In₂Se₃ is a polymorphous metal-chalcogenide system¹ where each phase exhibits dramatically different functional properties. α-In₂Se_3­ is a two-dimensional, ferroelectric semiconductor^2,3 and has shown promise in low-power, neuromorphic electronic devices^4,5. 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-In₂Se_3­ (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 ^oC^6,9. Another study reported MBE synthesis of metastable, strained, mixed phase β/γ-In₂Se_3­ films on Si(111) substrates at 180 ^oC¹⁰. One study from 2018 claims deposition of large-area α-In₂Se_{3­ ­}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.

(1) Okamoto, H. In-Se ( Indium-Selenium ). J. Phase Equilibria 2004, 25 (2).

(2) Küpers, M.; Konze, P. M.; Meledin, A.; Mayer, J.; Englert, U.; Wuttig, M.; Dronskowski, R. Controlled Crystal Growth of Indium Selenide, In₂Se₃, and the Crystal Structures of α-In₂Se₃. Inorg. Chem. 2018, 57, 11775–11781.

(3) Xiao, J.; Zhu, H.; Wang, Y.; Feng, W.; Hu, Y.; Dasgupta, A.; Han, Y.; Wang, Y.; Muller, D. A.; Martin, L. W.; et al. Intrinsic Two-Dimensional Ferroelectricity with Dipole Locking. Phys. Rev. Lett. 2018, 120 (22), 227601.

(4) Tang, B.; Tang, B.; Hussain, S.; Xu, R.; Cheng, Z.; Liao, J.; Chen, Q. Novel Type of Synaptic Transistors Based on a Ferroelectric Semiconductor Channel. ACS Appl. Mater. Interfaces 2020, 12 (22), 24920–24928.

(5) Wang, L.; Liao, W.; Wong, S. L.; Yu, Z. G.; Li, S.; Lim, Y. F.; Feng, X.; Tan, W. C.; Huang, X.; Chen, L.; et al. Artificial Synapses Based on Multiterminal Memtransistors for Neuromorphic Application. Adv. Funct. Mater. 2019, 29 (25), 1–10.

(6) Claro, M. S.; Grzonka, J.; Nicoara, N.; Ferreira, P. J.; Sadewasser, S. Wafer-Scale Fabrication of 2D β-In₂Se₃ Photodetectors. Adv. Opt. Mater. 2020, 2001034, 1–9.

(7) Ashok, A.; Regmi, G.; Velumani, S. Growth of In₂Se₃ Thin Films Prepared by the Pneumatic Spray Pyrolysis Method for Thin Film Solar Cells Applications. 2020 17th Int. Conf. Electr. Eng. Comput. Sci. Autom. Control. CCE 2020 2020, 2–7.

(8) Li, J.; Li, H.; Niu, X.; Wang, Z. Low-Dimensional In₂Se₃ Compounds: From Material Preparations to Device Applications. ACS Nano 2021, 15 (12), 18683–18707.

(9) Zhang, X.; Lee, S.; Bansal, A.; Zhang, F.; Terrones, M.; Jackson, T. N.; Redwing, J. M. Epitaxial Growth of Few-Layer β-In₂Se₃ Thin Films by Metalorganic Chemical Vapor Deposition. J. Cryst. Growth 2020, 533 (December 2019), 125471.

(10) Shen, Y. F.; Yin, X. B.; Xu, C. F.; He, J.; Li, J. Y.; Li, H. D.; Zhu, X. H.; Niu, X. Bin. Growth and Structural Characteristics of Metastable β-In₂Se₃ Thin Films on H-Terminated Si(111) Substrates by Molecular Beam Epitaxy. Chinese Phys. B 2020, 29 (5).