Characterization and Synthesis of BiSb Thin Films: MBE Growth, Pole Figures, Raman Spectroscopy, and Other Characterization Techniques

open_access
Open Access
Author:
Huang, Yu Sheng
Graduate Program:
Physics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 10, 2023
Committee Members:
Chaoxing Liu, Major Field Member
Nitin Samarth, Chair & Dissertation Advisor
Abhronil Sengupta, Outside Unit & Field Member
Irina Mocioiu, Professor in Charge/Director of Graduate Studies
Eric Hudson, Major Field Member
Keywords:
BiSb
spin-orbit-torque devices
molecular beam epitaxy
X-ray diffraction
pole figure measurement
Raman spectroscopy
Abstract:
The Bi1−xSbx alloy has garnered significant attention in the fields of semimetals and narrow bandgap semiconductors due to its unique properties in temperature and magnetic fields. The presence of topological surface states in bulk single crystals of the Bi1−xSbx alloy has sparked interest in exploring its potential for spin-orbit-torque (SOT) devices. Thin films of Bi1−xSbx exhibit high electrical conductivity, making them promising candidates for energy-efficient SOT devices. Previous studies have reported remarkable spin Hall angles and magnetization switching at ultralow current densities in Bi1−xSbx films interfaced with metallic ferromagnets, as well as giant unidirectional spin Hall magnetoresistance when interfaced with ferromagnetic semiconductors. These exciting findings have motivated further investigation into the growth of high-quality, single-crystal Bi1−xSbx films using molecular beam epitaxy (MBE) on various substrates. In this dissertation, we present a systematic study of epitaxial Bi1−xSbx thin films grown on (0001) sapphire substrates using MBE, incorporating a thin (Bi0.95,Sb0.05)2Te3 buffer layer. Various characterization techniques, including transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), atomic force microscopy (AFM), electrical transport, and Raman spectroscopy, were employed to investigate the microscopic structure, quality, and composition of the films. Raman spectroscopy provided valuable insights into the influence of strain on the phonon modes within the thin films. Our results demonstrate the successful growth of epitaxial Bi1−xSbx films with a well-defined growth direction, a crucial aspect for achieving high spin Hall angles in SOT devices. Furthermore, this dissertation emphasizes the effectiveness of characterization techniques for thin films, with a particular focus on XRD pole figure scans. Addressing the lack of standardized documentation in pole figure measurements, we present a clear and consistent approach, contributing to the development of a more standardized methodology in this area. To illustrate the capabilities of pole figure scans, analysis results from our own samples are presented. By comparing peak locations between the sample and substrate, we could assess the quality of epitaxial growth. Moreover, we suggest the potential for qualitative analysis of sample quality through pole figure measurements by examining the full width at half maximum (FWHM) values obtained from azimuthal scans at the maximum intensity, which may provide further insights into sample characteristics. Additionally, this dissertation provides an in-depth overview of two other characterization methods: XPS and Raman spectroscopy. XPS enables accurate analysis of elemental composition, oxidation states, and element migration in thin films. Precise determination of the composition ratio of Bi-Sb thin films using XPS measurements allows for adjustments in the growth process to enhance film quality. Raman spectroscopy, on the other hand, offers insights into bonding, crystal structure, contamination, strain, and material composition. By utilizing these complementary characterization techniques, a comprehensive understanding of Bi1−xSbx thin films can be achieved. Further investigations, such as detailed comparisons with Raman spectroscopy or the utilization of pole figure measurements, have the potential to enable the quantified reporting of strains and other challenging-to-measure information in thin film samples.