Article pubs.acs.org/crystal
Optimization of In2Se3/Si(111) Heteroepitaxy To Enable Bi2Se3/In2Se3 Bilayer Growth Somilkumar J. Rathi,† David J. Smith,‡ and Jeff Drucker*,‡ †
Materials Science and Engineering, School for Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, Arizona 85287-6106, United States ‡ Department of Physics, Arizona State University, Tempe, Arizona 85287-1504, United States S Supporting Information *
ABSTRACT: Systematic optimization of molecular beam epitaxy growth parameters enabled high quality heteroepitaxy of In2Se3 on Si(111) surfaces. Surfaces of the best epilayers were characterized by atomically flat terraces that extended laterally for several hundred nanometers. These terraces were separated by single quintuple layer high steps. These In2Se3 films were suitable for subsequent high quality epitaxy of Bi2Se3. The quality of the In2Se3/Bi2Se3 interface was confirmed using atomic resolution transmission electron microscopy.
I. INTRODUCTION
A promising route for mitigating these difficulties in order to fully realize the promise of these materials is to incorporate them into epitaxial heterostructures.25−27 This strategy has the potential to (i) stabilize the electronic properties of the surface via encapsulation at a suitable epitaxial interface, and (ii) enable facile integration with other materials. In addition, fabrication of thin Bi2Se3 layers can increase their surface-to-volume ratio and amplify electronic transport in the topologically protected surface bands relative to the bulk.28 Molecular beam epitaxy (MBE) is a proven technique for achieving these goals.29−32 Recent studies have shown that the growth of Bi2Se3 directly on Si(111) surfaces yields material with nonoptimal crystal and interface quality.33 As a consequence, we have performed an exploratory investigation into In2Se3/Si(111) heteroepitaxy for use as a buffer layer for subsequent Bi2Se3 growth with the eventual goal of fabricating epitaxial ...In2Se3/Bi2Se3/In2Se3... heterostructures. Stoichiometric In2Se3 is known to exist in several different crystalline phases: α, β, γ, δ, and κ.34−37 It is commonly believed that there are two major structural polymorphs, exemplified by the α and γ phases. Both α and β In2Se3 are hexagonal layered phases which are similar to Bi2Se3.36 γ and δ belong to a different defect Wurtzite structural classification.36,37 The stability regimes and transition temperatures between these phases have been extensively investigated but are still under discussion due to the difficulty of performing highquality structural determinations of the various polymorphs.
Layered chalcogenides are attracting significant attention due to their unique properties that may lead to diverse technological applications.1 Among these materials are the recently discovered topological insulators (TI), which are characterized by fully insulating bulk and metallic, topologically protected surface states.2−7 These topologically protected surface states exhibit spin-momentum locking that prohibits backscattering, which could lead to dissipationless surface transport, and suggests spintronic applications.8,9 When integrated with other materials such as ferromagnets or superconductors, additional exciting applications may result as a consequence of the novel physics that are predicted to emerge.10,11 These include solidstate Majorana modes that could lead to topologically protected quantum computation, the quantized magnetoelectric effect, and magnetic monopole-like image charges.6,12 Perhaps the most studied of these TI materials is Bi2Se3, due in part to its ∼0.3 eV bandgap, which is large for this class of materials, and its relatively simple surface electronic structure.13−16 The majority of experimental investigations into the electronic properties of Bi2Se3 as a TI have been performed on bulk crystals synthesized by a variety of methods.17,18 However, these materials can be plagued by a high point defect density that autodopes the bulk, which obscures transport in the surface bands.19 The surface electronic properties of these Bi2Se3 crystals also show a poorly understood gradual shift (aging) with respect to time.20−22 The surface electronic properties can also degrade either due to oxidation or absorption of other impurities once the Bi2Se3 crystals are exposed to atmosphere.23,24 © 2014 American Chemical Society
Received: May 16, 2014 Revised: June 17, 2014 Published: August 4, 2014 4617
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them with the structural quality of the In2Se3 layer. We find that In2Se3 films deposited and annealed at temperatures less than 300 °C on clean Si(111) always exhibit poor crystalline quality. These films are islanded and nonstoichiometric (In-deficient). Stoichiometry and crystallinity are however achieved by depositing films at 100 °C and then annealing at 400 °C. We find that In and Se codeposition at a substrate temperature of 400 °C followed by annealing at the growth temperature in a Se flux is crucial for obtaining the highest quality surfaces for subsequent Bi2Se3 heteroepitaxy. We also have conclusively identified the growth conditions leading to hexagonal layered phase In2Se3 growth on Si(111) and confirmed it using crosssectional transmission electron microscopy (TEM). The high quality of the Bi2Se3/In2Se3 interface is confirmed using crosssectional high-angle annular-dark-field scanning transmission electron microscopy (HAADF-STEM).
