Gated Four-probe Measurements on Pentacene Thin-film Transistors

JOURNAL OF APPLIED PHYSICS VOLUME 96, NUMBER 12 15 DECEMBER 2004

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Gated four-probe measurements on pentacene thin-film transistors:Contact resistance as a function of gate voltage and temperature

Paul V. Pesavento, Reid J. Chesterfield, Christopher R. Newman, and C. Daniel Frisbiea)

Department of Chemical Engineering and Materials Science, University of Minnesota,Minneapolis, Minnesota 55455

(Received 21 June 2004; accepted 20 August 2004)

We describe gated four-probe measurements designed to measure contact resistance inpentacene-based organic thin-film transistors(OTFTs). The devices consisted of metal source anddrain electrodes contacting a 300-Å-thick pentacene film thermally deposited on Al2O3 or SiO2

dielectrics with ap-doped Si substrate serving as the gate electrode. Voltage-sensing leads extendinginto the source-drain channel were used to monitor potentials in the pentacene film while passingcurrent during drain voltagesVDd or gate voltagesVGd sweeps. We investigated the potential profilesas a function of contact metallurgy(Pt, Au, Ag, and Ca), substrate chemistry,VG, and temperature.The contact-corrected linear hole mobilities were as high as 1.75 cm2/V s and the film sheetresistance and specific contact resistance were as low as 600 kV /h and 1.3 kV -cm, respectively,at high gate voltages. In the temperature range of 50–200 K, the pentacene OTFTs displayed anactivated behavior with activation energies of 15–30 meV. Importantly, the activation energyassociated with the contact resistance showed no dependence on contact metal type at high gatevoltage. Also, the activation energies of the contact resistance and film resistance wereapproximately the same. Above approximately 200 K and below 50 K, the mobility was essentiallytemperature independent. ©2004 American Institute of Physics. [DOI: 10.1063/1.1806533]

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I. INTRODUCTION

In the last few years, work by a number of reseagroups has shown that the properties of the source andcontacts in organic thin-film transistors(OTFTs) can have aimportant effect on the overall device performance.1–8 Ef-forts have been made to lower the contact resistances inFTs by doping the semiconductor at the metal-orginterface6 or by increasing contact area by switching fr“bottom contact” to “top contact” device geometries.7,8 How-ever, to date, there have been relatively few direct meaof contact resistance in OTFTs. Jackson and co-workercently published transmission line studies that showedspecific contact resistivities were approximately 40 kV -cmfor Au bottom contacts8 and 3 kV -cm for Au top contacts7

on pentacene.A powerful alternative to the transmission line metho

to map the potential profile across operating OTFTs, asbe done using Kelvin probe force microscopy(KFM).9–11

Sharp voltage drops, which are observed in the prohighlight the resistive bottlenecks to current flow. Forample, large contact resistances lead to large voltage drthe contact-semiconductor interfaces. Dividing the voldrop by the current yields the contact resistance. RecNicholset al.9 and Puntambekaret al.10 have reported potetial profiling of pentacene OTFTs by KFM. In additioBurgi et al. have demonstrated potential mapping of pthiophene OTFTs as a function of temperature, which ala better assessment of the carrier transport mechanismscontacts and in the film.11

Here, we report variable-temperature contact resist

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Electronic mail: [email protected]
0021-8979/2004/96(12)/7312/13/$22.00 7312

aded 18 May 2011 to 115.249.41.221. Redistribution subject to AIP licens

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measurements on pentacene OTFTs using a gated fourtechnique, in which the channel potential is sensed atpoints between the source and the drain. While this apprdoes not provide the exquisite detail of a full potential mping by KFM, it is relatively easy to implement and allovariable-temperature resistance measurements withouneed for a UHV-AFM. Four-terminal measurementsa-Si:H TFTs have been extensively reported,12–15 and recenstudies of field-effect transistors based on organic scrystals have also exploited four-probe geometries,16–19 butso far there have been very few four-terminal measuremon OTFTs. Recently, Yagiet al. reported an initial study ofour-terminal pentacene OTFTs.20 Our work presented heincludes a full study of contact resistance in top contacttacene OTFTs as a function of gate voltage, temperaturemetal work function. In addition, we report the temperadependence of the contact-corrected linear mobility fortacene films over the range of 4–300 K.

II. EXPERIMENT

A. Sample preparation

1. Substrate preparation

Pentacene thin films were grown on silica and alumsubstrates purchased from Silicon Valley MicroelectronInc (SVMI). The silica substrates consisted of 3000 Åthermal oxide onp-doped (Boron) silicon wafers. As received, the wafers had an oxide on each side: one sideished and the other unpolished. The polished side hadroughness not more than 2 Å(typically ,1.5 Å) in a 535 mm window as measured via atomic force microsc
(AFM). In order to use the wafers as backside gate elec-
© 2004 American Institute of Physics

e or copyright; see http://jap.aip.org/about/rights_and_permissions

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J. Appl. Phys., Vol. 96, No. 12, 15 December 2004 Pesavento et al. 7313

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trodes, the oxide on the unpolished side was removed aohmic contact was fabricated. First, to fabricate theelectrode, photoresist was spun on the polished sideprotective layer. The wafers were then dipped in a hydluoric solution to strip the oxide from the unpolished sThe photoresist was then stripped by an overnight acesoak, sonication in an acetone-methanol-isopropanoquence, followed by a de-ionized water spin rinse/dry anO2 plasma clean. An ohmic gate contact was fabricatedepositing 100 Å Al and 900 Å Au, then performing a rathermal anneal at 450 °C for 60 s to diffuse the Al intosilicon. The gate oxide capacitance was measured to bproximately 10 nF/cm2. The Al2O3 substrates consisted1500 Å of plasma-enhanced chemical vapor depo(PECVD) Al2O3 on p-doped(Boron) silicon wafers. As received from SVMI, the alumina had a rms roughness ofproximately 10–20 Å in a 535 mm window via AFM. Inorder to facilitate good film growth, the alumina was thpolished by Polishing Solutions International LLCchemical mechanical polishing until the rms roughnessless than 2.5 Å. After polishing, the alumina thicknessnominally 1330 Å±35 Å, being thicker at the center ofwafer and thinner at the edges. Because the Al2O3 dielectricwas deposited on the polished side of the wafer andwas no thermal oxide on the back, the fabrication ofohmic contact to the gate electrode backside required ometal deposition and a rapid thermal anneal. The gate ocapacitance was measured to be approximately 40 nF/2.

