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## Creator

Emi Kano, [Jun Nara](https://orcid.org/0000-0002-0486-2981), Koki Takenaka, Toshiki Yasuno, Keisuke Atsumi, Shuhong Shuhong Li, Tomonori Nishimura, Kaito Kanahashi, [Jun Uzuhashi](https://orcid.org/0000-0003-2023-8158), Kosuke Nagashio, [Yoshiki Sakuma](https://orcid.org/0000-0001-6804-7217), Nobuyuki Ikarashi

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This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Emi Kano, Jun Nara, Koki Takenaka, Toshiki Yasuno, Keisuke Atsumi, Shuhong Li, Tomonori Nishimura, Kaito Kanahashi, Jun Uzuhashi, Kosuke Nagashio, Yoshiki Sakuma, Nobuyuki Ikarashi; Direct observation of atomic configuration of a highly oriented MoS2 film/α-Al2O3 (0001) using atomic resolution electron microscopy. Appl. Phys. Lett. 22 December 2025; 127 (25): 252102. and may be found at https://doi.org/10.1063/5.0295643[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Direct observation of atomic configuration of a highly oriented MoS2 film/α-Al2O3 (0001) using atomic resolution electron microscopy](https://mdr.nims.go.jp/datasets/ca705cd9-bf97-46d7-be14-60b05606111e)

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Microsoft Word - re_manuscript_marked_20251029.docx1  Direct observation of atomic configuration of a highly oriented MoS2 film/-Al2O3 1 (0001) using atomic resolution electron microscopy 2  3 Emi Kano,1 Jun Nara,2 Koki Takenaka,3 Toshiki Yasuno,3 Keisuke Atsumi,4 Shuhong 4 Li,4 Tomonori Nishimura,4 Kaito Kanahashi,4 Jun Uzuhashi,5 Kosuke Nagashio,4 5 Yoshiki Sakuma,6 Nobuyuki Ikarashi1 6  7 1 Institute of Materials and Systems for Sustainability, Nagoya University, Nagoya, 8 Aichi 464-8601, Japan 9 2 Research Center for Materials Nanoarchitectonics, National Institute for Materials 10 Science, 1-1 Namiki, Tsukuba 305-0044, Japan 11 3 Graduate School of Engineering, Nagoya University, Nagoya, Aichi 464-8603, Japan 12 4 Department of Materials Engineering, The University of Tokyo, 7-3-1 Hongo, 13 Bunkyo-ku, Tokyo 113-8656, Japan 14 5 Electron Microscopy Unit, National Institute for Materials Science, 1-2-1 Sengen, 15 Tsukuba, Ibaraki 305-0047, Japan 16 6 Research Center for Electronic and Optical Materials, National Institute for Materials 17 Science, 1-1 Namiki, Tsukuba 305-0044, Japan 18  19 Corresponding author: Emi Kano 20 Email: kano@imass.nagoya-u.ac.jp 21  22 <Abstract> 23 We deposited a highly oriented MoS2 film on a 2-inch -Al2O3 (0001) wafer by metal-24 organic chemical vapor deposition and determined the atomic configuration of the 25 MoS2/-Al2O3 (0001) stacking structure by performing atomic resolution electron 26 microscopy observations along two orthogonal zone axis directions, i.e., the 〈1120〉 27 and 〈1100〉 directions of -Al2O3. The results show that, first, the in-plane positions of 28 Mo atoms coincide with those of the underlying Al and O atoms, and the [1120] 29 direction of monolayer 2H-MoS2 matches that of the -Al2O3 substrate. Second, the -30 Al2O3 surface was a reconstructed Al-I structure. Moreover, we performed the first-31 principles calculations using the observed in-plane atomic positions of the MoS2/-32 Al2O3 structure as a starting configuration and found that the MoS2 Al2O3 distance is 33 larger than the theoretical van der Waals distance. Because no ordered structures were 34 observed between the MoS2 film and the Al2O3 substrate, the experimental and 35 theoretical results strongly suggest that an amorphous interface layer exists between 36 them. Such an amorphous interface layer is likely to weaken the MoS2Al2O3 37 interaction that determines the stability of the MoS2/-Al2O3 (0001) structure. We thus 38 argue that controlling the interface layer is critical in fabricating highly oriented MoS2 39 films and is vital for improving the performance of field-effect transistors with MoS2 40 channels.  