Strong Exciton-Photon Coupling in Large Area MoSe 2 and WSe 2 Heterostructures Fabricated from Two-Dimensional Materials Grown by Chemical Vapor Deposition
Daniel J. Gillard, Armando Genco, Seongjoon Ahn, Thomas P. Lyons, Kyung Yeol Ma, A-Rang Jang, Toby Severs Millard, Aurelien A. P. Trichet, Rahul Jayaprakash, Kyriacos Georgiou, David G. Lidzey, Jason M. Smith, Hyeon Suk Shin, Alexander I. Tartakovskii
SStrong Exciton-Photon Coupling in Large Area MoSe and WSe HeterostructuresFabricated from Two-Dimensional Materials Grown by Chemical Vapor Deposition
Daniel J. Gillard † , Armando Genco ∗† , Seongjoon Ahn † , Thomas P. Lyons , Kyung Yeol Ma ,A-Rang Jang , Toby Severs Millard , Aur´elien A. P. Trichet , Rahul Jayaprakash , KyriacosGeorgiou , David G. Lidzey , Jason M. Smith , Hyeon Suk Shin , and Alexander I. Tartakovskii ∗∗ Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK Department of Energy Engineering, Department of Chemistry,and Low Dimensional Carbon and 2D Materials Center,Ulsan National Institute of Science and Technology, Ulsan 44919, South Korea Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, UK and † These authors contributed equally to this work.
E-mail addresses: dgillard1@sheffield.ac.uk, ∗ a.genco@sheffield.ac.uk, [email protected],t.lyons@sheffield.ac.uk, [email protected], [email protected], t.seversmillard@sheffield.ac.uk,[email protected], r.jayaprakash@sheffield.ac.uk, k.georgiou@sheffield.ac.uk,d.g.lidzey@sheffield.ac.uk, [email protected], [email protected], and ∗∗ a.tartakovskii@sheffield.ac.uk Corresponding authors: ∗ a.genco@sheffield.ac.uk, ∗∗ a.tartakovskii@sheffield.ac.uk (Dated: August 21, 2020) a r X i v : . [ phy s i c s . op ti c s ] A ug ABSTRACT
Two-dimensional semiconducting transition metal dichalcogenides embedded in optical microcavitiesin the strong exciton-photon coupling regime may lead to promising applications in spin and valleyaddressable polaritonic logic gates and circuits. One significant obstacle for their realization is theinherent lack of scalability associated with the mechanical exfoliation commonly used for fabrication oftwo-dimensional materials and their heterostructures. Chemical vapor deposition offers an alternativescalable fabrication method for both monolayer semiconductors and other two-dimensional materi-als, such as hexagonal boron nitride. Observation of the strong light-matter coupling in chemicalvapor grown transition metal dichalcogenides has been demonstrated so far in a handful of experi-ments with monolayer molybdenum disulfide and tungsten disulfide. Here we instead demonstrate thestrong exciton-photon coupling in microcavities comprising large area transition metal dichalcogenide/ hexagonal boron nitride heterostructures made from chemical vapor deposition grown molybdenumdiselenide and tungsten diselenide encapsulated on one or both sides in continuous few-layer boronnitride films also grown by chemical vapor deposition. These transition metal dichalcogenide / hexag-onal boron nitride heterostructures show high optical quality comparable with mechanically exfoliatedsamples, allowing operation in the strong coupling regime in a wide range of temperatures down to4 Kelvin in tunable and monolithic microcavities, and demonstrating the possibility to successfullydevelop large area transition metal dichalcogenide based polariton devices.
INTRODUCTION
Monolayers of transition metal dichalcogenides (TMDs) are promising semiconductors with unique electrical andoptical properties arising from the quantum confinement experienced by the electrons and holes in the two-dimensional(2D) structure . One of the main effects is the appearance of direct bandgap excitonic transitions showing featuresstrongly beneficial for optoelectronics, such as high binding energies and very large oscillator strengths . Moreover,the breaking of spatial inversion symmetry in the 2D lattice and a large spin-orbit coupling generate spin-valley lockedoptically addressable excitons at the K and K points of the momentum space . These exceptional properties canbe further enriched by integrating the TMDs within optical resonators enabling the strong exciton-photon couplingregime, where confined photons and excitons hybridize into new states called polaritons . Polaritons in TMDsacquire novel properties arising from the valley pseudo-spin degree of freedom of excitons , and further provideenhanced valley coherence for excitons strongly coupled with long-lived cavity photons . Efficient polariton prop-agation in TMDs has been recently observed, with diffusion lengths of up to 20 µ m in WS at room temperature ,while valley-dependent divergent polariton diffusion has been found in MoSe at cryogenic temperatures, wherebypolaritons spread in different in-plane directions owing to the exciton valley Hall effect . Polaritons in TMDs alreadyoffer the potential to create highly non-linear phenomena , with further effort directed at realization of Bose-Einstein condensation , polariton lasing and optical parametric oscillation so far observed in other materialsystems. United to the valley degree of freedom of TMD monolayers, these phenomena could be exploited to createlarge scale all-optical polariton circuits and quantum networks .However, TMD monolayers are usually fabricated by mechanical exfoliation, resulting in high quality but smallsized flakes, hindering the reproducibility and scalability of the devices. Chemical vapor deposition (CVD) offersan alternative growth and fabrication method that provides substrate-wide coverage of uniform monolayer islands ,as well as the ability to grow heterostructures in-situ , therefore completely bypassing the mechanical transfersnecessary in exfoliated equivalents and minimizing external contamination. CVD provides a very attractive andscalable method for the fabrication of large scale TMD based devices. As