Systematic Control of Hole-Injection Barrier Height with Electron

Feb 14, 2014 - The effective hole-injection barrier heights (ϕheffs) for [7]phenacene single-crystal FETs have been plotted as a function of the redo...
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Systematic Control of Hole-Injection Barrier Height with Electron Acceptors in [7]phenacene Single-Crystal Field-Effect Transistors Xuexia He,† Shino Hamao,† Ritsuko Eguchi,† Hidenori Goto,†,‡ Yukihiro Yoshida,§ Gunzi Saito,§ and Yoshihiro Kubozono†,‡,* †

Research Laboratory for Surface Science, Okayama University, Okayama 700-8530, Japan Research Center of New Functional Materials for Energy Production, Storage and Transport, Okayama University, Okayama 700-8530, Japan § Research Institute, Meijo University, Nagoya 468-8503, Japan ‡

S Supporting Information *

ABSTRACT: The interface between the single crystal and the Au source/drain electrodes in [7]phenacene single-crystal field-effect transistors (FETs) was modified using 14 electron acceptors with different redox potentials. The effective hole-injection barrier heights (ϕheffs) for [7]phenacene single-crystal FETs have been plotted as a function of the redox potential (Eredox) of the inserted electron acceptors, showing that the ϕheff decreases with increasing Eredox. The highest ϕheff occurs without inserted material (electron acceptors), and this deviates from the otherwise linear relationship between ϕheff and Eredox. We have investigated the temperature dependence of ϕheff in an attempt to determine why the ϕheff value without inserted material is so high, which suggests that no additional barrier, such as a tunneling barrier, is formed in the device. We conclude that the pure Schottky barrier in this FET is lowered very significantly by the insertion of an electron acceptor. The gate-voltage dependence of ϕheff suggests a slight reduction of Schottky barrier height owing to hole accumulation. Furthermore, the clear correlation between threshold voltage and redox potential suggests a relationship between threshold voltage and ϕheff. Controlling the interface between the single crystal and the source/drain electrodes in this FET produced a very high μ (∼6.9 cm2 V−1 s−1) and low absolute threshold voltage, i.e., excellent FET characteristics. The topological characterization of inserted materials on [7]phenacene single crystals are achieved using atomic force microscope (AFM) and X-ray diffraction (XRD). The results show that the single crystals are not completely covered with the inserted materials and the inhomogeneous modification of inserted materials for single crystals effectively leads to the drastic change of hole-injection barrier between source/drain electrodes and single-crystal active layer. V−1 s−1 in a [6]phenacene thin film FET,19 now exceeded by a value of 7.3 cm2 V−1 s−1.20 For comparison, the first singlecrystal FET was fabricated using picene,21 and had a μ value of 1.3 cm2 V−1 s−1 with an HfO2 gate dielectric. In the picene FET, the interface between the crystal and the source/drain electrodes was modified with 7,7,8,8-tetracyanoquinodimethane (TCNQ), which is a typical electron acceptor material, which significantly improved its FET properties. In particular, the concave character of output curves in the low range of absolute drain voltage, |VD|, changed to linear, suggesting a reduction of the hole-injection barrier height, ϕh. Subsequently, the FET properties of a [7]phenacene single-crystal FET were systematically investigated, and 2,3,5,6-tetrafluoro-TCNQ (F4TCNQ), which is a stronger electron acceptor than TCNQ, was inserted into the interface between the crystal and the source/drain electrodes.22 The μ value of the [7]phenacene single-crystal

1. INTRODUCTION Organic field effect transistors (FETs) are attractive candidates for applications such as active matrix displays, radio frequency identification tags, and compact sensors due to their flexibility and adaptable design.1−6 Studies of organic FETs and organic thin films have generally focused on their use in active layers.1−6 During the past decade, however, FETs based on organic single crystals have been investigated extensively because the intrinsic properties of their organic components can be evaluated quantitatively with relative ease and precision. This is due to single crystals’ relative freedom from complicating factors such as impurities, defects and grain boundaries, compared with thin films.7−15 Recently, thin film FETs were fabricated using phenacenetype molecules such as picene, [6]phenacene, and [7]phenacene;16−20 “phenacene” refers to a W-shaped structure (or multiple Ws) consisting of fused benzene rings (Figure 1(a)). They show excellent p-channel FET characteristics, including a field-effect mobility, μ, higher than 1.0 cm2 V−1 s−1. Until very recently, the highest reported μ value was 3.7 cm2 © 2014 American Chemical Society

Received: October 31, 2013 Revised: February 14, 2014 Published: February 14, 2014 5284

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Figure 1. Molecular structures of (a) picene, [6]phenacene and [7]phenacene, and (b) insertion materials used in this study. (c) AFM image of [7]phenacene single crystal and (d) device structure of a [7]phenacene single-crystal FET.

