Low resistance p-type contacts to monolayer WSe_2 through chlorinated solvent doping

Domain: physics
Published: 2026-01-20
Editorial outcome: minor_revision

Abstract
Tungstendiselenide(WSe2)isapromisingp-typesemiconductorlimitedbyhighcontactresistance(RC)andthelackofareliabledopingstrategy.Here,wedemonstratethatexposingWSe2tochloroformprovidessimpleandstablep-typedoping.InmonolayerWSe2transistorswithPdcontacts,chloroformincreasesthemaximumholecurrentbyover100×(>200µA/µm),reducesRCto~2.5kΩ·μm,andretainsanon/offratioof1010atroomtemperature.Theseimprovementspersistforover8months,survivea150°Cthermalanneal,andremaineffectivedownto10K,enablingacryogenicRCof~1kΩ·μm.DensityfunctionaltheoryindicatesthatchloroformstronglyphysisorbstoWSe2,inducingholedopingwithminimalimpactontheelectronicstatesbetweenthevalencebandandconductionbandedges.AugerelectronspectroscopyandatomicforcemicroscopysuggestthatchloroformintercalatesattheWSe2interfacewiththegateoxide,contributingtodopingstabilityandmitigatinginterfacialdielectricdisorder,thoughfurtherstudiesareneededtoconclusivelyconfirmthismechanism.Thisrobust,scalableapproachenableshigh-yieldWSe2transistorswithgoodp-typeperformance.

Introduction
Two-dimensional (2D) semiconductors, particularly transition metal dichalcogenides(TMDs),arepromisingcandidatesfornext-generation,high-density,complementary-metal-oxide-semiconductor(CMOS)1,2andlow-temperatureelectronics.However,thelargecontactresistance(RC)oftenobservedinnanoscaleTMDdevicesposesasignificantobstacletodeviceperformance,limitingtheon-statedraincurrent,ID,neededforpracticalcircuitapplications.Bothn-typeandp-typetransistorsarecriticalforlow-powerCMOS3,butprogressonminimizingRChaslargelybeenlimitedton-typedevices4,5.Developingscalable,low-RCp-typecontactsfor2Dtransistorsremainsacriticalchallenge.Additionally,RCtypicallyincreasesfurtheratlowtemperatures,impedingotherfundamentalchargetransportstudies6.

Various strategies have been explored to reduce RC to p-type WSe2 transistors,includingtransferredmetal7orsemimetal8,9contacts.However,metalcontactstypicallyformlargeSchottkybarriersatthemetal-2Dsemiconductorinterface(preventinglowRC),andsemimetalcontacts have yet to experimentally demonstrate superior performance for p-type devices. An alternative is to lower RC by implementingstablep-typedopingnearthecontacts.Substitutionaldopingwithelectronacceptors(e.g.,V10,Nb11,12)isstableduetocovalentchemicalbondingbutislikelytorequirecomplexfabricationwithmultiplematerialgrowthsteps.Incomparison,p-typesurfacechargetransferdoping(SCTD)13–24withdrawselectronsfromthe2DchannelusinghigherelectronegativitycappingoradsorbatelayerswithworkfunctionvaluesbelowtheFermi-leveloftheWSe2,suchastransitionmetaloxides(MoOx13andWOx14,17,18),NOx19,20,25,26,andhalidecompounds(HAuCl421,AuCl322,RuCl323,24,PtCl416).SCTDtypicallypreservesthehostlatticeandhasthepotentialtointroducefewerscatteringcenters27.However,thetemporalandthermalstabilityofthesemethodsremainsunclearduetothehighchemicalreactivityorlowthermalstabilityofthereagentsinvolved28. Furthermore, there is little consensusonthemechanismofhalide-baseddoping,withsomestudiessuggestingreactionsinwhichClatomssubstituteandpassivatechalcogenvacancies29–31,whileothersproposemolecularphysisorption22,32orintercalation33.

Solvent exposure can also unintentionally dope TMDs. For example, MoS2 and WSe2 are n-doped by exposure to the low-electronegativitysolventacetoneduringremovalofelectron-beam(e-beam)lithographyresistslikepoly(methylmethacrylate)(PMMA)34,35. Conversely,thehigh-electronegativitysolventchloroform(CHCl3)wasshowntop-dopesemimetallicgraphene33.Thissuggeststhatchloroformcouldserveasaneffectivep-typedopantfor2DsemiconductorssuchasWSe2,offeringasimpleandscalableapproachcomparedtoexistingdopingtechniques.However,solvent-baseddopingisoftenregardedastransient,andtheimpactofchloroformdopingontheelectricalperformanceofp-channelWSe2transistorshasnotyetbeenstudied.

Inthiswork,wedemonstratethatchloroformcaninducehigh-performance,stable,andhigh-yieldp-dopinginmonolayerWSe2transistors(Fig.1a,b).ExposingmonolayerWSe2transistorstochloroformincreasesIDbytwoordersofmagnitude,withholecurrentsreachingupto203μA/μmatVDS=−1V(Fig.1c).ThesedevicesalsomaintainlargeIon/Ioffratios(~1010)andalowRCof2.5kΩ·μm(atroomtemperature)and1.0kΩ·μm(at10K).Comparedtorecentapproachessuchascontactengineering(e.g.,Sb/Pt8,36,Ru37,38),oxide-baseddoping(WOx17,39,MoOx8,40,NOx19,39),andotherhalide-baseddopants(HAuCl421,RuCl323),chloroformdopingachievesoneofthehighestreportedvaluesforp-typetransistorcurrent(Fig.1b).Additionally,weobservethatchloroform-dopedtransistorsremainstableover8months(retaining81%ofinitialdopedID,max)andsurviveannealingat150°C.Densityfunctionaltheory(DFT)revealsthatchloroformbindsstrongly(>260meV,i.e.,>10kBTat296K)toWSe2withoutintroducingmid-gapstates.Atomic-forcemicroscopy(AFM),Augerelectronspectroscopy(AES),andX-rayphotoelectronspectroscopy(XPS)suggestthatchloroformintercalatesattheWSe2interfacewiththegateoxide,furtherstabilizingitsdoping.Thisstraightforwardapproachenhancesp-typeperformanceinWSe2transistorsandcomplementsothercontactandinterfaceengineeringtechniquesforadvancing2Dsemiconductortechnologies.

