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Te monolayer-driven spontaneous van der Waals epitaxy of two-dimensional pnictogen chalcogenide film on sapphire Jae-Yeol Hwang, Young-Min Kim, Kyu Hyoung Lee, Hiromichi Ohta, and Sung Wng Kim Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02737 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 14, 2017

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Te monolayer-driven spontaneous van der Waals epitaxy of two-dimensional pnictogen chalcogenide film on sapphire Jae-Yeol Hwang,† Young-Min Kim,†, ‡ Kyu Hyoung Lee,◊ Hiromichi Ohta,○,* and Sung Wng Kim†,* †

Department of Energy Science, SungKyunKwan University, Suwon 16419, Republic of Korea

‡

Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Suwon 16419,

Republic of Korea ◊

Department of Nano Applied Engineering, Kangwon National University, Chuncheon 24341,

Republic of Korea ○

Research Institute for Electronic Science, Hokkaido University, N20W10, Kita, Sapporo 001-

0020, Japan

KEYWORDS: van der Waals epitaxy, α-Al2O3, epitaxial films, pnictogen chalcogenides, 2D layered materials

ABSTRACT: Demands on high-quality layer structured two-dimensional (2D) thin films such as pnictogen chalcogenides and transition metal dichalcogenides are growing due to the findings of exotic physical properties and potentials for device applications. However, the difficulties in

ACS Paragon Plus Environment

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controlling epitaxial growth and the unclear understanding of van der Waals epitaxy (vdWE) for 2D chalcogenide film on three-dimensional (3D) substrate have been major obstacles for the further advances of 2D materials. Here, we exploit the spontaneous vdWE of high-quality 2D chalcogenide (Bi0.5Sb1.5Te3) film by the chalcogen-driven surface reconstruction of a conventional 3D sapphire substrate. It is verified that the in-situ formation of pseudomorphic Te atomic monolayer on the surface of sapphire, which results in a dangling bond-free surface, allows the spontaneous vdWE of 2D chalcogenide film. Since this route uses the natural surface reconstruction of sapphire with chalcogen under vacuum condition, it can be scalable and easily utilized for the developments of various 2D chalcogenide vdWE films through conventional thinfilm fabrication technologies.


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The rise of 2D chalcogenide materials (2DCMs) have evolved with the discoveries of exotic physical properties beyond 3D materials since an impact of predecessor graphene.1,2 Lately, explosive interests have been focused on 2DCMs, such as pnictogen chalcogenides (PCs) Pn2Ch3 (Pn = Sb or Bi and Ch = Se, Te, or S) and transition metal dichalcogenides (TMDs) TmCh2 (Tm = W or Mo) due to the findings of high performance thermoelectricity,3 superior carrier mobility,4

ultrafast

charge

transfer,5

topological

insulator,6,7 Weyl

semimetal,8

and

superconductivity.9 Such extraordinary properties of 2DCMs originate from their unique structural configuration and chemical bonding nature in which the crystal structure generally consists of weak cross-plane van der Waals (vdW) bonding between the interlayers with strong covalent and/or ionic bonding within intralayer, causing strong anisotropy in physical properties. The research focus on 2DCMs has shifted from the study of material itself to the use of material properties for various practical applications.3-5, 7, 9-13 Further advances and broad applications of 2DCMs can be realized by uniform and high-quality epitaxial films of 2DCMs with equivalent bulk properties on conventional 3D substrate. The epitaxial growth of 2DCMs on a dangling bond-free 2D substrate via weak vdW interaction, vdWE (Figure 1a), has long been considered as an ideal epitaxy preserving their own crystal structure and physical properties.14-16 In principle, vdWE of 2DCMs can be also made on a conventional 3D substrate by altering unsaturated surface structure into saturated one to fit for the vdWE (Figure 1b), whereas misfit dislocations and/or strain are inevitably generated when 2DCMs are heteroepitaxially grown on an unsaturated surface of the 3D substrate. Recently, many efforts have been focused on the epitaxial growth of 2DCMs, especially for PCs, on conventional 3D substrates (Si, α-Al2O3, GaAs, and BaF2) with small lattice mismatch,17 epitaxial buffer layer,18 or surface modification.19-23 However, the current methodologies for the development of epitaxial 2DCM


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films on conventional 3D substrates have been limited to moderate performance and complicated fabrication process.24-26 Mostly in common, additional procedures to generate artificially tailored 2D-like surfaces on 3D substrates are required for invoking the vdW epitaxial growth of 2DCMs on conventional 3D substrates.17,25-27 Surprisingly, except few studies about the TMDs,22,23 there is still missing in-depth knowledge and experimental verification about the interface structure and the epitaxial relation for the vdW epitaxial growth of 2DCM film on 3D substrates.