such, CVD-grown MoS and WS flakeshave already been employed in polaritonic devices working at room temperature . Nevertheless, in order to optimizethe polariton valley properties and optimize coherence, narrow exciton linewidths (low inhomogeneous broadening)and low structural disorder are needed, which up to now have only been shown by exfoliated MoSe and WSe monolayers at low temperatures . Another advantage of high quality structures and the possibility to operateat low temperature will be the access to highly non-linear trion-polaritons or Fermi-polaron-polaritons , requiringcontrol and stability of the charged excitons, as well as employment of suitable heterostructures comprising of TMDs,hexagonal boron nitride (hBN) and graphene.Here, we show large area MoSe and WSe monolayers encapsulated in hBN, all grown by CVD, with crystal domainsexceeding 100 µ m in size, which display high optical quality, rivalling exfoliated material. This is evidenced by theintense and narrow exciton peaks observed in photoluminescence (PL) measurements and in reflectance contrast (RC).Substrate-wide growth uniformity and a high degree of alignment within the ensemble of monolayer domains has beenachieved in these materials using CVD growth on sapphire for MoSe and directly on CVD-synthesized hBN for WSe .We have confirmed these favorable monolayer TMD crystal properties by using a recently developed substrate-widestatistical analysis of TMD crystal axes orientation and PL properties . Further testing of the samples in a tunablemicrocavity exposes clear evidence of strong exciton-photon coupling and formation of exciton-polariton states. Ananti-crossing is observed between the cavity mode and neutral exciton transition, with Rabi splittings of about 17 meVfor both MoSe and WSe , very similar in magnitude with exfoliated materials , highlighting similar opticalqualities. Finally, as a proof of concept for future large scale TMD-based polaritonic devices, a monolithic cavity isfabricated incorporating CVD grown hBN-encapsulated MoSe in a SiO spacer, sandwiched between two mirrors, inwhich strong coupling is observed with a large Rabi splitting of 34 meV at T=4 K and 31 meV at T=150 K. The resultspresented in this work demonstrate the possibility to fabricate large area polariton devices exploiting high qualityTMD based heterostructures made from CVD-grown materials, paving the way for future scalable TMD-polaritoniccircuits. RESULTS
Two types of encapsulated heterostructure (HS) have been fabricated from CVD-grown materials for this work(see further details in Methods). In the first heterostructure, HS1 (Fig.1(a)), MoSe monolayers are grown by CVDonto a sapphire substrate. Thousands of individual monolayer islands, positioned throughout the entire substrate,are then mechanically transferred using a polystyrene membrane onto a high reflectivity distributed Bragg reflector(DBR) composed of 13 pairs of SiO /Ta O with the high reflectance stop-band centered at a wavelength of 750 nm.A substrate-wide few-layer film of hexagonal boron nitride (hBN), also grown by CVD, is mechanically transferredon top of the MoSe flakes to complete the encapsulation. For the second heterostructure, HS2 (Fig.1(b)), WSe monolayers are grown by CVD directly on few-layer hBN, also grown by CVD. Both materials, WSe /hBN, arethen mechanically transferred, at once, onto the DBR and subsequently encapsulated with a few-layer film of CVDgrown hBN. It has been shown that for mechanically exfoliated TMDs, encapsulating with thin hBN provides uniformdielectric screening of the Coulomb interaction, reducing spatial inhomogeneity in the exciton, thereby narrowingthe emission linewidth . Furthermore, hBN protects the TMD layers in the heterostructure from damage andcontamination during the subsequent deposition of various dielectrics in microcavity structures relevant to our work .Moreover, the direct growth of TMD monolayers on hBN is also strongly beneficial as a route to single-crystal epitaxialgrowth , which, up until now, has been demonstrated with a limited range of TMD materials such as WS andMoS . Fully coalesced CVD grown WSe monolayer films on hBN were obtained very recently by Zhang et al ,through a careful control of nucleation and extended lateral growth time, and a strong improvement of optical andelectrical properties have been achieved compared to the same material grown on sapphire. HS1:
MoSe HS2:
WSe hBNMoSe DBRhBNWSe DBRhBN a cb d ef
FIG. 1: a, b)
Schematics of the CVD grown heterostructures (not to scale). a) HS1: MoSe monolayers weregrown by CVD on a sapphire substrate, mechanically transferred onto a DBR mirror and encapsulated withmonolayer hBN. b) HS2: WSe monolayers were grown by CVD on multilayer hBN, transferred together with thehBN onto a DBR mirror and encapsulated with multilayer hBN. The hBN multilayers were also grown by CVD. c,d) Optical microscope images of the photoluminescence (PL) from c) HS1 and d) HS2 taken at room temperatureusing a 50x objective lens (scale bar: 50 µ m). e, f ) Optical characterization of e) HS1 and f) HS2 at T ∼ X , and charged, X − , excitons, whereas onlyHS1 shows absorption from both. PL cw excitation conditions: λ exc = 660 nm, P = 20 µ W for HS1; λ exc = 532 nm,P = 20 µ W for HS2.As a first characterization step, the room temperature PL emission and the general morphology of the structureshave been analyzed under an optical microscope (see Methods). HS1 (Fig.1(c)) generally consists of large isolatedmonolayer islands with characteristic triangular shape and average lateral size of 8 µ m (see Supplementary Note I fordetails), along with a number of regions where multiple flakes merge to form monolayers with sizes exceeding 100 µ m.Similarly, HS2 (Fig.1(d)) shows large triangular monolayer flakes with average lateral widths of 11 µ m. Again, in areaswhere flakes merge, monolayers of over 100 µ m in width can be seen. Large areas of uniform coverage are necessaryfor constructing large arrays of identical heterostructure devices, such as transistors or photodetectors. Overall, thereis monolayer coverage of 14% for HS1 and 22% for HS2, calculated by dividing the total area of monolayer acrossthe sample by the total substrate area. The substrate-scale PL imaging analysis used to identify the monolayers isdiscussed in more detail below.Further optical characterization of the two heterostructures has been performed using a spectroscopic microscopysetup at both room ( ∼
