Heteroepitaxial van der Waals semiconductor superlattices

A broad range of transition metal dichalcogenide (TMDC) semiconductors are available as monolayer (ML) crystals, so the precise integration of each kind into van der Waals (vdW) superlattices (SLs) could enable the realization of novel structures with previously unexplored functionalities. Here we report the atomic layer-by-layer epitaxial growth of vdW SLs with programmable stacking periodicities, composed of more than two kinds of dissimilar TMDC MLs, such as MoS2, WS2 and WSe2. Using kinetics-controlled vdW epitaxy in the near-equilibrium limit by metal–organic chemical vapour depositions, we achieved precise ML-by-ML stacking, free of interlayer atomic mixing, which resulted in tunable two-dimensional vdW electronic systems. As an example, by exploiting the series of type II band alignments at coherent two-dimensional vdW heterointerfaces, we demonstrated valley-polarized carrier excitations—one of the most distinctive electronic features in vdW ML semiconductors—which scale with the stack numbers n in our (MoS2/WS2)n SLs on optical excitations. Kinetics-controlled van der Waals epitaxy in the near-equilibrium limit by metal–organic chemical vapour deposition enables precise layer-by-layer stacking of dissimilar transition metal dichalcogenides.

work, we report the direct growth of vdW SLs, heteroepitaxially stacked with MoS 2 , WS 2 and WSe 2 MLs, by metal-organic chemical vapour deposition (MOCVD) 33,34 . We achieved a precise ML-by-ML sequential stacking with atomically clean and sharp heterointerfaces by the kinetic control of heteronucleation in the near-equilibrium limit 33,35,36 . This ML-by-ML stacking epitaxy also enabled us to realize tunable vdW SL electronic structures with ML precision. We identified several atomic stacking orders at the vdW heterointerfaces by various methods. Compared with covalent semiconductors, the vdW ML semiconductors often host a distinctively larger and longer-lived spin-valley polarization of charge carriers, which can be efficiently generated by circularly polarized light. With the series of type II band alignments at coherent 2D vdW heterointerfaces in our (MoS 2 /WS 2 ) n SLs, where n is the bilayer stack numbers, we present scaling valley-polarized optical excitations.

Heteroepitaxial growth of vdW semiconductor SLs
Elemental variation in the MX 2 SLs can be achieved either in M-alteration (MX 2 /M′X 2 ) or X-alteration (MX 2 /MX′ 2 ) series. We demonstrated both series in MoS 2 /WS 2 and WS 2 /WSe 2 SLs with time-lapse precursor modulations by MOCVD. The first example of MoS 2 /WS 2 SLs is described in Fig. 1. The predetermined flow rates of the Mo(CO) 6 and W(CO) 6 precursors for each MoS 2 and WS 2 ML were set to 4 and 3 standard cubic centimetres per minute (sccm) on the (C 2 H 5 ) 2 S background flow of 2 sccm in a given growth sequence. The series of cross-sectional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images in Fig. 1a, taken in each growth step up to 7 ML stacks, demonstrates the precise ML-by-ML stacking of WS 2 and MoS 2 . We successfully achieved such vdW stacking epitaxy by kinetics-controlled growth in the near-equilibrium limit (that is, the lateral growth rate, ν growth , in each ML was ~0.15 nm min -1 to guarantee a full lateral coverage, Heteroepitaxial van der Waals semiconductor superlattices Gangtae Jin 1,2,5 , Chang-Soo Lee 1,2,5 , Odongo F. N. Okello 2 , Suk-Ho Lee 1,2 , Min Yeong Park 1,2 , Soonyoung Cha 1 , Seung-Young Seo 1,2 , Gunho Moon 1,2 , Seok Young Min 1,2 , Dong-Hwan Yang 2 , Cheolhee Han 1,2 , Hyungju Ahn 3 , Jekwan Lee 4 , Hyunyong Choi 4 , Jonghwan Kim 1,2 , Si-Young Choi 2 and Moon-Ho Jo 1,2 ✉ A broad range of transition metal dichalcogenide (TMDC) semiconductors are available as monolayer (ML) crystals, so the precise integration of each kind into van der Waals (vdW) superlattices (SLs) could enable the realization of novel structures with previously unexplored functionalities. Here we report the atomic layer-by-layer epitaxial growth of vdW SLs with programmable stacking periodicities, composed of more than two kinds of dissimilar TMDC MLs, such as MoS 2 , WS 2 and WSe 2 . Using kinetics-controlled vdW epitaxy in the near-equilibrium limit by metal-organic chemical vapour depositions, we achieved precise ML-by-ML stacking, free of interlayer atomic mixing, which resulted in tunable two-dimensional vdW electronic systems. As an example, by exploiting the series of type II band alignments at coherent two-dimensional vdW heterointerfaces, we demonstrated valley-polarized carrier excitations-one of the most distinctive electronic features in vdW ML semiconductors-which scale with the stack numbers n in our (MoS 2 /WS 2 ) n SLs on optical excitations.
