Comparative Study of the Microstructure of Bi2Se3 Thin Films Grown

Mar 9, 2012 - Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have been carried out to clarify the microstructure of Bi2...
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Comparative Study of the Microstructure of Bi2Se3 Thin Films Grown on Si(111) and InP(111) Substrates N. V. Tarakina,*,† S. Schreyeck,† T. Borzenko,† C. Schumacher,† G. Karczewski,‡,§ K. Brunner,† C. Gould,† H. Buhmann,† and L. W. Molenkamp† †

Experimentelle Physik III, Physikalisches Institut and Wilhelm Conrad Röntgen-Research Centre for Complex Material Systems, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany ‡ Institute of Physics, Polish Academy of Sciences, Al. Lotnikov 32/46, 02-668 Warsaw, Poland § Humboldt Visiting Professor at Experimentelle Physik III, Physikalisches Institut and Wilhelm Conrad Röntgen-Research Centre for Complex Material Systems, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany ABSTRACT: Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have been carried out to clarify the microstructure of Bi2Se3 thin films grown by molecular beam epitaxy (MBE) on Si(111) and InP(111) substrates. The film grown on InP displays much better overall quality at the microstructural level than does the film grown on the Si substrate. A layer of poor crystalline quality at the interface followed by well-crystallized Bi2Se3 has been observed for both substrates. The thickness of this interface layer is not uniform; it varies across the sample from zero, showing a sharp interface between the substrate and Bi2Se3, up to ∼1 nm and ∼1.8 nm for Bi2Se3/InP and Bi2Se3/Si, respectively. Formation of rotation twin domains and lamellar twins has been observed and is described in detail for both substrates.

1. INTRODUCTION Over the years, Bi2Se3 has been studied in the context of different potential applications: as a material for thermoelectric,1−4 photoelectrochemical,5 and optical recording6 devices. The recent prediction of the existence of threedimensional (3D) topological insulators7,8 has stimulated investigations of Bi2Se3 with special interest in the direct incorporation of this compound into semiconductor technology by growing thin films on nonconducting substrates. The possibility to grow Bi2Se3 epitaxial thin films on different substrates such as Si(111),9−12 Si(111) with GaSe, In2Se3, ZnSe buffers,12,13 SrTiO3(111),14,15 Al2O3(110),16 GaN(0001),12 GaAs(111)B,17 CdS,18 and double-layer graphene grown on SiC(0001)19 has been extensively investigated within the past few years. Till now, preparing high-quality epitaxial Bi2Se3 thin films is, as shown by several recent studies,9,10 not easy to realize. Different growth parameters have to be taken into account together with crystallographic features of the thin film and the substrate. The crystal structure of Bi2Se3 can be described as a cubic close-packing of Bi and Se atoms (Figure 1). Layers formed by Bi and Se are stacked along the c-direction in five layer packets Se−Bi−Se−Bi−Se, called quintuple layers (QL), which connect to each other by weak van der Waals bonds.20 In a Bi2Se3 thin film, the first QL forms van der Waals bonds to the substrate as well, resulting in a special case of heteroepitaxy known as van der Waals epitaxy.9,12,21−23 For our study, two different substrates with hexagonal surfaces, Si(111) and InP(111), were chosen. The former is widely used in the semiconductor industry and has been applied as a substrate for © 2012 American Chemical Society

Figure 1. Crystal structure of Bi2Se3. Projections along the (a) [100] and (b) [001] directions. Bi and Se atoms are represented by red and blue spheres, respectively.

the growth of Bi2Se3 before;9−12 the use of the latter reduces the lattice mismatch to ∼0.2% (cfr. ∼7.8% for Si(111)), which is expected to result in higher-quality films. A deep understanding of growth aspects specific to van der Waals heteroepitaxy, possible origins of defects, details of the Received: December 10, 2011 Revised: February 7, 2012 Published: March 9, 2012 1913

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microstructure of the thin film, and the quality of the interface are essential to realize a controlled growth process. So far, detailed microstructural studies using transmission electron microscopy (TEM) techniques were performed only for powders24,25 and nanostructures.3,26,27 Existing reports on heteroepitaxial Bi2Se3 thin films focus on the TEM crosssectional study of the quality of the interface and do not go deeply into the structural details of the obtained Bi2Se3 films.10,11,17,18 Here, we present a TEM study providing detailed insights into the microstructure of Bi2Se3 thin films, aimed to help to understand critical issues in sample preparation for optimizing growth conditions.