Table 1. Eredox of Inserted Materials and Their Abbreviations name of insertion material

abbreviation

Eredox

2,5-dihydroxy-p-benzoquinone 2,5-dichloro-3,6-dihydroxy-p-benzoquinone 2,5-dibromo-3,6-dihydroxy-p-benzoquinone 2,3,5,6-tetraiodo-p-benzoquinone 2,3,5,6-tetrachloro-p-benzoquinone 2,5-diecthyl-7,7,8,8-tetracyano-p-quinodimethane 2,5-dimethyl-7,7,8,8-tetracyano-p-quinodimethane 7,7,8,8-tetracyano-p-quinodimethane 9,9,10,10-tetracyano-2,6-naphthoquinodimethane 9-(dicyanomethylene)-2,4,5,7-tetranitrofluorene 2-fluoro-7,7,8,8-tetracyano-p-quinodimethane 2,5-difluoro-7,7,8,8-tetracyano-p-quinodimethane trifluoromethyl-7,7,8,8-tetracyanoquinodimethane 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-p-quinodimethane

Q(OH)2 QCl2(OH)2 QBr2(OH)2 QI4 QCl4 Et2TCNQ Me2TCNQ TCNQ TNAP DTENF FTCNQ F2TCNQ CF3TCNQ F4TCNQ

−0.38 −0.13 −0.12 −0.02 0.05 0.15 0.15 0.22 0.23 0.23 0.32 0.41 0.44 0.60

FET reached 4.7 cm2 V−1 s−1 with a SiO2 gate dielectric whose interface was modified by F4TCNQ. In the low |VDS| range, linear behavior was observed in output curves of an FET device with F4TCNQ inserted, as in one with TCNQ. Thus, interface control using an electron acceptor has been shown to reduce ϕh, the contact resistance. Therefore, it is of interest to evaluate quantitatively the ϕh in organic single-crystal FETs with electron acceptors. This should lead to a deeper understanding of the control of the hole-injection barrier. Interface control will let us improve not only the contact between the crystal and source/drain electrodes but also other FET properties such as μ, threshold voltage VTH, on−off ratio, and the subthreshold swing S. In this investigation, we have systematically explored the FET properties of [7]phenacene single-crystal FETs by inserting 14 electron acceptors with different redox potentials.

Their abbreviations and first redox potentials (vs. saturated calomel electrode23) are listed in Table 1 and the molecular structures are shown in Figure 1b. The ϕh values have been evaluated from the output curves of [7]phenacene single-crystal FETs with a thermionic emission model for double Schottky barriers,24 and their FET parameters were also determined with a conventional MOSFET formula in the saturation regime.25 For reference, a [7]phenacene single-crystal FET without interface control (no inserted material) was also fabricated and characterized.

2. EXPERIMENTAL SECTION A [7]phenacene sample with a purity of 99.9% was obtained from NARD Co Ltd.. Single crystals of [7]phenacene were prepared by a conventional physical-vapor-transport method, whose details are described elsewhere.21,22 The single crystals 5285

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were transparent-plate like, with the flat side (2.0 × 0.7 mm) parallel to the ab-plane and thickness (200−1000 nm) being grown along the c-axis, as in the single crystals used in the previous work.22 An atomic force microscope (AFM) image of a single crystal is shown in Figure 1c, displaying a very flat surface (root-mean-square roughness Rrms ∼ 0.09 nm over a 1 μm × 1 mm area), an Rrms smaller by 2 orders of magnitude than that, 4.8 nm, of [7]phenacene thin films.18 The AFM image of the surface (10 × 10 mm) shown in Figure 1c shows the presence of a step. The difference in height between two terraces (step height) is ∼1.7 nm, suggesting a difference of one monolayer. Thus, the thickness is almost constant across a single crystal. Nevertheless, since the thickness of individual crystals used in this study ranged from 200 to 1000 nm, an additional contribution from different crystal thicknesses may be found when evaluating the contact resistance between the crystal and the source/drain electrodes. This problem is also discussed in section 3.7. A [7]phenacene single-crystal FET was fabricated with a topcontact/bottom-gate structure, as seen in Figure 1d. The SiO2 (300 or 400 nm)/Si substrate was washed by the method reported previously.22 The SiO2 surface was coated with 30 nm of parylene to form a hydrophobic surface and to minimize the leakage of gate current IG. The [7]phenacene single crystal was carefully placed on the parylene-coated SiO2 surface. Then 50 nm thick Au source/drain electrodes were formed on the single crystal by thermal deposition under vacuum to form the top contact structure. To modify the interface between the crystal and the Au source/drain electrodes, an electron-acceptor material was thermally deposited on the crystal before the formation of the electrodes. The thickness of the inserted materials was fixed at 3 nm; their densities, which were needed for checking their thickness, are listed in Table S-1 in the Supporting Information. Most electron-acceptor materials were synthesized according to procedures reported elesewhere.23 Only four materials (TCNQ, FTCNQ, F2TCNQ, and F4TCNQ) were purchased, from either Aldrich Co. Ltd. or Tokyo Kasei Industry Co. Ltd. The names of all materials and their abbreviations as used in this text appear in Table 1. Channel length L and channel width W of the single-crystal FET devices fabricated in this study were 30 μm and 200−2000 μm, respectively. The capacitance per area, C0, was determined by their AC capacitance to be 11 nF cm−2 for parylene (30 nm)/SiO2 (300 nm) and 9.1 nF cm−2 for parylene (30 nm)/ SiO2 (400 nm). Most data were obtained from [7]phenacene single-crystal FETs with parylene (30 nm)/SiO2 (300 nm) gate dielectrics; only data for Figure 6 and Figure 7c were obtained with [7]phenacene single-crystal FETs with parylene (30 nm)/ SiO2 (400 nm) gate dielectrics. The FET characteristics at room temperature were measured using a probe system (HiSOR HMP-400) in an Ar-filled glovebox, while temperature-dependent FET characteristics were measured using a cryogenic microprobe system (Riko Instrumental Ltd. i-series); measurements were made in two-terminal measurement mode. All data were recorded using a semiconductor parametric analyzer (Agilent B1500A). AFM and X-ray diffraction pattern (XRD) were measured using AFM spectrometer (SII Nano Technology SPA400) and XRD diffractometer (RIGAKU Smart lab-pro), respectively.

