IEEE ELECTRON DEVICE LETTERS, VOL. 34, NO. 4, APRIL 2013
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Contact Thickness Effects in Bottom-Contact Coplanar Organic Field-Effect Transistors Yong Xu, William Scheideler, Chuan Liu, Francis Balestra, Senior Member, IEEE, Gerard Ghibaudo, Senior Member, IEEE, and Kazuhito Tsukagoshi
Abstract—Influences of contact thickness on bottom-contact and bottom-gate coplanar organic transistors are studied. In transistors with poor-quality pentacene films, thick contacts improve mobility and lower contact resistance. However, in transistors with high-quality pentacene films, thick contacts significantly degrade performance by disrupting molecular self-organization at the contact edge. These results highlight the importance of contact thickness to such organic transistors and reveal that semiconductor morphology should be considered in designing devices with minimal contact effects. Index Terms—Contact resistance, contact thickness, mobility, organic field-effect transistors (OFETs).
Fig. 1. (a) Architecture of the studied pentacene OFETs with different contact thicknesses tC . (b) and (c) Pentacene morphology in the channel observed by atomic force microscopy with Cytop and PMMA dielectric treatments, respectively.
I. I NTRODUCTION
O
RGANIC field-effect transistors (OFETs) are promising building blocks for future ecofriendly organic electronics, but they suffer from high contact resistance RC . Despite huge efforts devoted to this subject in the past decades, OFETs’ RC remains unacceptably high for many applications. Such high RC is a result of the poor charge injection at the contact–organic semiconductor (OSC) interface, as well as the poor charge transport at the contacts [1]. By properly choosing energy-matched contact material and delicately treating the contact–OSC interface, the charge injection barrier can be minimized [2]. Consequently, two key factors that determine RC are small injection area and hopping-dominated transport near the contacts. A larger contact with greater length has already proven effective to reduce contact limitations in staggered OFETs [3]–[5]. However, the contact thickness (or the contact electrode thickness tC ) would be another limit determining RC . For instance, in the commonly used bottom-contact (BC) and bottom-gate OFETs, thicker contacts afford a larger area for charge injection but increase consumption of contact material (often a noble metal, e.g., Au and Ag). Additionally, thick contacts cause inferior OSC molecular packing and thus degrade charge transport at the contacts. Therefore, evidence Manuscript received January 22, 2013; accepted January 25, 2013. Date of publication March 7, 2013; date of current version March 20, 2013. The review of this letter was arranged by Editor B.-L. Lee. Y. Xu, C. Liu, and K. Tsukagoshi are with the World Premier International Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba 305-0044, Japan (e-mail:
[email protected]). W. Scheideler is with the Department of Electrical and Computer Engineering, Duke University, Durham, NC 27708 USA. F. Balestra and G. Ghibaudo are with the Institut de Microélectronique Electromagnétisme et Photonique and Laboratoire d’Hyperfréquences et de Caractérisation, Grenoble Institute of Technology, Micro and Nanotechnology Innovation Centre, 38016 Grenoble, France. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2013.2244059
that illuminates the interplay between these two contradictory factors is highly desired.
