Abstract text (incl. figure legends and references)

1. Introduction

Since the beginning of the XXI century layered two-dimensional (2D) materials have been promising candidates for use in microelectronics, photonics, and photovoltaics. To grow high-quality heterostructures and superlattices, molecular beam epitaxy is used, but various defects are generated during the growth. Therefore, the growth of wafer-scale high-quality films is a fundamental issue of van der Waals (vdW) epitaxy that requires profound investigation of growth mechanism using in situ experimental techniques.

2. Objectives

In this work, we aim to visualize and study surface transformations during sublimation and van-der-Waals epitaxy of 2D metal chalcogenides by in situ reflection electron microscopy.

3. Materials & methods

We used an in situ reflection electron microscope (REM, 100 kV accelerating voltage) equipped with three separate evaporators for Se and metal (Bi, In, or Sn). Samples were prepared from a Bi₂Se₃ single crystal by cleavage along (0001) plane and were heated during experiment resistively by passing direct electric current. Above 100°C, the samples were exposed to a Se molecular beam (up to ~0.1 nm/s) to avoid noncongruent Bi₂Se₃ sublimation. Ex situ analysis of surface morphology was carried out by atomic force microscopy (AFM, Multimode 8, Bruker). The same equipment was used for creation of 15–30-nm-deep grooves by probe lithography. The crystal phase of obtained films was analyzed by ex situ Raman scattering measurements (XploRa Plus, Horiba, 532 nm laser).

4. Results

When Bi₂Se₃(0001) surface is heated above 400°C, in situ REM images show that atomic steps move in ascending direction (towards higher terraces), which corresponds to congruent crystal sublimation. During sublimation, many 2D vacancy islands nucleate, grow, and coalesce on wide (~1–100 μm) terraces between the steps. The grooves preformed by probe lithography widen and deepen, and the center of each groove acts as a source of atomic steps that generates new steps moving in the ascending direction away from the source. We used this phenomenon to create self-organized regularly-spaced 1-nm-high atomic steps on the Bi₂Se₃(0001) surface. The onset of Bi deposition reverses the step motion direction (towards lower terraces), which corresponds to the onset of epitaxial layer-by-layer Bi₂Se₃ growth. These processes were first observed on the Bi₂Se₃ surface by in situ REM [J. Phys. Conf. Ser. 1984 (2021) 012016].

When In (or Sn) is deposited onto the Se-exposed sublimating Bi₂Se₃(0001) surface, In₂Se₃ (or SnSe₂) heteroepitaxy begins with periodic 2D island nucleation and growth. After 3–5 nm growth, this mode switches to 3D growth (mounding). Ex situ Raman spectra of the grown films display a set of vibrational modes typical for the β-In₂Se₃ (or 1T-SnSe₂) crystal phase.

5. Conclusions

Our results show that in situ REM technique can be applied to study real-time surface dynamics of layered 2D materials during sublimation and van-der-Waals epitaxy. Using in situ REM, we have first visualized motion of atomic steps, evolution of lithographically patterned grooves, and 2D island nucleation on the Bi₂Se₃(0001) surface.

Acknowledgments

This research was performed on the equipment of CKP "Nanostruktury" and financially supported by Russian Science Foundation [grant number 22-72-10124].