2. EXPERIMENTAL SECTION Bi2Se3 thin films were grown by molecular beam epitaxy (MBE) in an ultrahigh vacuum MBE chamber with high purity Bi (99.9999%) and Se (99.9999%) source fluxes provided by standard effusion cells. For Bi2Se3 grown on a Si(111) substrate, Bi was deposited on a Si(111)-(7 × 7) surface at 200 °C and then heated up to 400 °C to obtain a Si(111)-(√3 × √3)-Bi surface similar to the procedure described in ref 8. Half of the sample was covered with an amorphous Se capping layer about 300 nm thick, at room temperature, in order to protect the interface from possible oxidation, as in ref 11. The subsequent growth was carried out at 300 °C. The growth of Bi2Se3 on InP(111)B has been realized using a two-temperature growth procedure on a substrate previously etched with HF acid. First, the substrate was heated up to 250 °C and about two quintuple layers of Bi2Se3 were deposited. Then the sample was heated up to 300 °C in Se atmosphere and annealed for 1 h. The subsequent growth was performed at 300 °C, the same temperature used for the Si(111) substrate. Scanning electron microscopy (SEM) experiments have been performed using a FEI Helios Nanolab Dual Beam system. TEM cross-sectional specimens have been prepared at the same facility by focused Ga+ ion beam milling. For both specimens, the wedge cut technique followed by an in situ welding lift-out and thinning was applied. Thinning using a 30 kV ion beam and currents down to 28 pA has been done from both sides of the specimen at a 1.5° incidence angle until it became electron transparent (thickness ∼100 nm). The final low-kV cleaning has been performed at 5 kV, 16 pA, and a 1° incidence angle. The TEM study has been carried out using a FEI Titan 80-300 (S)TEM operated at 300 kV and equipped with an EDAX energy dispersive X-ray (EDX) microanalysis detector. EDX spectra from thin films on wafers were also collected at incident beam energy of 10 keV using an X-max 50 mm2 SDD detector installed at a Gemini Zeiss scanning electron microscope.

Figure 2. SEM images of the surface of epitaxial Bi2Se3 films grown on a (A) Si(111) substrate, (B) InP(111) substrate. The small white dots at the surface are small contaminations introduced during cleavage and transportation of the samples from the MBE chamber. (C) Crosssectional HAADF-STEM image of the top layers of Bi2Se3 grown on Si. The sample has been covered with a Se layer of about 100 nm thick, so that the surface of the specimen is protected during sample preparation. The remains of an amorphous Se layer are visible at the top of the film.

Table 1. Average Size and Height of the Triangular Islands Estimated from SEM and TEM Imagesa substrate Si(111) InP(111)

SEM island size, nm island height, nm 294 ± 74 757 ± 171

4.6 ± 0.3 3.6 ± 0.9

TEM island size, nm 212 ± 74 474 ± 236

a

The island size has been estimated in the case of SEM images from measurements of the edge of about 60 triangular islands at the top surface. For the TEM data the exact position of the cross-section within the triangular islands can not be defined, which leads to values of the island size systematically lower than the values obtained from the SEM data.

Figure 2c, clearly shows that each surface step corresponds to one quintuple layer. The elemental composition of the Bi2Se3 films was determined from a quantitative analysis of EDX spectra collected at 10 different points for each film. The obtained average compositions show good agreement with the nominal element ratio and are Bi2.05(3)Se2.95(3) and Bi2.03(2)Se2.97(2) for the films on Si and InP substrates, respectively. Cross-section electron diffraction (ED) patterns of Bi2Se3/Si and Bi2Se3/InP specimens along [100]*Bi2Se3 are shown in Figure 3. The ED patterns are a superposition of the patterns from substrate and film (Figure 3a,d,e). Bi2Se3 reflections were indexed in a trigonal unit cell with parameters a = 4.2 Å, c = 28.7 Å. From the sharp spots on the selected-area electron diffraction (SAED) patterns, we confirm the epitaxial growth of Bi2Se3 on both substrates with the following crystallographic relations: normal to the growth plane [001]Bi2Se3∥[111]substrate, in-plane [100]Bi2Se3∥[11̅0]substrate or [110]Bi2Se3∥[11̅0]substrate (Figure 3). Often, SAED patterns of both thin films display additional spots coming from twinned domains which can be attributed to the formation of both rotation twin domains (with rotation 180° around the [001] axis) and lamellar twins (twin boundary parallel to the substrate). The indexing of the spots corresponding to two different domains is shown on the

