Surface Reconstruction-Induced Coincidence Lattice Formation

Letter pubs.acs.org/NanoLett

Surface Reconstruction-Induced Coincidence Lattice Formation Between Two-Dimensionally Bonded Materials and a ThreeDimensionally Bonded Substrate Jos E. Boschker,*,† Jamo Momand,‡ Valeria Bragaglia,† Ruining Wang,† Karthick Perumal,† Alessandro Giussani,† Bart J. Kooi,‡ Henning Riechert,† and Raffaella Calarco† †

Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

‡

S Supporting Information *

ABSTRACT: Sb2Te3 films are used for studying the epitaxial registry between two-dimensionally bonded (2D) materials and three-dimensional bonded (3D) substrates. In contrast to the growth of 3D materials, it is found that the formation of coincidence lattices between Sb2Te3 and Si(111) depends on the geometry and dangling bonds of the reconstructed substrate surface. Furthermore, we show that the epitaxial registry can be influenced by controlling the Si(111) surface reconstruction and confirm the results for ultrathin films. KEYWORDS: Coincidence lattices, van der Waals epitaxy, 2D materials, topological insulators, phase change materials, surface reconstructions

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This implies that there exists a dimensional mismatch between substrate and film and that the interfacial bonding can be expected to be reduced compared to the epitaxial growth of 3D bonded films. In classic epitaxy, defined as the growth of 3D bonded materials on 3D bonded substrates, interfacial bonding plays an important role during the growth of lattice mismatched materials. For example, it can result in strained films11 or lead to the formation of coincidence lattices when the lattice mismatch is large.12−15 On the other hand, when 2D bonded materials are grown on 2D bonded substrates, which is known as vdW epitaxy, a lattice mismatched epilayer grows with its own lattice constant from the beginning of the deposition.16 At present it is unclear to what extent these mechanisms influence the deposition of 2D bonded materials on 3D substrates. It is reasonable to expect that the epitaxial registry is similar to the growth of molecular layers on solid (3D) substrates, which depends strongly on the weak film−substrate interaction and displays both azimuthal rotations and incommensurism.17 Indeed, experiments have shown that Sb2Te3 films grown on Si(111)-(7 × 7) exhibit rotational domains,18 and that the first QL of Bi2Te3 on Si(111)-(7 × 7) is strained.19 Therefore, the direct growth of 2D chalcogenides on Si(111)-(7 × 7) cannot be considered idealized vdW epitaxy. However, it is unclear if this epitaxial growth system can be defined as classical epitaxy, due to the layered nature of

oosted by the success of carbon-based nanomaterials, a new intriguing field, that of 2D materials, has emerged in condensed matter physics. The impressive development in this field is attracting the interest of chemists and physicists working on synthesis and characterization and of the engineers aiming at new devices with improved functionalities.1−3 Graphene is without a doubt the best known and most successful 2D material.4 However, very recently 2D materials with functional properties complementary to graphene have been identified.1,3 Those comprise 2D oxides, graphene-like materials and chalcogenides. The latter two are distinguished by the presence of weak van der Waals (vdW) bonds between layers. This makes it possible to isolate individual 2D material sheets and stack them on top of each other. These layered sequences of 2D materials, known as vdW heterostructures, have revealed new phenomena and unusual properties.1 In this respect, Sb2Te3 is an exemplary 2D material since it is formed by quintuple layers (QL) which are bonded to each other by vdW bonds, it possesses robust topological insulating (TI) surface states,5−8 and thanks to its 2D nature it serves as a building block of interfacial phase-change memory9 and thermoelectric superlattices.10 Molecular beam epitaxy (MBE) allows for the fabrication of (multi)layers with a high degree of perfection on single crystalline substrates and thus offers a viable method for the synthesis of heterostructures containing 2D bonded materials. The bonding for most commonly used substrates is 3D, whereas layered materials are distinguished by the presence of weak vdW bonds between layers, and the bonding is thus 2D. © XXXX American Chemical Society

