MTL Annual Research Report 2013

Electronic Transport Studies of Thin Film Bi and Bi1-xSbx

Currently bulk Bi and Bi_xSb_1-x­ have the best known thermoelectric (TE) properties at cryogenic temperatures. These properties can be further improved in thin films through quantum confinement effects^[1]. The crystal orientation of the thin film is expected to impact the transport properties in Bi and Bi_xSb_1-x­ since they are known to be anisotropic. Previous theoretical calculations have suggested that the electronic structure of thin films of Bi_xSb_1-x­ with low Sb concentration may possess Dirac cones, similar to graphene, when grown in certain low symmetry directions^[2]. Unlike graphene, the Dirac cones in Bi_xSb_1-x­ can have an asymmetric slope depending on the crystal orientation, causing the Fermi velocity to be asymmetric as well^[3]. Further control of the crystal orientation is also predicted to open a band gap, changing the material from a semi-metal to semiconductor, which is useful for TE applications.

Clearly, the crystal orientation of Bi and Bi_xSb_1-x has a strong impact on the electronic structure and transport properties. By depositing thin films of Bi or Bi_xSb_1-x on different substrates, we can control the grain texture and orientation of the film. On substrates (e.g., mica, sapphire, BaF₂) that are well lattice-matched to Bi, grains are almost all oriented along the trigonal axis out of plane from the film. On substrates that are poorly lattice-matched (e.g., SiO₂) or where Bi wets poorly to the surface (e.g., graphene), each grain is randomly oriented. We fabricate van der Pauw devices from thin films of various thicknesses and substrates to quantify the contributions from different grain crystal orientations. Both the single contribution from each grain and the collective behavior of many grains are investigated. Devices are also gated to electrostatically dope the films and investigate the presence of the predicted Dirac cones.

1. L. D. Hicks and M.S. Dresselhaus, “Effect of quantum-well structures on the thermoelectric figure of merit,” Physical Review B, vol. 47, pp. 12727-12731, May 1993. [↩]
2. S. Tang and M.S. Dresselhaus, “Constructing anisotropic single-Dirac-cones in Bi_1-xSb_x thin films,” Nano Letters vol. 9, p. 2021, Mar. 2012. [↩]
3. S. Tang and M. S. Dresselhaus, “Phase diagrams of Bi_1-xSb_x thin films with different growth orientations,” Physical Review B, vol. 86, p. 075436, Aug. 2012. [↩]