2. Pentacene purification

Pentacene source material was purchased from SAldrich. It has been shown that source material puritykey issue in device performance. The as-received pentwas only 97% pure so it was purified in a temperatgradient sublimation apparatus similar to that used pously to grow single crystals of organic semiconductors21,22

The experimental apparatus consisted of an outer quartzapproximately 60 cm in length and 5 cm in diamewrapped with heating tape and insulation. One end otube was fed with an ultrapure carrier gas and the otheited to a mineral oil bubbler. A small inner quartz tube seras a holder for the source, whereas another longer one sas the location for material deposition. Key-operating paeters used to purify the material were carrier gas(Ar99.999%), pressure s1 atmd, Ar flow rate 50 mL/min),source temperatures280–290 °Cd, and deposition temperture (approximately 250 °C). The sublimed material was dposited in several distinct bands, those with higher subltion temperatures depositing first and those with losublimation temperatures traveling further down the tPurified pentacene deposited approximately 10–15downstream from the source. The first crop of pentaceneharvested from the growth tube and was used as the smaterial for a second gradient sublimation. A total of thsublimations were carried out and the final purified mat
was used as the source for the thin-film evaporations.

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3. Pentacene film deposition

The growth of pentacene thin films was accomplisvia vacuum sublimation. The experimental apparatussisted of a vacuum thermal evaporation system from DeVacuum(model DV-502) equipped with a diffusion pump,Radak I source furnace from Luxel Corp., and a water coand resistively heated copper substrate block. Cartridgeheaters from Watlow were employed for substrate bheating. The temperature of the substrate block wassured by a K-type thermocouple embedded within thestrate block and controlled by a Eurotherm on/off style tperature controller. Substrate temperature was held wapproximately ±1 °C of the desired deposition temperaThe purified pentacene source charge was placed in amina crucible and loaded into the Radak furnace. The sand alumina substrates were cleaved into approximat31 cm pieces. Silicon dust generated by the cleavingcess was blown from the surface by a stream of dry nitrgas. Silicon and dust particle removal was verified by AStainless-steel shadow masks were clipped to the sampdefine the area of deposition. Film masking was perforto reduce leakage current to the gate and to provideproper potential probe alignment, as described afterwThe thickness during deposition was monitoredin situ via anInficon quartz crystal microbalance(QCM) thickness measurement system. The shutter was kept closed untisource temperature was approximately 120 °C. Koperating parameters of the deposition process were chapressure(typically 1310−6 and 4310−7 Torr at the start anend of deposition, respectively), substrate temperatus80 °Cd, source temperature(150 and 180 °C at the start aend of deposition, respectively), deposition rat(,0.1 Å/s for the first 60 Å, slow ramp to 0.4 Å/s100 Å, then 0.4 Å/s to 300 Å), and final thickness300 Åd. Immediately after reaching a thickness of 300the shutter was closed and substrate heating was termiThe substrate reached 25 °C in approximately 5 min.

4. Metal contact deposition

Deposition of the top contact metal electrodes(Au, Pt,Ag, and Ca) was accomplished via thermal evaporation.and Pt were deposited in a turbo-pumped vacuum theevaporation system from Denton Vacuum(model BenchtoTurbo III), which was equipped with a wound tungsten fiment source from R. D. Mathis and a water-cooled subsblock. Ca and Ag, being more reactive, were depositedcustom built cryo-pumped vacuum thermal evaporationtem equipped with a Radak II source furnace from LuCorp. and a water-cooled substrate block. Etched sishadow masks were clipped to the samples to define theof contact deposition. Thickness during the depositionmonitored in situ via a QCM from Inficon. The keyoperating parameters of the deposition process were chapressure(Au and Pt: 2310−6 and 5310−6 Torr at the starand end of deposition, respectively, Ca and Ag: 1310−7 and5310−7 Torr at start and end of deposition, respectiv)and substrate temperatures25 °Cd. For Au, Ag, and Ca, th
deposition rate was 0.1 Å/s for the first 10 Å, then ramped

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7314 J. Appl. Phys., Vol. 96, No. 12, 15 December 2004 Pesavento et al.

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to 0.4 Å/s for 50–400 Å. For Pt, the deposition rate wheld at approximately 0.05 Å/s during the entire deposto minimize heating effects on the pentacene film due tohigh source temperature. For Pt, Ca, and Ag, all metalscapped with a layer of Au. The thicknesses used in eachwere Au s400 Åd, Pt:Au s50:350 Åd, Ca:Au s200:200 Åd,and Ag:Aus200:200 Åd. Ca and Ag were capped with Auprevent oxidation, whereas Pt was capped with Au to ca mechanically robust contact because of difficultiesevaporating greater thicknesses of Pt. Figure 1(a) depicts thefinal bottom gate, inverted-staggered(top contact) TFT con-figuration with the in-channel probes used to measurechannel potential of the operating TFT. Figure 1(b) shows atop view optical microscope image of the completed de(width and length of the device active area are 1000100 mm, respectively). Figure 1(c) illustrates how the channel probe contacts were aligned with the edge of the mapentacene film. Here, the potential probes only contacfilm directly in and near to the channel area. If the pro

FIG. 1. (a) Schematic cross section of inverted-staggered(top contact) de-vice geometry. Also shown are the potential probe positions at 1/3 L2/3 L. (b) Optical micrograph of the completed OTFT device strucshowing the dielectric, patterned film, source and drain electrodes, andnel potential probes.(c) Circuit schematic for the gated four-probe Tmeasurement configuration. Channel dimensions areW=1000mm, L=100mm, LV1=33 mm, andLV2=66 mm. During ID-VG sweeps, the sourcelectrode is held at ground andVD is held constant asVG is swept. Curreninto the source and out of the drain electrodes is monitored. The chpotential is measured via high-impedances.1014 Vd electrometers. Alsshown is the close alignment of the film to the potential probes wensures that the potential monitored atV1 andV2 is only that of the channregion.

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other than in or near the channel, the potential sensed belectrometers would include fringing field effects becathe potential sensed is an average of every portion of thewith which the probes were in contact. Proper positioninthe channel probes ensures only the channel potentreported.

B. Film characterization

Characterization of the pentacene thin films was caout by AFM and X-ray diffraction(XRD) under ambienconditions. AFM imaging of film morphology was accoplished with a Digital Instruments Dimension 3100. Topraphy and amplitude images were acquired while imaginTapping Mode with scan sizes of 10–20mm and a scan raof 2 Hz using NSC12 “B” cantilever tips purchased frMikroMasch. The “B” cantilever typically tuned at a resnant frequency near 350 kHz.

XRD was used to determine the filmd-spacing anphase composition using a Siemens D500 X-ray difframeter with a Cu Ka s1.54 Åd X-ray source. The samplwere approximately 131 cm2 in size with a pentacene fildeposited across the entire surface. The line sourcebeam struck the sample with an approximate beam of1 cm31 mm. Standardu-2u scans were acquired from 3°31° 2u at a step size of 0.05° 2u and a dwell time of 4 s/steThe number of counts at the maximum of the thin-film phpeak was typically within the range of 13104–33104

counts for a 4 s dwell time. All the XRD data analysis wperformed in the software packageJADE from MaterialsData, Inc.