which is far slower than that of ~1,500 nm min -1 in the usual thermal chemical vapour deposition growth 34 ) by setting the lower growth temperatures (550 °C) and precursor partial pressures (~10 −7 torr) of the metal-organic precursors (Supplementary Table 1). This optimized growth condition essentially suppressed the unwanted overgrowth and interlayer mixing 37,38 (see Supplementary Figs. 1-4 for detailed growth optimizations). Here we stress that the basal planes of the first MLs with the highly preferred in-plane orientations are a prerequisite for the ML-by-ML growth. For this, we grew the first ML films on the step-and-terrace terrains of c-plane sapphire  (c-sapphire) substrates, which were then mechanically transferred onto the SiO 2 /Si substrates as the growth templates for the subsequent stacking growth. We fully discuss these features later (see Fig. 3). The layered structures of MoS 2 /WS 2 SLs are presented in greater detail in Fig. 1b, which shows a total of nine MLs (1:1 alternation of five WS 2 MLs and four MoS 2 MLs) captured by the Z contrast in the HAADF-STEM image and by energy dispersive X-ray (EDX) spectra. One can clearly identify the distinct intensity contrast between the WS 2 and MoS 2 MLs, which arises from the atomic number (Z) difference of Z Mo = 42 and Z W = 7; the brighter MLs are WS 2 and the darker MLs are MoS 2 . In addition, the periodically alternating W Lα and Mo Kα peak intensities across each vdW gap confirms such SL modulations. Although the image in Fig. 1b cannot be atomically resolved with a fixed transmission electron microscopy (TEM) zone axis in the entire area, due to the finite in-plane orientation variants of each ML, when the zone axis is aligned to <1120> of the lowest ML, the local atomic structures can be identified in the bright-field STEM images. For example, we observed either AC (translation) or AA′ (180° rotation) stacking sequences between WS 2 and MoS 2 MLs with a >90% area coverage (Fig. 1c)-we call these stacking polytypes 'coherent stacks' , as classified in Supplementary Fig. 5. We also found local areas with some mixtures of random rotation stacks with <10% area coverage (Fig. 1d)-we call these random-orientation stacking polytypes 'incoherent stacks' . Interestingly, we found different values of the interlayer distances for coherent and incoherent stacks, 0.606 and 0.647 nm, respectively, which arose from different vdW gap sizes, consistent with the calculations 39 . We also note that the value at the coherent stack, in which individual atoms tend to reside at the thermodynamically stable coordinates, is clearly smaller than the average value of the mechanical stacking cases 40 . Synchrotron grazing-incidence wide-angle X-ray diffraction (GI-WAXD) was employed to obtain a larger-scale crystallographic coherence in both the out-of-plane (that is, q z ) and in-plane (that is, q xy ) directions of our SLs, as in Fig. 1e. The 2D pattern of GI-WAXD at the incidence angle of 0.12° in Fig. 1f shows a series of sharp Bragg spots in reciprocal space, which indicates the highly ordered P6 3 /mmc structures of our MoS 2 /WS 2 SLs (9 ML stacks). For reference, we compared GI-WAXD patterns from the homoepitaxial 9 ML MoS 2 , which possesses smaller grain sizes