Results
Characterizationofchloroform-dopedWSe2

OpticalspectroscopyprovidesinsightsintothephysicalandchemicalinteractionsbetweentheWSe2andchloroform.RamanspectraofmonolayerWSe2soakedovernightinchloroformshownosignificantchangesintheE’/A’peakintensityratio(Fig.1d),whichsuggeststhatlong-termchloroformexposuredoesnotsignificantlyincreasetheWSe2defectivity.WealsodonotobserveanincreaseinLA(M)or2LA(M)peakintensityassociatedwiththedisruptionoftheWSe2lattice(SupplementaryFig.1).Thephotoluminescence(PL)spectrumafterchloroformdoping(Fig.1e)showslowerintensitythanfortheundopedsample.ThisPLquenchingisconsistentwithachloroform-inducedincreaseintheholeconcentration,leadingtomorenon-radiativerecombinationviapositivetrions34. Additionally, the negligible change in surface roughness and morphology after doping indicates that residue adsorption does not play a significant role (SupplementaryFig.2).

To investigate the electrical performance of chloroform-doped WSe2,continuousmonolayerCVD-grownWSe2wastransferredontoanarrayofprefabricated~5nmHfO2localbackgates(Fig.2a).Thelocalbackgatesweredefinedbyphotolithographyandlift-offof2/8nmTi/Pt,followedbythermalatomic-layerdepositionofHfO2gatedielectricwithequivalentoxidethicknessof1.23nm.TheWSe2channelwaspatternedbyXeF2etching.Finecontactregionswerealsodefinedusinge-beamlithographywithabilayerPMMAresiststack.Pd/Au(20/20nm)weredepositedbyevaporationat~10-7Torr,followedbylift-offinacetoneovernight,thenrinsedinisopropanol(IPA).Electricalmeasurementswereconductedinavacuumat~10-4Torr.Afterinitialdevicemeasurements,thedevicesweresoakedinchloroformfor>8handre-measuredinvacuum(seeMethodsformoredetails).

Figure2bpresentsthetransfercharacteristicsof101transistorsbeforeandafterp-dopingatVDS=−1V,withchannellengths(Lch)rangingfrom100nmto1μm.Priortodoping,thedevicesexhibitahighlynegativeVTaround−2.6V.Afterdoping,theVTshiftspositivelyandthemaximumIDuniformlyincreasesby~100×acrossalldevices,from~1μA/μmto>100μA/μm(Fig.2c).Thelowdevice-to-devicevariationafterdopingdemonstratesthereproducibilityofthisdopingmethod.Noticeably,theelectronbranchisstronglysuppressedafterdoping(Fig.2b).ThissuppressionlikelyarisesfromthepositiveVTshiftfromincreasedholeconcentration(thusrequiringhigherVGSforelectroninjection)andfromtheincreaseinelectronSchottkybarrierheight,whichtogetherhinderelectroncurrent.

WecanfurtherunderstandtheoriginoftheseimprovementsbyexaminingtheeffectsofchloroformuponthestatisticaldistributionsofthemaximumholecurrentID,max(atVGS=−3.4V)andtheon-stateholecurrentIonatafixedoverdrivevoltage(Vov=|VGS−VT|),bothshowninFig.2c.Interestingly,devicesofallchannellengthsshowconcurrentincreasesinmaxID,andinIonatfixedVov=1.3V.Evidently,thechloroformdopingshiftsVTpositively,buttheobservedincreaseinIonatfixedVovinbothlong-andshort-channeldevicessuggeststheimprovementisacombinedeffectofincreasedmobilityandreducedRC.Themaximumtransconductance(gm)ofeachdeviceshowsa30.5×medianincrease(from1.07to33μS/μm)forLch=1μmdevices(SupplementaryFig.3a),consistentwiththisinterpretation.

All devices demonstrate a positive VT shift (VT extracted at a constantcurrent10nA/μm)41,withamedianshiftvalueof1.0V(from−2.6Vto−1.6V),consistentwithp-doping(Fig.2d).ThisVTshiftcorrespondstoroughly0.9–1.8×1013/cm2carriersinducedfromthisdopingtechnique(SupplementaryFig.3b).PrecisequantificationoftheinitialdopingconcentrationintheWSe2isdifficultasfabrication-inducedeffects(e.g.,adsorbates,annealing,processinghistory,etc.)canaltertheconcentration.Weestimateaninitialelectronconcentrationontheorderof1012cm-2priortodoping(seeSupplementaryNote1).Thus,uponchloroformdoping,theinducedholedensityof~1013cm-2cancompensatethisinitialdopinganddominatethefinalcarrierconcentration,shiftingthedevicetop-typeoperation.

Additionally, unlike other SCTD methods13,19,21, the chloroform-dopedtransistorsdidnotexhibitanydegradationinoff-statecurrentevenattheshortestLchof100nm.WeextracttheRCofourPd-contacteddopedmonolayerWSe2devicesusingthetransferlengthmethod(TLM),yieldinganRCof2.5(2.8)kΩ·μmforourbest(median)pseudo-TLMstructure(asdescribedinMethods)(Fig.2e).Incomparison,theinitialRCbeforechloroformexposurewas168kΩ·μm(SupplementaryFig.3c).TheimprovedRClikelystemsfromthedopingoftheWSe2regionnearthecontacts,whichnarrowsthemetal-semiconductorenergybarrierwidthandenhancesthecontributionoftunneling42,43. We estimate a low Schottky barrier height (SBH) of ~100meV after doping, consistent with previous theoretical calculations44.However,directextractionofSBHfromtemperature-dependentdataremainschallengingduetothesmallscreeninglengthandlargecontributionoftunnelingcurrentacrossthenarrowbarrier.RigoroussimulationstoextractSBHareanimportantfuturetopictounderstandtheRCimprovementintheseandotherdopedWSe2devices.Notably,thisRCvaluerepresentsthebestreportedforPdcontactsonmonolayerWSe2andiscomparabletothehighest-performingcontactschemesreportedtodate(e.g.,Sb/PtwithMoOxdoping36,WOxandNOxdoping39).SupplementaryTable1benchmarkstheperformanceofp-typemonolayerWSe2,highlightingthatourdevicesachievestate-of-the-artRCandperformancemetrics.

