Formation of Twin-Free\nSingle Phase β‑In<sub>2</sub>Se<sub>3</sub> Layers via\nSelenium Diffusion into InP(111)B\nSubstrate

Indium selenide, In<sub>2</sub>Se<sub>3</sub>, has recently\nattracted\ngrowing interest due to its remarkable properties, including room\ntemperature ferroelectricity, outstanding photoresponsivity, and exotic\nin-plane ferroelectricity, which open up new regimes for next generation\nelectronics. In<sub>2</sub>Se<sub>3</sub> also provides the important\nadvantage of tuning the electrical properties of ultrathin layers\nwith an external electrical and magnetic field, making it a potential\nplatform to study novel two-dimensional physics. Yet, In<sub>2</sub>Se<sub>3</sub> has many different polymorphs, and it has been challenging\nto synthesize a single phase material, especially using scalable growth\nmethods, as needed for technological applications. We recently reported\nthe growth of twin-free ultrathin layers of In<sub>2</sub>Se<sub>3</sub> prepared by a diffusion driven molecular beam epitaxy approach,\nand twin-free Bi<sub>2</sub>Se<sub>3</sub> layers grown on these unique\nvirtual substrates. In this paper, we use aberration-corrected scanning\ntransmission electron microscopy to characterize the microstructure\nof these materials. We emphasize features of the In<sub>2</sub>Se<sub>3</sub> layer and In<sub>2</sub>Se<sub>3</sub>/InP interface which\nprovide evidence for understanding the growth mechanism that leads\nto the twin-free and single phase In<sub>2</sub>Se<sub>3</sub>. We\nalso show that this In<sub>2</sub>Se<sub>3</sub> layer provides an\nideal substrate for growth of twin-free Bi<sub>2</sub>Se<sub>3</sub> with a nearly defect-free interface. This approach for growing high-quality\ntwin-free single phase two-dimensional crystals using InP substrates\nis likely to be applicable to other technologically important materials.