ARTICLE
Received2Jun2014|Accepted17Sep2014|Published20Oct2014
T owards intrinsic charge transport in monolayer molybdenum disulfide by defect and interface engineering
Zhihao Yu1,*,Yiming Pan2,*,Yuting Shen3,Zilu Wang4,Zhun-Yong Ong5,T ao Xu3,Run Xin1,Lijia Pan1, Baigeng Wang2,Litao Sun3,Jinlan Wang4,Gang Zhang5,Yong Wei Zhang5,Yi Shi1&Xinran Wang1
Molybdenum disulfide is considered as one of the most promising two-dimensional
semiconductors for electronic and optoelectronic device applications.So far,the charge
transport in monolayer molybdenum disulfide is dominated by extrinsic factors such as
charged impurities,structural defects and traps,leading to much lower mobility than the
intrinsic limit.Here we develop a facile low-temperature thiol chemistry route to repair the
sulfur vacancies and improve the interface,resulting in significant reduction of the charged
impurities and traps.High mobility480cm2VÀ1sÀ1is achieved in backgated monolayer
molybdenum disulfidefield-effect transistors at room temperature.Furthermore,we develop
a theoretical model to quantitatively extract the key microscopic quantities that control the
transistor performances,including the density of charged impurities,short-range defects and
traps.Our combined experimental and theoretical study provides a clear path towards
intrinsic charge transport in two-dimensional dichalcogenides for future high-performance
device applications.
1National Laboratory of Solid State Microstructures,School of Electronic Science and Engineering and Collaborative Innovation Center of Advanced Microstructures,Nanjing University,Nanjing210093,China.2School of Physics,Nanjing University,Nanjing210093,China.3SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of Ministry of Education,Southeast University,Nanjing210096,China.4Department of Physics,Southeast University,Nanjing 211189,China.5Institute of High Performance Computing,1Fusionopolis Way,Singapore138632,Singap
ore.*These authors contributed equally to this work. Correspondence and requests for materials should be addressed to L.S.(email:slt@seu.edu)or to Y.S.(email:yshi@nju.edu)or to X.W.
(email:xrwang@nju.edu).
D
espite its great promise as a two-dimensional (2D)channel material for logic transistors 1,2,integrated circuits 3,4and photodetectors 5,6,the charge transport in
monolayer of molybdenum disulfide (MoS 2)is still far away from the intrinsic limit.Theoretically,the phonon-limited mobility is B 410cm 2V À1s À1at room temperature and is weakly dependent on carrier density until B 1013cm À2,where electron–electron scattering starts to play important roles 7.Experimentally,however,very different behaviour is observed regardless of the sample preparation method.First,the electron mobility in backgated monolayer MoS 2devices is limited to B 40cm 2V À1s À1at room temperature,an order of magnitude lower than the phonon limit 2,8–10.Second,the mobility is found to increase with carrier density even beyond 1013cm À2(refs 2,9).Third,at low carrier density,the charge transport is dominated by hopping mechanism 11,12.Fourth,a metal–insulator transition (MIT)is observed at high carrier densities on the order of 1013cm À2,but the underlying mec
hanism remains debated 2,8,9.These observations clearly point to the existence of extrinsic factors that dominate charge transport in monolayer MoS 2.Charged impurities (CI)2,13,short-range defects 11and localized states 11,14,15,among others,are believed to strongly influence electron transport.However,a comprehensive physical picture that explains all of above-mentioned transport phenomena and provides quantitative and microscopic insights into the impurities is still lacking.
Owing to its atomic thickness,electrons in monolayer MoS 2are more susceptible to impurities both inside MoS 2and at the dielectric interface.Therefore,the engineering of defects and interface represents a logical route to further improve MoS 2device performance.Recently,the use of thiol-terminated SiO 2(ref.16),boron nitride 17and suspended structure 18were found to improve the mobility of monolayer MoS 2devices by several folds,showing the importance of interface.However,their mobility values are still much lower than the intrinsic limit,indicating that other sources of impurities,most likely inside MoS 2,are still present.
