Shuang Tang

Constructing Anisotropic Single-Dirac-Cones in Bi1–xSbx Thin Films

The electronic band structures of Bi(1-x)Sb(x) thin films can be varied as a function of temperature, pressure, stoichiometry, film thickness, and growth orientation. We here show how different anisotropic single-Dirac-cones can be constructed in a Bi(1-x)Sb(x) thin film for different applications or research purposes. For predicting anisotropic single-Dirac-cones, we have developed an iterative-two-dimensional-two-band model to get a consistent inverse-effective-mass-tensor and band gap, which can be used in a general two-dimensional system that has a nonparabolic dispersion relation as in the Bi(1-x)Sb(x) thin film system.

Electronic properties of nano-structured bismuth-antimony materials

Bismuth antimony (Bi1−xSbx) is one of the most important materials systems for fundamental materials science, condensed matter physics, low temperature thermoelectrics, infrared applications, and beyond. The bulk materials have been studied for many decades. Recently, nanoscience and nanotechnology has been introduced to this materials system, which has brought much more interest and attention for both research and applications. The present article reviews the up-to-date achievements on electronic properties of nanostructured Bi1−xSbx, and points out future directions for scientific research in this field.

Electronic phases, band gaps, and band overlaps of bismuth antimony nanowires

We have developed an iterative one dimensional model to study the narrow band-gap and the associated non-parabolic dispersion relations for bismuth antimony nanowires. An analytical approximation has also been developed. Based on the general model, we have developed, we have calculated and analyzed the electronic phase diagrams and the band-gap/band-overlap map for bismuth antimony nanowires, as a function of stoichiometry, growth orientation, and wire width.

Phase diagrams of Bi1−xSbxthin films with different growth orientations

A closed-form model is developed to evaluate the band-edge shift caused by quantum confinement for a two-dimensional non-parabolic carrier-pocket. Based on this model, the symmetries and the band-shifts of different carrier-pockets are evaluated for BiSb thin films that are grown along different crystalline axes. The phase diagrams for the BiSb thin film systems with different growth orientations are calculated and analyzed.

Anisotropic transport for parabolic, non-parabolic, and linear bands of different dimensions

We have developed a robust analytical methodology for modeling the anisotropic transport distribution function, which can be then used to describe various transport properties of anisotropic systems, including the electrical conductivity, carrier mobility, Seebeck coefficient, and thermal conductivity. Our methodology has considered the general cases for 3-, 2-, and 1-dimensional systems with parabolic, non-parabolic, and linear dispersion relations. Calculations are made using both the relaxation time approximation and the mean free path approximation. We have found that the Onsager relation can be violated under certain conditions. Furthermore, the methodology developed in the present work is compared with the traditionally used numerical methodology.

Thickness dependence oscillations of transport properties in thin films of a topological insulator Bi91Sb9

The dependences of the electrical conductivity, Hall coefficient, magnetoresistance, and Seebeck coefficient on the thickness d (d = 15–400 nm) of the topological insulator Bi91Sb9 thin films grown on mica substrates were obtained at room temperature. In addition to the oscillations with a period Δd = (105 ± 5) nm in the thickness range d = 100–400 nm which are attributed to the quantization of the semiconductor electron energy spectrum, oscillations with a period Δd = (8 ± 2) nm in the range d = 15–60 nm were also revealed. It is suggested that the existence of the high-frequency oscillations in the thin films may be connected with the quantization of the metallic surface states energy spectrum.

Induced electronic anisotropy in bismuth thin films

We use magneto-resistance measurements to investigate the effect of texturing in polycrystalline bismuth thin films. Electrical current in bismuth films with texturing such that all grains are oriented with the trigonal axis normal to the film plane is found to flow in an isotropic manner. By contrast, bismuth films with no texture such that not all grains have the same crystallographic orientation exhibit anisotropic current flow, giving rise to preferential current flow pathways in each grain depending on its orientation. Extraction of the mobility and the phase coherence length in both types of films indicates that carrier scattering is not responsible for the observed anisotropic conduction. Evidence from control experiments on antimony thin films suggests that the anisotropy is a result of bismuths large electron effective mass anisotropy.