Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

Strong exciton-photon coupling in large area MoSe2 and WSe2 heterostructures fabricated from two-dimensional materials grown by chemical vapor deposition

Strong exciton-photon coupling in large area MoSe2 and WSe2 heterostructures fabricated from... Two-dimensionalsemiconductingtransitionmetaldichalcogenidesembeddedinoptical microcavitiesinthestrongexciton-photoncouplingregimemayleadtopromisingapplicationsin spinandvalleyaddressablepolaritoniclogicgatesandcircuits.Onesignificantobstaclefortheir realizationistheinherentlackofscalabilityassociatedwiththemechanicalexfoliationcommonly usedforfabricationoftwo-dimensionalmaterialsandtheirheterostructures.Chemicalvapor depositionoffersanalternativescalablefabricationmethodforbothmonolayersemiconductors andothertwo-dimensionalmaterials,suchashexagonalboronnitride.Observationofthestrong light-mattercouplinginchemicalvaporgrowntransitionmetaldichalcogenideshasbeen demonstratedsofarinahandfulofexperimentswithmonolayermolybdenumdisulfideand tungstendisulfide.Hereweinsteaddemonstratethestrongexciton-photoncouplingin microcavitiescomposedoflargeareatransitionmetaldichalcogenide/hexagonalboronnitride heterostructuresmadefromchemicalvapordepositiongrownmolybdenumdiselenideand tungstendiselenideencapsulatedononeorbothsidesincontinuousfew-layerboronnitridefilms alsogrownbychemicalvapordeposition.Thesetransitionmetaldichalcogenide/hexagonalboron nitrideheterostructuresshowhighopticalqualitycomparablewithmechanicallyexfoliated samples,allowingoperationinthestrongcouplingregimeinawiderangeoftemperaturesdownto 4Kelvinintunableandmonolithicmicrocavities,anddemonstratingthepossibilitytosuccessfully developlargeareatransitionmetaldichalcogenidebasedpolaritondevices. 1.Introduction strengths [3,4]. Moreover, the breaking of spatial inversionsymmetryinthe2Dlatticeandalargespin– Monolayers of transition metal dichalcogenides orbit coupling generate spin-valley locked optically (TMDs) are promising semiconductors with unique addressable excitons at the K and K points of the electrical and optical properties arising from the momentum space [3, 5]. These exceptional proper- quantum confinement experienced by the electrons tiescanbefurtherenrichedbyintegratingtheTMDs and holes in the two-dimensional (2D) structure withinopticalresonatorsenablingthestrongexciton- [1, 2]. One of the main effects is the appearance of photoncouplingregime,whereconfinedphotonsand direct bandgap excitonic transitions showing fea- excitons hybridize into new states called polaritons tures strongly beneficial for optoelectronics, such [6–12]. Polaritons in TMDs acquire novel properties as high binding energies and very large oscillator arisingfromthevalleypseudo-spindegreeoffreedom ©2020TheAuthor(s). PublishedbyIOPPublishingLtd 2DMater.8(2021)011002 of excitons [13, 14], and further provide enhanced growthonsapphireforMoSe anddirectlyonCVD- valley coherence for excitons strongly coupled with synthesizedhBNforWSe .Wehaveconfirmedthese long-livedcavityphotons[15,16].Efficientpolariton favorablemonolayerTMDcrystalpropertiesbyusing propagation in TMDs has been recently observed, a recently developed substrate-wide statistical ana- withdiffusionlengthsofupto20 µminWS atroom lysis of TMD crystal axes orientation and PL prop- temperature [17], while valley-dependent divergent erties [30]. Further testing of the samples in a tun- polaritondiffusionhasbeenfoundinMoSe atcryo- able microcavity exposes clear evidence of strong genic temperatures, whereby polaritons spread in exciton-photon coupling and formation of exciton- differentin-planedirectionsowingtotheexcitonval- polaritonstates.Ananti-crossingisobservedbetween leyHalleffect[18].PolaritonsinTMDsalreadyoffer the cavity mode and neutral exciton transition, with the potential to create highly non-linear phenomena Rabisplittingsof17meVforbothMoSe andWSe , 2 2 [7,19,20], with further effort directed at realization very similar in magnitude with exfoliated materi- ofBose–Einsteincondensation[19,21],polaritonlas- als [8,9,13,15], highlighting similar optical qualit- ing[22,23]andopticalparametricoscillation[24]so ies. Finally, as a proof of concept for future large far observed in other material systems. United to the scale TMD-based polaritonic devices, a monolithic valley degree of freedom of TMD monolayers, these SiO cavity is fabricated incorporating CVD grown phenomena could be exploited to create large scale hBN-encapsulated MoSe , in which strong coupling all-optical polariton circuits and quantum networks isobservedwithalargeRabisplittingof34meVatT [25,26]. =4Kand31meVatT =150K.Theresultspresen- However, TMD monolayers are usually fabric- tedinthisworkdemonstratethepossibilitytofabric- atedbymechanicalexfoliation,resultinginhighqual- atelargeareapolaritondevicesexploitinghighquality ity but small sized flakes, hindering the reproducib- TMDbasedheterostructuresmadefromCVD-grown ility and scalability of the devices. Chemical vapor materials, paving the way for future scalable TMD- deposition (CVD) offers an alternative growth and polaritoniccircuits. fabricationmethodthatprovidessubstrate-widecov- erage of uniform monolayer islands [27], as well 2.Resultsanddiscussion as the ability to grow heterostructures in-situ [28], thereforecompletelybypassingthemechanicaltrans- Twotypesofencapsulatedheterostructure(HS)have fers necessary in exfoliated equivalents and minim- been fabricated from CVD-grown materials for this izing external contamination. CVD provides a very work(seefurtherdetailsinMethods).Inthefirsthet- attractive and scalable method for the fabrication of erostructure, HS1 (figure 1(a)), MoSe monolayers largescaleTMDbaseddevices.Assuch,CVD-grown are grown by CVD onto a sapphire substrate. Thou- MoS and WS flakes have already been employed 2 2 sands of individual monolayer islands, positioned in polaritonic devices working at room temperature throughout the entire substrate, are then mechanic- [6, 29]. Nevertheless, in order to enhance the polari- ally transferred using a polystyrene membrane onto ton valley properties and optimize coherence, nar- a high reflectivity distributed Bragg reflector (DBR) row exciton linewidths (low inhomogeneous broad- composed of 13 pairs of SiO /Ta O with the high 2 2 5 ening)andlowstructuraldisorderareneeded,which reflectance stop-band centered at a wavelength of uptonowhaveonlybeenshownbyexfoliatedMoSe 2 750nm.Asubstrate-widefew-layerfilmofhexagonal and WSe monolayers at low temperatures [7–11, boronnitride(hBN),alsogrownbyCVD,ismechan- 13, 15]. Another advantage of high quality struc- ically transferred on top of the MoSe flakes to com- tures combined with the possibility to operate at plete the encapsulation. For the second heterostruc- low temperature will be access to highly non-linear ture, HS2 (figure 1(b)), WSe monolayers are grown trion-polaritons[7,20]orFermi-polaron-polaritons by CVD directly on few-layer hBN, also grown by [9], requiring control and stability of the charged CVD. Both materials, WSe /hBN, are then mechan- excitons, as well as employment of suitable hetero- ically transferred, at once, onto the DBR and sub- structures [9] composed of TMDs, hexagonal boron sequentlyencapsulatedwithafew-layerfilmofCVD nitride(hBN)andgraphene. grown hBN. It has been shown that for mechan- Here,weshowlargeareaMoSe andWSe mono- 2 2 ically exfoliated TMDs, an encapsulation with thin layers encapsulated in hBN, all grown by CVD, with hBN provides uniform dielectric screening of the crystaldomainsexceeding100 µminsize,whichdis- Coulomb interaction, reducing spatial inhomogen- playhighopticalquality,rivalingexfoliatedmaterial. eity in the exciton, thereby narrowing the emission This is evidenced by the intense and narrow exciton linewidth [27, 31]. Furthermore, hBN protects the peaksobservedinphotoluminescence(PL)measure- TMD layers in the heterostructure from damage and ments and in reflectance contrast (RC). Substrate- contamination during the subsequent deposition of wide growth uniformity and a high degree of align- various dielectrics in microcavity structures relevant ment within the ensemble of monolayer domains to our work [32]. Moreover, the direct growth of has been achieved in these materials using CVD TMD monolayers on hBN is also strongly beneficial 2 2DMater.8(2021)011002 Figure1.(a),(b)SchematicsoftheCVDgrownheterostructures(nottoscale).(a)HS1:MoSe monolayersaregrownbyCVDon asapphiresubstrate,mechanicallytransferredontoaDBRmirrorandencapsulatedwithhBN.(b)HS2:WSe monolayersare grownbyCVDonmultilayerhBN,transferredtogetherwiththehBNontoaDBRmirrorandencapsulatedwithmultilayerhBN. ThehBNmultilayersarealsogrownbyCVD.(c),(d)Opticalmicroscopeimagesofthephotoluminescence(PL)from(c)HS1 and(d)HS2takenatroomtemperatureusinga50xobjectivelens(scalebar:50 µm).(e),(f)Opticalcharacterizationof(e)HS1 and(f)HS2atT~4K.InsetsshowPLspectraatroomtemperature.PLandreflectancecontrast(RC)spectraareshowninblack 0 − andred,respectively.AtlowT,bothsamplesshowemissionfrombothneutral,X ,andcharged,X ,excitons,whereasonlyHS1 showsabsorptionfromboth.PLcwexcitationconditions: λ =660nm,P =20 µWforHS1; λ =532nm,P =20 µWfor exc exc HS2. as a route to single-crystal epitaxial growth [30, 33], PL imaging analysis [30] used to identify the mono- which, up until now, has been demonstrated with a layersisdiscussedinmoredetailbelow. limitedrangeofTMDmaterialssuchasWS [34]and Further optical characterization of the two het- MoS [35].FullycoalescedCVDgrownWSe mono- erostructures has been performed using a spectro- 2 2 layer films were obtained very recently by Zhang et scopic microscopy setup at both room (~290 K) and al [33], through a careful control of nucleation and low (~4 K) temperature (figures 1(e) and (f)). PL extended lateral growth time, and a strong improve- measurementsarecarriedoutbyexcitingthesamples ment of optical and electrical properties have been with an off resonant continuous wave (cw) laser at achieved compared to the same material grown on a power of 20 µW (see Methods). The consider- sapphire. ableroomtemperatureexcitonicPLemissionshown As a first characterization step, the room tem- in the insets in figures 1(e), (f) highlights the large perature PL emission and the general morpho- exciton binding energy associated with TMD mono- logy of the structures have been analyzed under layers [4]. In HS1 the exciton PL peak is located at an optical microscope (see Methods). HS1 (fig- 1.579 eV with a linewidth of 40 meV, and HS2 dis- ure 1(c)) generally consists of large isolated mono- plays the exciton peak at 