C. Electrical measurements

1. Current-voltage characterization

Electrical characterization was carried out by sweethe drain voltagesVDd at a constant gate voltagesVGd orsweepingVG at a constantVD while monitoring the draicurrentsIDd and probe potentialssV1,V2d. Figure 1(c) illus-trates the electrical connections for the completed gatedprobe structure. Keithley 236 source-measure unitsused to applyVD and to measure the sourcesISd and draincurrents. Keithley 6517A high-impedances.1014 Vd elec-trometers were used to monitorV1 andV2 via channel probeand to sourceVG. All leads except the gate used guardshielded triaxial cables instead of shielded coaxial cablminimize electrical noise and leakage currents. A compconnected to all instrumentation via a general purposeface bus running custom National Instruments LabView cwas used to control the operation of all units. The dataexported to and analyzed in customMATLAB code. TypicaID–VG parameters used are as follows: for Al2O3 dielectric,VD=−4 or −5 V, VG= +10–−40 V or 0–−50 V, voltagstep=0.5 V, sweep rate,1 s/step whenID,10−9 A and,0.25 s/step whenID.10−9 A, COX=40 nF/cm2; for SiO2

dielectric,VD=−10 V, VG= +25–−75 V or 0–−100 V, voltage step=2 V, and COX=10 nF/cm2, all other paramete

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2. Variable-temperature transport measurements

All variable-temperature characterization was carriedin a turbo-pumped Desert Cryogenics vacuum probe stin ambient light with a chamber base vacuum in10−6–10−7 Torr range. No noticeable differences wereserved upon operating the devices in the dark. The dewere left in vacuum overnight before characterization beBeginning at room temperatures295 Kd, the temperature wadecreased to 280 K, then down to 10 K in steps of 10°nally reaching 4.2 K. After the completion of charactertion at 4.2 K, the temperature was increased back to 29checking the devices at several temperature intervals tsure that their performance was not affected by the temture cycling. Data from the devices that underwent sigcant degradation were excluded from the study.

III. RESULTS

A. Structural characterization: AFM and XRD

Figure 2 shows AFM topographs of pentacene thin fideposited on(a) dry, thermal SiO2 and(b) PECVD Al2O3 ata substrate temperature of 80 °C. There are several didifferences in the film morphology on the two different sstrate dielectrics. On Al2O3, the grain size is larger thanSiO2, the grains appear more angular, the width-to-heaspect ratio(W:H) is greater by approximately a factor osW:H=silica−10mm:40 nm;alumina−40mm:20 nmd,and cracking is evident to a greater degree. Close exation of individual grains reveals a distinct layered structTerrace heights of each layers17.5±4 Åd examined by AFMagree within experimental error with XRD measuremgiven afterwards.

Figure 3 shows the diffraction spectrum of340-Å-thick pentacene film on Al2O3 and SiO2. The twocommonly listed “thin-film” phases were present at 2u val-ues of 5.72° and 6.12° with correspondingd-spacings of 15.and 14.4 Å, respectively(the bulk structure of pentacene23

has ad-spacing of 14.1 Å). The areas of the individual difraction peaks indicated that the film was largely compoof the 15.4 Å phase(74.8% on Al2O3 and 85.8% on SiO2).

B. Electrical measurements

1. Current-voltage characterization

In order to examine the effects of metal work functon contact resistance in pentacene OTFTs, we studiecurrent-voltage characteristics using Au, Pt, Ag, and Cacontacted devices. The metals were found to fall in twoegories: those that produced low-resistance contacts(Au, Pt,and Ag) and high-resistance contacts(Ca). In Figs. 4–9, Auand Ca-contacted devices have been chosen to illustraamples of low- and high-resistance contacts, respectFigure 4 shows the output characteristicssID–VDd and transfer characteristicssID–VGd of OTFTs consisting of a pentcene thin film deposited on SiO2 and Al2O3 dielectrics withAu and Ca top contacts. In Figs. 4(a) and 4(b), there are twomajor differences between Au and Ca top-contacted deon SiO2 and Al2O3. First, the magnitude of the current
high VG produced by the devices with Ca contacts is signifi-

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cantly less than the devices with Au contacts[ID for Ca con-tacts is scaled by a factor of 53 sSiO2d and 103 sAl2O3d forvisibility ]. Second, the Ca-contacted devices exhibit grecurrent hysteresis between forward and reverse drain vosweeps, indicating increased charge-carrier trapping.

Devices fabricated on SiO2 obey a nearly ideal squarlaw behavior in saturation(i.e., ID

SAT~VG2), as shown in

Fig. 4(a), whereas devices fabricated on Al2O3 show deviations at highVG, as evident in Fig. 4(b). Linear field-effecmobilities for Au-contacted and Ca-contacted devicesAl2O3 are 1.75 and 0.6 cm2/V s, whereas for the devicesSiO2, mobilities are 0.5 and 0.25 cm2/V s, respectivelyThus, part of the differences in the maximum currents caascribed to variations in film mobility. Although pentacefilms for all of our devices were deposited in the same mner, we typically observed run-to-run variations in mob

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FIG. 2. AFM topographs of pentacene thin films deposited at a subtemperature of 80 °C on(a) SiO2 and(b) Al2O3. Films grown on(b) Al2O3

substrates(that have not seen any processing) show large grains which haa larger width-to-height aspect ratio and show sharper facets thangrown on(a) SiO2 (that has been exposed to photoresist). Both show evidence of cracking due to thermal-expansion differences between thand the substrate when cooling from 80 °C to room temperature aftedeposition. Maximum grain sizes observed on(a) SiO2 and (b) Al2O3 areapproximately 10 and 40mm, respectively.

in the range of 0.2–1.75 cm/V s on bare Al2O3 and SiO2.


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Figures 4(c) and 4(d) show that there are significant dferences in the onset voltages(V0, the gate voltage at whicfree carriers are first present at the pentacene-insulatorface) for the Au- and Ca-contacted devices. TheV0 valuesfor Au-, Ag-, and Pt-contacted devices were approximathe same on both substrate dielectrics. It can also be seethere is a large current hysteresis between forward an

FIG. 3. Thin-film XRD (Log Counts vs 2u) of pentacene thin films depoited on SiO2 and Al2O3 dielectrics. The traces are displaced verticallyclarity. Both thin-film phases are visible. Good film ordering is indicatethe four additional higher-order peaks observed.

FIG. 4. Current vs voltage plots for pentacene thin films with Au anddifferent gate voltagessVGd. Note that for the Ca top contact devices,ID hasreverse sweep directions are indicated by the arrows for all plots. Ea5–10 min perID-VD set (at all VG). (c) and (d) Drain currentsIDd as a
approximately 1–2 min each.

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verse traces. Note that in the “on” regionsVG.V0d, the for-ward trace current is greater than the reverse trace cubut in the “off” regimesVG,V0d, the opposite is true, i.ethe reverse trace current is greater than the forwardcurrent. This behavior is indicative of carrier trapping infilms.

