Tuning Topological Surface States by Charge Transfer

Author

Zhiyi Chen, The Graduate Center, City University of New YorkFollow

Date of Degree

9-2016

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

Lia Krusin-Elbaum

Committee Members

Maria C. Tamargo

Vinod Menon

Vadim Oganesyan

James Hone

Subject Categories

Condensed Matter Physics

Keywords

Topological insulator, molecular beam epitaxy, charge transfer, superlattice, topological surface states, transport measurement

Abstract

Three-dimensional (3D) topological insulators (TIs), Bi₂Se₃, Bi₂Te₃, Sb₂Te₃, are a class of materials that has non-trivial bulk band structure and metallic surface states. Access to charge transport through Dirac surface states in TIs can be challenging due to their intermixing with bulk states or non-topological two-dimensional electron gas quantum well states caused by bending of electronic bands near the surface. The band bending arises via charge transfer from surface adatoms or interfaces and, therefore, the choice of layers abutting topological surfaces is critical. Surfaces of these 3D TIs have also been proposed to host new quantum phases at the interfaces with other types of materials, provided that the topological properties of interfacial regions remain unperturbed.

This thesis presents a systematic experimental study of both bulk conducting and surface charge transfer problems. We started with optimizing growth condition of Bi₂Se₃ on various substrates, to achieve best quality of Bi₂Se₃ single layers we can get. We then move on to growth of Bi₂Se₃/Zn_xCd_1-xSe bilayers. Here we improved lattice mismatch between Bi₂Se₃ and Zn_xCd_1-xSe layers by tuning lattice parameter of Zn_xCd_1-xSe. After that, we achieved molecular beam epitaxial growth of Bi₂Se₃/Zn_xCd_1-xSe superlattices that hold only one topological surface channel per TI layer. The topological nature of conducting channels is supported by π-Berry phase evident from observed Shubnikov de Haas quantum oscillations and by the associated two-dimensional weak antilocalization quantum interference correction to magnetoresistance. Both density functional theory calculations and transport measurements suggest that a single topological Dirac cone per TI layer can be realized by asymmetric interfaces: Se-terminated Zn_xCd_1-xSe interface with the TI remains `electronically intact', while charge transfer occurs at the Zn-terminated interface. Our findings indicate that topological transport could be controlled by adjusting charge transfer from non-topological spacers in hybrid structures.

The first chapter contains a brief introduction to TIs. It describes basic concepts and notations used later in the bulk of the thesis. These include the topological surface states of a TI, crystal structure of 3D TIs, the origin of defects and their effects on transport study.

The second chapter presents experimental techniques employed for growth and for structural, and electrical characterization of the 3D TIs thin films and superlattices. First, every component of our custom-designed molecular beam epitaxy system will be described in detail, and then the important in situ surface morphology monitoring tool -- RHEED will also be mentioned, as well as high resolution X-ray diffraction (XRD). In the second part, a standard procedure for device fabrication will be presented. The last part will focus on the electron transport measurement setup and various techniques for characterization.

In the third chapter we present explorations of different substrates for growth of Bi₂Se₃ thin films, describe growth of Bi₂Se₃ thin films on sapphire, GaAs(111), InP(001) and InP(111), then optimize growth conditions accordingly. The quality of films are investigated to study the effects of substrates on quality of the films.

The fourth chapter is a growth study of superlattice of a TI with a traditional II-VI semiconductor, Bi₂Se₃/Zn_xCd_1-xSe. we explore II-VI semiconductor family and study the optimal material to grow on top of Bi₂Se₃. Then we focus on the growth of Bi₂Se₃/Zn_xCd_1-xSe superlattice and structural study.

The fifth chapter studies charge transfer at the interface between Bi₂Se₃ layer and Zn_xCd_1-xSe layer. We start by looking at the result of charge transport study of our superlattice. Then we will present the result of our density functional theory (DFT) calculation, which showed completely different charge transfer between Bi₂Se₃ sits on top of Zn_xCd_1-xSe and Zn_xCd_1-xSe on top of Bi₂Se₃. This will provide a perfect explanation of our experimental results. Then we designed experiment using transport measurement to test and confirm out explanation.

The sixth chapter gives a short summary of this thesis work and a proposal for future work.

Recommended Citation

Chen, Zhiyi, "Tuning Topological Surface States by Charge Transfer" (2016). CUNY Academic Works.
https://academicworks.cuny.edu/gc_etds/1554