3. RESULTS AND DISCUSSION High-resolution SEM surface images are shown in Figure 2a,b. The surfaces of both Bi2Se3 thin films display coalesced islands of triangular shape, oriented with respect to each other consistent with the 3-fold symmetry of Bi2Se3.20 Triangles of opposite orientation (180° in-plane rotation domains with crystallographic relations [110] Bi 2 Se 3 ∥[11̅ 0 ] substrate and [100]Bi2Se3∥[11̅0]substrate) can be observed on both films, resulting in the formation of twins and twin boundaries; the latter are indicated by arrows in Figure 2. The average edge length and height of the triangular islands were estimated from SEM images (the edges of about 60 triangular islands have been measured) and were found to be 757 ± 171 nm, 3.6 ± 0.9 nm, and 294 ± 74 nm, 4.6 ± 0.3 nm for Bi2Se3 on InP and Si, respectively (Table 1). The film grown on the InP(111) substrate thus displays flatter islands about twice as large as the film on Si(111). The cross-sectional STEM image, presented in 1914

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Figure 3. Selected area electron diffraction patterns taken along [100]Bi2Se3: (a) Bi2Se3 on InP(111), blue indices correspond to InP reflections, (b) a twin boundary region observed in Bi2Se3 grown on InP(111), white and red indices correspond to A and B twins, respectively, (c) 180° rotation or lamellar twins in Bi2Se3 (calculated), (d) Bi2Se3 on Si(111), blue indices correspond to Si reflections, (e) enlargement from (d) with reflections of two twins marked by white and red indices, (f) two twins of Bi2Se3 grown on Si(111), tilted with respect to each other.

experimental and simulated ED patterns (Figure 3). In the case of the growth of Bi2Se3 on Si(111) the 00l rows of spots are split into two components; the magnitude of the splitting increases with increasing distance from the direct beam. This splitting indicates a tilt (about 1°) of the rotation twins with respect to each other in the bc plane. The distribution of lamellar and rotation twins in the film is clearly seen from the dark field image obtained from the 012 reflection of Bi2Se3 grown on InP(111), Figure 4. The size of

at the surface was observed (Figure 5). We point out that the thickness of the observed layer is not uniform across the sample, but varies from almost zero (showing a sharp interface between substrate and Bi2Se3) up to ∼1 nm and ∼1.8 nm for Bi2Se3/InP and Bi2Se3/Si, respectively. In spite of the much weaker contrast, one can still see planes of Bi atoms in this layer on the HAADF-STEM images. These facts together allow us to propose that the first layer does not cover the surface continuously but most probably is composed of nucleation islands (of one QL height) which are immediately followed by a second Bi2Se3 QL of good quality. This second QL has a wellpronounced Z-contrast and a clearly resolved periodic structure on HAADF-STEM images (Figure 5). Such a formation of an interface layer has been reported previously for the growth of Bi2Se3 on Si(111).9−11 There is no agreement in the literature about the character of this layer: Li et al. reported the formation of an about 2 QLs thick amorphous Bi2Se3 layer at the interface;9 in the work of He et al.10 a ∼1.7 nm thick SiSe2 layer was found at the interface by EDX analysis; Bansal et al.11 observed the formation of a ∼4 nm SiO2 amorphous layer at the interface for a sample which was stored at ambient pressure for one month and also reported that a 300 nm thick Se capping layer can prevent such oxidation. In our case, the EDX analysis performed in the transmission electron microscope for both samples reveals neither the presence of O nor an increase of the Se or Bi concentrations at the interface. There were no Ga peaks observed in the spectra, indicating the absence of Ga+ ion implantation at the sidewalls of the specimen during focused ion beam (FIB) sample preparation. The quality of the interface is the same for the 110 nm thick Bi2Se3 thin film on Si with and without Se capping layer. In combination with the data from HAADF-STEM and EDX spectra, we can conclude that the layers indeed show van der Waals epitaxy of Bi2Se3 on

Figure 4. (a) Bright-field image of Bi2Se3 grown on a Si(111) substrate, (b) dark-field image obtained from the 012 reflection of Bi2Se3 grown on a InP(111) substrate. Grain boundaries are marked by white arrows.

the islands observed at the surface is in reasonable agreement with the size of the domains observed on cross-sectional bright and dark field images taken at low magnifications (Table 1, Figure 4). Systematically lower values of the island size measured from TEM images arise due to the impossibility of defining the exact position of the cross-section within the triangular islands. From cross-sectional high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images of the interfaces, the presence of a layer of poor crystalline quality 1915

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Figure 5. Cross-sectional HAADF-STEM images of Bi2Se3 grown on Si(111) (a, b) and InP(111) (c, d), showing the presence of a layer of poor crystalline quality at the interface. Well-crystallized parts, marked by black arrows, alternate with almost completely amorphous areas.

