Study of 6,13-bis(tri-isopropylsilylethynyl) pentacene (TIPS-pentacene crystal) based organic field effect transistors (OFETs)

Synthetic Metals 194 (2014) 146–152

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Study of 6,13-bis(tri-isopropylsilylethynyl) pentacene (TIPS-pentacene crystal) based organic field effect transistors (OFETs) Ghulam Murtaza a,∗ , Ishtiaq Ahmad a , HongZheng Chen b , Jiake Wu b a b

Department of Physics, Bahauddin Zakariya University, Multan 60800, Pakistan Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310037, PR China

a r t i c l e

i n f o

Article history: Received 23 January 2014 Received in revised form 15 April 2014 Accepted 30 April 2014 Keywords: Tri-isopropylsilyethynyl pentacene Organi field effect transistors (OFETs) Divinyltetramethyldisiloxanebis(benzocyclobutene) BCB

a b s t r a c t Organic field-effect transistors are fabricated with the help of an active organic material 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene). The conduction parameters such as mobility due to electrons, holes, threshold voltage, sub-threshold swing, the maximum density of surface states and standard deviation are calculated. The average mobility due to electron in the single crystal having concentrations 0.8 mg/ml, 0.4 mg/ml is 0.206 cm2 /V s and 2.452 cm2 /V s. The average hole mobility of a crystal having concentrations 0.8 mg/ml, 0.4 mg/ml is 0.25 cm2 /V s and 2.42 cm2 /V s. The performance of the devices is investigated with respect to surface morphology. The crystals are prepared by the droplet pinned crystallization (DPC) method. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The highest hole mobility of the TIPS-pentacene at room temperature has made it promising among organic materials and polycrystalline [1]. Its application is much as like Polyacene and Anthracene [2] single crystals and thin films of Tetracene [3], Anthracene [4] as well as thin film structures [5] are also investigated. The latest material including P3HT poly (3-hexylthiophene), TIPS-pentacene are P-type because the contact metal Fermi level is very close to the highest occupied molecular orbital (HOMO) level instead of lowest unoccupied molecular orbital level (LUMO): hence on the basis of this the P-type semiconducting materials are well developed and more stable in atmosphere than the N-type materials [6–8]. The reported dielectric material for organic field effect transistors gate are polyvinyl alcohol (PVA), polymethylmethacrylate (PMMA), polyvinylpyrrolidone (PVP), benzocyclobutene (BCB) [9–13]. Although, these materials have low dielectric constant but need to posses dense layer to avoid pinholes. The only drawback of these materials is their low gate capacitance, so they need a high device operating voltages. The materials which have a high dielectric constant are not suitable for gate insulator because they are mostly based on inorganic material which are brittle and require an etchant. As these materials have poor mechanical properties so these are not suitable in the flexible organic electronics. Moreover,

∗ Corresponding author. Tel.: +92 61 9210199; fax: +92 61 9210168. E-mail addresses: [email protected], [email protected] (G. Murtaza). http://dx.doi.org/10.1016/j.synthmet.2014.04.034 0379-6779/© 2014 Elsevier B.V. All rights reserved.

for the preparation of high dielectric constant materials, a high annealing process is required which is impossible in plastic. The applications of organic field effect transistors (OFETs) are in the active layer of matrix display, biological sensing, chemicals, and radio frequency identification (RFID) tags and in flexible electronics [6–12]. The organic semiconductor field effect transistors are achieving the highest mobility as compared to amorphous silicon material under low voltage operation. Many methods have been developed to obtain a high performance organic semiconductors [13] and dielectric materials [14–16]. The devices having top contacts are preferred due to their high performance as compared to bottom contact. The diminished performance of bottom contact devices is due to the incompatibility between organic semiconducting, metal electrode and the dielectric interfaces [17]. Though, certain organic materials are selected for the dielectric as a gate insulating but most of the organic FETs are still made with BCB covered SiO2 as a gate dielectric. To realize low cost and large area electronics, it is desired that SiO2 may be substituted with an organic material for making a high desirous dielectric material. The previous studies show that the charge transport properties of organic semiconductor are intimately dependent on their crystal structure and morphology [18–23]. The operation of a transistor is just like a capacitor, when a voltage is applied between the source and gate electrode. A charge starts to be injected from the source electrode into semiconducting layer. When the injected charge is accumulated at the semiconductor insulator interface, a conducting channel will be formed between the source and drain electrode. When a drain voltage is applied a current starts to flow from source to drain through the conducting channel. So, the transistor is also