1.670 eV with a similar layer islands with characteristic triangular shape and linewidth of 45 meV, typical of MoSe and WSe 2 2 average lateral size of 8 µm (see Supplementary monolayersoperatingatroomtemperatures. Note I (stacks.iop.org/TDM/08/011002/mmedia) for Decreasing the temperature to ~4 K produces a details), along with a number of regions where mul- narrowed and blue shifted neutral exciton PL peak, tiple flakes merge to form monolayers with sizes X , at 1.671 eV in HS1 and 1.759 eV in HS2, and a exceeding100 µm.Similarly,HS2(figure1(d))shows secondpeakattributedtothechargedexciton(trion) largetriangularmonolayerflakeswithaveragelateral transition, X , which appears at 1.639 eV in HS1 widthsof11 µm.Again,inareaswhereflakesmerge, and 1.726 eV in HS2, about 30 meV below the neut- monolayers of over 100 µm in width can be seen. ral exciton [8, 13, 36]. The relative intensity of the − 0 Large areas of uniform coverage are necessary for X peak, when compared to the X peak, is heav- constructing large arrays of identical heterostructure ily influenced by the free carrier densities present in devices, such as transistors or photodetectors. Over- the structures [3]. In HS2, PL seen at lower ener- all, there is monolayer coverage of 14% (1.75 mm ) gies(below1.70eV)haspreviouslybeenattributedto for HS1 and 22% (3.67 mm ) for HS2, calculated various excitonic complexes in WSe including spin by dividing the total area of monolayer across the dark excitons [37], exciton-phonon side-bands [38] samplebythetotalsubstratearea.Thesubstrate-scale and localized states [39]. The samples show neutral 3 2DMater.8(2021)011002 Figure2.(a)Schematicshowingflakeorientationsatdifferentanglesrelativetothehorizontalaxisofthemicroscopeimages.(b), (c)Analysisofflakeorientation.IslandsofmonolayerTMDareidentifiedandtheorientationextractedusingmethodologyas describedinpreviousinvestigations[30].Both(b)HS1and(c)HS2showtwomainpeakssituated60 apart,whileHS1also showstwoextrapeakssituatedat30 fromthemainpeaks. exciton linewidths of 13 meV and 21 meV for HS1 microscopePLimages,anexampleofonesuchimage and HS2 respectively, and 12 meV and 20 meV for isshowninfigures1(c)and(d).Byemployinganalyt- thechargedexcitonofthetwosamples.Generally,the ical methods detailed in our previous work [30], the linewidth of an excitonic transition in TMD mono- flakeorientationrelativetothehorizontalaxisofthe layersisstronglyaffectedbythelevelofstructuraldis- microscopeimagecanbefound.Inordertomaximize orderanddensityofdefects[29].Thespectralshapes accuracy, only islands with shape close to equilateral and linewidths demonstrated by the CVD grown trianglesareanalyzedintermsofangularorientation. samplesinvestigatedinthisworkimproveuponthose Of the 16999 (14205) individual monolayer islands reported by Zhang et al [33] and Lippert et al [40], identifiedinHS1(HS2),8089(8391)satisfythiscon- and are similar to exfoliated flakes operating at low dition.Measuredintermsofarea,thiscorrespondsto temperaturewithoutencapsulation(seeSupplement- 21%(17%)ofthetotalmonolayercoverage.Thehis- aryNoteIV)[8,15,27,31,36].ThisshowsthatCVD tograms in figures 2(b) and (c) detail the number of growth can produce heterostructures of comparable islands identified as a function of orientation angle, optical quality to mechanically exfoliated flakes. The showing that both the samples feature a very high role of hBN is mostly to provide a high quality sub- degree of island orientation uniformity, a signature stratefortheTMDsynthesis[30,33],butalsotoactas of epitaxial growth. For WSe grown directly onto a buffer layer protecting TMDs from damage during hBN (figure 2(c)), two main orientations have been the deposition of additional layers in order to com- found, 60 apart. This is expected from equilateral pleteamicrocavityorwaveguidestructure. triangular islands growing in two preferential direc- We also measure reflectance contrast spectra tionsat60 relativetooneanother.Thesetwoprefer- using a broad band white light source and calcu- ential directions are directly related to the hexagonal latedas ∆R/R = (R −R )/R ,whereR isthe crystal structure of the growth substrate and have sub HS sub HS reflectance of the heterostructure, and R is the alsobeenobservedinpreviousstudiesofMoS ,WS , sub 2 2 reflectanceofthebaresubstrateinbetweentheTMD and WSe grown by CVD on hBN [30,33–35]. For flakes. These spectra (red lines in figures 1(e), (f)) MoSe grown onto c-plane sapphire (figure 2(b)), reveal a strong absorption peak attributed to the four peaks in the angular distribution are observed. X transition in both heterostructures and a lower Twomainpeaksshowthepreferredflakeorientation, intensity peak at lower energy attributed to X in situated at 60 relative to each other, along with two HS1.Therelativepeakheightisstrictlyrelatedtothe less populated angles at 30 relative to the two main oscillator strength of individual transitions, with the peaks. Both two, and four preferential growth direc- neutral exciton being much more intense than the tions have been seen in TMDs grown via CVD onto trioninHS1duetoarelativelylowdopinglevel.The c-plane sapphire [41, 42]. Control over the relative X absorptionpeakofHS2isslightlybroaderthanin angleoftheflakesatthesynthesisstageoffabrication HS1,similartothetrendobservedforthePLspectra, will provide the basis to build scalable heterostruc- which is an indication of the greater structural dis- tureswith controloverthe relativeinterlayercrystal- orderpresentinCVD-grownWSe . lographicorientation. As can be inferred in figures 1(c) and (d), the Aftertheopticalcharacterizationstep,theencap- bright triangular monolayer islands appear to have a sulated heterostructures are tested in a tunable open preferredorientation.Toextractthesizeandshapeof cavitysetupwhichconsistsofatopconcaveDBRmir- monolayer flakes, shape recognition techniques were ror distanced 2–3 µm from a planar bottom DBR used on a full substrate map comprised of multiple mirror (figures 3(a) and (b)), upon which the HSs 4 2DMater.8(2021)011002 Figure3.(a)Schematicofthetunableopenmicrocavityincludingthesetofpiezoactuatorsusedtoalignthemirrorsandperform thePLscansoftheheterostructure.(b)Schematicoftheopticalmicrocavity.ThecavityiscomposedofaplanarDBR,upon whichtheHSisplaced,andaconcavetopDBRconfiningtheopticalcavitymodein3dimensions.(c),(d)PLemissionfrom(c) HS1and(d)HS2displayedasafunctionofphotonenergyandexciton-photondetuning(∆ =E −E ).Clearanti-crossingsof c X thecavitymodewiththeexcitonareobservedinbothheterostructures.PLspectraarefittedusingaLorentzianpeak(see SupplementaryNoteII)andatwolevelcoupledoscillatormodelisusedtoextractthelower(bluecurve)andupper(yellow curve)polaritonbranches,excitonicresonances(whitehorizontallines),andLG photonicmode(greendiagonalline).Rabi splittingsof17meVarefoundforbothHS1andHS2.Samplesareopticallyexcitedusinga637nmcwlaser. are placed [8, 9, 13, 15, 43]. The mirrors are posi- strengthofthetriontransition.Theabsenceofmode tioned using piezo-actuator stages (figure 3(a)). Free broadening in HS2 (figure 3(d)) cavity scans at the spaceopticalaccessfromabovethetopconcaveDBR trionenergyisanindicationthattheabsorptionofthe allowslaserexcitationandopticaldetection,usingan WSe trion resonance, occurring in HS2 at 1.726 eV, achromatic doublet objective lens. The cavity length istooweak,asalsoconfirmedinfigure1(f). can be tuned by slowly moving the bottom mirror The second regime of exciton-photon coupling, along the z-axis, thus allowing the tuning of the cav- known as the strong coupling, presents itself as an itymode(diagonalgreendashedlinesinfigures3(c), anti-crossingoftheLG cavitymodeandtheexciton (d)). The PL signal collected from the TMDs, as the energieswithacharacteristicRabisplitting,2ℏΩ ,at cavity length is reduced, is displayed in figures 3(c) the resonance. This behavior can be clearly observed and (d) as a function of detuning (the energy dif- in both HS1 (figure 3(c)) and HS2 (figure 3(d)) ference between the cavity mode and unperturbed as the LG is tuned into resonance with the neut- exciton, X ). The three dimensional optical confine- ral exciton, at 1.673 eV for HS1 and 1.770 eV for ment provided by the concave top mirror generates HS2. The peak positions of the lower (LPB) and asetoftransversemodesforeachlongitudinalmode upper (UPB) polariton branches have been extrac- [8,43]alsovisibleinbothfigures. ted using a Lorentzian peak fitting, and used to fit a Whenthefundamentallongitudinalcavitymode, two-level coupled oscillator model (detailed in Sup- LG (greendiagonaldashedlinesinfigures3(c)and plementary Note II) in order to determine 2ℏΩ as 00 R (d)) which ensures the highest light confinement, is shown overlaid in figures 3(c), (d). We find a value tunedintoresonancewiththeexcitontransitionener- of 17.2 ± 3.3 meV for HS1 and 16.8 ± 3.1 meV gies,lightmattercouplingcanmanifestinoneoftwo forHS2. ways,bothofwhichareobservedinHS1(figure3(c)). These measurements demonstrate large Rabi As the cavity mode is tuned into resonance with the splittings closely comparable to the values trion, X , at 1.638 eV, the cavity mode is broadened observed in exfoliated monolayer MoSe and WSe 2 2 and brightened, a demonstration of the weak coup- placed in zero-dimensional tunable microcavities ling[8,13,15,44]arisingfromthereducedoscillator [9,13,15,43]. This further confirms, thanks to the 5 2DMater.8(2021)011002 Figure4.(a),(b)AngleresolvedPLimagingofHS1monolithiccavity.(a)At5K,thecavityhasanegativedetuningat0 of ∆ ≈ −3.5meV,showinganti-crossingsat ±7 .ThePLintensityintheregionwithinthereddashedlineshasbeenmultipliedbya factorof10forclarity.