Figure 2. (a) Transfer and (b) output curves of [7]phenacene singlecrystal FET with SiO2 gate dielectric, in which Et2TCNQ is inserted into the space between the crystal and the electrodes.

transfer and output curves of a [7]phenacene single-crystal FET with a SiO2 gate dielectric, in which thin films of 2,5-diethyl− TCNQ (Et2TCNQ) are inserted into the interface between the crystal and the source/drain electrodes. Clear p-channel FET properties are visible in these curves, in which a switch-on drain current is produced by applying negative VG. The hysteresis between forward and reverse transfer curves decreases after long storage of the FET in an Ar-filled glovebox, indicating that the presence of water molecules between the crystal and the SiO2 causes the hysteresis. Similar behavior has been noted previously in a [7]phenacene single-crystal FET.22 The values of μ, |VTH|, on−off ratio, and S were determined to 6.9 cm2 V−1 s−1, 50 V, 1.0 × 109, and 9.3 × 10−1 V decade−1, respectively. The μ value of 6.9 cm2 V−1 s−1 is the highest so far reported in a [7]phenacene single-crystal FET, and is approximately 1.5 times higher than the previous high of 4.7 cm2 V−1 s−1 in a [7]phenacene single-crystal FET with F4TCNQ inserted.22 The |VTH| (=50 V) is larger than the 18 V reported for a [7]phenacene single-crystal FET with F4TCNQ.22 The on− off ratio is consistent with that of 2.2 × 109 in the earlier device.22 Thus, the insertion of Et2TCNQ into the interface between the crystal and the source/drain electrodes is what results in the high μ value. The value of Eredox is 0.15 V, which is smaller than the 0.60 V of F4TCNQ, indicating that Et2TCNQ is a weak electron acceptor. The correlation between μ and Eredox is fully discussed in section 3.6. 3.2. Evaluation of ⟨ϕheff⟩ in [7]Phenacene SingleCrystal FETs with/without Inserted Materials. The output curves at |VD| = 0−20 V and |VG| = 120 V are shown in Figure 3a. The |ID|s of each output curve are normalized to |VD| = 20 V, so that the |ID| at |VD| = 20 V is consistent. In this graph, the output curves are plotted for five typical FET devices with four inserted materials (Q(OH) 2 , Et 2TCNQ, FTCNQ, and F4TCNQ) and one without inserted materials (no insertion), to make the differences clear. The difference in concavity can be clearly seen in these output curves. The highest concavity, implying the largest hole-injection barrier, is observed in the FET without inserted material, and the concavity decreases

3. RESULTS AND DISCUSSION 3.1. Typical FET Characteristics of a [7]Phenacene Single-Crystal FET. Parts a and b of Figure 2 show the 5286

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Figure 3. (a) Output curves at VG = −120 V of [7]phenacene single-crystal FETs with four insertion materials (Q(OH)2, Et2TCNQ, FTCNQ, and F4TCNQ) and without insertion materials (no insertion). The |ID| is normalized at VD = −20 V. Energy diagrams of Au and [7]phenacene (b) before and (c) after their contact, where VD = 0 V. (d) Energy diagram of Au and [7]phenacene after their contact, in which negative VD is applied. In part d, the source electrode is grounded. Schematic of the band bending with/without insertion materials is shown in part d. The solid and dashed lines refer to the band bending without insertion materials and with electron acceptors, respectively.

