Suppressing Twin Domains in Molecular Beam Epitaxy Grown Bi2Te3

Article pubs.acs.org/crystal

Suppressing Twin Domains in Molecular Beam Epitaxy Grown Bi2Te3 Topological Insulator Thin Films Jörn Kampmeier,*,† Svetlana Borisova,† Lukasz Plucinski,‡ Martina Luysberg,§ Gregor Mussler,† and Detlev Grützmacher† †

Peter Grünberg Institut 9, Forschungszentrum Jülich & JARA Jülich-Aachen Research Alliance, D-52425 Jülich, Germany Peter Grünberg Institut 6, Forschungszentrum Jülich & JARA Jülich-Aachen Research Alliance, D-52425 Jülich, Germany § Peter Grünberg Institut 5, Forschungszentrum Jülich & JARA Jülich-Aachen Research Alliance, D-52425 Jülich, Germany ‡

ABSTRACT: The structural perfection of the topological insulator (TI) Bi2Te3 is a key issue for its employment in future device applications. State of the art TIs, featuring exotic electronic properties, predominantly suffer from structural defects such as twin domains. A suppression of such domains in molecular beam epitaxy-grown Bi2Te3 thin films on Si(111) substratesmeasured by X-ray diffraction pole figure scansis presented in this paper. A numerical analysis of van der Waals potentials was performed, revealing the nucleation collinear with the Si(311) reflections of the Si(111) substrate to be energetically preferred.

layer (QL). The five single atomic layers forming a QL are bonded chemically, whereas adjacent QLs are linked by van der Waals forces. Bi2Te3 crystallizes in the space group R3̅m and features a rhombohedral structure. A description in terms of a hexagonal primitive cell is also possible. Thereby one unit cell is built from three QLs in the sequence of ABCAB-CABCABCABC, stacked along the c-axis, as schematically displayed in Figure 1a. For convenience Te atoms are depicted in green whereas Bi atoms are in purple. Early investigations on the TI behavior of Bi2Te3 have mainly been performed studying the surfaces of bulk crystals realized by means of stoichiometric melt-growth or self-flux methods.6 Because of local composition fluctuations and the formation of defects originating in these growth methods, the produced bulk crystals exhibit a high bulk charge carrier concentration, masking the contribution of the Dirac surface states. Very recently, however, molecular-beam epitaxy (MBE) has been employed to grow high-quality Bi2Te3 thin films, aiming to drastically reduce the bulk charge carrier density. Because of the weak vdW force between epilayer and substrate, the lattice mismatch between Bi2Te3 and the substrate is not crucial. A large variety of substrates with high lattice mismatch can be used, including CdTe,7 Al2O3,8,9 BaF2,10,11 InP,12 and Si.3,13,14 To exemplarily prove the high crystalline and morphological quality of our MBE-grown Bi2Te3 thin films, scanning transmission electron microscopy (STEM) and atomic force microscopy (AFM) images are presented in Figure 1, panels b and c, respectively. The provided high-resolution high-angle annular dark field STEM image in Si(110) projection displays Bi and Te atoms to be assembled in a well-ordered, crystalline

T

he narrow gap semiconductor Bi2Te3 is well-known as a classical thermoelectric material offering a high figure of merit.1,2 Recently this material system has been shown to exhibit topological insulator (TI) behavior as well.3,4 A threedimensional TI features an energy gap in bulk, but a spinpolarized linear energy dispersion at its surface. It is predicted that the surface carriers of a TI have unparalleled properties, such as extremely high mobilities and dissipationless spinlocked transport. Taking into account these unique features, TIs offer great potential for future device applications in spintronics and quantum computation processing.5 The extraordinary TI characteristics are based on its crystalline structure consisting of a sequence of five alternating atomic layers (Te−Bi−Te−B−Te; Figure1a), called quintuple

Figure 1. (a) Schematic hexagonal primitive Bi2Te3 cell. (b) Bi2Te3 STEM image. (c) Bi2Te3 AFM image. (d) Bi2Te3 XRD scan. (e) Bi2Te3 ARPES scan. © 2014 American Chemical Society