Additionally, there are conflicting reports regarding epitaxial stabilization of the different phases due to variability between growth conditions, substrates, and substrate orientations. Recently, the crystal structure of Bi-stabilized α-In2Se3 has been shown to be very similar to Bi2Se3.37 The structural parameters for Bi2Se3 and a representative hexagonal layered phase of In2Se3, α-In2Se3, are listed in Table 1.38−40 In common Table 1. Crystal Structure Parameters for a Representative Hexagonal Layered Phase of In2Se3 and Bi2Se3 material
a (Å)
c (Å)
crystal structure
QL spacing (Å)
α-In2Se3 Bi2Se3
4.0338,39 4.1440
19.238,39 28.640
hexagonal hexagonal
9.638,39 9.540
with many other layered chalcogenides, they are composed of covalently bonded Se-M-Se-M-Se (M = Bi or In) quintuple layers (QLs) that assemble into three-dimensional crystals mediated by weak van der Waals interlayer interactions. The crystal structure is schematically depicted in Figure 1. Figure 1a
II. EXPERIMENTAL METHODS The In2Se3 and Bi2Se3 films were grown using physical vapor deposition in a home-built MBE system under ultrahigh vacuum (UHV) conditions. The growth chamber was evacuated to pressures of 1.1 × 10−10 Torr using a magnetically levitated Seiko-Seiki STP-451C turbomolecular pump backed by a rotary vane pump. Selenide-based thin films were grown on 0.4 cm × 3.3 cm Si strips. These strips were cut from n-type, on-axis Si (111) (to within 1°) wafers with resistivity ρ = 0.01−0.05 Ω cm. Oxide free surfaces for growth were obtained in situ by first degassing the Si (111) strips at 650 °C for 12 h, followed by rapid flashing at 1250 °C for an integrated duration of 30 s in the growth chamber. After the flashing step, the Si (111) substrate was allowed to cool down for about 2 h prior to room temperature deposition. To assess the effect of different surface preparation on film growth, we also performed selected growth experiments on miscut (4° tilted toward [112̅]) and on H terminated Si surfaces. H-termination was achieved by dipping the Si strips into dilute HF/H2O solution (5%) for 2 min immediately before inserting into a vacuum. The substrates were resistively heated with direct current. Substrate temperature, T, was measured using a two-color infrared pyrometer (Omega IR 2C) with an accuracy of ±10 °C. The lowest substrate temperature that can be measured using our optical pyrometer in twocolor mode is 360 °C. Temperatures lower than 360 °C were estimated by the following process. Immediately after the flashing step, the sample was heated to T = 360 °C and the corresponding heating current was noted. Then, T was increased in increments of 10 °C up to 430 °C, and the corresponding heating current values were noted. Using this T vs I data, we can then extrapolate to the lower temperatures of relevance for our growth experiments. In2Se3 and Bi2Se3 films were grown either by sequential or codeposition of the elemental constituents. Ultra high purity (>99.999%) Bi, In, and Se pellets obtained from Alfa Aesar Inc. were used as source materials and were evaporated from alumina coated W wire baskets. Each deposition source is individually shuttered enabling termination of the target material flux to the substrate and facilitating the switch between the In2Se3 and Bi2Se3 layers. There are line-of-sight shutters between sources to minimize cross-contamination. Deposition rates were correlated with source currents using postgrowth Rutherford backscattering spectrometry (RBS) and monitored during the deposition using a quartz crystal microbalance. Both the In and Bi deposition rates were held constant at 0.2 ML/min. The Se deposition rate was ∼0.6 ML/min, providing 3 times overpressure during growth employing codeposition of the metal and chalcogen components of the target layer. One monolayer or ML is referred to the atomic density of the bulk-terminated Si(111) surface, which is 7.83 × 1014 atoms/cm2. Sample morphology was analyzed by field emission scanning electron microscopy (FESEM) (Hitachi S-4700 operated at 15 kV) and ambient atomic force microscopy (AFM) in tapping mode. High resolution transmission electron microscope (TEM) images were acquired using a JEM-400EX operated at 400 keV with a structural
Figure 1. (a) Projection of a single quintuple layer of Bi2Se3 along a showing the stacking order of Bi and Se. The Se2 layer is the inversion plane for the Se1/Bi and Se1′/Bi′ layers as shown in the projection along c displayed in (b). Se2 occupies C sites, Bi and Se1′ occupy B sites, and A sites are occupied by Se1 and Bi′.
