Topological Insulator Bi2Se3 Nanowire Field Effect Transistors

Topological insulators (TI) have insulating band gaps in the bulk but gapless surface states topologically protected by time-reversal symmetry [1, 2]. Bi₂Se₃, a well-known semiconductor and thermoelectric material, is a TI with a bulk band gap of 0.35 eV and a single Dirac cone on the surface [2, 3]. Most current experimental research focus on the surface states of thin TI samples exfoliated from bulk material [4]. A few groups have reported modification of the surface conduction of such exfoliated TI samples by doping, applying a vertical electric field or polarized light. However, high-performance microelectronic devices based on topological insulators such as the analog of metal oxide semiconductor field effect transistors (MOSFETs) have not yet been reported.

MOSFETs are the basic building blocks for digital and analog circuits. In this work, we fabricated and measured surrounding-gate Bi₂Se₃ nanowire (NW) field-effect transistors. The current-voltage (I-V) characteristics were measured at different temperatures, exhibiting excellent performance: well-saturated output I-V curves and large on/off ratio. The surface metallic conduction can be separated from the bulk conduction with gate electric field at different temperatures. We found that the surface conduction increases as temperature decreases, while the bulk conductance decreases with decreasing temperature. This indicates that the carriers at the surface and the carriers inside the bulk Bi₂Se₃ body have different properties. In addition, the Bi₂Se₃ FET exhibits I-V characteristics different to conventional MOSFETs in the weak inversion region. The drain current (I_DS) increases significantly beyond 3ф_t (ф_t is thermal potential). This trend indicates that the Bi₂Se₃ nanowire FET current does not follow the same diffusion current model as that describes conventional MOSFETs. We believe I_DSin the weak/moderate conductive regions is also dominated by drift current.

To further study the topological insulator properties, we have performed magneto-transport measurements and observed Aharonov-Bohm (AB) oscillation that arise from the surface conduction channel that surrounds the Bi₂Se₃ NW in these FETs. The AB oscillations are obtained at low temperatures with a magnetic field oriented along the channel direction. The devices exhibit the anomalous AB effect [5] while maintaining high FET performance. The AB oscillations have a period of h/e and can be significantly tuned by the top surrounding gate. The resistance has a peak at B = 0 and decreases as the magnetic field increases. This phenomenon indicates that the surface states may be bound together by their spin in the circle and activated for conduction by the magnetic field along the axial direction of the circle.

In summary, we have fabricated and measured Bi₂Se₃nanowire field effect transistors with excellent I-V characteristics. The devices exhibit anomalous AB oscillation, which may lead to new discovery of spin coupling in topological insulators.

References:

1. J. E. Moore, Nature 464, 194 (2010)

2. M. Z. Hasan et al., 82, 3045 (2010).

3. X.-L. Qi et al., Rev. of Modern Phys. 83, 1057 (2011).

4. Y.L. Chen et al., Science 325, 178-181 (2009).

5. Y. Zhang and A. Vishwanath, Phys. Rev. Lett. 105, 206601 (2010).