Low-Temperature Sintering of In-Plane Self-Assembled ZnO Nanorods for Solution-Processed High-Performance Thin Film Transistors

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1 , 111, Published on Web 12/04/2007 Low-Temperature Sintering of In-Plane Self-Assembled ZnO Nanorods for Solution-Processed High-Performance Thin Film Transistors Baoquan Sun,*, Rebecca L Peterson, and Henning Sirringhaus CaVendish Laboratory, J. J. Thomson AVenue, Cambridge, CB3 0HE United Kingdom Downloaded via on March 17, 2019 at 13:23:32 (UTC). See for options on how to legitimately share published articles. Introduction Kiyotaka Mori Panasonic Europe Ltd., Panasonic R&D Centre of Europe, Cambridge Liaison Office, Cambridge, CB3 0AX United Kingdom ReceiVed: September 26, 2007; In Final Form: NoVember 12, 2007 ZnO is an attractive active semiconducting material for thin-film transistor (TFT) applications due to its high band gap, high mobility, ease of forming Ohmic contacts, and low toxicity. Here we present a process for solution fabrication of ZnO TFTs based on a simple, double-layer spin-coating process during which a dense layer of in-plane zinc oxide nanorods is deposited first, followed by coating of a chemical precursor solution and low-temperature annealing. First, a lower ZnO nanorod concentration can lead to the self-assembly of nanorods along the in-plane direction to form a relatively dense semiconductor layer. Then the chemical precursor solution sinters the nanorods and improves the contact between them. The n-channel TFTs exhibit high ON/OFF ratio of , mobilities of 1.2 cm 2 V -1 s -1 and low threshold voltages of about -4 V with low hysteresis. We show that the quality of the semiconductor film and the minimum annealing temperature depends sensitively on the thickness and composition of the two layers. Recently, thin film transistors (TFTs) based on poly crystalline zinc oxide (ZnO) 1,2 and related amorphous metal oxide 3-6 semiconductors have attracted significant interest since their performance is comparable with micro- or poly crystalline silicon. ZnO-based TFTs have been widely studied since high quality poly crystalline layers can be formed at low temperatures, 7,8 which is a particular advantage for flexible substrate technologies. ZnO is an n-type semiconductor with a band gap of ev, a hexagonal wurtzite structure, and a high melting point. 9 Since ZnO is transparent in the visible spectrum, it is less sensitive than small band gap materials to ambient light. Also ZnO is an economical and environmentally friendly material. So far, most ZnO layers for TFTs have been deposited under vacuum by radio frequency magnetron sputtering, 2,8 ionbeam sputtering, 1 or pulsed laser deposition, 10 so there is only limited cost reduction compared with micro- or poly crystalline silicon. Solution-based deposition processes for ZnO such as spin-coating 11,12 and chemical bath deposition 13,14 could use simple, inexpensive facilities and thus offer a lower operating cost. Ong 12 has found that ZnO can form a preferable crystal orientation via controlling thermal annealing of solutiondeposited zinc precursor to achieve ZnO TFT field effect mobilities up to 5 cm 2 V -1 s -1. Routes to other metal oxide semiconductors such as zinc tin oxide and zinc indium oxide * To whom correspondence should be addressed. baoquan@ lanl.gov. Present address: Chemistry Division, Los Alamos National Laboratory, New Mexico /jp077740f CCC: $37.00 have been developed by spin coating or ink-jet printing a metal chloride and then firing the film to decompose it into a metal oxide. 15,16 To date, all solution-based techniques except chemical bath deposition require high temperatures (up to 600 C) to convert the metal precursor into metal oxide and promote crystallization. 11,12,15,16 We have previously used a combination of spin-coating and chemical bath deposition (also called hydrothermal growth) to achieve high mobility ZnO TFTs at low annealing temperature. 