(b)At150K,thecavityhasapositivedetuningat0 of ∆ ≈10meV.WeoverlaythefittedLPB(blue)and UPB(yellow)alongwiththeextractedparaboliccavitydispersion(green)andneutralexcitonresonance(white).ARabisplitting of34 ±4meVand31 ±4meVisobservedat5Kand150K,respectively.Thesampleisopticallyexcitedusinga660nmcwlaser atapowerof50 µW. reduced structural disorder in the presented hetero- modepenetrationintothemirrors.Asshownbelow, structures,thehighopticalqualityofthehBNencap- thisleadstoahighermagnitudeoftheRabisplitting. sulatedCVDgrownTMDmonolayers,henceproving Since monolithic cavities are not tunable in size, the thevalidityforCVDgrowthtechniqueswhendesign- temperature dependence of the X transition energy ingscalabledevices. (furtherdiscussedinSupplementaryNoteIII)isused Further advantages of large area TMDs can be totunetheexcitonintoresonancewiththephotonic exploited in monolithic cavities, providing a plat- mode which has a negligible dependence of its fre- formtoformvarioustopologicaldesignstoadaptor quencywithtemperature. enhance device functionality towards polariton cir- ThePLcollectedfromthemonolithiccavitywhile cuits [25, 26]. For these devices, the protective func- being optically excited by non-resonant continuous tionofthehBNencapsulationisofparticularimport- wave laser in the linear power regime, as imaged anceasthetopdielectricmirrorneedstobedeposited by angle resolved spectroscopy probing the exciton ontopoftheTMDlayers[32].Asaproofofconcept, and photon states at different k-vectors, is shown in we deposited 98 nm of SiO via e-beam deposition, figure4. In figure 4(a) at a temperature of 5 K, the followed by a semi-transparent layer of 50 nm gold, X transition is at 1.648 eV, while in figure 4(b) at on top of HS1 to fabricate a λ/2 monolithic cavity 150 K, the X red-shifts to 1.636 eV. The monolithic (see Supplementary Note III). The oxide deposition cavity shows strong exciton-photon coupling signa- process has been carried out at room temperature in tures in PL at both the temperatures, owing to the ordertopreservetheopticalintegrityoftheemitting protective capability of the CVD grown hBN which materialsasmuchaspossible[32,45]. helpedshieldtheMoSe monolayersfromthepoten- Inthetunablecavity,thephotonicmodesarecon- tiallydamagingSiO depositionprocess. finedinallthreedimensions,resultinginasetofdis- At a temperature of 5 K, the exciton is negat- crete cavity modes, with k ~ 0, which are tuned ively detuned from the cavity mode at 0 by - 3.5 x,y,z inenergybyalteringthecavitylength.Bycontrast,in meV,suchthattheLPBismuchmorevisiblethanthe a monolithic two-dimensional cavity, as in our case, UPB. To show the upper polariton branch in figure the photonic mode is confined only in the vertical z 4(a) the collected intensity values have been multi- direction, and thus a cavity mode energy dispersion plied by a factor of 10 between 1.658 and 1.705 eV, as a function of continuous k values is observed as outlined by red dashed lines. By fitting the PL x,y [44]. This dispersion can be probed by measuring emissionspectrawithLorentzianpeaksandapplying angle-resolvedPLorreflectivityspectraasafunction the extracted peak positions to a two level coupled- of angle measured from the normal to the sample oscillator model (see Supplementary Note II) [44] (correspondingtok =0)[44].Inthecavityusedin we obtain a large Rabi splitting of 34.1 ± 4.0 meV x,y our experiment, a stronger light confinement can be at 5 K. The exciton-photon coupling strength in the achievedthaninthetunabledevicespresentedearlier, monolithic cavity is higher than in the open cav- due to a smaller vertical size of the cavity and lower ity due to the increased light confinement. The two 6 2DMater.8(2021)011002 polariton branches, cavity dispersion, and exciton 4.2.GrowthofsinglelayerMoSe onc-plane energy obtained from the coupled-oscillator model sapphire are shown overlaying figures4(a)and(b), with an MoO (99.97%, Sigma Aldrich) and Se (99.999%, anti-crossingclearlyseenat ±7 whenthedeviceisat AlfaAesar)wereusedasprecursorsforthegrowth.60 5K.Thestronglycoupledcavityperformswellupto mg of MoO powder was placed in the center of the 150K,whentheexcitonicmodeispositivelydetuned furnaceand150mgofSewasplacedattheupstream (∆(0 ) = +10 meV), leading to a Rabi splitting of entry. To minimize the intense evaporation of the 30.5 ±3.9meV. MoO ,thecruciblethatcontainedtheprecursorwas partially covered by a SiO /Si wafer. The target sub- strate of c-plane sapphire was placed beside the cru- 3.Conclusion ciblecontainingMoO . The tube furnace was evacuated for 30 min and In summary, high quality substrate-wide MoSe , subsequentlyrepletedwiththeArgas.Ambientpres- andWSe ,TMDmonolayersencapsulatedwithlarge sure is reached before the furnace was heated to area hBN were fabricated using CVD growth tech- 600 C for 18 minutes under a steady flow of Ar gas niques, and subsequently embedded in tunable and (60sccm)andH gas(12sccm).Asthefurnacetem- monolithicmicrocavitydeviceswherestrongexciton- perature reached 600 C, the upstream entry of the photon coupling was observed. The heterostructures ◦ tube was heated to 270 C, using a heating belt, as show optical properties comparable with exfoliated to vaporize Se. Finally, the furnace temperature was materials, and consequently exhibit similar values of ◦ raised to 700 C, and maintained for 1 hour to facil- polaritonRabisplittingstopreviouslystudiedhetero- itate the MoSe growth. Upon completion, the tube structuresmadefromexfoliatedflakes[8,9,13,15]. furnacewascooledtoroomtemperaturewhilstmain- Furthermore, the demonstrated CVD growth on taining Ar flow, without H . Polystyrene was used to sapphire and hBN produced highly orientated TMD transfer MoSe on top of the DBR in order to main- islands, and is thus suitable for the fabrication of tainsamplequality. large scale TMD/TMD heterostructures with highly controlled interlayer twist angle [46] to be embed- 4.3.GrowthofsinglelayerWSe directlyonhBN ded in microcavities. Together with additional hBN Initially, hBN was grown on c-plane sapphire (see and graphene layers these structures could provide a Methods). WO (99.998%, Alfa Aesar) and Se viableroutetorealizationofhighlytunableandnon- (99.999%, Alfa Aesar) were used as precursors for lineardipolarpolaritons[47]inlargescaledevices. the growth of single layer WSe . 120 mg of WO 2 3 This work demonstrates the possibility to fabric- powder,mixedwithasmallamountofNaCltoreduce ate large scale polaritonic devices based on TMD- the influence of humidity, was placed in the center basedheterostructures.Furtherdevelopmentoflarge of the furnace and 300 mg of Se was placed at the scale monolayer semiconductor growth techniques, upstream entry. The target substrate of multilayer most notably directly onto hBN which provides hBN on sapphire was placed beside the crucible that highly co-orientated TMD domains, will inevit- contained WO . The tube furnace was evacuated for ably lead to heterostructures that can reliably and ◦ more than 30 min before being heated to 800 C for repeatedlycompetewith,orout-perform,thosebuilt 24 minutes under a steady flow of Ar gas (120 sccm) with exfoliated flakes due to the unprecedented and H gas (20 sccm). As the furnace temperature scalabilitythatisgranted. reached 800 C, the upstream entry of the tube was heated to 270 C, using a heating belt, as to vapor- 4.Methods ize Se. Finally, the furnace temperature was raised to, and maintained at, 870 C for 1 hour to facilitate 4.1.Dielectricmirrorfabrication WSe growth.Uponcompletion,thetubefurnacewas cooled to room temperature whilst maintaining Ar HighlyreflectingdistributedBraggreflectors(DBRs) are deposited on silica substrates by ion beam sput- flow, without H . In order to maintain sample qual- ity, polystyrene was used to transfer the WSe /hBN tering.TheDBRsarecomprisedof13pairsofquarter wavelengthSiO /Ta O layers,terminatingwithSiO , stacktotheDBR. 2 2 5 2 of thicknesses 129 and 89 nm (refractive index 1.45 and 2.10, respectively). The DBRs are designed for a 4.4.GrowthoflargeareahBN center wavelength of 750 nm and a stop-band width MultilayerhBNwithanAA’stackingorderwasgrown of200nm. byremoteinductivelycoupledplasmachemicalvapor Using focused Gallium ion beam milling into a deposition method. A 2-inch c-plane sapphire was planarfusedsilicasubstrate,theconcaveshapedtem- used as a substrate for the hBN growth. The sub- plateforthetopmirrorisformed[48].Thenominal strate was placed in the center of a 2-inch alumina radiusofcurvatureoftheconcavemirrorwas20 µm, tube furnace of CVD. A borazine (Gelest, Inc.) pre- prompting a beam waist of around 1 µm [8, 43] on cursor flask was placed in a water bath at −15 C. thefocalplane. The bath temperature before the growth of hBN was 7 2DMater.8(2021)011002 increasedupto25 C.Beforethegrowthofmultilayer the x, y, and z translational motion, while another hBN,thefurnacewasheatedto1220 Cunderflowof two stages control the tilt alignment. The top mir- Ar gas (10 sccm). Plasma was generated at a power ror is positioned using a 3-axis piezo-actuator stage of 30 W under a flow of borazine (0.2 sccm) and controlling the x, y, and z translational motion. Ar (10 sccm) gases for 30 mins. Atomic force micro- Optical PL scans were completed with the samples scopy and transmission electron microscopy meas- placed in a helium bath cryostat system at a tem- urements confirmed that the thickness of hBN was perature of 4.2 K using a 637 nm continuous- 1.2nm,approximately3layers.Inaddition,thehBN wavelaserdiode(VortranStradus),focusedontothe sample shows quite good thickness uniformity over sample using an achromatic lens. The collected PL the2-inchsapphiresubstrateaccordingtotheRaman is focused onto a single mode fiber and guided into andUVabsorptionspectrameasuredatninerandom a 0.75 m spectrometer (SP-2-750i, Princeton Instru- pointsoverthe2-inchhBNfilm. ments) and a high-sensitivity charge-coupled device (PyLoN:400BR,PrincetonInstruments)foremission 4.5.Opticalmeasurements collection. The photoluminescence images of the CVD samples wereacquiredusingacolorcamera(DS-Vi1,Nikon) 4.7.Measurementofmonolithiccavities equipped to a modified bright-field microscope We performed the Fourier space spectral imaging of (LV150N, Nikon). A 550 nm short-pass filter thePLemittedbythemonolithiccavitybyemploying (FESH0550, Thorlabs) blocked the near-infrared a 2D CCD array (PyLoN:100BR, Princeton Instru- emission from the white-light source, and a 600 ments)coupledtoa300gr/mmgratingspectrometer nm long-pass filter (FELH0600, Thorlabs) was used (SP-2-500i, Princeton Instruments). We focused a to isolate the photoluminescence signal from the 30 cm lens onto the back plane of a 50x Mitutoyo samples. A fully detailed description is available in infinity corrected objective to obtain the Fourier Ref[49]. planeimageofthesample,whichwasthenprojected Spectrally-resolved photoluminescence and ontheslitofthespectrometerbyusinga10cmlens. reflectance contrast measurements were implemen- WeusedtheslittoselectonlythesectionoftheFour- ted using a custom-built micro-PL setup. For pho- ier space at k = 0, resulting in a final image on the toluminescence, the samples were excited using two CCD displaying the PL as a function of k on the y- diode-pumped solid-state lasers (CW532-050 and axis and energy on the x-axis. The conversion from ADL-66505TL, Roithner), centered at 2.33 eV and k toangleshasbeencarriedoutbyconsideringk ≈ y y 1.88eV.Forreflectivity,astabilizedtungsten-halogen sinθ and knowing that the maximum k detected by white-light source (SLS201L, Thorlabs) was used. A oursetupisequaltotheobjectiveNA =0.55 50x objective lens (M Plan Apo 50X, Mitutoyo) was used to focus the excitation light onto the sample. 