Mode of Growth of Ultrathin Topological Insulator ... - ACS Publications

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Mode of Growth of Ultrathin Topological Insulator Bi2Te3 Films on Si (111) Substrates Svetlana Borisova,*,† Julian Krumrain,† Martina Luysberg,‡ Gregor Mussler,† and Detlev Grützmacher† †

Peter Grünberg Institute −9, Forschungszentrum Jülich, D-52425, Jülich, Germany; JARA, Fundamentals of Future Information Technologies ‡ Peter Grünberg Institute −5, Forschungszentrum Jülich, D-52425, Jülich, Germany; Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich, D-52425, Jülich, Germany S Supporting Information *

ABSTRACT: Layered materials such as graphene, bi-, and multilayer graphene as well as various compounds of topological insulators are currently in the focus of interest due to their extraordinary physical properties related to Dirac surface states. The ability to grow thin films of these complex layered materials is the key to explore their fundamental phenomena giving insights into modern solid-state physics. However, complex materials composed of layers only weakly bonded via van der Waals forces offer unmatched challenges for the deposition of thin epitaxial films. Here, we report on the growth of Bi2Te3 ultrathin films on Si (111) substrates using molecular beam epitaxy. Special emphasis is put on the nucleation phenomena and growth dynamics studied in detail by in situ scanning tunnelling microscopy and high-resolution scanning transmission electron microscopy. The morphology of the Bi2Te3 surface and the structure of the Si(111)/Bi2Te3 interface as well as the formation of threading dislocations and crystal domains are studied at the atomic level. Our data indicate that the film is formed via the nucleation of islands, which float on the substrate; thus, the islands are only weakly bonded to the substrate and rather mobile. Apparently, these floating islands are able to arrange themselves by moving in the x−y direction to perfectly coalesce and form a continuous film. The results present a crucial step toward understanding growth and defect formation in this class of materials and thus pave the avenue to a higher control over both their structural and electronic properties, in order to study the electronic properties of the Dirac surface states.

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INTRODUCTION

temperature spintronics applications are offered. Additionally, these materials are of great interest to explore topological phenomena, such as the image monopole effect and Majorana fermions.3 Whereas for two-dimensional (2D) topological insulators, with one-dimensional (1D) edge states, the predicted4 spin-polarized currents could be unambiguously demonstrated by the observation of the quantum spin Hall

The prediction of exceptional properties of surface states of three-dimensional (3D) topological insulators such as Bi2Te3, Bi2Se3, Sb2Te3, and their alloyed compounds1 has inspired a whole new field of activity to explore the properties of this new state of matter. These materials have a band gap like ordinary insulators in the bulk and a single Dirac cone on the surface, that is, a state with a linear energy dispersion. The strong spin− orbit coupling leads to spin-polarized states, which are protected toward spin flipping scattering events by time reversal symmetry.1,2 Hence, new paths for future room© 2012 American Chemical Society

Received: August 27, 2012 Revised: October 4, 2012 Published: October 24, 2012 6098

