IBA analysis of Iron-doped Bi2Se3 topological insulator

Sorbonne Universités, UPMC Univ Paris 06, CNRS UMR 7588, INSP, F-75005, Paris, France

In three-dimensional Topological Insulators (TIs), strong intrinsic spin-orbit coupling generates an insulator band inversion with spin-momentum locking leading to creation of protected Topological Surface States (TSS) due to time-reversal symmetry. These surface states are described by helical Dirac fermions with a topological order, meaning that opposing electron spin states generate opposing currents [1, 2]. The topological nature with its spin texture is extremely interesting from the perspective of using these materials in electronic technology: such as quantum computing and spintronic devices [3]. However for such applications it is necessary to be able to form a ferromagnetic TSS, therefore time-reversal symmetry has to be broken. This may be attempted through doping with a magnetic dopant.

In this work, we have studied growth by MBE on GaAs (111) substrates of epitaxial Bi2Se3, a topological insulator with a simple band structure, doped with Fe. Rutherford Backscattering Spectroscopy (RBS), ion channelling, and heavy ion Elastic Recoil Detection Analysis (ERDA) were applied. Nominal Fe/Bi atomic ratios were varied from 0 to 0.3, for films of 100 nm-thickness, either capped or uncapped with a thin Se or ZnSe layer to protect from oxidation.

In the undoped capped samples RBS shows stoichiometry very close to Bi2Se3, however iron doping introduces substantial variations of the Bi:Se ratio, and both lateral and depth composition inhomogeneity. Furthermore, in these doped samples significant light element contamination is deduced from the RBS observations, including capped samples. ERDA measurements currently underway will be presented to confirm this interpretation of the RBS spectra.

The RBS spectrum fitting was performed with the NDF DataFurnace, which allowed extraction of the layered composition, showing interface zones between the TI layers and both the capping layer and the substrate. RBS showed significant disagreement with the nominal Fe doping and layer thicknesses. Synchrotron X-Ray Fluorescence (XRF) is therefore used as a complementary method to determine Fe areal density. The implications of these observations for the MBE growth processes of doped films, and the prospects for obtaining intrinsically ferromagnetic TSS are discussed.

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[2] C. X. Liu, X. L. Qi, H. Zhang, X. Dai, Z. Fang, and S. C. Zhang, “Model Hamiltonian for topological insulators,” Phys. Rev. B - Condens. Matter Mater. Phys., vol. 82, pp. 1–19, 2010.
[3] P. Roushan, J. Seo, C. V Parker, Y. S. Hor, D. Hsieh, D. Qian, A. Richardella, M. Z. Hasan, R. J. Cava, and A. Yazdani, “Topological surface states protected from backscattering by chiral spin texture.,” Nature, vol. 460, no. 7259, pp. 1106–1109, 2009.