Output Properties of C60 Field-Effect Transistors with Au Electrodes

Apr 25, 2007 - Field-effect transistors (FETs) with thin films of C60 have been fabricated with Au electrodes modified by a series of 1-alkanethiols. ...
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J. Phys. Chem. C 2007, 111, 7211-7217

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Output Properties of C60 Field-Effect Transistors with Au Electrodes Modified by 1-Alkanethiols Takayuki Nagano,†,‡ Michiko Tsutsui,† Ryo Nouchi,‡ Naoko Kawasaki,† Yohei Ohta,† Yoshihiro Kubozono,*,†,‡ Nobuya Takahashi,§ and Akihiko Fujiwara‡,§ Department of Chemistry, Okayama UniVersity, Okayama 700-8530, Japan, CREST, Japan Science and Technology Agency, Kawaguchi, 322-0012, Japan, and Japan AdVanced Institute of Science and Technology, Ishikawa 923-1292, Japan ReceiVed: February 1, 2007; In Final Form: March 14, 2007

Field-effect transistors (FETs) with thin films of C60 have been fabricated with Au electrodes modified by a series of 1-alkanethiols. All C60 FETs show n-channels of normal FET properties. It has been found that the output properties for the FETs with the Au electrodes modified by 1-alkanethiols with long alkyl chains are largely affected by the carrier-injection barrier (i.e., the current vs drain/source voltage plots exhibited concaveup nonlinearity at low voltage regions). The output properties are substantially dominated by an additional tunneling barrier of 1-alkanethiols inserted into the junction of the Au electrode and C60 thin films, and the parameters associated with the junction barrier height and tunneling efficiency were determined from the output properties based on the thermionic emission model for double Schottky barriers.

Introduction Field-effect transistor (FET) devices with thin films of organic molecules have attracted special attention because of structural flexibility, low temperature/low cost processing, and large area coverage.1,2 However, the field-effect mobility, µ, is lower than that, ∼1000 cm2 V-1 s-1, of conventional inorganic FETs. Recently, the FET devices with thin films of C60 showed a very high µ value of 4.9 cm2 V-1 s-1 by use of pentacene as a buffer layer between C60 thin films as the active layer and SiO2 as the gate insulator.3 This epoch-making success is based on the improvement of the crystallinity of C60 thin films realized by the use of a pentacene layer (i.e., the improvement of the interface between C60 and SiO2 was a key point). Important interfaces other than that between active layer and gate insulator exist in the organic FET device. Especially, the interface between source/drain electrodes and active layer is known to be important for the carrier-injection. Actually, the control of work function, φm, of the source/drain electrodes can produce various types of FET properties as n-channel, ambipolar, and p-channel operations, even if the active layer is invariant.4-6 This result originates from a matching between the Fermi level of electrodes and the energy level of the conduction/valence band of the active layer. Recently, we observed normally-on FET properties in the C60 FET device with the Eu electrodes, and the µ value was relatively high at 0.5 cm2 V-1 s-1.7 This can be explained on the basis of the fact that the lowest unoccupied molecular orbital (LUMO) energy level (ELUMO ) -3.6 eV) of C60 is lower than the Fermi level (EF ) -2.5 eV) of Eu (i.e., the ohmic contact is formed between C60 thin films and Eu electrodes). This implies that the electrons can be smoothly injected into the LUMO band of C60 by an application of drain/source voltage * Corresponding author. E-mail: [email protected]. † Okayama University. ‡ Japan Science and Technology Agency. § Japan Institute of Science and Technology.