II. D EVICE FABRICATION AND C HARACTERIZATIONS Pentacene OFETs were fabricated on heavily doped Si(100) wafers covered with a 50-nm-thick SiO2 [see Fig. 1(a)]. After ultrasonic cleaning by acetone and isopropyl alcohol, the substrates were immersed in a mixture of sulfuric acid and hydrogen peroxide to eliminate organic contaminants and then were spin coated with Cytop (20%) or polymethylmethacrylate (PMMA, 20%), first at 500 r/min for 5 s and next at 4000 r/min for 120 s, forming 320-nm Cytop or 40-nm PMMA layers (measured by surface profiler KLA Tencor P-16+). The coated Cytop and PMMA were dried in N2 at room temperature overnight, and the gold source and drain electrodes were deposited in vacuum onto the pretreated SiO2 through a metal mask, yielding a uniform channel width W = 1000 μm and different channel lengths L varying from 50 to 350 μm with an interval of 50 μm. We did not deposit adhesive metal (e.g., Ti) as it has been observed to deteriorate charge injection [6] and would affect analysis of contact thickness. By monitoring the thickness of the evaporated gold film, diverse tC = 10, 20, 30, 40, 60, 80, and 100 nm were obtained. Finally, an 88-nmthick pentacene film was deposited in vacuum through a metal stencil mask (0.01 nm/s at room temperature), simultaneously for all substrates of different tC . The current–voltage (I–V ) and capacitance–voltage (C–V ) characterizations were carried out at room temperature in air using an HP4156C semiconductor parameter analyzer and an Agilent E4980A LCR meter, respectively. The areal gate capacitance Ci was extracted from the C–V characteristics by the split C–V technique. It was found to be Ci = 5 × 10−9 F/cm2 for Cytop and Ci = 3.5 × 10−8 F/cm2 for PMMA. This Ci was utilized to calculate the carrier mobility.
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IEEE ELECTRON DEVICE LETTERS, VOL. 34, NO. 4, APRIL 2013
Fig. 3. Pentacene morphology in the contact-near region for three OFETs treated by (a), (c), and (e) Cytop and (b), (d), and (f) PMMA, with different contact thicknesses tC = 20, 60 and 100 nm, respectively. All scales are equivalent. Fig. 2. (a) and (b) Transfer characteristics at the drain voltage VD = −10 V of three OFETs (W = 1000 μm, L = 200 μm) with different contact thicknesses for Cytop and PMMA dielectric treatments, respectively. (c) and (d) Low-field mobility (at VD = −0.1 V) evaluated in the two groups of OFETs. Each point corresponds to the average mobility calculated from at least ten transistors on a single substrate. The solid fitting line guides to general contact thickness dependence.
III. R ESULTS AND D ISCUSSION In Fig. 2(a) and (b), the OFETs with the PMMA dielectric treatment exhibited greater performance than the Cytop-treated ones. This is because the quasi-amorphous pentacene films were obtained by using Cytop [see Fig. 1(b)] and the PMMA favors ordered growth of pentacene, resulting in polycrystalline films with large-size grains [≈ μms; see Fig. 1(c)]. The charge transport in the high-quality pentacene OFETs with the PMMA dielectric is thus much more efficient than in the OFETs with Cytop. This result slightly differs from the previous reports in which the Cytop dielectric often produced high performance in pentacene OFETs [7], presumably due to the absence of Cytop baking at proper temperatures after coating here. Interestingly, the influence of variable tC is very different between the two groups. As tC increases from 30 to 100 nm, the ON current is elevated around one order of magnitude in the Cytop-treated group, but this is opposite in the devices with the PMMA dielectric. It implies that different factors govern the charge injection in the two sets of transistors. We evaluated their low-field mobility μ0 [8] [cf. Fig. 2(c) and (d)]. For the OFETs with the Cytop dielectric, mobility is linearly related to tC . To understand this trend, we examined the pentacene morphology near the contacts. As shown in Fig. 3, contacts form an abrupt shelf for the subsequent pentacene deposition. The molecular self-organization is disrupted by the presence of the contacts, causing poor molecular packing in the vicinity of contacts. In Fig. 3(a), (c), and (e), this influence of diverse tC is nearly indistinguishable. Instead, the improved mobility at higher tC could be explained by an additional conduction path at the contact edge provided by thicker contacts [9]. This result indicates that charge injection due to the small contact side area is the primary limitation to the overall transport for the quasi-amorphous pentacene OFETs. It also explains the discontinuity of charge distribution at the channel ends that was observed in coplanar OFETs [10], [11]. The charge carriers
