Gate Voltage Dependent Resistance of a Single Organic

C.D.F. thanks the National Science Foundation (DMR-9624154), the David and Lucile Packard Foundation, and the Camille and Henry Dreyfus Foundation for...
0 downloads 0 Views 89KB Size
Download PDF
4538

J. Phys. Chem. B 2001, 105, 4538-4540

Gate Voltage Dependent Resistance of a Single Organic Semiconductor Grain Boundary Tommie W. Kelley and C. Daniel Frisbie* Department of Chemical Engineering and Materials Science, UniVersity of Minnesota, 421 Washington AVenue SE, Minneapolis, Minnesota 55455 ReceiVed: December 18, 2000

Conducting probe atomic force microscopy (CP-AFM) was used to examine electrical transport through an individual grain boundary (GB) in the organic semiconductor sexithiophene (6T, Egap ∼ 2.3 eV). The sample consisted of a pair of grains grown by vapor deposition onto an SiO2/Si substrate. A variable channel length transistor was constructed using a microfabricated Au electrode contacting one grain, a Au-coated AFM tip as a positionable electrode, and the doped Si substrate as a gate. The GB resistance was found to be gate voltage dependent and large, on the order of 109-1010 Ω for a 1 µm boundary length. Resistances across single 6T grains were an order of magnitude lower. The results indicate that GBs can be the principal bottleneck to charge transport in polycrystalline organic semiconductor films, particularly at low gate fields, consistent with a recent model that proposes potential barriers exist between grains. We estimate the GB barrier height to be on the order of 100 meV.

We report the use of conducting probe atomic force microscopy (CP-AFM)1 to measure resistances associated with individual grain boundaries in the organic semiconductor sexithiophene (6T, Egap ∼ 2.3 eV).2 Grain boundaries (GBs) play an important role in the transport properties of field effect transistors (FETs) based on polycrystalline organic semiconductor films.3 In particular, it is expected that GBs trap and scatter mobile charge carriers, increasing the overall film resistance. Understanding of GB effects and the extent to which they dominate transport in organic semiconductors can be improved by microscopic measurements on indiVidual grains and GBs. In the work described here, we have taken advantage of the submicron resolution of CP-AFM to show that GB resistances can be an order of magnitude higher than single grain resistances in sexithiophene. Furthermore, the GB resistances are gate voltage dependent, as predicted by a recently proposed transport model.3b Our studies employ a commercial AFM4 and the experimental configuration shown in Figure 1. We use a Au-coated AFM tip as a positionable electrical contact to 6T grains that are also contacted by a fixed microfabricated Au electrode. The doped Si substrate serves as a gate. Current-voltage (I-V) measurements are made in air as a function of the probe position and gate field by applying negative voltages to the microfabricated electrode while keeping the probe tip at ground. Effectively, the structure in Figure 1 constitutes a FET in which the source (hole injecting) contact is an AFM tip that can be positioned at arbitrary points on the grain. Figure 2A is an AFM topograph showing two 6T grains on SiO2/Si sharing a common boundary approximately 1 µm in length. Both grains display typical 2.3 nm thick terraces corresponding to single layers of 6T molecules. Near the GB, the total thickness of each crystal is 18 nm, or 8 6T layers stacked on top of one another. The well-defined facets on the grains allow identification of their crystallographic orientations; there is a 10° misalignment between the [011] directions shown in the figure. The grain in the upper half of the figure is * Address all correspondence to this author. E-mail: [email protected].

Figure 1. Scheme of the CP-AFM experiment in which a Au-coated AFM probe (1) is used to make electrical contact to a thin crystal of sexithiophene (6T) grown by vapor deposition on SiO2/Si. A microfabricated Au wire (2) serves as the second contact and the doped Si substrate (3) is a gate. This configuration is effectively a variable channel length field effect transistor where the probe and wire serve as the source and drain contacts, respectively.

contacted by a 250 nm wide Au wire, which serves as the drain electrode. The electrode contacting the grain in the lower half of the figure was floated. Figure 2B shows the drain current-drain voltage (ID-VD) traces as a function of gate voltage, VG, obtained when the conducting probe was positioned at the “x” shown in 2A. The curves are characteristic of 6T-based FETs.2 ID increases as VG becomes more negative, consistent with hole conduction. Topographic images taken in tapping mode with the same conducting probe before and after recording the ID-VD curves showed no evidence of damage to the sample. Figure 2 demonstrates that the gate-modulated conductance of the crystals can be recorded with our CP-AFM methodology. The key result is shown in Figure 3. The conducting probe was placed at points 1-7 labeled in Figure 3A, and the ID-VD characteristics were recorded at each point. From the linear portion of the ID-VD data, the differential resistances (dVD/ dID) were determined as a function of gate voltage. Figure 3B shows plots of differential resistance as a function of both probe

10.1021/jp004519t CCC: $20.00 © 2001 American Chemical Society Published on Web 03/31/2001

