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Contact-metal dependent current injection in pentacene thin-filmtransistors

S. D. Wang, T. Minari, T. Miyadera, K. Tsukagoshi,a and Y. Aoyagi RIKEN, Wako, Saitama 351-0198, Japan and CREST-JST, Kawaguchi, Saitama 332-0012, Japan

Received 24 July 2007; accepted 26 October 2007; published online 13 November 2007

Contact-metal dependent current injection in top-contact pentacene thin-film transistors is analyzed,

and the local mobility in the contact region was found to follow the Meyer-Neldel rule. Anexponential trap distribution, rather than the metal/organic hole injection barrier, is proposed to bethe dominant factor of the contact resistance in pentacene thin-film transistors. The variabletemperature measurements revealed a much narrower trap distribution in the copper contactcompared with the corresponding gold contact, and this is the origin of the smaller contact resistancefor copper despite a lower work function. © 2007 American Institute of Physics.DOI: 10.1063/1.2813640

Large contact resistance  RC  in organic thin-film tran-sistors OTFTs is one of the serious problems for practicalapplications,1–3 and the fundamental origin of  RC  is still un-clear. Since the energy difference between the Fermi level

 E F  of metal electrode and the highest occupied molecularorbital HOMO of organic semiconductor is commonly con-sidered as an origin of  RC  in p-type OTFTs, gold Au hasbeen widely used as source and drain electrodes due to itshigh work function. On the other hand, RC  can be treated astwo parts in series see Fig. 1a,4 the part due to the metal/ organic hole injection barrier  Rint and the other part forcharge accessing from the metal/organic interface to thechannel  Racc. In particular, the charge transport in the latterpart was found to follow the Meyer-Neldel rule,4 indicatingthat trap distribution in the contact region has great contri-bution to RC .

In this letter, we present that copper Cu, which is one

of the low-cost metals and highly desirable in device manu-facturing, can produce smaller RC  than Au in top-contactpentacene TFTs despite a lower work function. Thetemperature-varying measurements revealed that RC  is domi-nated by the accessing resistance rather than the hole injec-tion barrier, and small RC  for Cu is ascribed to the narrowtrap distribution at the Cu contact.

The heavily doped silicon wafers, with 200 nm silicondioxide SiO2 layers, were employed as the transistor sub-strates. The substrates were coated with a self-assemblingmonolayer of beta-phenethyltrichlorosilane  -PTS Ald-rich, 95% prior to the deposition of pentacene.5 PentaceneAldrich, purified with temperature gradient sublimation

thin films with 40 nm thickness were deposited at 0.1 Å s−1

under 610−5 Pa. Cu and Au top electrodes were subse-quently deposited by thermal evaporation at 0.3 Å s −1 in thesame chamber, where a shadow mask was used to defineseven transistors see Fig. 1a with the same channel widthW =750 m and varied channel lengths  L =50–350 mwith 50 m interval on an identical pentacene film. Notethat all the processes except the final metal deposition wereperformed at the same time for all the devices. The transistorcharacteristics were measured in a high vacuum 8

10−5 Pa system equipped with temperature controller.Figure 1b shows the transfer characteristics of the pen-

tacene TFTs. The drain current  I  D in the Cu TFT is muchlarger at low gate bias V GS compared with the Au TFT; this

feature is highly reproducible in about 100 TFTs. The tran-sistor total resistance  Rtot in the linear regime can be ex-pressed as

 Rtot = Rch + RC  =L

WC iV GS − V T ch+ RC , 1

where Rch and ch are the channel resistance and channelmobility, respectively, C i is the capacitance per unit area of SiO2 layer, and V T  is the threshold voltage. In the linearregime V GS−V T V DS, we have Rsource Rdrain= RC  /2,where Rsource and Rdrain are the resistances at source and draincontacts, respectively. The transfer line method TLM was

employed here to estimate RC  based on Eq. 1.