4.8.Flakeorientationanalysis A 0.5 m spectrometer (SP-2-500i, Princeton Instru- OpticalmicroscopePLimageswereanalyzedinMAT- ments)withanitrogen-cooledcharge-coupleddevice LAB using the image processing toolbox functions camera (PyLoN:100BR, Princeton Instruments) is [50]. A color thresholding application was used to used to detect the signal from the samples, collected visually separate monolayer material in a typical PL inthebackwardsdirection.700nmand650nmlong- image.Thisthresholdingisappliedtoallimagesana- passfilters(FEL0700andFEL0650,Thorlabs)isused lyzedbytheshaperecognitionprogram.Theprogram to isolate the PL signal. The reflectivity spectra were identified 8089 objects in HS1 and 8391 objects in determined by comparing the collected white-light HS2.Acompleteexplanationoftheanalysisandfur- reflected from the DBR substrate and the sample, as therinformationregardingtheimageprocessingcan ∆R/R = (R −R )/R , where R (R ) is the sub HS sub HS sub befoundinRef[30]. intensity of acquired light reflected by the sample (substrate). 4.9.Fabricationofmonolithiccavities Room-temperature measurements were carried The monolithic cavity has been fabricated by depos- outinambientconditions.Acontinuous-flowliquid itingaSiO filmof98nmontopoftheCVD-grown helium cryostat with a cold finger at a base tem- monolayers, which were previously transferred on a perature of ~5 K was employed to obtain the low- 13pairsSiO /Ta O DBRgrownbyionbeamassisted 2 2 5 temperaturemeasurements. sputteringonasapphiresubstrate.Inordertominim- izethepotentialdamagesonthemonolayers,thesilica 4.6.Tunablemicro-cavity layer covering the TMDs has been grown at room Theopticalcavitywasformedusinganexternalcon- temperature by using an e-beam deposition system. cave mirror, with nominal radius of 20 µm, to pro- ducea0Dtunablecavity[43]. For the top mirror, a semi-transparent layer of Au The bottom mirror is controlled by a 5-axis (thickness: 50 nm) has been thermally evaporated, piezo-actuator stack, the first three stages control completingthecavity. 8 2DMater.8(2021)011002 Acknowledgments References [1] Novoselov KS,JiangD,SchedinF,BoothTJ, D J G, A G, T S M, and A I T acknow- KhotkevichVV,MorozovSVandGeimAK2005 ledge funding by EPSRC (EP/P026850/1 and Two-dimensionalatomiccrystalsProc.NatlAcad.Sci.USA EP/S030751/1). This work was supported by the 10210451–3 research funds (NRF-2019R1A4A1027934 and NRF- [2] MakKFandShanJ2016Photonicsandoptoelectronicsof 2DsemiconductortransitionmetaldichalcogenidesNat. 2017R1E1A1A01074493) through National Research Photon.10216–26 Foundation by the Ministry of Science and ICT, [3] MakKF,HeK,LeeC,LeeGH,HoneJ,HeinzTFand Korea. R J, K G, and D G L thank the financial ShanJ2013TightlyboundtrionsinmonolayerMoS2Nat. support of EPSRC via programme grant ’Hybrid Mater.12207–11 [4] HeK,KumarN,ZhaoL,WangZ,MakKF,ZhaoHand Polaritonics’ (EP/M025330/1). T P L acknow- ShanJ2014TightlyboundexcitonsinmonolayerWSe2 ledges financial support from the EPSRC Doc- Phys.Rev.Lett.1131–5 toral Prize Fellowship scheme Grant Reference [5] XuX,YaoW,XiaoDandHeinzTF2014Spinand EP/R513313/1. A I T thanks the financial support pseudospinsinlayeredtransitionmetaldichalcogenidesNat. Phys.10343–50 of the Graphene Flagship under grant agreements [6] LiuX,GalfskyT,SunZ,XiaF,LinE-c,LeeY-H, 785219and881603. K´ena-CohenSandMenonVM2014Stronglight–matter couplingintwo-dimensionalatomiccrystalsNat.Photon. Dataavailabilitystatement 930–4 [7] DharaS,ChakrabortyC,GoodfellowKM,QiuL, Thedatathatsupporttheplotswithinthispaperand O’LoughlinTA,WicksGW,BhattacharjeeSandVamivakas AN2018Anomalousdispersionofmicrocavity otherfindingsofthisstudyareavailablefromthecor- trion-polaritonsNat.Phys.14130–3 respondingauthoruponreasonablerequest. [8] DufferwielSetal2015Exciton-polaritonsinvanderWaals heterostructuresembeddedintunablemicrocavitiesNat. Authorcontributions Commun.61–7 [9] SidlerM,BackP,CotletO,SrivastavaA,FinkT,KronerM, DemlerEandImamogluA2017Fermipolaron-polaritons DJG,AG,andSAcontributedequallytothiswork. incharge-tunableatomicallythinsemiconductorsNat.Phys. DJG,AG,TPLandTSMcarriedoutopticalinvest- 13255–61 igations.TMDmonolayersweregrownviaCVDand [10] LowTetal2017Polaritonsinlayeredtwo-dimensional materialsNat.Mater.16182–94 samplesfabricatedbySAandHSS.hBNlayerswere [11] SchneiderC,GlazovMM,KornT,HöflingSandUrbaszekB grownviaCVDbyKYMandA-RJ.Themonolithic 2018Two-dimensionalsemiconductorsintheregimeof cavitywascompletedbyRJ,KG,andDGL.Thecon- stronglight-mattercouplingNat.Commun.92695 cave mirrors were made by A A P T and J M S. Data [12] KrolMetal2020Exciton-polaritonsinmultilayerWSe ina planarmicrocavity2DMater.715006 wasanalyzedbyDJGandAG.Themanuscriptwas [13] DufferwielSetal2017Valley-addressablepolaritonsin writtenbyDJGwithmajorinputfromAGandfur- atomicallythinsemiconductorsNat.Photon.11497–501 thercontributionsfromallco-authors.AITprovided [14] ChenY-J,CainJD,StanevTK,DravidVPandSternNP management of various aspects of the project, and 2017Valley-polarizedexciton–polaritonsinamonolayer semiconductorNat.Photon.11431–5 contributed to the analysis and interpretation of the [15] DufferwielSetal2018Valleycoherentexciton-polaritonsin dataandwritingofthemanuscript.AIT,HSS,DG amonolayersemiconductorNat.Commun.94797 L and J M S provided management of relevant parts [16] QiuL,ChakrabortyC,DharaSandVamivakasAN2019 oftheproject.AITconceivedandoversawthewhole Room-temperaturevalleycoherenceinapolaritonicsystem Nat.Commun.101513 project. [17] BarachatiFetal2018Interactingpolaritonfluidsina monolayeroftungstendisulfideNat.Nanotechnol.13906–9 Competinginterests [18] LundtNetal2019OpticalvalleyHalleffectforhighly valley-coherentexciton-polaritonsinanatomicallythin Theauthorsdeclarenocompetinginterests. semiconductorNat.Nanotechnol.14770–5 [19] WaldherrMetal2018Observationofbosoniccondensation inahybridmonolayerMoSe2-GaAsmicrocavityNat. ORCIDiDs Commun.93286 [20] EmmanueleRPAetal2020Highlynonlinear DanielJGillard https://orcid.org/0000-0002- trion-polaritonsinamonolayersemiconductorNat. 0001-4918 Commun.113589 [21] KasprzakJetal2006Bose-Einsteincondensationofexciton ArmandoGenco https://orcid.org/0000-0002- polaritonsNature443409–14 1292-2614 [22] ChristopoulosSetal2007Room-temperaturepolariton ThomasPLyons https://orcid.org/0000-0001- lasinginsemiconductormicrocavitiesPhys.Rev.Lett. 5569-7851 98126405 [23] BhattacharyaP,FrostT,DeshpandeS,BatenMZ,HazariA Aur´elienAPTrichet https://orcid.org/0000-0002- andDasA2014Roomtemperatureelectricallyinjected 0114-3340 polaritonlaserPhys.Rev.Lett.112236802 AlexanderITartakovskii https://orcid.org/0000- [24] AmoA,Lefr`ereJ,PigeonS,AdradosC,CiutiC,CarusottoI, 0002-4169-5510 Houdr´eR,GiacobinoEandBramatiA2009Superfluidityof 9 2DMater.8(2021)011002 polaritonsinsemiconductormicrocavitiesNat.Phys. [37] RobertCetal2017Finestructureandlifetimeofdark 5805–10 excitonsintransitionmetaldichalcogenidemonolayersPhys. [25] LiewTCH,KavokinAV,OstatnickýT,KaliteevskiM, Rev.B96155423 ShelykhIAandAbramRA2010Exciton-polariton [38] LindlauJetal2018Theroleofmomentum-darkexcitonsin integratedcircuitsPhys.Rev.B8233302 theelementaryopticalresponseofbilayerWSe2Nat. [26] BallariniD,DeGiorgiM,CancellieriE,Houdr´eR, Commun.92586 GiacobinoE,CingolaniR,BramatiA,GigliGandSanvittoD [39] JadczakJ,Kutrowska-GirzyckaJ,Kapu´scinski ´ P,HuangYS, 2013All-opticalpolaritontransistorNat.Commun.41778 W´ojsAandBryjaL2017Probingoffreeandlocalized [27] ShreeSetal2019HighopticalqualityofMoS2monolayers excitonsandtrionsinatomicallythinWSe2,WS2,MoSe2 grownbychemicalvapordeposition2DMater.715011 andMoS2inphotoluminescenceandreflectivity [28] ZhangY,YaoY,SendekuMG,YinL,ZhanX,WangF,Wang experimentsNanotechnology28395702 ZandHeJ2019RecentprogressinCVDgrowthof2D [40] LippertSetal2017Influenceofthesubstratematerialonthe transitionmetaldichalcogenidesandrelated opticalpropertiesoftungstendiselenidemonolayers2D heterostructuresAdv.Mater.311901694 Mater.4025045 [29] GebhardtCetal2019Polaritonhyperspectralimagingof [41] DumcencoDetal2015Large-areaepitaxialmonolayerMoS two-dimensionalsemiconductorcrystalsSci.Rep. 2ACSNano94611–20 913756 [42] ZhangXetal2018Diffusion-controlledepitaxyoflargearea [30] SeversMillardT,GencoA,AlexeevEM,RandersonS, coalescedWSe2monolayersonsapphireNanoLett. AhnS,JangA-R,ShinHSandTartakovskiiAI2020Large 181049–56 areachemicalvapourdepositiongrowntransitionmetal [43] SchwarzSetal2014Two-dimensionalmetal-chalcogenide dichalcogenidemonolayersautomaticallycharacterized filmsintunableopticalmicrocavitiesNanoLett.147003–8 throughphotoluminescenceimagingNpj2DMater.Appl. [44] KavokinA,BaumbergJJ,MalpuechGandLaussyFP2007 412 Microcavities(Oxford:OxfordUniversityPress)pp1–432 [31] AjayiOAetal2017Approachingtheintrinsic [45] GencoA,GiordanoG,CaralloS,AccorsiG,DuanY, photoluminescencelinewidthintransitionmetal GambinoSandMazzeoM2018Highqualityfactor dichalcogenidemonolayers2DMater.4031011 microcavityOLEDemployingmetal-freeelectricallyactive [32] DelPozo-ZamudioOetal2020Electricallypumped BraggmirrorsOrg.Electron.62174–80 WSe -basedlight-emittingvanderWaalsheterostructures [46] AlexeevEMetal2019Resonantlyhybridizedexcitonsin embeddedinmonolithicdielectricmicrocavities2DMater. moir´esuperlatticesinvanderWaalsheterostructuresNature 731006 56781–6 [33] ZhangXetal2019Defect-controllednucleationand [47] CristofoliniP,ChristmannG,TsintzosSI,DeligeorgisG, orientationofWSe2onhBN:Aroutetosingle-crystal KonstantinidisG,HatzopoulosZ,SavvidisPGand epitaxialmonolayersACSNano133341–52 BaumbergJJ2012Couplingquantumtunnelingwithcavity [34] OkadaM,SawazakiT,WatanabeK,TaniguchT,HibinoH, photonsScience336704–7 ShinoharaHandKitauraR2014Directchemicalvapor [48] DolanPR,HughesGM,GraziosoF,PattonBRand depositiongrowthofWS2atomiclayersonhexagonalboron SmithJM2010FemtolitertunableopticalcavityarraysOpt. nitrideACSNano88273–7 Lett.353556 [35] YuHetal2017PreciselyalignedmonolayerMoS2 [49] AlexeevEMetal2017Imagingofinterlayercouplinginvan epitaxiallygrownonh-BNbasalplaneSmall131603005 derwaalsheterostructuresusingabright-fieldoptical [36] RossJSetal2013Electricalcontrolofneutralandcharged microscopeNanoLett.175342–9 excitonsinamonolayersemiconductorNat.Commun. [50] MathWorks2017ImageProcessingToolboxUser’sGuide 41473–6 195-207 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png 2D Materials IOP Publishing

Strong exciton-photon coupling in large area MoSe2 and WSe2 heterostructures fabricated from two-dimensional materials grown by chemical vapor deposition

Download PDF
10 pages

Loading next page...
