Elsevier

Applied Surface Science

Volume 361, 15 January 2016, Pages 185-189
Applied Surface Science

Growth of niobium on the three-dimensional topological insulator Bi2Te1.95Se1.05

https://doi.org/10.1016/j.apsusc.2015.11.163Get rights and content

Highlights

  • We grew niobium on topological insulator at different substrate temperatures.

  • Local density of states is modified by deposited Nb islands.

  • We found a downward shift of the Dirac point, since niobium acts as a donor.

  • Nb grew in layer-by-layer growth mode up to an annealing temperature of 450 °C.

  • We applied a new cleaving method allowing for sample heating of flux-grown TI.

Abstract

While applying a new cleaving method, we investigated the growth of Nb on the three-dimensional (3D) topological insulator (TI) Bi2Te1.95Se1.05 by scanning tunneling microscopy and spectroscopy. After the deposition of nearly a full monolayer of Nb by high-energy electron-beam evaporation, we observed a downshift of the bands and the Dirac point on the TI surface, which is the result of an n-type doping of the TI by transition metal adatoms. Extra peaks in the spectroscopy results upon Nb deposition might indicate a Rashba-split of the bulk bands. Nb grew in small 10 nm wide islands upon sub-monolayer growth and in a layer-by-layer growth mode up to an annealing temperature of 450 °C.

Introduction

Topological insulators (TIs) have drawn much interest owing to the intriguing properties of the surface states, in which the spin is locked to the TI momentum and the charge carriers are protected against backscattering by time-reversal symmetry [1], [2], [3], [4], [5], [6]. There have been many suggestions on how to utilize these unique properties to create a new science [7], [8], find applications in future solid-state quantum computing [9], and identify the Majorana fermion experimentally [10], [11], [12]. Fu and Kane suggested that Majorana fermions are found on a topological insulator at the vortex of a magnetic field, penetrating the topological superconducting region created by an s-wave superconductor [10]. Therefore, investigation of nanoscale flat-top superconducting islands in the vicinity of a clean TI surface is the first crucial step. To date, TIs have mostly been grown on top of other materials or as a part of heterostructures [13], [14], [15]; however little is known of how TIs behave as substrates. In this paper, we present the growth and electronic properties of Nb islands grown on TI substrates. We studied the growth of high-energy electron-beam-evaporated Nb using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). We applied a cleaving technique that allowed for sample annealing and investigated the growth mode at different temperatures and coverages.

Section snippets

Experimental details

The selenium-rich grown ternary compound Bi2Te1.95Se1.05 (BTS) was used as a substrate. It has been reported to be one of the most bulk-insulating and robust TI materials to date [16], [17]. The BTS sample was grown using a standard flux-method from a high-purity melt, as described elsewhere [18]. The Fermi level lies inside the band gap so that only topologically protected surface states are present [19], [20].

For the s-wave superconductor we chose Nb for deposition, because it has the highest

Results and discussion

Fig. 1(b) shows a topographic image of a pure BTS substrate surface, revealing that the surface consists of two different kinds of atoms. The brighter contrast depicts tellurium atoms and the darker sites were identified as selenium atoms [26]. After the confirmation of the atomically flat and clean sample surface, 0.12 monolayer (ML) Nb was deposited at room temperature [Fig. 1(c)]. Small Nb islands that were approximately 5–10 nm wide and 0.6 nm high [about twice the lattice constant of Nb (aNb 

Conclusion

By applying a new, conductive-epoxy-free cleaving method that allows for sample heating, we grew Nb on a TI and post-annealed at different temperatures. At sub-ML deposition of Nb with high surface coverage, a shift of the DP of the TI was observed. Nb acts as a donor, which leads to downwards bending of the bands and possibly a Rashba-split of the BCB. With multi-ML deposition, Nb showed a layer-by-layer growth mode. Even after annealing at 450 °C, no island formation has been observed. No

Acknowledgements

We would like to thank Sungmin Kim and Minjun Lee for valuable discussions. PM and SFF gratefully acknowledge financial support by DFG SPP 1666. This work is supported in part by the National Research Foundation of Korea (NRF) Grant (NRF-2006-0093847, NRF-2010-00349). JP and JSK were supported by the NRF through SRC (Grant No. 2011-0030785) and Max Planck POSTECH/KOREA Research Initiative Programs (Grant No. 2011-0031558).

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