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147

Si

Si

S Si

O

Si

Fig. 1. Chemical structure of divinyltetramethyldisiloxane-bis(benzocyclobutene).

known as organic field effect transistor. The field effect mobility in the TIPS-pentacene FET depends strongly on the monolayer growth of the active materials grains. If the crystal contains some impurities or cracks, the conductivity decreases which results in decreasing mobility. It also increases with the increase of channel length. In the present paper the comparison of two crystal having different ribbon heights and annealing temperature is done by fabricating devices on them and then evaluating their performance we use BCB coated SiO2 as an insulator because BCB possesses a best quality for hydroxyl free interfacing to organic semiconducting material and it posses a high dielectric breakdown strength which is more than 3 MV/cm. The idea of organic electronics is helpful against the costly photolithographic structuring by simple droplet pinned crystallization method. The current–voltage (I–V) characteristics are studied with the help of Agilent Semiconductor Parameter Analyzer 4155 C, the optical images were taken with the help of Optical microscope (Leica DM4000 m) and the thickness (step height) of the single crystals are measured by using an atomic force microscopy (AFM), Veeco Di 3000. 2. Experimental 2.1. Droplet pinned crystallization method TIPS-pentacene (Sigma–Aldrich) crystals were grown by using the droplet pinned crystallization (DPC) method. The crystals were grown on the substrate for organic field effect transistors (OFETs). The substrates were BCB covered highly doped SiO2 , having size (1 cm2 ). Divinyltetramethyldisiloxane-bi (benzocyclobutene) belongs to a family of thermosetting polymer materials. It is the product of The Dow Chemical Company. The manifestication of BCB are such as low moisture absorbtion, super dielectric, easy to process, excellent gap filling properties, polarization and rapid curing (cross linking) process [24]. The monomer of benzocyclobutene (BCB) cures thermally activated to form a polymer network. One hour is required to cure it at 250 ◦ C and it can be cured completely in a minutes at 300 ◦ C [25]. The dispersion in the dielectric constant of BCB is negligible over a wide temperature and frequency range, having dielectric constant value about 2.65 [26–28]. The chemical structure of benzocyclobutene is shown in Fig. 1. The TIPS-pentacene solution 10 ␮l with different concentration 0.2 mg/ml, 0.4 mg/ml, 0.6 mg/ml, 0.8 mg/ml and 1.0 mg/ml were prepared by mixing two solvents m-xylene and carbon tetrachloride (CCl4 ) of Sigma–Aldrich. The solution of each concentration having volume ratio 1:1 was dropped onto a silicon substrate (1 cm2 ) with a smaller piece of silicon wafer (0.4 cm × 0.4 cm) called pinner to pin the solution droplet. The BCB covered SiO2 substrate was placed on a Teflon slide inside a Petri dish (35 mm × 10 mm) sealed with para film allowing the solvent (concentration 0.4 mg/ml) to evaporate slowly on a hotplate. The annealing temperature for the single crystal having concentration 0.8 mg/ml,

Fig. 2. Optical images of a single crystal of TIPS-pentacene with concentration (a) 0.4 mg/ml and (b) 0.8 mg/ml.