of ~80-100 nm ( Supplementary Fig. 6). The 1D line-cut profiles were analysed in both the q z and q xy directions, as shown in Fig. 1g,h. The q z line-cut profile at q xy = 2.3 Å −1 clearly shows four diffraction peaks from the heteroepitaxial MoS 2 /WS 2 SLs (purple) and homoepitaxial MoS 2 9 MLs (grey), which verifies the highly ordered stacking textures-the more pronounced peaks from the MoS 2 /WS 2 SLs indicate a higher degree of ordering due to the larger grain sizes. The interlayer distances extracted from the (103) diffractions were 0.616 nm, consistent with the value at the coherent stack regions measured from the STEM images. Similarly, we found this to be 0.617 nm from the 9 ML MoS 2 (see Supplementary Table 2 for the complete diffraction analyses). The vertical coherence length, L c(103) , which strictly quantifies the number of coherently repeating layers along the q z direction, could also be extracted as 1.87 nm by the Scherrer equation-it was 1.49 nm in 9 ML MoS 2 . The in-plane diffraction peaks of (100), (110) and (200) were also distinct and much more pronounced in the MoS 2 /WS 2 SLs, which suggests highly ordered in-plane textures with preferred orientations. We extracted the (100) interplanar distance to be 0.273 nm from the (100) diffractions in the q xy line-cut profiles at q z = 0 Å −1 (Supplementary Table 3).
Overall, we define the crystalline textures of our vdW SLs as largely coherent vdW vertical stacks, composed of in-plane-oriented polycrystalline MLs.

Designer vdW semiconductor SLs with tunable periodicities
The established heteroepitaxial stacks of MoS 2 /WS 2 SLs by a ML-by-ML mode enabled us to achieve designed SLs with arbitrary periodicities. Figure 2a,b demonstrates 1:2 MoS 2 /WS 2 SLs and 2:2 MoS 2 /WS 2 SLs (see Supplementary Fig. 7 for the ML-by-ML mode). This growth tunability in our vdW SL heteroepitaxy markedly contrasts with previous covalent-bonded SLs, which intrinsically suffer from the strict requirements of lattice-matching heteroepitaxy on stacking. The larger-scale homogeneity and absence of interlayer intermixing were verified by systematic atomic force microscopy images and Raman spectra and mapping on our (MoS 2 /WS 2 ) 1-4 SLs ( Supplementary Figs. 8-10). We also show an example of X-alteration SLs in MX 2 , that is, WSe 2 /WS 2 SLs, in Fig. 2c, for which we employed (C 2 H 5 ) 2 S and (C 2 H 5 ) 2 Se precursors in each time lapse on the continuous W(CO) 6 background flow for the WS 2 and WSe 2 MLs (see Supplementary Fig. 11 for the growth optimizations). Although the intensity contrast between the WS 2 and WSe 2 MLs was less obvious in the Z contrast in the HAADF-STEM image, due to a smaller Z sensitivity compared with that for the MoS 2 and WS 2 SLs, the periodic oscillation of S Kα and Se Kα peaks in the EDX spectra clearly validates the SL modulations. Thus, we successfully established both the M-and X-modulated SLs in heteroepitaxial WSe 2 /MoS 2 /WS 2 trilayers, as shown in Fig. 2d (see Supplementary Fig. 12 for the optical absorption spectra and photoluminescence spectra). In addition, our heteroepitaxial growth can be extended to include graphene, which was intermittently inserted by an ex situ dry-transfer method ( Fig. 2e and Supplementary Fig. 13).