Figure 6. HAADF-STEM cross-sectional image of Bi2Se3 grown on InP(111): (a) overview image of the film, (b) enlargement from the middle part of the film with rotation twin boundary, arrows indicate the presence of dislocations, (c) enlargement from the interface region at the start of the twin boundary, (d) enlargement from the middle part of the film with lamellar twin boundary. White borders indicate unit cell of Bi2Se3 (e−g) structural models for the lamellar twin with twin boundaries at the position of Se(1), Bi, and Se(2) atoms, positions marked by arrows, (h) structural model for the rotation twin boundary. Bi atoms are represented by red spheres and Se atoms by blue spheres.

domains in the bc plane of about 1° relative to each other, Figure 3f). This tilting is most likely the result of the lattice mismatch between substrate and film (∼7.8%). Probably due to this tilt the interface layer is much broader than is the case for InP(111), where a tilt of the film with respect to the substrate has not been observed (probably due to the almost perfect lattice match, the mismatch is about 0.2%). The first few QLs

both Si and InP, but with a first layer of poor crystalline quality at the interface, which consists of alternate well-crystallized parts and almost completely amorphous areas. In addition, in the case of Bi2Se3 on Si(111) it has been found that in some regions the layers are not exactly parallel to the substrate. This is observed in HAADF-STEM images of the interface (Figure 5) and in diffraction patterns (tilt of twin 1916

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Figure 7. HAADF-STEM cross-sectional image of Bi2Se3 grown on Si(111): (a) overview image of the film at the region of a rotation twin boundary; left twin is parallel to the substrate, right twin is slightly tilted with respect to the substrate, (b) enlargement from the upper part of the rotation twin boundary, (c) enlargement from the middle part of the film with dislocation and lamellar twin boundary marked.

be slightly twisted in the ab plane. This small misorientation as well as the slight tilt of layers with respect to the substrate surface, described before, most probably helps to reduce the high interfacial energy which appears due to the lattice mismatch. As a result, the first few QLs and the rotation twin boundaries have to accommodate more dislocations for the film relaxation than observed in the case of Bi2Se3 on InP (Figures 6 and 7). Since for both Si(111) and InP(111) substrates, irrespective of the lattice mismatch, twins have been observed, we conclude that the formation of twins should be considered more as an intrinsic crystal feature rather than be directly attributed to the choice of substrate or growth conditions. By intrinsic crystal features, we mean atomic interaction energies within and between QLs, which are of essential importance for the understanding of the crystal growth mechanisms in both bulk and thin films. Thus, the conclusion made above is more relevant for lamellar twins, which can be directly associated with crystal structure features and are comparable to planar twins in bulk compounds since they do not depend on the presence of a substrate. Rotation twins are more sensitive to the growth conditions, since their formation will strongly depend on the number of nucleation points, which can be controlled by different parameters such as growth temperature, preparation of the substrate surface, and substrate miscut.

contain more dislocations than the upper layers of the film, which make them slightly “wavy”, but in total the film displays high crystalline quality and less defects compared to the film on the Si(111) substrate. The crystal quality of both films increases with the distance from the interface. Since both Bi2Se3 films display the formation of twins, we studied this aspect in more detail. In Figure 6 an overview HAADF-STEM image of Bi2Se3/InP with enlarged regions at the interface and at the upper layers of the twin domains is shown. The formation of rotation twins in Bi2Se3 can be explained as follows: after the first atomic layer of Se, a Bi layer can grow in two equally possible cubic close-packed atomic configurations in two separate regions. These regions are related to each other by a 180° rotation around the [001] axis, and form a twin boundary perpendicular to the growth plane when they meet. The observed rotation twins do not display sharp boundaries. Lamellar twins, the other observed type of twins, form a twinned domain boundary parallel to the growth plane. Such twins have aligned close-packed planes and display a 180° rotation around the c-axis with respect to each other. Since the Bi2Se3 crystal structure has three nonequivalent atomic positions, three possibilities to form a lamellar twin exist. To be consistent with previously published results on the crystal structure of Bi2Se3, we use Greek and Latin letters to denote the Se and Bi layers in a cubic close-packed atomic arrangement and to distinguish between different possibilities to form a lamellar twin in the structure (Figure 6e−h). A comparison of the proposed models with the experimentally observed lamellar twins shows the formation of twins at the upper Se layer in the QL, which connects to the following layer only by van der Waals interactions (Figure 6d). The formation of such a twin is probably most plausible from an energetic point of view. The same type of planar twins has been observed experimentally and predicted by ab initio calculations for bulk Bi2Te3.28 We cannot directly compare results obtained for Bi2Te3 bulk crystals with Bi2Se3 thin films, but since these compounds are isostructural, we assume that similar twins as in Bi2Te3 should be observed both experimentally and theoretically in bulk bismuth selenide as well. In general, the formation of twins in Bi2Se3 grown on Si(111) goes via the same mechanism and has the same structural origin as for Bi2Se3/InP(111). However, it can be noticed from HAADF-STEM images of Bi2Se3/Si (Figure 7) that in comparison with the film on InP the left and the right twins cannot be equally resolved at the same time; a slight tilt of ∼1° perpendicular to the grain boundary is required to bring the left twin perfectly oriented along the [100] direction. We propose that in the case of Bi2Se3 grown on Si(111) twinned grains can