Figure 1. Schematic illustrations for possible epitaxial growths of 2D structures on different substrates. (a) 2D structure on the 2D substrate via vdWE, (b) 2D structure on clean surfaced 3D substrate terminated with an atomic layer of chalcogen via quasi-vdWE, and (c) the unit cells with atomic arrangements for 2D chalcogenides, such as TMDs and PCs, and α-Al2O3.

Among various 3D substrates, we conceive that sapphire (α-Al2O3; rhombohedral structure, space group R3 c ) can be a versatile structural platform for vdW epitaxial growth of 2DCMs due to the lack of dangling bonds on the intrinsically-generated α-Al2O3 (001) surface,28 in which the crystal symmetry and atomic arrangements (Figure 1c) are similar to most PCs (rhombohedral structure, space group R3 m ) and TMDs (hexagonal structure, space group P63/mmc) in addition to chemical, thermal, and mechanical stabilities. In particular, commercially available cheap αAl2O3 substrate is now widely used in modern device technologies such as light emitting diode


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(LED),29 frequency agile devices,30 and high electron mobility transistors (HEMTs).31 Hence, the realization of high-quality vdWE film on α-Al2O3 will give a huge impact on the monolithic integration of 2DCMs for practical device applications. In particular, it is crucial to produce a dangling bond-free surface of the α-Al2O3 substrate in the atomic scale for invoking the uniform and scalable vdW epitaxial growth of 2D chalcogenide film. Furthermore, the stacking control of 2D chalcogenide film on the 3D substrate affecting physical properties is of major concern. Here, we report a new approach for the epitaxial growth of high-quality 2DCMs on 3D substrate via spontaneous vdWE enabling large-area uniformity and low defect density without any buffer layer or additional treatment. As a proof, we demonstrate the high-quality epitaxial growth of 2D pnictogen chalcogenide Bi0.5Sb1.5Te3 (BST) film on α-Al2O3 substrate exploiting the naturally-assembled surface reconstruction of substrate. We verify that the in-situ formation of pseudomorphic Te monolayer on the surface of α-Al2O3 promotes a spontaneous vdW epitaxial growth of 2DCMs maintaining the structural correlation between vdWE film and substrate, which results in the high-quality vdWE BST film with a superior carrier mobility of 339 cm2 V – 1 s – 1 at 300 K comparable to bulk single crystal BST.


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Figure 2. (a) X-ray diffraction (XRD) patterns through ω-2θ scans. An asterisk (*) indicates αAl2O3 substrate peak. (b) the optimized growth rate of BST film, (c) X-ray reflectivity (XRR) data (values indicate the thicknesses of films), (d) rocking curves for BST (0 0 6) peak (values indicate full width at half maximum values), and (e) ϕ-scans of BST 0 1 5 reflections for Bi0.5Sb1.5Te3 films deposited on α-Al2O3 substrates with different thicknesses, respectively. Two distinguishable in-plane atomic arrangements (marked as † and ‡) of BST film, azimuthally rotated from each other by 60°, were visualized in (e).

Figure 2a shows XRD patterns for the BST films deposited on α-Al2O3 substrates by PLD. Only intense (0 0 l) diffraction peaks of BST are observed without any secondary phase in the ω2θ scans. Pendellösung fringes around every BST (0 0 l) peak indicate strong c-axis orientation of BST films and the formation of the clean interface between α-Al2O3 and BST layer. The optimized growth rate for BST film was 0.3 Å/shot (Figure 2b) and the thickness of BST film


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was determined by XRR (Figure 2c). The degree of orientation for BST film along cross-plane direction was ranging from 153 to 320 arcseconds implying a good crystallinity (Figure 2d) comparable to molecular beam epitaxy (MBE)-grown film.32 Six-fold symmetrical peaks in ϕscan prove that the BST films were epitaxially grown on α-Al2O3 substrates (Figure 2e). In the ϕ-scan of BST, there are two sets of three-fold symmetric peaks denoted as † and ‡. One set of BST peaks (†) corresponds to 0 1 3 reflections of α-Al2O3 substrate at the same ϕ angles. This signifies that in-plane orientation of BST film is correlated with the atomic arrangement of the in-plane lattice of α-Al2O3. The other set (‡) is azimuthally 60° rotated from the set of †. The epitaxial relation between BST film and α-Al2O3 substrate was confirmed as [1 2 0] (0 0 1) BST || [1 2 0] (0 0 1) α-Al2O3 (Figure S1). The in-plane lattice mismatch (dfilm – dsubstrate / dsubstrate) between BST (a = 4.302 Å) and α-Al2O3 (a = 4.754 Å; ICSD #9771) is −9.5 %. Assuming BST film is directly grown on top of α-Al2O3 substrate via conventional heteroepitaxy, the variation in lattice parameter and the formation of dislocation are inevitably generated in the epitaxial BST film to relieve misfit strain, inherited from the considerable in-plane lattice mismatch between overlayer film and substrate due to the strong covalent bonding between dangling bonds on substrate and constituent atoms of film at the heterointerface (Figure S2). However, no considerable change in lattice parameter of the as-grown epitaxial BST film on the α-Al2O3 substrate was found compared to bulk BST in XRD analysis. This might be related to the peculiar epitaxial growth of layer structured BST film on α-Al2O3, suggesting that unconventional quasi-vdWE occurs as depicted in Figure 1b.