shows a single QL projected along the a direction. Figure 1b shows a projection of the hexagonal lattice along c, which identifies the three different sites labeled A, B, and C. The ∼3% lattice mismatch between these two materials can be effectively accommodated via van der Waals epitaxy.26 Additionally, the large bandgap of α-In2Se3, 1.2−1.3 eV relative to Bi2Se3, provides wide latitude for its use as a barrier layer for integration of ferromagnetic metals or superconductors in order to exploit the novel behavior that is expected to occur.36,41 Wang and co-workers have demonstrated the possibility of Bi2Se3 growth on In2Se3/Si(111).26 However, they did not correlate the starting surface quality with that of the underlying In2Se3 layer or its dependence on growth parameters. Here, we demonstrate that high quality Bi2Se3 epitaxy can be achieved on planar regions of In2Se3 films grown on Si(111). Thus, the primary experimental challenge is to achieve high quality In2Se3 epitaxy on Si(111). As a consequence, we place particular emphasis on optimizing growth parameters for high quality In2Se3 buffer layers on Si(111). We have systematically explored Si(111) surface processing, growth on miscut surfaces, substrate temperature, and annealing conditions and correlated 4618
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resolution of 1.7 Å. HAADF-STEM images were obtained using a probe-corrected JEOL ARM-200F operated at 200 keV with a probe size of 0.8 Å. Elemental coverage and confirmation of film stoichiometry was quantified using RBS. AFM images and RBS data were computer analyzed using off-line image analysis and RUMP software, respectively.
III. RESULTS AND DISCUSSION We commence our discussion by first identifying the growth conditions leading to the highest quality In2Se3 films on Si(111). Then we demonstrate that excellent Bi2Se3 epitaxy can be obtained on atomically flat regions of the best In2Se3 films. Growth optimization of In2Se3 was achieved by systematic variation of the growth parameters. Specifically, we investigated the effects of substrate temperature, deposition sequence (i.e., codeposition of In and Se or In first and then Se or vice versa), Si(111) substrate orientation, and surface pretreatment as well as post growth annealing temperature and duration, on the quality of the resulting In2Se3 layer. Because of the paucity of literature addressing In2Se3 growth on Si(111), a systematic investigation was necessary to obtain the highest quality In2Se3 films. Subsequent high quality Bi2Se3/In2Se3 epitaxy was comparatively straightforward to achieve. A. In2Se3 Growth Optimization. A.1. Growth at Substrate Temperature ≤200 °C. Subsequent to preparing clean, nominally on-axis Si(111) surfaces in UHV, we investigated the morphology of films grown at T ≤ 200 °C by sequential deposition of In then Se and by In and Se codeposition. The initial experiments deposited In onto Si(111) at room temperature to coverages in the 3.0−4.6 ML range. The In flux was terminated, while the substrate temperature was simultaneously raised to 200 °C. The sample was immediately exposed to a Se flux and annealed for 8 min. Hereafter, we will refer to annealing in Se flux as “selenization”. Figure 2a is an AFM image showing the surface morphology of a sample grown following this procedure. It is evident that the surface is covered with islands in three distinct size ranges. The smallest islands appear at a density of ∼1010 islands/cm2 and have an average diameter near 5 nm and average height near 2 nm. The intermediate size islands have an average diameter of 60 nm and height of 15 nm, and their density is approximately 5 × 108 islands/cm2. The biggest islands have an average diameter of 150 nm, height of 45 nm, and density of ∼1 × 107 islands/cm2. Elemental analysis using RBS indicates that the film is not stoichiometric In2Se3, with 4.6 and 7.7 ML of In and Se, respectively, on the surface. Similar experiments were carried out with different selenization durations, but the outcomes were very similar to that shown in Figure 2a. Figure 2b depicts the AFM image of a sample