4. CONCLUSIONS The microstructure of Bi2Se3 thin films grown by MBE on Si(111) and InP(111) substrates has been studied in detail using TEM, STEM, and SEM techniques. In both cases, the films display high-crystal quality. For both substrates, the tendency to form rotation twin domains and lamellar twins has been observed. The twins were characterized using crosssectional TEM and SEM. The size of the domains of Bi2Se3 grown on InP (about ∼700 nm) was found to be about twice as large than on Si. It has been shown that for both films there is a layer of poor crystalline quality at the interface followed by well-crystallized Bi2Se3 layers. The thickness of this layer varies throughout the sample. It ranges from zero (sharp interface between substrate and Bi2Se3) to ∼1 nm and ∼1.8 nm for Bi2Se3/InP and Bi2Se3/Si, respectively. Neither the presence of oxygen nor an increase in Se or Bi concentration was detected at the interface. We believe that optimizing the growth start and the MBE parameters can help to improve the poor crystallinity of the starting layer and to reduce the quantity of rotation twins; in addition the latter can be suppressed by using vicinal substrates as was done in ref 9 or substrates which do not exhibit plane mirror symmetry. Overall, for very similar growth 1917

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J. F.; Zou, J.; Wang, K. L. Appl. Phys. Lett. 2011, 98, 242102-1− 242102-3. (19) Song, C.-L.; Wang, Y.-L.; Jiang, Y.-P.; Zhang, Y.; Chang, C.-Z.; Wang, L.; He, K.; Chen, X.; Jia, J.-F.; Wang, Y.; Fang, Z.; Dai, X.; Xie, C.-X.; Qi, X.-L.; Zhang, S.-Z.; Xue, Q.-K.; Ma, X. Appl. Phys. Lett. 2010, 97, 143118−1−143118−3. (20) Nakajima, S. J. Phys. Chem. Solids 1963, 24, 479−485. (21) Koma, A. J. Cryst. Growth 1999, 201−202, 236−241. (22) Jaegermann, W.; Rudolph, R.; Klein, A.; Pettenkofer, C. Thin Solid Films 2000, 380, 276−281. (23) Koma, A. Thin Solid Films 1992, 216, 72−76. (24) Frangis, N.; Kuypers, S.; Manolikas, C.; Van Tendeloo, G.; Van Landuyt, J.; Amelinckx, S. J. Solid State Chem. 1990, 84, 314−334. (25) Frangis, N.; Kuypers, S.; Manolikas, C.; Van Landuyt, J.; Amelinckx, S. Solid State Commun. 1989, 69, 817−819. (26) Qui, X.; Burda, C.; Fu, R.; Pu, L.; Chen, H.; Zhu, J. J. Am. Chem. Soc. 2004, 126, 16276−16277. (27) Dang, W.; Peng, H.; Li, H.; Wang, P.; Liu, Z. Nano Lett. 2010, 10, 2870−2876. (28) Medlin, D. L.; Ramasse, Q. M.; Spataru, C. D.; Yang, N. Y. C. J. Appl. Phys. 2010, 108, 043517-1−043517-6.

conditions, the thin film obtained on InP displays much better quality at the microstructural level than the one grown on the Si substrate. The main reason for this is the almost perfect lattice match between the hexagonal InP(111) surface and Bi2Se3. A bigger misfit results in quasi-van der Waals epitaxy at the early stage of the growth, which considerably lowers the quality of the film. A significant improvement of the Bi2Se3 thin film quality has also been observed in the case of other substrates with a small mismatch in lattice parameters with bismuth selenide, such as GaAs(111)B17 and CdS.18

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been funded by the EU ERC-AG Program (project 3-TOP), and by the Alexander von Humboldt Foundation. N.V.T. acknowledges funding by the Bavarian Ministry of Sciences, Research and the Arts.

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REFERENCES

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dx.doi.org/10.1021/cg201636g | Cryst. Growth Des. 2012, 12, 1913−1918