13 However, this process generally requires two steps: first the nucleation seeds in the form of nanoparticles must be deposited onto the substrate, and then the substrate is immersed in a water bath for hydrothermal seed growth. The hydrothermal growth step is undesirable from a manufacturing point of view, and its replacement by a simple spin-coating step would greatly simplify the ZnO TFT fabrication process. In our previous report, 17 organic octylamine is utilized as a ligand to coat the ZnO nanorod surface, enhancing nanorod dispersion and favoring self-assembly. After removal of the octylamine, there is poor contact between the nanorods which could cause significant grain boundary resistance and an associated reduction in field-effect mobility. It has been shown that the electron mobility in undoped poly crystalline ZnO thin films is mainly limited by grain boundaries for films fabricated by a variety of deposition techniques. 18,19 One way to fill the internanorod space and thus weaken grain boundary scattering is through further ZnO growth by chemical bath deposition. However, it is time-consuming to regrow the ZnO and then rinse to remove extra ions introduced by the regrowth process. 13,14 Here an alternative, more convenient method is presented by 2007 American Chemical Society

2 18832 J. Phys. Chem. C, Vol. 111, No. 51, 2007 Letters spin coating a thin zinc precursor layer on top of the first nanorod film. During annealing, the octylamine ligand is removed. Meanwhile the zinc precursor is decomposed into ZnO, which grows on the surface of the nanorods. This second precursor layer fills in the inter-nanorod space and thus sinters the nanorods and enhances the connectivity between them. ZnO TFTs made by this double layer spin-coating technique can achieve excellent performance with mobilities of 1.2 cm 2 V -1 s -1, which is comparable to our previously developed two-step method of spin-coat depositing the nanorods followed by chemical bath growth, but the process is more reproducible and simplified significantly. Experimental Methods Synthesis of ZnO Nanorods. Colloidal ZnO nanorods chelated with octylamine are synthesized as described in previous reports. 17,20 In brief, zinc acetate was dissolved into methanol with a small amount of water and then heated to 60 C under magnetic stirring. Potassium hydroxide in methanol was dropped into zinc acetate. It took 2 h and 30 min to form a turbid solution containing spherical nanopartitcles. Nitrogen flow was utilized to condense the solution up to approximately one-fifth of its initial volume. At this point, the solvent should be totally transparent and colorless. It was reheated at 60 C for another 5htoobtain nanorods. The ZnO nanorods were washed twice with methanol and dispersed into chloroform/ methanol (3/1, V/V). Octylamine was added as a ligand to coat the nanorods surface. ZnO nanorod concentrations were varied from 7.5 to 50 mg/ml. TFT Device Fabrication. The substrate is n ++ silicon coated with 300-nm silicon dioxide and treated with hexamethyldisilazane (HMDS). Semiconductor films are formed by spin casting these solutions, using a spin speed of 1500 rpm and a spin time of 1 min. Precursor solutions containing 0.05 M zinc acetate dehydrate and 0.05 M aminoethanol in methoxyethanol are spin coated atop the nanorod layer using the same spin-coating conditions. All films are annealed at 270 C in ambient air to remove residual organic components and to convert the zinc precursor into ZnO. To fabricate thin film transistors, 100 nmthick T-shaped aluminum source/drain electrodes are thermally evaporated through a shadow mask onto the ZnO film. The device structure is that of a bottom-gate top-contact TFT, as illustrated in the inset image of Figure 2b. The channel has a width (W) of 3 mm and a length (L) of90µm (W/L ) 33.3/1). All of the TFT devices are characterized in nitrogen to avoid the possible effects of humidity and oxygen. Results and Discussion Nanorod Orientation Dependence on ZnO Concentration. The as-cast ZnO nanorod films described here show in-plane self-assembly of the nanorods which is needed in order to achieve high performance devices. 17 Since the as prepared ZnO nanorods are approximately nm long and 6-15 nm wide, it is possible to characterize the nanorod orientation by scanning electron microscopy (SEM). As shown in our earlier report, 17 if the proper nanorod concentration is used, during spin coating the nanorod assembly nucleates at the air-liquid interface and micrometer-sized self-assembled areas form in the final film with in-plane, parallel alignment of the nanorods (Figure 1, panels b and d). However, the nanorods could assemble out of plane if the concentration is too high (Figure 1, panels a and c). SEM images of ZnO films made from 50 mg/ml and 25 mg/ml solutions are shown in Figure 1, panels a and b. In Figure 1a, it is clear that the surface ZnO nanorods