with increasing Eredox in the four FET devices with inserted materials, suggesting that the insertion of electron acceptors decreases the hole-injection barrier. The hole-injection barrier, which produces the contact resistance between the crystal and the source/drain electrodes, corresponds approximately to the Schottky barrier that is formed by the energy difference between the HOMO of [7]phenacene, εHOMO, and the Fermi level of the Au electrode, εF. As the [7]phenacene single-crystal FET operates in the pchannel (with holes), the energy level of the HOMO is associated with hole conduction. The energy levels, εHOMO (−5.7 eV) of [7]phenacene and εF (−5.1 eV) of Au,18 are drawn in Figure 3b. The fact that the εF is higher by 0.6 eV than εHOMO can produce a Schottky barrier. The energy difference substantially corresponds to the Schottky barrier height, i.e., the potential barrier height from Au to [7]phenacene; this barrier height is defined as ϕh. When the [7]phenacene comes into contact with Au electrodes, an alignment of Fermi levels must take place between Au and [7]phenacene to form the band curvature shown in Figure 3c. The curvature is strictly maintained owing to the formation of a depletion region through the movement of holes to Au due to the alignment of Fermi levels. Here the Fermi level of [7]phenacene is assumed to be located between the εF of Au and the HOMO, i.e., the εF is near the HOMO, although the Fermi level of [7]phenacene is still unclear. This assumption appears reasonable because [7]phenacene is a p-type semiconductor. The actual FET device has two Schottky barriers, as shown in Figure 3d. When a negative VD is applied to the drain electrode, the new band bending shown in Figure 3d would be expected to take place, showing that the actual hole-injection barrier is produced only at the source electrode. That is, the ϕh at the drain electrode does not act as an effective barrier for holes, but only the ϕh at the source electrode is significant.

Not only this electronic band bending but also a tunneling barrier between the crystal and the electrodes may contribute to the formation of the actual potential barrier. The tunneling barrier is probably due to the insulating region produced by the difficulty of forming an ideal contact between the organic crystal and the Au electrodes. Thus, the ϕheff value can be expressed as24,26−30 ϕh eff = ϕh + kBTβl

(1)

where T, kB, β, and l refer to temperature, Boltzmann constant, tunneling efficiency, and length of tunneling barrier, respectively. The value of β is known to be proportional to ϕt1/2, where ϕt refers to tunneling barrier height,31 and a larger β value results in a larger tunneling barrier height. The first and second terms in eq 1 are contributions from electronic band bending (Schottky barrier) and the tunneling barrier, respectively; the kBTβl term corresponds to the height of the tunneling barrier represented on the same scale as ϕh. In this paper, we call kBTβl the tunneling barrier height. We determined the ϕheff values by a least-squares fitting to the output curves at |VG| = 120 V with a formula based on the thermionic emission model for double Schottky barriers.23 The formula is expressed as eV ) ⎤ ⎛ −ϕ eff ⎞⎡ sinh( h ⎟⎢ 2kBT ⎥ ⎜ ID = WtAT exp⎜ ⎟⎢ cosh(eV ) ⎥ ⎝ kBT ⎠⎢⎣ 2nk T ⎥⎦ 2

B

(2)

where W, t, A*, e, and n are the channel width, the thickness of the carrier accumulation region, the Richardson constant, elementary charge, and ideality factor, respectively (T = 300 K). This equation is expanded from a single Schottky barrier.26−30 The t was fixed to the lattice constant c (=1.78 nm) of [7]phenacene,18 i.e., the thickness of a herringbone layer (ablayer). The A* (≡4πemh*kB2/h3) was also fixed at 1.2 × 102 A 5287

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cm−2 K−2, where the effective mass, mh*, is assumed to be equal to the rest mass, m0; we showed the exact mh of picene to be 2.24m0,32 which is fairly near 1.0m0. From the fitting, the ϕh and n values can be definitely determined for each FET device. As typical examples of fitting, the experimental and fitting curves for Q(OH)2 and F4TCNQ are shown in Figure 4, parts a and b,

Figure 5. ⟨ϕheff⟩ plots as a function of the Eredox of each insertion material, as well as the no-insertion case; the ⟨ϕheff⟩ for no-insertion is conveniently plotted at Eredox = 0 V for a comparison with other data although actual Eredox cannot be defined. The solid circle and square refer to plots for a [7]phenacene single-crystal FET in the negative and positive Eredox range. The solid triangle refers to plot for a [7]phenacene single-crystal FET without insertion materials. Numbers indicate the inserted material: 1 = Q(OH)2, 2 = QCl2(OH)2, 3 = QBr2(OH)2, 4 = QI4, 5 = QCl4, 6 = Et2TCNQ, 7 = Me2TCNQ, 8 = TCNQ, 9 = TNAP, 10 = DTENF, 11 = FTCNQ, 12 = F2TCNQ, 13 = CF3TCNQ, and 14 = F4TCNQ. The lines are included as guides for the eye.

