Molecular beam epitaxy and characterization of doped topological insulators for spintronics and quantum anomalous hall effect applications

Topological insulators are highly valued for their distinctive electronic properties, particularly their topologically protected surface states. Among TIs, Bi2Te3 stands out due to its large bulk band gap. The realization of topologically protected surface states is largely dependent on the quality of deposited thin films. However, the epitaxial growth modes, thickness, and phase segregation of Bi2Te3 and its Sb-doped variant remain inadequately understood. Our study aims to elucidate the effects and importance of growth kinetics on the quality of thin films deposited, including deposition rates, annealing time, substrate temperature, and growth duration. We employed techniques such as X-ray diffraction, X-ray reflectivity, and atomic force microscopy for structural characterization, X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy for electronic characterization, and Raman spectroscopy for optical characterization. We explored two main avenues. First, we investigated the impact of growth rate on strain in Bi2Te3 thin films by varying the flux of deposited material. Second, we examined the effect of doping levels in Sb-doped Bi2Te3. In both investigations, all other growth parameters were kept constant. In our analysis we observed an undesirable secondary phase peak at 015 in both Bi2Te3 and Sb-doped Bi2Te3 films. For Bi2Te3 films, reducing the growth rate nearly eliminated this secondary phase. In Sb-doped Bi2Te3, increasing the Sb flux achieved a similar result. In both cases, pronounced thickness fringes indicated coherence between the film surface and the substrate. Our research underlines the capability of our deposition system to achieve high-quality growth and fine-tuned control over the properties of Bi2Te3 and its Sb-doped films, demonstrating significant potential for future electronic and spintronic applications.