Figure 1. Scanning electron microscopy images of a ZnO nanorod film on a silicon dioxide/silicon substrate:(a) Top view, 50 mg/ml. (b) Top view, 25 mg/ml, inset shows the fast Fourier transform of the image. (c) Cross-sectional view, 50 mg/ml. (d) Cross-sectional view, 25 mg/ml. (e) Top view, single-layer film fabricated from 10 mg/ml ZnO nanorod solution, then annealed at 270 C in air. (f) Top view, double-layer film fabricated from 10 mg/ml ZnO nanorods solution, and 0.05 M zinc acetate precursor solution, then annealed at 270 C in air. are tilted out of substrate plane, and dozens of nanorods are packed into self-assembled bundles. To determine the nanorod orientation at the substrate interface, a cross-sectional SEM image of the same film is shown in Figure 1c. Obviously, the bottom nanorods have the same tendency as the top ones to be roughly normal to substrate. The film thickness is around 220 nm comprising three to four nanorod layers in the vertical direction. In sharp contrast, when the initial nanorod concentration is 25 mg/ml or lower, the nanorods lie in the plane of the substrate, as shown in a top view in Figure 1b and cross-sectional view in Figure 1d. When the nanorods are aligned in the plane of the substrate, they can form large selfassembled structures. The fast Fourier transform of Figure 1b (inset) confirms the largely unidirectional in-plane orientation. Moreover, the in-plane orientation of the nanorods is preserved throughout the film depth, as shown in the crosssectional image of Figure 1d. As we reported earlier, 17 ZnO nanorods with octylamine ligands prefer to be trapped at the air-liquid interface since the long carbon chain has a low surface energy. Because self-assembly nucleates at the top airliquid interface, the bottom nanorod direction is consistent with the top direction.

3 Letters J. Phys. Chem. C, Vol. 111, No. 51, Figure 2. Typical TFT transfer and output characteristics based on a ZnO nanorod film made from 10 mg/ml colloidal solution followed by zinc precursor coating, and annealing at 270 C in air. Panel a shows drain current and gate leakage current versus gate voltage, for constant drain biases. Panel b shows drain current versus drain voltage with constant gate bias. The inset image in panel b illustrates the TFT device structure. It is well-known that the thickness of a spin-cast film may be controlled by varying the solution concentration while fixing other spin-coating parameters. Here we discuss how the solution concentration also affects the nanorod assembly orientation. In the spin coating process, the solvent will gradually evaporate with time. When the solvent still surrounds the nanorods such that the liquid layer is thicker than the nanorod length, the nanorods are free to assemble in a variety of directions. As the solvent evaporates, the liquid layer thins and the nanorod assembly has to nucleate in a restricted space, typically forcing orientation in the plane of the liquid layer. According to the Onsager-Flory rigid rod model, self-assembly of nanorods can nucleate when the nanorod concentration is above a critical value. The space confinement of assembly nucleation may explain the observed nanorod orientation dependence on the initial colloidal concentration, because the initial concentration determines the liquid thickness at the point of critical concentration. For high initial colloidal nanorod concentrations, this critical point will be achieved early during the solvent evaporation process. At this point the liquid layer would be rather large compared with the long dimension of nanorods. Therefore, the liquid layer supplies enough space for the nanorods to be oriented either in-plane or out-of-plane. Once the nanorod assembly has begun, the nanorod orientation, either in or outof-the plane, propagates down through the film thickness. For solutions with lower concentrations, the liquid layer may become too thin to permit assembly nucleation in the out of plane orientation before the critical concentration is reached. The highenergy barrier at the liquid-gas interface keeps the nanorods confined to the liquid, and thus when the liquid layer thickness is less than the long dimension of the nanorods, vertical assembly is prevented. Therefore, the dilute nanorods assemble in the plane of the substrate. When very dilute solutions are used to make a film, self-assembly may not occur at all if the critical point is never achieved