Thin Solid Films 519 (2011) 4313–4318

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Experimental and modeling study of the capacitance–voltage characteristics ofmetal–insulator–semiconductor capacitor based on pentacene/parylene

W.T. Wondmagegn a,⁎, N.T. Satyala a, I. Mejia-Silva b, D. Mao b, S. Gowrisanker b, H.N. Alshareef c,H.J. Stiegler b, M.A. Quevedo-Lopez b, R.J. Pieper a, B.E. Gnade b

a Department of Electrical Engineering, University of Texas at Tyler, Tyler, TX 75799, USAb Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX 75080, USAc Department of Materials Science & Engineering, King Abdullah University of Science & Technology, Thuwal 23955–6900, Saudi Arabia

⁎ Corresponding author. Tel.: +1 903 566 7109.E-mail address: wwondmagegn@uttyler.edu (W.T. W

0040-6090/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.tsf.2011.02.014

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 March 2010Received in revised form 30 October 2010Accepted 8 February 2011Available online 18 February 2011

Keywords:PentaceneInterfaceDefectsPoole–FrenkelCapacitanceDispersion

The capacitance–voltage (C–V) characteristics of metal–insulator–semiconductor (MIS) capacitors consistingof pentacene as an organic semiconductor and parylene as the dielectric have been investigated by experi-mental, analytical, and numerical analysis. The device simulation was performed using two-dimensional drift-diffusion methods taking into account the Poole–Frenkel field-dependent mobility. Pentacene bulk defectstates and fixed charge density at the semiconductor/insulator interface were incorporated into thesimulation. The analysis examined pentacene/parylene interface characteristics for various parylenethicknesses. For each thickness, the corresponding flat band voltage extracted from the C–V plot of the MISstructure was more negative than −2.4 V. From the flat band voltage the existence of a significant mismatchbetween the work functions of the gate electrode and pentacene active material has been identified.Experimental and simulation results suggest the existence of interface charge density on the order of3×1011 q/cm2 at the insulator/semiconductor interface. The frequency dispersion characteristics of the deviceare also presented and discussed.

ondmagegn).

l rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Significant progress in organic thin-film transistors (OTFTs) hasbeen reported in the last decade [1,2]. Successful demonstrations havealso been made in the areas of active matrix displays [3], chemicalsensors [4–6], and electronic paper [7]. Some of the fabricationprocesses used include shadow masking [8,9], inkjet printing [10,11],and vacuum or plasma enhanced chemical vapor deposition [12,13].However, improvements in reproducibility [14–16], threshold voltage[17], mobility [17,18], and contact resistance [19,20] are still necessaryfor full implementation of organic field effect transistors. Recently, wehave reported a photolithography-based process to fabricate complexcircuits based on complementary metal oxide semiconductor deviceswith stable threshold voltage (Vth) and lifetime [21,22].

Interface traps [23,24] and gate dielectric surface roughness[25,26] are important parameters which can contribute to variationsin the threshold voltage and mobility in OTFTs. In order to controlthreshold voltage and carrier field-effect mobility it is necessary tocharacterize and control the processes that take place at thesemiconductor/insulator interface [27]. Capacitance–voltage (C–V)characteristics of metal–insulator–semiconductor (MIS) structures

have been one useful method to investigate interface properties [28].MIS capacitors are the two-terminal counterparts of thin filmtransistors sharing the same basic layer structure. Therefore, to fullyunderstand different physical phenomena in the bulk and at insulator/semiconductor interfaces, a systematic analysis of pentacene/pary-lene MIS capacitors has been performed.

Depending on the sign of the gate voltage the MIS capacitor can bein one of the three states: accumulation, depletion, and inversionregimes. The total capacitance of the MIS structure can be describedby a series connection of two capacitors: the capacitance of theinsulator and the capacitance of the semiconductor layer. The totalcapacitance is given by

1C

=1Ci

+1Cs

ð1Þ

where Cs is the semiconductor capacitance and Ci is the insulatorcapacitance which is given by:

Ci =ε0εiti

⋅A ð2Þ

where A is the capacitor area, ε0 is the vacuum permittivity, εi is theinsulator (parylene) dielectric constant and ti is the insulator thickness.In accumulation, Cs≫Ci and the total capacitance is approximately

4314 W.T. Wondmagegn et al. / Thin Solid Films 519 (2011) 4313–4318

equal to the insulator capacitance [29]. In depletion regime Cs=εsA/W.