2. Potential measurements

During the standard three-terminal current-voltage msurements, the channel potential was monitoredin situ at twopoints in the channel via the potential probes. At a conVD, VG was swept and the channel potential was measureachVG step. Knowing the channel potential at two poiV1 and V2, a linear extrapolation of the potential profileeach contact was performed. From the difference betthe applied potential at each contact(VD=−4 V for Al2O3

andVD=−10 V for SiO2) and the extrapolated potential vues for the contacts, the potential drops at the sourcedrain, DVS and DVD, were calculated according to the flowing equations:

DVS= FV1 −sV2 − V1dsL2 − L1d

L1G − VS, s1d

op contacts.(a) and (b) Drain currentsIDd as a function of drain voltagesVDd atscaled by a factor of 53 sSiO2d and 103 sAl2O3d for visibility. Forward and

orward and reverse sweep was acquired in approximately 1–2 min ftion of gate voltagesVGd. Forward and reverse sweeps were acquire

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DVD = VD − FV2 +sV2 − V1dsL2 − L1d

sL − L2dG , s2d

where VS, VD, V1, and V2 are the voltages at the sourdrain, and potential probes, respectively.L1, L2, andL are thedistances from the source electrode to the first poteprobe, the second potential probe, and the drain electrespectively. Since we are also interested in examiningpotential drop across the film, we define

DVFilm =sV2 − V1dsL2 − L1d

L, s3d

whereDVFilm is the actual drain bias across the film(i.e., theapplied drain bias less the drops at the contacts).

Figures 5(a) and 5(b) represent plots of measured chnel potentialsV1 and V2 and calculatedDVS and DVD as afunction of VG for Au and Ca top-contacted devices. Hethe potential data are truncated just afterV0, where the device initially begins to conduct. Before this point, the volta

FIG. 5. Potential data and surface plots for devices with Au and Capotential probes as a function ofVG. Shown are the probe data,V1 andV2,DVD. The positions ofV0 andVT (forward sweep) are also indicated.(c) and(within the channel(d). The continuous surface shows the values of thedashed lines,VD,extrap and VS,extrap, on the left-front and right-rear facescontacts. The differences between these values and the respective ap(valid at VG@VD only).

probes measure potentials(e.g., +100 V) that are outside the


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range of the applied source or drain biases. We believethese results from static charge buildup on the pentacensurface and in the cables of the measurement systemthe film is relatively nonconductive. As soon as an apprebly conducting channel is established, the probes reportentials characteristic of the channel. Data on both SiO2 andAl2O3 dielectrics show the same trends and, for brevity,plots for devices fabricated on Al2O3 are shown. The valueof V0 andVT for the same device are indicated on eachure. The top scales of the plots in Figs. 5(a) and 5(b) showthe VG-induced carrier density forVG.V0. It is clear that aall carrier densities, the voltage drops at the Ca contactgreater than the voltage drops at the Au contacts.

These data are visualized differently in Figs. 5(c) and5(d), which show surface plots of channel potential afunction of both channel position and gate voltage. In thplots, a channel position of 0 and 100mm corresponds to thsource and drain electrodes, respectively. The sourcetrode is held at a potential of 0 V and the drain electrod

ontacts fabricated on Al2O3. (a) and (b) Potential data acquired via the chanwith the extrapolated contact potential differences at the contactsDVS and

rface plots of the channel potential as a function of gate voltage and ped source and drain potential and the channel potentialsV1 andV2. The solid andach plot represent the estimated film potential just after the source acontact potential,VD andVS, represent the potential drops due to contact resis

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held at −10 V for SiO2 and −4 V for Al2O3. Potential probes


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1 and 2 were at 32 and 69mm for the Au-contacted devicon Al2O3 and at 30 and 66mm for the Ca-contacted devion Al2O3. Each probe is approximately 5mm in width. At agiven VG, the straight lines connecting the source, poteprobe 1, potential probe 2, and the drain give a qualitaestimate of the channel potential profile. Also included isoutline of a surface generated by extrapolating the potebetween potential probes 1 and 2 to the source andpositions. It is the difference between the extrapolated va(VS,extrap andVD,extrap) and the applied potential at each ctact that gives us the values ofDVS and DVD. From thesurface plots, it is easy to see that the Ca contacts indmuch greater potential drop than do the Au contacts. Tthe contact effects in the pentacene OTFTs are signifibeing much larger for the reactive metal Ca than fornoble-metal Au. Note also thatDVD.DVS, contrary to whahas been observed in bottom-contacted polythiopTFTs.11

3. Contact resistance

With the knowledge of the total current flowing throuthe device and the potential drops across the film andtacts, the resistance of the source contact, drain contac

FIG. 6. Film resistance and source and drain contact resistances astion of VG for devices with(a) Au and (b) Ca top contacts. For Au tocontacts, the contact resistance(at high VG) is approximately an ordermagnitude less than that for Ca top contacts.

the film can be calculated using Ohm’s Law,Ri =DVi / ID,


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wherei is S, D, and Film. Figures 6(a) and 6(b) for Au andCa top-contacted devices, respectively, show plots of rtance as a function of gate voltage. Initially, at lowVG, thefilm resistancesRFilmd and contact resistances(RD andRS) allbegin at large values of resistance withRD much larger thaRS. As VG is increased, the resistance of both the filmcontacts decrease rapidly and finally saturate. Note thadata at lowVG (less thanVT) could be misleading becauwhenuVG−VTu, uVDu, the device is in saturation and pinchoccurs. The depletion region associated with pinchoff induces a high-resistance region near the drain electrodethe potential profile across the film is not linear. The gfour-probe technique presented here does not decoupresistance due to the saturation-induced depletion refrom the contact resistance. At lowVG, RD is effectively acombination of the two. The lowest observed contact rtance was approximately 0.7 kV -cm for Au top contacts anAl2O3 dielectric. Table I summarizes the calculated rotemperature values ofRFilm and the sum of the contact restancessRS+RDd for devices on Al2O3 and SiO2 and withcontact metals Au, Pt, Ag, and Ca. Values are given intrinsic units and intrinsic units. Devices with Ag top contaon Al2O3 dielectrics consistently did not yield reliable dvices and were omitted.

4. Variable-temperature measurements

a. Current-voltage behavior.Figure 7(a) shows ID-VG

plots at different temperatures for Au top-contacted devon Al2O3. All other devices yielded the same trend as thetop-contacted pentacene device fabricated on Al2O3 and, forbrevity, are not shown. As temperature was decreased

TABLE I. (a) Film resistancesRFilmd and (b) total contact resistancesRS

+RDd in extrinsic and intrinsic units(in parentheses) for all contact metaland substrate dielectrics examined. Extrinsic units for all values are insVd. Intrinsic units are given as(a) film sheet resistancefRFilmsW/Ld=V /hg and (b) contact resistivityfsRS+RDdW=V -cmg. Resistance valuewere acquired at sample biases of:VG=−40 V andVD=−4 V for Al2O3,VG=−75 V andVD=−10 V for SiO2.