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Figure 3. (a) HAADF-STEM image of vdWE BST on α-Al2O3. The red arrow indicates the direction of STEM-EDX line scan. (b) EDX line profile of Tellurium (Te) at the interface between BST and α-Al2O3. (c) STEM-EDX mappings for Sb, Bi, Te, and composite in the vdWE BST layer. (d) Illustrations for the surfaces of Al- and Te-terminated α-Al2O3 substrates (open circle: the outmost vacant Al sites, green ball: Te). (e)-(f) Magnified images of the boxed regions in (a) for the near the surface of α-Al2O3 (cyan box) and at the heterointerface (yellow box). The blue zigzag line indicates the bonding array of α-Al2O3 along c-axis shown in (d). Simulated atomic arrangements of α-Al2O3, quintuples of BST, and atomic Te layer were superimposed on the magnified STEM images.

In order to directly confirm the vdWE of BST film on α-Al2O3 substrate, the interfacial atomic arrangements between BST and α-Al2O3 were investigated by high-resolution scanning transmission electron microscopy (HR-STEM). The uniformly oriented epitaxial BST film consisting of quintuple layers separated by vdW gaps along [0 0 l] and the clean heterointerface


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are observed (Figure 3a). It should be noted that atomically thin Te monolayer (white spotted line) on the top of the α-Al2O3 substrate was clearly observed and identified by energy dispersive X-ray spectroscopy (EDX) line scan along [0 0 l] at the heterointerface (Figure 3b). In particular, there is a vdW gap between the first quintet of BST and Te-terminated α-Al2O3 substrate. The inplane lattice of BST for each quintuple layer was 4.302 ± 0.003 Å, almost identical to that of bulk BST (a = 4.302 Å). The chemical composition of as-grown BST film was identified as Bi:Sb:Te = 9.89%:29.6%:60.5% in EDX analysis. STEM-EDX mapping for each element proves that vdWE BST film consists of uniformly cascaded Te1-Bi/Sb-Te2-Bi/Sb-Te1 quintuple layers along c-axis without any detectable defect in atomic scale, such as stacking fault and antisite defect (Figure 3c). According to the structural simulations, Te atom can occupy the outmost vacant Al sites (open circle) without breaking crystal structure and the symmetry ( R3 c ) of αAl2O3 as depicted in Figure 3d. This is verified in the comparison between simulated atomic arrangements and the magnified high angle annular dark field (HAADF)-STEM image of αAl2O3. As shown in Figure 3e, there is a huge Z-contrast between the atomic array (white dots) on the top surface of α-Al2O3 and the constituent atoms of α-Al2O3. Since the intensity of the observed atom in HAADF-STEM is proportional to the nth power (n = 1.5-2) of its average atomic number Z and there is an apparent difference in Z value between Te, Al, and O (ZTe = 52, ZAl = 13, and ZO = 8), the white atomic array is clearly identified as a Te atomic monolayer, consistent with STEM-EDX analysis (Figure 3b). It is noted that the Te atoms coordinate at the outmost vacant Al sites along the same column of Al sites in the view of [1 2 0]. Indeed, an interatomic distance of Te-O (1.961 Å) is similar to that of the outmost vacant Al-O (1.966 Å), whereas the distance of inner Al-O is 1.854 Å. Figure 3f shows the atomic arrangement of BST lattice on α-Al2O3 at the heterointerface indicating that the Te atomic monolayer was naturally


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assembled on top of the α-Al2O3 substrate during the PLD process and then the spontaneous vdWE of BST film occurred on the Te monolayer.

