Molecular beam epitaxy growth of topological insulator Bi₄Br₄ on silicon for the infrared applications

Bi₄Br₄ is a material rich in intriguing topological properties. Monolayer Bi₄Br₄ film exhibits helical edge states characteristic of a quantum spin Hall insulator, while bulk Bi₄Br₄ represents a higher-order topological insulator with hinge states. However, direct exfoliation from single crystal can only obtain thin nanowires due to the weak van der Waals forces between Bi₄Br₄ chains, which limits its optical analysis and application, while the growth of Bi₄Br₄ thin films is also full of challenges due to the extremely narrow growth temperature range and the accurate control of the BiBr₃ flux. Here, we reported the controlled growth of α-Bi₄Br₄ thin films on intrinsic silicon substrates using molecular beam epitaxy. The growth temperature, BiBr₃ flux, and the flux ratio of Bi and BiBr₃ were accurately controlled. Then, the morphology, composition, and bonding of the prepared films were investigated using atomic force microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. The growth of large, uniform thin films provides an ideal material platform for studying the physical properties of Bi₄Br₄. Additionally, we utilized Fourier-transform infrared spectroscopy to explore the film’s infrared characteristics, revealing strong absorption in the low frequency range due to the high proportion of one-dimensional topological edge states and laying the groundwork for further exploration of its potential applications in the optoelectronic field.