Date of Degree

9-2022

Document Type

Dissertation

Degree Name

Ph.D.

Program

Chemistry

Advisor

Maria Tamargo

Committee Members

Ilona Kretzschmar

Stephen O'Brien

Subject Categories

Materials Chemistry

Keywords

Topological Insulators, Molecular Beam Epitaxy, Bi2Se3, Sb2Te3, Short period superlattice, MnSb2Te4

Abstract

Topological insulators (TIs) have been the subject of much research since their discovery in 2009. The unique band structure of the TIs makes them attractive in research of exotic physical phenomena, such as the realization of the quantum anomalous Hall effect, as well as novel applications, including quantum computing, spintronics, and more. From a crystal growth perspective, their van-der-Waals structure also provides new opportunities, for example, by enabling the growth on a large selection of substrates. This thesis presents the growth of three-dimensional TI layers, heterostructures and magnetic TIs by molecular beam epitaxy (MBE). First, we show the improvement of the growth of Bi­2Se3 and Bi2Te3 single layers on sapphire (Al2O3) substrates. By modifying the substrate surface through a pre-growth procedure, we show a marked decrease in the defect density in the crystals, which could lead to improved quality of the materials including their transport properties. We used this new technique as the basis of our subsequent MBE growths. To further improve the transport properties of the TIs, we apply band engineering principles to these novel materials, by growing short-period superlattices of Bi2Se3/Sb2Te3. We show that with these “designer” type-III superlattices we can lower the carrier density of the TIs, which can be explained by bandgap enhancement. Theoretical predictions also show that the gap enhancement occurs without suppressing the topological surface states. This result is promising for future research toward achieving truly insulating TIs.

We also grow and investigate a special class of TIs known as magnetic topological insulators. These materials garnered a huge amount of interest recently, following the discovery of intrinsically magnetic topological insulators of the form MnBi2Te4 and MnSb2Te4. Although some research has been performed on MnBi2Te4, little work exists on its analogue, MnSb2Te4, the material investigated in this thesis. We first present a systematic study of the controlled growth of self-assembled (MnSb2Te4)x(Sb2Te3)1-x structures by MBE. We grew samples ranging from all-Sb2Te3 to all-MnSb2Te4 based on the Mn flux used during growth and present a modified growth sequence to improve the incorporation of Mn to form MnSb2Te4. The transport and magnetic properties of the samples are also analyzed. We see that the samples are all ferromagnetic, with Curie temperatures that depend on the sample’s composition. We show that samples with certain compositions have a distinct Hall resistance dependence on temperature, suggesting two Curie temperature regions. Some of these samples exhibit Curie temperatures as high as 80K, the highest value reported to date. High Curie temperatures are essential for high temperature applications of the TIs’ unique properties.

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