Molecular Beam Epitaxy of Doped Topological Insulators for Spintronics and Quantum Anomalous Hall Effect Applications

Topological insulators stand out in many fundamental magnetotransport, spintronic and quantum anomalous Hall effect (QAHE) studies due to their unique electronic band structures. Their bulk band gap, topologically protected surface states with Dirac cone features, dopant-tuning of Fermi level and large spin-orbit coupling characteristics enable the exploration of a wide range of new condensed matter phenomena, materials physics and new quantum device applications. Among these applications, QAHE is an important basic scientific and applied area of condensed matter physics, which helps define the fundamental units of resistivity and help develop metrology-grade extremely sensitive and precise magnetic field and current sensors. An important issue in this field is the requirement of 30-50 mK range of magnetotransport measurements, which limits the field to fundamental scientific studies and prevents using this highly promising mechanism for sensing or other device applications. Several materials breakthroughs are needed to enable QAHE at higher temperatures such as few-Kelvin ranges.

In this study, we present molecular beam epitaxial growth and optimization of topological insulator Bi₂Te₃ and Sb-doped Bi₂Te₃ and a detailed structural and magnetotransport analysis of the films grown expitaxially on sapphire. Bi₂Te₃ is notable among TIs because of its substantial bulk band gap. The quality of these topologically protected surface states is heavily influenced by the quality of the deposited thin films. However, the epitaxial growth behaviors, thickness, and phase segregation of Bi₂Te₃ and its Sb-doped variant are not well understood. Our study clarifies the effects and significance of growth kinetics on the quality of thin films, focusing on deposition rates, annealing time, substrate temperature, and growth duration. We used X-ray diffraction (XRD), X-ray reflectivity (XRR), and atomic force microscopy (AFM) for structural characterization, X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDX) for electronic characterization, and Raman spectroscopy for optical characterization.

To elucidate the effect of stoichiometry, defects, and strain on the TI quality, we pursued two main research paths. First, we investigated the effect of elemental flux of deposited material in MBE on strain and Bi₂Te₃ thickness. Second, we investigated the influence of different doping levels in Sb in Bi₂Te₃, keeping all other growth parameters constant. In our analysis, we detected an unwanted secondary phase peak at 015 and Bi₂Te phase in both Bi₂Te₃ and Sb-doped Bi₂Te₃ and films. Reducing the growth rate for Bi₂Te₃ films almost eliminated this 015 secondary phase. Increasing the Sb flux also significantly reduced this secondary phase. In both cases, pronounced thickness fringes indicated coherence between the film surface and the substrate. Our growths identified that the flux ratios of bismuth to tellurium must be optimized for achieving full film growth. A higher bismuth flux rate than for tellurium causes hexagonal island growth and no films.

Finally, we present a broader analysis describing the intrinsic and extrinsic effects that hold the magnetic ordering temperature for QAHE low in the mK ranges. We identify that the intrinsic effects are material chemistry related and those effects are magnetic ordering temperature (Curie temperature), doping limitations (V, Sb and the associated dilution of magnetic moments), material defects and nonuniform dopant distributions, the weak interaction between magnetic dopants the host materials' electronic states, and a likely competition between spin-orbit coupling and robust magnetic interactions limits QAHE in the mK ranges. Extrinsic effects include finite size effects, interference from bulk states, and materials engineering challenges. We conclude that MBE growth optimization is necessary for precise control of the properties of high-quality Bi₂Te₃ and its Sb-doped films.