scaled completely by Eredox. Thus, a bit of complicated ⟨ϕheff⟩ − Eredox behavior may originate in the difference in tunneling barrier height. Furthermore, the ⟨ϕh⟩ in a [7]phenacene FET without inserted material may fall in the ⟨ϕh⟩s in FETs with inserted materials. These scenarios are examined in section 3.4. The mechanism of the decrease in Schottky barrier height can be well explained based on the band bending shown in Figure 3d. As seen from Figure 3d, the electron acceptor produces a separation between negative and positive charges that reduces the potential barrier for a hole near the electrodes. The dipole moment formed between an electron acceptor and a [7]phenacene single crystal can maintain the new band bending, which reduces the potential barrier height. However, the interface between an electron acceptor and an Au drain electrode cannot produce a depletion layer because of an enrichment of electrons in both the electron acceptor and Au, stimulating the destruction of the Schottky barrier. These effects cooperate to reduce the Schottky barrier. To establish the mechanism presented above, we have to determine the pure Schottky barrier heights, ϕhs, in [7]phenacene single-crystal FETs with/without inserted materials. In general, they must be determined to investigate the effect of any inserted material. 3.4. Temperature Dependence of ϕheff: A Presence of Tunneling Barrier. To distinguish the contributions to the potential barrier height of the Schottky barrier and the tunneling barrier, we plotted the temperature dependence of ϕheff for a [7]phenacene single-crystal FET without any inserted material. The ϕh should be definitely determined by an extrapolation of the ϕheff − T plot to 0 K since the ϕheff is expressed with eq 1. Furthermore, the tunneling barrier can be quantitatively evaluated from the slope of the ϕheff − T plot because the slope corresponds to the tunneling barrier term, kBβl. The ϕheff − T plot of [7]phenacene single-crystal FET without any inserted material is shown in Figure 6. However, the ϕheff − T plot does not show a sloping, but is almost constant. This is inconsistent with the prediction that a clearly varying linear relationship should be obtained. This result (Figure 6) suggests that a tunneling barrier is not formed between the crystals and electrodes, i.e., ϕheff ≈ ϕh. When the

Figure 4. Output curves of [7]phenacene single-crystal FETs with (a) Q(OH)2 and (b) F4TCNQ insertion. The fitting curves are shown as solid lines.

respectively. The ϕheff values are evaluated from at least two FET devices for each inserted material, and the average value, ⟨ϕheff⟩, is used in the discussion in section 3.3. 3.3. Variation of ⟨ϕheff⟩ for Inserted Materials with Different Redox Potentials. Figure 5 shows the ⟨ϕheff⟩ plot as a function of the Eredox of each inserted material. The highest ⟨ϕheff⟩ (=0.40(4) eV) is obtained for the [7]phenacene singlecrystal FET with no inserted material, i.e., the insertion of electron acceptors can reduce ⟨ϕheff⟩. The strongest electron acceptor, F4TCNQ, provides the smallest ⟨ϕheff⟩ among the inserted materials used in this study. The ⟨ϕheff⟩ decreases substantially with increasing Eredox, i.e., the ⟨ϕheff⟩ in FETs with inserted materials can be scaled by Eredox. However, the slope of the ⟨ϕheff⟩ - Eredox plot is steeper in the negative Eredox range, i.e., with weak electron acceptors . These results suggest the following scenarios for the effect of inserted materials on ⟨ϕheff⟩. (1) The ⟨ϕheff⟩ may be associated with two barriers, a Schottky barrier and a tunneling barrier, as suggested in section 3.2. (2) Electron acceptors can reduce the tunneling barrier through improved contact between the crystal and the electrodes; the contribution to the ⟨ϕheff⟩ from the tunneling barrier may not relate to the Eredox. (3) The ⟨ϕheff⟩ decreases in strict correspondence with any increase in Eredox, if the contribution from the tunneling barrier for each inserted material is exactly eliminated in ⟨ϕheff⟩; i.e., the ⟨ϕh⟩ may be 5288

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implies that the ⟨ϕheff⟩ decreases gradually with increasing |VG|. The ⟨ϕheff⟩ − |VG| plots for [7]phenacene single-crystal FETs with Q(OH)2 and F4TCNQ, and without any inserted material, are shown in Figure 7. The ⟨ϕheff⟩ decreases linearly with the application of negative VG only for the FETs with F4TCNQ and Q(OH)2 inserted. The accumulation of holes in the [7]phenacene single-crystal FET changes the ⟨ϕheff⟩ from 0.40(9) eV at VG = −15 V to 0.41(5) eV at VG = −120 V with no inserted material, changes the ⟨ϕheff⟩ from 0.16(2) eV at VG = −15 V to 0.12(1) eV at VG = −120 V with F4TCNQ, and changes the ⟨ϕheff⟩ from 0.36(2) eV at VG = −30 V to 0.32(6) eV at VG = −120 V with Q(OH)2. Because of difficulty in fitting in the output curve at VG = −15 V for the [7]phenacene singlecrystal FET with Q(OH)2, the ⟨ϕheff⟩ at VG ≥ −30 V, is shown in Figure 7a. The slope, d⟨ϕheff⟩/d|VG|, estimated from the

