Title: Self‐organized van der Waals epitaxy of layered chalcogenide structures
Abstract: physica status solidi (b)Volume 252, Issue 10 p. 2151-2158 Original PaperOpen Access Self-organized van der Waals epitaxy of layered chalcogenide structures Yuta Saito, Corresponding Author Yuta Saito [email protected] Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 4, 1-1-1 Higashi, Tsukuba 305-8562, Japan CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, JapanCorresponding author: e-mail [email protected], Phone: +81-29-861-8013, Fax: +81-29-851-2902Search for more papers by this authorPaul Fons, Paul Fons Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 4, 1-1-1 Higashi, Tsukuba 305-8562, Japan CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, JapanSearch for more papers by this authorAlexander V. Kolobov, Alexander V. Kolobov Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 4, 1-1-1 Higashi, Tsukuba 305-8562, Japan CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, JapanSearch for more papers by this authorJunji Tominaga, Junji Tominaga Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 4, 1-1-1 Higashi, Tsukuba 305-8562, Japan CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, JapanSearch for more papers by this author Yuta Saito, Corresponding Author Yuta Saito [email protected] Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 4, 1-1-1 Higashi, Tsukuba 305-8562, Japan CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, JapanCorresponding author: e-mail [email protected], Phone: +81-29-861-8013, Fax: +81-29-851-2902Search for more papers by this authorPaul Fons, Paul Fons Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 4, 1-1-1 Higashi, Tsukuba 305-8562, Japan CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, JapanSearch for more papers by this authorAlexander V. Kolobov, Alexander V. Kolobov Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 4, 1-1-1 Higashi, Tsukuba 305-8562, Japan CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, JapanSearch for more papers by this authorJunji Tominaga, Junji Tominaga Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 4, 1-1-1 Higashi, Tsukuba 305-8562, Japan CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, JapanSearch for more papers by this author First published: 11 August 2015 https://doi.org/10.1002/pssb.201552335Citations: 55AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract Highly oriented SbTe films were successfully deposited by RF-magnetron sputtering on both crystalline and amorphous substrates. A novel deposition mechanism and method are proposed based on van der Waals epitaxy. Due to the selective reactivity of the top surface atoms of the substrate with sputtered atoms, a Te monolayer is the first layer formed on the substrate, resulting in the subsequent layer-by-layer growth of the SbTe film independent of the crystallinity of the substrates. We believe that this method can be applied to the mass production of a wide range of various van der Waals solids, such as transition metal dichalcogenides and topological insulators for future electronics devices. 1 Introduction Two-dimensional-layered materials beyond graphene, such as topological insulators (TIs) and transition metal dichalcogenides (TMDCs), have been attracting great attention and extensive research is being carried out 1-3. Despite the fascinating features of these materials, most of them have been limited to fundamental research mainly due to the difficulties of fabricating two-dimensional materials uniformly over a large area, since large area growth is one of the most essential requirements to achieve mass production. TMDCs have recently gained increased attention due to a number of unique properties, such as the formation of a direct optical gap in monolayers with concomitantly strong photoluminescence 4, 5, strong spin-orbit coupling, and a lack of inversion symmetry resulting in spin-valley locking (6), extraordinarily large exciton bonding energies 7, 8, and strong light–matter interaction 9, 10, to name a few. These unique properties, combined with the very high strength of monolayers and their flexibility, make them of great interest for applications in nanoelectronics and nano-optoelectronics (11). Bulk TMDCs possess a layered structure where covalently bonded monolayers (triple layers terminated by chalcogen atomic planes) are held together by van der Waals interaction (12). At the same time, SbTe and BiTe chalcogenides have attracted attentions for a number of applications such as thermoelectric devices and phase change memory devices. These alloys are well known to be among the most