during the solution drying process. For thin film transistor applications, concentrations of mg/ml are used to obtain well-assembled in-plane ordering of nanorods in a continuous semiconductor film. Also, since there is a space gap produced after ligand removal, a second precursor layer is applied to fill the space and get a more dense film. The solvent chosen for the precursor does not dissolve the nanorods, and the in-plane self-assembled nanorod structure is preserved after the deposition of the precursor solution (Figures 1, panels e and f), except that the roughness of the nanorod surface increases slightly after precursor coating. After the deposition of the precursor layer, an annealing process is necessary to remove the organic ligand from the surface of nanorods. During this annealing process at 270 C the nanorods grow due to decomposition of the zinc precursor. Careful analysis of transmission electron micrographs shows that, compared to samples without any precursor, the precursor process increases nanorod diameters by about 1 nm. TFT Device Performance. For TFT device applications, the nanorods in-plane alignment is preferred for enhanced device performance. Therefore, a lower concentration nanorod solution was utilized to fabricate devices. The resulting TFT devices operate in n-type enhancement mode. Transfer and output characteristics are illustrated in Figure 2, panels a and b, respectively, where I d, I g, V d, and V g are drain current, gate current, drain voltage, and gate voltage, respectively, with reference to the source, which is grounded. The electrical parameters used to characterize a field effect transistor include drain current ON/OFF ratio, threshold voltage (V th ), and channel mobility. For these devices, clean transfer curves are observed with large drain current on/off ratios of , small current hysteresis (<1 V at a current of 10 na), and low gate leakage current. The output curves show current saturation at large V d, which is highly desirable for most circuit applications. Saturated region mobility (µ sat ) and threshold voltage are extracted by fitting a straight line to a plot of the square root of I d verses V g in the saturated regime (V d ) 40 V), using eq 1, where C i is the capacitance per unit area of the gate insulator (for 300 nm SiO 2, C i ) 11.5 nf cm -2 ) I d ) µ WC i sat 2L (V g - V th ) (1) ( I d ) W V g)vd)const L µ lin C i V d (2) The saturated mobility is 1.2 cm 2 V -1 s -1 and V th is -4.2 V. The linear mobility (µ lin ) is calculated using the transconductance (eq 2), for V d ) 5 V. For this device, µ lin has a maximum value of 1.3 cm 2 V -1 s -1. Note that the linear mobility reaches a maximum shortly after turn-on (at V g ) +9V) and then decreases with gate-bias. Comparison of TFTs without precursor treatment indicates that the mobility is improved by a factor of about five, and the drain current on/off ratio is slightly enhanced after precursor treatment. The transfer curves for both cases, with V d ) 40 V, are plotted in Figure 1S. The conductivity of the ZnO film increases approximately forty times from 0.11 to 4.6 ms cm -1, which is ascribed to improved connectivity between the nanorods. The film conductivity was estimated from the slope of the output curve near zero drain voltage when the gate and source are both grounded. 21 With precursor treatment, the turn-

4 18834 J. Phys. Chem. C, Vol. 111, No. 51, 2007 Letters on voltage shifts to a more negative value, from -5 to-12v: a larger negative voltage is needed to deplete the extra free electrons and turn the transistor off. These results indicate that the increase in conductivity at zero gate voltage is due to a combination of increased charge carrier concentration and increased mobility. For this double-layer solution deposition method, the thickness of the individual layers plays a very important role in determining the quality of the active channel layer. For the bottom layer, the nanorod film should be as thin as possible to enable the precursor to penetrate to the active interface between the silicon dioxide and zinc oxide. However, on the other hand the nanorod film should be sufficiently dense and have continuous coverage over the substrate in order to provide efficient carrier conduction paths along the channel region. As described above, the nanorod film exhibits the favorable inplane nanorod alignment if the nanorod concentration is less than 25 mg/ml. Below this concentration, the only difference between the films is the number of in-plane oriented nanorod layers, which account for the variable film thickness. When a gate bias is applied, electrons are accumulated by the gate field near the dielectric/semiconductor interface. In principle, the device performance should be independent of the thickness of the semiconducting layer as long as the film is thicker than the typical thickness of the accumulation layer. 