Molecular beam epitaxy growth of wafer-scale SnSe van der Waals ultrathin layers

Tin selenide (SnSe) is a van der Waals layered post-transition metal monochalcogenide compound which is promising for a wide range of device applications when its thickness is reduced to a few layers. Hence, developing a mature synthesis technique to obtain wafer-scale, high-quality ultrathin SnSe layers is crucial. In this work, we present a comprehensive study on the effect of growth parameters on the material quality of ultrathin SnSe thin films grown by molecular beam epitaxy (MBE). A growth window including substrate temperature of 210–270 °C and low Se/Sn flux ratio with Se valve position of 10–30 mils has been identified which results in SnSe films with root mean square roughness as low as 0.6 nm and full-width-at-half-maximum of 0.1° in SnSe (400) x-ray diffraction rocking curve. Finally, using a three-step growth approach, we demonstrate wafer-scale coalesced ultrathin SnSe layers with thicknesses from 20 down to 5 nm, with good crystallinity, structural quality, and surface morphology. This work establishes a growth-condition framework for MBE-grown SnSe and presents a viable route for developing wafer-scale single-layer films, unlocking the potential of this highly promising material for advanced device integration.