3,4

As anexample, normalized Rtot of a set of Cu TFTs is plotted as afunction of  L in Fig. 1c.

aAuthor to whom correspondence should be addressed. Electronic mail:tsuka@riken.jp

FIG. 1. Color online a Optical micrograph of a set of Cu TFTs left andschematic transistor cross section describing RC  right. b Transfer charac-teristics in the linear regime of pentacene TFTs  L =50 m, 300 K with Cuand Au top electrodes. V DS is drain bias. c W -normalized transistor totalresistance vs L TLM plots of a set of Cu TFTs.

APPLIED PHYSICS LETTERS 91, 203508 2007

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Figure 2a shows that RC  for Cu at 300 K is smallercompared with Au, especially at low V GS. On the other hand,the Rch values of the both TFTs data not shown are very

close, indicating the similarity of the channel region. Thetemperature dependence of  RC  is shown in Fig. 2b. It isclear that the charge transport at both contacts is thermallyactivated. These results suggest that RC  is not dominated bythe hole injection barrier. The reasons are as follows. 1 Thereported hole injection barrier height at the Cu/pentaceneinterface 0.95 eV is higher than that at the Au/pentacene0.47 eV interface.6,7 Therefore, if  RC  is dominated by thehole injection barrier, the Cu contact should have larger RC .2 If  RC  is determined by the tunneling process acrossthe hole injection barrier, RC  should be temperatureindependent.8

According to the above discussion, we define the localmobility in the contact region as contact mobility C ,which satisfies

 RC  =2 D

WC iV GS − V T C 

, 2

where D is the thickness of pentacene films, and the factor 2comes from RC = Rsource+ Rdrain. As expected from Eq. 2, anapproximately linear increase of  RC  with D was observedwhen the pentacene films are not very thin  D10 nm. Inorder to precisely extract V GS-dependent C , we define aV GS-dependent function V GS = 1/  RC  /  V GS, which canbe obtained from the data in Fig. 2b. Assuming C  followsthe empirical V GS dependence,9,10 that is,

C  =  V GS − V T  , 3

we have 1 /  RC  V GS−V T  +1 and V GS  +1C  from

Eqs. 2 and 3, and accordingly 1/  RC  / V GS= V GS

−V T  /  +1. We derived the exponent   from the linearplots of  1/  RC  / V GS vs V GS, whose values are tempera-ture dependent about −0.2 and 1.0 at 300 K for Cu and Au,respectively. Eventually, C  is calculated from V GS and . It is worth pointing out that the present method for mo-bility extraction is quite general. For instance, in the case of constant mobility  =0 this method is equivalent to theconventional extrapolation of transfer curve.

The Arrhenius plots of C  and the corresponding activa-

tion energy  E  A for Cu and Au are shown in Figs. 3a and3b, respectively. The fitting straight lines in Figs. 3a and

3b show a common intersection point in both cases, imply-ing that C  follows the Meyer-Neldel rule11,12

C  = C 0 exp−E  A

kT  = C 00 exp E  A

 E MNexp−

E  A

kT 

= C 00 exp E  A1

 E MN −

1

kT  , 4

where C 0 Y  intercept and C 00 intersection point are theexponential prefactors, E MN is the Meyer-Neldel energy, k  isthe Boltzmann constant, and T  is the temperature. E MN canbe calculated from the intersection point. E MN for Cu is aslow as 22 meV, which is lower than k T at room temperature26 meV. From Eq. 4, C  is expected to increase and de-crease with E  A when kT  E MN and kT  E MN, respectively.This behavior is clearly presented in Fig. 3a for Cu. Theeffect of the C  decrease with increasing V GS for instance,Cu at 300 K may counteract the effect of the charge densityincrease, and result in a weak V GS dependence of RC  as dem-

onstrated in Fig. 2a. In contrast, the high E MN 55 meV forAu will lead to a strong V GS dependence of  RC  at all mea-sured temperatures Fig. 2b.

The observation of the Meyer-Neldel rule in the presentstudy can be interpreted with an exponential distribution of trap states in the contact region.4,10,13 The physical meaningof E MN is the width of trap distribution. Thus the trap distri-bution at the Cu contact is much narrower compared with theAu contact see Fig. 4. On the other hand, C 00

FIG. 2. Color online a Experimental data of W -normalized contact resis-tance as a function of gate bias at 300 K markers. Solid line denotescalculation data based on the theory in Ref. 10, using RC = C V GS− V T 

− −1 inwhich  = E MN / kT −1 and C  is an adjustable constant. b W -normalizedcontact resistance vs gate bias at different temperatures from 160 to 300 Kwith 20 K interval. Each line contains 20 data of  RC  obtained from TLMplots.

