Molecular Beam Epitaxy of in2Se3 Thin Films
Authors: Voigt Cooper Augustus , Wagner Brent K. , Vogel Eric M. 
Published: November 2022

In2Se3 is a polymorphous metal-chalcogenide system1 where each phase exhibits dramatically different functional properties. α-In2Se3 is a two-dimensional, ferroelectric semiconductor2,3 and has shown promise in low-power, neuromorphic electronic devices4,5. β-In2Se3 is a two-dimensional, centrosymmetric semiconductor and has shown promise in photodetectors6. γ-In2Se3 is a three-dimensional, defect- wurtzite wide bandgap semiconductor that has shown promise as a precursor layer for CuInSe2 solar cells7. Considered together, the bandgaps of these materials range from 1.3eV to 2.5eV8. Understanding how to synthesize phase-pure thin-films of In2Se3 is of interest to harness the properties of these materials for applications in electronic devices.
α-In2Se3 is thermodynamically stable below 200 oC and transitions to the 2D, centrosymmetric β-In2Se3 phase upon heating above 200 oC1. While the β to α-In2Se3 phase transition is achievable in bulk synthesis via slow cooling2, the β to α-In2Se3 phase transition does not occur as readily in thin-films. Recent studies on thin-film synthesis of In2Se3 have demonstrated β- and/or γ-In2Se3 films at temperatures above the thermodynamic stability range of α-In2Se3 (200 oC). Two reports from the last two years demonstrate large-area synthesis of β-In2Se3 thin-films metal-organic chemical vapor deposition and molecular beam epitaxy (MBE) on sapphire at temperatures greater than 300 oC6,9. Another study reported MBE synthesis of metastable, strained, mixed phase β/γ-In2Se3 films on Si(111) substrates at 180 oC10. One study from 2018 claims deposition of large-area α-In2Se3 thin-films on graphene at a substrate temperature of 250 oC via molecular beam epitaxy; however, it is difficult to distinguish the α from β phase by the Raman spectra and TEM micrograph along the c-axis.
In this study we demonstrate thin-film synthesis of In2Se3 over a wide array of synthesis conditions via molecular beam epitaxy. We systematically investigate the effects of substrate temperature, cooling rate after deposition, substrate surface, Se/In flux ratio, and precursor type on the structure and properties of In2Se3 films. We characterize the crystalline phase, stoichiometry, and crystal morphology via Raman Spectroscopy, X-ray Photoelectron Spectroscopy and Atomic Force Microscopy, respectively.
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