Figure 4. (a) Simulated in-plane atomic arrangements of BST and α-Al2O3. (b) STEM-EDX line profiles for each element of BST were superimposed on the HAADF-STEM cross-sectional image. 11 times of in-plane BST lattice is well-matched to 10 times of in-plane α-Al2O3 (AO) lattice. (c) Pseudomorphically coordinated Te atoms (green) on the surface of α-Al2O3. (d) Electrical conductivity (σ), Hall mobility (µH) and carrier concentration (nc) of vdWE BST film on α-Al2O3 substrate and single crystal BST as a function of temperature. (e) Comparisons in electronic transport properties of BST (a-b plane) between single crystal,33 vdWE film on a 2D substrate34 and vdWE film on a 3D substrate (this work) at 300 K, respectively.


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Interestingly, as we expected from the simulated atomic arrangements (Figure 4a), BST/Teterminated α-Al2O3 with 11/10 matching in in-plane lattice alignment was clearly identified like a domain matching epitaxy35, 36 as shown in Figure 4b. Consistently with the observations of the ϕ-scans for BST film and α-Al2O3 substrate, this can be interpreted that there is a considerable structural correlation between vdWE BST film and α-Al2O3 substrate even though the first BST quintet is separated by vdW gap since the surface Te atoms create pseudomorphic monolayer by coordinating at the outmost vacant Al sites without perturbing the structure of α-Al2O3 (Figure 4c). It is clear that this peculiar quasi-vdWE (2D on 3D) is distinct from the conventional vdWE (2D on 2D) and 3D heteroepitaxial growths. Figure 4d shows the temperature dependence of inplane electrical conductivity (σ), carrier concentration (nc), and Hall mobility (µH) for vdWE BST film on α-Al2O3 and single crystal BST. As-grown vdWE BST film shows p-type conduction as the BST bulk with the same composition. In-plane µH of vdWE BST on α-Al2O3 film (339 cm2 V –1 s –1 at 300 K) was superior to that of single crystal BST (312 cm2 V –1 s –1 at 300 K)33 while σ of single crystal BST is higher than that of vdWE film due to the higher nc for whole investigated temperature range.30 The difference in nc signifies that vdWE BST film has lower atomic defect density than a single crystal indicating that the generation of extrinsic carriers, inherited from pnictogen/chalcogen antisite defects and chalcogen vacancies in PCs,37,38 are successfully suppressed. In particular, vdWE BST film on a 3D substrate (α-Al2O3) exhibits an equivalent performance comparable to vdWE BST film on a 2D substrate (graphene)34 in terms of crystal quality and electronic transport properties (Figure 4e). This proves that vdWE film with high crystal quality and the low defect density can be grown on a conventional α-Al2O3


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substrate without any 2D template by utilizing the chalcogen monolayer-driven pseudomorphic surface reconstruction of α-Al2O3.

The vdWE growth mechanism of 2D chalcogenides on the 3D substrate can be understood from the natural formation of Te atomic monolayer on the α-Al2O3 substrate. The single Alterminated surface is the most stable α-Al2O3 (0001) surface with non-polarity in nature.39,40 Hydroxylation of clean α-Al2O3 (0001) surfaces results in further lowering of the energies since the attachment of hydrogen to the surface drastically lowers the Gibbs free energy and the work function in the realistic environment.40 In our experiment, no surface treatment was performed on the α-Al2O3 and the substrate was handled in air prior to deposition. Therefore, OH groups are always present on the nature-driven bare α-Al2O3 surface.41,42 During the PLD process, a plume with the Te-rich composition can be easily made in the vacuum chamber due to the relatively higher vapor pressure of Te than others (Te > Sb > Bi). Also, the oxygen affinity of Te is larger than other elements. Te adatom can be actively attached on the surface of α-Al2O3 by capturing through surface OH group (and/or exposed surface oxygen) and stabilized by coordinating on the topmost vacant Al site, resulting in a stable Te-O bonding.43 Thus, a dangling bond-free α-Al2O3 surface suitable for a vdWE is realized by the natural formation of a Te atomic monolayer on the α-Al2O3 substrate. Hence, the growth of the deposited layers can be mainly determined by the surface energy of the grown 2D material like conventional vdWE (2D on 2D). Theoretically, the surface energy is the lowest for the (0 0 1) surface of BST among low Miller index surfaces and the lowest interfacial energy between two basal planes is expected for outmost Te-Te interface in BST structure.44 When the ablated clusters or plasma species arrive at heated substrates, atoms tend to form new surfaces with lower surface energies. Accordingly, after the surface of α-Al2O3


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is uniformly transformed into monolayer Te, the epitaxial growth of BST film along c-axis can be initiated by the formation of the first Te layer on the surface of Te-terminated α-Al2O3 via vdW interaction. We found that the epitaxy of BST film can be grown at substrate temperature range from 190 oC to 350 oC whereas BST films deposited at a substrate temperature below ~190 o

