscopy. We present a combined experimental and theoretical study of the structure of the valence band of trilayer ${\mathrm{WSe}}_{2}$. Nanoangle-resolved photoemission spectroscopy and density-functional theory calculation show that trilayer electrons populate two distinct subbands associated with the K and \ensuremath{\Gamma} valleys, with effective masses along the \ensuremath{\Gamma}M direction about 0.27 and $0.5\phantom{\rule{0.16em}{0ex}}{m}_{\mathrm{e}}$, respectively (${m}_{\mathrm{e}}$ is the bare electron mass).