In-plane crystalline textures of vdW semiconductor SLs
The heteroepitaxial evolution of the in-plane crystal textures of our vdW SLs was investigated in the WSe 2 /WS 2 /MoS 2 trilayers, in which the chemical compositions vary for both M and X, and the in-plane lattice constants also vary-they are known to be 0.315 nm for bulk WS 2 and MoS 2 , and 0.328 nm for bulk WSe 2 (ref. 41 ). Figure 3a-c shows schematic descriptions of each stacking during the successive growth, in which the initially formed multiple triangular facet crystals merge to form continuous MLs. Statistical variation in the in-plane crystal orientations can be verified in the six-fold periodic clustering of the (1010) diffraction patterns with some degree of angular spread (Fig. 3d inset). The first bottom MoS 2 ML was formed by multiple nucleation on the regular step and terrace terrains of the c-sapphire substrate to initiate the preferred in-plane crystal orientations with a typical grain size of ~0.1-1 μm, which are either 0 or 60° rotated with respect to each other, as captured by a series of atomic force microscopy images ( Fig. 3d and Supplementary Figs. 14-16). To promote the subsequent ML-by-ML epitaxy, the first bottom ML templates were mechanically transferred onto SiO 2 /Si substrates ( Supplementary Fig. 17). We observed that the direct successive SL growth on the sapphire substrates without the transfer of the first MLs often led to unwanted local overgrowth, because the periodic steps and terraces of the c-sapphires still act as additional nucleation sites for the second MLs other than the homogeneous nucleations on the first bottom layers. In this way, we were able to guide the preferred orientations of each ML during the remaining successive heteroepitaxy. This ML-by-ML growth proceeded on the successive stacking growth for the second (WS 2 ) (Fig. 3e) and third (WSe 2 ) (Fig. 3f) MLs with smaller grains of 90-100 nm (see the in-plane TEM images in Supplementary Fig. 18 and grain-size distribution in Supplementary Fig. 19). The larger lattice parameter of 0.331 nm in the third WSe 2 ML was also verified from the (1120) diffraction patterns, and we also measured those of the underlying MoS 2 and WS 2 to be 0.322 nm (Fig. 3f inset). Atomic-scale images of such in-plane crystalline textures were directly captured by in-plane HAADF-STEM observations of partially covered bilayers of WS 2 /MoS 2 , as in Fig. 3g-k. We verified that the top MLs were preferentially oriented with the basal planes of the bottom MLs, and the stacking polytypes were mostly either AA′ (Fig. 3g and Supplementary Figs. 20-22) or AC (Fig. 3h) coherent stacking (see Supplementary Fig. 23 for the quantitative portion of the AA′ and AC stacks). Such textures introduce grain boundaries (GBs), mainly identified as 0 or 60° GBs, which were interfaced between either AC and AC domains (Fig. 3k) or AC and AA′ domains ( Fig. 3j and Supplementary Fig. 24). We also observed random interlayer twists, which showed moiré interference patterns ( Fig. 3i and Supplementary Fig. 25). According to first-principles calculations 42 , the formation energies of small-angle (<5°) and 60° GBs are relatively smaller than those of random-angle GBs, which suppress the unwanted local overgrowth of the second MLs-for example, the second ML preferentially nucleates at the random-angle GBs (other than the 0 or 60° GBs), which leads to non-ML-by-ML growth. Whereas on the first MLs with 0 or 60° GBs, the nucleation is not locally concentrated, which leads to the ML-by-ML growth. We indeed observed such growth patterns, with the initial nuclei of the second WS 2 predominantly populated at the smaller-grained MoS 2 ML templates with random GBs (Supplementary Fig. 3) on the larger-grained first ML with 0 or 60° GBs, the initial nuclei were uniformly distributed on the entire surface. This suggests that the preferred orientation growth here is critical to maintain the coherent ML-by-ML growth.