important thermoelectric materials 13, 14, and the superlattice BiTe/SbTe exhibits the highest thermoelectric figure of merit at room temperature (15) which is characterized by the electrical conductivity, Seebeck coefficient and thermal conductivity. In non-volatile electrical memory applications, SbTe is an important component along with GeTe for phase change memory (PCM) to enable switching between amorphous and crystalline states 16, 17. Recently it was demonstrated that a GeTe/SbTe superlattice can significantly reduce the switching energy of PCM (18). Furthermore, SbTe and BiTe are typical TI materials 19-22. TIs possess conductive edge states on their surfaces characterized by a Dirac cone, while the bulk is an insulator with an energy band gap (23). TIs are expected to open new horizons in spintronics and quantum computing. Representative TI materials, such as SbTe and BiTe, also have a layered structure where covalently bonded quintuple layers are connected by van der Waals forces. For all the applications, high-quality bulk single crystals or highly oriented thin films are required to exploit the appealing physical properties. In thermoelectrics, a variety of methods have been reported for growth of high-quality SbTe or BiTe films, including co-evaporation (24), sputtering (25), MOCVD 26-28, and BiTe/SbTe superlattices fabricated using molecular beam epitaxy (MBE) (15). In non-volatile memory, GeTe and SbTe single crystal films have been deposited by MBE on surface-treated Si single crystal substrates 29, 30 and GeTe/SbTe superlattice films have successfully been fabricated by sputtering using alloy targets (18). In topological insulators, MBE has been widely used to grow single crystal films. (31) Various preparation methods such as chemical exfoliation (32), CVD 33, 34, or MBE (35) have also been developed for TMDCs. However, once lab-based experiments are transformed to an industrial scale, the sputtering technique is more suitable than many alternatives from the viewpoint of cost and productivity since sputtering has long been used for thin film production, such as for semiconductor devices and optical media. In this work, SbTe was selected as an example among a multitude of two-dimensional materials, in order to unveil the deposition mechanism of layered chalcogenide materials, and sputtering was adopted as the deposition method because it is an industry-friendly technique. In this work, the deposition mechanism of highly oriented SbTe films is unraveled from the viewpoint of van der Waals epitaxy and a novel deposition method is proposed. 2 Experimental methods Films were prepared on Si(100), Si(111), and SiO glass substrates using radio-frequency (RF) magnetron sputtering using alloy targets (GeTe, SiTe, and SbTe). In addition, for some samples, tungsten and silicon layers were initially deposited on the substrates. First, the effect of sputter-cleaning of the substrate surface was investigated. Figure 1a shows a schematic of the sample preparation process with and without surface sputtering. First a 50-nm-thick tungsten film was deposited onto an Si substrate (i), followed by exposure to air (ii), leading to surface oxidation of the tungsten film. For one sample, a chalcogenide film was deposited directly onto the substrate (iii), while for another piece of the same sample, surface sputtering using Ar was carried out prior to film deposition ((iv) and (v)). It should be noted that not only was an SbTe single film deposited but also GeTe/SbTe and SiTe/SbTe superlattice films were fabricated in order to test the application of this method for the fabrication of PCRAM 18, 36. In all cases, an SbTe film was initially deposited to allow for the growth of highly oriented GeTe or SiTe compounds (37). Additionally, in order to understand the deposition mechanism, tungsten or silicon layers were deposited on the substrate prior to SbTe film deposition. We define the sample names used in this work as shown in Table 1. The sputtering pressure and deposition temperature used for chalcogenide film growth were in the range 0.4–0.5 Pa and 200 to 250C, respectively. X-ray diffraction (XRD) measurements were carried out using a Cu K source (= 1.542 Å) in mode along the surface normal. The microstructure of the samples was observed by transmission electron microscopy (TEM) with an acceleration voltage of 200 kV. The TEM samples were prepared by ion milling at liquid nitrogen temperature. Table 1. Sample list Figure 1Open in figure viewerPowerPoint (a) Schematic images of the sample preparation process. (i) W deposition on an Si substrate, (ii) exposure to air, (iii) deposition of a chalcogenide (Ch.) film on the oxidized surface, (iv) Ar–plasma cleaning to remove surface oxides, and (v) deposition of a chalcogenide film on the cleaned substrate. (b) XRD spectra for [GeTe/SbTe]/SbTe(5 nm) films deposited on a W(50 nm)/Si substrate without (A) and with (B) sputter-cleaning of the W surface. 