Received: October 2, 2014 Revised: November 18, 2014 Published: November 19, 2014 390

dx.doi.org/10.1021/cg501471z | Cryst. Growth Des. 2015, 15, 390−394

Crystal Growth & Design

Article

580 °C to set up different growth rates between v = 2.7 nm/h and v = 120 nm/h.3 Bi2Te3 growth has to take place in a Te overpressure regime.3,13 Therefore, the tellurium temperature was increased from TTe = 290 °C to TTe = 320 °C for low and high growth rates, respectively. The substrate temperature TSub = 300 °C was kept constant for all experiments. To qualitatively and quantitatively investigate the existence of twin domains, one- and two-dimensional (1D and 2D) pole figure measurements around the Bi2Te3(105) reflection were performed utilizing a triple-axis high-resolution Bruker D8 discover diffractometer. The formation of twin domains is a key challenge in epitaxial growth of tetradymite-type crystals on hexagonal symmetric surfaces, like Bi2Te3 on Si(111). Figure 2a features a STEM

arrangement, representing the exact structure of a rhombohedral crystal and evidencing its high crystalline perfection. The atomic stacking sequence of three Bi2Te3 QLs (one unit cell) is additionally illustrated by a schematic drawing in Figure 1b. An AFM image presented in Figure 1c yields the topographical perfection of a typical Bi2Te3 film grown by MBE. The surface features atomically flat terraces separated by steps of 1 nm in height, representing the height of a single QL. Here, the authors would like to stress that, despite carbon contamination as well as rapid oxidation of the Bi2Te3 surface due to the exposure to air,15 the morphology of the Bi2Te3 surfaces is not affected; i.e., the AFM images look the same irrespective of how long the samples have been exposed to air. X-ray diffraction (XRD) investigations reveal the layer to be perfectly (00l)oriented on a Si(111) substrate (with l = 3, 6, 15, 18; Figure 1d). Confirming the findings gained from the STEM image in Figure 1c, the XRD pattern features sharp peaks indicating a high quality of the Bi2Te3 crystal. Furthermore, Figure 1e depicts an angle resolved photoemission spectroscopy (ARPES) scan representing the band structure of pure Bi2Te3 and its typical electronic properties.4,6,16 The Dirac Point is buried in the bulk valence band (vb), whereas the Fermi level (EF) lies in the bulk conduction band (cb). Thus, Bi2Te3 appears to be naturally n-type doped; the bulk charge carriers mostly originate from point defects like TeBi antisite donor defects.17 The structural perfection of a TI is a key issue for its employment in future device applications. Even state-of-the-art MBE grown TIs predominantly suffer from structural defects such as twin domains.14 With respect to its hexagonally symmetric surface atom arrangement, Si(111) is dedicated to be a suitable substrate to deposit TIs of a rhombohedral crystal structure like Bi2Te3, despite the large lattice mismatch of 14%.3 Within the scope of this paper we present an approach to create an ideally twin domain-free TI by gradually reducing the growth rate of Bi2Te3 and thereby enhancing the adatom diffusion length and time to settle at an energetically preferred position. To obtain the preferential nucleation configuration, a numerical analysis of van der Waals (vdW) potentials for Bi2Te3 adatoms on a two monolayer thick Si(111) surface, passivated by one fully strained Te monolayer, was performed taking into account two different epilayer domain orientations. High quality Bi2Te3 thin films were deposited by means of a solid source MBE system on 100 mm Si(111) wafers under ultrahigh vacuum conditions utilizing Knudsen effusion cells. Each sample was grown on an individual Si wafer, taken from the same batch. Prior to the growth, the Si substrates were wet chemically cleaned by a RCA HF-last (H2SO4: H2O2 2:1) procedure to remove organic contaminations as well as the native oxide. The dip in 1% HF additionally passivates the Si(111) surface by hydrogen to protect it from rapid oxidation during the subsequent ex-situ transport to the MBE growth chamber. The Si(111) wafers were heated in situ to 700 °C for 10 min to desorb remaining hydrogen atoms. The surface configuration was in situ monitored by reflection high energy electron diffraction, indicating a (1 × 1) reconstructed Si surface. Subsequently the Si surface was flushed with Te to saturate its dangling bonds. The saturation of dangling bonds is hereby crucial to finally employ the van der Waals growth mode.18−21 The base pressure of the growth chamber was 5 × 10−10 mbar, but never exceeded 9 × 10−10 mbar during growth. The beam-equivalent pressure of Bi was varied by applying different cell temperatures in the range of TBi = 480 °C to TBi =