FIG. 3. Color online Arrhenius plots of contact mobility for a Cu and b

Au contacts. The activation energy is calculated from the slope of fittingstraight line, E MN and C 00 are derived from the intersection point.

FIG. 4. Color online Schematics of trap distribution and trap filling in thecontact region at a low gate bias and b high gate bias. N T  E 

=  N T 0 /  E MNexp− E  /  E MN denote exponentially distributed density of trapstates solid line, where E MN is derived from Fig. 3 and N T 0 is estimatedfrom C 00. nT  E  = N T  E  f  E  denote density of trapped charges brokenline, where f  E  is the Fermi-Dirac distribution at 300 K. The E F  position

vertical dotted line is selected for setting the total density of trappedcharges integration of nT  equal in both cases of Cu and Au.

203508-2 Wang et al. Appl. Phys. Lett. 91, 203508 2007

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= f  N HOMO /  N T 0 based on the multiple trapping and releasemodel,10,14 where  f  is the free carrier mobility along thevertical direction, and N HOMO and N T 0 are the density of HOMO states and the total density of trap states integrationof trap distribution in the contact region, respectively. Thereare two factors that may lead to the low C 0010−3 cm2 /V s as observed in Figs. 3a and 3b. One is therelatively low  f  in the present case because the vertical

direction is perpendicular to the 

-overlap direction. Theother one is high N T 0. For instance, assuming  f 

=0.1 cm2 /V s and N HOMO=1018 cm−3, N T 0 is in the order of 1020 cm−3 for both contacts.

Using the derived E MN and estimated N T 0, the density of trap states  N T  in the contact region is schematically plottedin Fig. 4. The trapped charges density nT  is dependent on N T  and the E F  position, that is, nT  E = N T  E  f  E , where E  isthe energy and f  E  is the Fermi-Dirac distribution. At lowV GS, there are much fewer deep traps for Cu, so that E F  iscloser to the HOMO as shown in Fig. 4a. As a conse-quence, the Cu TFT shows smaller RC  and accordingly larger I  D. The increase of  V GS will move E F  toward the HOMO by

trap filling, leading to an increase of both the free carrierdensity n f  and nT  see Fig. 4b. However, C  f n f  / nT  isdetermined by the ratio n f  / nT ,

10,14 and the change of thisratio with E F  is strongly dependent on the trap distributionprofile. If the trap distribution is very steep  E MNk T, asthe situation of Cu at 300 K, the nT  increase due to the E F 

shift will be in excess of the corresponding n f  increase. Con-sequently, C  will decrease with increasing V GS as shown inFig. 3a. Finally, an exponential trap distribution actuallyimplies a V GS-dependent mobility with the same form of Eq.3, where  = E MN / kT −1.10 According to this theory, thecalculation for RC  at 300 K is shown in Fig. 2a, and a goodconsistency is seen. Therefore, the physical picture in Fig. 4can well interpret the experimental results.

High N T 0 in the contact region might arise from thedense structural defects close to the metal/organic interface.The possible causes of the defects include original surface

defects before the metal deposition, vacancy and distortionafter the metal deposition, and molecular polarization due tothe image force. More experiments are needed to clarify theunderlying mechanism of the different trap distributions be-tween the Cu and Au contacts.

In conclusion, top-contact pentacene TFTs with Cu elec-trodes show smaller RC  in comparison with the correspond-ing Au TFTs. We suggest that the large RC  in OTFTs is due

not to the hole injection barrier but to a low mobility regionclose to the metal/organic interface. Different exponentialdistributions of trap states at the Cu and Au contacts are welldefined by our analysis, and the small RC  of the Cu contact isattributed to the narrow trap distribution.

This work was supported in part by the Grant-In-Aid forScientific Research Nos. 16GS50219, 17069004, and18201028 from the Ministry of Education, Culture, Sport,Science and Technology of Japan.

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