W is the variable depletion layer width which is given by

W =2εsϕs

qNA

� �1=2ð3Þ

where εs and ϕs are the permittivity and the surface potential of thesemiconductor respectively. NA is the doping concentration of thesemiconductor and q is the electronic charge. The depletion capacitancedependson thedopingconcentration in the semiconductor. Theeffect ofdoping concentration on the C–V characteristics can be better seen inthe Schottky–Mott analysis [29]. This states that the derivative of C−2

with respect to the gate bias is inversely proportional to the dopingconcentration:

∂C−2

∂V =2

εsqNAA2 : ð4Þ

In inversion, the capacitance becomes independent of the gatevoltage. The low frequency capacitance equals the insulator capaci-tance. But the high frequency capacitance is obtained from the seriesconnection of the oxide capacitance and the capacitance of thedepletion layer having its maximum width, Wm

Wm = 2εsϕF

qNA

� �1=2ð5Þ

where ϕF is the Fermi potential of the semiconductor.

2. Experiment

The MIS structures were fabricated on p-type silicon wafers as thesubstrate with a 200 nm layer of Si3N4 deposited on the surface inorder to isolate silicon from the rest of the structure. A chromeadhesion layer was deposited on the nitride, followed by a 120 nmgold layer which acts as the base contact for the capacitor structure.Pentacene from Sigma-Aldrich Co. was then deposited on the goldbottom contact layer by thermal evaporation at a pressure lower than6.7×10−5 Pa, obtaining a 150 nm thickness layer. The pentacene wasfollowed by deposition of the parylene dielectric with four differentthicknesses: 130 nm, 192 nm, 290 nm and 400 nm. The structure wascompleted with a 120 nm gold layer deposited by shadow mask ontop of parylene to serve as the top contact, different capacitor areaswere defined by circles ranging from 500 μm, 300 μm, 200 μm, 100 μmand 50 μm in diameter. The completed structure is shown in Fig. 1(a)which is the main structure of an organic TFT fabricated with paryleneas dielectric and pentacene as semiconductor [20,21].

Also, metal–insulator–metal (MIM) parylene structures werefabricated with gold contacts (Fig. 1(b)) following similar process inorder to extract the parylene dielectric constant at differentfrequencies. Capacitances were measured with a HP 4284A LCR

Substrate Cr/SiN4/p-Si

Au

Parylene

Au

Pentacene

a

Fig. 1. Schematic cross section of a) MIS a

meter and the dielectric constant was extracted using Eq. (1). As canbe seen from Fig. 2, the dielectric constant at 1 kHz is 3.5 and as thefrequency increases the value tends to decrease until it saturates atapproximately 3 for 100 kHz and remains the same up to 1 MHz.

3. Simulation

Simulation based on physical models is an important tool forobtaining an understanding of the fundamental device physics,material properties and device geometries. In this work, we havesimulated the capacitance–voltage characteristics of a MIS deviceshown in Fig. 1(a). Thematerial parameters used in the simulation arelisted in Table 1. The device simulation was performed using thestandard Poisson, continuity, and drift-diffusion transport equationsas implemented in Silvaco's ATLAS device simulator [30].

Most organic semiconductors have a disordered structure in bothposition and energy. The experimentally observed linear relationbetween log(μ) and √E in disordered organic systems is modeled bythe Poole–Frenkel transport model [28,30]. In the Poole–Frenkelmodel, the field-dependent mobility μ(E) is given by Eq. (6):

μ Eð Þ = μ0 exp − ΔkT

+βkT

−γ� � ffiffiffi

Ep� �

ð6Þ

where μ0 is the zero-field mobility, E is the electric field, Δ is the zerofield activation energy, β is the Poole–Frenkel factor, and γ is a fittingparameter. Generally, these parameters depend on the fabricationconditions and temperature. Poole–Frenkel conduction is originatedby field-enhanced thermal excitation of trapped charge carriers. Thecharge carriers localized around the traps reduce current density inthe low field region. The field assisted delocalization of these trappedcharge carriers results in a large increase in the drain current of thedevice [30–32]. In the work being presented, the Poole–Frenkelmobility model was used in the simulation of the pentacene activelayer.

The charge carrier mobility of the semiconductor might restrict thefrequency of the external signal used for capacitance measurement. Inthe capacitance measurement of the devices, the majority chargecarriers can follow the external signal when the dielectric relaxationtime (τd) is much less than the characteristic period of the probingexternal sinusoidal signal (Tmeas=1/ fmeas) [33].