 
/lp/iop-publishing/strong-exciton-photon-coupling-in-large-area-mose2-and-wse2-EdLxFV1DWu

References (51)

Copyright
Copyright © 2020 The Author(s). Published by IOP Publishing Ltd
eISSN
2053-1583
DOI
10.1088/2053-1583/abc5a1
Publisher site
See Article on Publisher Site

Abstract

Two-dimensionalsemiconductingtransitionmetaldichalcogenidesembeddedinoptical microcavitiesinthestrongexciton-photoncouplingregimemayleadtopromisingapplicationsin spinandvalleyaddressablepolaritoniclogicgatesandcircuits.Onesignificantobstaclefortheir realizationistheinherentlackofscalabilityassociatedwiththemechanicalexfoliationcommonly usedforfabricationoftwo-dimensionalmaterialsandtheirheterostructures.Chemicalvapor depositionoffersanalternativescalablefabricationmethodforbothmonolayersemiconductors andothertwo-dimensionalmaterials,suchashexagonalboronnitride.Observationofthestrong light-mattercouplinginchemicalvaporgrowntransitionmetaldichalcogenideshasbeen demonstratedsofarinahandfulofexperimentswithmonolayermolybdenumdisulfideand tungstendisulfide.Hereweinsteaddemonstratethestrongexciton-photoncouplingin microcavitiescomposedoflargeareatransitionmetaldichalcogenide/hexagonalboronnitride heterostructuresmadefromchemicalvapordepositiongrownmolybdenumdiselenideand tungstendiselenideencapsulatedononeorbothsidesincontinuousfew-layerboronnitridefilms alsogrownbychemicalvapordeposition.Thesetransitionmetaldichalcogenide/hexagonalboron nitrideheterostructuresshowhighopticalqualitycomparablewithmechanicallyexfoliated samples,allowingoperationinthestrongcouplingregimeinawiderangeoftemperaturesdownto 4Kelvinintunableandmonolithicmicrocavities,anddemonstratingthepossibilitytosuccessfully developlargeareatransitionmetaldichalcogenidebasedpolaritondevices. 1.Introduction strengths [3,4]. Moreover, the breaking of spatial inversionsymmetryinthe2Dlatticeandalargespin– Monolayers of transition metal dichalcogenides orbit coupling generate spin-valley locked optically (TMDs) are promising semiconductors with unique addressable excitons at the K and K points of the electrical and optical properties arising from the momentum space [3, 5]. These exceptional proper- quantum confinement experienced by the electrons tiescanbefurtherenrichedbyintegratingtheTMDs and holes in the two-dimensional (2D) structure withinopticalresonatorsenablingthestrongexciton- [1, 2]. One of the main effects is the appearance of photoncouplingregime,whereconfinedphotonsand direct bandgap excitonic transitions showing fea- excitons hybridize into new states called polaritons tures strongly beneficial for optoelectronics, such [6–12]. Polaritons in TMDs acquire novel properties as high binding energies and very large oscillator arisingfromthevalleypseudo-spindegreeoffreedom ©2020TheAuthor(s). PublishedbyIOPPublishingLtd 2DMater.8(2021)011002 of excitons [13, 14], and further provide enhanced growthonsapphireforMoSe anddirectlyonCVD- valley coherence for excitons strongly coupled with synthesizedhBNforWSe .Wehaveconfirmedthese long-livedcavityphotons[15,16].Efficientpolariton favorablemonolayerTMDcrystalpropertiesbyusing propagation in TMDs has been recently observed, a recently developed substrate-wide statistical ana- withdiffusionlengthsofupto20 µminWS atroom lysis of TMD crystal axes orientation and PL prop- temperature [17], while valley-dependent divergent erties [30]. Further testing of the samples in a tun- polaritondiffusionhasbeenfoundinMoSe atcryo- able microcavity exposes clear evidence of strong genic temperatures, whereby polaritons spread in exciton-photon coupling and formation of exciton- differentin-planedirectionsowingtotheexcitonval- polaritonstates.Ananti-crossingisobservedbetween leyHalleffect[18].PolaritonsinTMDsalreadyoffer the cavity mode and neutral exciton transition, with the potential to create highly non-linear phenomena Rabisplittingsof17meVforbothMoSe andWSe , 2 2 [7,19,20], with further effort directed at realization very similar in magnitude with exfoliated materi- ofBose–Einsteincondensation[19,21],polaritonlas- als [8,9,13,15], highlighting similar optical qualit- ing[22,23]andopticalparametricoscillation[24]so ies. Finally, as a proof of concept for future large far observed in other material systems. United to the scale TMD-based polaritonic devices, a monolithic valley degree of freedom of TMD monolayers, these SiO cavity is fabricated incorporating CVD grown phenomena could be exploited to create large scale hBN-encapsulated MoSe , in which strong coupling all-optical polariton circuits and quantum networks isobservedwithalargeRabisplittingof34meVatT [25,26]. =4Kand31meVatT =150K.Theresultspresen- However, TMD monolayers are usually fabric- tedinthisworkdemonstratethepossibilitytofabric- atedbymechanicalexfoliation,resultinginhighqual- atelargeareapolaritondevicesexploitinghighquality ity but small sized flakes, hindering the reproducib- TMDbasedheterostructuresmadefromCVD-grown ility and scalability of the devices. Chemical vapor materials, paving the way for future scalable TMD- deposition (CVD) offers an alternative growth and polaritoniccircuits. fabricationmethodthatprovidessubstrate-widecov- erage of uniform monolayer islands [27], as well 2.Resultsanddiscussion as the ability to grow heterostructures in-situ [28], thereforecompletelybypassingthemechanicaltrans- Twotypesofencapsulatedheterostructure(HS)have fers necessary in exfoliated equivalents and minim- been fabricated from CVD-grown materials for this izing external contamination. CVD provides a very work(seefurtherdetailsinMethods).Inthefirsthet- attractive and scalable method for the fabrication of erostructure, HS1 (figure 1(a)), MoSe monolayers largescaleTMDbaseddevices.Assuch,CVD-grown are grown by CVD onto a sapphire substrate. Thou- MoS and WS flakes have already been employed 2 2 sands of individual monolayer islands, positioned in polaritonic devices working at room temperature throughout the entire substrate, are then mechanic- [6, 29]. Nevertheless, in order to enhance the polari- ally transferred using a polystyrene membrane onto ton valley properties and optimize coherence, nar- a high reflectivity distributed Bragg reflector (DBR) row exciton linewidths (low inhomogeneous broad- composed of 13 pairs of SiO /Ta O with the high 2 2 5 ening)andlowstructuraldisorderareneeded,which reflectance stop-band centered at a wavelength of uptonowhaveonlybeenshownbyexfoliatedMoSe 2 750nm.Asubstrate-widefew-layerfilmofhexagonal and WSe monolayers at low temperatures [7–11, boronnitride(hBN),alsogrownbyCVD,ismechan- 13, 15]. Another advantage of high quality struc- ically transferred on top of the MoSe flakes to com- tures combined with the possibility to operate at plete the encapsulation. For the second heterostruc- low temperature will be access to highly non-linear ture, HS2 (figure 1(b)), WSe monolayers are grown trion-polaritons[7,20]orFermi-polaron-polaritons by CVD directly on few-layer hBN, also grown by [9], requiring control and stability of the charged CVD. Both materials, WSe /hBN, are then mechan- excitons, as well as employment of suitable hetero- ically transferred, at once, onto the DBR and sub- structures [9] composed of TMDs, hexagonal boron sequentlyencapsulatedwithafew-layerfilmofCVD nitride(hBN)andgraphene. grown hBN. It has been shown that for mechan- Here,weshowlargeareaMoSe andWSe mono- 2 2 ically exfoliated TMDs, an encapsulation with thin layers encapsulated in hBN, all grown by CVD, with hBN provides uniform dielectric screening of the crystaldomainsexceeding100 µminsize,whichdis- Coulomb interaction, reducing spatial inhomogen- playhighopticalquality,rivalingexfoliatedmaterial. eity in the exciton, thereby narrowing the emission This is evidenced by the intense and narrow exciton linewidth [27, 31]. Furthermore, hBN protects the peaksobservedinphotoluminescence(PL)measure- TMD layers in the heterostructure from damage and ments and in reflectance contrast (RC). Substrate- contamination during the subsequent deposition of wide growth uniformity and a high degree of align- various dielectrics in microcavity structures relevant ment within the ensemble of monolayer domains to our work [32]. Moreover, the direct growth of has been achieved in these materials using CVD TMD monolayers on hBN is also strongly beneficial 2 2DMater.8(2021)011002 Figure1.(a),(b)SchematicsoftheCVDgrownheterostructures(nottoscale).(a)HS1:MoSe monolayersaregrownbyCVDon asapphiresubstrate,mechanicallytransferredontoaDBRmirrorandencapsulatedwithhBN.(b)HS2:WSe monolayersare grownbyCVDonmultilayerhBN,transferredtogetherwiththehBNontoaDBRmirrorandencapsulatedwithmultilayerhBN. ThehBNmultilayersarealsogrownbyCVD.(c),(d)Opticalmicroscopeimagesofthephotoluminescence(PL)from(c)HS1 and(d)HS2takenatroomtemperatureusinga50xobjectivelens(scalebar:50 µm).(e),(f)Opticalcharacterizationof(e)HS1 and(f)HS2atT~4K.InsetsshowPLspectraatroomtemperature.PLandreflectancecontrast(RC)spectraareshowninblack 0 − andred,respectively.AtlowT,bothsamplesshowemissionfrombothneutral,X ,andcharged,X ,excitons,whereasonlyHS1 showsabsorptionfromboth.PLcwexcitationconditions: λ =660nm,P =20 µWforHS1; λ =532nm,P =20 µWfor exc exc HS2. as a route to single-crystal epitaxial growth [30, 33], PL imaging analysis [30] used to identify the mono- which, up until now, has been demonstrated with a layersisdiscussedinmoredetailbelow. limitedrangeofTMDmaterialssuchasWS [34]and Further optical characterization of the two het- MoS [35].FullycoalescedCVDgrownWSe mono- erostructures has been performed using a spectro- 2 2 layer films were obtained very recently by Zhang et scopic microscopy setup at both room (~290 K) and al [33], through a careful control of nucleation and low (~4 K) temperature (figures 1(e) and (f)). PL extended lateral growth time, and a strong improve- measurementsarecarriedoutbyexcitingthesamples ment of optical and electrical properties have been with an off resonant continuous wave (cw) laser at achieved compared to the same material grown on a power of 20 µW (see Methods). The consider- sapphire. ableroomtemperatureexcitonicPLemissionshown As a first characterization step, the room tem- in the insets in figures 1(e), (f) highlights the large perature PL emission and the general morpho- exciton binding energy associated with TMD mono- logy of the structures have been analyzed under layers [4]. In HS1 the exciton PL peak is located at an optical microscope (see Methods). HS1 (fig- 1.579 eV with a linewidth of 40 meV, and HS2 dis- ure 1(c)) generally consists of large isolated