are injected through a tiny contact edge into the OSC. However, their mobility is so small that they are unable to diffuse away from the contacts quickly, forming a “depletion-like” region at the source contact [1], [10]. Thicker contacts provide a larger area for charge diffusion, alleviating the charge discontinuity as well. In contrast to Cytop, the μ0 of high-quality pentacene OFETs treated by PMMA shows a general decreasing tendency with tC [see Fig. 2(d)]. By observing the pentacene morphology shown in Fig. 3(b), (d), and (f), one can see that relatively small grains (even voids) are produced close to the contacts, as compared with in the channel, and this small-grain region extends into the channel at greater tC . The large numbers of traps distributed in this defect-rich zone degrade the charge injection and, concurrently, the channel transport [8], manifesting as reduced mobility with tC . This effect could be particularly significant to the OFETs with short channels and using high-quality OSC films, e.g., the pentacene OFETs on the PMMA dielectric here. As a result, this degrading effect becomes predominant in devices with thicker contacts, and μ0 is decreased by one order of magnitude. The contact resistance RC was also evaluated for different contact thicknesses using the transfer-line method (TLM) [cf. Fig. 4(a) and (b)]. RC is strongly altered by tC , changing more than one decade in both cases for a nearly one decade change in tC . Note that the gate-voltage VG dependence of RC in the Cytop group is clearly stronger than for the PMMA one. In coplanar OFETs, the VG lowered RC was interpreted as a thinning “depletion-like” region at the contacts [12], due to the greater charge induced by increasing VG . In our pentacene OFETs with the Cytop dielectric, the “depletion-like” region would be thicker, and in turn, its response to VG is more significant. In addition, one needs to consider the mobility’s VG dependence. In a highly disordered system incorporating many localized states (e.g., the Cytop-treated pentacene OFETs), the carrier mobility often exhibits strong VG enhancement [13]. This behavior is drastically weakened when disorder and charge trapping are reduced (e.g., the PMMA-treated pentacene OFETs). The RC at a fixed VG is plotted against tC to visualize the dependence on contact thickness [see Fig. 4(c) and (d)]. For the OFETs with the Cytop dielectric, RC decreases with tC , but it no longer follows a linear variation as observed above
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morphology, mobility, and contact resistance on contact thickness is similar to devices with Au contacts. Therefore, the observed effects of the contact thickness appear intrinsic to the architecture of the BC coplanar organic transistors. IV. C ONCLUSION The OSC film morphology is vital in determining an optimal contact thickness that minimizes contact effects in BC coplanar OFETs. In OFETs using poor-quality OSCs such as polymers, relatively thick contacts facilitate charge injection and alleviate contact limitations to the overall charge transport. However, for the OFETs fabricated with high-quality OSCs such as singlecrystal or polycrystalline films, thin contacts are indispensable for avoiding performance degradation. ACKNOWLEDGMENT Fig. 4. (a) and (b) Contact resistance extracted by TLM at VD = −0.1 V. Each curve corresponds to a set of transistors measured on the same substrate. For clarity, only three contact thicknesses are shown here, and the different biasing ranges are for the comparable charge density due to the different Ci . (c) and (d) Contact resistance at a fixed gate voltage, where the fitting solid line is guide for the eyes.
Fig. 5. (a)–(d) Pentacene morphology near copper contacts for different contact thicknesses. The pentacene OFETs were fabricated on PMMA-treated dielectrics using the same process as above aside from the copper contact material. (e) and (f) Mobility and contact resistance measured at VD = −1 V.
for the mobility μ0 . This is reasonable because μ0 reflects the transport in the channel that is limited mainly by a tiny injection area in the Cytop case. However, RC represents both components concerning the interface injection and the contact access transport [14]. The latter should be closely linked to the underlying carrier mobility, i.e., μ0 , in spite of its rather different magnitude at the contacts and in the channel. Increasing tC , the two contributions are all lowered, giving the result shown in Fig. 4(c). For the pentacene OFETs with the PMMA dielectric treatment, however, the degrading effect completely dominates, and RC is thus increased as tC is enlarged. To confirm this inference, we fabricated pentacene OFETs with copper contacts [cf. Fig. 5]. The dependence of pentacene
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