41 2   1 Si metal-semiconductor-oxide field-effect transistors (MOS-FETs) have been scaled 2 down to improve device performance at reduced power and cost. Along the geometrical 3 scaling path, the MOS-FET structure has changed from bulk-planar to gate-all-around, 4 which could eliminate short-channel effects at reduced gate lengths.1 2 On the other 5 hand, the mobility of charge carriers in conventional semiconductors, such as Si and 6 SiGe, decreases as the channel body thickness is made smaller.3 Here, transition metal 7 dichalcogenides (TMDs) show great potential as channel materials for deeply scaled 8 MOS-FETs because they can be used to make an atomically thin body and are 9 theoretically predicted to have high carrier mobility.3 4 5 6 7 8 9 10 For fabricating large-scale integrated circuits with TMD channels, wafer-scale 11 TMD films with improved transport properties are necessary. Previous studies have 12 indicated that defects in TMD films, such as grain boundaries have a major impact on 13 transport properties.10 11 We have grown monolayer MoS2 film on -Al2O3 by metal-14 organic chemical vapor deposition (MOCVD) and have shown that highly oriented or 15 single crystalline MoS2 films exhibit phonon-limited mobility, while polycrystalline 16 MoS2 films exhibit thermally activated mobility.12 These indicate that control of the 17 crystalline orientation of TMD films is critical to controlling their transport properties. 18 The orientation of TMD films is substantially influenced by the crystallographic 19 structure of the substrate,13 14 15 and various models have been proposed for the 20 TMDsubstrate interaction that determines the orientation of the films. However, the 21 underlying mechanism is still under debate. For example, some models assume an 22 interaction through an interface layer, such as a crystalline thin film or a layer of 23 regularly arranged atoms or molecules,16 17 18 19 20 21 while others assume no interface 24 layer and instead postulate a direct van der Waals interaction.14 22 23 In addition, some 25 studies14 16 22 23 on TMDs grown on -Al2O3 (0001) substrates have assumed an Al-I 26 surface structure24, while others18 19 21 have assumed an Al-II surface structure. -Al2O3 27 (0001) wafers have often been used because of their chemical stability under TMD 28 growth conditions and the availability of large-diameter wafers. This variety of 29 proposed TMD/substrate structural models is mainly due to the lack of experimental 30 results showing the atomic configuration of the TMD/substrate stacking structure. That 31 is, the structural features that determine the stability of the structure, such as the 32 positions of atoms of a TMD film along the TMD/substrate interface (or the in-plane 33 atomic positions of a TMD film on a substrate), the surface structure of the substrate, 34 and the structure of the interface layer, if it exists, have yet to be clarified. 35 3  Here, we report a definitive structural analysis on the atomic configuration of the 1 MOCVD-grown MoS2/-Al2O3(0001) stacking structure. We prepared a highly 2 oriented, almost single-crystalline, monolayer MoS2 film with reduced electrically 3 active defects and analyzed it using atomic resolution transmission electron microscopy 4 (TEM) and first-principles calculations. The experimental results clarified the in-plane 5 atomic positions of the MoS2 film/-Al2O3 structure, the surface structure of -Al2O3 6 substrate, and the MoS2-Al2O3 distance. Moreover, the experimental and theoretical 7 analyses strongly suggest the existence of an amorphous interface layer. We thus argue 8 that controlling the interface layer is critical for fabricating highly oriented TMD films 9 and improving the performance of field-effect transistors with MoS2 channels. 10 A MoS2 film was grown by MOCVD, where MoO2Cl2 and H2S were used as 11 precursors, and N2 was used as carrier gas. We used a 2-inch -Al2O3 (0001) wafer as a 12 substrate and annealed it at 1150°C in air for 1 hour prior to the growth of MoS2. After 13 annealing in air at temperatures above 1000 °C, Al2O3 (0001) surfaces reportedly 14 exhibit a terrace-step morphology with atomically flat terraces,17 25 26  and the surface 15 is crystalline.25 Thus, no amorphous layer was likely to exist on the substrate surface 16 before the MoS2 growth, although we did not examine the atomic structure of the 17 substrate surface. The substrate temperature was 950℃ during the growth. Figure 1 18 shows a plan-view TEM image and a transmission electron diffraction (TED) pattern of 19 the MoS2 film transferred onto a holey carbon TEM grid (Quantifoil 1.2/1.3), and the 20 holes are empty. The TED pattern (inset) shows a single set of six 1100 spots of MoS2 21 and no spots due to rotational domains, indicating that the film was almost single 22 crystalline and that the portion of the rotational domains were very small. The 23 micrograph shows a 1100 dark-field (DF) image taken from a specimen area where 24 rotational domains were observed. The