Received: March 28, 2014 Revised: May 7, 2014

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the epilayer and the large influence of surface preparation on the achieved crystalline quality.20−22 It is thus likely that the epitaxy of 2D bonded materials on 3D bonded materials differs from classic epitaxy and vdW epitaxy. In view of the potential application of 2D materials it is of major relevance to understand the epitaxial registry of 2D bonded materials with silicon surfaces. In this work, we use Sb2Te3 thin films on Si(111) substrates as an example to study the epitaxial registry of 2D bonded materials on 3D bonded substrates. In contrast to the growth of 3D bonded materials on 3D substrates, it is found the geometry and dangling bonds of the reconstructed substrate surface determine the epitaxial registry between the two materials. These observations suggest that Si surface reconstructions can survive the deposition of 2D materials and that interface structures and bonding configurations can form between 2D and 3D bonded materials that depend on substrate surface reconstructions. Prior to the deposition of Sb2Te3 2 × 2 cm2 (111)-oriented Si substrates (B-doped) with a resistance of 1−10 Ω·cm were cleaned using standard procedures, as described elsewhere,5,23,24 and introduced in the MBE system. The Si(111)-(7 × 7) reconstruction was obtained by annealing the substrates at 720 °C for 10 min, and Sb-terminated Si(111) surfaces were fabricated by slight modifications to the procedure described by Andrieu.25 Reflection high-energy electron diffraction (RHEED) images were taken prior to the deposition to confirm the formation of these surface reconstructions (Supporting Information, Figure 1). The deposition of Sb2Te3 and GeTe-Sb2Te3 alloys was performed at a substrate temperature of 227 and 250 °C, respectively. The Sb2Te3 films studied have a thickness of ∼40 and ∼1 nm. More details about the growth can be found elsewhere.5,23,24 In situ characterization was performed by angular-resolved RHEED (ARHEED).26 Due to the presence of a custom-made sample holder the measurement is not possible for the full rotation. For sake of clarity, data obtained at ±60° are plotted replacing the missing rotations. Ex situ structural characterization of the films was performed by four-circle, highresolution X-ray diffraction (XRD) using Cu Kα1 radiation (λ = 1.540598 Å). The diffractometer (PANalytical X′Pert Pro) was equipped with a Ge (220) monochromator. For ω-2ϑscans a 0.1 mm detector slit was used, whereas φ-scans were performed without slit and in skew geometry. For transmission electron microscopy (TEM) imaging the Sb2Te3 samples were cleaved along the Si(111) ⟨110̅ ⟩ viewing directions. These slices are stacked inside brass tubes with a 3 mm diameter and cut into TEM sample disks. The disks are grinded, dimpled, and ion milled (using a Gatan PIPS II with Ar sputtering from ±6° at voltages of 4 kV to as low as 0.1 kV) to electron transparency. The high-resolution TEM (HRTEM) images were obtained using a JEOL 2010F operating at 200 kV. Simulations of HRTEM images were obtained using MacTempas PPC Version 1.7.8. Matching simulations were found using sample thicknesses around 25 nm and microscope defocus values around −16 nm. XRD and TEM investigations of the Sb2Te3 films are in agreement with previous reports5,6 and show that the Sb2Te3 layers consist of domains with a (00.1) out-of-plane orientation (Supporting Information, Figure 2). A detailed picture of the interface structure between the domains and Si(111) surface is obtained by HRTEM investigation (Figure 1a). The image shows clear intensity modulations, consistent with quintuple

Figure 1. (a) HRTEM images of a Sb2Te3 film grown on Si(111). The inset shows the simulated HRTEM image. (b) φ-scans around the Si(004) and Sb2Te3(01.5) diffraction peaks for Sb2Te3 deposited on Si(111)-(7 × 7). (c) φ-scans around the Sb2Te3(01.5) diffraction peaks for films grown on Si(111)-(7 × 7) (upper curve), Si(111)(5√3 × 5√3)R30°-Sb (middle curve), and Si(111)-(√3x√3)R30Sb (lower curve).