VDS. Furthermore, the weakly n-doped C60 thin films by Eu metal in the formation of electrode flow bulk the current immediately by an application of VDS. Thus, the energy levels of electrodes and the active layer dominate the carrier-injection in the FET device. Consequently, we stress that the control of electronic structure near the interface between metal electrodes and organic semiconductor is very significant for the performance control of organic devices. In addition, the formation of the tunneling barrier near the interface may open a way toward new types of organic devices because of the possibility of the fine control of carrier-injection, and this will produce a comprehensive understanding of the electronic structure of the interface in the organic devices. In the present study, we have fabricated C60 FET devices with Au electrodes modified by various types of 1-alkanethiols and studied their transport properties, to clarify quantitatively the electronic structure produced by a contact of C60 thin films and the electrodes. The large carrier-injection barrier was observed in the C60 FET devices with the Au electrodes modified by 1-alkanethiols with long alkyl chains. Experimental Procedures The device structure used in this study is shown in Figure 1a; the bottom contact-type device structure is adopted for all FET devices. Commercially available C60 (99.98%) was used for the formation of the active layers, and a commercially available Si(100)/SiO2 wafer was used as a substrate after cleaning and hydrophobic treatment of the surface with hexamethyldisilazane. The source/drain electrodes with a thickness of 55 nm were formed by thermal deposition under 10-8 Torr; 50 nm of Au was deposited on 5 nm of Cr, as shown in Figure 1a. The surface of the Au electrodes was modified by 1-alkanethiols by immersing the Si/SiO2/Cr/Au substrates into the ethanol (EtOH) solution of 1-alkanethiols (10-1 mol L-1) for 17 h; as an example of 1-alkanethiol used in this study, the molecular structure of 1-hexadecanethiol is shown in Figure 1b. The substrates were washed with EtOH and ultrapure H2O. The

10.1021/jp0708751 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/25/2007

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Figure 1. (a) Device structure of C60 thin-film FET; green and blue lines refer to 1-alkanethiol and hexamethyldisilazane, respectively. (b) Molecular structure of 1-hexadecanethiol. (c) Schematic representation of the surface of Au electrodes modified by 1-alkanethiols. (d) Schematic representation of the energy band diagram expected for Au electrode/alkanethiol/C60 thin film junction.

surfaces of the Au electrodes modified by 1-alkanethiols were confirmed to be hydrophobic from an increase in the contact angle of H2O in comparison with that of the pristine Au electrode. The expected structure of 1-hexadecanethiol on the Au surface8,9 is shown in Figure 1c. The thin films of C60 with a thickness of 150 nm were formed on the substrate by the thermal deposition under 10-8 Torr. During thermal deposition, the temperature of the substrate was maintained at room temperature. The change and damage of 1-alkanethiols by thermal deposition of C60 were not recognized based on the reason described in the third section of the Results and Discussion. The thickness of the SiO2 layer was 400 nm, and the capacitance per unit area, C0, was determined by an LCR meter to be 8.6 × 10-9 F cm-2. The channel length, L, and the channel width, Z, of all devices were 30 and 3000 µm, respectively. The devices were transferred from a vacuum chamber to another vessel for measurements of FET properties. The characteristics of the FET devices were measured under a vacuum of 10-6 Torr after annealing the devices under 10-6 Torr for 12 h to eliminate O2 and H2O adsorbed into the C60 thin films during transferring the device to the measurement vessel. The work function φm values of the electrodes were determined from photoemission (PE) spectra measured with a a PE spectrometer (Riken AC-2). Results and Discussion ID - VDS Plots of C60 FET with Au Electrodes Modified by 1-Alkanethiols. The plots of the drain current ID versus VDS of the C60 FET device with the Au source/drain electrodes

Figure 2. (a) ID - VDS plots of C60 FET device with Au electrodes modified by 1-hexadecanethiol. Inset of panel a: ID - VDS plots of C60 FET device with pristine Au electrodes. ID - VG plot at VDS of (b) 10 V, (c) 40 V, and (d) 100 V.

modified by 1-hexadecanethiol (CnH2n+1SH: n ) 16) measured at 300 K show n-channel normally-off properties (Figure 2a). The ID - VDS plot shows concave-up nonlinearlity in the low VDS region, showing that the plots are remarkably affected by the carrier-injection barrier. For comparison, the ID - VDS plots in the C60 FET device with the pristine Au electrodes are shown in the inset of Figure 2a. The ID - VDS plots (inset of Figure 2a) increase linearly in the low VDS region. On the other hand, the ID increases linearly in the intermediate region for the C60 FET with the modified Au electrodes (Figure 2a) and saturates in the high VDS region. Thus, the ID - VDS plots of the C60 FET