dark and light gray regions (arrowed) are 25 rotational and 180 rotational domains, respectively, and the white and gray triangles 26 are double-layer regions. Thus, Fig. 1 shows that most of the MoS2 film was a single 27 crystal with a monolayer thickness, although a small part of the film was rotational or 28 double-layer domains with nanometer-scale lateral dimensions. The electrically active 29 defects in the MoS2 film were well reduced, and the film exhibited phonon-limited 30 mobility.12 31 Atomic resolution observations were performed using high-angle annular dark-field 32 scanning TEM (HAADF-STEM). The acceleration voltage was 200 kV, and the 33 spherical aberration coefficient was less than 1 m. The inner and outer angles of the 34 4  annular dark-field detector were 50 and 150 mrad, respectively. Cross-sectional TEM 1 specimens were prepared by mechanical thinning and Ar ion milling. 2 We performed first-principles calculations based on density-functional theory 3 (DFT) 27 and pseudo-potential schemes using the PHASE/0 code28. The generalized 4 gradient approximation of Perdew, Burke, and Ernzerhof was used as the exchange-5 correlation energy functional.29 As for the van der Waals interactions, the DFT-D2 6 method was applied.30 We used a slab model consisting of monolayer MoS2 on an -7 Al2O3 (0001). The in-plane periodicity of the 3×3 MoS2 and 2×2 -Al2O3 unit cells 8 were fixed. The Al2O3 slab contains twelve Al layers, and the top and bottom of the slab 9 were terminated with a single Al layer in accordance with the present experimental 10 results. Details of the model structure are described in the following. The cut-off 11 energies for the wavefunctions and charge density were 56 Ry and 506 Ry, respectively. 12 The number of k points sampled in the Brillouin zone was more than 2×2 per surface 13 unit cell of -Al2O3 (0001). All the models were optimized to meet the force criteria of 14 0.01 eV/Å. 15 HAADF-STEM images of the MoS2/-Al2O3 (0001) structure viewed in the 16 〈1100〉 and 〈1120〉 directions of -Al2O3 are shown in Fig. 2(a) and 2(b). Bright dots 17 in the images are at the positions of Mo, S, Al, and O atoms. O atoms of the -Al2O3 18 substrate are only weaky seen in Fig. 2(b), because the number of atoms at each O atom 19 position is only half of that of the Al atoms in the incident beam direction. In Fig. 2(a), 20 Mo atoms are located atop the Al (1120) planes indicated by the vertical dotted lines. In 21 Fig. 2(b), they are atop the Al (1100) planes indicated by the vertical dashed lines. In 22 addition, in Fig. 2(b), the lateral (in the 〈1100〉 direction) distance between a Mo and 23 neighboring S atoms is smaller on the left-hand side of the Mo atom than on the right-24 hand side, directly indicating that the [1120] direction of monolayer 2H-MoS2 25 matches that of the -Al2O3 substrate. Thus, the in-plane positions of the Mo and S 26 atoms of the MoS2 film on the -Al2O3 substrate are as depicted in Fig. 2(c), which 27 schematically shows the MoS2/-Al2O3 structure viewed in the 〈0001〉 direction. The 28 dashed and dotted lines in the figure correspond to the positions of the Al planes 29 indicated by the dotted or dashed lines in Fig. 2(a) and 2(b). Thus, Fig. 2(c) shows that 30 the in-plane positions of Mo atoms coincide with those of the underlying Al and O 31 atoms.  32 In the lateral direction in Fig. 2(a) and (b), the three-fold length of the period of the 33 MoS2 lattice image coincides with the two-fold length of the period of the Al2O3 lattice 34 image. This implies that the in-plane atomic spacing of the MoS₂ film is +0.2% of its 35 5  literature value in both the 〈1100〉 and 〈1120〉 directions, provided that the lattice 1 constant of Al2O3 remains unchanged.31 32 However, further experiments are needed to 2 clarify the strain of the MoS₂ film quantitatively. 