290 K) and low ( ∼ µ W (see Methods). Theconsiderable room temperature excitonic PL emission shown in the insets in Figs.1(e, f) highlights the large excitonbinding energy associated with TMD monolayers . In HS1 the exciton PL peak is located at 1.579 eV with a linewidthof 40 meV, and HS2 displays the exciton peak at 1.670 eV with a similar linewidth of 45 meV, typical of MoSe andWSe monolayers operating at room temperatures.Decreasing the temperature to ∼ X , at 1.671eV in HS1 and 1.759 eV in HS2, and a second peak attributed to the charged exciton (trion) transition, X − , whichappears at 1.639 eV in HS1 and 1.726 eV in HS2, about 30 meV below the neutral exciton . The relative intensityof the X − peak, when compared to the X peak, is heavily influenced by the free carrier densities present in thestructures . In HS2, PL seen at lower energies (below 1.70 eV) has previously been attributed to various excitoniccomplexes in WSe including spin dark excitons , exciton-phonon side-bands and localized states . The samplesshow neutral exciton linewidths of 13 meV and 21 meV for HS1 and HS2 respectively, and 12 meV and 20 meV for thecharged exciton of the two samples. Generally, the linewidth of an excitonic transition in TMD monolayers is stronglyaffected by the level of structural disorder and density of defects . The spectral shapes and linewidths demonstratedby the CVD grown samples investigated in this work improve upon those reported by Zhang et al and Lippert etal , and are similar to exfoliated flakes operating at low temperature without encapsulation . This showsthat CVD growth can produce heterostructures of comparable optical quality to mechanically exfoliated flakes. Therole of hBN is mostly to provide a high quality substrate for the TMD synthesis , but also to act as a bufferlayer protecting TMDs from damage during the deposition of additional layers in order to complete a microcavity orwaveguide structure.We also measure reflectance contrast spectra using a broad band white light source and calculated as ∆ R/R =( R sub − R HS ) /R sub , where R HS is the reflectance of the heterostructure, and R sub is the reflectance of the baresubstrate. These spectra (red lines in Figs.1(e, f)) reveal a strong absorption peak attributed to the X transition inboth heterostructures and a lower intensity peak at lower energy attributed to X − in HS1. The relative peak heightis strictly related to the oscillator strength of individual transitions, with the neutral exciton being much more intensethan the trion in HS1 due to a relatively low doping level.As can be inferred in Figs.1(c, d), the bright triangular monolayer islands appear to have a preferred orientation.To extract the size and shape of monolayer flakes, shape recognition techniques were used on a full substrate mapcomprised of multiple microscope PL images, an example of one such image is shown in Figs.1(c, d). By employing º º º a b c FIG. 2: a)
Schematic showing flake orientations at different angles relative to the horizontal axis of the microscopeimages. b, c)
Analysis of flake orientation. Islands of monolayer TMD are identified and the orientation extractedusing methodology as described in previous investigations . Both b) HS1 and c) HS2 show two main peaks situated60 ◦ apart, equivalent to two opposite growth configurations rotated by 180 ◦ due to the three fold symmetry ofequilateral triangles. HS1 also shows two extra peaks situated at 30 ◦ from the main peaks.analytical methods detailed in our previous work , the flake orientation relative to the horizontal axis of the micro-scope images can be found (Fig.2(a)). In order to maximize accuracy, only islands with shape close to equilateraltriangles are analyzed in terms of angular orientation. Of the 16999 (14205) individual monolayer islands identifiedin HS1 (HS2), 8089 (8391) satisfy this condition. Measured in terms of area, this corresponds to 21% (17%) of thetotal monolayer coverage. The histograms in Figs.2(b, c) detail the number of islands identified as a function of ori-entation angle, showing that both the samples feature a very high degree of island orientation uniformity, a signatureof epitaxial growth. For WSe grown directly onto hBN (Fig.2(c)), two main orientations have been found. This isexpected from a sample with a three-fold symmetric triangular morphology presenting two possible opposite growthdirections at 180 ◦ to one another. These two preferential directions are directly related to the hexagonal crystalstructure of the growth substrate and have also been observed in previous studies of MoS , WS , and WSe grown byCVD on hBN . For MoSe grown onto c-plane sapphire (Fig.2(b)), four peaks in the angular distribution areobserved. Two main peaks show the preferred flake orientation, situated at 60 ◦ relative to each other, along with twoless populated angles at 30 ◦ relative to the two main peaks. Both two, and four preferential growth directions havebeen seen in TMDs grown via CVD onto c-plane sapphire . Control over the relative angle of the flakes at thesynthesis stage of fabrication will provide the basis to build scalable heterostructures with control over the relativeinterlayer crystallographic orientation.After the optical characterization step, the encapsulated heterostructures are tested in a tunable open cavity setupwhich consists of a top concave DBR mirror distanced 2-3 µ m from a planar bottom DBR mirror (Fig.3(a, b), uponwhich the HSs are placed . The mirrors are positioned using piezo-actuator stages (Fig.3(a)). Free space xyz φ θ xyz ac bd cts/scts/s FIG. 3: a)
