Transport study in graphene and topological insulator materials
Abstract
Two Dirac fermion systems recently realized in solid state physics are believed promising for novel physics and device applications. One is graphene, whose linear electrical band dispersion results from graphene's unique honeycomb lattice structure. The other one is topological insulator (TI) materials, which process topologically non-trivial surface states (TSS) as a result of interplay between strong spin-orbital coupling and bulk band inversion. Different from free electron model described by Schrö dinger equation, the charge carriers arsing from the Fermi surfaces close to the charge neutral points in graphene and TSS obey Dirac equation derived to combine special relativity and quantum mechanics. This thesis reports electronic transport properties of graphene and TI materials. We first focus on graphene grown by chemical vapor deposition (CVD) on metals. We observed ambipolar field effect and weak localization in graphene grown on Cu and Ni. Unique Berry phase π of Dirac fermions was revealed by integer quantum Hall effect measured in graphene grown on Cu. Our results demonstrated large scale CVD growth of graphene could be achieved, and the sample quality was approaching the intrinsic exfoliated graphene. Graphene grown by CVD method provide a platform for not only industrial applications but also physics research. Electronic transport measurements in TI materials are still challenging to date because of multiple parallel transport channels (e.g., bulk, trivial 2D electron gas induced by surface band bending in addition to the TSS). We have measured various TI materials, and were able to distinguish different conduction channels. In highly doped Bi2 Se3 bulk crystals (with 3D carrier concentrations on the order of 1019 cm-3), we found that the observed quantized Hall effect arise from the bulk quintuple layers acting as parallel 2D electron systems. In Bi2 Te2Se (BTS221) bulk crystals, we have observed TSS coexisting with trivial surface 2D electron gas, and studied their transport signatures at both high B (Shubnikov de Haas oscillations) and low B regime (weak anti-localization). Other than magneto-transport, gate tunable thermoelectric transport properties were also studied in Bi0.04Sb 0.96Te3 TI thin films. We observed an ambipolar behavior in thermopower (Seebeck coefficient), and found bulk carriers are responsible for the thermoelectric power generation. We also demonstrate a large enhancement (more than 10 times) in the power factor, where TSS are believed to play a substantial role. Our results not only highlight the first observation of ambipolar thermoelectric behavior in TI materials, but also open a new route to field effect device based ZT improvement, which could potentially benefit thermoelectric applications of TI materials.
Degree
Ph.D.
Advisors
Chen, Purdue University.
Subject Area
Electrical engineering|Nanoscience|Condensed matter physics
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