Letters

J. Phys. Chem. B, Vol. 105, No. 20, 2001 4539

Figure 2. (A) AFM topographic image of a pair of 6T grains sharing a 1 µm grain boundary. The GB and crystallographic orientations of the grains are labeled. A microfabricated Au wire contacts the grain in the upper right. Terraces corresponding to single monolayers of 6T are visible. (B) The ID - VD characteristics as a function of gate voltage obtained when the AFM probe was positioned at the “X” in (A). The inset shows the conductance at VD ) -3 V as a function of VG.

position and VG. It is evident that between points 4 and 5, which straddle the GB, there is a substantial increase in resistance on the order of 109-1010 Ω. We take the difference in resistance between points 4 and 5 to be the GB resistance, RGB. This GB resistance is gate voltage dependent. As VG becomes more negative, RGB drops from 25 to 4 GΩ, as shown in Figure 4. A gate voltage dependent RGB has been predicted for organic semiconductors by Scho¨n and Batlogg,3b in analogy with work on polycrystalline Si.5 The hypothesis is that an electrostatic barrier to hole transport arises at GBs due to the presence of localized (trapped) charge. The GB charge is positive if the GBs contain electron donor states; the charge is negative if the GBs contain electron acceptor states. Direct determination of trapped charge at organic semiconductor GBs has so far not been reported, and the sign of the trapped charge is an outstanding question. However, the presence of any trapped charge at GBs will produce an energy barrier EB to hole transport due to localized band bending. The hole conductivity, σ, depends on the GB barrier height, EB, as shown in

σ ) σ0 exp(-EB/kT)

(1)

where EB is expected to be gate voltage dependent because the gate voltage modifies the GB charge concentration. Our measurements show that RGB decreases and therefore σ increases

Figure 3. (A) Magnified AFM image of the GB region shown in Figure 2A. The probe was positioned at points 1-7. (B) Resistance as a function of gate voltage and probe position. The resistance is actually the differential resistance (dVD/dID) obtained as a function of VG in the linear regime of the ID-VD characteristic at each probe position. The GB resistance is taken to be the difference in resistance between points 4 and 5, which straddle the GB. The resistance between points 1 and 4 is the resistance across the 6T grain.

Figure 4. Grain boundary resistance, RGB, vs gate voltage.

as VG becomes more negative. This means that EB decreases as VG becomes more negative. Using eq 1, we estimate from the RGB plot in Figure 4 that ∆EB ) -45 meV upon changing VG from 0 to -10 V, corresponding to about a 6-fold increase in conductivity through the GB.6 Equation 1 can also be used to approximate the absolute barrier height, EB. From the ID-VD characteristics taken at VG ) 0 on both sides of the GB, we estimate that the conductivity differs by a factor of at least 30 across the GB. This translates to a GB barrier height on the order of 100 meV at VG ) 0.7 Most importantly, Figure 3 shows that over the 0-10 V gate voltage range, RGB is much larger than the resistance of the individual grains. The significance of RGB becomes clearer by observing that the resistance increase between points 1 and 4, which is the resistance across the grain, is on the order of 108

4540 J. Phys. Chem. B, Vol. 105, No. 20, 2001 Ω. At VG ) -10 V, RGB is at least an order of magnitude larger and increases by almost another order of magnitude as VG decreases to 0 V. We have made measurements on five other pairs of grains and found similar results. Our measurements indicate that GBs can dominate transport in polycrystalline organic semiconductor films. A key point is that the maximum gate-induced carrier density in this study is ∼1 × 1012/cm2.8 This concentration corresponds to the minimum or threshold concentration needed to turn on a typical polycrystalline 6T device.2 Thus, our experiment has been carried out with gate fields in the sub-threshold to near-threshold regime for typical devices.9 At higher gate fields, carrier densities will be greater and RGB may be more comparable to single grain resistances. However, because we find RGB dominates transport at threshold carrier densities, we can conclude that the threshold voltage (VT) in 6T FETs is essentially determined by GB potential barriers. Effectively, a device turns on once the GB barriers have been minimized such that charge can flow. The connection of VT to GBs is potentially important for practical device preparation where control of the gate threshold voltage is critical. However, the origin of GB potential barriers has not been identified. It seems likely that chemical contaminants, e.g. adsorption of O2 to the grain surfaces, play a role; GB potential barriers may exist only in the presence of such contaminants or will at the very least be modified by them. We note that many devices reported in the literature are characterized in air or at least exposed to air after the organic film has been deposited. Consequently, variations in VT from device to device may result from small variations in impurity concentrations at GBs. The ability to probe transport on local length scales by CPAFM is a broadly applicable approach to quantifying the effects of microstructure and contacts on the performance of organic semiconductor devices. In this study, the ability to probe transport through individual GBs allowed the microscopic effect of gate voltage on transport to be observed in a particularly transparent way. Further work can be done to determine the effects of grain orientation on GB resistances in these materials. In addition, high-resolution AFM potentiometry experiments may allow detection and spatial imaging of localized potential barriers near GBs. Acknowledgment. C.D.F. thanks the National Science Foundation (DMR-9624154), the David and Lucile Packard