3 Results and discussion Figure 1b compares the XRD peak intensities for samples A and B. It is clear that the peak intensities of the sputter-cleaned sample are significantly enhanced compared to the non-cleaned sample. It should be noted that the peak at 28, which is the strongest powder diffraction peak of SbTe (015), was absent in both samples (38). It can thus be concluded that both films are highly oriented with the (001) plane normal perpendicular to the substrate surface, and the degree of the orientation can be significantly improved by the sputter-cleaning of the surface. Microstructural differences between samples A and B were observed by TEM. Figures 2a and b show low-magnification cross-sectional micrographs. The surface morphology with sputter cleaning is noticeably better than that without cleaning. From Figs. 2c and d, in addition, one can confirm the misalignment of grains in the sample A. The corresponding electron diffraction patterns (see inset to (c) and (d)) are in good agreement with XRD results. These results suggest that the grain structure is the origin of the lower intensity of the XRD signal as shown in Fig. 1b. In Fig. 2c, a WO amorphous-like thin layer was identified at the interface of sample A, while it was not observed in sample B as shown in Fig. 2d. Figure 2Open in figure viewerPowerPoint Cross-sectional TEM images of samples A and B. (a) and (b) are low-magnification views of the samples without/with plasma cleaning, respectively, and zoomed images are shown in (c) and (d), respectively. Insets are selected area electron diffraction images at the marked points in (c) and (d). Very similar results were observed using an Si substrate. Figure 3 shows XRD patterns for sample C for different sputter-cleaning times. While the peak intensities of the uncleaned sample are very weak, they increase significantly with increasing cleaning time. It is noted that only (00) reflections were observed from the as-grown films, confirming their strong orientation. Figure 3Open in figure viewerPowerPoint Sputtering-cleaning time dependence of the intensities of the XRD peaks of SbTe (50 nm) films on the Si substrate (C). The effect of Si substrate orientation was then compared using samples D and E. The inset in Fig. 4a shows the XRD patterns for untreated Si(111) and Si(100) substrates. The substrate peaks appear at different positions due to the different orientation of the Si substrates. Rather unexpectedly, the intensities of all peaks originating from samples D and E almost perfectly matched each other. If the orientation of the superlattice film was dominated by the atomic arrangement of the Si surface as in conventional MBE, the degree of orientation would have been expected to be different between the (111) and (100) planes. However, no meaningful difference was identified in the XRD results regardless of substrate orientation. To understand such unusual behavior, a cross-section of sample D was observed using TEM (Fig. 4b). A few nm thick and uniform interface-layer was clearly observed just above the Si(100) surface. Since lighter elements show white-tinged contrast in a bright-field image, it is speculated that the interface-layer consists of Si. Moreover, the interface-layer is homogeneous throughout the film and no grain boundaries can be observed. In addition, energy dispersive x-ray spectroscopy (EDX) revealed that the oxygen concentration was at a contamination level, so that the formation of silicon oxide may be excluded. Therefore, it is suggested that the interface-layer is an amorphous-Si (a-Si) layer formed by Ar ion bombardment, which breaks certain bonds on the Si substrate surface, resulting in its transformation to the amorphous phase. According to these results, the degree of orientation of the superlattice film does not depend on the Si substrate orientation. A crucial question still remains: why is the superlattice, which maintains a rigid order atomic arrangement, formed even on an amorphous underlayer? Figure 4Open in figure viewerPowerPoint (a) XRD patterns of [SiTe/SbTe]/SbTe(5 nm) films on different Si substrates ((100) (sample D) and (111) (sample E)). Inset XRD of the substrates. (b) Cross-sectional TEM image of the superlattice film on Si(100). Below, we propose a novel deposition mechanism based on the selective reactivity