Comparison of the subthreshold swing (SS) before and after doping reveals a decrease in SS for the doped devices, down to 81.4mV/decfrom144mV/decatroomtemperature(Fig.2fandSupplementaryFig.3d).ThisimprovementinSSspanstheentiresubthresholdrangeofIDfromdoping.(Fig.2fandSupplementaryFig.3e),whichmaybeduetopassivationofinterfacialdefectsandcouldalsoexplaintheincreaseinmobility45.

HighRCalsolimitstheoperationofWSe2transistorsatcryogenictemperatures,impedingthestudyofWSe2inquantumtransportdevices.Chloroform-dopedmonolayerWSe2transistorsat10K(Fig.2g)showconsistentlyhighholecurrentacrossalldevices,upto403μA/μmatVDS=−1VforLch=0.1μm(SupplementaryFig.4),withrelativelylinearIDvs.VDS(Fig.2h).ThecryogenicRC~1.0kΩ·μmwasextractedfromapseudo-TLMfittodevicesrangingfromLch=100nmto1μm(Fig.2i).Toourknowledge,thisisthelowestp-typeRCreportedtodateforcryogenictemperatures.

Chargetransfermechanismofchloroform-dopedWSe2

Temperature-dependent PL measurements of WSe2 before (Fig. 3a) andafter(Fig.3b)chloroformexposureprovideadditionalevidenceforchargetransfer.Inbothsamples,thepeaksat1.75eVand1.71eVcorrespondtoneutralexcitons(X)andtrions(T),respectively,withthe40meVdifferencematchingthereportedtrionbindingenergy46,47. Additionally, the relative intensity of the trion peak is greaterthantheexcitonpeakinthechloroform-exposedWSe2(SupplementaryFig.5),asexpectedforincreaseddoping48.

InbothdopedandundopedWSe2,thethreelower-energypeaks—labeledL1(~1.67eV),L2(~1.64eV),andL3(~1.60eV)—resemblepreviousreportsofexcitonicboundstates48whichdisplaysublinearexcitationpowerdependence(SupplementaryFig.5)andrapidlyquenchabove100K49.Thesecharacteristicsareconsistentwithweakly-bounddefector-donorstatesnearthevalencebandmaxima49,50.TheintensityoftheL1,L2,andL3peaksissignificantlyhigherinthedopedsample(Fig.3a,b),whichindicatesincreasedradiativerecombinationofelectronsandholesboundtodifferentsitesandcouldbeexplainedbytheFermi-levelmovingtowardsthevalencebandafterchloroformdoping51,52. The decrease in exciton intensity and emergence of bound states suggest that the passivation of WSe2 defects is unlikely. Techniques53–55 that passivate defects commonly exhibit an increase in quantum yield and the suppression of low-energy defect peaks at low temperatures.

Toinvestigatethep-dopingmechanism,wemodeledtheinteractionsbetweenchloroformandWSe2usingdensityfunctionaltheory(DFT).Thechloroformabsorptionsitewasdeterminedbyrelaxationagainstarigid5×5WSe2supercell,consideringgeometrieswherethehydrogenatomfacedtowards(H-facing)orawayfrom(Cl-facing)themonolayerWSe2.AdditionalcomputationaldetailsareprovidedintheMethodssection.

TheCl-facingandH-facingchloroformorientationsexhibitfavorableadsorptionenergies(Eads)of−468and−351meV,respectively,indicatingstrongphysisorption(|Eads|≫kBT)totheWSe2withoutinducingcovalentchemicalmodification(Fig.3c,d).Thisgreatlyexceedsadsorptionenergiesbetweensomesmallmoleculesandgraphene(|Eads|<100meV)56andareonthehighendofvaluescalculatedforotheradsorbatesonTMDs(50to333meV)57,58.Incomparison,chemisorptionischaracterizedbystrongerbindingenergies(≥1eV)andshorterdistances(<2Å)59.Thus,theadsorptionenergiesandequilibriumdistancefrom2.5–3.2Å(Fig.3d)resemblestrongphysisorption.

Bader charge analysis reveals that the chloroform withdraws electronsfromtheWSe2inbothconfigurations(Fig.3c).InboththeCl-facingandH-facingorientations,theadsorbedchloroformmoleculegainsanetchargeof0.0095and0.0351excesselectrons,respectively,confirmingp-doping.Theseelectrontransfersarecomparabletothatbetweenseveralwell-knownTMDSCTDsystems,including:(i)MoOx,awell-establishedp-dopant,andMoS2(~0.077electrons60transferredfromMoS2toMoOxperunitcellofMoS2,assumingfullsurfacecoverage),(ii)MoS2andacetone(~0.039electronstransferredtoMoS2permoleculeofacetone61),whichisknowntostronglyn-dope2DTMDs34,35,and(iii)nitricoxide(NO)andWS2(0.018electronspermoleculeofNO57),whichisknowntobeanexcellentp-dopantfor2DTMDs19,39.Forathresholdvoltageshiftof1V,thecalculatedBaderchargetransferof0.0095(0.0351)holesperchloroformmoleculecorrespondstoasurfacecoverageof1.0(0.27)chloroformmoleculesperWSe2unitcell.ThecalculatedBaderchargetransfercouldbeincreasedbyuptoafactoroffourduetosubstrateinteractions61,whichwouldfurtherenhancethepredictedefficiencyofholedopingduetochloroformadsorption.