Figure 2. (a) STEM image of a Si/Bi2Te3 interface featuring twin domains, indicated by two differently hatched patterns marked by the white lines. For clarity, Domain II is in green. (b) Schematic atomic model of a domain boundary. Between Domain II and Domain I the stacking sequence changes from (ABCAB) to (CBACB), equivalent to an in-plane rotation of the initial Bi2Te3 unit cell by 60°. (c) High resolution TEM image of the Si/Bi2Te3 interface. (d) Line scans across the Si/Bi2Te3 interface revealing its atomic structure. The data are averaged parallel to the interface within the blue and red marked areas, respectively. The figures were adapted from ref 14.

image displaying contributions of two domains. Domain II (green) is hereby completely encapsulated by Domain I without causing any defects on the surface (not shown here). Because of the weak vdW interaction potential, the in-plane domain boundary is observed at the interface of two Te sheets between single QLs, as indicated by a red dashed line. The atomic stacking sequences of the domains are mirrored with respect to each other along the domain boundary. A schematic illustration is presented in Figure 2b. At the in-plane domain boundary the atomic crystal stacking sequence abruptly changes from ABCAB to CBACB. This change is equivalent to an in-plane rotation of the initial Bi2Te3 unit cell by 60° with respect to the Si substrate (Figure 3a). Additionally a faceted out-of-plane boundary between Domain I and Domain II is observed. Single atoms at the boundary region are in-plane 391



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Figure 3. (a) Schematic illustration of 60° in-plane rotated Bi2Te3 nucleation QLs on a Te passivated Si(111) substrate, representing two possible domains. The twin domains are collinear with the Si(311) reflections of the Si(111) substrate or in-plane rotated by 60°. (b) Schematic illustration of the first QL of a Bi2Te3 crystal deposited on a Te passivated Si(111) substrate. The passivation layer is fully strained on the Si substrate, whereas the first QL is already completely relaxed. The interatomic spacings are extracted from Figure 2c,d. Figure 4. 2D and 1D pole figure plots around the (105) reflection of Bi2Te3. 2D pole figure plots of films deposited with a growth rate of v = 26 nm/h (a) and v = 2.7 nm/h (b). Red crosses indicate the Si(311) reflections. (c) 1D pole figure plots of thin films deposited with growth rates of v = 120 nm/h (purple), v = 26 nm/h (green), v = 17.6 nm/h (red), and v = 2.7 nm/h (black). Decreasing the growth rate increases the dominance of one Bi2Te3 domain collinear with the Si(311) reflections.

displaced with respect to their native positions, again induced by the 60° in-plane rotation of the confronted domains (see Figure 2b). A change of the out-of-plane lattice constant is not noticed. The out-of-plane domain boundary only spreads over a region of a few atom distances and does not form any threading dislocations. All obtained STEM images implicate Domain I to be on average only present within the first six QLs of the Bi2T3 film. Above this limit, the film is dominated by Domain II, encapsulating Domain I completely. In principle a Bi2Te3 film can start nucleating with any of the three QLs building its primitive cell on a 60° symmetric, Te saturated Si(111) surface (see Figure 1a). Each of it can be stacked normal or reverse, providing six possible kinds of nucleation QLs. Three configurations of each kind differ only by a linear displacement. Note that due to the large lattice mismatch toward the Si(111) substrate a linear displacement occurs anyway and has to be overcome upon coalescence of nuclei.14 Thus, two domains rotated by 60° toward each other are left, because nucleation seeds rotated by 60° cannot be transformed into each other by a simple displacement in the x− y plane. An in-plane rotation of 60° is required, which apparently does not occur upon coalescence of growth nuclei/ islands. Furthermore, the saturation of Si(111) dangling bonds by a Te flush prior to the TI growth is crucial to employ the van der Waals growth mode. In this context, TEM investigations presented in Figure 2c show that the passivating Te layer exhibits the same in-plane lattice constant as the underlying Si(111) substrate.14 Thus, the first Te layer is fully strained on the Si(111). In contrast, the second Te layer, representing the first monolayer of a Bi2Te3 QL, is completely relaxed and only attached by vdW bonds to the Te passivation layer (see Figure 3b). The atom−atom distances of each considered layer (initial values for numerical calculations, presented later) are additionally displayed in Figure 3b, representing a cross section of the Si/Bi2Te3 interface including the fully strained Te passivation layer. The inter atomic spacings were obtained from the TEM investigations depicted in Figure 2c,d. To evidence the existence of twin domains, 1D and 2D pole figure measurements around the Bi2Te3(105) reflection were performed (Figure 4). The expected trigonal symmetry of an epitaxial film with rhombohedral crystal structure on a hexagonal symmetric surface is in distinct contrast to the 6fold symmetry observed in Figure 4a and can be attributed to the suggested existence of Bi2Te3 twin domains.