grown at 200 °C by the codeposition of In and Se, with the total deposition time of 20 min. RBS indicates that this film is also not stoichiometric In2Se3. The In and Se coverages are 3.6 and 6.9 ML, respectively. A high density of islands (∼8 × 108 islands/ cm2) with an average diameter near 60 nm and height near 25 nm are evident on the surface. A set of experiments were also performed where Se was first deposited at room temperature followed by In deposition and then the 200 °C selenization step. Nonstoichiometric, islanded surfaces were obtained for these growth experiments as well. The linear contrast features running from lower left to upper right in Figure 2a,b are the bilayer high (3.13 Å) steps expected for Si surfaces very close to the (111) orientation.42 The average terrace width in these
Figure 2. 5 μm × 5 μm AFM images of (a) islanded surface resulting from In deposition onto Si(111) held at room temperature and selenized at 200 °C for 8 min. The surface has 4.6 and 7.7 ML of In and Se, respectively. Arrows show typical intermediate (I) and big (B) sized islands. The smallest islands appear at higher density. (b) Sample grown by codeposition of In and Se onto Si(111) held at 200 °C for 20 min, resulting in an islanded surface. 3.6 and 6.9 ML of In and Se, respectively, are present on the surface. In both (a) and (b), the linear features running from lower left to upper right are steps on the substrate. The white dashed lines are guides to identifying the steps.
images indicates that the starting surface for growth of In2Se3 is well within the Si(111) ± 1° tolerance specified. We conclude that these growth parameters lead to islanded and nonstoichiometric films, which are clearly unsuitable for subsequent Bi2Se3 epitaxy. Additionally, we find that the total island volume is insufficient to account for the total volume of deposited In and Se. A complete accounting of the total material volume requires further investigation. The 3D island morphology evident in Figure 2a may result from island formation during the room temperature In deposition stage of growth. Previous results published by our group and others have shown that In growth on Si(111) at temperatures lower than the In melting point (156 °C) follows the Stranski− Krastanov (SK) mode for which 3D islands grow atop a thin planar layer.43,44 Therefore, it is likely that the 3D islands observed in Figure 2a are the outcome of this growth mode. That is, the 3D islands form during room temperature In deposition and persist after selenization. A factor that may contribute to the Se-rich composition of these films could be 4619
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films obtained for growth on these three surfaces. We also explored annealing at 300 °C, but the resultant films were nonstoichiometric and the surface was islanded. Figure 3a displays the AFM image of a 19.5-nm-thick In2Se3 layer grown on H-passivated, nominally on-axis Si(111). Other material systems have been grown on H-passivated Si(111) via van der Waals epitaxy,46,47 but we believe this report is the first for In2Se3/H-passivated Si(111). The film consists of equilateral triangular islands with an average width of ∼200 nm. Similar triangular features have also been observed in other materials systems such as Bi2Se3 when deposited on Hpassivated Si(111) surfaces.46,47 The majority of these triangular features are oriented along a common crystallographic direction. In addition, their size is comparable to the terrace width observed on these nominally on-axis wafers (see the step-related contrast in Figure 2a,b). The inset in Figure 3a displays the height profile along the dashed line in the AFM micrograph. The film roughness is in the several nanometers range. Figure 3b displays an AFM image of an In2Se3 film grown on Si(111) miscut by 4° toward [112̅]. Our rationale for this attempt was provided by the recent report by Li et al. who showed that the high step density on miscut Si(111) influenced the growth of Bi2Se3.46 For growth on this miscut surface, we observe a similar influence, as shown in the AFM image of