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

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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 ( 2h 00)-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.

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