Figure 6. ϕheff−T plots of [7]phenacene single-crystal FETs without any inserted material (no insertion).

tunneling barrier is not formed, the large ⟨ϕheff⟩ recorded in [7]phenacene single-crystal FET without inserted materials may be contributed only from the Schottky barrier. If this is the case, we must conclude that the ⟨ϕheff⟩ formed in FETs without inserted material is quite different from the behavior of ⟨ϕheff⟩ in other FETs with inserted materials. The experimental ϕheff − T plot (Figure 6) suggests that the ⟨ϕheff⟩ in a [7]phenacene single-crystal FET without inserted material corresponds to the pure Schottky barrier height ϕh. If this is the case, the pure Schottky barrier may be destroyed by the insertion of an electron acceptors, owing to the absence of a depletion layer, i.e., the formation of a carrier-enriched region in [7]phenacene single-crystal FETs with inserted material. Actually, it is expected that the pure Schottky barrier height ϕh is 0.6 eV (= εF(Au) − εHOMO), which is close to the observed ⟨ϕheff⟩ = 0.40(4) eV in a [7]phenacene single-crystal FET without inserted material. The ⟨ϕheff⟩ in a [7]phenacene singlecrystal FET without inserted material cannot be compared with the ⟨ϕheff⟩s with inserted material because the former corresponds to the pure Schottky barrier height and the latter corresponds to the Schottky barrier destroyed. Consequently, we have to recognize the ⟨ϕheff⟩ without inserted material as a special case, which is different from the ⟨ϕheff⟩ with inserted material. Therefore, it is reasonable that only ⟨ϕheff⟩ values in [7]phenacene single-crystal FETs with inserted material can be scaled by Eredox, and the variation of band curvature shown in Figure 3d should be scaled by Eredox. The difference in slope of the ⟨ϕheff⟩ − Eredox plot between electron acceptors with negative and positive Eredox may imply that the band curvature is not modified in the same way. Thus, we have exhaustively demonstrated that no tunneling barrier is formed in a [7]phenacene single-crystal FET without inserted material, and that the Schottky barrier may be destroyed by the insertion of an electron acceptor. Therefore, the ⟨ϕheff⟩s (=Schottky barrier height) with inserted material can be correlated with Eredox, except for the ⟨ϕheff⟩ without any inserted material. To sum up, the insertion of material with high Eredoxs can effectively destroy the Schottky barrier. 3.5. VG Dependence of ⟨ϕheff⟩ and Influence of Accumulated Holes in Channel Region. Here we investigated the VG dependence of ⟨ϕheff⟩ in order to clarify the effect on the potential barrier of accumulated holes in the channel region. Simply considering the height of the Schottky barrier formed in the source electrodes, the enrichment of carriers by applying VG may destroy the Schottky barrier which is formed through the contact of the semiconductor and the metal. The carriers enriched in the semiconductor may result in an Ohmic-like contact with metal because of the decrease in the depletion region (area of low carrier concentration). This

Figure 7. ⟨ϕheff⟩−|VG| plots for [7]phenacene single-crystal FETs with (a) Q(OH)2 and (b) F4TCNQ and (c) with no inserted material (no insertion). The solid lines are determined by the least-squares method.

⟨ϕheff⟩ − |VG| plots shown in parts a−c in Figure 7 are −4.1 × 10−4 with Q(OH)2, −5.0 × 10−3 with F4TCNQ and 9.8 × 10−5 with no inserted material. Namely, the slope is negative for insertion, and positive but extremely small for no insertion. The results imply that the lowering of the potential barrier caused by hole accumulation from the application of VG is different between FETs with and without inserted material, and that the lowering of ⟨ϕheff⟩ due to hole accumulation is small. 3.6. Correlation between FET Parameters and Eredox. Figure 8a shows the plot of the averaged μ, ⟨μ⟩, as a function of Eredox in a [7]phenacene single-crystal FET with/without 5289