Schematic of the tunable open microcavity including the set of piezo actuators used to align themirrors and perform the PL scans of the heterostructure. b) Schematic of the open optical microcavity. The cavityis composed of a planar DBR, upon which the HS is placed, and a concave top DBR confining the optical cavitymode in 3 dimensions. c, d)
PL emission from c) HS1 and d) HS2 displayed as a function of photon energy andexciton-photon detuning (∆ = E c − E X ). Clear anti-crossings of the cavity mode with the exciton are observed inboth heterostructures. PL spectra are fitted using a Lorentzian peak (see Supplementary Note II) and a two levelcoupled oscillator model is used to extract the lower (blue curve) and upper (yellow curve) polariton branches,excitonic resonances (white horizontal lines), and LG photonic mode (green diagonal line). Rabi splittings ofabout 17 meV are found for both HS1 and HS2. Samples are optically excited using a 637 nm cw laser.optical access from above the top concave DBR allows laser excitation and optical detection, using an achromaticdoublet objective lens. A cavity length can be tuned by slowly moving the bottom mirror along the z -axis, thusallowing the tuning of the cavity mode (diagonal green dashed lines in Figs.3(c, d)). The PL signal collected from theTMDs, as the cavity length is reduced, is displayed in Figs.3(c, d)) as a function of detuning (∆, the energy differencebetween the cavity mode and unperturbed exciton, X ). The three dimensional optical confinement provided by theconcave top mirror generates a set of transverse modes for each longitudinal mode also visible in both figures.When the fundamental longitudinal cavity mode, LG (green diagonal dashed lines in Figs.3(c) and (d)) whichensures the highest light confinement, is tuned into resonance with the exciton transition energies, light mattercoupling can manifest in one of two ways, both of which are observed in HS1 (Fig.3(c)). As the cavity mode is tuned cts/s a b cts/s FIG. 4: a, b)
Angle resolved PL imaging of HS1 monolithic cavity. a) At 5 K, the cavity has a negative detuningat 0 ◦ of ∆ ≈ -3.5 meV, showing anticrossings at ± ◦ . The PL intensity in the region within the red dashed lineshas been multiplied by a factor of 10 for clarity. b) At 150 K, the cavity has a positive detuning at 0 ◦ of ∆ ≈ ± ± µ W.into resonance with the trion, X − , at 1.638 eV, the mode is broadened and brightened, a demonstration of the weakcoupling between the cavity and the trion transition with small oscillator strength. The absence of modebroadening in HS2 (Fig.3(d)) cavity scans at the trion energy is an indication that the absorption of the WSe trionresonance, occurring in HS2 at 1.726 eV, is too weak, as also confirmed in Fig.1(f)).The second regime of exciton-photon coupling, known as the strong coupling, presents itself as an anti-crossing ofthe LG cavity mode and the exciton energies with a characteristic Rabi splitting, 2¯ h Ω R , at the resonance. Thisbehaviour can be clearly observed in both HS1 (Fig.3(c)) and HS2 (Fig.3(d)) as the LG is tuned into resonance withthe neutral exciton, at 1.673 eV for HS1 and 1.770 eV for HS2. The peak positions of the lower (LPB) and upper(UPB) polariton branches have been extracted using a Lorentzian peak fitting, and used to fit a two-level coupledoscillator model (detailed in Supplementary Note II) in order to determine 2¯ h Ω R as shown overlaid in Figs.3(c, d).We find a value of 17.2 ± ± and WSe placed in zero-dimensional tunable microcavities . This further confirms, thanksto the reduced structural disorder in the presented heterostructures, the high optical quality of the hBN encapsulatedCVD grown TMD monolayers, hence proving the validity for CVD growth techniques when designing scalable devices.Further advantages of large area TMDs can be exploited in monolithic cavities, providing a platform to form0various topological designs to adapt or enhance device functionality towards polariton circuits . For these devices,the protective function of the hBN encapsulation is of particular importance as the top dielectric mirror needsto be deposited on top of the TMD layers . As a proof of concept, we deposited 98 nm of SiO via e-beamdeposition, followed by a semi-transparent layer of 50 nm gold, on top of HS1 to fabricate a λ /2 monolithic cavity(see Supplementary Note III). The oxide deposition process has been carried out at room temperature in order topreserve the optical integrity of the emitting materials as much as possible .In the tunable cavity, the photonic modes are confined in all three dimensions, resulting in a set of discrete cavitymodes, with k x,y,z ∼
0, which are tuned in energy by altering the cavity length. By contrast, in a monolithic two-dimensional cavity, as in our case, the photonic mode is confined only in the vertical z direction, and thus a cavitymode energy dispersion as a function of continuous k x,y values is observed . This dispersion can be probed bymeasuring angle-resolved PL or reflectivity spectra as a function of angle measured from the normal to the sample(corresponding to k x,y =0) . In the cavity used in our experiment, a stronger light confinement can be achieved thanin the tunable devices presented earlier, due to a smaller thickness of the cavity spacer and lower mode penetrationinto the top mirror. As shown below, this leads to a higher magnitude of the Rabi splitting. Since monolithic cavitiesare not tunable in size, the temperature dependence of the X transition energy (further discussed in SupplementaryNote III) is used to tune the