mono- plays the exciton peak at 1.670 eV with a similar layer islands with characteristic triangular shape and linewidth of 45 meV, typical of MoSe and WSe 2 2 average lateral size of 8 µm (see Supplementary monolayersoperatingatroomtemperatures. Note I (stacks.iop.org/TDM/08/011002/mmedia) for Decreasing the temperature to ~4 K produces a details), along with a number of regions where mul- narrowed and blue shifted neutral exciton PL peak, tiple flakes merge to form monolayers with sizes X , at 1.671 eV in HS1 and 1.759 eV in HS2, and a exceeding100 µm.Similarly,HS2(figure1(d))shows secondpeakattributedtothechargedexciton(trion) largetriangularmonolayerflakeswithaveragelateral transition, X , which appears at 1.639 eV in HS1 widthsof11 µm.Again,inareaswhereflakesmerge, and 1.726 eV in HS2, about 30 meV below the neut- monolayers of over 100 µm in width can be seen. ral exciton [8, 13, 36]. The relative intensity of the − 0 Large areas of uniform coverage are necessary for X peak, when compared to the X peak, is heav- constructing large arrays of identical heterostructure ily influenced by the free carrier densities present in devices, such as transistors or photodetectors. Over- the structures [3]. In HS2, PL seen at lower ener- all, there is monolayer coverage of 14% (1.75 mm ) gies(below1.70eV)haspreviouslybeenattributedto for HS1 and 22% (3.67 mm ) for HS2, calculated various excitonic complexes in WSe including spin by dividing the total area of monolayer across the dark excitons [37], exciton-phonon side-bands [38] samplebythetotalsubstratearea.Thesubstrate-scale and localized states [39]. The samples show neutral 3 2DMater.8(2021)011002 Figure2.(a)Schematicshowingflakeorientationsatdifferentanglesrelativetothehorizontalaxisofthemicroscopeimages.(b), (c)Analysisofflakeorientation.IslandsofmonolayerTMDareidentifiedandtheorientationextractedusingmethodologyas describedinpreviousinvestigations[30].Both(b)HS1and(c)HS2showtwomainpeakssituated60 apart,whileHS1also showstwoextrapeakssituatedat30 fromthemainpeaks. exciton linewidths of 13 meV and 21 meV for HS1 microscopePLimages,anexampleofonesuchimage and HS2 respectively, and 12 meV and 20 meV for isshowninfigures1(c)and(d).Byemployinganalyt- thechargedexcitonofthetwosamples.Generally,the ical methods detailed in our previous work [30], the linewidth of an excitonic transition in TMD mono- flakeorientationrelativetothehorizontalaxisofthe layersisstronglyaffectedbythelevelofstructuraldis- microscopeimagecanbefound.Inordertomaximize orderanddensityofdefects[29].Thespectralshapes accuracy, only islands with shape close to equilateral and linewidths demonstrated by the CVD grown trianglesareanalyzedintermsofangularorientation. samplesinvestigatedinthisworkimproveuponthose Of the 16999 (14205) individual monolayer islands reported by Zhang et al [33] and Lippert et al [40], identifiedinHS1(HS2),8089(8391)satisfythiscon- and are similar to exfoliated flakes operating at low dition.Measuredintermsofarea,thiscorrespondsto temperaturewithoutencapsulation(seeSupplement- 21%(17%)ofthetotalmonolayercoverage.Thehis- aryNoteIV)[8,15,27,31,36].ThisshowsthatCVD tograms in figures 2(b) and (c) detail the number of growth can produce heterostructures of comparable islands identified as a function of orientation angle, optical quality to mechanically exfoliated flakes. The showing that both the samples feature a very high role of hBN is mostly to provide a high quality sub- degree of island orientation uniformity, a signature stratefortheTMDsynthesis[30,33],butalsotoactas of epitaxial growth. For WSe grown directly onto a buffer layer protecting TMDs from damage during hBN (figure 2(c)), two main orientations have been the deposition of additional layers in order to com- found, 60 apart. This is expected from equilateral pleteamicrocavityorwaveguidestructure. triangular islands growing in two preferential direc- We also measure reflectance contrast spectra tionsat60 relativetooneanother.Thesetwoprefer- using a broad band white light source and calcu- ential directions are directly related to the hexagonal latedas ∆R/R = (R −R )/R ,whereR isthe crystal structure of the growth substrate and have sub HS sub HS reflectance of the heterostructure, and R is the alsobeenobservedinpreviousstudiesofMoS ,WS , sub 2 2 reflectanceofthebaresubstrateinbetweentheTMD and WSe grown by CVD on hBN [30,33–35]. For flakes. These spectra (red lines in figures 1(e), (f)) MoSe grown onto c-plane sapphire (figure 2(b)), reveal a strong absorption peak attributed to the four peaks in the angular distribution are observed. X transition in both heterostructures and a lower Twomainpeaksshowthepreferredflakeorientation, intensity peak at lower energy attributed to X in situated at 60 relative to each other, along with two HS1.Therelativepeakheightisstrictlyrelatedtothe less populated angles at 30 relative to the two main oscillator strength of individual transitions, with the peaks. Both two, and four preferential growth direc- neutral exciton being much more intense than the tions have been seen in TMDs grown via CVD onto trioninHS1duetoarelativelylowdopinglevel.The c-plane sapphire [41, 42]. Control over the relative X absorptionpeakofHS2isslightlybroaderthanin angleoftheflakesatthesynthesisstageoffabrication HS1,similartothetrendobservedforthePLspectra, will provide the basis to build scalable heterostruc- which is an indication of the greater structural dis- tureswith controloverthe relativeinterlayercrystal- orderpresentinCVD-grownWSe . lographicorientation. As can be inferred in figures 1(c) and (d), the Aftertheopticalcharacterizationstep,theencap- bright triangular monolayer islands appear to have a sulated heterostructures are tested in a tunable open preferredorientation.Toextractthesizeandshapeof cavitysetupwhichconsistsofatopconcaveDBRmir- monolayer flakes, shape recognition techniques were ror distanced 2–3 µm from a planar bottom DBR used on a full substrate map comprised of multiple mirror (figures 3(a) and (b)), upon which the HSs 4 2DMater.8(2021)011002 Figure3.(a)Schematicofthetunableopenmicrocavityincludingthesetofpiezoactuatorsusedtoalignthemirrorsandperform thePLscansoftheheterostructure.(b)Schematicoftheopticalmicrocavity.ThecavityiscomposedofaplanarDBR,upon whichtheHSisplaced,andaconcavetopDBRconfiningtheopticalcavitymodein3dimensions.(c),(d)PLemissionfrom(c) HS1and(d)HS2displayedasafunctionofphotonenergyandexciton-photondetuning(∆ =E −E ).Clearanti-crossingsof c X thecavitymodewiththeexcitonareobservedinbothheterostructures.PLspectraarefittedusingaLorentzianpeak(see SupplementaryNoteII)andatwolevelcoupledoscillatormodelisusedtoextractthelower(bluecurve)andupper(yellow curve)polaritonbranches,excitonicresonances(whitehorizontallines),andLG photonicmode(greendiagonalline).Rabi splittingsof17meVarefoundforbothHS1andHS2.Samplesareopticallyexcitedusinga637nmcwlaser. are placed [8, 9, 13, 15, 43]. The mirrors are posi- strengthofthetriontransition.Theabsenceofmode tioned using piezo-actuator stages (figure 3(a)). Free broadening in HS2 (figure 3(d)) cavity scans at the spaceopticalaccessfromabovethetopconcaveDBR trionenergyisanindicationthattheabsorptionofthe allowslaserexcitationandopticaldetection,usingan WSe trion resonance, occurring in HS2 at 1.726 eV, achromatic doublet objective lens. The cavity length istooweak,asalsoconfirmedinfigure1(f). can be tuned by slowly moving the bottom mirror The second regime of exciton-photon coupling, along the z-axis, thus allowing the tuning of the cav- known as the strong coupling, presents itself as an itymode(diagonalgreendashedlinesinfigures3(c), anti-crossingoftheLG cavitymodeandtheexciton (d)). The PL signal collected from the TMDs, as the energieswithacharacteristicRabisplitting,2ℏΩ ,at cavity length is reduced, is displayed in figures 3(c) the resonance. This behavior can be clearly observed and (d) as a function of detuning (the energy dif- in both HS1 (figure 3(c)) and HS2 (figure 3(d)) ference between the cavity mode and unperturbed as the LG is tuned into resonance with the neut- exciton, X ). The three dimensional optical confine- ral exciton, at 1.673 eV for HS1 and 1.770 eV for ment provided by the concave top mirror generates HS2. The peak positions of the lower (LPB) and asetoftransversemodesforeachlongitudinalmode upper (UPB) polariton branches have been extrac- [8,43]alsovisibleinbothfigures. ted using a Lorentzian peak fitting, and used to fit a Whenthefundamentallongitudinalcavitymode, two-level coupled oscillator model (detailed in Sup- LG (greendiagonaldashedlinesinfigures3(c)and plementary Note II) in order to determine 2ℏΩ as 00 R (d)) which ensures the highest light confinement, is shown overlaid in figures 3(c), (d). We find a value tunedintoresonancewiththeexcitontransitionener- of 17.2 ± 3.3 meV for HS1 and 16.8 ± 3.1 meV gies,lightmattercouplingcanmanifestinoneoftwo forHS2. ways,bothofwhichareobservedinHS1(figure3(c)). These measurements demonstrate large Rabi As the cavity mode is tuned into resonance with the splittings closely comparable to the values trion, X , at 1.638 eV, the cavity mode is broadened observed in exfoliated monolayer MoSe and WSe 2 2 and brightened, a demonstration of the weak coup- placed in zero-dimensional tunable microcavities ling[8,13,15,44]arisingfromthereducedoscillator [9,13,15,43]. This further confirms, thanks to the 5 2DMater.8(2021)011002 Figure4.(a),(b)AngleresolvedPLimagingofHS1monolithiccavity.(a)At5K,thecavityhasanegativedetuningat0 of ∆ ≈ −3.5meV,showinganti-crossingsat ±7 .ThePLintensityintheregionwithinthereddashedlineshasbeenmultipliedbya factorof10forclarity.