Figure 3b. Triangular features evident on the film surface appear to be templated by the substrate steps. A similar morphology was reported for Bi2Se3 growth atop miscut Si(111).46 The AFM height profile shown in the inset indicates that the surface roughness is about twice that of the film depicted in Figure 3a. Moreover, in comparison to growth on the H passivated surface discussed in the preceding paragraph, the size of the crystallographically oriented features has decreased. We attribute this decrease to the reduced terrace size on the miscut surface that has templated growth of the crystallographically oriented triangular features. On the basis of the surface morphology, we believe that growth on miscut substrates does not provide an In2Se3 surface of sufficient quality for subsequent epitaxy of Bi2Se3. It has been shown that very thin Bi layers can remove the Si(111)-7×7 surface reconstruction and potentially provide a higher quality starting surface for epitaxy.25,48 On the basis of this result, we deposited 1/3 ML Bi on Si(111) at room temperature prior to In2Se3 growth. As shown in Figure 3c, the 18.5-nm-thick In2Se3 film consists of connected hexagonal and triangular domains in addition to several similarly shaped 3D features appearing on the film surface. Planar regions of the film are rough on a similar scale to the surfaces shown in Figure 3a,b but there are 3D islands. One possible explanation for the observation of these islands may be reduction of the Si(111) surface energy by Bi exposure, which could promote 3D island growth instead of layer by layer film growth. A.3. Se Surface Passivation and Annealing at 400 °C. We found that the best In2Se3 films resulted from a strategy shown by Bansal et al., to produce high quality Bi2Se3 epitaxy on Si(111).32 They showed that Se deposition onto Si(111) held at 100 °C led to a self-limiting surface passivation at a Se coverage of 1 ML. Deposition at lower T accumulated excess Se, while deposition at higher temperature allowed Se reaction with the Si substrate and formation of an amorphous layer. Following this passivation scheme, they obtained superior quality Bi2Se3 films. On the basis of their success, we adopted this as the first step in our growth procedure.
the accumulation of excess Se at the Si(111) substrate/film interface, as observed, for Se deposition at T < 70 °C by Bansal et al.32 A.2. Effect of Si(111) Surface Pretreatment and Step Density on In2Se3 Film Morphology. Further efforts to optimize In2Se3 film quality explored growth on miscut Si(111) surfaces to assess the effect of step density on film morphology. We also investigated whether surface pretreatments including H-passivation or Bi predeposition significantly improved the quality of the In2Se3 epilayer. For these experiments, we also modified the growth process. We began by codepositing In and Se at 100 °C for 4 min and then ramped the substrate temperature to 400 °C and continued depositing for another 42 min. After the deposition fluxes were terminated, the resulting film was annealed for 3 min with no Se flux at 400 °C. Also, on the basis of the findings of Lu and co-workers45 who demonstrated complete crystallization of thermally deposited amorphous In2Se3 films via annealing at 380 °C, we selected 400 °C as the annealing temperature. Films annealed at 400 °C were stoichiometric In2Se3, as measured using RBS. Figure 3 compares the surface morphology of In2Se3
Figure 3. 5 μm × 3 μm AFM images of In2Se3 films grown by the codeposition of In and Se for 4 min at 100 °C and 42 min at 400 °C, on (a) H passivated Si(111), (b) Si(111) miscut by 4° toward [112̅], and (c) Si(111) that was predeposited with 0.3 ML Bi. The film thicknesses were 19.5, 24.9, and 18.5 nm for the films shown in panels (a), (b), and (c), respectively. Insets show the height profile of each film along the indicated white dashed lines. 4620