exciton into resonance with the photonic mode which has a negligible dependence of itsfrequency with temperature.The PL collected from the monolithic cavity while being optically excited by non-resonant continuous wave laser, asimaged by angle resolved spectroscopy, is shown in Fig.4. In Fig.4(a) at a temperature of 5 K, the X transition is at1.648 eV, while in Fig.4(b) at 150 K, the X red-shifts to 1.636 eV. The monolithic cavity shows strong exciton-photoncoupling signatures in PL at both the temperatures, owing to the protective capability of the CVD grown hBN whichhelped shield the MoSe monolayers from the potentially damaging SiO deposition process.At a temperature of 5 K, the exciton is negatively detuned from the cavity mode at 0 ◦ by - 3.5 meV, such that theLPB is much more visible than the UPB. To show the upper polariton branch in Fig.4(a) the collected intensity valueshave been multiplied by a factor of 10 between 1.658 eV and 1.705 eV, as outlined by red dashed lines. By fitting thePL emission spectra with Lorentzian peaks and applying the extracted peak positions to a two level coupled-oscillatormodel (see Supplementary Note II) we obtain a large Rabi splitting of 34 ± ± ◦ when the device is at 5 K. Thestrongly coupled cavity performs well up to 150 K, when the excitonic mode is positively detuned (∆(0 o ) = + 10meV), leading to a Rabi splitting of 31 ± DISCUSSION
In summary, high quality substrate-wide MoSe , and WSe , TMD monolayers encapsulated with large area hBNwere fabricated using CVD growth techniques, and subsequently embedded in tunable and monolithic microcavitydevices where strong exciton-photon coupling was observed. The heterostructures show optical properties comparablewith exfoliated materials, and consequently exhibit similar values of polariton Rabi splittings to previously studiedheterostructures made from exfoliated flakes .Furthermore, the demonstrated CVD growth on sapphire and hBN produced highly orientated TMD islands, andis thus suitable for the fabrication of large scale TMD/TMD heterostructures with highly controlled interlayer twistangle to be embedded in microcavities. Together with additional hBN and graphene layers these structures couldprovide a viable route to realization of highly tunable and non-linear dipolar polaritons in large scale devices.This work demonstrates the possibility to fabricate large scale polaritonic devices based on van-der-Waals het-erostructures. Further development of large scale monolayer semiconductor growth techniques, most notably directlyonto hBN which provides highly co-orientated TMD domains, will inevitably lead to heterostructures that can reliablyand repeatedly compete with, or out-perform, those built with exfoliated flakes due to the unprecedented scalabilitythat is granted. METHODS
Dielectric mirror fabrication.
Highly reflecting distributed Bragg reflectors (DBRs) are deposited on silicasubstrates by ion beam sputtering. The DBRs are comprised of 13 pairs of quarter wavelength SiO /Ta O layersof thicknesses 129 and 89 nm (refractive index 1.45 and 2.10 respectively), terminating with SiO . The DBRs aredesigned for a center wavelength of 750 nm and a stop-band width of 200 nm.The concave-shaped template for the top mirror is produced by focused ion beam milling in a smooth fused silicasubstrate. Gallium ions are accelerated onto a precise position of the silica substrate achieving an accuracy of around5 nm with an r.m.s. roughness below 1 nm . The nominal radius of curvature of the concave mirror was 20 µ m,2leading to a beam waist on the planar mirror of around 1 µ m . Growth of single layer MoSe and transfer to SiO /Si. MoSe was grown on c-plane sapphire by CVD. Twoprecursors, MoO (99.97%, Sigma Aldrich) and Se (99.999%, Alfa Aesar), were used for the growth. 150 mg of Sewas placed at the upstream entry of the furnace and 60 mg of MoO powder was placed at the centre of the furnace.A crucible containing MoO was partially covered by a SiO /Si wafer to reduce intense evaporation of the precursor.The sapphire substrate was located next to the crucible that contained MoO . Before the tube furnace was heated,the tube was evacuated for 30 min and filled with the Ar gas achieving ambient pressure. The temperature of thefurnace was increased up to 600 ◦ C for 18 min under a steady flow of Ar gas (60 sccm) and H gas (12 sccm). Whenthe furnace reached 600 ◦ C, Se was vaporized by heating the upstream entry of the tube up to 270 ◦ C using a heatingbelt. Finally, temperature of the tube furnace was increased to 700 ◦ C and maintained for 1 hr for the MoSe growth.Afterwards, the tube furnace was cooled down to room temperature while the Ar flow was maintained without H .To transfer MoSe on top of the DBR, polystyrene was used to maintain the sample quality. Growth of single layer WSe on hBN. To fabricate WSe on hBN, multilayer hBN was initially grown onc-plane sapphire (see Methods). WO (99.998%, Alfa Aesar) and Se (99.999%, Alfa Aesar), were used for the WSe growth. 300 mg of Se was placed at the upstream entry of the furnace and 120 mg of WO powder was placed at thecenter of the furnace. To reduce the influence of humidity, a small amount of NaCl was mixed with WO powder. Themultilayer hBN on sapphire substrate was positioned next to the crucible containing WO . Before the tube furnacewas heated, the tube was evacuated for more than 30 min. Then, the temperature of the tube furnace was increasedto 800 ◦ C for 24 min under a steady flow of Ar gas (120 sccm) and H gas (20 sccm). When the furnace reached800 ◦ C, the Se was vaporized by heating the upstream entry of the tube to 270 ◦ C using a heating belt. Finally,temperature of the tube furnace was increased to 870 ◦ C and maintained for 1 hour for the WSe growth. Afterwards,the tube furnace was cooled down to room temperature under Ar flow. Growth of large area hBN.