According to the atom-resolved projected density of states (pDOS),thechloroformorbitalsarelocatedmorethan1eVbelowthevalencebandedgeorabovetheconductionbandedgeofmonolayerWSe2,andthechloroformmoleculedoesnotintroduceelectronicstatesnearthebandextremaorinthebandgap(Fig.3eandSupplementaryFig.6).ThissuggeststhatthechargetransferbetweenchloroformandWSe2occurswithoutcovalentbondformationororbitalhybridization,whichisconsistentwiththeadsorptionenergieswecalculateforthechloroform/WSe2system(additionalstatesassociatedwithchemisorptionaretypicallyaccompaniedbyanadsorptionenergy<−500meV62).TheabsenceofstatesformedinornearthebandgapsuggeststhatchloroformdopingavoidsintroducingscatteringsitesthatcoulddegradethemobilityofWSe2.

Temporalandthermalstabilityofchloroform-dopedWSe2

Toevaluatethelong-termstabilityofchloroformdoping,weregularlymeasuredthechargetransportcharacteristicsofdopedmonolayerWSe2transistorsformorethan8months.Figure4ashowstheevolutionofIDvs.VGSsweepsforasingledopedmonolayerWSe2deviceover243days(>8months).ThemaximumdraincurrentID,maxatVGS=−3.4V,slightlydecreasesafter243daysfrom113μA/μmto97.4μA/μm.Figure4bplotstheforwardandbackwardsweepVTfordeviceswithLch=1μm,revealingamediannegativeshiftof−0.18Voverthecourseoflong-termtesting.ThissmallnegativeshiftinVTcouldindicateaslightreductioninp-dopingduetochloroformdesorption.

Figure4csummarizestheevolutionofIDin1µmlongdevicesasID,maxforVGS=−3.4VandatVov=1.5V.Chloroformdopingremainsremarkablystableovertime,withthemedianID,maxretaining>96%after6daysand>81%after243days.WenotethatthedecreaseinID,maxcanbepartiallyattributedtothenegativeVTshiftbecausetheIonremainsrelativelystable.After8months,thefinalID,maxisstill76.1×higherthantheinitialundopedID,maxforthesamesetofdevices.Thisdemonstratesthattheimprovedp-typeperformancefromchloroformdopingishighlystableovertime.Incontrast,otherdopingtechniques(e.g.MoOx13,63,O3oxidation64)degraderapidlyinair,losingfunctionalityoverthecourseofseveralhoursordays.Thenext-bestreportedexample,nitricoxide,maintainedperformanceafter24days25.Additionally,thelowRCfromchloroformdopingwasmaintainedafter8months(SupplementaryFig.7).ThisstabilityofchloroformdopingovertimeisconsistentwiththestrongphysisorptionpredictedfromourDFTsimulations(Fig.3c,d).

Thermal stability is also critical to enable further processing. Weexaminedthethermalstabilityofundopedandchloroform-dopedWSe2transistorsbysequentiallyannealingtheminvacuumfor30minat100°C,150°C,and200°C.Figure4dshowstheIDvs.VGSofacontroldevice,displayingaprogressivedecreaseinholecurrentaftereachannealingstep.Thisdeclinemayresultfromthedesorptionofweakly-boundwatermolecules,whichalsocontributetop-doping65,66. Figure 4e plots the ID vs.VGS evolutions for a chloroform-doped device after the same annealing sequence.Similartotheundopedcontroldevice,thedopeddeviceexhibitedaslightreductioninholecurrentafterannealingat100°Cand150°C.However,afterthe200°Canneal,theID,maxofthedopeddevicedroppedsignificantlyto3×10-3μA/μm,comparabletothecontroldeviceundersimilarannealingconditions.Thissuggeststhatchloroformdesorbsatelevatedtemperatures,revertingthedevicetoanundopedstate.Thisissupportedbyactivationenergycalculations(Fig.3d),whichshowthatthereisnoadditionalbarriertodesorption,leadingtomuchfasterdesorptionathighertemperatures.

AsummaryofID,maxacrossallannealingstages(i.e.,theinitialstateandannealsat100°C,150°C,and200°C)showsthatthedopeddevicesremain>140×higherinID,maxafterannealingat100°Cand150°C(Fig.4fandSupplementaryFig.8).However,followingthe200°Canneal,therewasasharpdropinholecurrentofthedopeddevices,consistentwiththedesorptionofchloroformandareversaltotheundopedstate.Thissequentialannealingproceduresuggeststhat150°Ccanbetreatedasasafeupper-boundforthethermalstabilityofchloroformdopingonWSe2,althoughfasterthermalrampingandcoolingmayrevealathermalbudgetforhighertemperatures.Whilethisthermalbudgetcouldbeaconcernfordirect-current(DC)operationwithsignificantself-heating67,68,devicesoperatingunderhighfrequencieswillheatupless.Thisisbecausethedeviceswitchingspeedishigherthanthe2Ddevicethermaltimeconstant(typicallyfrom30to300ns69).Inanycase,this150°Cthermalbudgetforchloroformstabilityenablescompatibilitywithoxideencapsulationbyatomic-layerdeposition(ALD),whichoftenoccursbetween100and200°C.Thismayalsofurtherenhancethethermalstabilityofthep-typedoping,enablefabricationoftop-gateddevices,andallowforconcurrentapplicationofotherdopingtechniques,suchassolidchargetransferlayers(e.g.,MoOₓ,WOₓ).