In this work,we show that sulfur vacancies (SVs),which are the main type of intrinsic defects in MoS 2(ref.11),can be effectively repaired by (3-mercaptopropyl)trimethoxysilane (MPS)under mild annealing,resulting in significant reduction of CI and short-range scattering.Monolayer MoS 2with both sides treated by MPS exhibits a record-high mobility of 81cm 2V À1s À1(320cm 2V À1s À1)at room te
mperature (low temperature),much higher than untreated samples.In addition,we show that MIT in MoS 2is due to localized trap states,which can be modulated by improving the sample and interface quality.A theoretical model that takes into account the major scattering sources (phonon,CI and short-range defects)and localized charge traps is developed to quantitatively understand the scaling of mobility,conductivity and MIT in monolayer MoS 2.By fitting the experimental data,we are able to extract key microscopic quantities including the density of CI,short-range defects and charge traps,as well as derive a transport phase diagram for MoS 2.The quantitative information also allows us to discuss the possible origins of these extrinsic factors,which serve as the basis for further device optimization.
Results
Repair of SVs by thiol chemistry .The monolayer MoS 2samples studied here are obtained by mechanical exfoliation from bulk crystals (Supplementary Fig.1).A high density of SV exists in as-exfoliated MoS 2as demonstrated in earlier works 11,19.These defects,which can act as catalytic sites for hydrodesulfurization reactions 20,21,are chemically reactive.Therefore,it is possible to repair the SV by thiol chemistry.Here,we choose a specific molecule MPS (Fig.1inset)for two reasons.(i)The S-C bond in MPS is weaker than other thiol molecules like dodecanethiol due to the acidic nature of CH 3-
O-groups,leading to a low energy barrier for the reaction 22.(ii)The trimethoxysilane groups in MPS react with the SiO 2substrate to form a self-assembled monolayer 23(SAM,Supplementary Fig.2).The SAM layer can effectively passivate the MoS 2/SiO 2interface,while the outstanding thiol group can also repair the SV on the bottom side of MoS 2,which is otherwise difficult to access.This unique property of MPS allows us to systematically compare three types of MoS 2samples:as-exfoliated on SiO 2,top-side (TS)treated on SiO 2and double-side (DS)treated.For the MPS treatment,we used a liquid-phase process 22,followed by 350°C annealing in forming gas to repair the SV (see Methods and Supplementary Fig.3for details of sample preparation and characterization).We first study the reaction kinetics of SV and MPS by density functional theory (Fig.1).The reaction can be de-scribed by HS(CH 2)3Si(OCH 3)3þSV -CH 3(CH 2)2Si(OCH 3)3(Supplementary Fig.4;Supplementary Movie 1),which is exothermic with the enthalpy change of À333.3kJ mol À1of MPS.The reaction process comprises two steps with an energy barrier of 0.51eV and 0.22eV,respectively.In the first step (Supplementary Fig.5),MPS chemically absorbs onto an SV through the sulfur atom,and then it cleaves the S-H bond and forms a thiolate surface intermediate.The dissociated H atom is bonded to a neighbouring S atom.The second step involves cleavage of the S-C bond and hydrogenation of the thiolate intermediate to form the final product trimethoxy(propyl)silane (Supplementary Fig.6).We note that the two-step process agrees well with earlier thiol absorption experiments on defective MoS 2(refs 20,22).The relatively low energy barriers also facilitate the reaction to occur at low temperature.
To quantify the effect of MPS treatment on sample quality,we perform aberration-corrected transmission electron microscopy (TEM)on as-exfoliated and TS-treated monolayer MoS 2.Figure 2compares high-resolution TEM images of a typical as-exfoliated
SH
H 3CO
Si
OCH 3
OCH 3MPS
0.22 eV
3.4 eV
0.51 eV
0.06 eV
Figure 1|Kinetics and transient states of the reaction between a
single SV and MPS.There are two energy barriers,the first one (0.51eV)is due to the S-H bond breaking,and the second one (0.22eV)is due to S-C bond breaking.(a –e )Plots the initial,transient and final states of the reaction.The SV in the initial state is illustrated by dashed line in (a ).The inset shows the chemical structure of MPS.