(b)At150K,thecavityhasapositivedetuningat0 of ∆ ≈10meV.WeoverlaythefittedLPB(blue)and UPB(yellow)alongwiththeextractedparaboliccavitydispersion(green)andneutralexcitonresonance(white).ARabisplitting of34 ±4meVand31 ±4meVisobservedat5Kand150K,respectively.Thesampleisopticallyexcitedusinga660nmcwlaser atapowerof50 µW. reduced structural disorder in the presented hetero- modepenetrationintothemirrors.Asshownbelow, structures,thehighopticalqualityofthehBNencap- thisleadstoahighermagnitudeoftheRabisplitting. sulatedCVDgrownTMDmonolayers,henceproving Since monolithic cavities are not tunable in size, the thevalidityforCVDgrowthtechniqueswhendesign- temperature dependence of the X transition energy ingscalabledevices. (furtherdiscussedinSupplementaryNoteIII)isused Further advantages of large area TMDs can be totunetheexcitonintoresonancewiththephotonic exploited in monolithic cavities, providing a plat- mode which has a negligible dependence of its fre- formtoformvarioustopologicaldesignstoadaptor quencywithtemperature. enhance device functionality towards polariton cir- ThePLcollectedfromthemonolithiccavitywhile cuits [25, 26]. For these devices, the protective func- being optically excited by non-resonant continuous tionofthehBNencapsulationisofparticularimport- wave laser in the linear power regime, as imaged anceasthetopdielectricmirrorneedstobedeposited by angle resolved spectroscopy probing the exciton ontopoftheTMDlayers[32].Asaproofofconcept, and photon states at different k-vectors, is shown in we deposited 98 nm of SiO via e-beam deposition, figure4. In figure 4(a) at a temperature of 5 K, the followed by a semi-transparent layer of 50 nm gold, X transition is at 1.648 eV, while in figure 4(b) at on top of HS1 to fabricate a λ/2 monolithic cavity 150 K, the X red-shifts to 1.636 eV. The monolithic (see Supplementary Note III). The oxide deposition cavity shows strong exciton-photon coupling signa- process has been carried out at room temperature in tures in PL at both the temperatures, owing to the ordertopreservetheopticalintegrityoftheemitting protective capability of the CVD grown hBN which materialsasmuchaspossible[32,45]. helpedshieldtheMoSe monolayersfromthepoten- Inthetunablecavity,thephotonicmodesarecon- tiallydamagingSiO depositionprocess. finedinallthreedimensions,resultinginasetofdis- At a temperature of 5 K, the exciton is negat- crete cavity modes, with k ~ 0, which are tuned ively detuned from the cavity mode at 0 by - 3.5 x,y,z inenergybyalteringthecavitylength.Bycontrast,in meV,suchthattheLPBismuchmorevisiblethanthe a monolithic two-dimensional cavity, as in our case, UPB. To show the upper polariton branch in figure the photonic mode is confined only in the vertical z 4(a) the collected intensity values have been multi- direction, and thus a cavity mode energy dispersion plied by a factor of 10 between 1.658 and 1.705 eV, as a function of continuous k values is observed as outlined by red dashed lines. By fitting the PL x,y [44]. This dispersion can be probed by measuring emissionspectrawithLorentzianpeaksandapplying angle-resolvedPLorreflectivityspectraasafunction the extracted peak positions to a two level coupled- of angle measured from the normal to the sample oscillator model (see Supplementary Note II) [44] (correspondingtok =0)[44].Inthecavityusedin we obtain a large Rabi splitting of 34.1 ± 4.0 meV x,y our experiment, a stronger light confinement can be at 5 K. The exciton-photon coupling strength in the achievedthaninthetunabledevicespresentedearlier, monolithic cavity is higher than in the open cav- due to a smaller vertical size of the cavity and lower ity due to the increased light confinement. The two 6 2DMater.8(2021)011002 polariton branches, cavity dispersion, and exciton 4.2.GrowthofsinglelayerMoSe onc-plane energy obtained from the coupled-oscillator model sapphire are shown overlaying figures4(a)and(b), with an MoO (99.97%, Sigma Aldrich) and Se (99.999%, anti-crossingclearlyseenat ±7 whenthedeviceisat AlfaAesar)wereusedasprecursorsforthegrowth.60 5K.Thestronglycoupledcavityperformswellupto mg of MoO powder was placed in the center of the 150K,whentheexcitonicmodeispositivelydetuned furnaceand150mgofSewasplacedattheupstream (∆(0 ) = +10 meV), leading to a Rabi splitting of entry. To minimize the intense evaporation of the 30.5 ±3.9meV. MoO ,thecruciblethatcontainedtheprecursorwas partially covered by a SiO /Si wafer. The target sub- strate of c-plane sapphire was placed beside the cru- 3.Conclusion ciblecontainingMoO . The tube furnace was evacuated for 30 min and In summary, high quality substrate-wide MoSe , subsequentlyrepletedwiththeArgas.Ambientpres- andWSe ,TMDmonolayersencapsulatedwithlarge sure is reached before the furnace was heated to area hBN were fabricated using CVD growth tech- 600 C for 18 minutes under a steady flow of Ar gas niques, and subsequently embedded in tunable and (60sccm)andH gas(12sccm).Asthefurnacetem- monolithicmicrocavitydeviceswherestrongexciton- perature reached 600 C, the upstream entry of the photon coupling was observed. The heterostructures ◦ tube was heated to 270 C, using a heating belt, as show optical properties comparable with exfoliated to vaporize Se. Finally, the furnace temperature was materials, and consequently exhibit similar values of ◦ raised to 700 C, and maintained for 1 hour to facil- polaritonRabisplittingstopreviouslystudiedhetero- itate the MoSe growth. Upon completion, the tube structuresmadefromexfoliatedflakes[8,9,13,15]. furnacewascooledtoroomtemperaturewhilstmain- Furthermore, the demonstrated CVD growth on taining Ar flow, without H . Polystyrene was used to sapphire and hBN produced highly orientated TMD transfer MoSe on top of the DBR in order to main- islands, and is thus suitable for the fabrication of tainsamplequality. large scale TMD/TMD heterostructures with highly controlled interlayer twist angle [46] to be embed- 4.3.GrowthofsinglelayerWSe directlyonhBN ded in microcavities. Together with additional hBN Initially, hBN was grown on c-plane sapphire (see and graphene layers these structures could provide a Methods). WO (99.998%, Alfa Aesar) and Se viableroutetorealizationofhighlytunableandnon- (99.999%, Alfa Aesar) were used as precursors for lineardipolarpolaritons[47]inlargescaledevices. the growth of single layer WSe . 120 mg of WO 2 3 This work demonstrates the possibility to fabric- powder,mixedwithasmallamountofNaCltoreduce ate large scale polaritonic devices based on TMD- the influence of humidity, was placed in the center basedheterostructures.Furtherdevelopmentoflarge of the furnace and 300 mg of Se was placed at the scale monolayer semiconductor growth techniques, upstream entry. The target substrate of multilayer most notably directly onto hBN which provides hBN on sapphire was placed beside the crucible that highly co-orientated TMD domains, will inevit- contained WO . The tube furnace was evacuated for ably lead to heterostructures that can reliably and ◦ more than 30 min before being heated to 800 C for repeatedlycompetewith,orout-perform,thosebuilt 24 minutes under a steady flow of Ar gas (120 sccm) with exfoliated flakes due to the unprecedented and H gas (20 sccm). As the furnace temperature scalabilitythatisgranted. reached 800 C, the upstream entry of the tube was heated to 270 C, using a heating belt, as to vapor- 4.Methods ize Se. Finally, the furnace temperature was raised to, and maintained at, 870 C for 1 hour to facilitate 4.1.Dielectricmirrorfabrication WSe growth.Uponcompletion,thetubefurnacewas cooled to room temperature whilst maintaining Ar HighlyreflectingdistributedBraggreflectors(DBRs) are deposited on silica substrates by ion beam sput- flow, without H . In order to maintain sample qual- ity, polystyrene was used to transfer the WSe /hBN tering.TheDBRsarecomprisedof13pairsofquarter wavelengthSiO /Ta O layers,terminatingwithSiO , stacktotheDBR. 2 2 5 2 of thicknesses 129 and 89 nm (refractive index 1.45 and 2.10, respectively). The DBRs are designed for a 4.4.GrowthoflargeareahBN center wavelength of 750 nm and a stop-band width MultilayerhBNwithanAA’stackingorderwasgrown of200nm. byremoteinductivelycoupledplasmachemicalvapor Using focused Gallium ion beam milling into a deposition method. A 2-inch c-plane sapphire was planarfusedsilicasubstrate,theconcaveshapedtem- used as a substrate for the hBN growth. The sub- plateforthetopmirrorisformed[48].Thenominal strate was placed in the center of a 2-inch alumina radiusofcurvatureoftheconcavemirrorwas20 µm, tube furnace of CVD. A borazine (Gelest, Inc.) pre- prompting a beam waist of around 1 µm [8, 43] on cursor flask was placed in a water bath at −15 C. thefocalplane. The bath temperature before the growth of hBN was 7 2DMater.8(2021)011002 increasedupto25 C.Beforethegrowthofmultilayer the x, y, and z translational motion, while another hBN,thefurnacewasheatedto1220 Cunderflowof two stages control the tilt alignment. The top mir- Ar gas (10 sccm). Plasma was generated at a power ror is positioned using a 3-axis piezo-actuator stage of 30 W under a flow of borazine (0.2 sccm) and controlling the x, y, and z translational motion. Ar (10 sccm) gases for 30 mins. Atomic force micro- Optical PL scans were completed with the samples scopy and transmission electron microscopy meas- placed in a helium bath cryostat system at a tem- urements confirmed that the thickness of hBN was perature of 4.2 K using a 637 nm continuous- 1.2nm,approximately3layers.Inaddition,thehBN wavelaserdiode(VortranStradus),focusedontothe sample shows quite good thickness uniformity over sample using an achromatic lens. The collected PL the2-inchsapphiresubstrateaccordingtotheRaman is focused onto a single mode fiber and guided into andUVabsorptionspectrameasuredatninerandom a 0.75 m spectrometer (SP-2-750i, Princeton Instru- pointsoverthe2-inchhBNfilm. ments) and a high-sensitivity charge-coupled device (PyLoN:400BR,PrincetonInstruments)foremission 4.5.Opticalmeasurements collection. The photoluminescence images of the CVD samples wereacquiredusingacolorcamera(DS-Vi1,Nikon) 4.7.Measurementofmonolithiccavities equipped to a modified bright-field microscope We performed the Fourier space spectral imaging of (LV150N, Nikon). A 550 nm short-pass filter thePLemittedbythemonolithiccavitybyemploying (FESH0550, Thorlabs) blocked the near-infrared a 2D CCD array (PyLoN:100BR, Princeton Instru- emission from the white-light source, and a 600 ments)coupledtoa300gr/mmgratingspectrometer nm long-pass filter (FELH0600, Thorlabs) was used (SP-2-500i, Princeton Instruments). We focused a to isolate the photoluminescence signal from the 30 cm lens onto the back plane of a 50x Mitutoyo samples. A fully detailed description is available in infinity corrected objective to obtain the Fourier Ref[49]. planeimageofthesample,whichwasthenprojected Spectrally-resolved photoluminescence and ontheslitofthespectrometerbyusinga10cmlens. reflectance contrast measurements were implemen- WeusedtheslittoselectonlythesectionoftheFour- ted using a custom-built micro-PL setup. For pho- ier space at k = 0, resulting in a final image on the toluminescence, the samples were excited using two CCD displaying the PL as a function of k on the y- diode-pumped solid-state lasers (CW532-050 and axis and energy on the x-axis. The conversion from ADL-66505TL, Roithner), centered at 2.33 eV and k toangleshasbeencarriedoutbyconsideringk ≈ y y 1.88eV.Forreflectivity,astabilizedtungsten-halogen sinθ and knowing that the maximum k detected by white-light source (SLS201L, Thorlabs) was used. A oursetupisequaltotheobjectiveNA =0.55 50x objective lens (M Plan Apo 50X, Mitutoyo) was used to focus the excitation light onto the sample. 