of the constituent elements of the film (Sb, Te) and the substrate materials giving rise to van der Waals epitaxy. Figure 5 shows the binary phase diagrams of (a) Sb–W, (b) Te–W, (c) Sb–Si, (d) Te–Si, (e) Sb–O, and (f) Te–O systems 39-43. These diagrams can be divided into two classes; (a) and (c) having no compounds or solid solutions, while (b), (d), (e), and (f) compounds are formed: WTe, SiTe, and oxides, respectively. In other words, Sb does not form a compound with W and Si, while Te does, which results in significantly different sticking coefficient. On the other hand, O reacts with both Sb and Te, resulting in a non-exclusive reaction, as shown in Figs. 5e and f. Therefore, it is expected that once the Si or W surface is cleaned and surface oxides are removed, the surface will be covered with a quasi-monolayer of Te, when Sb and Te atomic species are co-deposited onto the substrate. On the other hand, it has been reported that the Sb-terminated Si(111)-(77) reconstructed surface plays an important role in the growth of a single crystal film by MBE (30). In that case, the surface reactivity of the reconstructed Si(111) surface may exhibit different reactivity than the unreconstructed Si surface. Meanwhile, on the amorphous Si layer, we believe a Te layer is preferentially deposited and covers the substrate. As mentioned earlier, SbTe and related V–VI compounds have a layered crystal structure, in which five atomic planes that form a quintuple layer (QL), are weakly bonded by van der Waals forces (Fig. 6a) (44). Figure 5Open in figure viewerPowerPoint Binary phase diagrams of (a) Sb–W, (b) Te–W, (c) Sb–Si, (d) Te–Si, (e) Sb–O, and Te–O 39-43. Figure 6Open in figure viewerPowerPoint Self-organized van der Waals epitaxy model, (a) crystal structure of SbTe consisting of a quintuple layer (QL) bonded by van der Waals forces. (b) Schematic of the initial stage of the sputter deposition process using the alloy targets. Without (c) / with (d) the selective reactivity. Such materials can be grown on the substrate even in the presence of a large lattice mismatch under condition known as van der Waals epitaxy 31, 45-47, as depicted in Fig. 6. If we consider this mechanism together with selective reactivity, the following mechanism can be proposed. When the surface selectivity is absent (e.g., on the oxide), the first layer is a mixture of Sb and Te, resulting in the loss of QL formation ability (Fig. 6c). On the other hand, due to the reactive selectivity of the surface, the surface is terminated by a Te atomic layer, which is followed by the formation of the first QL by the formation of weak van der Waals bonds (Fig. 6d). This is the same situation as occurs in van der Waals epitaxy onto a surface terminated with dangling bonds, where a “quasi” van der Waals gap is formed between the surface-terminating Te layer and the first Te layer of the QL (46). After growth of the first QL is complete, the process continues. Te atoms again dominated the layer formation to form the bottom layer of the second QL. As a result, highly oriented layer-by-layer deposition occurs. This mechanism is very reasonable and suitable for mass production of SbTe films, since successive layer growth naturally occurs in the sequence Te/Sb/Te/Sb/Te once the initial Te layer covers the W or Si surface. We define this deposition mechanism as “self-organized van der Waals epitaxy (SO–vdW epitaxy).” It should be noted that even though a strong out-of-plane orientation of the film can be successfully achieved by sputtering, this technique does not ensure the orientation in the in-plane direction, namely the as-grown film is not a single crystal but instead has a fiber texture with grains exhibiting a random in-plane orientation and the in-plane grain size is about 100 nm on average. Based on SO–vdW epitaxy, we searched for other materials that would give rise to the same characteristic surface selectivity for Sb and Te co-deposition. Figure 7 shows the Periodic table for such elements. The different colors describe the classification. Intriguingly, there is a rather limited number of elements (highlighted in red in Fig. 7) that show selectivity for Sb and Te besides Si and W. On the other hand, most of other elements produce compounds or solid solutions with both Sb and Te. Since oxygen belongs to the latter group, the SbTe QL cannot be formed on an oxide layer, as shown in Figs. 1b and 3. Therefore, the removal of oxide layers is essential not only to form the high-quality SbTe-oriented film, but also for the growth of superlattices of SbTe and GeTe. Here, it should be noted that although the importance of chemical