research papers

ISSN 1600-5767

Received 26 August 2016 Accepted 11 January 2017

Twin domain imaging in topological insulator Bi2Te3 and Bi2Se3 epitaxial thin films by scanning X-ray nanobeam microscopy and electron backscatter diffraction Dominik Kriegner,a* Petr Harcuba,a Jozef Vesely´,a Andreas Lesnik,b Guenther Bauer,c Gunther Springholzc and Va´clav Holy´a a

Edited by G. Renaud, CEA-Grenoble DSM/ INAC/SP2M/NRS, Grenoble, France Keywords: twinning; electron backscatter diffraction; scanning X-ray diffraction; topological insulators.

Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 121 16 Prague 2, Czech Republic, bInstitut fu¨r Experimentelle Physik, Otto-von-Guericke Universita¨t Magdeburg, Universita¨tsplatz 2, 39106 Magdeburg, Germany, and c Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz, Altenbergerstrasse 69, 4040 Linz, Austria. *Correspondence e-mail: [email protected]

The twin distribution in topological insulators Bi2Te3 and Bi2Se3 was imaged by electron backscatter diffraction (EBSD) and scanning X-ray diffraction microscopy (SXRM). The crystal orientation at the surface, determined by EBSD, is correlated with the surface topography, which shows triangular pyramidal features with edges oriented in two different orientations rotated in the surface plane by 60 . The bulk crystal orientation is mapped out using SXRM by measuring the diffracted X-ray intensity of an asymmetric Bragg peak using a nano-focused X-ray beam scanned over the sample. By comparing bulkand surface-sensitive measurements of the same area, buried twin domains not visible on the surface are identified. The lateral twin domain size is found to increase with the film thickness.

1. Introduction Three-dimensional topological insulators represent a new state of matter with bulk band gap and Dirac cone-like surface states (Zhang et al., 2009). Bi2X3 with X = Se and Te are prime members of this material class and were shown to exhibit the predicted exotic topological properties (Chen et al., 2009). For electrical devices, large-area high-quality thin films are required. However, epitaxial films as well as bulk crystals generally suffer from a large number of structural defects. The most common defects are twin boundaries that arise from the rhombohedral structure of Bi2X3, which consists of three X– Bi–X–Bi–X quintuplet layers separated by a van der Waals gap. The stacking of the (000.1)1 basal planes can be described by a face-centred cubic (f.c.c.)-like AbCaBC . . . stacking as shown in Fig. 1(a), where lower- and upper-case letters correspond to Bi and X atomic layers, respectively. Because of the weakly bonded van der Waals gap, the Bi chalcogenides are highly prone to twin formation; a twinned domain (TD) exhibits the inverted AcBaCB . . . stacking and can be described as being rotated by 60 around the [000.1] c axis of the crystal with respect to that of the ordinary domain (OD). The boundary dividing the OD and TD, i.e. the twin boundary, is the most common planar defect in these materials and it can be parallel or inclined to the sample surface. Twin boundaries perpendicular to the c axis, which is the usual growth direction, are particularly common since the change of the stacking can 1

We use Miller indices (hkl) and Bravais indices (hki.l) with i ¼ h  k to denote the rhombohedral structure.