(a)Film resistancesRFilmd

Substrate CasVd

sV /hd

AgsVd

sV /hd

AusVd

sV /hd

PtsVd

sV /hdAl2O3 2.503105

s2.503106d(…)(…)

6.003104

s6.003105d1.003105

s1.003106dSiO2 5.003105

s5.003106d2.003105

s2.003106d3.003105

s3.003106d1.503105

s1.503106d

(b)Total contact resistancesRS+RDd

Substrate CasVd

sV -cmd

AgsVd

sV -cmd

AusVd

sV -cmd

PtsVd

sV -cmdAl2O3 3.753105

s3.753104d(…)(…)

1.303104

s1.303103d4.003104

s4.003103dSiO2 8.503105

s8.503104d5.503104

s5.503103d2.253104

s2.253103d6.503104

s6.503103d

c-

295 to 4.2 K, several important changes were noticed in the


in-easxt,

the

andse

oxi-atur

eure.

-bot

i-

n in

em-trate

orFig.

e

d Pt-proxi-anceactedm re-

and,ases.atvolt-

catedde-rease

ction

de-from


Downlo

current-voltage characteristics. First, the on currentcreased, peaked at approximately 270 K, and then decrby approximately four orders of magnitude by 4.2 K. Nenoticeable shifts in V0 are also seen betweenID-VG traces at different temperatures. Plots ofV0 andVT asa function of temperature for pentacene devices with AuCa top contacts and Al2O3 substrate dielectric show thechanges in more detail in Fig. 7(b). It was observed thatV0

and VT were nearly temperature independent until apprmately 150 K and then increased rapidly as the temperdecreased.

b. Mobility as a function of temperature.We examined thcontact-corrected film mobility as a function of temperatFigure 8(a) shows the linear field-effect hole mobility(cor-rected and uncorrected for contact resistance) for a Au-contacted pentacene TFT on Al2O3. As temperature is decreased, the most dramatic feature of the data is thattraces remain constant(or increase slightly) and then drastcally decrease below 200 K. Figure 8(b) and its inset showArrhenius plots of the same corrected mobility data showFig. 8(a). The slope in the 50,T,200 K regime(see inset)yields activation energies in the range ofEA<15–25 meVfor pentacene films on Al2O3 and 20–30 meV on SiO2. Fig-

FIG. 7. (a) The temperature evolution ofID–VG curves from 295 to 4.2 Kfor a device with Au top contacts fabricated on Al2O3. (b) V0 andVT as afunction of temperature for devices with Au and Ca top contacts fabrion Al2O3. Both V0 and VT remain fairly constant as the temperaturecreases until approximately 100–150 K, where they begin to incrapidly.

ure 8(b) also shows a third region of transport below ap-


ed

e

h

proximately 50 K where the mobility becomes nearly tperature independent. All other contact metal and subscombinations yield the same trends.

c. Contact resistance as a function of temperature.Figure9(a) showsRFilm, RS, andRD as a function of temperature fa pentacene device with Au top contacts. The data in9(a) reflect the resistance values found at the maximumVG

value at each temperature(i.e., in the linear region of devicoperation). We observed for all devices thatRFilm, RS, andRD

increase with decreasing temperature. For Au-, Ag-, ancontacted devices, the contact resistances were apmately an order of magnitude less than the film resistthroughout the entire temperature range. For Ca-contdevices, the contact resistance is comparable to the filsistance at all temperatures.

Figure 9(b) shows an Arrhenius plot ofRD at varyingVG

for the same Au-contacted device as in Fig. 9(a). Betweenapproximately 180 and 50 K, the resistance is activatedthen it saturates below 50 K(not shown). As VG is increasedRD drops and the slope of the activated region decreData forRS andRFilm yield the same trend. If we now lookhow the activated region slope varies with applied gate

FIG. 8. (a) Uncorrected and contact-corrected linear mobilities as a funof temperature for an Au top-contacted device on Al2O3 dielectric.(b) Ac-tivation plot showing log linear mobility as a function of 1/T. At very lowtemperaturess,40 Kd, the mobility appears to be nearly temperature inpendent. The inset shows a close-up of the activated region50 to 295 K.

age in Fig. 9(b), we can construct a plot of activation energy


nergothwit

easeageance

ved.met-s

nedices

as ex-par-r

vail-iffer-indi-ncesrepa-o-

Aln

ions

rtantostduequentnta-

dBuu-l-s indi-ase.allyfor-elevi-

e ofenta-

ucedpping

iersg thecen-shift

urn,

ice

l


Downlo

as a function of gate voltage, as given in Fig. 9(c). We ob-served that at moderate gate voltages, the activation efor Ca contacts was slightly larger than that for Au. For bCa and Au top contacts, the activation energy decreasedincreasing gate voltage. The value for Ca contacts decrfaster than for Au such that, at the maximum gate voltactivation energy values for the film and contact resist

FIG. 9. (a) Plot of RFilm, RS, andRD as a function of temperature for a devwith Au top contacts on pentacene thin films on an Al2O3 dielectric. (b)Activation plot of logRD vs 1/T at varyingVG. Plots for all contact metaand substrate dielectric combinations yielded similar behavior.(c) EA for thefilm, source resistance, and drain resistance as a function ofVG for Ca andAu top-contacted devices fabricated on Al2O3.

were all within the range of 15–25 meV on Al2O3. This is


y

hd

,

somewhat different from the results obtained by Yagiet al.,where an increased contact activation energy was obser20

Remarkably, measurements performed with all contactals yielded similar values. On SiO2, the activation energieof the film and contact resistance at largeVG were slightlyhigher, 20–40 meV, for all contact metals. Data gleafrom an analysis of Au, Pt, Ag, and Ca top-contacted devfabricated on SiO2 is nearly identical and is not shown.

IV. DISCUSSION

A. Structural characterization: AFM and XRD

Before pentacene deposition, each substrate type wamined via AFM. It was observed that the adventitiousticle density was approximately 1 per 535 mm area foSiO2, whereas it was approximately 1 per 10310 mm areaon Al2O3. An increased particle density increases the aable sites for grain nucleation and can account for the dence in grain size if not the different appearance of thevidual grains. The grain shape and topography differeare effects of processing that occurs during substrate pration (i.e., the SiO2 film growth surface is exposed to phtoresist and solvent-cleaning procedures, whereas the2O3

surface is not). This was confirmed by growing films opristine SiO2 substrates(before the backside gate fabricatstep). Pentacene film growth on these SiO2 substrates wanearly identical in grain size and shape to that on Al2O3.Thus, surface cleanliness after processing is an impofactor in the resulting film morphology. Cracking is mlikely caused by the large thermal-expansion differencesto the elevated substrate temperature and the subsecooling to room temperature upon completion of the pecene film deposition.

The thin-film phases visible withd-spacings of 15.4 an14.4 Å are consistent with measurements performed bychomset al.24 and Knippet al.25 on films deposited at eevated substrate temperature. The integrated peak areacated that the films consisted mainly of the 15.4 Å phThe smaller 14.4 Å peak in the XRD spectrum was typicbut not always accompanied by needle- or pyramid-likemations(not shown) observed in AFM images. A high levof structural ordering was present within the film, as isdent by the large number of higher-order reflections.