C show (0 0 l)-textured polycrystalline. This indicates that the formation of pseudomorphic Te

monolayer on the α-Al2O3 surface is initiated at the temperature over 190 oC. Substrate temperature was limited by the chemical stability of Te, which has the lowest melting temperature (~450

o

C) among constituent elements. In vacuum condition, the melting

temperature of Te element might be lower than the atmospheric condition. Indeed, over 350 oC of substrate temperature, the growth of BST film is deteriorated due to the instability of Te. Finally, Bi2Te3 and Sb2Te3 with the same crystal symmetry ( R3 m ) and similar atomic arrangements were epitaxially grown on α-Al2O3 substrates via vdWE maintaining the structural correlation of in-plane orientation in the same manner (Figure S6). This denotes that air-stable αAl2O3 surface can be easily utilized for the spontaneous vdWE of 2D chalcogenides with the help of natural formation of pseudomorphic Te monolayer through various thin-film fabrication technologies. In summary, we demonstrated a new concept and promising methodology for the spontaneous vdWE of 2DCMs on a conventional 3D substrate by rendering the high-quality vdWE BST films on α-Al2O3 through utilizing the naturally-assembled surface structure of substrate. The surface reconstruction driven by the in-situ formation of pseudomorphic Te monolayer on the α-Al2O3 during PLD process promotes the spontaneous vdWE of 2D chalcogenide BST film preserving good crystallinity, high carrier mobility, and the in-plane structural correlation between vdWE film and substrate. Our findings provide new insights into


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the importance of hetero-interface structure attributing vdWE and the fundamental properties of 2D chalcogenide vdWE film on 3D substrate for exploring unprecedented physics in the vdW heterostructure.

Methods. Thin film preparation. High-purity (>99.999%) Bi, Sb, and Te granules were weighted according to the composition Bi0.5Sb1.5Te3 and loaded into a vacuum-sealed quartz tube of 10 mm, and the contents were melted and homogeneously mixed in a rocking furnace for 10 h at 1073 K. The acquired ingot was pulverized into powder by ball milling and a 2 in. diameter BST target was prepared by spark plasma sintering under 70 MPa at 480 °C for 3 min. Using this target, BST thin films were deposited on α-Al2O3 (001) substrate (10 mm × 10 mm) by PLD under the conditions at the substrate temperature of 200−230 °C, Ar partial pressure of 10 mTorr, and KrF excimer laser (λ = 248 nm) with the fluence of 1.25−1.5 J cm–2. Material characterization. XRD analyses of ω-2θ, ω-, and ϕ-scans, and XRR were performed for vdWE films using an X-ray diffractometer (Rigaku Smartlab, Rigaku Co.) in parallel beam mode with monochromatized Cu Kα1 radiation (λ = 1.540592 Å). The thickness of the BST film was characterized by XRR measurement. Structural information including interplanar distances between arbitrary lattice planes was extracted from the optimized diffraction conditions for symmetric and asymmetric scans through numerical calculations based on least square method. Sample preparation for STEM was carried out using a dual-beam focused ion beam (FIB, AURIGA CrossBeam Workstation, Carl Zeiss) slicing and lift-out technique. The microscope used for atomic resolution HAADF-STEM imaging was a probe-corrected STEM (JEMARM200F, JEOL) equipped with a cold field emission source, operating at 200 kV. To improve the image quality and the resolution, successive images were acquired with short time intervals and averaged for the same investigated area. An EDX (JEM-ARM200F, JEOL) in the STEM


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imaging mode was used to obtain atomic resolution chemical mapping data. Electronic transport properties as a function of temperature were measured using four-point probe geometry by physical parameter measurement system (PPMS).

ASSOCIATED CONTENT

Supporting Information. The following supporting information is available free of charge on the ACS Publication website. Epitaxy relation between vdWE BST film and α-Al2O3 substrate, conventional heteroepitaxy of 2D on 3D, structural characterizations through EDX, AFM, and STEM, chemical composition of vdWE BST film at the near hetero-interface, and the realization of vdWE for Bi2Te3 and Sb2Te3 on α-Al2O3 (PDF)

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

ACKNOWLEDGMENT J.-Y.H. was supported by Basic Science Research Program through the National Research Foundation

of

Korea

(NRF)

funded

by

the

Ministry


of

Education

(NRF-

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2017R1A6A3A11032364). Y.-M.K. was supported by the Institute for Basic Science (IBSR011-D1). This research was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MSIP) (NRF-2015R1A5A1036133) and by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2015M3D1A1070639).

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