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value, instead of the contact resistance. In other words, by inserting high Eredox materials the contact resistance is minimized but the channel of the [7]phenacene device probably remains unchanged, so that the channel conductance governs the μ in devices that provide a low ϕheff (or have high Eredox inserted materials), which should lead to the saturation of μ . However, it remains unclear why ⟨μ⟩ values are too low for some inserted materials with an Eredox in the range of 0.2 to 0.45 V. Figure 8b shows the plot of average |VTH|, ⟨|VTH|⟩, as a function of Eredox in [7]phenacene single-crystal FETs with/ without inserted material; all VTH values were negative. Except for ⟨|VTH|⟩ for the [7]phenacene single-crystal FET without inserted material, all ⟨|VTH|⟩s lie essentially on the straight line, showing a monotonic decrease in |VTH| with increasing Eredox. This is closely related to the decrease in ϕheff with increasing Eredox. That is, the smooth hole-injection from the source electrode to the [7]phenacene crystal should cause a rapid switch-on (or a small |VTH|), because the VTH (or switch-on voltage) in the transfer curve is affected by two factors in twoterminal measurement mode: trap-filling by carrier-accumulation (the main, or direct factor), and carrier-injection from the electrode (indirect factor). Thus, the correlation between |VTH| and Eredox is closely associated with that between ϕheff and Eredox. The ⟨|VTH|⟩ in a [7]phenacene single-crystal FET without insertion material is plotted at Eredox = 0 V for a sake of convenience, whose value does not fall on the linear relationship shown in Figure 7 and 8b; i.e., this value is larger than the ⟨|VTH|⟩ values for all inserted materials. It can be explained by the fact that the ϕheff in a [7]phenacene single-crystal FET without insertion material is too large to be scaled by Eredox. The large ϕheff without inserted material, which deviates from the linear relationship in the ϕheff - Eredox plot (Figure 5), must make it difficult to inject holes effectively, increasing the |VTH|. Thus, the |VTH| relates directly to ϕheff. 3.7. Effect of Crystal Thickness on Hole-Injection Barrier Height. We have found what appear to be clear correlations of ϕheff and VTH with the Eredox of insertion materials. To confirm the validity of these correlations, we have to take into account various problems that may affect ϕheff and VTH. In this study, [7]phenacene single crystals with a thickness ranging from 200 to 1000 nm were used in the evaluation of FET parameters (μ, VTH, on−off ratio, and ϕheff). Therefore, the effects of the thickness of such crystals on ϕheff should be fully investigated. As an example, the plot of ϕheff as a function of crystal thickness in a [7]phenacene single-crystal FET with source/drain electrodes modified with F2TCNQ is shown in Figure 9; the ⟨ϕheff⟩ value (= 0.17(2) eV) for F2TCNQ shown in Figure 5 is evaluated from the ϕheff values plotted in Figure 9. The plot shown in Figure 9 shows the independence of ϕheff and crystal thickness, i.e., the thickness of the crystal does not affect the ϕheff or contact resistance within the thickness range of 200−1000 nm. This implies that the variation of FET characteristics clarified in this study was produced exclusively by the selection of insertion material. In other words, this study evidently evaluates FET characteristics caused by the insertion of electron acceptors. 3.8. Topological Characterization of Inserted Electron Acceptor Layer. In this study, various electron acceptors are inserted between Au source/drain electrodes and [7]phenacene single crystals. In the device fabrication, 3 nm thick electron acceptor is deposited on single crystal, and subsequently Au is

Figure 8. (a) ⟨μ⟩ and (b) ⟨|VTH|⟩ as a function of Eredox in [7]phenacene single-crystal FETs with/without inserted material. Arabic numbers indicate the inserted material as in Figure 5. The lines are included as guides for the eye. The ⟨ϕheff⟩ value for no-insertion is conveniently plotted at Eredox = 0 V for a comparison with other data although the actual Eredox cannot be defined.

inserted material. The plots show no clear correlation between μ and Eredox; two data (inserted materials 5 and 6) possessed huge error bars. The μ value should reflect not only channel conductance but also contact resistance (or injection barrier), since the measurement of FET characteristics is performed in two-terminal measurement mode. Therefore, it is first assumed that μ increases monotonically with increasing Eredox because of the decreases in ϕheff. In the plot shown in Figure 8a, except for the ⟨μ⟩ for no insertion, ⟨μ⟩ seems to increase with increasing Eredox in the low Eredox range from negative to 0.15 V, regardless of huge error bars of two data. This is easily understood based on the decrease in ϕheff. Actually, the μ for a [7]phenacene single-crystal FET without insertion material is too low, as seen from the plot (Figure 8a), which is also explained based on the extremely high ϕheff. However, the ⟨μ⟩ values appear to scatter broadly in the Eredox range above 0.15 V. The ⟨μ⟩ values exhibit a maximum estimate (∼4 cm2 V−1 s−1) at around Eredox = −0.1−0.15 V, and as described in section 3.1, the highest μ value (= 6.9 cm2 V−1 s−1) appears in a [7]phenacene singlecrystal FET with source/drain electrodes modified with Et2 TCNQ (Eredox = 0.15 V). The ⟨μ⟩ value for the [7]phenacene single-crystal FET with Et2TCNQ shown in Figure 8a is also approximately the maximum. The scattering of ⟨μ⟩ found in the positive Eredox range cannot simply be explained based on the contact resistance since the ⟨ϕheff⟩ decreases substantially with increasing Eredox in the positive Eredox range above 0.15 V. The μ values for F2TCNQ (Eredox = 0.41 V) and F4TCNQ (Eredox = 0.6 V) are very similar to the maximum value. This may be interpreted by a scenario in which the channel conductance dominates the μ 5290

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F4TCNQ and 50 nm for ET2TCNQ, indicating the aggregation of the inserted materials and inhomogeneous coverage. The XRD patterns of F4TCNQ (3 nm) and Et2TCNQ (3 nm) deposited [7]phenacene single crystals were measured. In the XRD patterns, only 00l reflections ascribable to [7]phenacene were recorded and no reflections due to the above inserted materials are observed, indicating the no formation of homogeneous layers of the inserted materials, consistent with the indication obtained from AFM images (Figure 10, parts b and c). Therefore, the modification of electronic state between electrodes and single crystal (or modification of hole-injection barrier height) is effectively achieved by such an inhomogeneous deposition that most of the area in single crystal is not covered with electron acceptor. These results enable ones to imagine that the inhomogeneous deposition may be profitable because of formation of incomplete insulating layer (electronacceptor layer).

