Formation of Twin-Free Single Phase β-In2Se3 Layers via Selenium Diffusion into InP(111)B Substrate

Indium selenide, In₂Se₃, has recently attracted growing interest due to its remarkable properties, including room temperature ferroelectricity, outstanding photoresponsivity, and exotic in-plane ferroelectricity, which open up new regimes for next generation electronics. In₂Se₃ also provides the important advantage of tuning the electrical properties of ultrathin layers with an external electrical and magnetic field, making it a potential platform to study novel two-dimensional physics. Yet, In₂Se₃ has many different polymorphs, and it has been challenging to synthesize a single phase material, especially using scalable growth methods, as needed for technological applications. We recently reported the growth of twin-free ultrathin layers of In₂Se₃ prepared by a diffusion driven molecular beam epitaxy approach, and twin-free Bi₂Se₃ layers grown on these unique virtual substrates. In this paper, we use aberration-corrected scanning transmission electron microscopy to characterize the microstructure of these materials. We emphasize features of the In₂Se₃ layer and In₂Se₃/InP interface which provide evidence for understanding the growth mechanism that leads to the twin-free and single phase In₂Se₃. We also show that this In₂Se₃ layer provides an ideal substrate for growth of twin-free Bi₂Se₃ with a nearly defect-free interface. This approach for growing high-quality twin-free single phase two-dimensional crystals using InP substrates is likely to be applicable to other technologically important materials.

Cross-sectional scanning transmission electron microscopy image showing atomic resolution of single phase β-In₂Se₃ layer grown via selenium (Se) passivation of InP(111)B substrate. Atomic models of β-In₂Se₃ and InP(111)B are overlaid with the real lattice depicting that indium atoms remain at the same positions in both lattices as shown by yellow box with the same dimensions. This also shows the result of selenium diffusing into InP and substituting for phosphorus (P) atoms while preserving the zinc-blende InP lattice right below the In₂Se₃/InP interface. These features provided evidence to uncover the growth mechanism which led to the formation of ultrathin layers of untwinned single phase β-In₂Se₃.