dx.doi.org/10.1021/cg301236s | Cryst. Growth Des. 2012, 12, 6098−6103

Crystal Growth & Design

Article

effect,5 the characterization of transport properties of the surface states in 3D topological insulators is hampered by the vast amount of free carriers in the bulk of these materials. Most of the studies on Bi2Te3 and Bi2Se3 are carried out on cleaved surfaces of 3D crystals prepared by zone melting. The large carrier density observed in these crystals is attributed to structural defects, such as missing atoms, antisite defects, dislocations, and stacking faults.6 However, the presence of Dirac states could be confirmed using surface sensitive techniques such as angle resolved photoemission spectroscopy (ARPES).7 Recently, the spin-polarized nature of these states was reported using this technique.8 Magnetotransport measurements of Bi2Se3 nanoribbons confirm the presence of a 2D electron gas at the surface states.9 Thin film technology may provide layers with a lower structural defect density, and additionally, back side gates may be used to deplete the carriers in the films of topological insulators. Topological insulators Bi2Te3, Bi2Se3, and similar materials consist of quintuple layers (QL), which are only weakly bonded among each other; thus, they are suitable candidates for van der Waals epitaxy,10 which provides completely relaxed films even for a large lattice mismatch between the film and the substrate in contrast to the common epitaxy. At the same time, the films grown in this mode are aligned to the substrate. Indeed, single crystalline films of different layered materials have been obtained on substrates despite a large lattice mismatch11,12 and different crystal structure.13−15 Epitaxial growth of Bi2Te3 and Bi2Se3 films via van der Waals epitaxy has been successfully employed if the substrate surface does not possess any dangling bonds and has the same rotational symmetry as the film. Recently, different substrates, such as Si (111),16,17 InP (111),17 epitaxial graphene on SiC (0001),18 or exfoliated graphene on SiO2/Si,19 have been used to study the growth of thin Bi2Se3 films. All these papers report the observation of the same characteristic growth with spiral cores pinned at the substrate steps,18 resulting in triangular pyramids of two twin domains.16,17 Moreover, frequently the presence of an amorphous Bi2Se3 layer at the Si (111)/ Bi2Se3 interface is found.16,17 First reports on the growth of Bi2Te3 by molecular beam epitaxy (MBE) on Si (111) surfaces indicate continuous films aligned to the Si substrate.20,21 Surface analysis by atomic force microscopy and scanning tunneling microscopy (STM) of reasonably thick films (∼100 nm) shows that these are free of the triangular spirals and they apparently grow in a 2D layer-by-layer mode.20,21 ARPES proves Dirac surface states in these films.20,22 Here, we focus on the mode of growth of Bi2Te3 starting from the nucleation on the Si (111) surface to the formation of continuous films providing large atomically flat terraces. Special emphasis is put on the nucleation phenomena and the atomic structure of the Si (111)/ Bi2Te3 interface. In this study, ultrathin Bi2Te3 films on Si (111) were prepared by MBE. In situ STM is used to analyze the nucleation and growth dynamics at a very early stage of growth. Additionally, high-resolution scanning transmission electron microscopy (STEM) has been employed to gather insights of the interface structure between film and substrate and of structural defects within the film with atomic resolution. The challenge of the epitaxial growth of thin films of Bi2Te3 is the complex lattice structure consisting of three QLs bound only weakly to each other via van der Waals forces. Figure 1 sketches 1 unit cell covering 15 atomic layers in the z direction, arranged in 3 units, QLs of 1 nm width. Upon nucleation of an aligned to

Figure 1. Crystal structure of Bi2Te3. Spatial view and (0001) projection of a Bi2Te3 unit cell. The unit cell consists of three QLs, which are bonded by weak van der Waals forces. A QL consists of five strongly bound alternating hexagonal Bi (violet balls) and Te (green balls) atomic sheets, shifted relative to each other.

the substrate Bi2Te3 film, the layer can start to grow with each of those three quintuple layers, and for each of them, two stacking orders, normal and reverse, exist. Thus, in principle, six different kinds of islands separated into two groups: three with normal and three with reverse stacking order are able to nucleate on a surface. Within the groups, the islands differ only by a linear displacement on the surface, as indicated in Figure 1, whereas the two groups can be transformed by a 60° rotation on the surface into each other. In the case of the growth on Si (111) surfaces, the Bi2Te3 has a large lattice mismatch of 14% on the Si surface; thus, considering a relaxed single crystal Bi2Te3 film, the Bi2Te3 atoms can be found at any position with respect to the Si substrate. This immediately raises the question about the atomic structure at the interface between two islands after island coalescence. Moreover, for successful epitaxy of Bi2Te3 films, it has to be considered that the Si (111) surface has in its unreconstructed state dangling bonds, whereas the Bi2Te3 film consists of weakly bonded quintuple layers with the preference to attach via van der Waals bonds to the surface. Nevertheless, epitaxy of Bi2Te3 on Si (111) is successfully obtained. The latter implies that the surface of the Si (111) substrate has to be converted into one suitable for van der Waals epitaxy at an early stage of the process.