4.8.Flakeorientationanalysis A 0.5 m spectrometer (SP-2-500i, Princeton Instru- OpticalmicroscopePLimageswereanalyzedinMAT- ments)withanitrogen-cooledcharge-coupleddevice LAB using the image processing toolbox functions camera (PyLoN:100BR, Princeton Instruments) is [50]. A color thresholding application was used to used to detect the signal from the samples, collected visually separate monolayer material in a typical PL inthebackwardsdirection.700nmand650nmlong- image.Thisthresholdingisappliedtoallimagesana- passfilters(FEL0700andFEL0650,Thorlabs)isused lyzedbytheshaperecognitionprogram.Theprogram to isolate the PL signal. The reflectivity spectra were identified 8089 objects in HS1 and 8391 objects in determined by comparing the collected white-light HS2.Acompleteexplanationoftheanalysisandfur- reflected from the DBR substrate and the sample, as therinformationregardingtheimageprocessingcan ∆R/R = (R −R )/R , where R (R ) is the sub HS sub HS sub befoundinRef[30]. intensity of acquired light reflected by the sample (substrate). 4.9.Fabricationofmonolithiccavities Room-temperature measurements were carried The monolithic cavity has been fabricated by depos- outinambientconditions.Acontinuous-flowliquid itingaSiO filmof98nmontopoftheCVD-grown helium cryostat with a cold finger at a base tem- monolayers, which were previously transferred on a perature of ~5 K was employed to obtain the low- 13pairsSiO /Ta O DBRgrownbyionbeamassisted 2 2 5 temperaturemeasurements. sputteringonasapphiresubstrate.Inordertominim- izethepotentialdamagesonthemonolayers,thesilica 4.6.Tunablemicro-cavity layer covering the TMDs has been grown at room Theopticalcavitywasformedusinganexternalcon- temperature by using an e-beam deposition system. cave mirror, with nominal radius of 20 µm, to pro- ducea0Dtunablecavity[43]. For the top mirror, a semi-transparent layer of Au The bottom mirror is controlled by a 5-axis (thickness: 50 nm) has been thermally evaporated, piezo-actuator stack, the first three stages control completingthecavity. 8 2DMater.8(2021)011002 Acknowledgments References [1] Novoselov KS,JiangD,SchedinF,BoothTJ, D J G, A G, T S M, and A I T acknow- KhotkevichVV,MorozovSVandGeimAK2005 ledge funding by EPSRC (EP/P026850/1 and Two-dimensionalatomiccrystalsProc.NatlAcad.Sci.USA EP/S030751/1). This work was supported by the 10210451–3 research funds (NRF-2019R1A4A1027934 and NRF- [2] MakKFandShanJ2016Photonicsandoptoelectronicsof 2DsemiconductortransitionmetaldichalcogenidesNat. 2017R1E1A1A01074493) through National Research Photon.10216–26 Foundation by the Ministry of Science and ICT, [3] MakKF,HeK,LeeC,LeeGH,HoneJ,HeinzTFand Korea. R J, K G, and D G L thank the financial ShanJ2013TightlyboundtrionsinmonolayerMoS2Nat. support of EPSRC via programme grant ’Hybrid Mater.12207–11 [4] HeK,KumarN,ZhaoL,WangZ,MakKF,ZhaoHand Polaritonics’ (EP/M025330/1). T P L acknow- ShanJ2014TightlyboundexcitonsinmonolayerWSe2 ledges financial support from the EPSRC Doc- Phys.Rev.Lett.1131–5 toral Prize Fellowship scheme Grant Reference [5] XuX,YaoW,XiaoDandHeinzTF2014Spinand EP/R513313/1. A I T thanks the financial support pseudospinsinlayeredtransitionmetaldichalcogenidesNat. Phys.10343–50 of the Graphene Flagship under grant agreements [6] LiuX,GalfskyT,SunZ,XiaF,LinE-c,LeeY-H, 785219and881603. K´ena-CohenSandMenonVM2014Stronglight–matter couplingintwo-dimensionalatomiccrystalsNat.Photon. Dataavailabilitystatement 930–4 [7] DharaS,ChakrabortyC,GoodfellowKM,QiuL, Thedatathatsupporttheplotswithinthispaperand O’LoughlinTA,WicksGW,BhattacharjeeSandVamivakas AN2018Anomalousdispersionofmicrocavity otherfindingsofthisstudyareavailablefromthecor- trion-polaritonsNat.Phys.14130–3 respondingauthoruponreasonablerequest. [8] DufferwielSetal2015Exciton-polaritonsinvanderWaals heterostructuresembeddedintunablemicrocavitiesNat. Authorcontributions Commun.61–7 [9] SidlerM,BackP,CotletO,SrivastavaA,FinkT,KronerM, DemlerEandImamogluA2017Fermipolaron-polaritons DJG,AG,andSAcontributedequallytothiswork. incharge-tunableatomicallythinsemiconductorsNat.Phys. DJG,AG,TPLandTSMcarriedoutopticalinvest- 13255–61 igations.TMDmonolayersweregrownviaCVDand [10] LowTetal2017Polaritonsinlayeredtwo-dimensional materialsNat.Mater.16182–94 samplesfabricatedbySAandHSS.hBNlayerswere [11] SchneiderC,GlazovMM,KornT,HöflingSandUrbaszekB grownviaCVDbyKYMandA-RJ.Themonolithic 2018Two-dimensionalsemiconductorsintheregimeof cavitywascompletedbyRJ,KG,andDGL.Thecon- stronglight-mattercouplingNat.Commun.92695 cave mirrors were made by A A P T and J M S. Data [12] KrolMetal2020Exciton-polaritonsinmultilayerWSe ina planarmicrocavity2DMater.715006 wasanalyzedbyDJGandAG.Themanuscriptwas [13] DufferwielSetal2017Valley-addressablepolaritonsin writtenbyDJGwithmajorinputfromAGandfur- atomicallythinsemiconductorsNat.Photon.11497–501 thercontributionsfromallco-authors.AITprovided [14] ChenY-J,CainJD,StanevTK,DravidVPandSternNP management of various aspects of the project, and 2017Valley-polarizedexciton–polaritonsinamonolayer semiconductorNat.Photon.11431–5 contributed to the analysis and interpretation of the [15] DufferwielSetal2018Valleycoherentexciton-polaritonsin dataandwritingofthemanuscript.AIT,HSS,DG amonolayersemiconductorNat.Commun.94797 L and J M S provided management of relevant parts [16] QiuL,ChakrabortyC,DharaSandVamivakasAN2019 oftheproject.AITconceivedandoversawthewhole Room-temperaturevalleycoherenceinapolaritonicsystem Nat.Commun.101513 project. [17] BarachatiFetal2018Interactingpolaritonfluidsina monolayeroftungstendisulfideNat.Nanotechnol.13906–9 Competinginterests [18] LundtNetal2019OpticalvalleyHalleffectforhighly valley-coherentexciton-polaritonsinanatomicallythin Theauthorsdeclarenocompetinginterests. semiconductorNat.Nanotechnol.14770–5 [19] WaldherrMetal2018Observationofbosoniccondensation inahybridmonolayerMoSe2-GaAsmicrocavityNat. ORCIDiDs Commun.93286 [20] EmmanueleRPAetal2020Highlynonlinear DanielJGillard https://orcid.org/0000-0002- trion-polaritonsinamonolayersemiconductorNat. 0001-4918 Commun.113589 [21] KasprzakJetal2006Bose-Einsteincondensationofexciton ArmandoGenco https://orcid.org/0000-0002- polaritonsNature443409–14 1292-2614 [22] ChristopoulosSetal2007Room-temperaturepolariton ThomasPLyons https://orcid.org/0000-0001- lasinginsemiconductormicrocavitiesPhys.Rev.Lett. 5569-7851 98126405 [23] BhattacharyaP,FrostT,DeshpandeS,BatenMZ,HazariA Aur´elienAPTrichet https://orcid.org/0000-0002- andDasA2014Roomtemperatureelectricallyinjected 0114-3340 polaritonlaserPhys.Rev.Lett.112236802 AlexanderITartakovskii https://orcid.org/0000- [24] AmoA,Lefr`ereJ,PigeonS,AdradosC,CiutiC,CarusottoI, 0002-4169-5510 Houdr´eR,GiacobinoEandBramatiA2009Superfluidityof 9 2DMater.8(2021)011002 polaritonsinsemiconductormicrocavitiesNat.Phys. [37] RobertCetal2017Finestructureandlifetimeofdark 5805–10 excitonsintransitionmetaldichalcogenidemonolayersPhys. [25] LiewTCH,KavokinAV,OstatnickýT,KaliteevskiM, Rev.B96155423 ShelykhIAandAbramRA2010Exciton-polariton [38] LindlauJetal2018Theroleofmomentum-darkexcitonsin integratedcircuitsPhys.Rev.B8233302 theelementaryopticalresponseofbilayerWSe2Nat. [26] BallariniD,DeGiorgiM,CancellieriE,Houdr´eR, Commun.92586 GiacobinoE,CingolaniR,BramatiA,GigliGandSanvittoD [39] JadczakJ,Kutrowska-GirzyckaJ,Kapu´scinski ´ P,HuangYS, 2013All-opticalpolaritontransistorNat.Commun.41778 W´ojsAandBryjaL2017Probingoffreeandlocalized [27] ShreeSetal2019HighopticalqualityofMoS2monolayers excitonsandtrionsinatomicallythinWSe2,WS2,MoSe2 grownbychemicalvapordeposition2DMater.715011 andMoS2inphotoluminescenceandreflectivity [28] ZhangY,YaoY,SendekuMG,YinL,ZhanX,WangF,Wang experimentsNanotechnology28395702 ZandHeJ2019RecentprogressinCVDgrowthof2D [40] LippertSetal2017Influenceofthesubstratematerialonthe transitionmetaldichalcogenidesandrelated opticalpropertiesoftungstendiselenidemonolayers2D heterostructuresAdv.Mater.311901694 Mater.4025045 [29] GebhardtCetal2019Polaritonhyperspectralimagingof [41] DumcencoDetal2015Large-areaepitaxialmonolayerMoS two-dimensionalsemiconductorcrystalsSci.Rep. 2ACSNano94611–20 913756 [42] ZhangXetal2018Diffusion-controlledepitaxyoflargearea [30] SeversMillardT,GencoA,AlexeevEM,RandersonS, coalescedWSe2monolayersonsapphireNanoLett. AhnS,JangA-R,ShinHSandTartakovskiiAI2020Large 181049–56 areachemicalvapourdepositiongrowntransitionmetal [43] SchwarzSetal2014Two-dimensionalmetal-chalcogenide dichalcogenidemonolayersautomaticallycharacterized filmsintunableopticalmicrocavitiesNanoLett.147003–8 throughphotoluminescenceimagingNpj2DMater.Appl. [44] KavokinA,BaumbergJJ,MalpuechGandLaussyFP2007 412 Microcavities(Oxford:OxfordUniversityPress)pp1–432 [31] AjayiOAetal2017Approachingtheintrinsic [45] GencoA,GiordanoG,CaralloS,AccorsiG,DuanY, photoluminescencelinewidthintransitionmetal GambinoSandMazzeoM2018Highqualityfactor dichalcogenidemonolayers2DMater.4031011 microcavityOLEDemployingmetal-freeelectricallyactive [32] DelPozo-ZamudioOetal2020Electricallypumped BraggmirrorsOrg.Electron.62174–80 WSe -basedlight-emittingvanderWaalsheterostructures [46] AlexeevEMetal2019Resonantlyhybridizedexcitonsin embeddedinmonolithicdielectricmicrocavities2DMater. moir´esuperlatticesinvanderWaalsheterostructuresNature 731006 56781–6 [33] ZhangXetal2019Defect-controllednucleationand [47] CristofoliniP,ChristmannG,TsintzosSI,DeligeorgisG, orientationofWSe2onhBN:Aroutetosingle-crystal KonstantinidisG,HatzopoulosZ,SavvidisPGand epitaxialmonolayersACSNano133341–52 BaumbergJJ2012Couplingquantumtunnelingwithcavity [34] OkadaM,SawazakiT,WatanabeK,TaniguchT,HibinoH, photonsScience336704–7 ShinoharaHandKitauraR2014Directchemicalvapor [48] DolanPR,HughesGM,GraziosoF,PattonBRand depositiongrowthofWS2atomiclayersonhexagonalboron SmithJM2010FemtolitertunableopticalcavityarraysOpt. nitrideACSNano88273–7 Lett.353556 [35] YuHetal2017PreciselyalignedmonolayerMoS2 [49] AlexeevEMetal2017Imagingofinterlayercouplinginvan epitaxiallygrownonh-BNbasalplaneSmall131603005 derwaalsheterostructuresusingabright-fieldoptical [36] RossJSetal2013Electricalcontrolofneutralandcharged microscopeNanoLett.175342–9 excitonsinamonolayersemiconductorNat.Commun. [50] MathWorks2017ImageProcessingToolboxUser’sGuide 41473–6 195-207

Journal

2D Materials – IOP Publishing

Published: Nov 21, 2020

There are no references for this article.