Letters Foundation, and the Camille and Henry Dreyfus Foundation for financial support of this work. Funding from the McKnightLand Grant Professorship program at the University of Minnesota is also acknowledged. References and Notes (1) (a) Kelley, T. W.; Frisbie, C. D. J. Vac. Sci. Technol, B. 2000, 18, 632. (b) Kelley, T. W.; Granstrom, E. L.; Frisbie, C. D. AdV. Mater. 1999, 11, 1. (c) Loiacono, M. J.; Granstrom, E. L.; Frisbie, C. D. J. Phys. Chem. B 1998, 102, 1679. (d) Olbrich, A.; Ebersbergerm, B.; Boit, C. Appl. Phys. Lett. 1998, 73, 3114. (e) Dai, H.; Wong, E. W.; Lieber, C. M. Science 1996, 272, 523. (f) Tanimoto, M.; Kanisawa, K.; Shinohara, M. Jpn. J. Appl. Phys. 1996, 35, 1154. (g) Houze, F.; Meyer, R.; Schneegans, O.; Boyer, L. Appl. Phys. Lett. 1996, 69, 1975. (h) Klein, D.; McEuen, P. Appl. Phys. Lett. 1995, 66, 2478. (i) Alperson, B.; Cohen, S.; Rubenstein, I.; Hodes, G. Phys. ReV. B. 1995, 52, R17017. (j) De Wolf, P.; Snauwaert, J.; Hellemans, L.; Clarysse, T.; Vandervorst, W.; D’Olieslaeger, M.; Quaeyhaegens, D. J. Vac. Sci. Technol. A 1995, 13, 1699. (k) Heddleson, J.; Weinzierl, S.; Hiullardm, R.; Rai-Choudhury, P.; Mazur, R. J. Vac. Sci. Technol. B 1994, 12, 317. (2) (a) Torsi, L.; Dodabalapur, A.; Rothberg, L. J.; Fung, A. W. P.; Katz, H. E. Phys. ReV. B 1998, 57, 2271. (b) Scho¨n, J. H.; Kloc, Ch.; Laudise, R. A.; Batlogg, B. Phys. ReV. B 1998, 58, 12952. (c) Schoonveld, W. A.; Vrijmoeth, J.; Kapwijk, T. M. Appl. Phys. Lett. 1998, 73, 3884. (d) Horowitz, G. AdV. Mater. 1998, 10, 365. (e) Lovinger, A. J.; Davis, D. D.; Dodabalapur, A.; Katz, H. E.; Torsi, L. Macromolecules 1996, 29, 4952. (f) Kloc, Ch.; Simpkins, P. G.; Siegrist, T.; Laudise, R. A. J. Cryst. Growth 1997, 182, 416. (g) Torsi, L.; Dodabalapur, A.; Rothberg, L. J.; Fung, A. W. P.; Katz, H. E. Science 1996, 272, 1462. (h) Dodabalapur, A.; Torsi, L.; Katz, H. E. Science 1995, 268, 270. (i) Garnier, F.; Hajlaoui, R.; Yassar, A.; Srivastava, P. Science 1994, 265, 1684. (3) (a) Horowitz, G.; Hajlaoui, M. E.; Hajlaoui, R. J. Appl. Phys. 2000, 87, 4456. (b) Scho¨n, J. H.; Batlogg, B. Appl. Phys. Lett. 1999, 74, 260. (c) Gundlach, D. J.; Jackson, T. N.; Schlom, D. G.; Nelson, S. F. Appl. Phys. Lett. 1999, 74, 3303. (d) Nelson, S. F.; Lin, Y.-Y.; Gundlach, D. J.; Jackson, T. N. Appl. Phys. Lett. 1998, 72, 1854. (e) Salih, A. J.; Marshall, J. M.; Maud, J. M. Philos. Mag. Lett. 1997, 75, 169. (f) Biscarini, F.; Samori, P.; Greca, O.; Zamboni, R. Phys. ReV. Lett. 1997, 78, 2389. (g) Lovinger, A. J.; Rothberg, L. J. J. Mater. Res. 1996, 11, 1581. (h) Dimitrakopolous, C. D.; Brown, A. R.; Pomp, A. J. Appl. Phys. 1996, 80, 2501. (4) Digital Instruments Multimode AFM equipped with a Keithley model 617 electrometer and 236 source-measure unit. (5) (a) Nussbaumer, H.; Baumgartner, F. D.; Willeke, G.; Bucher, E. J. Appl. Phys. 1998, 83, 292. (b) Pike, G. E.; Seager, C. H. J. Appl. Phys. 1979, 50, 3414. (6) ∆RGB in Figure 4 is inversely proportional to ∆σ over the 0 to -10 V gate voltage range, so ∆EB ) kT ln(RGB,-10V/RGB,0V) ) kT ln(4/25) ) -45 meV. (7) At VG ) 0 V, ID changes by a factor of 30 on either side of the grain boundary. Taking this change to be the change in conductivity, EB is obtained from eq 1: EB ) kT ln(30) ∼ 100 meV. (8) The capacitance of the 150 nm thick SiO2 layer is ∼20 nF/cm2. Thus, for VG ) -10V, the number of induced carriers is 1012/cm2. (9) Gate voltage excursions more negative than -10 V resulted in a drain-to-gate short for this particular device.