τd =ε0ε

enmμmð7Þ

where nm and μm are the concentration and mobility of the majoritycarriers respectively. In our devices pentacene has hole majoritycarrier concentration in the order of 1017 cm−3. The maximum fieldeffect mobility extracted from pentacene transistor structure is in theorder of 0.1 cm2/V-s. Using those values and dielectric constant of 3

Substrate Cr/SiN4/p-Si

Au

Parylene

Au

b

nd b) MIM capacitors. (Not to scale.).

-15 -10 -5 0 5 10 15

6.0x10-9

8.0x10-9

1.0x10-8

1.2x10-8

1.4x10-8

1.6x10-8

1.8x10-8

2.0x10-8ExperimentSimulation

Cap

acita

nce

(F/c

m2 )

Voltage (V)

400 nm

290 nm

192 nm

130 nm

Fig. 3. C–V characteristics of the MIS structure.

1k 10k 100k 1M

3.0

3.1

3.2

3.3

3.4

3.5D

iele

ctric

Con

stan

t

f [Hz]

Parylene

Fig. 2. Dielectric constant frequency dependence.

4315W.T. Wondmagegn et al. / Thin Solid Films 519 (2011) 4313–4318

for pentacene, the condition τd≪Tmeas is fulfilled for all measuringfrequencies we used in this study.

4. Results and discussion

Simulated and measured C–V characteristics of MIS capacitor at1 MHz are shown in Fig. 3 for various parylene thicknesses. Theaccumulation and depletion regions are clearly identifiable in the C–Vcharacteristics of the device, with the decrease in capacitance in thedepletion region due to the variable depletion capacitance of theactive semiconductor layer in series with the parylene dielectric layercapacitance. The capacitance in the accumulation region dependsprimarily on the parylene thickness and permittivity, whereas thecapacitance in the depletion region is also dependent on the proper-ties of the semiconductor such as doping concentration, permittivityand interface charge density at the interface with parylene. These arethe parameters which have been investigated in comparing thepresent simulation study with experiment.

Poole–Frenkelmodel parameters previously reported by our groupand others [34–37] are used in the simulation. These parameters arelisted in Table 2. In the literature, values of the effective density of thestates, Nc and Nv, ranging from 1.0×1019 cm−3 to 6×1021 cm−3 havebeen reported for pentacene [38–40]. In our simulation, we have setNc=2.4×1021 cm−3 and Nv=2.4×1021 cm−3. This is a reasonableassumption as those values need to be close to the value of the densityof molecules. Reported values of pentacene parameters such as energygap, affinity, and dielectric permittivity are used [40–42]. The acceptordoping concentration of 1×1017 cm−3 was extracted from the slopeof the experimental 1/C2 vs. V curve using Eq. (4). This same dopingconcentration value with an exponentially decaying energy distribu-tion (Eq. (8)) has been used to match C–V simulation with C–Vexperiment and this number is in the range of previously reportedvalues. Depending on the trap energy level, pentacene doping con-

Table 1Material parameters used in the simulation.

Pentacene Band gap 2.2 eVAffinity 2.7 eVPermittivity 3.0Nc, Nv 2.4×1021 cm−3

Doping Concentration andcharacteristic energy

1×1017 cm−3eV−1

0.15 eVInterface trap Concentration and characteristic

energy1×1011 cm−2eV−1

0.06 eVParylene Permittivity 3.0Gold Work Function 5.1 eV

centration from 1.0×1015 cm−3 to 6×1017 cm−3 has been reportedin literatures [43,44].

gTA Eð Þ = NTA expE−ECð ÞWTA

� �ð8Þ

where E is the energy measured from the valence band edge, and NTAandWTA are the coefficient and the characteristic energy width of theexponential trap distribution, respectively. EC is the conduction band(LUMO level). Values of NTA and WTA are 1×1017 cm−3 eV−1 and0.15 eV respectively.

From the simulated and experimental data, theflat band capacitancewas first calculated using the flat band capacitance Eq. (9) [29]:

CFB =ε0εi

ti +εiεs

LD⋅A ð9Þ

LD =

ffiffiffiffiffiffiffiffiffiffiffiffiffiffikTε0εsq2NA

sð10Þ

where LD is Debye screening length. Given the calculated flat bandcapacitance, the flat band voltage can then be determined graphicallyfrom the horizontal axis of the C–V plots. The calculated flat bandvoltage as function of parylene thicknesses is shown in Fig. 4 and ascan be seen, the flat band voltage has dependence on the insulatorthickness. This dependence of the flat band voltage on insulatorthickness indicates the existence of charges which are fixed or locked-in at the interface between the gate insulator and the semiconductor[45]. The value of those interface charges can be extracted from theslope of a linear fit of the flat band voltage versus insulator thicknessaccording to Eq. (11):

VFB = ϕMS−Q f tiεiε0

ð11Þ

where ϕMS is the gate metal–semiconductor work function differenceper electron and Qf is the interface fixed charge.