and TS-treated sample.The SVs can be clearly distinguished by analysing the intensity profile 11as shown in Supplementary Fig.9.Statistical analysis (from more than 15areas in each case)indicates that the density of SV is reduced from B 6.5Â1013cm À2for the as-exfoliated samples to B 1.6Â1013cm À2for the TS-treated samples (Supplementary Fig.9c,d).Owing to the difficulty in making TEM samples,we are not able to characterize the DS-treated MoS 2,where further reduction of SV is expected.During the course of TEM experiment,we have paid great attention to minimize the knock-on damage and lattice reconstruction caused by electron beam irradiation 24by limiting the exposure time to o 30s and the current density to below 106e nm À2S À1(ref.11).The SV generation rate induced by electron irradiation under our experimental conditions was very low,B 5.6Â1010cm À2S À1(ref.11).Therefore,the SVs in Fig.2are believed to be intrinsic rather than induced by electron irradiation.
Electrical transport properties .Next,we systematically investigated the effect of MPS treatment on electrical transport properties of MoS 2.We fabricated backgated field-effect transistors (FETs)on as-exfoliated,TS-treated and DS-treated monolayer MoS 2samples,and carried out electrical measure-ments in high vacuum with a standard four-probe technique,unless otherwise stated (see Methods for details of device fabri-cation and measurement).All the devices exhibited very small hysteresis at room temperature,which became even smaller as they were cooled down (Supplementary Fig.10).Therefore,in the following we only present electrical data from the forward sweep.Figure 3a shows the four-probe conductivity s ¼(I ds )/(D V )(L )/(W )as a function of backgate voltage V g for three representative devices at room temperature (300K),where I ds is the source-drain current;D V ,L and W are the voltage difference,distance and sample width between the two voltage probes,respectively.The MPS-treated samples show improved s compared with the as-exfoliated one,indicating higher sample and interface quality.At carrier density n ¼C g V g ¼7.1Â1012cm À2(C g ¼11.6nF cm À2is the gate capacitance for 300nm SiO 2dielectrics),the DS-treated sample shows s ¼1.52e 2h À1and field-effect mobility m ¼(d s )/(C g d V g )¼81cm 2V À1s À1.To our best knowledge,this is the highest room-temperature field-effect mobility reported so far for monolayer MoS 2regardless of the device geometry 2,8–10.
To gain further insights into the charge transport physics,we performed variable-temperature electrical measurements down to 10K.Surprisingly,the three types of devices exhibit very different
behaviour (Fig.3;Supplementary Fig.11).For the as-exfoliated sample,s monotonically decreases during cooling over the entire range of n ,indicating an insulating behaviour (Fig.3d;Supplementary Fig.11a).For the DS-treated sample,the transfer curves all intersect near V g ¼80V (corresponding to n B 5.7Â1012cm À2,Supplementary Fig.11c),which is a signature of MIT.Metallic and insulating behaviours are observed down to the base temperature for n 46.6Â1012cm À2and n o 3.5Â1012cm À2,respectively.At intermediate n ,the s -T characteristics are not monotonic,and MIT occurs within our experimental temperature range (solid symbols in Fig.3f).For the TS-treated sample,MIT is observed for n 44.7Â1012cm À2(solid symbols in Fig.3e),while insulating behaviour is always observed at low temperatures (Fig.3e;Supplementary Fig.11b).As we shall see later,these distinct transport behaviours precisely reflect the quality of the MoS 2and interface.
The scaling behaviour of m is also very different for the three samples (Fig.3b,c).The as-exfoliated sample shows the lowest m among the three samples.The m -T curve is not monotonic with a peak value of 31cm 2V À1s À1near 175K.In contrast,m monotonically increases for the MPS-treated samples on cooling.For T 4100K,m B T Àg ,where g ¼0.72and 0.64for the DS-treated and TS-treated
samples,respectively.For T o 100K,m gradually saturates.At T ¼10K,m ¼320cm 2V À1s À1for the DS-treated sample,which is B 3(22)times higher than the TS-treated (as-exfoliated)one.