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flux. Figure 4b shows an AFM image of a stoichiometric In2Se3 film grown using this procedure. The equivalent thickness of this film is 15 nm. Apparently, the temperature ramp to 400 °C combined with annealing in a Se flux qualitatively alters the surface morphology. The film comprises atomically flat planar regions that extend laterally for several hundred nanometers. As seen in the AFM height profile shown in the inset to Figure 4b, these atomically flat planar regions differ in height by integral multiples of In2Se3 QLs (∼0.9 nm). Figure 4c displays a crosssectional TEM image of this film showing that it comprises domains of crystalline, layered In2Se3. The In2Se3 layer floats atop an amorphous layer at the substrate/film interface. We speculate that this amorphous layer formed via oxygen diffusion through the In2Se3 layer and subsequent reaction with the Si substrate to form amorphous SiO2. A similar sequence of events was characterized by Bansal et al. for Bi2Se3 films grown following a similar procedure.32 Another possibility is that the amorphous layer is an artifact of TEM specimen preparation, since regions of the TEM specimen that are too thick for acquisition of atomically resolved images show little evidence of this amorphous layer. B. Bi2Se3 Growth Atop In2Se3. Of the In2Se3 growth procedures we investigated, we identified the one leading to the film depicted in Figure 4b as being the best candidate for subsequent Bi2Se 3 heteroepitaxy. Immediately after we annealed the codeposited In2Se3 layer at 400 °C in a continuous Se flux, we reduced the substrate temperature to 100 °C and began simultaneous deposition of Bi and Se. After 4 min, the substrate temperature was raised to 220 °C and the Bi and Se codeposition was continued for a further 42 min. Following deposition, the sample was cooled to room temperature in about 1 h prior to removal from vacuum for characterization. Figure 5a shows an AFM image of a 27 nm equivalent thickness Bi2Se3 film grown on a 16 nm thick In2Se3 layer. RBS analysis for this sample is shown in the Supporting Information, Figure S1. The triangular growth mounds commonly observed in MBE-grown Bi2Se3 layers are evident in Figure 5a.29−32,46,49 These mounds comprise atomically smooth terraces of 0.5−1 μm width that are usually separated in height by a single QL, as shown in the inset to Figure 5a. Occasionally, these terraces can be separated by sub-QL, ∼0.5 nm high steps. These mounds have 3-fold symmetry and are aligned along the ⟨111⟩ directions of the substrate. The origin of these triangular growth mounds is not clear, but their formation has been attributed to either spiral growth around a screw dislocation at their core or a recently proposed “reflection” at substrate steps mechanism.46,49 Complete understanding of their origin and whether or not they can be eliminated by variation of growth parameters or improved quality of the substrate surface is a topic for further investigation. We characterized the interface between the Bi2Se3 and In2Se3 films using cross-sectional HAADF-STEM. On the basis of the differences in their atomic numbers, Bi atomic columns appear with brighter contrast than In atomic columns. As shown in Figure 5b, the interface between the two selenide layers is commensurate, and high quality layer-by-layer growth of Bi2Se3 has been achieved. On the basis of the close similarity in the structures of the In2Se3 and Bi2Se3 layers evident in Figure 5b, we believe that the In2Se3 has grown in one of its hexagonal layered phases. We have been unable to conclusively identify whether the In2Se3 layer is α or β phase.
Figure 4a depicts an AFM image of a sample grown by codeposition of In and Se for 20 min at 100 °C onto Se-
Figure 4. (a) 1 μm × 1 μm AFM scan of an In2Se3 film grown by codepositing In and Se for 20 min at 100 °C and annealing at 400 °C. Total film thickness is 5.9 nm. (b) 1 μm × 1 μm AFM image of an In2Se3 film grown by codepositing In and Se for 4 min at 100 °C and 42 min at 400 °C, and then annealing at 400 °C for 3 min in a Se flux. Total film thickness is 15 nm. Insets show the height profile of each film along the indicated white lines. (c) A cross-sectional high resolution TEM image of layer structure phase viewed along [110] of In2Se3 film shown in (b).