Table 2Poole-Frenkel model parameters.

Parameter Value

β 4.37×10−5 eV(cm/V)1/2

μ0 0.081 cm2/V-sΔ 0.16 eVγ 10−5 (cm/V)1/2

100 150 200 250 300 350 400-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0F

lat b

and

volta

ge (

V)

Parylene thickness (nm)

ExperimentSimulation

Fig. 4. Flat band voltage for different thicknesses of parylene.

4316 W.T. Wondmagegn et al. / Thin Solid Films 519 (2011) 4313–4318

Applying Eq. (11) to the data of Fig. 5, values of 9.8×1010 q/cm2

and 1.5×1011 q/cm2 were obtained for experiment and simulation,respectively. ϕMS extracted from the intercepts of the linear fits aremuch higher than the anticipated value. The values obtained were−2.9 V for experiment and −1.7 V for simulation. Assuming a work

-8.0-7.5-7.0-6.5-6.0-5.5-5.0-4.5-4.0-3.5-3.0-2.5-2.0-1.5-1.0-0.50.0

Fla

tban

d vo

ltage

(V

)

Parylene thickness (nm)

Experimental dataLinear fit

a

0 50 100 150 200 250 300 350 400 450

0 50 100 150 200 250 300 350 400 450-8.0-7.5-7.0-6.5-6.0-5.5-5.0-4.5-4.0-3.5-3.0-2.5-2.0-1.5-1.0-0.50.0

Fla

tban

d vo

ltage

(V

)

Parylene thickness (nm)

Simulation dataLinear fit

b

Fig. 5. Linear fit to flat band voltage plots a) experiment b) simulation.

function of 5.1 eV for gold and 4.9 eV for the highest occupiedmolecular orbital (HOMO) level of pentacene, a much lower absolutevalue of mismatch would be expected. In the case of strong p-typedefect doping such as typical of pentacene, the semiconductor workfunction level, which is known to coincide with the Fermi level, willapproximately line up with the HOMO level. Based on the known ruleto predict the work function difference for MIS structure [29], theabsolute value of the difference between gold work function andpentacene HOMO level is only 0.2 eV. Previous reports indicated thework function modification when gold is in contact with pentacenedue to non-ideal interfacial effects such as polarization and dipoleeffect [46–49]. However further investigation is needed if suchphenomenon exists at the gold/parylene interface as well, whichresulted to such a difference between extracted and expected valuesof ϕMS.

In addition to extracting the average interface charge from thethickness dependent flat band voltage as shown above, the interfacecharge for each thickness has been determined and is presented inFig. 6. In the case of simulation, the interface charge was used as avariable during the simulation of C–V characteristics for eachthickness and determined from the best match, whereas in the caseof experiment the interface charge was extracted from the fitting ofthe measured C–V data to an analytical equation. As shown in Fig. 6,the interface charge shows slight variation with thickness of paryleneand all values are slightly higher than the value extracted from the flatband voltage. For parylene thickness variations from 130 nm to400 nm, the interface charge increases only by about a factor of 2. Thisindicates that the parylene film thicknesses and surfaces were fairlyuniform. Since the thicknesses we have used are large compared toprocess variations, interface charge variations as a function ofthickness are not expected. However for ultrathin gate insulators,non-uniformities of insulator surfaces can cause significant variationsin interface charges and traps.

The frequency dependence of the measured and simulated C–Vcurves is shown in Fig. 7 for selected frequencies of an MIS structurewith 50 nm thick parylene and 150 nm thick pentacene. The anodebias was swept from −10 V to 10 V in 0.5 V increments with asuperposition of an AC signal at a particular frequency in Fig. 7. For10 kHz and 100 kHz frequencies, experimental and simulation plotsapproximately match in the accumulation and depletion regimes,with small deviation in the transition regime. However, at 1 MHzfrequency the simulation deviates from experiment in the accumu-lation regime. The simulationwas also performed at 1 kHz and did notshow significant difference from 10 kHz curve. For that reason the1 kHzwas not included. For all frequencies, the dispersion is greater in

100 150 200 250 300 350 400

2.0x1011

2.5x1011

3.0x1011

3.5x1011

4.0x1011

4.5x1011

5.0x1011

5.5x1011

6.0x1011

Inte

rfac

e ch

arge

, Qf (

q/cm

2 )

Parylene thickness (nm)

ExperimentSimulation

Fig. 6. Semiconductor/insulator interface charge for different thicknesses of parylene.