Multilayer hBN with an AA’ stacking order was grown by remote inductively coupledplasma chemical vapor deposition method. A 2-inch c-plane sapphire was used as a substrate for the hBN growth.The substrate was placed in the center of a 2-inch alumina tube furnace of CVD. A borazine (Gelest, Inc.) precursorflask was placed in a water bath at -15 ◦ C. The bath temperature before the growth of hBN was increased up to 25 ◦ C.Before the growth of multilayer hBN, the furnace was heated to 1220 ◦ C under flow of Ar gas (10 sccm). Plasma wasgenerated at a power of 30 W under a flow of borazine (0.2 sccm) and Ar (10 sccm) gases for 30 mins. Atomic forcemicroscopy and transmission electron microscopy measurements confirmed that the thickness of hBN was 1.2 nm,3approximately 3 layers. In addition, the hBN sample shows quite good thickness uniformity over the 2-inch sapphiresubstrate according to the Raman and UV absorption spectra measured at nine random points over the 2-inch hBNfilm.
Optical measurements.
The photoluminescence images of the CVD samples were acquired using a modifiedbright-field microscope (LV150N, Nikon) equipped with a color camera (DS-Vi1, Nikon). The near-infrared emissionfrom the white-light source was blocked with a 550-nm short-pass filter (FESH0550, Thorlabs), and a 600-nm long-passfilter (FELH0600, Thorlabs) was used to isolate the photoluminescence signal from the samples. The full descriptionof the system is available in Ref. .Spectrally-resolved photoluminescence (PL) and reflectance contrast (RC) measurements were performed using acustom-built micro-PL setup. For PL, the excitation light centered at 2.33 eV and 1.88 eV was generated by twodiode-pumped solid-state lasers (CW532-050 and ADL-66505TL, Roithner). For RC, a stabilized tungsten-halogenwhite-light source (SLS201L, Thorlabs) was used. The excitation light was focused onto the sample using a 50xobjective lens (M Plan Apo 50X, Mitutoyo). The PL and RC signals collected in the backwards direction weredetected by a 0.5 m spectrometer (SP-2-500i, Princeton Instruments) with a nitrogen-cooled charge-coupled devicecamera (PyLoN:100BR, Princeton Instruments). The PL signal was isolated using 700 nm and 650 nm long-pass filters(FEL0700 and FEL0650, Thorlabs). The RC spectra were derived by comparing the spectra of white light reflectedfrom the sample and the substrate as ∆ R/R = ( R sub − R HS ) /R sub , where R HS ( R sub ) is the intensity of lightreflected by the sample (substrate). The room-temperature measurements were performed in ambient conditions.The low-temperature measurements were carried out using a continuous-flow liquid helium cryostat, in which thesample was placed on a cold finger with a base temperature of ∼ Tunable micro-cavity.
The optical cavity was formed using an external concave mirror, with nominal radius of20 µ m, to produce a 0D tunable cavity .The bottom mirror is controlled by a 5-axis piezo-actuator stack, the first three stages control the x , y , and z translational motion, while another two stages control the tilt alignment. The top mirror is positioned using a 3-axis piezo-actuator stage controlling the x , y , and z translational motion. Optical PL scans were completed with thesamples placed in a helium bath cryostat system at a temperature of 4.2 K using a 637 nm continuous-wave laser diode(Vortran Stradus), focused onto the sample using an achromatic lens. The collected PL is focused onto a single modefiber and guided into a 0.75 m spectrometer (SP-2-750i, Princeton Instruments) and a high-sensitivity charge-coupleddevice (PyLoN:400BR, Princeton Instruments) for emission collection.4 Measurement of monolithic cavities.
