Growth and Characterization of Bismuth Selenide Thin Films by Chemical Vapor Deposition

Open Access
Author:
Brom, Joseph Edward
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 01, 2014
Committee Members:
  • Joan Marie Redwing, Dissertation Advisor
  • Nitin Samarth, Committee Member
  • Joshua Alexander Robinson, Committee Member
  • Suzanne E Mohney, Committee Member
Keywords:
  • CVD
  • Bi2Se3
  • bismuth selenide
  • topological insulator
Abstract:
Topological insulators are a recently discovered class of materials that have garnered much interest due to their unique surface states. With its relatively high band gap (0.3eV) and nearly ideal band structure, Bi2Se3 has been a primary material of interest in the study of topological insulating behavior. However, several factors have made this study difficult. Bi2Se3 typically has a high native selenium vacancy concentration, and selenium vacancies act as donors in the material, leading to a high bulk electron concentration. The surface of Bi2Se3 has also been shown to be susceptible to environmental doping when exposed to ambient air. Combining these two factors means that Bi2Se3 is usually highly n-type doped, making it difficult to study the surface conducting states by transport measurements. This study investigated the use of two different chemical vapor deposition (CVD) techniques for the growth of Bi2Se3 thin films on sapphire (001): hybrid physical-chemical vapor deposition (HPCVD) and metal-organic chemical vapor deposition (MOCVD). HPCVD is a process which combines the evaporation of elemental selenium with the thermal decomposition of trimethylbismuth (TMBi). The use of elemental selenium immediately around the substrate provides a high overpressure of selenium, allowing for reduction of the selenium vacancy concentration. Bi2Se3 films grown on sapphire were epitaxial and highly oriented parallel to the substrate giving rise to narrow X-ray rocking curves (full-width-at-half-maximum=160 arcsecs for (006) reflection) and 6-fold rotational symmetry as determined by phi scans. The structural properties were consistent with deposition via a van der Waals epitaxy process. The selenium to bismuth ratio (VI/V) ratio proved important for achieving a reduced electron concentration of <8x1018 cm-3 and room temperature mobilites of up to 800 cm2V-1s-1. MOCVD growth of Bi2Se3 was also investigated using trimethylbismuth (TMBi) dimethylselenide (DMSe) as precursors. Epitaxial Bi2Se3 films were also produced by MOCVD on sapphire, however, the electron concentrations were generally higher (1-3x1019 cm 3) and the mobilities were lower (~250 cm2V-1s-1) than films grown by HPCVD. This difference is likely due to the higher VI/V ratios more easily achievable with HPCVD growth compared to MOCVD growth. The primary advantage of MOCVD compared to HPCVD, however, was the flexibility that it afforded to grow multilayer structures. This was demonstrated through the deposition of Bi2Se3/MgB2 heterostructures on sapphire for potential use in the study of proximity effect induced topological superconductivity. The effects of different ambient environments on the surface chemistry and electrical properties of Bi2Se3 were also studied. Hall measurements performed over time in air, N2, H2O vapor, and O2 ambient environments showed that the type of ambient gas has a significant impact on the electrical properties of Bi2Se3. Samples held in air and water vapor showed a 25-30% increase in carrier concentration over 10 hours, while a sample held in N2 showed no increase. A sample held in O2, however, showed an initial 20% decrease in carrier concentration followed by a steady increase with time eventually reaching a value 15% above the initial value after 10 hours. Water vapor was determined to be the major contributing factor to the oxidation of Bi2Se3 in air over time, reacting with the Bi2Se3 surface and leading to an increase in free electrons, increasing the carrier concentration. This was supported by x-ray photoelectron spectroscopy (XPS) showing oxygen bonding on the surface for samples held in air and oxygen, but not in nitrogen. Angle resolved photoemission spectroscopy (ARPES) shows that nitrogen was able to suppress oxidation over a period of several weeks compared to a sample stored in air.