layers stacked along the (00.1) direction. Simulations of the HRTEM image using a Sb2Te3 structure (see inset) reveal that the dark bands can be associated with the vdW gap between neighboring QL. The presence of a dark band at the interface between Si and Sb2Te3 thus suggests that the interface is formed by a vdW gap. However, the exact interface structure, and possible presence of covalent bonding across the interface, cannot be deduced from the presented data. Additional insight into the interface structure is obtained by investigating the orientation of the observed domains with respect to the silicon lattice. Figure 1b shows XRD φ-scans on the Sb2Te3 (01.5) and Si(004) reflections. Two differences between the film and substrate can be observed. First, the film has six-fold symmetry, whereas the substrate has three-fold symmetry. Since Sb2Te3 also has a three-fold symmetry, this observation indicates that the film is twinned. Twinning is typical for chalcogenide thin films grown on Si(111) substrates20,21,27 and can be attributed to weak interaction across the interface. This implies that the Sb2Te3 film only interacts with the first layer of the Si(111) surface that has point symmetry of 6mm, resulting in twinning.28,29 The second difference between the film and the substrate is the observation of additional peaks at a separation of ±6.7° and ±16° from the central peaks (Figure 1b,c). Φ-scans performed around the (02.10) peak confirmed that these peaks are at a constant angle. Therefore, they are the result of rotational domains in the Sb2Te3 films. Coincidence lattices are a possible explanation for the presence of rotational domains, if a large lattice mismatch is present in the epitaxy of two 3D bonded materials. However, an analysis of a coincidence lattice between Sb2Te3 and the Si(111)-(1 × 1) surface (Supporting Information, Figure 3) shows that the formation of a coincidence lattice would result in domains rotated by 3.2° and 8.9°, in disagreement with observation. This indicates that the mechanism for the formation of coincidence lattices between Si(111) and 2D materials is different from that of Si(111) and 3D materials. As discussed above, the observation of six-fold symmetry in φ-scans on Sb2Te3 indicates that the interfacial interaction B

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These findings suggest that the domain orientations in the film depend on the substrate surface reconstruction. In order to verify this assertion, we prepared Si(111)-(5√3 × 5√3)R30Sb and Si(111)-(√3x√3)R30-Sb surfaces to use as substrates for Sb2Te3 growth. It is important to note that the two surfaces display not only different reconstructions but also dissimilar starting surfaces. The Si(111)-(5√3 × 5√3)R30°-Sb surface consists of a Si(111)-(5 × 5) dimer-adatom-stacking-faulted base with Sb adatoms,31 whereas the Si(111)-(√3x√3)R30-Sb consists of unreconstructed Si(111)-(1 × 1) surface with Sb adatoms.32 Figure 1c shows φ-scans from the films grown on the three different substrate surfaces. For the films grown on Si(111)-(7 × 7) and Si(111)-(5√3 × 5√3)R30°-Sb surfaces multiple diffraction peaks are observed. The additional peaks for films grown on the Si(111)-(5√3 × 5√3)R30°-Sb have a separation of ±3.2° and ±9.8°. These rotational angles are different from the angles observed in films on the Si(111)-(7 × 7) surface. A close inspection of the Si(111)-(5√3 × 5√3)R30°-Sb surface structure reveals that these angles can be related to the positions of dangling bonds and Sb adatoms (Figure 2b). The distances c and d are given in the Supporting Information, Table 1, and approximately correspond to integer numbers of the Sb2Te3 in-plane lattice constant. This suggests that the mechanism that determines the epitaxial registry on this surface is the same as for Sb2Te3 deposited on Si(111)-(7 × 7). However, we note that the rotations of ±3.2° also can be related to the formation of a coincidence lattice with the Si(111)-(1 × 1) surface, as discussed above. Nevertheless, these observations confirm that the azimuthal orientation of domains depend on the substrate surface reconstruction. On the other hand, films grown on Si(111)-(√3x√3)R30°Sb surface exhibit a single diffraction peak. Similar results were obtained for Sb2Te3 films grown on Si(111)-H and Si(111)-(2 × 1)-Sb surfaces and GeTe-Sb2Te3 alloys on Si(111)(√3x√3)R30°-Sb (Supporting Information, Figure 4a). These surfaces have in common that they are based on an unreconstructed Si(111)-(1 × 1) structure, suggesting that this is a prerequisite for obtaining a single, well-defined in-plane domain orientation. It is well-known that strain relaxation generally occurs with increasing film thickness.11 It is thus a priori not clear if the results obtained for films with a thickness of ∼40 nm are equivalent to that of a single QL. In order to address this point, Sb2Te3 films with a thickness of approximately 1 QL were deposited on Si(111)-(7 × 7) and on Si(111)-(√3x√3)R30°Sb, and investigated by in situ ARHEED directly after the deposition. For the film grown on Si(111)-(7 × 7) the diffracted intensity lies on a circle, and broad maxima with sixfold symmetry can be observed (Figure 3a). Detailed analysis shows that these broad maxima consist of four peaks with a separation of ±6.8° and ±16° from the main peak. This confirms the results obtained from XRD measurements on thicker films. In addition, an increase in intensity is observed at a separation of 30° from the main peaks, indicating that domains rotated by 30° are also present in ultrathin films. The latter is not observed by XRD, suggesting that these domain orientations are probably overgrown during continued deposition. For the film grown on Si(111)-(√3x√3)R30°-Sb the ARHEED pattern (Figure 3b) exhibits sharp diffraction peaks, consistent with the results obtained by XRD. From the separation between the diffraction peaks an in-plane lattice parameter of 4.26 Å is calculated, showing that the film is fully relaxed. The data thus confirm that the formation of