Figure 9. ϕheff as a function of the thickness of the single crystal in [7]phenacene single-crystal FETs with F2TCNQ inserted. The dashed line is included as a guide for the eye.

deposited on the electron acceptor to form source/drain electrodes. Therefore, it is of importance to clarify the topological feature of electron-acceptor layer which may enable ones to understand the influence of topological feature other than redox potential of electron acceptor on electronic states at the interface. The topological characterization of F4TCNQ and Et2TCNQ were carried out using AFM and XRD. The AFM image of surface of [7]phenacene single crystal is shown in Figure 10a, together with those of [7]phenacene single crystals on which F4TCNQ and Et2TCNQ are deposited by 3 nm (Figures 10, parts b and c). The AFM image of [7]phnacene single crystal (Figure 10a) is similar to that shown in Figure 1c, showing the terraces and steps. Small islands ascribable to F4TCNQ (bright spots and stripes) are randomly observed in the AFM image (Figure 10b), showing that F4TCNQ does not cover all area of single crystal (incomplete layer-formation). This implies that the direct contact is formed in most of the interface between Au source/drain electrodes and single crystals. Furthermore, a regular arrangement of islands such as the aggregation at the step lines does not seem to appear. As seen from Figure 10c, Et2TCNQ is deposited in the confined area of surface of [7]phenacene single crystal. We deposited 3 nm Et2TCNQ on [7]phenacene single crystal twice and measured AFM images. Both AFM images showed the same topological pattern. This also implies that the Au electrodes contact directly to [7]phenacene single crystals even if 3 nm thick Et2TCNQ is deposited. The heights of the islands are as high as 15 nm for

4. CONCLUSION Hole injection from electrodes has been precisely controlled by the insertion of various materials (electron acceptors) into the space between a [7]phenacene crystal and source/drain electrodes. The hole-injection barrier formed between the crystal and the electrodes has been closely correlated with the redox potential of the inserted material. The FET characteristics in a [7]phenacene single-crystal FET without the addition of any electron acceptor suggest the presence of a pure Schottky barrier. This conclusion follows because the ϕheff value is much larger than would be consistent with the ϕheff − Eredox plot in [7]phenacene single-crystal FETs with inserted materials, despite the fact that no additional tunneling barrier is formed, as suggested by the temperature dependence of ϕheff. At the present stage, we must conclude that all ϕheffs in [7]phenacene single-crystal FETs with inserted materials can be substantially scaled by Eredox. The |VTH| value can be correlated with Eredox, except for |VTH| in an FET with no inserted material. This result may be reasonable because the decrease in ϕheff due to an increase in Eredox should cause the smooth hole-injection to result in a decrease in |VTH|. In addition, the large ϕheff with no inserted material produces a large |VTH|. In other words, the |VTH| can be meaningfully related to ϕheff. Thus, this study not only shows the effectiveness of interface control using electron acceptors but also leads to its quantitative understanding. Furthermore, controlling the interface with inserted material produced a very

Figure 10. (a) AFM image of [7]phenacene single crystal without any inserted materials. AFM images of (b) F4TCNQ and (c) Et2TCNQ deposited [7]phenacene single crystals. F4TCNQ and Et2TCNQ are deposited thermally by 3 nm. 5291

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high μ value (= 6.9 cm2 V−1 s−1) in a [7]phenacene singlecrystal FET with Et2TCNQ, which is the highest so far reported in phenacene single-crystal FETs. Finally, we characterized the surface of [7]phenacene single crystals on which 3 nm thick inserted materials were deposited, indicating no formation of homogeneous layers between Au source/drain electrodes and single crystals. This may be profitable for modification of the interface between electrodes and single crystal.

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ASSOCIATED CONTENT

S Supporting Information *

Table of densities of inserted materials and complete reference 19. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(Y.K.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to express their gratitude to Profs. Y. Kuroda, T. Kambe and K. Kawaguchi of Okayama University for their valuable discussions during the preparation of this paper. This study was partly supported by Grants-in-Aid (22244045, 24654105, and 23684028) from MEXT, by the Strategic International Collaborative Research Program (JST/SICORP, EU-JPN LEMSUPER) and the Advanced Catalytic Transformation for Carbon Utilization Program (JST/ACT-C) in Japan Science and Technology Agency (JST), by the Program for Promoting the Enhancement of Research Universities from MEXT, and a by Special Project of Okayama University.

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