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Molecular-beam epitaxy of topological insulator Bi2Te3 on Si (111) substrates

Recently a new state of matter called topological insulator (TI) has been theoretically predicted and experimentally observed in a number of materials [1]. Topological insulators are characterized by gapless surface states that show a linear energy dispersion, similar to relativistic particles. Hence, carriers at the surface of topological insulators have unparalleled properties, such as extremely high mobilities, or dissipationless spin-locked transport, and consequently these features may lead to new applications in the field of spintronics or quantum computing.

Concerning the Bi2Te3 material system, this narrow gap semiconductor has been traditionally investigated as a thermoelectric material. However, very recently a TI behavior has been observed at the surface of Bi2Te3. To date, the Bi2Te3 material used to study TI behavior have mainly been carried out in the form of bulk crystals realized by means of the melt-growth or self-flux method [2,3], which results in heavily n-type doped material due to the formation of defects. To compensate the n-type doping, the Bi2Te3 material is usually heavily doped by Sn or Ca, which strongly degrades transport properties. In order to investigate TI properties of Bi2Te3, it is therefore desirable to grow intrinsic thin films of Bi2Te3. Besides studying fundamental properties of TI Bi2Te3, the realization of high quality Bi2Te3 epilayers on low-cost substrates, such as silicon, is highly beneficial for device applications.

Here we focus on the realization of Bi2Te3 thin films grown by molecular-beam epitaxy (MBE) onto Si (111) substrates [4]. By a careful optimization of the growth parameters, we are able to realize high quality single-crystal Bi2Te3 epilayers. Figure 1a depicts a symmetric XRD curve of a 20 nm thin Bi2Te3 film on a Si (111) substrate. Besides the XRD peaks due to the Si substrate, numerous narrow XRD peaks originating from the Bi2Te3 epilayer are observed, indicating a (001)-oriented single crystal Bi2Te3 films commensurately grown on a Si(111) substrate. Atomic force microscopy measurements were carried out, as depicted in figure 1b. The AFM image shows atomic steps with step heights of 1.017 nm (see figure 1c), which represents the thickness of a single Bi2Te3 quintuple layer. Most importantly, figure 1d illustrates ARPES measurements of the Bi2Te3 surface. A clear linear energy dispersion is seen, evidencing the TI behavior of the MBE-grown Bi2Te3 film. Right now we are in the process of carrying out electrical measurements in order to observe carrier transport at the surface of the Bi2Te3 epilayer. A substantial increase of the carrier mobility is expected due to the linear energy dispersion is expected.

These research activities are part of the Virtual Institute of Topological Insulators (VITI).

BiTeFigure 1: a) symmetric XRD curve of a Bi2Te3 epilayer grown on a Si (111) substrate, b) atomic force micrograph of the Bi2Te3 surface showing atomic steps, c) height profile of these atomic steps, indicating a constant step height of 10.17 A, which represents the height of a single Bi2Te3 quintuple layer, d) ARPES measurements revealing the linear energy dispersion behavior at the Bi2Te3 surface.



References:

[1] J. E. Moore, The birth of topological insulators, Nature 464, 194 (2010)

[2]  D. Hsieh, D. Qian, L. Wray, Y. Xia, Y. S. Hor, R. J. Cava, and M. Z. Hasan,
A topological Dirac insulator in a quantum spin Hall phase,
Nature 452 970 (2008)

[3] Y. L. Chen, J. G. Analytis, J.-H. Chu, Z. K. Liu, S.-K. Mo, X. L. Qi, H. J. Zhang, D. H. Lu, X. Dai, Z. Fang, S. C. Zhang, I. R. Fisher, Z. Hussain, and Z.-X. Shen,
Experimental realization of a three-dimensional topological insulator Bi2Te3,
Science 325, 178 (2009)

[4] J. Krumrain, G. Mussler, S. Borisova, T. Stoica, L. Plucinski, C. M. Schneider, D. Grützmacher,
MBE growth optimization of topological insulator Bi2Te3 films
Journal of Crystal Growth, 324 (2011), 115 - 118


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