We stress that all the above-mentioned transport phenomena are reproducible among different samples.In Supplementary Fig.12,we show three additional sets of data for two-terminal devices.Although the mobility is lower than their corresponding four-terminal counterparts due to contact resistance,all the key transport properties,including MIT,scaling of mobility and conductivity,are qualitatively reproduced.
Theoretical modelling of charge transport .Although some theories have been proposed to explain the mobility of monolayer MoS 2(refs 13,25),a more complete physical model remains to be developed to fully understand the charge transport.In particular,the model should establish the correlation between different transport regimes and the underlying microscopic scattering mechanisms,provide quantitative information about the samples,and project device performance based on realistic parameters.We start by calculating the mobility of monolayer MoS 2.According to Matthiessen’s rule,the mobility for free carriers is expressed as
m 0n ;T ðÞÀ1¼m ph T ðÞÀ1þm CI n ;T ðÞÀ1þm À1
sr
ð1Þ
where m ph ,m CI and m sr are mobility limited by phonons,CI and short-range scatterings,respectively (the calculation of each term is described in Methods).The incorporation of m sr in our model is motivated by (i)TEM characterization that clearly shows the existence of short-range SV defects and (ii)the saturation of m at low temperatures.We do not consider surface optical phonon scattering because the relatively high energy of the SiO 2phonon modes makes them irrelevant to the low-field mobility phenomenon considered here 25.
The experimentally measured ‘effective’field-effect mobility m is not equal to the free-carrier mobility m 0,due to the presence of charge traps that limits the free carrier population.Recently,localized trap states within the bandgap have been observed in both exfoliated and CVD monolayer MoS 2(refs 11,14,15).The impurity band from these trap states can introduce a mobility edge that strongly affects the charge transport 26.In a simple picture 27,transport is carried only by electrons in the extended states,that is,states above the mobility edge.This model,
which
Mo S
Figure 2|High-resolution aberration-corrected TEM images.
(a )As-exfoliated and (b )TS-treated monolayer MoS 2sample,showing the significant reduction of SV by MPS treatment.The SVs are highlighted by red arrows.The overlaid blue and yellow symbols mark the position of Mo and S atoms,respectively.Scale bar,1nm.Detailed intensity profile analysis and histogram of SV density are shown in Supplementary Fig.9.
does not account for the hopping between the localized states,has been very successful in modelling organic FETs27,28.Here we adopt the same model to extract important physical quantities such as the density of CI(N i)and trap states(N tr),while avoiding the complexity of dealing with the energy-dependent mobility and percolation effects in hopping transport27(the detailed calculation is described in Methods).The model is further justified by its excellent agreement with the experimental data over a broad range of temperature and carrier density.
Discussion
With the above model,we can now quantitatively understand the obtained experimental data.The solid lines in Fig.3are the best fitting results using the parameters listed in Table1.Remarkably, the scaling of mobility and conductivity with temperature and carrier density is well reproduced with a single set of parameters, suggesting that our model captures the essential physics.At low temperature and low carrier density,the calculated s and m are lower than experiments(Fig.3b,d–f),presumably due to the omission of hopping transport in our model.Hence,the discrepancy is the largest for the as-exfoliated sample(Fig.3b,d) as N tr is the highest.
Thefitting parameters in Table1give considerable insights into the microscopic origin of impurities in MoS2.In all of our samples,m is much lower than m ph,indicating that phonon scattering does not play a significant role.Rather,the mobility is largely limited by CI and short-range scatterings.We notice that N i and m sr are reduced by MPS treatment and are correlated for each sample,suggesting that CI partially shares the microscopic origin with short-range defects,most likely SV.CI also partially comes from the interface,as the DS-treated sample has much lower N i than TS-treated one.For the DS-treated sample,N i becomes comparable to that of SiO2substrate(0.24–2.7Â1011cmÀ2)(refs29–31),indicating that a large portion of SV is repaired.N tr is also partially due to SVs as it can be reduced by MPS treatment.However,N tr is an order of magnitude higher than N i for all the samples,pointing to
contributions from additional sources.This could be due to the ambient species absorbed between MoS2and SiO2during exfoliation,which act as charge traps as in the case of graphene29,31,32.
