Molecular beam epitaxy of van der Waals magnet Cr₂Ge₂Te₆ thin films in wafer-scale

Ko-Hsuan Mandy Chen^1*, Hsuan-Ning Chen¹, Zih-Ping Huang², Wei-Nien Chen¹, Jui-Min Chia¹, Jyun-Fong Wong¹, Chao-Kai Cheng², Shang-Fan Lee³, Chia-Hung Hsu⁴, Minghwei Hong², Jueinai Kwo¹

¹Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan
²Graduate Institute of Applied Physics and Department of Physics, National Taiwan University, Taipei 10617, Taiwan
³Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
⁴National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan

* Presenter:Ko-Hsuan Mandy Chen, email:khchen@phys.nthu.edu.tw

Cr₂Ge₂Te₆ (CGT), a van der Waals semiconductor, is a ferromagnet with a bulk Curie temperature (T_C) of 61 K and perpendicular magnetic anisotropy. Possessing a layered structure with long-range ferromagnetic order down to bilayer, CGT is a promising material for the investigation of two-dimensional magnetism and proximity effect with other layered materials such as topological insulators. In this study, we present the growth of uniform CGT thin films on 2-inch sapphire (0001) substrates by using molecular beam epitaxy. A 3 nm (Bi,Sb)₂Te₃ (BST) layer was adopted as a buffer layer for good CGT growth. In contrast to the polycrystalline CGT growth directly on sapphire, streaky and single crystalline reflection high energy electron diffraction patterns were observed for the growth using a van der Waals layered BST buffer. Dependence on the substrate temperatures, Ge/Cr flux ratios, and post-annealing temperatures have been conducted. The narrow CGT(006) rocking curve with a full width at half maximum (FWHM) of 0.014° and clear Pendellösung fringes in x-ray diffraction manifested the excellent crystallinity of our CGT films with sharp interfaces. Layered CGT islands were found with a roughness of ~0.6 nm in the 1×1 μm² atomic force microscopy image. We will also present the investigation of the electrical transport and magnetic properties and their correlation to various growth parameters.

The authors would like to thank the support from National Science and Technology Council (NSTC), Taiwan, with project number NSTC 111-2112-M-007-043-, Center for Quantum Technology, Taiwan, with project number NSTC 111-2634-F-007-006-, and NSTC project of NTU with TSMC, Taiwan, with project number NSTC 111-2622-8-002-001-. Ko-Hsuan Mandy Chen, Hsuan-Ning Chen, and Zih-Ping Huang contributed equally to this work.

Keywords: magnetic vdW materials, thin films and heterostructures, molecular beam epitaxy