B. Bias stress and contact effects at roomtemperature

The transport data in Fig. 4 shows ample evidencboth bias stress and contact resistance effects in our pcene TFTs. For example, theID-VG traces in Figs. 4(c) and4(d) display significant hysteresis ascribed to gate-indbias stress. Bias stress effects involve charge-carrier traon a time scale faster than the total sweep time(approxi-mately 1–2 min). As the forward sweep progresses, carrare captured in trapping sites simultaneously screeningate electric field, reducing the overall free-carrier contration, and creating scattering centers. This results in aof V0 to largerVG as the sweep progresses which, in tgives rise to a reducedID upon reversing theVG sweep(be-
cause the differenceVG−V0 is smaller in the reverse trace at

hwed in

s.ughals-

r-Auor to

impl, andmeticonc-l be

deg o

en a

ges

sily

n is

lm

rnne

edly ahigo Cs, thil-

ngdraiwa

elyvice

deseentedulaye

sede ofd de-cantnate

eatureof or-with-

tststed byacesrface. In thebarrier


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the same values ofVG). It is evident that for our slowsweeps, we observeV0 shifts of approximately 5 V for botAu- and Ca-contacted devices. Pt and Ag devices shosimilar effects. Bias stress effects are well documentea-Si TFTs and have been reported by Knippet al.,25 Matterset al.,26 Yuanet al.,27 and Zilkeret al.28 for pentacene TFTThus, there is significant trapping in pentacene films thothe nature of these traps is currently not known. We notethat the larger off currents on the reverseVG sweeps is indicative of slow extraction of trapped carriers.

In Figs. 4(c) and 4(d), there is also a noticeable diffeence inV0 between the two different metal contact types,and Ca. The negative shift ofV0, when comparing Ca- tAu-contacted devices, is consistent with a larger barrieinject holes at the Ca-pentacene interface. However, a senergy level alignment at metal-pentacene contactsshown in Fig. 10(a), is not accurate. Interface dipoles ainterface states have been found to exist at pentacene-contact interfaces that strongly affect the metal and semductor energy-level alignments.29–31 The role that these fators may play in charge-carrier injection and transport wildiscussed later.

Measurable contact resistance is present to varyinggrees in all of our top-contact pentacene TFTs dependinthe contact metal type. The potential data in Figs. 5(a) and5(b) show that finite potential drops(DVS andDVD) exist atthe contacts for both Au- and Ca-contacted devices evhigh VG. In the case of Au top-contacted devices[Fig. 5(a)],the final values reached at high gate volta(DVS=0.35 V, DVD=0.38 V; at VG=−40 V) are relativelysmall. However, for Ca top-contacted devices[Fig. 5(b)], thedeviation is greater(DVS=0.93 V, DVD=1.32 V; at VG=−40 V). The larger contact potential drops are more easeen in the surface-potential plots in Figs. 5(c) and 5(d). It isnoteworthy that for both devices, the drop at the draigreater than the drop at the source(only slightly for Au),indicating it is more difficult to extract carriers from the fithan to inject them.

The extracted resistance data shown in Figs. 6(a) and6(b) illustrate that at largeVG suVG−VTu@ uVDud, the transistoenters the linear regime where it obeys the gradual chaapproximation, andRD approaches the same value asRS (butremains slightly larger). Au-contacted devices were observto have a contact resistance that is less by approximateorder of magnitude than that of Ca-contacted devices atVG. This was also the case when comparing Pt and Ag ttop-contacted devices. For Au- and Ca-contacted deviceRFilm difference is well within the device-to-device variabity with identical film growth conditions employed duridevice fabrication. Comparing the observed source andcontact resistance with the total device resistance, itfound that at highVG, the contacts account for approximat55% of the total device resistance for Ca-contacted deand 15% of the total device resistance for Au-contactedvices. Possible origins for significant contact resistancein these devices may include an injection barrier preseby nonohmic contacts, interface states, or a contributionto the series resistance of the ungated semiconductor
underneath the contacts. The role that these factors may pla

d

o

es

al-

-n

t

l

nhae

ns

s-nder

in charge-carrier injection and transport will be discuslater. However, our essential finding is that the magnitudthe contact resistance in organic semiconductor-basevices is a function of the contact metal and can be signifienough, in the case of low work-function metals, to domidevice performance.

C. Variable-temperature measurements

From Fig. 7(a), it is apparent that at largeVG, ID firstincreases then decreases with temperature. This is a foften seen in our variable-temperature measurementsganic semiconductors, suggesting at least two regimesdiffering transport mechanisms. TheID decrease with tem

FIG. 10. Schematic of metal and pentacene energy-level alignmen(a)before contact,(b)–(e) after contact.(b) and (c) Ohmic and barrier contacformed by the Au- and Ca-pentacene contact interfaces as construcMott-Schottky theory.(d) and(e) Au-pentacene and Ca-pentacene interfas modified by interface dipoles and interface in-gap states. If no intestates were present, holes would be injected via path 1 into pentacenepresence of interface states, holes may be able to travel through thevia path 2.

yperature below approximately 270 K is indicative of an ac-


dlow

iersthe

ct-of

o---oy

ow-ermizedtions o

caseand-hthe

on 2ap

rvedfromThetheraprface

ra-ningal ea-ntly

ab-l typteal t

t detal-ctivgesatet theetaliouss fu

ten-

ts

r thedia-uctorolemicone

arrierc-this

n en-sure-t the, this

non-pho-

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it-pos-n inely in-ndingeastandtal.hin,n be

s thef

aredCafunc-a-n-thet theare

dnjec-emea-

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the

as isition

esultte the


Downlo

tivated mechanism. Correspondingly, Fig. 7(b) shows a rapiincrease ofV0 and VT as temperature decreased be100 K. The decrease in drain current and shift inV0 andVT isconsistent with the trapping of gate-induced hole carrAdditional evidence for trap-limited transport is seen invariable-temperature mobility data.

In Figs. 8(a) and 8(b), the variable-temperature, contacorrected mobility data reveal three different regimestransport behavior,(1) below approximately 50 K, the mbility is nearly temperature independent,(2) between approximately 50 and 200 K[see inset of Fig. 8(b)], the transport is activated, and(3) from approximately 200 K up troom temperature(see inset), the mobility is again relativeltemperature independent. In region 1, 4.2 K,T,50 K, thethermal energy of the system is very ls0.36 meV,kBT,4.3 meVd. At these extremely low temperatures, most holes no longer have the necessary thenergy to escape from the traps in which they are localTraps in pentacene TFTs have been modeled as distribuof gap states with depths of several tens to hundredmeV.32 The most probable transport mechanism in thisis no longer hopping out of the traps into a conducting bbut rather a tunneling mechanism.33,34 At 4.2 K, the magnitude of the mobility is still quite higs,0.005 cm2/V sd. The temperature dependence andhigh mobility suggest a large density of trap states. Regiillustrates a regime of activated transport extending fromproximately 50 to 200 K. The activation energies obseare consistent with the trapping and release of carriersan extremely shallow distribution of trapping states.slight difference inEA seen in the devices fabricated ontwo different dielectrics is most likely due to an altered tenergy distribution caused by surface chemistry or sucleanliness differences. In region 3, as seen in Fig. 8(a) andin the inset of Fig. 8(b), the mobility plateaus as the tempeture is increased above approximately 200 K. The flatteof the temperature dependence suggests that the thermergy (approximately 17 meV at 200 K) has become comprable to the trap energies in the film and that, consequethe fraction of trapped carriers is decreasing rapidly.