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device with the modified Au electrodes are different from those with the pristine Au electrodes. Furthermore, the plots of ID - VG at a VDS of 10 V (low VDS region) and 40 V (intermediate VDS region) are shown in Figure 2b,c, respectively. The increase in ID for the application of VG at a VDS of 10 V is much smaller than that at a VDS of 40 V. Thus, the FET operation is suppressed in the low VDS region. Consequently, in the C60 FET device with the modified Au electrodes even under the high gate voltage VG above the threshold voltage VT, the injection-barrier between electrodes and thin films of C60 dominates the ID - VDS curve at the low VDS region, and above the intermediate VDS region, the curve is dominated by the resistance of the channel region of the C60 thin films, and it reaches gradually the drain saturation current, IDsat, by a channel pinch-off in the high VDS region. The values of µ and VT were determined from the (IDsat)1/2 VG plot (Figure 2d) in the saturation region (VDS ) 100 V) to be 5.9 × 10-3 cm2 V-1 s-1 and 38 V, respectively. The µ value was smaller by 2 orders of magnitude than the value (0.1-0.3 cm2 V-1 s-1) of C60 FET with Au electrodes fabricated under normal conditions.10,11 This low µ value is due to the existence of a significant carrier-injection barrier between electrodes and thin films of C60 because the µ value of the FET device is affected from both intrinsic mobility of the semiconductor and carrier-injection barrier.7 The normalized ID - VDS plots for the C60 FET devices with the Au electrodes modified by 1-alkanethiols (CnH2n+1SH: n ) 4, 6, 10, 12, and 16) at VG ) 100 V are shown in Figure 3a; as a reference, the plot for the FET device with pristine Au electrodes (n ) 0) is also shown. Concave-up characteristics in normalized ID - VDS plots are clearly found in the FET devices with 1-alkanethiols with long alkyl chains (n ) 10, 12, and 16). The concave-up characteristics are analyzed with a thermionic emission model for double Schottky barriers in the subsequent section. Analyses of Concave-Up ID Curves Based on Thermionic Emission Model for Double Schottky Barriers. The thermionic emission model for a single Schottky barrier in the junction of the metal-insulator-semiconductor can be expressed as12-20

J ) A*T2 exp(-βl) exp(-φB/kBT) exp(eVDS/nkBT)[1 exp(-eVDS/kBT)] ) A*T2 exp(-(φB + kBTβl)/kBT) exp(eVDS/nkBT)[1 - exp(-eVDS/kBT)] (1) and

A* ) 4πemn*kB2/h3

(2)

where J, A*, e, kB, n, mn*, and h are current density, effective Richardson constant, elementary charge, Boltzmann constant, ideality factor, effective mass of electron, and Planck’s constant, respectively. Here, φB is the barrier height for the metal-C60 junction and exp(-βl) is the attenuation factor acting on A* by the tunneling process through an insulator such as the alkyl chain at the interface. β and l are the tunneling constant that refers to the tunneling efficiency of electrons through the insulator and the thickness of the insulator, respectively; the smaller β value can lead to a higher efficiency of carrier tunneling through the insulator. This model substantially refers to the emission of electrons from a hot metal surface (i.e., eq 1 is the Schottky diode equation modified to incorporate the voltage dependence of the barrier height given by eq 3 and the effect of the tunneling barrier produced by the inserted insulator).12-20

φB(V) ) φB + eV(1 - 1/n)

(3)

Figure 3. (a) ID/IDO vs VDS plots at VG ) 100 V for the C60 FET devices with the Au electrodes modified by 1-alkanethiols. The letter n refers to the number of C in 1-alkanethiols (CnH2n+1SH). IDO is the ID value of the device with each 1-alkanethiol at VDS ) 100 V and VG ) 100 V. (b) J - VDS plot in VDS at 0-20 V for the C60 FET with the Au electrode modified by 1-hexadecanethiol. The red line is drawn by a least-squares curve fit with the thermionic emission model for double Schottky barriers (eq 14). (c) VG dependence of φBeff determined by the leastsquares curve fit for the J - VDS plot at each VG. The red line is drawn by a linear least-squares fit, and the φB0eff was determined from the intercept of the line.

