Characterization and Control of the Surface of the Topological Insulator Bi2Se3

Abstract

The field of topological insulator (TI) materials is new. The ideal TI contains surface states in helical Dirac cones that can be used for spintronics or interconnect applications. Of the TI class, Bi2Se3 is the most promising for applications due to its stoichiometric composition, its relatively large band gap (0.3 eV), and the central (??-point) position of the Dirac cone in its 2D surface band structure. Although the theoretical solid-state models that the TI field has produced are powerful and unique, their novel emergent physical properties are not universally observed in every sample. These materials are difficult to grow and maintain under ambient conditions. Growths tend to either not be applicable to wafer-scale production or produce high polycrystallinity, and all samples experience natural oxidation, band bending, and intrinsic n-doping, which generates spin-degenerate or bulk conduction.

This thesis contains a primer on topologically non-trivial materials, and two studies aimed at understanding and minimizing defects at the surface of Bi2Se3. In the first, the aging process of Bi2Se3 when exposed to air at room temperature is investigated. The time scale and topographic changes of the oxidation process at micromechanically exfoliated surfaces are measured, and an optical model of the bulk and oxide layers are developed. The surface appears to oxidize starting at 2 hours after exfoliation, and continuing through 1.5 weeks, by which time, the oxide layer growth has reached an asymptote of 1.9 nm. New optical characterization methods are developed to monitor the orientation of the crystal (via second harmonic generation) and to measure the oxide growth at the surface (using spectroscopic ellipsometry and the derived dielectric functions of the bulk and oxide layers). The goal of the second study is to assess the use of Se capping and subsequent thermal decapping to preserve a pristine surface and maintain a constant Fermi level. This was measured by annealing samples in a UHV environment to successively higher temperatures until the Bi2Se3 film decomposed, and measuring the surface crystallinity, topography, surface chemistry, and Fermi level between each anneal. Thermally decapping samples has no measurable effect on crystallinity, minimal effect on surface topography, reveals the expected Bi-Se surface bonds, and retains a mid-gap Fermi level. This may serve as a reference to improve the fabrication process of devices that include Bi2Se3.