Thickness-modulated crystal structure and band gap of 2D SnSe deposited by molecular beam epitaxy

Realizing the potential for 2D SnSe optoelectronics requires understanding the thickness dependence of structure, defects, and optical properties. We investigate the thickness-dependent crystal structure, band gap, and carrier lifetime of SnSe films deposited by molecular beam epitaxy (MBE) on (100) MgO. MBE enables stoichiometric (2h00)-oriented SnSe films with tunable thicknesses from 80 nm down to 4 nm. As thickness decreases, out-of-plane covalent bonds contract, while in-plane bonding and the van der Waals gap expand with a concurrent increase in stacking fault density, consistent with theoretical predictions of reduced stacking fault energies. Below 8 nm, the band gap transitions from indirect to direct, increasing from 1.4 eV to 1.8 eV, primarily driven by a combination of structural changes and confinement effects. Our results demonstrate how the thickness and structural distortion of 2D materials can be used to modulate the optical properties relevant to optoelectronics.