22,23 The accumulation layer thickness in these ZnO TFTs in the ON state is expected to be less than 10 nm (ref 24), i.e., comparable with the diameter of one ZnO nanorod. In practice, however, the thinnest films suffer from limited film connectivity, and it was found that the optimal bottom layer thickness is around 40 nm consisting of 2-3 layers of in-plane assembled nanorods. For such a small thickness, the subsequently deposited precursor solution is still able to permeate the internanorod space. For such a film, the nanorod solution concentration should be in the range of mg/ml. Both film thickness and nanorod orientation play an important role in ZnO TFT device performance. Table 1S compares the electrical characteristics of TFTs with in-plane and out-of-plane nanorod orientations. The films with in-plane alignment show a 4-fold increase in both linear and saturated mobilities and a slight increase in the drain current on/off ratio, compared to films with out-of-plane nanorods. Only devices without precursor treatment are presented in Table 1S because it was found that for thick nanorods films, such as those with out-of-plane orientation, the chemical precursor treatment has a very limited effect on TFT electrical properties. When the nanorod film is thick, it is difficult for the precursor to penetrate to the dielectric/ nanorod interface where the active channel is formed, and thus, the use of the precursor does not strongly affect transistor performance. Properties of the Precursor Layer. The top precursor layer must be thick enough to effectively fill the internanorod spaces while being thin enough to maintain a low resistance contact between ZnO nanorod layer and the source and drain electrodes. If the top film is too thick, the precursor layer comprises a layer of tiny nanocrystals instead of simply filling the space between the nanorods. Such films exhibit reduced field-effect mobility. The optimal zinc precursor solution concentration is about 0.05 M. To compare the effectiveness of the precursor approach with that of other potential methods for filling the space between nanorods, we have also investigated depositing a film of small colloidal ZnO nanospheres 13 with a diameter about 3 nm on top of a nanorod film. TFT devices made on the resulting films Figure 3. (a) FTIR spectrum of zinc oxide or zinc acetate precursor film on double-side polished silicon substrate: (I) film formed from an unannealed precursor solution; (II) film formed from precursor solution annealed at 270 C in air; (III) double-layer film formed from nanorods and precursor solution then annealed at 270 C in air. (b) Thermogravimetric analyses of zinc compounds under flowing air: curve (a) is zinc acetate chelated with aminoethanol gel and curve (b) is zinc acetate dihydrate powder. did not show any improvement in mobility, suggesting that the nanoparticles are too big to fill any gaps and to improve the contact between the nanorods. However, film conductivity increased undesirably, since the spheres formed a separate conducting layer on top of the nanorod film. The composition of the precursor solution also plays an important role in determining the annealing temperature required to achieve a good performance of the ZnO TFTs. Here, aminoethanol acts as a ligand which is chelated with zinc ions to enhance the solubility of zinc acetate dihydrate. However, when the precursor film is annealed in air, aminoethanol also behaves as a catalyst to accelerate the reaction of zinc ions with water to form ZnO. As shown in the FTIR spectrum of Figure 3a, vibration signals in range of and cm -1 originally present in the as-spun precursor film (spectrum I) disappear after annealing, indicating that the annealed film is free from organic components, with only inorganic ZnO remaining. Moreover, the octylamine ligands which coat the nanorods were removed by the annealing process even when the extra precursor layer coated the nanorod film. The FTIR spectrum of the annealed single precursor layer (spectrum II) is almost identical to that of the combined nanorod and precursor layer (spectrum III), providing evidence that the double layer films do not contain a significant concentration of organic residues. As mentioned above, the precursor decomposes into ZnO which grows along the nanorod surface as the octylamine ligand escapes. Here the annealing temperature is kept at 270 C. At this temperature complete conversion of the