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EXPERIMENTAL SECTION

Four inch n-type doped (3−6 Ohm·cm) Si (111) wafers were chemically cleaned in piranha (H2SO4/H2O2) solution in an ultrasonic bath. Subsequently, the substrates were dipped in 5% hydrofluoric acid resulting in the surface passivation by H atoms. Next, the substrates were placed in the UHV MBE chamber and annealed at 750 °C for 10 min to desorb the H atoms. Atomic Bi and Te were evaporated using standard effusion cells at temperatures TBi = 500 °C and TTe = 340 °C, which corresponds to a Te/Bi partial pressure ratio of 20. The epitaxial growth takes place in a single-step process at a base pressure of 1 × 10−9 mbar and a substrate temperature TS = 550 °C. The growth sequence is started by opening the Te shutter 2 s prior to the Bi shutter. The thickness of the deposited films is controlled by the variation of the growth time. The growth rate of the Bi2Te3 films is 0.14 nm/min. Directly after deposition, the samples were transferred to the in situ room-temperature STM chamber (base pressure better than 1 × 10−10 mbar) without breaking the vacuum. Then, the STM was placed on vibration isolation supports, and STM measurements were performed. Tunneling parameters are in a wide range (|Utun| = 500 mV...4 V, Itun = 40...290 pA), which is attributed to changes in band 6099



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Figure 2. Growth dynamics of Bi2Te3 films by in situ STM. In situ STM images of (0001) Bi2Te3 growth surfaces for film thicknesses (a) 30 nm, (b) 13 nm, (c) 4 nm, and (d) 1 nm. Sub-QL islands are depicted with white arrows; dislocation cores are depicted with black arrows; (e, f) height profiles along the lines in panels a and c, respectively. The profiles indicate the one QLs steps typically seen at the thicker films (the red line), slight layer bending between a pair of dislocations (black line in e), and sub-QL steps at the 4 nm thick film. structure of the tunneling tip due to possible Bi2Te3 uptake, during the scanning. Structural investigations on the atomic scale were performed with an aberration corrected scanning transmission electron microscope (FEI Titan 80−300) on cross-sectional specimen. Specimens were prepared by conventional grinding techniques and final ion milling with Ar ions.

recognized. The statistical analysis of several STM images of these two samples yields that the number of dislocations per surface unit is approximately the same and is in the range of 1 × 109 cm−2. Examining the surface roughness of both samples does not indicate any significant difference. Thus, the images give evidence for an equilibrium layer-by-layer growth mode of thick Bi2Te3 films with a high structural perfection. In contrast, the STM image of the 4 nm thick Bi2Te3 film (Figure 2c) shows significantly smaller sizes of terraces and a lower dislocation density. The islands indicated in Figure 2c by arrows possess the step height of 0.4 QL and are not related to threading defects (Figure 2f). The large number of steps with a small 0.4 nm step height leads to a smaller root mean square (rms) roughness than that obtained for the 13 and 30 nm thick samples. Obviously, the growth of ultrathin films occurs in a sub-QL mode. As shown in Figure 2d, at a nominal film thickness of 1 nm, the formation of small (10−20 nm) separated islands of sub-QL heights is observed; that is, the growth begins by formation of a number of independent nucleation centers at the surface, which subsequently coalesce to form a continuous film. Apparently, the diffusion length of ad-atoms on the substrate surface is small leading to the nucleation of a high density of islands with a sub-QL height. Upon coalescence, bigger units are formed. Frequently, islands with one completed QL and a second layer nucleated with a sub-QL thickness on top are obtained (marked by arrows in Figure 2c). Only after the substrate is completely covered with a QL of Bi2Te3 does the growth turn into the layer-by-layer growth within the deposition of a few monolayers. This observation leads to the conclusion that the diffusion length on the completed QL is much larger than initially on the substrate and that ad-atoms arriving on still separated islands avoid the incorporation at the