J. Appl. Cryst. (2017). 50, 369–377

https://doi.org/10.1107/S1600576717000565

369

research papers easily occur in the weakly bound van der Waals gap formed by the X–X double layers, as shown in Fig. 1(a). However, in thin films twin boundaries inclined to the c axis also occur owing to the simultaneous nucleation of two different domains on lattice type mismatched substrate materials (Schreyeck et al., 2013; Tarakina et al., 2014). Since it is the (000.1) surface of Bi2X3 that features the topological surface states, the impact of such defects depends on their orientation. For example, (000.1) twin planes parallel to the surface were shown to influence the electronic properties of the surface state when they are in close vicinity to the surface (Aramberri et al., 2015). On the other hand, little is known about the impact of the lateral twins with inclined twin boundaries which cross the surface and therefore might perturb the surface state as well. Up to now, only a few studies have dealt with the real structure and defects in epitaxial Bi2X3 films, which will obviously depend strongly on the substrate material and growth conditions. Richardella et al. (2015) studied crystal defects in epitaxial layers of (Bi,Sb)2(Te,Se)3 on InP(111)B and SrTiO3 substrates using atomic force microscopy (AFM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). These authors detected the presence of twin domains and interpreted the broadening of the XRD maxima as lattice mosaicity; they concluded that lattice mismatch between the substrate and the layer is the main factor determining the quality of the epitaxial growth and consequently the presence of twin domains. However, no information on the domain size and their arrangement was obtained. Reduced twin formation in Bi2Se3 was reported by Tarakina et al. (2014) by deposition on artificially roughened InP(111)B substrates. Another strategy for the twin suppression was chosen by Kampmeier et al. (2015), where Bi2Te3 layers were deposited by van der Waals epitaxy on Te-passivated Si(111) substrates using an extremely slow growth rate of 2.7 nm h1. The

deposited layer followed the threefold symmetry of the substrate surface and no twin domains were detected. For vicinal InP(111)A substrates Guo et al. (2013) have found that the miscut steps play a stabilizing role in the step-flow growth mode which suppresses the twin domains as well. In the mentioned work the existence of twin domains was detected by XRD pole-figure measurements and microscopically observed by cross-sectional high-resolution scanning transmission electron microscopy. However, no information on the sizes of twin domains and their vertical and lateral arrangement in the layer could be derived. The aim of this study is to address and clarify this issue by real-space imaging of individual twin domains by electron backscatter diffraction (EBSD), TEM and scanning X-ray nanobeam diffraction microscopy (SXRM). The latter is based on reciprocal-space mapping using a highly focused primary X-ray beam with a footprint of about 200 nm, which is laterally scanned over the sample, as has been previously used for investigation of various thin layers and nanostructures (see, for instance, Stangl et al., 2009; Hrauda et al., 2011; Chahine et al., 2014, 2015; Scha¨fer et al., 2016). Here, we show that EBSD and SXRM are able to directly determine the local crystalline orientation with high spatial resolution. With EBSD, however, only the material in close vicinity to the surface is probed. The EBSD measurements are therefore correlated with the surface topography, whereas SXRM yields information from the whole film thickness. Combining the results with crosssectional TEM investigations we obtain a complete picture of the twin domain/defect structure of both topological insulator Bi2Se3 and Bi2Te3 films. The paper is structured as follows: after description of the sample growth and the methods employed, we present our results for Bi2Te3 films and describe the evolution of twinning in Bi2Se3 as a function of the layer thickness. Our results reveal for the first time the twin distribution in such films and demonstrate that EBSD and SXRM are key tools for characterization and optimization of van der Waals bonded topological crystalline insulator heterostructures.

2. Experimental 2.1. Sample growth and characterization

Figure 1 Sketch of the atomic structure of Bi2X3 (X = Se, Te). Panel (a) shows the stacking of the basal planes for the two twin modifications. A unit cell consists of 15 atomic planes which are grouped into three quintuples. A dashed line marks a horizontal twin plane which separates twins 1 and 2 and marks the moment of a change of the stacking order, which is indicated. Panel (b) shows the top view of the two twin modifications. The unit-cell parameters c and a are indicated in panels (a) and (b), respectively.

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Thin film samples of Bi2X3 (X = Se, Te) were grown using molecular beam epitaxy on cleaved BaF2(111) substrates from compound BiSe/BiTe and elemental Se and Te beam flux sources. By adjusting the Se and Te fluxes (Caha et al., 2013), the growth of the pure Bi2X3 phase is achieved with lattice parameters of a = 0.414, c = 2.865 nm and a = 0.438, c = 3.050 nm for Bi2Se3 and Bi2Te3, respectively. The lattice mismatch between the (111) BaF2 surface and the (000.1) c planes of Bi2X3 is 5.5% and

Twin domain imaging in topological insulator Bi2Te3 and Bi2Se3 epitaxial thin films by scanning X-ray nanobeam microscopy and electron backscatter diffraction.

The twin distribution in topological insulators Bi2Te3 and Bi2Se3 was imaged by electron backscatter diffraction (EBSD) and scanning X-ray diffraction...
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