To investigate the twin domain formation dynamics and to explore how to suppress the domain formation, several Bi2Te3 samples with different growth rates between v = 120 nm/h and v = 2.7 nm/h were fabricated. Comparing the 2D pole figure plots in Figure 4a,b, the observed 6-fold symmetry of a layer deposited with a high growth rate (Figure 4a) is turned into a 3-fold symmetry (Figure 4b) by reducing the epilayer growth rate to v = 2.7 nm/ h. This effect is even more apparent in the 1D pole figure scans presented Figure 4c, revealing a gradual transition from six peaks to three peaks with decreasing growth rates. Hence the reduction of the epilayer growth rate causes a suppression of the Bi2Te3 twin domain formation. Taking into account the underlying Si(111) substrate, the orientation of high intensity peaks for high growth rates (Figure 4a) as well as the reflections of the remaining domain for low growth rates (Figure 4b) are found to be always collinear with the Si(311) reflections (top of Figure 4a), suggesting this orientation to be the preferential orientation of a Bi2Te3 epilayer on a Te saturated Si(111) surface. To clarify this change in nucleation dynamics with respect to the growth rate and thereby the diffusion length/time, numerical calculations of the local vdW potential wvdW ≈ −C/r6 of the nucleation sites were performed. To obtain the preferred nucleation site of a QL, triangular Te networks were positioned on random positions of a (111) oriented, Te passivated Si lattice (Figure 5) and subsequently displaced within its surface unit cell (blue rhombs in Figure 5). To evidence each domain orientation, the Te networks were rotated by 60° (Figure 5a,b). In order to realize lattice matched conditions, seven periods of a triangular shaped Te network were placed on eight, Te saturated Si unit cells (7aBi2Te3 = 8aSi(111)). Hence, these networks can be regarded as building blocks of each domain, and they can be employed to calculate vdW potentials to find their energetically most favorable nucleation site on the Si(111) substrate. 392



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ing the growth rate and consequently enhancing the diffusion length and time of the adatoms is a suitable approach to suppress the domain formation in Bi2Te3 epitaxial thin films on Si(111) substrates. Despite the fact that the twin domains are substantially suppressed at low growth rates, we find no correlation between the bulk charge carrier concentration and the existence of twin domains. All Bi2Te3 samples exhibit a large n-type doping (n ≈ 1019 cm−3), irrespective of the growth rates. The existence of bulk electrons is typical for MBE-grown Bi2Te3.7,9,10 Proven by density functional theory calculations, it is attributed to point defects, in particular TeBi antisites.17 Hence, we assume the twin domains not to be the main origin of the large n-type doping. The density of the TeBi antisite defects is not affected by the growth rate. In conclusion we have evidenced that high quality single crystalline and twin domain free Bi2Te3 thin films can be realized by means of MBE on Te passivated Si(111) substrates utilizing the vdW growth mode. The existence of 60° rotated twin domains was obtained by 1D and 2D pole figure measurements around the (105) reflection of Bi2Te3 thin films. Furthermore, an equally distributed nucleation of both possible Bi2Te3 domain orientations was observed for high growth rates. Reducing the growth rate and enhancing the adatom diffusion length and time forces the Bi2Te3 QLs to exclusively nucleate in the energetically most appreciated position, collinear with the Si(311) reflections. This nucleation behavior was investigated and confirmed by numerical calculations of the vdW potential for Te adatoms on a Te terminated Si(111) substrate, taking into account the two uppermost Si(111) monolayers. These findings highlight the unique properties of vdW epitaxy, which are remarkable distinct from conventional epitaxy featuring strong directional covalent bonds of nearest neighbors. Hence the vdW growth provides exceptional freedom to grow high-quality layered films on a large variety of substrates, even without lattice-matched conditions, in order to explore the fascinating properties inherent in 2D materials and layered films such as TIs.