DeterminationofchloroformlocationinaWSe2/oxidestack

To clarify the mechanism and stability of chloroform doping, weinvestigateditslocationrelativetotheWSe2.Figure5apresentsa20×20μm2atomic-forcemicroscopy(AFM)topographyimageofanexfoliatedWSe2flakewithvariouslayerthicknesses.LdenotesthenumberofWSe2layers.Thebilayer(2L)tofour-layer(4L)regionwasmeasuredin2×2μm2scansbeforeandafterdoping(SupplementaryFig.9),yieldingtheheightdistributionsshowninFig.5b.ThepeaksmarktheheightoftheSiO2,2L,3L,and4LWSe2regions.ThereisnonoticeablechangeinspacingbetweentheWSe2layers,buttheheightdifferencebetweentheSiO2and2LWSe2increasesby>0.15nm.ThissuggeststhatchloroformdoesnotintercalatebetweentheWSe2layers,butratherthatchloroformeither(i)insertsattheSiO2/WSe2interface,or(ii)adsorbsontopofeveryWSe2layer.X-raydiffraction(XRD)revealsthattheinterplanarspacingremainedconstantat0.645nm-afterdoping(SupplementaryFig.10),supportingtheconclusionthatchloroformdoesnotintercalatebetweenWSe2layers.

We measured the dependence of chloroform adsorption on WSe2thicknessusingAugerelectronspectroscopy(AES).Figure5cshowsascanningelectronmicroscopy(SEM)imageoftheexfoliatedWSe2flakefromFig.5a,whileFig.5d–fdisplayAESelementalmapsofSe,Si,andCl,respectively.Figure5gplotstheAESsignalintensitiesalongthelineinFig.5c.AsasurfacesensitivetechniquewithanAugerelectronescapedepthofapproximately5to50Å,AESconfirmsthattheSesignalintensityscaleswiththeWSe2thickness(Fig.5d).Incontrast,theClsignalisnegligibleoutsidetheWSe2region,peakswithinthe2LWSe2terrace,anddiminishessignificantlyforthickerWSe2layers(Fig.5f,g).

AFM height mapping indicates uniform increases in height across the2L,3L,and4LWSe2regionsafterchloroformexposure(Fig.5b),whileAESmappingshowsthehighestClsignalinthe(thinnest)2Lregion(Fig.5f,g).ThissuggeststhatchloroformintercalatesattheWSe2/oxideinterface,asreportedforgrapheneonSiO233,withthickerWSe2regionsattenuatingtheAESsignalsfromClbeneaththeWSe2.ThisinterfacialchloroformmayenhancetheWSe2deviceperformancebyincreasingtheoxide/WSe2separationandreducingtheinfluenceofinterfacialoxiden-doping70andtrapstates—whichmaycontributetotheobservedreductioninSSfordopedWSe2devices(Fig.2f).Althoughthisdatasetisconvincing,furtherexperimentalworkisrequiredtodefinitivelyconfirmtheintercalationofchloroform.Notably,thecorrelationbetweentheClsignalandWSe2regionssuggeststhatWSe2isnecessaryforchloroformadsorption(Fig.5f).XPSisconsistentwiththisobservation,detectingaClpeakonlyinsubstrateregionscoveredbymonolayerWSe2(SupplementaryFig.11).Incontrast,thereisnoapparentClpeakinthebaresubstrateregionsofchloroform-soakedsamples.

We further assess the substrate dependence of our doping strategy and its implications for CMOS compatibility. While chloroform doping demonstrates reproducible p-type doping withoutleavingmetallicresiduesonthesurface,thesensitivitytostandardwet-processingsolvents(e.g.,acetone)posesintegrationchallenges(SupplementaryFig.12a).Additionally,selective-areadopingrequiresthedevelopmentofprotectivestratgies,suchasencapsulation,thatpreventdopantdesorption(SupplementaryFig.12b,c).Furtherexperimentalworkisneededtooptimizeencapsulationstrategiesforlocalizeddoping.Tofurtherclarifytheinfluenceofthesubstrate,weextendourexperimentstoincludeWSe2devicesonSiO2(SupplementaryFig.13).These-showcomparableperformanceenhancementasthoseonHfO2(~100×increaseofID,positiveVTshift,andsimilarchangeincarrierconcentration).Thissuggeststhatthedopingmechanismisrobustacrossdifferentamorphousdielectrics.

Overall,thisworkpresentsastraightforwardandstablep-dopingmethodtoachievehigh-performancemonolayerWSe2transistors,whileprovidingnewmechanisticinsightsintosolvent-baseddopingtechniques.Byachievingsignificantimprovementsinholecurrent,RC,anddevicestability,thismethodoffersaviablepathforfuturelow-power2Dsemiconductorapplications.

Methods
Dopingprocess
TheWSe2samplewasimmersedinas-purchasedundilutedchloroform(SIGMA-Aldrich,No.650498),inawatchglass-coveredborosilicatebeakeratstandardroomtemperature(20-24°C)andrelativehumidity(approx.25%-65%)inaventilatedfumehood.Thechloroformsolventwasusedwithoutmodification(as-purchased≥99.9%chloroform).Unlessindicatedotherwise,thedopingprocessoccurredovernight(>8h).Sampleswereexposedonlytopurechloroform,asdilutionwithothersolvents(e.g.,acetone,IPA)couldintroduceco-dopanteffects,complicatingtheinterpretationofconcentration-dependenttrends.Fordevicemeasurements,thedopingprocesswasperformedaftertheinitialdevicefabricationprocesswascompleted.

Materialcharacterization
RamanmeasurementsweretakenontheHoribaLabramHREvolutionRamansystemintheStanfordNanofabricationSharedFacility,using532nmlaserexcitationat1%nominallaserpower(120μW)andaspotsize<1μmindiameter.Theseparameterswereselectedtoensureminimalsampleheatingduringmeasurement.ForRamanandPL,asolid-sourcechemicalvapordeposition(CVD)monolayerofWSe2grownonsapphirewastransferredonto100nmSiO2beforemeasurement.XPSwascarriedoutusingaPHIVersaProbe4,equippedwithamonochromatizedAlKαsource(1486eV)withabeampowerof50Wandbeamenergyof15kV,basepressureof1.2×10-7Pa,andpassenergyof224eV(stepsize:0.8eV)and55eV(stepsize:0.1eV)forsurveyandhigh-resolutionacquisitions,respectively.