This voltage dependence is caused by the mirror-force effect.13,14 The schematic representation of the energy band diagram of the junction of the Au electrode-(1-alkanethiol)-C60 thin films is shown in Figure 1d. The total effective barrier height, φBeff, is defined as16-20

φBeff ) φB + kBTβl

(4)

The φBeff contains both φB and the additional barrier height imposed by the thin insulator 1-alkanethiol. Eq 1 is rewritten with eq 4 as follows:

J ) A*T2 exp(-φΒeff/kBT) exp(eVDS/nkBT)[1 exp(-eVDS/kBT)] (5) Therefore, as in Figure 1d, if a device is composed of a single junction with the φBeff value, the efficiency of electron tunneling through 1-alkanethiol at the interface of the Au-C60 junction can be estimated on the basis of eq 4 from the temperature dependence of φBeff determined by eq 5.

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Figure 4. Energy diagrams for double Schottky barriers at an applied bias voltage of (a) 0 and (b) V () V1 + V2). The voltage dependence by mirror-charge effect is not considered.

As seen from Figure 1a, the source/drain electrodes of the C60 FET device are covered with the same 1-alkanethiol (i.e., this device contains two Schottky barriers). Therefore, it requires the analyses based on the thermionic emission model for double Schottky barriers. The schematic representation for double Schottky barriers is shown in Figure 4a; the barrier height corresponds to the effective Schottky barrier φBeff, which contains not only the Schottky barrier originating from the junction of Au-C60 but also the tunneling barrier through the insulator inserted. When VDS is applied into this device, the energy diagram should change to what is shown in Figure 4b. Here, most of VDS is assumed to be on two junctions between C60 thin films and electrodes. As is seen from this energy diagram (Figure 4a), the net J in the junction of the drain electrode and semiconductor (barrier 1), which is named J1, is a forward current because the built-in potential, eVBi1, decreases to e(VBi1 - V1) in the semiconductor side, and the Schottky barrier height for barrier 1, φB1eff, never changes. On the other hand, the net J in the junction of the source electrode and semiconductor (barrier 2), which is named J2, is a reverse current because eVBi2 increases to e(VBi2 + V2) in the semiconductor side, and even in this case, the Schottky barrier height for barrier 2, φB2eff, never changes. Here, V1 and V2 are voltages applied for barriers 1 and 2, respectively. The direction of J1 and J2 is the same, and these are expressed as

J1 ) Js1[exp(eV1/kBT) - 1]

(6)

J2 ) -Js2[exp(-eV2/kBT) - 1]

(7)

and

where Js1 and Js2 are the reverse saturation current densities.12-15 The total current density JT can be derived based on JT ) J1 ) J2 and V ) V1 + V221

JT ) 2Js1Js2 sin h (eV/(2kBT))/{Js1 exp(eV/2kBT) + Js2 exp(-eV/2kBT)} (8) Because one can expect the relationship, φB1eff ) φB2eff ) φBeff, from the device structure (Figure 1a)

Js1 ) Js2 ) A*T2 exp(-φB1eff/kBT) ) A*T2 exp(-φB2eff/kBT) ) A*T2 exp(-φBeff/kBT) (9) Therefore, eq 8 is rewritten by substituting eq 9 into eq 8 as follows:

JT ) A*T2 exp(-φBeff/kBT) sin h (eV/(2kBT))/ cos h (eV/(2kBT)) (10)

We have tried to determine the φBeff value by the least-squares fit to the J - VDS plot in the low VDS region of 0-20 V at VG ) 100 V for the C60 FET device (Figure 3b) with eq 10. However, the parameter fitting could not be achieved because of the difference of the shape of the function of eq 10. As seen from eq 10, this formula should lead to the saturation of J at high VDS. Therefore, this equation cannot apply to this device. Furthermore, we have modified eq 10 by considering the voltage dependence of φBeff induced by the mirror-charge effect.13,14 The voltage dependence of φB1eff and φB2eff are given by

φB1eff(V1) ) φB1eff + eV1(1 - 1/n)

(11)

φB2eff(V2) ) φB2eff - eV2(1 - 1/n)