A central observation of this study is that while thesolute value of contact resistance does depend on meta(e.g., Ca contacts are worse than Au contacts), the contacresistance activation energies are nearly equal at largVG,regardless of the metal type, and are essentially identicthe activation energy of the film(see Figs. 8 and 9). Giventhis, the activated behavior observed in Fig. 9 does norive from the potential barriers existing at the mepentacene interface. The close correspondence of the ation energies for the film and contact resistances sugthat the contact resistance arises from thermally activtransport in the film near the contacts. We conclude thadifferent contact resistances observed for dissimilar marise from differences in the detailed structure of the varmetal-pentacene contacts. We elaborate on these issuether, beginning with a consideration of how large the potial barriers are at metal-pentacene interfaces.

Figures 10(a)–10(c) show the energy-level alignmen
and band-bending processes for the semiconductor pentace

.

al.sf

,

-

n-

,

e

o

-

a-tsd

s

r-

and the top contact metals, Au and Ca, before and aftemetal-semiconductor contact is formed; in drawing thegrams, we have simply aligned the metal and semicondFermi levels. In Fig. 10(a), assuming pentacene is a hconductor, it is apparent that Au and Ca should form ohand blocking contacts, respectively. In the case of Ca,expects the injected holes must surmount a large bs,2.2 eVd, as given in Fig. 10(c), and that the observed ativated behavior of the contact resistance would reflectlarge barrier. Given that the contact resistance activatioergies extracted from our variable-temperature meaments in Fig. 9 yielded barriers of 15–40 meV, and thaenergies do not depend on the top contact metal utilizedsimple electronic structure model is contradictory.

We believe that our data are better explained by aidealized metal-pentacene contact interface. In detailedtoemission studies, several major differences from thehave been found to be present in real metal-organic ifaces. First, interface dipoles have been found to existween the metal and organic layers, the magnitude and dtion of which depend upon the metal utilized and the ordethe organic/metal deposition.29,30,35,36For example, deposing Au on pentacene yields a −0.3 eV dipole, whereas deiting Ca on pentacene gives a +0.35 eV dipole, as showFigs. 10(d) and 10(e). The dipoles shift the energy-levalignments of the metal-semiconductor interface and macrease or decrease the existing injection barriers depeon the dipole’s direction. The interface dipoles derive at lin part from the interaction of the organic semiconductorthe electron cloud tail present at the surface of a me37

Second, it was also found that the work function of very tnoncontinuous metal films deposited on pentacene casignificantly less than that of the bulk metal.30 A 30 Å film ofAu on pentacene has a work function of 4.2 eV, whereawork function of bulk Au is 5.1 eV. Similarly, a 30 Å film oAg on pentacene has a work function of 3.8 eV compwith 4.5 eV in the bulk. Thin films of the reactive metaldeposited on pentacene, on the other hand, have worktions that match the bulk metal(presumably because the Cpentacene interface is abrupt). These differences also cotribute to additional energy-level misalignments fromideal case. Energy-level diagrams that take into accounrecent ultraviolet photoemission spectroscopy resultsshown in Figs. 10(d) and 10(e), which show that both Au anCa contacts to pentacene should be blocking with hole ition barriers of 1.0 and 1.7 eV, respectively.30 These largbarriers are not observed in our temperature-dependentsurements, so the charge injection at the metal-pentaceterface cannot be the limiting bottleneck. Thus, the devare not injection limited. This conclusion is also consiswith the fact that the source and drain contact resistancecomparable.

Another important consideration is the fact thatmetal-pentacene interfaces may not be sharp. Figure 11[con-tact (a)] shows an idealized metal-pentacene contactoften drawn in device schematics. However, the deposof metal at high temperatures on organic films may not rin an abrupt interface, but rather, the metal may penetra
neorganic surface and create clusters of metallic and nonmetal-

in

thetalsoretions

0theto1 i

by

eruldthethemay, asscto

ll beidthghectlyg are iandndeu-tac

ateurgy

anonhat

tha0:1rk-canto a

etalrfor-ce theÅ

weres.-entstionticalthatfilm

tioncon-

at thisicro-eciseinter-

lo,port.NSFgram

hys.

P. C.

.

olid-

. A.

.

. Lett.

hys.

.

, T.

nd B.

nd B.

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owth

anter-u-irect


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lic metal atoms(depending on size and geometry), as shownin Fig. 11 [contact(b)]. This has been shown to be truestudies of metals deposited on a small molecule38,39 andpolymer films,40 where noble metals tend to diffuse intofilm and form clusters, whereas lower work-function mechemically react with the organic layer and form a mabrupt interface. Clusters, along with any chemical reacthat might occur due to a highly reactive metal(such as Ca),could form in-gap interface states, as indicated in Figs. 1(d)and 10(e). If the distribution of interface states spannedwidth of the barrier, carriers which would ordinarily needbe thermalized over the metal-pentacene barrier via pathFigs. 10(d) and 10(e) may be able to bypass the barriertunneling or hopping through the in-gap states31 by path 2 inFigs. 10(d) and 10(e). This effectively removes the barrifrom the hole injection process. Alternatively, path 1 costill occur, but hopping in the film near the contact israte-limiting step. Another possibility is that, duringmetal contact deposition, the incoming metal atomspenetrate the film completely to the dielectric surfacesuggested in Fig. 11[contact(c)]. If there exists a continuoumetal contact that reaches completely to the semiconduinsulator interface, the semiconductor-metal interface wiin direct contact with the conducting channel and the wof any injection barriers(and the rate of tunneling throuthem) at the metal-pentacene interface could then be dirmodulated by the varying hole concentration yieldinstrongly VG-dependent contact resistance. Such a pictuconsistent with our observation that both the sourcedrain contact resistances are strongly gate voltage depe(see Fig. 6). However, further investigation into what inflence the extent of metal-organic intermixing has on conresistance properties is needed.