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RESULTS AND DISCUSSION The Bi2Te3 growth was performed under optimized growth conditions, reported previously,21 leading to layer-by-layer grown single crystal films by MBE. Atomically flat surfaces of stoichiometric Bi2Te3 with wide ∼100 nm terraces separated by one QL steps are obtained for films exceeding a thickness of 30 nm. X-ray diffraction and the pole-figure measurements confirm that the obtained Bi2Te3 (0001) films are relaxed, single crystalline, and of high quality matching with the Si substrate orientation21 (see the Supporting Information, Figure 1). In contrast to previous reports,20,23 these films were grown on nonreconstructed Si (111) substrate surfaces, due to the low annealing temperature of 750 °C prior to growth. Four topographical in situ STM images of the MBE grown Bi2Te3 films with different thickness in the range 1−30 nm are shown in the same lateral scale in Figure 2a−d. The STM images provide information concerning nucleation and growth dynamics of the obtained films. Flat single crystalline terraces separated by small steps are easily recognized in the images. However, their morphology varies quite strongly as the growth proceeds from 1 to 30 nm. The 13 and 30 nm thick films have similar topography (Figure 2a, b). Particularly, they possess wide atomically smooth terraces separated by one QL steps (Figure 2e). However, a few dislocations, marked by arrows in Figure 2a, b, accompanied by sub-QL steps can also be 6100



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Te signal prevails at the interface. Electron energy loss spectroscopy (EELS)25 measurements clearly confirm that these first two atomic layers predominantly contain Te atoms (see the Supporting Information, Figure 2). For the blue area, these two atomic Te layers are marked by arrows in the line scan. Within the red area, a third peak of higher intensity than the Si appears, indicating a third atomic layer that, in parts, contains Te. This can be explained by the low temperature bake (750 °C) of the Si substrate, since a roughness in the atomic scale prevails on the surface. The data indicates that first sites of missing Si atoms in the surface are filled with Te atoms. Since the growth was performed under Te overflow and additionally the Bi shutter has been opened 2 s after the Te shutter has been opened, we suppose the Si surface dangling bonds are terminated by a one monolayer of Te and thus prepare the Si surface for van der Waals epitaxy. In fact, careful measurements of the in-plane distance between neighboring atoms of the first atomic layer of Te reveal a value of 0.33 ± 0.01 nm, which agrees with the value expected for Si. The second monolayer of Te is already part of the Bi2Te3 film; thus, the second Te monolayer is attached to the first one only by van der Waals bonds. The latter can be concluded from the distance of the two interfacial Te layers, which matches the distance between quintuple layers within the Bi2Te3 crystal. Moreover, it can be stated from the Z-contrast images that an amorphous or polycrystalline layer at the interfaces is absent. The calculation of the Si/Bi2Te3 in-plane lattice constant ratio close to the Si/Bi2Te3 interface yields exactly a lattice mismatch of 14%. Thus, the Bi2Te3 film is fully relaxed even within the first QL. A careful examination of STEM images over larger sections shows the presence of two different domains at the initial growth stage separated by both in-plane and out-of-plane boundaries. An example is given in Figure 4a, where a Fourier-filtered image shows the contributions of two reflections belonging to two different domains. Domain II (green), which extends over about 60 nm, is encapsulated by domain I (red). It has to be noted that the presence of two domains is found by STEM only within the first five to six monolayers, at a later stage, when the growth switched to the QL by QL mode of growth of only one domain dominates. Figure 4b displays the domain wall at the right side of domain II in higher magnification. The vertical wall reveals small facets. The horizontal boundary is found at a double Te layer separating adjacent QLs. Domain II is mirrored relative to the domain boundary and in-plane shifted relative to domain I (see Figure 4c and the Supporting Information, Figure 3). The domains depicted in the STEM images are larger than the islands formed by the nucleation of the film (see Figure 2d), and the density of the domains is significantly smaller than the island density. Due to the high nucleation density on the atomically rough Si surface, the islands are small. Due to the double layer of Te at the interface, the islands are only weakly bonded to the substrate; thus, the small islands float on the substrate. Upon coalescence of the islands, small shifts in the x−y direction of the islands brings them into exact matching conditions, and no domain boundary is formed upon coalescence. Note that, due to the large lattice mismatch between Bi2Te3 and Si, the atoms of the first Bi2Te3 QL can take any position in the x−y plane with respect to the Si lattice. As mentioned before, the islands can nucleate in two different groups, which cannot be transformed into each other by a shift in the x−y plane, only by a rotation of 60°, which is equivalent

step edge of the island toward the substrate. Instead, the formation of a sub-QL nucleus on the island surface is preferred, despite the fact that the diffusion length on a completed QL is apparently larger than the island size. It is remarkable that no obvious atomic steps attributed to the substrate are visible in contrast to a report24 on MBE growth of Bi2Se3 film on the Bi-covered Si (111) surface. However, as mentioned above, the islands can nucleate in six different stacking orders, which immediately raises the question what occurs upon coalescence. In this context, it is interesting to note that the density of dislocations is orders of magnitude smaller than the density of nucleated islands. Consequently, high-resolution STEM has been performed to unravel the peculiarities at the interface toward the substrate and to search for boundaries indicating the coalescence of islands. A high-resolution high-angle annular dark field (Z-contrast) image of a Bi2Te3 film is shown in the (110) projection of the Si substrate in Figure 3a. According to their difference in atomic