For the MPS-treated samples,the high-temperature mobility is limited by CI,leading to the usual TÀg scaling behaviour. Reference13predicts g B1for the range of carrier densities studied here.However,both short-range scattering(temperature independent)and thermal excitation from charge traps(which leads to the opposite trend since the density of electrons in the extended states n c increases with temperature)can decrease the effective g as observed here.Therefore,one cannot reliably infer the scattering mechanism solely by analysing g.When the charge traps become dominant(which usually happens at low temperature and low carrier density),the mobility even exhibits an insulating behaviour as commonly observed in backgated devices2,11,12and in the as-exfoliated sample here(Fig.3b). The mobility increases with carrier density for all three samples from the combined effect of CI and charge traps(Fig.3c).Below a threshold carrier density(equivalent to mobility edge),m becomes negligible due to the localized nature of electrons in charge traps. The mobility edge is found to decrease with temperature (Supplementary Fig.13)because of the thermal excitation of electrons to the extended states.At T¼80K,the threshold density is about2Â1012,3Â1012and5Â1012cmÀ2for the DS-treated,TS-treated and as-exfoliated samples,respe
ctively (Fig.3c).These values are on the same order of magnitude with N tr,further supporting the charge trap
model.
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100246
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(
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reaction to a book or an article(
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Figure3|The effect of defect and interface engineering on monolayer MoS2charge transport.(a)Typical s-V g characteristics for as-exfoliated (black),TS-treated(blue)and DS-treated(red)monolayer MoS2at T¼300K.(b)m-T characteristics for the three devices at n¼7.1Â1012cmÀ2. Solid lines are the best theoreticalfittings.The dashed red line shows TÀ0.72scaling.(c)m-n characteristics for the three devices at T¼80K.Solid lines are the best theoreticalfittings.(d–f)Arrhenius plot of s(symbols)and theoreticalfittings(lines)for the as-exfoliated(d),TS-treated(e)and DS-treated (f)MoS2.From top to bottom,n¼7.0,6.0,5.0and4.0Â1012cmÀ2,respectively.The critical points of the MIT are highlighted by solid symbols in (e)and(f).Insets in(d–f)show the cartoon illustration of the corresponding MoS2samples undergone different treatments.
Table1|Thefitting parameters in our theoretical model for
the three devices in Fig.3.
As-exfoliated TS-treated DS-treated
N i(1012cmÀ2)0.70.590.24
N tr(1012cmÀ2)8.1 6.16 5.22
D E tr(meV)754658
m sr(cm2VÀ1sÀ1)127161410
Finally,we can also understand MIT in the framework of charge traps.Strong electron–electron correlation in 2D electron gas has been proposed to explain the MIT in monolayer MoS 2,
which gives a universal threshold density n MIT B 1013cm À2(ref.2).In our experiments,however,n MIT is apparently dependent on sample quality (Fig.3).The MIT can be intuitively understood from equation (6)(Methods).When n o N tr ,the density of conducting electrons in the extended states (n c )is exponentially dependent on temperature due to thermal activa-tion,the temperature dependence of s is dominated by n c ,showing thermally activated insulating behaviour 2,11,12,14.For even smaller n and T ,transport is dominated by variable-range hopping because n c can be ignored 11,14(Fig.4a).When n 4N tr ,the Fermi level is above the mobility edge and n c is independent of temperature.Thus,the temperature dependence of s is dominated by m 0,showing metallic behaviour.Therefore,MIT occurs when n E N tr .Using the trap and CI parameters of the DS-treated sample in Table 1,we numerically calculated s (n,T)(equation (6))and obtained the n and T of each critical point as in Fig.3f.Figure 4a sho
ws the calculated transport phase diagram with metallic and insulating regions.Excellent agreement with experiment is achieved without any fitting parameters,showing the consistency of our model.From Fig.4a,n MIT slowly increases with decreasing temperature and converges to n 0at T ¼0K.When n 4n 0,metallic behaviour is always expected.When n o n 0,MIT is always expected at finite temperature.Under realistic trap and CI parameters,n 0is linearly proportional to,but slightly higher than N tr (Fig.4b;Supplementary Fig.14).Therefore,n 0can be used as a rough estimate of N tr .After careful comparison with literature 2,9,14,33,we find that the N tr of our MPS-treated samples is indeed the lowest (Fig.4b),indicating the highest sample quality.