V. CONCLUSIONS

In conclusion, we have studied pentacene TFTs via gfour-probe measurements as a function of contact metallsubstrate chemistry, gate bias, and temperature. AFMXRD have shown the growth of highly crystalline filmsboth Al2O3 and SiO2 dielectrics. We have demonstrated tthe source and drain contact resistances can be morehalf of the total device resistance for devices with a 1W/L ratio, depending on the contact metal. For a low wofunction metal (Ca), the observed contact resistancescause the measured field-effect mobility to differ by up

FIG. 11. (a) Illustration of the idealized metal-organic top contact withabrupt interface.(b) Metal-organic top contact with metal clusters inmixed in the film thickness.(c) Top contact where the electrically continous metal contact reaches the semiconductor-dielectric interface and dcontacts the conducting channel.

factor of 2 from the contact-corrected film mobility. Even for


n

r-

s

nt

t

d,d

n

Au contacts in a top contact configuration, the contact mand device geometry most often employed for high pemance pentacene TFTs, the contact resistance can reduapparent linear field-effect mobility by 10%–15% for 300thick pentacene films. Specific contact resistancesfound to be in the range of 1.3–85 kV -cm at high gate biaBias stress effects are strongly evident inID-VG curves acquired over 1–2 min. Variable-temperature measuremover the range of 4.2–295 K show that transport activaenergies for the contacts and film are essentially idenand are small, on the order of 15–40 meV, indicatingthe contact resistance is determined by transport in thein the vicinity of the contacts. Though the contact activaenergies do not depend substantially on metal type, thetact resistances do depend on the metal. We suggest this due to the details of the metal-pentacene interfacial mstructure, but more work is needed to determine the prconnection between the structure of metal-pentacenefaces and their transport properties.

ACKNOWLEDGMENTS

The authors would like to thank T. W. Kelley(3M) andA. Kahn (Princeton) for helpful discussions and J. A. MerS. E. Fritz, and K. P. Puntambekar for equipment supThis work was supported by the 3M Company and theMaterials Research Science and Engineering Center Pro(DMR# 0212302).

1G. B. Blanchet, C. R. Fincher, M. Lefenfeld, and J. A. Rogers, Appl. PLett. 84, 296 (2004).

2R. J. Chesterfield, J. C. McKeen, C. R. Newman, C. D. Frisbie,Ewbank, K. R. Mann, and L. L. Miller, J. Appl. Phys.95, 6396(2004).

3B. H. Hamadani and D. Natelson, J. Appl. Phys.95, 1227(2004).4D. J. Gundlach, L. Jia, and T. N. Jackson, IEEE Electron Device Lett22,571 (2001).

5R. Schroeder, L. A. Majewski, and M. Grell, Appl. Phys. Lett.83, 3201(2003).

6R. Schroeder, L. A. Majewski, and M. Grell, Appl. Phys. Lett.84, 1004(2004).

7P. V. Necliudov, M. S. Shur, D. J. Gundlach, and T. N. Jackson, SState Electron.47, 259 (2003).

8H. Klauk, G. Schmid, W. Radlik, W. Weber, L. Zhou, C. D. Sheraw, JNichols, and T. N. Jackson, Solid-State Electron.47, 297 (2003).

9J. A. Nichols, D. J. Gundlach, and T. N. Jackson, Appl. Phys. Lett83,2366 (2003).

10K. P. Puntambekar, P. V. Pesavento, and C. D. Frisbie, Appl. Phys83, 5539(2003).

11L. Burgi, T. J. Richards, R. H. Friend, and H. Sirringhaus, J. Appl. P94, 6129(2003).

12C.-Y. Chen and J. Kanicki, IEEE Electron Device Lett.18, 340 (1997).13C.-S. Chiang, C.-Y. Chen, and J. Kanicki, IEEE Electron Device Lett19,

382 (1998).14S.-D. Liu, A. Shih, S.-D. Chen, and S.-C. Lee, J. Vac. Sci. Technol. B21,

677 (2003).15C. E. Parman, N. E. Israeloff, and J. Kakalios, Phys. Rev. B44, 8391

(1991).16V. C. Sundar, J. Zaumseil, V. Podzorov, E. Menard, R. L. Willett

Someya, M. E. Gershenson, and J. A. Rogers, Science303, 1644(2004).17C. Goldmann, S. Haas, C. Krellner, K. P. Pernstich, D. J. Gundlach, a

Batlogg, eprint cond-mat 1(2004).18J. Takeya, C. Goldmann, S. Haas, K. P. Pernstich, B. Ketterer, a

Batlogg, J. Appl. Phys.94, 5800(2003).19V. Podzorov, S. E. Sysoev, E. Loginova, V. M. Pudalov, and M. E.

shenson, Appl. Phys. Lett.83, 3504(2003).20I. Yagi, K. Tsukagoshi, and Y. Aoyagi, Appl. Phys. Lett.84, 813 (2004).21R. A. Laudise, C. Kloc, P. G. Simpkins, and T. Siegrist, J. Cryst. Gr

ly

187, 449 (1998).


wth

T. T

ijk,

et.

ppl.

Phys

urf.

nd A.

nson,

d O.


Downlo

22C. Kloc, P. G. Simpkins, T. Siegrist, and R. A. Laudise, J. Cryst. Gro182, 416 (1997).

23C. C. Mattheus, A. B. Dros, J. Baas, A. Meetsma, J. L. de Boer, andPalstra, Acta Crystallogr., Sect. C: Cryst. Struct. Commun.57, 939(2001).

24I. P. M. Bouchoms, W. A. Schoonveld, J. Vrijmoeth, and T. M. KlapwSynth. Met. 104, 175 (1999).

25D. Knipp, R. A. Street, A. Volkel, and J. Ho, J. Appl. Phys.93, 347(2003).

26M. Matters, D. M. De Leeuw, P. T. Herwig, and A. R. Brown, Synth. M102, 998 (1999).

27J. Yuan, J. Zhang, J. Wang, D. Yan, and W. Xu, Thin Solid Films450, 316(2004).

28S. J. Zilker, C. Detcheverry, E. Cantatore, and D. M. De Leeuw, APhys. Lett. 79, 1124(2001).

29P. G. Schroeder, C. B. France, J. B. Park, and B. A. Parkinson, J.
Chem. B 107, 2253(2003).

.

.

30N. J. Watkins, L. Yan, and Y. Gao, Appl. Phys. Lett.80, 4384(2002).31Y. Hirose, C. I. Wu, V. Aristov, P. Soukiassian, and A. Kahn, Appl. S

Sci. 113, 291 (1997).32A. R. Volkel, R. A. Street, and D. Knipp, Phys. Rev. B66, 195336(2002).33G. Horowitz, Synth. Met.138, 101 (2003).34G. Horowitz, Adv. Funct. Mater.13, 53 (2003).35N. Koch, J. Ghijsen, R. L. Johnson, J. Schwartz, J. J. Pireaux, a

Kahn, J. Phys. Chem. B106, 4192(2002).36I. G. Hill, A. Rajagopal, A. Kahn, and Y. Hu, Appl. Phys. Lett.73, 662

(1998).37N. Koch, A. Kahn, J. Ghijsen, J. J. Pireaux, J. Schwartz, R. L. Joh

and A. Elschner, Appl. Phys. Lett.82, 70 (2003).38A. C. Durr, F. Schreiber, M. Kelsch, H. D. Carstanjen, H. Dosch, an

H. Seeck, J. Appl. Phys.93, 5201(2003).39C. Shen, A. Kahn, and J. Schwartz, J. Appl. Phys.89, 449 (2001).40F. Faupel, R. Willecke, and A. Thran, Mater. Sci. Eng., R.R22, 1 (1998).


Gated Four-probe Measurements on Pentacene Thin-film Transistors

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