-10 -5 0 5 10

1.5x10-8

2.0x10-8

2.5x10-8

3.0x10-8

3.5x10-8

4.0x10-8

4.5x10-8

5.0x10-8

5.5x10-8

ExperimentSimulation

Cap

acita

nce

(F/c

m 2 )

Volatge (V)

1 MHz

100 kHz

10 kHz

Fig. 7. Frequency dispersion characteristics of the device.

4317W.T. Wondmagegn et al. / Thin Solid Films 519 (2011) 4313–4318

the accumulation region than in the depletion region. The capacitancedecreases in accumulation regime when high frequency is used, whilethe capacitance in depletion regime remains almost the same.

The frequency dependence of C–V characteristics originates fromdifferent responses of trapped and mobile charges between theinterface region and bulk of the semiconductor [50]. This difference inresponse to the applied signal is directly related to the differentcarrier-lifetimes and carrier-generation rates of disordered organicsemiconductors [28,51]. The shallow states respond to high frequencyvoltages whereas deep and localized states do not respond to thosevoltages at high frequency [52]. Lower values of the capacitance athigher frequencies indicate that mobile charges do not respond atthose high frequencies, behaving similar to fixed or trapped charges.

Typically, the capacitance in the accumulation regime has shown arapid decrease when the frequency becomes higher than 100 kHz.This indicates that the response of the holes to the gate signal above100 kHz decreases rapidly. The holes' response to the gate signal ismainly constrained by large bulk resistance of pentacene. In contrast,the depletion capacitance remains almost constant. Since holes aredepleted in pentacene, the whole device can be considered as a seriesof two capacitors, one from the insulator and the other from thedepletion region, with no resistance which constrains the response ofthe charges along the capacitors [50].

Frequency dispersion in the transition region between themaximum and minimum capacitances is due to the presence ofinterface traps between parylene and pentacene. Interface traps of8×1011 cm−2 with exponential energy distribution of characteristicdecay energy 0.06 eV have been used to match simulation withexperiment. The slope of the transition region from depletion toaccumulation is affected by those interface traps whose energy existsin the band gap of pentacene [52]. As Lindner et al. [33] have shown intheir simulation work on MIS C–V characteristics, interface traps canhave significant effect on the C–V characteristics. This is particularlysignificant when the traps have deep and discrete energy level, wheretheir effect can be seen as maxima in the transition region betweenthe accumulation and depletion. However the interface traps weincorporated in the simulation have an exponential energy distribu-tion, which did not result in irregularities in the C–V characteristics.

In addition to the C–V characteristics, the I–V characteristic of thedevice with 200 nm parylene and 110 nm pentacenewas simulated toexamine the leakage through parylene. Two tunneling models: trapassisted tunneling and Fowler–Nordheim tunneling have been tested.However in both cases the leakage current was insignificant, which isin the order of pA, because of the high thickness of parylene.

5. Conclusion

In this work, Metal–Insulator–Semiconductor capacitors based onpentacene and parylene have been studied by experimental and nu-merical analysis. The impact of the insulator–semiconductor interfaceand the bulk properties of the materials on the C–V characteristic ofthe device have been investigated. The interface charge and the flatband voltage of the device were extracted. The flat band voltageextracted from the C–V plot of MIS structures ranges from −2.4 V to−5 V. From the flat band voltage, the existence of a significantmismatch between the work functions of the gate electrode andpentacene active material was identified. The thickness dependenceof the flat band voltage indicates the existence of fixed and trappedcharges at the interface between the insulator and the semiconductor.An interface charge on the order of 3×1011 q/cm2 between pentaceneand parylene was calculated from simulation and experimental data.An interface trap of 8×1011 cm−2 with an exponential energydistribution of characteristic energy 0.06 eV has been used for thesimulation. The frequency dispersion of the device has also beenpresented. The accumulation, depletion and transition regions aredistinctively shown for all frequencies. Experiment and simulationagree in all regions except for 1 MHz in which simulation showsdeviation from experiment in the accumulation region.

Acknowledgment

We gratefully acknowledge the Department of Defense for support-ing this work (Army Research Office Contract W911NF-07-2-0059).

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