From the above discussion,it is clear that the transport in current state-of-the-art monolayer MoS 2samples is still limited by charge traps,CI and short-range defects.To reach the real potential of monolayer MoS 2in high-performance devices,continuous improvement of sample and interface quality is still needed.In Fig.4c,we project the room-temperature mobility as a function of N tr in the ideal case,that is,without CI and short-range scattering.The mobility at low carrier density is rapidly degraded by even a small N tr .At a modest n ¼7Â1012cm À2,N tr has to be lower than 8.8Â1011cm À2,which is B 6times lower than our DS-treated device,to achieve mobility 4400cm 2V À1s À1.
In conclusion,we have shown that thiol chemistry is an effective approach to engineer the defects and i
nterface in monolayer MoS 2towards intrinsic charge transport.A physical model that includes charge traps and major scattering sources has been developed to comprehensively describe the charge transport in MoS 2and to quantify the density of CI and charge traps in the samples.We believe that our model captures the essential charge transport physics for monolayer MoS 2and can be readily extended to other 2D semiconductors 33–35.Methods
Density functional theory calculations .Density functional theory calculations were performed using the Vienna ab initio simulation package (VASP)36,37.Projector-augmented-wave 38potentials were used to describe ion–electron
interactions,and the exchange correlation potential was represented by the local density approximation 39.We used the climbing-image nudged elastic band method to locate the minimum energy paths and the transition states 40.The defective MoS 2sheet was modelled by a 4Â4supercell of MoS 2with a single SV.A k-point sampling of 5Â5Â1was used for all the calculations.
MoS 2sample preparation and MPS treatment .In this work,we exfoliated monolayer MoS 2from natural bulk flakes (SPI Supplies).The as-exfoliated samples were directly exfoliated on 300nm SiO 2/Si substrate.Before device fabrication,the samples were annealed in a mixture of H 2/Ar at 350°C to
remove organic residue.For the TS-treated samples,we first exfoliated monolayer MoS 2on 300nm SiO 2/Si substrate,followed by annealing in a mixture of H 2/Ar at 350°C to remove organic residue.The sample was then dipped in a fresh solution of 1/15
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Hopping Insulating
Metallic
250300350
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n (1012 c m –2)
n 0 (1012 c m –2)
(c m 2V –1 s –1)
N tr (1012 cm –2)
N tr (1012 cm –2)
n 0
Figure 4|Theory of charge transport in MoS 2.(a )Phase diagram of
charge transport in monolayer MoS 2.The solid black line plots the
calculated MITcritical points (using the parameters of DS-treated sample in T able 1)that separate the metallic and insulating regimes.The red symbols are experimental MIT points extracted from Fig.3f.The lower left corner of the phase diagram illustrates the hopping transport regime (not calculated).(b )The solid black line is the calculated n 0as a function of N tr using the parameters of DS-treated sample in T able 1.The MIT curves under different N tr are plotted in Supplementary Fig.14.The blue and red symbols are experimental points from the TS-treated and DS-treated samples in Fig.3,respectively.We use the highest n that exhibits MIT as n 0.The horizontal dashed lines are MIT critical density estimated from refs 2,9,respectively.The intersections with the solid line represent the estimate of N tr in their devices (black arrows).(c )Theoretical calculation of m as a function of N tr at T ¼300K without any CI or short-range scatterings (using the
parameters of DS-treated sample in T able 1).From top to bottom,n ¼2.0,1.2,0.7and 0.16Â1013cm À2,respectively.The phonon-limited value of 410cm 2V À1s À1is recovered at N tr ¼0.
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