5 Letters J. Phys. Chem. C, Vol. 111, No. 51, aminoethanol containing precursor solution into ZnO is possible, as shown by the thermogravimetric analysis in Figure 3b. For pure zinc acetate dehydrate, a higher temperature of about 300 C is required for decomposition into ZnO. The addition of aminoethanol decreases the decomposition temperature significantly, which is advantageous for fabricating ZnO TFT devices on a plastic (e.g., polyimide) substrate by a solutionbased process. The likely mechanism for this catalytic activity of the aminoethanol is by providing the hydroxyl groups that are needed for the formation of intermediate Zn(OH) 2 in the reaction of Zn acetate into ZnO. Conclusion In summary, ZnO TFT devices are fabricated by spin coating an in-plane self-assembled ZnO nanorod solution followed by a zinc precursor film and sintering the nanorod film at a modest annealing temperature of 270 C. High performance TFTs are achieved with mobilities of 1.2 cm 2 V -1 s -1 and on/off ratios of Control of layer thicknesses by adjusting the solution concentration is critical to forming a dense layer of interconnected in-plane nanorods at the active interface. Selection of the appropriate precursor composition is important to minimize the annealing temperature. A ZnO film prepared via this simple and cost-effective wet chemical process at low temperature is potentially a promising material for TFTs on plastic substrates. Acknowledgment. We gratefully acknowledge collaboration with Panasonic. Supporting Information Available: Figure 1S shows transfer curves of ZnO TFT devices with and without precursor treatment, and Table 1S compares electrical properties of TFT devices with different nanorod orientations. This material is available free of charge via the Internet at References and Notes (1) Hoffman, R. L.; Norris, B. J.; Wager, J. F. Appl. Phy. Lett. 2003, 82, 733. (2) Carcia, P. F.; McLean, R. S.; Reilly, M. H.; Nunes, G. Appl. Phy. Lett. 2003, 82, (3) Nomura, K.; Ohta, H.; Takagi, A.; Kamiya, T.; Hirano, M.; Hosono, H. Nature 2004, 432, 488. (4) Jackson, W. B.; Hoffman, R. L.; Herman, G. S. Appl. Phy. Lett. 2005, 87, (5) Chiang, H. Q.; Hong, D.; Hung, C. M.; Presley, R. E.; Wager, J. F.; Park, C. H.; Keszler, D. A.; Herman, G. S. J. Vac. Sci. Technol. B 2006, 24, (6) Yaglioglu, B.; Yeom, H. Y.; Beresford, R.; Paine, D. C. Appl. Phy. Lett. 2006, 89, (7) Ellmer, K.; Diesner, K.; Wendt, R.; Fiechter, S. Solid State Phenom. 1996, 51-52, 541. (8) Fortunato, E. M. C.; Barquinha, P. M. C.; Pimentel, A. C. M. B. G.; Goncalves, A. M. F.; Marques, A. J. S.; Pereira, L. M. N.; Martins, R. F. P. AdV. Mater. 2005, 17, 590. (9) Landolt-Bornstein, Semiconductors; Springer: Berlin, 1982; Vol. 17, p 35. (10) Nishii, J.; Hossain, F. M.; Takagi, S.; Aita, T.; Saikusa, K.; Ohmaki, Y.; Ohkubo, I.; Kishimoto, S.; Ohtomo, A.; Fukumura, T.; Matsukura, F.; Ohno, Y.; Koinuma, H.; Ohno, H.; Kawasaki, M. Jpn. J. Appl. Phys. 2003, 42, L347. (11) Norris, B. J.; Anderson, J.; Wager, J. F. Keszler, D. A. J. Phys. D 2003, 36, L105. (12) Ong, B. S.; Li, C. S.; Li, Y. N.; Wu, Y. L.; Loutfy, R. J. Am. Chem. Soc. 2007, 129, (13) Sun, B. Q.; Sirringhaus, H. Nano Lett. 2005, 5, (14) Cheng, H. C.; Chen, C. F.; Tsay, C. Y. Appl. Phy. Lett. 2007, 90, (15) Chang, Y. J.; Lee, D. H.; Herman, G. S.; Chang, C. H. Electrochem. Solid State Lett. 2007, 10, H135. (16) Lee, D. H.; Chang, Y. J.; Herman, G. S.; Chang, C. H. AdV. Mater. 2007, 19, 843. (17) Sun, B. Q.; Sirringhaus, H. J. Am. Chem. Soc. 2006, 28, (18) Ellmer, K. J. Phys. D 2001, 34, (19) Minami, T. MRS Bull. 2000, 25, 38. (20) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem. Int. Ed. 2002, 41, (21) Horowitz, G.; Hajlaoui, R.; Bouchriha, H.; Bourguiga, R.; Hajlaoui, M. AdV. Mater. 1998, 10, 923. (22) Hoshino, S.; Kamata, T.; Yase, K. J. Appl. Phys. 2002, 92, (23) Barquinha, P.; Pimentel, A.; Marques, A.; Pereira, L.; Martins, R.; Fortunato, E. J. Non-Cryst. Solids 2006, 352, (24) Horowitz, G. J. Mater. Res. 2004, 19, 1946.