0.4 mg/ml was 60 ± 1 ◦ C and 30 ± 1 ◦ C, respectively. Aligned crystals were formed within one hour. Single crystal images were taken by the help of optical microscope. Two samples having concentrations 0.4 mg/ml, 0.8 mg/ml were selected for the detailed study. The DPC method has been described by Hanying et al. [29–31]. The optical microscopic images of these samples are shown in Fig. 2(a and b). The optical image of a crystal having concentration 0.4 mg/ml is comparatively better than that of crystal with concentration 0.8 mg/ml because the ribbons of the single crystals having concentration 0.4 mg/ml are continuous and uniformly distributed. 2.2. Fabrication of a device In order to make devices, the thermal vapor deposition and shadow mask deposition technique were used. Thermal vapor deposition technique was used to deposit metal films under a vacuum pressure of 10−6 Pa to avoid the unwanted oxidation. The channel length and width are 60 ␮m, 1000 ␮m for concentration 0.4 mg/ml and 50 ␮m and 1000 ␮m for concentration 0.8 mg/ml respectively. To complete the FET structure shadow mask deposition was used to selectively coat a material for making the contacts of the device, gold source and drain electrodes were deposited over the TIPS-pentacene layer by a shadow mask under a pressure of 10−6 Pa at a rate of 0.01–0.02 nm/s. The gold electrodes had a thickness of 50 nm. In order to make the devices, eleven and fourteen contacts were made on the single crystal having concentrations 0.4 mg/ml and 0.8 mg/ml respectively. The chemical structure of TIPS-pentacene is shown in Fig. 3. The surface morphology was investigated using an atomic force microscope (AFM) Veeco D1 3000 operating in Tapping ModeTM having tip HQ: NSC 15/AI BS with force constant 46 N/m. In order to measure the accurate height sufficiently high resolution was assured by recording 512 points/line at a slow scan rate of one line/s. The step height of a single crystal having concentration 0.4 mg/ml and 0.8 mg/ml was 107.13 nm and was 258.99 nm respectively, as shown in Fig. 4(a and b). 3. Results and discussion The electrical properties of an organic semiconductor are studied to measure the current–voltage characteristics. The electrical characterization of the devices is done in Argon (Ar) atmosphere inside the glove box with the help of Agilent analyzer 4155C. The mobility which represents the case with which charges can travel

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H3 C H3 C

Si

H3 C

H3 C H3 C H3 C

C CH3 CH3 CH3

Si

CH3 CH3 C CH 3

Fig. 5. Schematic diagram of top source, drain contacts with bottom gate.

Fig. 3. Chemical structure of TIPS-pentacene.

When we apply a positive gate bias the organic semiconductor is depleted of holes and the transistor is switched off. If the bias between source and drain is kept fixed and the bias applied to the gate electrode is swept from positive to negative value, the current curve obtained is called a transfer curve. The transistors made of inorganic material shows the gate bias threshold voltage value at which charge inversion occurs in the channel of the transistor but organic transistors do not show charge inversion. The threshold voltage VT in case of OFETs can be empirically defined as the intercept of the extrapolated linear part of the

(a)

-35

VG(0V) -30

Drain Current ID ( μA)

through the material is determined from the electrical properties of the organic semiconductor. The charge carrier mobility in single crystal is more as compared to those in thin films. The decrease in mobility is caused by the inherent disorder in the structure of a material. Top contact bottom gate transistors have source and drain contact on top whereas at the bottom is gate contact. Such type of configuration is also known as staggered configuration. A device with top contacts of source and drain are fabricated with gold and a bottom contact is made of silicon as shown in Fig. 5. In the current–voltage curve, on increasing the drain voltage (VD ), a linear and saturation region is observed which indicate the presence of low contact resistance between the gold electrode and organic semiconducting layer. Fig. 6(a and b) shows the current–voltage characteristics in the linear and saturation regime. On applying a negative bias to the gate electrode, charge accumulation takes place near the semiconductor insulator interface. On applying a bias to the drain electrode, the charges present in the accumulation layer start to move in the direction of decreasing potential, which give rise to source drain current.

VG(-40V)

VG(-20V) VG(-60V)

VG(-80V)

-25

VG(-100V)

-20 -15 -10 -5 0 0

-10

-20

-30

-40

-50

Drain Voltage (V)

Drain Current ID (μ A)

(b)

-0.25

VG(0V) -0.20

VG(-20V)

VG(-40V)

VG(-60V)

VG(-80V)

VG(-100V)

-0.15

-0.10

-0.05

0.00 0

-10

-20

-30

-40

-50

Drain Voltage (V) Fig. 4. AFM images of the single crystal ribbons having a concentration of (a) 0.4 mg/ml and (b) 0.8 mg/ml.

Fig. 6. Drain current–voltage characteristics of TIPS-pentacene (OFET) having concentration (a) 0.4 mg/ml and (b) 0.8 mg/ml.