(12)

and

In this case,12-20 by assuming φB1eff ) φB2eff ) φBeff, eq 8 can be rewritten as

JT ) 2A*T2 exp(-φBeff/kBT) exp(e(V2 - V1)/2kBT) exp(-e(V2 - V1)/nkBT) sin h (eV/(2kBT))/{exp(eV1/nkBT) + exp(-eV2/nkBT)} (13) One assumes V1 ) V2 ) V/2 because the channel resistance in the FET device should be reduced in the case of an application of VG (>VT). In this case, eq 13 becomes

JT ) A*T2 exp(-φBeff/kBT) sin h (eV/(2kBT))/cos h (eV/2nkBT) (14) Here, V in eq 14 corresponds to VDS of the FET device. We have further tried to fit the J - VDS curve in the low VDS region of 0-20 V at VG ) 100 V with eq 14, which is the formula for the double Schottky barriers that involves the voltage dependence of the Schottky barrier height. The J - VDS plot in the low VDS region of 0-20 V at VG ) 100 V for the C60 FET device with Au electrodes modified by 1-hexadecanethiol is shown together with the fitted line in Figure 3b. The φBeff value was determined to be 0.591 ( 0.002 eV by a least-squares fitting to the J - VDS plot (0-20 V) with the thermionic emission model (eq 14) because the plot in the low VDS region is affected exclusively from the carrier-injection barrier. The mn* value in the C60 thin films was fixed to be 1.2m0 based on the theoretical value, 1.21m0;22 m0 is the electron rest mass. Therefore, A* is fixed in the parameter fitting, which should lead to reliable φBeff values because the parameter correlation can be eliminated. The value of n was 1.0071. This value is close to the ideal value of 1, suggesting that the junction of this device consists of almost ideal Schottky diodes. This n value is close to the ideal value in comparison with those, 1.3-3.5, of the metal-(alkyl chains)-Si junction reported recently.20 Here, it is very significant to notice that the slight deviation of n () 1.0071 ( 0.0001) from 1 produces the operation of the FET device because in the case of n ) 1, the output properties of the FET device are completely dominated by the saturation of J caused by the junctions between metal electrodes and C60 thin films as predicted from eq 10. The φBeff value determined from the J - VDS plot at each VG is shown as the φBeff - VG plot in Figure 3c. As is seen from Figure 3c, the φBeff value was independent of the applied VG. Here, it should be noted that the φBeff value was almost constant below and above the VT value of 38 V. This implies that the

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Figure 5. ID - VDS plots at VG ) 100 V at temperatures of 210-320 K; the plots are shown at the 10 K step. (b) Temperature dependence of φBeff. The red line is drawn by a fit of a linear relationship (eq 4).

Figure 6. (a) PE spectrum of Au electrodes modified by 1-hexadecanethiol (CnH2n+1SH: n ) 16). (b) Energy diagram of EF () -φm) of Au electrodes and ELUMO and EHOMO of C60. χs is the electron affinity of C60.

electron accumulation in the channel region of C60 never affects the J - VDS plot in the low VDS region. In other words, the parameters determined from the J - VDS plot in the low VDS region reflect only the electronic structures of the two junctions of Au-(1-hexdecanethiol)-C60 in the C60 FET device. The φBeff value at VG ) 0 V, φB0eff, was determined to be 0.596 ( 0.002 eV from the linear extrapolation of φBeff - VG plot at VG. This value is the net effective carrier-injection barrier height for the C60 FET device with the Au-(1-hexadecanethiol)-C60 junction. Temperature Dependence of ID -VDS Plot for the FET with Au Electrodes Modified by 1-Alkanethiols. The ID -VDS plots at VG ) 100 V at 210 - 320 K are shown in Figure 5a. The concave-up nonlinearlity in the low VDS region is observed in the plots. The φBeff value was determined from the J - VDS plot at VDS ) 0-20 V at the fixed VG value of 100 V at each temperature based on the thermionic emission model for double Schottky barriers (eq 14). The temperature dependence of the φBeff value is shown in Figure 5b. As seen from Figure 5b, the φBeff increases linearly with an increase in temperature up to 320 K. This result shows a significant influence of the tunneling barrier produced by the insertion of 1-alkanethiol between Au electrode and C60 thin films, as is expected from eq 4. The β value associated with the efficiency of electron tunneling can be estimated from the slope of the φBeff - T plot with eq 4. Since l is 18.6 Å for 1-hexadecanethiol (Figure 1c), the β value is determined to be 1.12 ( 0.06 Å-1; alkanethiols are known to stand on the Au surface at a tilt angle of 35° (Figure 1c).9 The β value is consistent with those, 0.43-1.3 Å-1, of Si-C linked organic monolyer systems.20 Especially, it should be noted that this β value of 1.12 Å-1 is close to that, 0.87 Å-1, of the junction of n-Si-(hexadecane)-Hg.20 This consistency suggests that the change and damage of 1-alkanthiols by thermal deposition of C60 does not occur because the n-Si-(hexadecane)-Hg junction is formed without thermal deposition.20 Thus, the tunneling efficiencies of alkanethiols can be easily determined from the temperature dependence of φBeff. Here, it should be noted that the ID value increases with an increase in temperature (Figure 5a) in spite of the enhancement of φBeff