Valley-polarized interlayer excitations in type II vdW SLs
As for the electronic structures of our vdW SLs, we estimated them to be a series of type II band alignments across the vdW gaps to the lowest order. From optical absorption spectra, collected from a series of SLs and each individual ML (Fig. 4a), one can identify additive features of optical absorption with increasing ML stacks-the spectra of (MoS 2 /WS 2 ) n SLs are linear sums of the WS 2 and MoS 2 MLs, according to Voigt fitting ( Supplementary   Fig. 26; also see Supplementary Figs. 27 and 28 for a series of Raman scattering spectra and photoluminescence spectra, obtained from the same SL batches). One of the most distinctive electronic features that only pertain in TMDC ML heterostructures is the large and long-lived spin-valley polarization of charge carriers, as observed previously in (mechanically transferred) WS 2 /WSe 2 and WSe 2 /MoS 2 bilayers by pump-probe spectroscopy 43 . The ultrafast interlayer charge-transfer process across the type II alignment dramatically suppresses the exciton exchange interaction, which is the major relaxation channel of valley polarization in TMDC MLs. We indeed observed scaling of such a temporal population of the valley-polarized carriers, which arose from a series of type II band alignments in our MoS 2 /WS 2 SLs. Figure 4b,c illustrates   In-plane HAADF-STEM images with the corresponding atomic models (the blue, red and yellow spheres represent Mo, W and S atoms, respectively) obtained from a WS 2 /MoS 2 bilayer at the regions of the AA′ stack (g), AC stack (h) and incoherent stack (i) and at the interfaces between the AA′ and AC domains (j) and AA′ and AA′ domains (k). MTB, mirror twin boundary. Scale bars, 1 nm.
such valley-polarized excitations in real space and momentum space, respectively, with optical circular dichroism (CD), using time-resolved pump-probe spectroscopy. First, the pump pulses with a right-handed circular polarization (σ + ) (blue arrows in Fig. 4b,c) create the valley-polarized excitons on K valleys in the MoS 2 MLs. Then, the immediate interlayer hole transfer occurs across the type II alignments from the MoS 2 ML valence bands to those of the WS 2 MLs within an ultrafast timescale of less than 50 fs (ref. 44 ). Initially, we verified such interlayer charge separation from the substantially extended lifetime of the CD dynamics in our MoS 2 /WS 2 bilayers, compared with that of the MoS 2 MLs (Supplementary Fig. 29). Note that our SL series shows a shorter lifetime of valley polarization (approximately nanoseconds), presumably due to more defect densities compared with those in mechanically exfoliated cases. Nevertheless, the valley-polarized electron selectively remains on the K valley of the MoS 2 MLs (upper panel of Fig. 4c). After the pump excitation, the delayed probe pulse (red arrows) was focused on the SLs, and by measuring the pump-induced differential reflectance ∆R with both σ + and the left-handed (σ -) circular polarization of the probe beam, we evaluated CD = ΔR σ + − ΔR σ − ) /R 0 , where R 0 is the probe reflectance without the pump. Such a valley-polarized electron gives rise to a helicity-dependent absorption difference (that is, CD). Then, the transient CD response must scale to the amount of the residual valley-polarized electrons in the MoS 2 MLs. Figure 4d displays such time-resolved CD responses from a series of (MoS 2 /WS 2 ) n SLs at 77 K, when the pump and probe photon energy were set to the A exciton resonance of MoS 2 MLs with the pump fluence of 5.7 μJ cm -2 . We observed linearly proportional CD signals with increasing n in (MoS 2 /WS 2 ) n SLs, as in Fig. 4d.

Conclusions
We report the growth realization of vdW semiconductor SLs with tunable periodicities by ML-by-ML heteroepitaxy. We verified coherent atomic stacking orders at the vdW heterointerfaces in SLs. Such coherent vdW SLs present a new set of tunable 2D electronic systems, which offers challenging opportunities to investigate diverse solid-state phenomena for unexplored properties and functionalities. As an example, we present scaling valley-polarized optical excitations that only pertain to a series of 2D type II band alignments in our vdW SLs. Designer vdW SLs by epitaxy could serve as scalable platforms for a new type of QW states with distinct vdW interfaces.

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