G. Murtaza et al. / Synthetic Metals 194 (2014) 146–152

√ Fig. 7. Plots of drain current |ID | and square root of drain current | ID | versus gate voltage having concentration (a) 0.4 mg/ml and (b) 0.8 mg/ml.

transfer curve with the gate voltage axis. Fig. 7(a and b) shows the drain current on left, square root of drain current on right and gate voltage along the horizontal axis. The transfer curves are shown in Fig. 8(a and b). A transistor is said to be operating in the linear regime, when the applied source–drain bias VSD , is much smaller than the bias on the gate, VGD , and it is said to be operating in the saturation regime when the source–drain bias is much larger than the gate bias. In this ways the charge density in the accumulation layer remains no longer uniform so the charge density decreases continuously to zero. For both regimes, an approximate expression for source–drain current can be derived [32]. In linear regime, where |VSD |  |VGD − VT |, the source drain current, ISD , can be written as ISD = lin

Ci W L



(VGD − VT )VSD −

1 2 V 2 SD

 (1)

where W and L are the width and length of the channel, Ci the gate dielectric capacitance per unit area, lin the mobility of the charge carriers in the linear regime. On increasing the applied source–drain bias to a value such that VSD = (VGD − VT ), the carrier density in the accumulation layer goes to zero near the drain electrode. For VSD  (VGD − VT ), the source drain current does not increase with the applied source–drain bias. In the saturation regime, where |VSD | ≥ |VGD − VT |, the source–drain current can therefore be obtained from Eq. (1) by substituting VSD = (VGD − VT ) as given below ISD = sat

Ci W (VGD − VT )2 2L

(2)

149

Fig. 8. Transfer characteristics of TIPS-pentacene (OFETs) having concentration (a) 0.4 mg/ml and (a) 0.8 mg/ml.

The mobility in the linear regime in can be derived from Eq. (1) by taking the first derivative of the source–drain current with respect to the gate bias.



lin =



 ∂ISD  L   Ci WVD  ∂VGD 

(3)

The mobility in the saturation regime can be derived by Eq. (2).

sat

2L = Ci W

  2 ∂ ID ∂VGD

(4)

The mobility in the linear regime can be calculated by using Eq. (3) and the mobility in the saturation regime can be calculated by using Eq. (4). In case of saturation regime, the square of the slope of the square root of drain current and gate voltage is taken then it is multiplied by the factor (2L/Ci W). The field effect conductivity  can be calculated using the following Eq. (5) [33]. The highest field effect conductivity was 1.29 × 10−9 (S/cm) and 1.14 × 10−10 (S/cm) in the single crystal having concentration 0.4 mg/ml and 0.8 mg/ml. (VG ) =

L ID × W VD

(5)

The mobility is strongly dependent on the applied gate bias. For the organic semiconductor these values depend on the trap states at the surface of the gate dielectric, the permittivity of the gate dielectric, chemical impurities, and the type of contacts used in the device architecture. The inverse sub threshold slope S can be calculated by the following relation.

   d log(ID ) −1  dV

S=

G

(6)

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Fig. 10. Mobility due to electron versus number of devices having concentration 0.4 mg/ml and 0.8 mg/ml.

Fig. 9. Graph of mobility versus gate voltage at concentration (a) 0.4 mg/ml and (b) 0.8 mg/ml.

Further analysis is possible from the sub threshold slope calculation. The maximum number of interface traps can be calculated using Eq. (7), assuming that the densities of deep bulk states and interface states are independent of energy [34]. kT/q

  Ci

− 1

q

(7)

max is the maximum number of interface states, k is the where NSS Boltzman constant and T is the absolute temperature. The hole mobility can be calculated by using the following relation

=2

m2 L WCi



Conc.0.4mg/ml Conc.0.8mg/ml

10 8

2



12

Hole Mobility (Cm /Vs)

  S log(e)

max NSS =

be due to scattering at structural defects in the thin film phase, in third region the decrease in mobility is due to the scattering phenomenon which may arises due to the presence of structural defects at the interface. We have observed two regions instead of three. The initial rise of mobility is interpreted by the multiple trap and release (MTR) model. The mobility degradation of TIPS-pentacene is found at different gate voltages. The degradation in mobility is due to the ohmic losses at the source and drain electrodes or it may be due to structural defects which are present near the interface traps in the semiconductor insulator interface. The existence of the shallow traps also degrades the mobility and it can also be occurred due to space charge limited current. A significant variation in the charge carrier mobility may occur due to the different conditions under which crystals are grown. The crystals which are annealed at high temperature of 60 ± 1 ◦ C have more roughness due to the increase of grain size as it is clear from the step height and it possesses more structural defects as compared to the crystal which are grown at a low temperature as reported by Dong-Su Kim et al. [37]. Devices are fabricated on substrate by varying the contact position of the source and drain. Figs. 10 and 11 show the mobility of different devices due to electrons and holes. The device number 6 has the highest performance as compared to other devices due to the existence of the strong ohmic contact at the source and drain contact. The mobility of electrons in a crystal having step height 107.13 nm is more as compare to the mobility values of a