accompanied by the elevated temperature, which is caused by the additional tunneling barrier kBTβl. This is caused by an increase in the hot electrons, which transfer across the barrier thermally (i.e., the effect of enhancement of the number of hot electrons by raising temperature exceeds that of the suppression of ID by the increase in φBeff). Furthermore, the φB value, which is the barrier height for the Au electrode-C60 junction without 1-hexadecanethiol, can be estimated to be 0.09 ( 0.03 eV from the extrapolation of the plot to 0 K. We have recently estimated the transport properties of the contact resistance, RC, in C60 FET with Au electrodes from device-size dependence of the resistance R between the source/drain electrodes.23 From this preliminary experiment on the temperature dependence of RC, the Schottky barrier height was found to be 0.07 eV for the Au-C60 junction. The φB of 0.09 eV for the Au-C60 junction determined from the J - VDS plot is almost consistent with that, 0.07 eV, from the preliminary experiment. Therefore, we have concluded that the Schottky barrier height for the Au-C60 junction is ∼0.1 eV. Typical output properties (linear and saturation) observed in the C60 FET device with pristine Au electrodes (inset of Figure 2a) can be reasonably explained by the fact that only a very small barrier (∼0.1 eV) is formed in the device. Here, it is assumed that the surface structures of Au and C60 thin films are not changed by the contact with insulator, and this assumption seems to be reliable because of a small difference in the observed φB value between pristine Au and modified Au surfaces, as described in the following section. This assumption is also accepted in the Si-C linked organic monolayer systems.20 Electronic Structure Produced by a Contact of Au Electrodes Modified by 1-Alkanethiols and C60 Thin Films. The values of φm for the used electrodes can experimentally be determined from the onset of the PE spectra. The PE spectrum for the Au electrode modified by 1-hexadecanethol is shown in Figure 6a. The onset energy of the PE spectrum, which corresponds to φm, is 4.9 eV, and this value is slightly smaller than that, 5.1 eV, of the Au electrode. The EF values of the Au

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electrodes modified by 1-alkanethiols, and the ELUMO and EHOMO of the C60 thin films, are shown in Figure 6b. The Schottky barrier height for the contact of C60 and Au modified by 1-hexadecanethiol (or pristine Au without modification) is predicted to be 1.3 eV (or 1.5 eV) from the difference between the EF of -4.9 eV (or -5.1 eV) for the electrode and ELUMO of C60 (-3.6 eV).24-26 The φB value () 0.09 eV), which is associated with the net barrier height for the Au-C60 junction, is less than 1/10 of the predicted value (1.3-1.5 eV) for AuC60. This implies that the actual barrier height is remarkably lowered. This origin is discussed later. The net Schottky barrier height, φBeff(V), for the junction of Au-C60 must be varied from the expected Schottky barrier height by the mirror-charge effect,13,14 as described in previously. Furthermore, the φB value in φBeff(V) [) φB(V) + kBTβl ) φB + eV(1 - 1/n) + kBTβl] should also be lowered. The vacuum level shift, ∆, due to the formation of the electric double layer produced by the contact between electrodes and thin films of C60, may also be necessary to be considered for the estimation of the actual Schottky barrier height.25 If the ∆ value is negative, the actual φB value should be lowered. Furthermore, the existence of interface states can lower φB because the alignment of the Fermi levels is not produced by the electron transfer from semiconductor to metal but from the interface states to the metal.13,14 Actually, a large number of permitted states (i.e., interface states) should be produced in the band gap of the thin films of C60 near the interface between Au electrodes and thin films; the interface states are produced by the disturbance of the periodic nature of the C60 lattice near the interface, and electron trapping in the interface states originates from the weak n-character of the C60 thin films. These factors may lead to the actual lowering of φB in the Au-C60 junction; this is expected to be the origin of a remarkable decrease in φB from the expected value of 1.3-1.5 eV. In spite of the very small barrier height of ∼0.1 eV in the Au-C60 junction, the actual barrier height, φBeff, reaches a maximum value of 0.685 ( 0.006 eV at 320 K, as discussed in the third section of the Results and Discussion. This can be attributed to the formation of the tunneling barrier by the insertion of 1-hexadecanethiol between Au electrode and C60 thin films. This tunneling barrier is the origin for the concaveup nonlinearlity in the low VDS region. Consequently, the φBeff value is an indicator of the tunneling barrier produced by the insertion of the alkyl chains with high resistance.