3 In addition, the HAADF-STEM images of the -Al2O3 surface indicate that the 4 surface is Al-I terminated. In Fig. 2(b), bright dots are at Al atom positions in the Al2O3 5 substrate. The topmost Al layer comprises a single Al atomic layer, denoted as T, while 6 closely spaced pairs of Al atomic layers, denoted as D1, D2, and D3, are in the bulk. This 7 indicates that the substrate surface is Al-I terminated, differing from the Al-II and O-I 8 terminated structures24, as well as from the O-terminated or OH-terminated structures 9 proposed as the surface structures under CVD growth conditions23. Moreover, no 10 periodic structures can be seen between the MoS2 film and the Al2O3 surface in the 11 HAADF-STEM images in Figs. 2. This strongly suggests that the MoS2 film and the 12 Al2O3 substrate are separated by a vacuum layer or an amorphous layer, not by a layer 13 with the ordered structures that have been suggested in previous studies.16 17 18 19 20 21 14 To quantitatively determine the atomic layer spacing in the MoS2/-Al2O3 15 structure, we measured the image intensities in the HAADF-STEM images. Figure 3(a) 16 shows a magnified image of the MoS2/-Al2O3 structure, and Fig. 3(b) shows the image 17 intensity profile in the depth direction ([0001] direction), averaged over 2 nm parallel 18 to the interface. The peak denoted as Mo in the diagram represents the position of the 19 Mo layer. The peak denoted as T is at the positions of the surface Al layer and those 20 denoted as Dn (n = 1 to 7) are at the closely spaced Al atomic layer pairs. Gaussian fits 21 to these peaks (Fig. 3(c)) were used to determine the peak positions, from which the 22 layer spacings were extracted (Fig. 3(d)). The spacing between neighboring Dn peaks is 23 0.22 nm on average and is nearly constant, corresponding to the spacing between Al-24 layer pairs of -Al2O3.31 Minor fluctuations are attributed to experimental noise. The 25 MoT spacing is 0.86  0.06 nm on average across five measurements in distinct 26 observation areas. In addition, the TD1 spacing is 0.16 nm, i.e., 72% of the spacing 27 between the Dn peaks. These spacings are listed in Table I together with those of the 28 theoretical models described below.  29  30  31  32 6  Table I  Experimental and theoretical (model A and B) atomic layer spacings in nanometer. 1 Experimental values show average and standard deviation of layer spacings measured on HAADF-2 STEM images. 3 Layer spacing Experimental Model A B MoT 0.86  0.06 0.43  TD1 0.16  0.02 0.11 0.11 DnDn+1 0.22  0.01 0.22 0.22  4 We further explored the atomic configurations of the MoS2/-Al2O3 structure by 5 performing first-principles calculations using an atomic model with the experimentally 6 determined in-plane positions of the MoS2/-Al2O3 structure as a starting configuration 7 (model A). No interface layer was assumed in the calculation. We also performed 8 structural optimization of a model without a MoS2 layer, i.e., the clean Al-I surface 9 model (model B). The atomic configurations of the relaxed structures of the models are 10 depicted in Fig. 4 and the atomic layer spacings are listed in Table I. We used the 11 positions of the atoms of each atomic layer (Mo, ST, and Dn layers) averaged in the 12 〈0001〉 direction as the atomic layer positions, from which their spacings were extracted. 13 Table I shows that the observed MoT spacing is larger than that of model A. This 14 indicates that the space between the MoS2 film and the substrate in the present sample is 15 not a vacuum, but an interface layer is likely to exist between them. Thus, the observed 16 MoT spacing and HAADF-STEM image in Fig. 2 indicate that an amorphous interface 17 layer exists between the MoS2 film and the substrate. In addition, the observed TD1 18 spacing differs from that of model A and also differs from that of the truncated (pristine) 19 Al-I surface (0.19 nm). This indicates that the surface Al-I layer is reconstructed 20 through an interaction between the surface Al-I layer and the interface layer on it. 21 Therefore, the MoT and TD1 layer spacings in Table I strongly suggest that an 22 amorphous interface layer exists between the MoS2 film and substrate and it affects the 23 Al-I surface structure of the substrate. 24 The present analysis has clarified the atomic configuration of the highly oriented 25 MoS2/-Al2O3 structure. The epitaxial relationship on the atomic scale shows that the 26 interaction between the MoS2 film and the substrate plays a pivotal role in determining 27 7  the MoS2 orientation. On the other hand, the amorphous interface layer found between 1 the film and substrate is likely to weaken the interaction, and thus, when an interface 2 layer that further weakens the interaction forms, the portion of the rotational domains in 3 the MoS2 film would increase. Previous studies have shown that the orientation of MoS₂ 4 changes depending on substrate surface conditions and the MoS₂ growth conditions16- 5 23. These suggest that the properties of the interface layers can be controlled through 6 these conditions. Therefore, controlling the interface layer during MoS2 growth is 7 critical for fabricating highly oriented MoS2 films and for improving their transfer 8 characteristics. 