Bulk WSe2 crystals were exfoliated with scotch tape onto oxygen-plasma cleaned silicon wafers with 100nm thermal oxide. The exfoliated WSe2 was probed for Auger electron spectroscopy (AES),X-raydiffraction(XRD),andatomic-forcemicroscopy(AFM)imagesinFig.5aandSupplementaryFig.9.AESmapping,compositionanalysis,andlinescansonexfoliatedWSe2wereperformedonaPHI700ScanningAugerNanoprobe.XRDmeasurementswereconductedusingaPANalyticEmpyreansystemwithaCu-Kαsource.ExfoliatedWSe2flakeswereprobedwithsymmetric2θ/ωscans.AFMwasconductedonboththeexfoliatedWSe2andonCVD-grownWSe2onsapphireusingaBrukerDimensionIconinpeakforcemodewithanNSC19AlBSprobe(nominalspringconstant=0.5N/m).

Localback-gatedevicefabricationonHfO2andelectricalmeasurements
Continuous2-inchCVD-grownmonolayerWSe2onsapphirewaspurchasedfrom2Dsemiconductorsandtransferredontolocalbackgatesof5.3nmHfO2withCox=2.8μF/cm2.Thelocalbackgatesweredefinedbylift-off2nm/8nmTi/PtfollowedbytheHfO2gatedielectricbythermalatomic-layerdepositionat200°C.Coarsecontactpadswerethendefinedbylift-off2/20nmTi/Pt.Polystyrene(PS)wasspin-coatedontopoftheWSe2andthentransferredinDIwater.AnO2plasmatreatment(100W,1min)oftheHfO2dielectricwasdonebeforetransferringthePS/WSe2filmtomodifythesubstrate’ssurfaceenergy.ThePSwassthenremovedintoluene.Channeldefinitionwasdoneusingelectron-beamlithographyandetchedbyXeF2(2.5T,30s,3cycles)todefineachannelwidthof1μm.Electron-beamlithographywasusedtopatternthefinecontacts.Pd/Au(20/20nm)wase-beamevaporatedat∼10−8Torr.Electricalmeasurementswereperformedat296KinaJanisST-100vacuumprobestationat∼10−4Torr,usingaKeithley4200semiconductorparameteranalyzer.

CryogenicmeasurementswereconductedinaLakeshorecryoprobestationat~10-6Torr,usingaKeithley4200semiconductorparameteranalyzer.Thesamplewasslowlycooledandlefttostabilizeovernightat10Kbeforeelectricaltesting.

Forcontactresistance(RC)extraction,apseudo-transferlengthmethod(TLM)wasused,asdevicesmadeweresingledeviceswithvaryingchannellengths.Inthismethod,alldevicesatacertainchannellengthwereusedforRCextraction.ThetotalresistanceinkΩ·μm(normalizedbythechannelwidth)canbeexpressedasRTOT=2RC+Rch=2RC+RshLch,whereRshisthesheetresistanceofthechannelandRchisthechannelresistance.RCisevaluatedbyplottingRTOTversusLchanddrawingalinearfitthroughalldatapoints,andthey-interceptatLch=0gives2RC.TheRCisextractedforeachgateoverdriveVov=|VGS–VT|,withVTfromtheconstant-currentmethodatID=10–2μA/μm.

Lowtemperaturephotoluminescence
Low-temperaturephotoluminescence spectroscopy wasconductedwitha532nmexcitationlaser,∼1μmspotsize,and600l/mmspectrometergrating.Thelaserpowerwasfixedat60μW,unlessotherwisenoted.Theemissionwascollectedusinga50×objectivewithanumericalapertureof0.55,with2sacquisitiontimesand2accumulations.Thesamplewascooledtoabasetemperatureof~6.7K,thenwarmedupusingaresistiveheaterfortemperature-dependentmeasurements.For-temperature-dependentexperiments,thesamplesatfor30minatthedesiredtemperaturetostabilizebeforecollectingthespectra.Forthisexperiment,CVD-grownWSe2waswettransferred(asdescribedabove)onto100nmSiO2/p++Si,thenhalfofthechipwascleavedandsubjectedtoanovernightchloroformsoak.Severalspotsacrossboththecontrolanddopedsampleswereexaminedtoensurepeakshapeconsistency.Finally,peakfittingwasconductedinOriginusingaGaussian-Lorentzianblend.

Temporalandthermalstabilitytesting
For time stability measurements, devices were stored atroomtemperatureinanitrogendryboxundercontinuousN2purge,withrelativehumiditymaintainedat~5%.Forthermalstabilitytesting,initialelectricalmeasurementswereperformedat296Kin-aJanisST-100vacuumprobestationat∼10−4Torr,usingaKeithley4200semiconductorparameteranalyzer.Thesampleswerethenin-situannealedat100°C,heldfor30min,thenlefttocooldownfor>5h.Electricalmeasurementsweretakenat300Kinavacuum.Thisprocesswasthenrepeatedat150°Cand200°C,respectively,withdevicemeasurementsinbetween,withoutbreakingthevacuum.Atpresent,theimpactofambienthumidityandoxygenonthelong-termstabilityofchloroformdopingremainsanopenquestionandwarrantsfutureinvestigation,particularlyinthecontextofencapsulatedorintegrateddevicearchitectures.