between Sb2Te3 and Si is weak. This weak interaction can also have consequences for the formation of coincidence lattices. Since the Sb2Te3 films were deposited on a reconstructed Si(111) surface, we considered the possibility of the formation of a coincidence lattice between the Si(111)-(7 × 7) surface and Sb 2 Te 3 . The formation of the Si(111)-(7 × 7) reconstruction reduces the number of dangling bonds from 49 to 19.30 It is therefore likely that any coincidence lattice present here depends on the dangling bonds on the Si(111)-(7 × 7) surface. Figure 2a shows a schematic of the Si(111)-(7 ×

Figure 2. Schematics of the (a) Si(111)-(7 × 7) and (b) Si(111)(5√3 × 5√3)R30°-Sb surfaces. Each large triangle corresponds to one (un)faulted half of the reconstructed surface. The dangling bonds on the surface are marked by small blue dots and the Sb adatoms by black squares. The red and green dashed lines are rotated by 6.6° and 16.1° (9.8°) with respect to the ⟨11−2⟩ direction, respectively. The dangling bonds or Sb adatoms lying on these lines are marked with large red and green dots. The distances between these dots are indicated.

7) surface. The (un)faulted half’s of the Si(111)-(7 × 7) are illustrated by large triangles, and for simplicity only Si atoms with dangling bonds are marked by blue dots. Interestingly, we find that the observed rotation angles correspond to directions on the Si(111)-(7 × 7) surface where Si atoms with dangling bonds are aligned with each other (dashed lines in Figure 2a). For each rotational angle, two spacings between the dangling bonds on the surface are obtained, a1, a2, b1, and b2. The calculated distances are given in Supporting Information, Table 1, and are compared with the Sb2Te3 (10.0) lattice spacing. It shows that an integer number of Sb2Te3 unit cells can be matched on a1 and a2 and on b1 + b2. The higher number of coincidence sites for a given distance for a 6.6° rotation suggests that this is energetically more favorable and thus more likely to occur than a rotation by 16.1°, in agreement with our observation. Furthermore, we have observed different rotational angles in GeTe-Sb2Te3 alloy films grown on Si(111)-(7 × 7) (Supporting Information, Figure 4a), which can be related to the smaller in-plane lattice constant of the alloy. This suggests that the domains are rotated in order to reduce the lattice mismatch with the dangling bonds of the Si(111)-(7 × 7) surface (Supporting Information, Figure 4b). C

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sites as discussed in the main text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

J.E.B. performed the growth, XRD and RHEED measurements of Sb2Te3. GeTe-Sb2Te3 alloys were grown and characterized by V.B. and R.W. K.P. and A.G. contributed to the growth optimization and characterization. TEM measurements and analysis were conducted by J.M. and B.J.K. Analysis of data was mostly carried out by J.E.B. The paper was written by J.E.B and R.C., with the help and through contributions from all coauthors. All authors have given approval to the final version of the manuscript. The project was initiated and conceptualized by R.C.

Figure 3. Angular dependence of the RHEED intensity for Sb2Te3 films consisting of 1 QL grown on (a) Si(111)-(7 × 7) and (b) Si(111)-(5√3 × 5√3)R30°-Sb.