(8)

where m is the slope of the plot of the square root of drain current versus gate source voltage Ci is the specific capacitance of the gate dielectric, W is the effective width of the channel and L is the length of the channel. The effect of the gate voltage on the mobility is shown in Fig. 9(a and b). According to M. Voigt et al. [36] the effect of gate voltage on mobility have been divided in to the three regions, In region first the mobility increases by the increase of gate voltage, in second region the mobility saturate and in third region the mobility decrease on increasing the gate voltage. In region one the gate voltage dependent mobility is described by the Multiple Trap Release Model due to the partial saturation of traps and in the second region the observed limitation of the charge transport must

6 4 2 0 0

2

4

6

8

10

12

14

16

Device Number Fig. 11. Mobility due to holes versus number of devices having concentration 0.4 mg/ml and 0.8 mg/ml.

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151

Table 1 Threshold voltage, sub threshold slope S, Ion /Ioff , number of interface traps, mobility due to electrons, mobility due to holes and standard deviation of mobility for devices having concentration 0.4 mg/ml and 0.8 mg/ml. Device no.

VT (V)

S (mV/decades)

Ion Ioff

max Nss (cm−2 eV−1 )

Mobility (cm2 /V s) ␮electron

Standard deviation of mobility (cm2 /V s)

Hole mobility (cm2 /V s) ␮hole

Concentration 0.4 1 2 3 4 5 6 7 8 9 10 11

−1.6 −20.7 −3 −12 3.8 13.2 6.4 −30 −20 9.4 −25

−19 −20 −25 −22 −24 −24 −28 −18 −15 −28 −17

102 102 102 102 102 102 102 102 102 102 102

1.98 × 1012 2.08 × 1012 2.61 × 1012 2.30 × 1012 2.50 × 1012 2.50 × 1012 2.92 × 1012 1.88 × 1012 1.56 × 1012 2.92 × 1012 1.77 × 1012

.023 .328 .150 .238 .048 1.46 .029 .020 .077 .228 .052

0.066 0.098 0.045 0.072 0.014 0.441 0.008 0.006 0.023 0.068 0.015

0.24 3.34 1.52 2.42 0.49 14.84 0.29 0.20 0.78 2.32 0.53

−29 −37 −29 −28 −39 −32 −41 −25 −28 −31 −45 −22 −34 −39

103 103 103 103 103 103 103 103 103 103 103 103 103 103

3.03 × 1012 3.87 × 1012 3.03 × 1012 2.92 × 1012 4.08 × 1012 3.35 × 1012 4.29 × 1012 2.61 × 1012 2.92 × 1012 3.24 × 1012 4.70 × 1012 2.30 × 1012 3.56 × 1012 4.08 × 1012

0.002 0.004 0.003 0.003 0.002 0.005 0.006 0.005 0.007 0.005 0.004 0.007 0.005 0.006

0.14 0.17 0.14 0.13 0.12 0.23 0.26 0.24 0.31 0.23 0.13 0.33 0.23 0.23

Concentration 0.8 −0.5 1 5.6 2 3 −0.17 −14.4 4 1.8 5 4.1 6 −7.7 7 13.5 8 −5.7 9 10 6.5 −10 11 18 12 2.7 13 14.3 14