We performed the Fourier space spectral imaging of the PL emittedby the monolithic cavity by employing a 2D CCD array (PyLoN:100BR, Princeton Instruments) coupled to a 300gr/mm grating spectrometer (SP-2-500i, Princeton Instruments). We focused a 30 cm lens onto the back plane of a50x Mitutoyo infinity corrected objective to obtain the Fourier plane image of the sample, which was then projectedon the slit of the spectrometer by using a 10 cm lens. We used the slit to select only the section of the Fourier spaceat k x = 0, resulting in a final image on the CCD displaying the PL as a function of k y on the y-axis and energy onthe x-axis. The conversion from k y to angles has been carried out by considering k y ≈ sin θ and knowing that themaximum k y detected by our setup is equal to the objective NA = 0.55 Flake orientation analysis.
Optical microscope PL images were analyzed in MATLAB using functions from theimage processing toolbox . The color thresholding application was used to isolate monolayer material in a typicalPL image and was applied to each combination of monolayer and substrate. The program identified 8089 triangularobjects in HS1 and 8391 triangular objects in HS2. Further details of the image processing and a more completeexplanation of the analysis can be found in Ref. . Fabrication of monolithic cavities.
The monolithic cavity has been fabricated by depositing a SiO film of 98nmon top of the CVD-grown monolayers, which were previously transferred on a 13 pairs SiO /Ta O DBR grown byion beam assisted sputtering on a sapphire substrate. In order to minimize the potential damages on the monolayers,the silica layer covering the TMDs has been grown at room temperature by using an e-beam deposition system. Forthe top mirror, a semi-transparent layer of Au (thickness: 50 nm) has been thermally evaporated, completing thecavity.
ACKNOWLEDGMENTS
D. J. G., A.G., T.S.M., and A.I.T. acknowledge funding by EPSRC (EP/P026850/1). This work was supported bythe research funds (NRF-2019R1A4A1027934 and NRF-2017R1E1A1A01074493) through National Research Foun-dation by the Ministry of Science and ICT, Korea. R. J., K. G., and D. G. L. thank the financial support of EPSRCvia programme grant ’Hybrid Polaritonics’ (EP/M025330/1). T. P. L. acknowledges financial support from the EP-SRC Doctoral Prize Fellowship scheme Grant Reference EP/R513313/1. A.I.T. thanks the financial support of theGraphene Flagship under grant agreements 785219.5
Author Contributions
D. J. G., A. G., and S. A. contributed equally to this work. D. J. G., A. G., and T. P. L. carried out opticalinvestigations. TMD monolayers were grown via CVD, and samples fabricated, by S. A. and H. S. S. hBN layers weregrown via CVD by K. Y. M. and A-R. J. The monolithic cavity was completed by R. J., K. G., and D. G. L. Theconcave mirrors were made by A. A. P. T. and J. M. S. Data was analyzed by D. J. G. and A. G. The manuscriptwas written by D. J. G. with major input from A. G. and further contributions from all co-authors. A. I. T. providedmanagement of various aspects of the project, and contributed to the analysis and interpretation of the data andwriting of the manuscript. A. I. T., H. S. S., D. G. L., and J. M. S. provided management of relevant parts of theproject. A. I. T. conceived and oversaw the whole project.
Data Availability
The data that support the plots within this paper and other findings of this study are available from the correspondingauthor upon reasonable request.
Competing interests
The authors declare no competing interests.
Supplementary Information
Supplementary information are available upon reasonable request.
BIBLIOGRAPHY Novoselov, K. S. et al.
Two-dimensional atomic crystals.
Proceedings of the National Academy of Sciences of the UnitedStates of America , 10451–10453 (2005). Mak, K. F. & Shan, J. Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides.
Nature Photonics , 216–226 (2016). Mak, K. F. et al.
Tightly bound trions in monolayer MoS 2.
Nature Materials , 207–211 (2013). He, K. et al.
Tightly bound excitons in monolayer WSe2.
Physical Review Letters , 1–5 (2014). Xu, X., Yao, W., Xiao, D. & Heinz, T. F. Spin and pseudospins in layered transition metal dichalcogenides.
Nature Physics , 343–350 (2014). Liu, X. et al.
Strong lightmatter coupling in two-dimensional atomic crystals.
Nature Photonics , 30–34 (2014). Dhara, S. et al.
Anomalous dispersion of microcavity trion-polaritons.
Nature Physics , 130–133 (2018). Dufferwiel, S. et al.
Exciton-polaritons in van der Waals heterostructures embedded in tunable microcavities.
NatureCommunications , 1–7 (2015). Sidler, M. et al.
Fermi polaron-polaritons in charge-tunable atomically thin semiconductors.
Nature Physics , 255–261(2017). Low, T. et al.
Polaritons in layered two-dimensional materials.
Nature Materials , 182–194 (2017). Schneider, C. et al.
Two-dimensional semiconductors in the regime of strong light-matter coupling.
Nature Communications , 2695 (2018). Krol, M. et al.
Exciton-polaritons in multilayer WSe 2 in a planar microcavity.
2D Materials , 15006 (2020). Dufferwiel, S. et al.
Valley-addressable polaritons in atomically thin semiconductors.
Nature Photonics , 497–501 (2017). Chen, Y.-J., Cain, J. D., Stanev, T. K., Dravid, V. P. & Stern, N. P. Valley-polarized excitonpolaritons in a monolayersemiconductor.
Nature Photonics , 431–435 (2017). Dufferwiel, S. et al.
Valley coherent exciton-polaritons in a monolayer semiconductor.