Figure 5. Schematic illustration of triangular Te atom networks consisting of seven Bi2Te3 unit cells in each direction positioned on a (111) oriented, Te passivated Si lattice. The Te atom networks represent the first Te sheet of the Bi2Te3 QL related to the domain collinear with the Si(311) reflections (b) and in-plane rotated by 60° (a), respectively. The Si(111) surface unit cell is indicated by blue rhombs.

Thus, for each Te network position, the configuration with the lowest vdW potential was determined, taking into account the next nearest neighbor of the Te saturation layer, as well as the four nearest neighbors of the Si lattice. The calculations were performed considering an arbitrary value of C =1 for both the Te−Te and the Te−Si vdW potentials. Furthermore, the Si−Te (3.8 Å) and the Te−Te (3.3 Å) distances depicted in Figure 3b have been used. For both networks, the calculations reveal configurations exhibiting the highest symmetry (i.e., the outermost Te atoms are positioned right above the Teterminated Si atoms, as depicted in Figure 5) to feature the lowest vdW potential. To quantitatively analyze the vdW potential, two cases were distinguished. First, only the Te passivation layer and the topmost Si layer were taken into account. Second, the Te passivation layer, the topmost Si layer, as well as the first Si layer underneath were considered. Regarding the first case, no difference between the Te networks 1 and 2 was obtained, due to the 60° symmetry of the Te passivation layer and the Si surface atoms (compare the identical arrangement of Si surface atoms, in red, with respect to the Te adatom positions in row 3 of both networks). However, including the two topmost Si atomic layers into the calculations, a difference between the calculated vdW potential of the investigated networks is found, owing to the 120° symmetry of the Si surface atoms and the first Si layer underneath (compare the different arrangements of Si atoms underneath the surface, in purple, with respect to the Te adatom positions in row 3 of networks 1 and 2). The calculations reveal network 2 to feature a slightly lower vdW potential than network 1. Hence network 2, which is collinear with the (311) reflection, is energetically more favorable than network 1. The existence of an energetically preferential orientation of the Bi2Te3 layers on the Si substrate can be employed to explain the growth-rate dependent suppression of the domains. In case of a large growth rate, the high adatom density leads to a rapid nucleation rate and fast island growth, and a critical island size is achieved before the relaxation into an energetically preferred site occurs. Thus, the probability to settle at an energetically favored nucleation site is suppressed. Because of the small energy difference of both configurations the Te adatoms forming the first sheet of a QL settle in such a configuration and the second domain is formed. Decreasing the Bi2Te3 growth rate and thereby decreasing the adatom density facilitates their nucleation probability collinear with the energetically preferred Si(311) reflections. Therefore, decreas-

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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 The authors gratefully acknowledge financial support by the German Science Foundation (DFG) regarding the priority program SPP1666 “Topological Insulators”, as well as the Helmholtz Association concerning the “Virtual Institute for Topological Insulators” (VITI).

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ABBREVIATIONS TI, topological insulator; MBE, molecular beam epitaxy; vdW, van der Waals; Ql, quintule layer; STEM, scanning transmission microscopy; XRD, X-ray diffraction; ARPES, angle resolved photoemission spectroscopy

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REFERENCES

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Suppressing Twin Domains in Molecular Beam Epitaxy Grown Bi2Te3

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