passivated Si(111). Following In and Se deposition, the sample was annealed for 20 min at 400 °C with no Se flux. Analysis of this film by RBS indicates that the film is stoichiometric In2Se3; i.e., the In-to-Se ratio is 2-to-3. RBS also shows the equivalent film thickness is 5.9 nm. From the AFM image and inset height profile, it is evident that the film comprises planar regions with pits. Within the vertical resolution of AFM, these planar regions are atomically flat. About 90% of the sample surface area is covered by planar regions, and the remaining is occupied by ∼2−3 QL deep pits. The white circular features evident on the film surface are 3D islands that formed during AFM image acquisition. These features were absent during the first few AFM scans and appeared after sequential scans of the same area. We investigated longer duration anneals of identically grown films. Doubling the annealing time to 40 min produced stoichiometric In2Se3 films with rougher planar regions and increased the areal coverage and depth of the pits. We also investigated films grown for longer times that resulted in 18 and 40 nm equivalent thickness layers of stoichiometric In2Se3. Both of these thicker films were annealed for 20 min. The film quality progressively decreased as the equivalent thickness increased as evidenced by increasing roughness of the planar regions and increasing areal density and depth of the pits. To further improve the film quality, following Se passivation at 100 °C, we codeposited In and Se at the same temperature for 4 min and then ramped the temperature to 400 °C and continued the deposition for another 42 min. After terminating the In flux, the film was annealed at 400 °C for 3 min in a Se 4621
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hundred nanometers. These terraces differed in height by single quintuple layers. These In2Se3 epilayers were suitable for subsequent high-quality heteroepitaxy of Bi2Se3. The Bi2Se3/ In2Se3 interface was commensurate, as sensed by atomic resolution HAADF-STEM. Triangular growth mounds were evident in AFM images of the Bi2Se3 surface. These mounds consisted of flat terraces separated by quintuple layer high steps. Our results provide a useful starting point for growth of In2Se3/Bi2Se3/In2Se3 heterostructures that may stabilize the electronic properties of the topologically protected Bi2Se3 surface states. Alternatively, Bi2Se3 surfaces cleaved from bulk crystals could be employed as substrates for In2Se3/Bi2Se3 growth. Atomically flat terraces with widths near 1 μm have recently been produced using this technique.50 Finally, the growth strategy presented here may enable straightforward integration of Bi2Se3 with ferromagnets and superconductors in order to investigate novel physics and exploit predicted technological applications.
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ASSOCIATED CONTENT
* Supporting Information S
RBS analysis demonstrating stoichiometry of the Bi2Se3/In2Se3 heterostructure shown in Figure 5a. 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: jeff
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Seed Funding from the College of Liberal Arts and Sciences. We also acknowledge use of facilities in the LeRoy Eyring Center for Solid State Science at Arizona State University and thank Dr. Toshihiro Aoki for assistance with operation of the JEM-ARM200F.
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REFERENCES
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Figure 5. (a) 1 μm × 1 μm AFM image of Bi2Se3 film grown on In2Se3. The respective thicknesses of the Bi2Se3 and In2Se3 films are 27 and 16 nm. Inset shows the height profile along the indicated line revealing atomically flat terraces separated by single QL high steps on the surface. (b) Cross-sectional high-angle annular dark-field scanning transmission electron microscopy image of Bi2Se3/In2Se3 heterostructure grown on Si(111) showing excellent epitaxy at Bi2Se3/In2Se3 interface. Sample surface morphology is shown in (a).
IV. CONCLUSIONS In summary, we have used MBE to grow Bi2Se3/In2Se3 bilayers on Si(111). Systematic optimization of growth parameters enabled heteroepitaxy of hexagonal layer phase In2Se3/Si(111). The highest quality epilayers resulted from a growth sequence that began with Se passivition of the Si(111) substrate at 100 °C. In2Se3 growth was via codeposition initially at 100 °C, followed by a temperature ramp to 400 °C. The final step was a 400 °C anneal in a Se flux. The best layers were characterized by atomically flat terraces that extended laterally for several 4622
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Crystal Growth & Design
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dx.doi.org/10.1021/cg500722n | Cryst. Growth Des. 2014, 14, 4617−4623