Nature communications , 4797(2018). Qiu, L., Chakraborty, C., Dhara, S. & Vamivakas, A. N. Room-temperature valley coherence in a polaritonic system.
NatureCommunications , 1513 (2019). Barachati, F. et al.
Interacting polariton fluids in a monolayer of tungsten disulfide.
Nature nanotechnology , 906–909(2018). Lundt, N. et al.
Optical valley Hall effect for highly valley-coherent exciton-polaritons in an atomically thin semiconductor.
Nature nanotechnology , 770–775 (2019). Waldherr, M. et al.
Observation of bosonic condensation in a hybrid monolayer MoSe2-GaAs microcavity.
Nature Commu-nications , 3286 (2018). Emmanuele, R. P. A. et al.
Highly nonlinear trion-polaritons in a monolayer semiconductor.
Nature Communications ,3589 (2020). Kasprzak, J. et al.
Bose-Einstein condensation of exciton polaritons.
Nature , 409–414 (2006). Christopoulos, S. et al.
Room-temperature polariton lasing in semiconductor microcavities.
Physical Review Letters ,126405 (2007). Bhattacharya, P. et al.
Room temperature electrically injected polariton laser.
Physical Review Letters , 236802 (2014). Amo, A. et al.
Superfluidity of polaritons in semiconductor microcavities.
Nature Physics , 805–810 (2009). Liew, T. C. H. et al.
Exciton-polariton integrated circuits.
Physical Review B , 33302 (2010). Ballarini, D. et al.
All-optical polariton transistor.
Nature Communications (2013). Shree, S. et al.
High optical quality of MoS 2 monolayers grown by chemical vapor deposition.
2D Materials , 15011 (2019). Zhang, Y. et al.
Recent Progress in CVD Growth of 2D Transition Metal Dichalcogenides and Related Heterostructures.
Advanced Materials , 1901694 (2019). Gebhardt, C. et al.
Polariton hyperspectral imaging of two-dimensional semiconductor crystals.
Scientific Reports , 13756(2019). Severs Millard, T. et al.
Large area chemical vapour deposition grown transition metal dichalcogenide monolayers automat-ically characterized through photoluminescence imaging. npj 2D Materials and Applications , 12 (2020). Ajayi, O. A. et al.
Approaching the intrinsic photoluminescence linewidth in transition metal dichalcogenide monolayers.
2D Materials , 31011 (2017). Del Pozo-Zamudio, O. et al.
Electrically pumped WSe 2 -based light-emitting van der Waals heterostructures embedded inmonolithic dielectric microcavities.
2D Materials , 31006 (2020). Zhang, X. et al.
Defect-Controlled Nucleation and Orientation of WSe 2 on hBN: A Route to Single-Crystal EpitaxialMonolayers.
ACS Nano , 3341–3352 (2019). Okada, M. et al.
Direct chemical vapor deposition growth of WS2 atomic layers on hexagonal boron nitride.
ACS Nano ,8273–8277 (2014). Yu, H. et al.
Precisely Aligned Monolayer MoS 2 Epitaxially Grown on h-BN basal Plane.
Small , 1603005 (2017). Ross, J. S. et al.
Electrical control of neutral and charged excitons in a monolayer semiconductor.
Nature Communications , 1473–1476 (2013). Robert, C. et al.
Fine structure and lifetime of dark excitons in transition metal dichalcogenide monolayers.
Physical ReviewB , 1–8 (2017). Lindlau, J. et al.
The role of momentum-dark excitons in the elementary optical response of bilayer WSe2.
Nature Commu-nications , 2586 (2018). Jadczak, J. et al.
Probing of free and localized excitons and trions in atomically thin WSe 2 , WS 2 , MoSe 2 and MoS 2 inphotoluminescence and reflectivity experiments.
Nanotechnology , 395702 (2017). Lippert, S. et al.
Influence of the substrate material on the optical properties of tungsten diselenide monolayers.
2D Materials , 025045 (2017). Dumcenco, D. et al.
Large-Area Epitaxial Monolayer MoS 2.
ACS Nano , 4611–4620 (2015). Zhang, X. et al.
Diffusion-Controlled Epitaxy of Large Area Coalesced WSe 2 Monolayers on Sapphire.
Nano Letters ,1049–1056 (2018). Schwarz, S. et al.
Two-dimensional metal-chalcogenide films in tunable optical microcavities.
Nano Letters , 7003–7008(2014). Kavokin, A., Baumberg, J. J., Malpuech, G. & Laussy, F. P.
Microcavities (Oxford University Press, 2007). Genco, A. et al.
High quality factor microcavity OLED employing metal-free electrically active Bragg mirrors.
OrganicElectronics , 174–180 (2018). Alexeev, E. M. et al.
Resonantly hybridized excitons in moir´e superlattices in van der Waals heterostructures.
Nature ,81–86 (2019). Cristofolini, P. et al.
Coupling Quantum Tunneling with Cavity Photons.
Science , 704–707 (2012). Dolan, P. R., Hughes, G. M., Grazioso, F., Patton, B. R. & Smith, J. M. Femtoliter tunable optical cavity arrays.
OpticsLetters , 3556 (2010). Alexeev, E. M. et al.
Imaging of Interlayer Coupling in van der Waals Heterostructures Using a Bright-Field OpticalMicroscope.
Nano Letters , 5342–5349 (2017). MathWorks.