Figure 3. Atomic structure of the Si/Bi2Te3 interface. (a) Highresolution Z-contrast image of the Si/Bi2Te3 interface. (b) Line scans across the interface averaged parallel to the interface within the blue and red marked areas reveal one monolayer of Te at the interface. In some areas, such as the red one, an additional Te layer is revealed.

number, Bi atoms appear brighter than Te and Si atoms. The (Te−Bi−Te−Bi−Te) QL structure is clearly visible, evidencing a high structural perfection of the grown Bi2Te3 films. First, we want to focus on two line scans across the interfaces, averaged parallel to the interface within the red and blue areas. The line scans reveal the different intensity levels of Bi, Te, and Si (see Figure 3b). The maxima within the Bi2Te3 layer coincide with the ideal positions schematically shown at the bottom of the graph. As the interface is approached, the intensities decrease, due to a reduced thickness produced by the ion milling (see the Supporting Information). However, clearly, in both areas, the 6101



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Figure 4. Atomic structure of domains in Bi2Te3. (a) Fourier-filtered image shows the contributions of two reflections, which belong to two different domains. Domain II (green), which extends over about 60 nm, is overgrown by domain I (red). (b) The right-hand side of domain II is shown in higher magnification. The vertical boundary reveals small facets. (c) The atom structure at the domain boundary. Domain II is mirrored relative to the domain boundary (black dotted line) and in-plane shifted relative to domain I (see the Supporting Information, Figure 3).

to the observed flipping of the stacking order from A−B−C to C−B−A within the two domains. Thus, a rotation of the islands by 60° upon coalescence does not occur. This simple model explains the observed growth behavior. Moreover, we speculate that the high nucleation density on the imperfect Si (111) surface as well as the large lattice mismatch are actually beneficial for the growth, since small islands with little lattice compliance will float easily on the substrate. In addition, small islands of a different domain can be easily overgrown during the QL step flow mode, as it is observed in our films. The observation and its interpretation also correlate with the presence of six peaks in the pole figure measured for the threefold symmetry (105) reflection of Bi2Te3,21 instead of predicted three peaks of equal intensity for the crystal structure of Bi2Te3 (see the Supporting Information, Figure 1). Particularly, the difference in the intensities of the peaks for both groups of three peaks is a clear sign for a difference in a volume percentage of domains I and II. The change of growth modes from 3D island growth to 2D QL step flow mode of growth is in accordance with the growth model suggested by LEED investigation on the (7 × 7) reconstructed Si surface.23 In that investigation, the 3D growth critical thickness was found to be nearly three QL, which is in agreement with our results. In addition, to the domain structure, one other type of defect has been observed, namely, out-of-plane stacking faults. The high-resolution Z-contrast image shown in Figure 5a reveals a stacking fault extending from the substrate to the surface of the Bi2Te3. An out-of-plane displacement of 0.4 nm is measured, which agrees with the STM measurements shown in Figure 2a, e. Possibly such a defect is caused by a step of one unit cell height of the Si substrate. Such domain boundaries are well-known from III/V epitaxy on Si, forming antiphase domains. Indeed, also in the case of Bi2Te3 on Si, we find Bi layers heading into Te layers and vice versa, as shown by red arrows in the schematic picture (see Figure 5b) of the atomic configuration of this defect. Interestingly, two of three Te layers within the QL match up with the neighboring domain, “stabilizing” this boundary. However, the observed density of these defects is much smaller than the average distance of atomic steps on the Si (111) surface, which is 50−100 nm according to STM measurements. Thus, we propose that most of the floating Bi2Te3 islands are capable to overgrow steps on the Si surface without defect formation. However, adjacent islands may have overgrown the step with and without a defect, and the combination of this does correspond to the observation shown in Figure 2a, namely, sub-QL steps having their start and end point at a dislocation. The area is defined on one side by