crystal having step height 258.99 nm as reported by Tian Xue-Yan et al. [35]. The mobility decreases by the increase of thickness (step height) and it also decreases due to the non-uniformity in the grown single crystal monolayer. Table 1 shows the calculated values of threshold voltage, sub threshold slope, Ion /Ioff , number of interface traps, mobility due to electrons, standard deviation of mobility and the hole mobility of TIPS-pentacene having concentration 0.4 mg/ml are in the range −30 V to 13.2 V, 15–28 mV/decade × 102 , 1.56 × 1012 –2.92 × 1012 cm−2 eV−1 , 0.020–1.46 cm2 /V s, 0.006– 0.441 cm2 /V s, 0.20–14.84 cm2 /V s respectively. The calculated values of threshold voltage, sub threshold slope, Ion /Ioff , number of interface traps, mobility due to electrons, standard deviation of mobility and the Hole mobility of TIPS-pentacene having concentration 0.8 mg/ml are in the range −14.4 V to 18 V, 22–45 mV/decade × 103 , 2.30 × 1012 –4.70 × 1012 cm−2 eV−1 , 0.0103–0.0274 cm2 /V s, 0.002–0.007 cm2 /V s, 0.12–0.33 cm2 /V s respectively. Among several transistor parameters, the field-effect mobility is the primary one because it governs the dynamic properties such as the driving frequency and response time for the various types of applications. The field effect mobility in the OFET strongly depends on the layer by layer growth and the resultant crystalline of an organic semiconductor film [38]. Therefore, it is very important to control the initial growth of an organic semiconductor layer on a variety of substrates for practical applications.

4. Conclusions The surface morphology has a great influence on the performance of TIPS-pentacene FETs. The highest value of the mobility due to electrons in the crystal having ribbon heights 107.13 nm, 258.99 nm is found to be 1.463 cm2 /V s and 0.0274 cm2 /V s respectively. The crystal which has more height shows low conductivity and charge carrier mobility. It was due to loss of ohmic contacts, surface morphology because the single crystals having more height

.0113 .0143 .0116 .0109 .0103 .0189 .0215 .0200 .0261 .0192 .0107 .0274 .0193 .0193

shows non-uniformly, discontinuity and cracks in the structure. The charge carrier mobility increases with the increase of channel length. In mobility versus gate voltage graph, the initial rise of mobility is interpreted by the multiple trap and release (MTR) model. The mobility degradation of TIPS-pentacene is found at different gate voltages. The degradation in mobility is due to the ohmic losses at the source and drain electrodes. The existence of the shallow traps in the semiconductor insulator interface also degrades the mobility and it may also be occurred due to space charge limited current. The highest hole mobility in the crystal having concentrations 0.4 mg/ml, 0.8 mg/ml was found to be 14.84 cm2 /V s and 0.33 Cm2 /V s respectively. It is concluded from the facts that the texture of the grown crystals has a remarkable impact in the performance of a device. Acknowledgements One of the authors (Ghulam Murtaza) is thankful to Higher Education Commission (HEC) of Pakistan for providing financial assistance through IRSIP scholarship program and the Zhejiang University of China for providing opportunity to work in Organic Semiconducting Lab (OSL). References [1] P.V. Necliudov, M.S. Shur, D.J. Gundlach, T.N. Jackson, J. Appl. Phys. 88 (2000) 6594. [2] J. Gyu Park, R. Vasic, J.S. Brooks, J.E. Anthony, J. Appl. Phys. 100 (2006) 044511. [3] G.H. Gelnick, H.E.A. Huitema, E.V. Veenendaal, E. Cantatore, L. Schrijnemakers, J.B.P.H. Van Der Putten, T.C.T. Genus, M. Beenhakers, J.B. Glesbers, B.-H. Huisman, E.J. Meijer, E.M. Benito, F.J. Touwslager, A.W. Marsman, B.J.E. Van Rens, D.M. De Leeuw, Nat. Mater. 3 (2004) 106–110. [4] P.F. Baude, D.A. Ender, M.A. Haase, T.W. Kelley, D.V. Muyres, S.D. Thesis, Appl. Phys. Lett. 82 (2003) 3964–3966. [5] H. Klauk, M. Halik, U. Zschieschang, F. Eder, G. Schmid, C. Dehm, Appl. Phys. Lett. 82 (2003) 4175–4177. [6] M. Yoshida, S. Uemura, S. Hoshino, N. Takada, T. Kodzasa, T. Kamata, J. Appl. Phys. 44 (2005) 3715–3720. [7] L.L. Chua, P.K.H. Ho, H. Sirringhaus, R.H. Friend, J. Appl. Phys. Lett. 84 (2004) 3400–3402.

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