similarity or surface reactions to the quality of epitaxial layer has been pointed out and the surface termination is well known to improve the quality of epitaxial films 47-49, in this work we for the first time specifically address the possible combination of the elements and the rule of chemical selectivity in enabling the selective growth process. Figure 7Open in figure viewerPowerPoint Periodic table of elements, colored by selectivity, red: selective, blue: non-selective of the substrates. Finally, to verify the proposed mechanism, an SbTe film was deposited onto an SiO glass substrate covered with a sputter-deposited 50-nm-thick amorphous Si film (samples F and G), and the XRD patterns were compared with a film deposited on a cleaned Si substrate (the sample C). Figure 8 shows the XRD results of three SbTe films deposited onto different substrates. The XRD intensity of the SbTe film on the glass (oxide) was very low indicating poor preferred orientation. However, when an amorphous silicon film was deposited on the glass substrate prior to SbTe deposition, the film exhibited much stronger XRD peak intensities, which were comparable to the intensities obtained for a film deposited on a sputter-cleaned Si substrate. Here, one can see a slight difference in the peak positions. This may arise from strain induced by the difference in thermal expansion coefficients of the Si and SiO substrates. Since the SbTe film is usually deposited at a high temperature (250C), followed by cooling to room temperature, the difference of the thermal expansion may leave different stresses in the film. Additionally, the relatively strong background at around 25 in the film on the glass is attributed to the broad peak of amorphous SiO itself. Figure 9 shows cross-sectional TEM images of sample H. In the bright field image, the sputter-deposited homogeneous amorphous Si layer is clearly identifiable above the glass substrate. Figure 9b is another image at the same position using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Since the contrast of light (such as Ge) and heavy (such as Sb or Te) elements are reversed in the HAADF image, the alternate contrast in the superlattice film reflects the alternate stacking of GeTe and SbTe layers. In the magnified image (c), the lattice fringes were observed parallel to the substrate surface. Furthermore, from a fast Fourier transform (FFT) analysis, it was confirmed that the superlattice film was clearly formed even on an amorphous Si layer (d). The results strongly support the validity of the proposed mechanism of the SO–vdW epitaxy. Figure 8Open in figure viewerPowerPoint XRD patterns of SbTe films deposited on sputter-cleaned Si (C), a glass substrate (F), and a glass covered with sputter-deposited amorphous Si (50 nm) (G). Figure 9Open in figure viewerPowerPoint Cross-sectional TEM images of a [GeTe/ SbTe]/SbTe(5 nm)/a-Si(50 nm)/glass sample (H). (a) Bright field image, (b) HAADF-STEM mapping, (c) bright field-magnified image, and (d) fast Fourier-transformed (FFT) image at the interface between the film and substrate. Heterostructures of different two-dimensional materials, such as molybdenum disulfide (MoS), other dichalcogenides, and layered oxides, including topological insulators, have great potential to enable novel electric devices beyond Moore's law 1-3. Since these materials are composed of layers, terminated by chalcogens, the proposed deposition method can be expanded to the broader field of two-dimensional materials. The large-area deposition of high-quality layered materials on a variety substrates may bridge the gap between industry and fundamental research. Furthermore, the proposed sputtering method requires a maximum process temperature of only 250C, a value much lower than required for the MBE process, making this method technologically friendly. We believe that self-organized van der Waals epitaxy by sputtering will contribute to the realization of novel devices using two-dimensional materials. 4 Conclusions In this work, self-organized van der Waals epitaxy of SbTe films was proposed and confirmed experimentally. The degree of film crystallographic orientation strongly depended on the chemical nature of the under-layer material rather than on its structure. Related binary phase diagrams could be categorized by the relative reactivity of Sb and Te. According to the proposed model, the choice of surface composition was found to be important as the substrate. Based on this model, highly oriented chalcogenide films were successfully fabricated even on an amorphous Si substrate. 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