Figure 5. Atomic structure of threading defects in Bi2Te3. (a) A highresolution STEM image of a stacking fault (along the line shown by the arrows) extending from the substrate to the surface. Possibly due to a step in the substrate (0.31 nm), an out-of-plane displacement by 0.4 nm between the left- and right-hand side is induced, leading to the steps of 0.4 nm in height at the sample surface (Figure 2 a, e). (b) Crystal model shown in (110) projection of Si explaining the atomic structure of the stacking fault. The out-of-plane stacking sequence is undisturbed, whereas the connecting Bi to Te and Te to Bi atom sheets are observed (red markers).

the submonolayer step that ends at the dislocations and on the other side by a soft bending of the surface as shown in the black line scan of Figure 2a. Since the dislocation density of the MBE grown films (∼10−9 cm−2) is a few orders of magnitude higher than for those grown by the Zone melting method (∼10−5 cm−2),26 all these defects probably appear as a consequence of the 3D growth in the nucleation phase.

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CONCLUSIONS Generalizing the information obtained by STM and STEM, we conclude that the growth can be separated in three phases consisting of the substrate passivation by Te, the 3D growth via islands, and the subsequent 2D QL step flow mode of Bi2Te3 films. The fact that even the first Bi2Te3 QL is fully relaxed indicates that the substrate only orients the crystallographic z axis of the island nuclei perpendicular to the substrate surface. 6102



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(7) Hsieh, D.; Xia, Y.; Qian, D.; Wray, L.; Dil, J. H.; Meier, F.; Osterwalder, J.; Patthey, L.; Checkelsky, J. G.; Ong, N. P.; Fedorov, A. V.; Lin, H.; Bansil, A.; Grauer, D.; Hor, Y. S.; Cava, R. J.; Hasan, M. Z. Nature 2009, 460, 1101. (8) Hsieh, D.; Xia, Y.; Wray, L.; Qian, D.; Pal, A.; Dil, J. H.; Osterwalder, J.; Meier, F.; Bihlmayer, G.; Kane, C. L.; Hor, Y. S.; Cava, R. J.; Hasan, M. Z. Science 2009, 323, 919. (9) Peng, H.; Lai, K.; Kong, D.; Meister, S.; Chen, Y.; Qi, X.-L.; Zhang, Sh.-Ch.; Shen, Zh.-X.; Cui, Y. Nat. Mater. 2010, 9, 225. (10) Koma, A. Thin Solid Films 1992, 216, 72. (11) Koma, A. J. Cryst. Growth 1999, 201/202, 236. (12) Koma, A.; Sunouchi, K.; Miyajima, T. Microelectron. Eng. 1984, 2, 129. (13) Rudolph, R.; Tomm, Y.; Pettenkofer, C.; Klein, A.; Jaegermann, W. Appl. Phys. Lett. 2000, 76, 1101. (14) Yamamoto, H.; Yoshii, K.; Saiki, K; Koma, A. J. Vac. Sci. Technol., A 1994, 12, 125. (15) Niu, G.; Vilquin, B.; Penuelas, J.; Botella, C.; Hollinger, G.; Saint-Girons, G. J. Vac. Sci. Technol., B 2011, 29, 041207. (16) Li, H. D.; Wang, Z. Y.; Kan, X.; Guo, X.; He, H. T.; Wang, Z.; Wang, J. N.; Wong, T. L.; Wang, N.; Xie, M. H. New J. Phys. 2010, 12, 103038. (17) Tarakina, N. V.; Schreyeck, S.; Borzenko, T.; Schumacher, C.; Karczewski, G.; Brunner, K.; Gould, C.; Molenkamp, L. W. Cryst. Growth Des. 2012, 12, 1913. (18) Liu, Y.; Weinert, M.; Li, L. Phys. Rev. Lett. 2012, 108, 115501. (19) Dang, W.; Peng, H.; Li, H.; Wang, P.; Liu, Zh. Nano Lett. 2010, 10, 2870. (20) Li, Y.-Y.; Wang, G.; Zhu, G.; Liu, M.-H.; Ye, C.; Chen, X.; Wang, Y.-Y.; He, K.; Wang, L.-L.; Ma, X.-C.; Zhang, H.-J.; Dai, X.; Fang, Zh.; Xie, X.-Ch; Liu, Y.; Qi, X.-L.; Jia, J.-F.; Zhang, Sh.-Ch.; Xue, Q.-K. Adv. Mater. 2010, 22, 4002. (21) Krumrain, J.; Mussler, G.; Borisova, S.; Stoica, T.; Plucinski, L.; Schneider, C. M.; Grützmacher, D. J. Cryst. Growth 2011, 324, 115. (22) Plucinski, L.; Mussler, G.; Krumrain, J.; Herdt, A.; Suga, S.; Grützmacher, D.; Schneider, C. M. Appl. Phys. Lett. 2011, 98, 222503. (23) Liu, H. W.; Yuan, H. T.; Fukui, N.; Zhang, L.; Jia, J. F.; Iwasa, Y.; Chen, M. W.; Hashizume, T.; Sakurai, T.; Xue, Q. K. Cryst. Growth Des. 2010, 10, 4491. (24) Zhang, G.; Qin, H.; Teng, J.; Guo, J.; Guo, Q.; Dai, X.; Fang, Zh.; Wu, K. Appl. Phys. Lett. 2009, 95, 053114. (25) Heidelmann, M.; Barthel, J.; Houben, L. Ultramicroscopy 2009, 109, 1447. (26) Jariwala, B.; Shah, D. V. J. Cryst. Growth 2011, 318, 1179.