coincidence lattices occurs during the initial growth of Sb2Te3 on Si(111). We have thus demonstrated that the epitaxial registry of 2D bonded materials deposited on reconstructed Si(111) surfaces differs from classic epitaxy, as coincidence lattices are formed with the bonding geometry of the reconstructed Si(111) surface and not with the Si(111)-(1 × 1) surface. Furthermore, we have demonstrated that the epitaxial relationship between silicon and Sb2Te3 can be controlled by changing the Si surface reconstruction. This shows that substrate−film bonding continues to play a significant role for the epitaxy of 2D bonded materials on 3D bonded substrates and that Si surface reconstructions do not completely reorder due to the deposition of 2D materials at the given substrate temperature. The survival of surface reconstructions can have a number of causes: First, it can be energetically favorable for the reconstructed silicon to be present at the interface; second, the driving force for the reordering of silicon is too low; or third, the energy barrier for silicon reordering is too high. Given the relatively weak interaction that occurs between a 3D bonded substrate and a 2D bonded material it is likely that the driving force for reordering is lower compared to the case of classical epitaxy and that this is the main cause for the observed effect. However, contributions of the other two causes to the survival of surface reconstructions cannot be ruled out, and further investigations will be needed in order to determine the energetics of this type of interface. To place our findings in a broader context, we conclude that the growth of 2D materials on reconstructed surfaces makes it possible to create solid−solid interfaces with interface structures and bonding configurations that are influenced by the surface reconstruction of 3D bonded substrates. The relatively easy fabrication of such interfaces using 2D materials and the wide range of 2D materials available makes it possible to study this type of solid−solid interface in great detail. Given the wide variety of electronic effects that occur at classical epitaxial interfaces, interesting effects can be expected to occur when 2D materials are joined with reconstructed substrates surfaces.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by EU within the FP7 project PASTRY (GA 317746). We thank S. Behnke and C. Stemmler for technical support and J. M. Wofford for careful reading of the manuscript.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

RHEED patterns of reconstructed Si(111) surfaces, XRD diffraction profile and cross-section TEM images of Sb2Te3 films, calculations on the lattice mismatch of Sb2Te3 with the Si(111)-(1 × 1) surface, Φ-scans for GeTe-Sb2Te3 grown on Si(111), and table with distances between coincidence lattice D

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(19) Liu, Y.; Wang, H.-H.; Bian, G.; Zhang, Z.; Lee, S. S.; Fenter, P. a.; Tischler, J. Z.; Hong, H.; Chiang, T.-C. Phys. Rev. Lett. 2013, 110, 226103. (20) He, L.; Kou, X.; Wang, K. L. Phys. Status Solidi RRL 2013, 7, 50−63. (21) Zhang, G.; Qin, H.; Teng, J.; Guo, J.; Guo, Q.; Dai, X.; Fang, Z.; Wu, K. Appl. Phys. Lett. 2009, 95, 053114. (22) Bansal, N.; Kim, Y. S.; Edrey, E.; Brahlek, M.; Horibe, Y.; Iida, K.; Tanimura, M.; Li, G.-H.; Feng, T.; Lee, H.-D.; Gustafsson, T.; Andrei, E.; Oh, S. Thin Solid Films 2011, 520, 224−229. (23) Rodenbach, P.; Calarco, R.; Perumal, K.; Katmis, F.; Hanke, M.; Proessdorf, A.; Braun, W.; Giussani, A.; Trampert, A.; Riechert, H.; Fons, P.; Kolobov, A. V. Phys. Status Solidi RRL 2012, 6, 415−417. (24) Katmis, F.; Calarco, R.; Perumal, K.; Rodenbach, P.; Giussani, A.; Hanke, M.; Proessdorf, A.; Trampert, A.; Grosse, F.; Shayduk, R.; Campion, R.; Braun, W.; Riechert, H. Cryst. Growth Des. 2011, 11, 4606−4610. (25) Andrieu, S. J. Appl. Phys. 1991, 69, 1366. (26) Braun, W. J. Vac. Sci. Technol., B: Microelectron. Nanometer Struct.−Process., Meas., Phenom. 1998, 16, 1507. (27) Tarakina, N. V.; Schreyeck, S.; Borzenko, T.; Schumacher, C.; Karczewski, G.; Brunner, K.; Gould, C.; Buhmann, H.; Molenkamp, L. W. Cryst. Growth Des. 2012, 12, 1913−1918. (28) Grundmann, M.; Böntgen, T.; Lorenz, M. Phys. Rev. Lett. 2010, 105, 146102. (29) Grundmann, M. Phys. Status Solidi 2011, 248, 805−824. (30) Neergaard Waltenburg, H.; Yates, J. T. Chem. Rev. 1995, 95, 1589−1673. (31) Park, K.-H.; Ha, J. S.; Yun, W. S.; Lee, E.-H.; Yi, J.-Y.; Park, S.-J. Phys. Rev. B 1997, 55, 9267−9270. (32) Elswijk, H. B.; Dijkkamp, D.; van Loenen, E. J. Phys. Rev. B 1991, 44, 3802.

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