Here, we raise a question as to whether all 1-alakanethiols form only single monolayers. The β value of 1.12 Å-1 obtained for C60-(1-hexadecanethiol)-Au FET is close to that for Hg(hexadecane)-Au junction where the single monolayer is formed.20 These results support that 1-hexadecanethiol forms a single monolayer on the Au electrode as in Hg-(hexadecane)Si. Finally, this study shows that the single monolayers of 1-alkanethiols with long alkyl chains can act effectively as the carrier-injection barrier. The analyses of the transport properties of the organic FET device with two tunneling barriers have never been achieved thus far, although a few studies on double Schottky diodes such as ZnO-V2O5 ceramics were reported.28 Therefore, this study is the first quantitative analysis for transport properties of organic FET devices with double Schottky barriers produced by formation of the tunneling barriers through insulators. Consequently, this study accelerates the quantitative understanding of the Schottky barrier on the Au electrodes modified by 1-alkanethiols or other insulators. The formation of the tunneling barriers at the interface between the electrodes and the active layers plays an important role in electronic devices such as spinelectronics and single electron transistors (SET). Actually, the effective spin injection is achieved by the formation of a tunneling barrier with aluminum oxide between ferromagnetic metal electrode and semiconductor in spinelectronics,29,30 and the formation of a thinner and defect-free tunneling barrier can further increase the spin injection efficiency. Therefore, the utilization of 1-alkanethiols for the formation of a tunneling barrier should enhance the spin-injection efficiency. Furthermore, 1-alkanethiols may be utilized for SETs, especially based on organic molecules, because the quantum dot with organic molecules and organic clusters can be stably maintained through the formation of double tunneling junctions with 1-alkanethiols. Furthermore, the formation of an effective carrier-injection barrier by 1-alkanethiols on Au electrodes shows a potential application toward the thin dielectric gate insulators in FET devices. This will realize a low gate voltage operation in the organic FET devices. Thus, the effective formation of the carrier-injection barrier in the organic devices can be closely associated with the development of future high performance electronics.

Conclusion

References and Notes

In conclusion, the insertion of 1-alkanethiols with long alkyl chains into the interface between the Au electrodes and thin films in the C60 FET device produced a large injection barrier (i.e., a concave-up nonlinear J - VDS curve in the low VDS region). The height of the charge-injection barrier has been determined on the basis of the thermionic emission model for double Schottky barriers, and it has been found that the carrierinjection barrier in the C60 FET device with the Au electrodes modified by 1-hexadecanethiol is mainly formed by a tunneling barrier of 1-hexadecanethiol inserted into the Au-C60 interface. In this study, we assumed the formation of single monolayers of 1-alkanethiols on Au electrodes based on a previous report.27 However, the β values for some alkanethiols other than 1-hexadecanethiol seem to be slightly different from that of 1-hexadecanthiol because the concavity in the low VDS region does not smoothly depend on the length of 1-alkanethiol or number of carbons. This may be due to either the different affinity or different structure of 1-alkanethiols on Au electrodes.

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Acknowledgment. This study was partly supported by a Grant-in-Aid (18340104) from MEXT, Japan.

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