Densityfunctionaltheory(DFT)simulations
First,avariablecellrelaxationwasperformedtooptimizethelatticecoordinateswithinthemonolayerWSe2primitivecell.Theoptimizedprimitivecellwasthenscaledtoa5×5supercell,interfacedwithachloroformmolecule,andthensubjectedtoafixedcellrelaxationtodeterminetheWSe2/chloroformatomiccoordinates.BoththeCl-facingandH-facingorientationswereconsidered,wherethechlorineorhydrogenatomofthechloroformmoleculewasorientedtowardstheWSe2.QuantumESPRESSO7.171wasusedforallDFTsimulations,andthevanderWaalsinteractionsbetweenthechloroformandWSe2monolayerweremodeledusingthevdW-DF-C09correction.Forallself-consistentcalculations,weuseΓ-pointsamplingforall5×5supercells,andk-pointgridsof3×3×1and7×7×1forthe3×3and2×2supercells,respectively.Weperformnon-self-consistentcalculationsona15×15×1k-pointgridpriortoextractingthedensityofstatesfortheWSe2+chloroformassemblies.AllDFTcalculationsuseprojector-augmentedwavepseudopotentialswithkineticenergycutoffsandchargedensitycutoffsof50and400Ry,respectively.Weusethe“Bader”code72forBaderchargeanalysis,PyProcar73forplottingprojecteddensityofstatesandbandstructures,andVESTA74forplottingisosurfaces.Wenotethatinthiswork,weconsideronlyinteractionsbetweenthechloroformandWSe2withoutaninsulatororsubstrate.IncludingtheamorphousSiO2andHfO2wouldimprovetheaccuracyofoursimulationsbycapturingchloroform-substrateinteractions;however,simulatingthisinterfaceiscomputationallydemanding,andweleaveitasapromisingresearchdirection.