9 In summary, a highly oriented monolayer-MoS2/-Al2O3 grown by MOCVD has 10 been analyzed by atomic resolution HAADF-STEM direct observations combined with 11 first-principles calculations. The observations have directly clarified the epitaxial 12 relation between MoS2 and -Al2O3: the in-plane positions of the Mo atoms coincide 13 with those of the underlying Al and O atoms, and the [11-20] direction of the monolayer 14 2H-MoS2 matches that of the -Al2O3 substrate. In addition, the observed -Al2O3 15 surface was a reconstructed Al-I surface. Moreover, the results showing that the 16 observed MoS2-Al2O3 distance was larger than the theoretical van der Waals distance 17 and that no ordered structures were observed between the MoS2 film and -Al2O3 18 substrate strongly suggest that an amorphous interface layer existed between the MoS2 19 film and -Al2O3 substrate. Such an interface layer is likely to weaken the interaction 20 between the MoS2 film and -Al2O3 substrate. We thus argue that control of the 21 interface layer is critical to growing a unidirectional MoS2 film on -Al2O3 substrate. 22  23 Supplementary material 24 The supplementary material describes the simulated STEM image of model A and the 25 energy dispersive X-ray spectroscopy line profile across the interface layer between 26 MoS₂ and Al₂O₃ for the sample deposited under growth conditions nearly equivalent to 27 those of the sample described in the main text. 28  29 Acknowledgements 30 This work was partly supported by JSPS KAKENHI (Grant Numbers: JP21H05237, 31 JP21H05232, JP22H04957), the JST-Mirai Program (Grant Number: JPMJMI22708192), and JST 32 CREST (Grant Number: JPMJCR24A3). Calculations were performed on the Numerical Materials 33 8  Simulator of NIMS and ES of JAMSTEC. 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Satow, “Synchrotron X‐ray study of 26 the electron density in α‐Al2O3,” Acta Crystallogr. Sect. B 49(6), 973–980 (1993). 27 32 R.W.G. Wyckoff, Crystal Structures, 2nd ed. (Interscience Publishers, New York, 1963). 28  29  30   31 12  <Figure>  1  2 Fig. 1 Example of a plan-view 1100 DF image of rotational domains of MoS2 film and a TED 3 pattern (inset) taken from a large specimen area. The TED pattern shows a single set of six 1100 4 spots, indicating that portion of the rotational domains are very small. Dark and light-gray regions in 5 the DF image (arrows) are rotational and 180 rotational domains, respectively. White and gray 6 triangles are double layer domains. These show that most of the MoS2 film was a single crystal of 7 monolayer thickness. 8   9  10   11 13   1  2 Fig. 2 (a) (b) HAADF-STEM image of monolayer-MoS2/-Al2O3 viewed in [1100] and [1120] 3 direction of -Al2O3. Atomic models of pristine MoS2 and Al2O3 are overlaid. In (b), T indicates the 4 Al2O3 surface, which is terminated with a single Al atomic layer (the Al-I structure), whereas D1 and 5 D2 indicate pairs of closely spaced Al atomic layers. The positions of Mo atoms coincide with those 6 of the Al (1120) planes on the dotted line in (a). The atomic positions of Mo coincide with those of 7 surface Al-I atoms on the vertical dashed lines in (b). In (b), downward arrows show that the lateral 8 distance between a Mo and neighboring S atoms is smaller on the left-hand side of the Mo atom than 9 on the right-hand side. (c) Atomic model of monolayer-MoS2 on -Al2O3 structure based on the 10 HAADF-STEM image in (a) and (b). The dotted and dashed lines indicate the Al atomic plane of -11 Al2O3 and correspond to the dotted lines in (a) and dashed line in (b). 12  13   14 14  Fig. 3 (0001) layer spacing in MoS2/-Al2O3 structure. (a) [1120] HAADF-STEM image. (b) Image 1 intensity distribution in the [0001] direction in (a). The peak denoted as T is the intensity peak due to 2 surface Al-I atoms. Peaks Dn are due to pairs of closely spaced Al-I and II layers. (c) Gaussian 3 curves fitted to peaks in (b). The centers of the Gaussian curves are used as the positions of peaks. 4 (d) Lattice spacings in (c).  5   6 15   1  2 Fig. 4 Theoretical atomic models of (a) fully relaxed MoS2/-Al2O3 structure (model A) and (b) -3 Al2O3 surface without MoS2 (model B). No interface layer was assumed in these calculations. Atoms 4 are marked using the same colors as in Fig. 2. 5  6