During the process of coalescence of islands, small in-plane displacements of the islands occur to find the correct positions. For larger islands, this may become more difficult, and coalescence may lead to the formation of dislocations. For the first time, we have imaged ultrathin films of Bi2Te3 by in situ STM and STEM and therefore have got a direct access to the structure of the Si/Bi2Te3 interface as well as the few first QLs on the atomic level. This enables us to conclude that a highly mismatched single crystalline growth of Bi2Te3 takes place via one-step van der Waals epitaxy on a Te-terminated Si (111) surface without formation of an amorphous or polycrystalline buffer layer. The grown film is completely relaxed from the first QL. Two types of domain boundaries are observed, one of them is due to inverse stacking orders of adjacent growth nuclei and the other one to antiphase domain walls initiated by atomic steps of the substrate. The fundamental understanding of defect formation in these ultrathin Bi2Te3 films paves the road to fabricate them with perfections suitable for device applications. A first step, using the step-flow mode of growth to overgrow domains has been demonstrated.

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ASSOCIATED CONTENT

S Supporting Information *

XRD and EELS data of Bi2Te3 films and atomic model of a domain boundary. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 0049 2461 612732; fax: 0049 2461 612333; e-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge Dr. András Kovács for fruitful discussions and support with TEM specimen preparation.

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ABBREVIATIONS ARPES, angle-resolved photoemission spectroscopy; MBE, molecular beam epitaxy; STM, scanning tunneling microscopy; STEM, scanning transmission electron microscopy; QL, quintuple layer; EELS, electron energy loss spectroscopy; LEED, low energy electron diffraction

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REFERENCES

(1) Xia, Y.; Qian, D.; Hsieh, D.; Wray, L.; Pal, A.; Lin, H.; Bansil, A.; Grauer, D.; Hor, Y. S.; Cava, R. J.; Hasan, M. Z. Nat. Phys. 2009, 5, 398. (2) Fu, L.; Kane, C. L. Phys. Rev. B 2007, 76, 045302. (3) Hasan, M. Z.; Kane, C. L. Rev. Mod. Phys. 2010, 82, 3045. (4) Kane, C. L.; Mele, E. J. Science 2006, 314, 1692. (5) König, M.; Buhmann, H.; Molenkamp, L. W.; Hughes, T.; Liu, Ch.-X.; Qi, X.-L.; Zhang, Sh.-Ch. J. Phys. Soc. Jpn. 2008, 77, 031007. (6) Horák, J.; Č ermák, K.; Koudelka, L. J. Phys. Chem. Solids 1986, 47, 805. 6103


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