OtherorientationsofchloroformonWSe2arecertainlypossible,althougharigoroustestforthemostfavorableorientationremainsoutsidethescopeofthisstudy.WeconfirmedthattheCl-facingorientationisstablecomparedtoothersimilarrandomorientationsbyrotatingthechloroformmoleculeby5degrees(i.e.,tiltingtheC-Hbond5degreesrelativetothesurfacenormal)andthenre-relaxingthesystem.Thechloroformmoleculereturnedclosetoitsoriginalpositionandtotalenergy(towithin2meV),suggestingthatthisorientationisindeedfavorable.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this work, the authors present a p-type doping strategy for monolayer WSe2 transistors using chloroform solvent, demonstrating significant performance enhancements including high hole current (>200 μA/μm), reduced contact resistance (~2.5 kΩ·μm), and maintained high current on/off ratio (1010) at room temperature. The study systematically investigates the physisorption mechanism and long-term stability (exceeding 8 months) through comprehensive characterization techniques (electrical measurements, optical spectroscopy, AFM, XPS) combined with DFT simulations. While this research provides valuable insights into 2D semiconductor polarity control, concerns regarding mechanistic clarity and application potential lead this reviewer to question the work's novelty relative to existing literature. Specifically, the manuscript's innovation appears limited given prior reports on WSe2 p-type doping techniques [Nano Lett. 2023, 23, 10236−10242; ACS Nano 2023, 17, 19709−19723; Nano Lett. 2024, 24, 13528−13533; Nano Lett. 2025, 25, 3571−3578; ACS Nano 2025, 19, 10244−10254]. The following issues require attention:\n1. Interfacial Mechanism Clarification:\n1) While AFM/AES data suggest chloroform intercalation at the WSe2/oxide interface, direct evidence linking intercalation to contact resistance reduction is lacking. Cross-sectional STEM-EDS or in situ TEM characterization would strengthen this claim.\n2) Although DFT indicates physisorption, potential chemical interactions (e.g., Cl substitution at Se vacancies) remain unaddressed. XPS depth profiling or TOF-SIMS analysis could differentiate between physical adsorption and chemical bonding.\n2. Computational Model Limitations:\n1) The DFT analysis appears oversimplified, considering only two molecular orientations (Cl-facing/H-facing) while neglecting coverage effects and substrate interactions (e.g., SiO2/WSe2 interface).\n2) Furthermore, the observed performance degradation at 200 °C annealing requires quantitative analysis through activation energy calculations or desorption kinetics modeling to elucidate thermal stability limitations.\n3) The Bader charge transfer (0.0125 - 0.0349 excess electrons) in DFT calculations seems inconsistent with the substantial device improvements. A detailed discussion reconciling this discrepancy is needed, particularly regarding potential roles of defect passivation versus band structure modulation.\n3. CMOS Compatibility:\n1) As the doping mechanism operates at the WSe2/oxide interface, its compatibility with standard CMOS processes remains questionable.\n2) The authors should address integration challenges and propose strategies for selective area doping or hybrid n-p device architectures (such as WS2, MoS2 n-type devices).\n4. Process Parameter Optimization:\nCritical fabrication parameters (chloroform concentration, immersion temperature, ambient conditions) require explicit documentation. Concentration-dependent performance studies would establish dopant dosage effects and process robustness.\n5. Environmental Stability Assessment:\nThe 8-month stability data necessitate clarification regarding storage conditions (inert vs. ambient environment). Additional analysis of humidity/oxygen impacts on doping efficacy would strengthen the stability claims.\n6. Electrical Properties:\nFor the monolayer WSe2 transistors using chloroform doping, the other electrical properties should be provided, such as transconductance, mobility, and Schottky barrier height.", "reviewer_2": "The manuscript 'Low Resistance P-Type Contacts to Monolayer WSe2 through Chlorinated Solvent Doping' reports a strategy to reduce the hole contact resistance for high performance p-type FETs based on 2D TMDs - an important step towards developing scalable next generation CMOS. I have the following issues with the paper that should be considered by the authors:\n1. Doping level - What was the original doping level of their WSe2 samples, and how much enhancement was obtained after solvent doping?\n2. Mobility - The authors suggest improvement in electron mobility to the increased drain current. Again, can they quantify the improvement in mobility as a result of the treatment?\n3. Substrate - Is there a role of the substrate/dielectric surface on the solvent induced doping? Would the doping levels be different for HfO2 and SiO2 as they are known to have different surface dopants to TMDs\n4. Contact Resistance - Could the authors provide a benchmark figure of their contact resistance compared to previous reports?\n5. The authors need to provide spectroscopic evidence of doping through XPS, and correlate changes in binding energy to doping levels estimated from FET measurements.\nAnswers to the above comments will allow others in the community to reliably use the technique proposed and achieve high performance WSe2 p-FETs.", "reviewer_3": "Here, Lauren Hoang et al. report a p-type doping method for 2D transistors, in which chloroform molecules are intercalated between monolayer WSe₂ channels and gate oxides in field-effect transistors, serving as dopants. The hole currents increase by 100 times to over 200 μA/μm, with an on/off ratio of 10¹⁰ after doping—comparable to the highest reported values for p-type 2D transistors.\nThe overall performance of the p-type 2D transistors is impressive, and the mechanism underlying the high doping efficiency is well explained. While there are several other studies on surface charge transfer doping of p-type WSe₂ transistors, reports achieving both high hole currents (greater than several μA/μm) and high on/off ratios are very limited (IEEE Electron Device Lett. 43, 319 (2022)). I believe researchers in the 2D transistor community will find this study of high interest. However, I have several concerns, as outlined below.\nMain Comments\n1. Versatility of the doping method\nCan this doping method be applied to WSe₂ with arbitrary interfaces while still achieving high doping efficiency and stability? The authors suggest that intercalation of dopants at the substrate–2D channel interface is key to attaining both. However, different interfaces may affect the intercalation process and the resulting doping efficiency. For instance, intercalation may be more difficult in non-transferred, as-grown WSe₂ films on dielectric substrates or on hexagonal boron nitride (hBN), which are useful for large-area electronics and high-mobility quantum devices, respectively. It would be valuable if the authors could provide supporting experimental data.\n2. Stability\nCompared to other doping processes such as substitutional doping or charge transfer doping using inorganic interlayers, one of the main drawbacks of molecular doping is limited stability. Can the p-type doping be retained through further fabrication steps, such as wet processing during lithography for CMOS integration? Currently, the chloroform doping affects the entire channel globally, without spatial control, resulting in a strong suppression of electron conduction (as shown in Fig. 2b). It indicates the likely need to fabricate n-type and p-type channels separately by sequential fabrication processes. These limitations should be discussed when evaluating the scalability and practicality of this doping approach for CMOS applications.\nAdditionally, the limited thermal stability (~200 °C) raises concerns regarding device operation. Joule heating during current flow in the channel could lead to dopant desorption. It would be helpful if the authors could comment on the retention characteristics of the doping over time under operational conditions.\n3. Origin of low contact resistance\nThe authors discuss the impact of chloroform doping in reducing contact resistance, but they do not provide Schottky barrier height (SBH) values for Pd/WSe₂ contacts before or after doping. It would strengthen the study to include temperature-dependent measurements or thermionic emission fits to extract SBH values and offer a more quantitative understanding of the band alignment at the contacts.\nOther Comments\n4. Related to the previous comment, does intercalation also occur beneath the metal-contacted regions of WSe₂? If so, would this directly affect the Fermi level at the contacts?\n5. As mentioned above, the doping strongly suppresses electron conduction (Fig. 2b). Could the authors provide insight into the mechanism behind this phenomenon?", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "We are grateful to the Reviewers for their time and careful evaluation. In this detailed Response Letter, we have carefully considered each Reviewer’s comments and have revised our manuscript accordingly. The original Reviewer comments are in blue italics, followed by our responses in black text. Revisions to the manuscript text are shown in red text, below and in the resubmitted manuscript and supplement.\n[... Author response text truncated for brevity as per instructions to include full text ...]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Overall, this work demonstrates a convenient method for p-type doping of WSe₂ using chloroform and shows improvements in contact resistance. While the authors have addressed my previous concerns by providing additional explanations, data, and details, I remain unconvinced that the modest electrical performance advances presented here are sufficient for publication in Nature Communications. In particular, the recent article on “Lan et al. In International Electron Devices Meeting (IEDM) 1–4 (IEEE, 2023)” reported significantly higher performance in enhancement-mode PMOS, including a higher current of 727 μA/μm and lower contact resistance of 0.875 kΩ·μm. In comparison, the values reported in this manuscript—2.5 kΩ·μm and approximately 200 μA/μm—though respectable, do not represent a substantial breakthrough. Furthermore, the practical applicability of this approach in CMOS integration remains uncertain. Therefore, I believe this study does not meet the high threshold for novelty and impact expected for Nature Communications.", "reviewer_2": "I appreciate the authors response to the concerns raised. I have the remaining issues with their responses:\n1. Doping level: My concern regarding the initial doping level is motivated by the goal of extending this technique to WSe2 obtained from alternative growth methods (MOCVD, ALD, exfoliation). Since process parameters for chloroform treatment may vary with the starting material, an estimate of the initial doping level in the present study would assist future researchers in adapting and reproducing the work. Without such context, reproducibility across different WSe2 sources may be challenging. Could the authors address this issue?\n2.The explanation regarding contact-limited behaviour is reasonable. However, since mobility values (even if underestimated) are routinely reported in WSe2 FETs, could the authors provide an approximate pre-doping mobility range or at least benchmark their post-doping values against reported literature?\n3. A major challenge with this work is the incompatibility of this technique with standard solvent based fabrication processes. The authors suggest encapsulation techniques might offer localised doping. However, their comment in Page 23 about chloroform intercalation through percolation underneath the contact interface challenges their previous assertion related to encapsulation, as lateral percolation would still occur. The authors should address this inconsistency.", "reviewer_3": "The authors have fully addressed my previous concerns. I now recommend the publication of this work in Nature Communications.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "We are grateful for the Reviewer’s time and constructive comments. The original Reviewer comments are in blue italics, followed by our responses in black text. Revisions to the manuscript text are shown in red text below.\n[... Author response text truncated for brevity ...]"}]

Decision letter