J. Phys. Chem. C 2008, 112, 13869–13872
13869
Substrate-Free, Self-Standing ZnO Thin Films Antonino Gulino,* Fabio Lupo, and Maria E. Fragala` Dipartimento di Scienze Chimiche, UniVersita` di Catania and I.N.STM UdR of Catania, V.le A. Doria 6, 95125 Catania, Italy ReceiVed: May 5, 2008; ReVised Manuscript ReceiVed: June 13, 2008
Substrate-free ZnO thin films have been obtained by metal organic chemical vapor deposition. Mild heating (37-51 °C) of the Zn(C5F6HO2)2 · 2H2O · CH3(OCH2CH2)nOCH3 (n ) 2, 3, 4) adduct precursors produced thermally stable liquid compounds that were easily evaporated. Rapid quenching to room temperature of the reactor quartz tube caused self-exfoliation of ZnO films deposited on the walls, thus giving flexible films having a few hundred nanometer thickness. No exfoliation was observed from films grown on flat silica substrates, even in the case of rapid quenching to room temperature. The obtained films were characterized by X-ray diffraction, UV-vis spectra, secondary electron microscopy, and transmission electron microscopy. The film thickness was evaluated by SEM cross-sections. Present ZnO films are semiconducting with an allowed direct transition at 3.3 eV. Introduction Control of nanodimensionality of metal oxide semiconductors remains a crucial goal for many technologies since particular morphologies might have distinguished properties for special applications.1 In this perspective, self-standing substrate-free films are of relevance since the substrate-film interface affects optical, electrical, and mechanical properties of thin crystalline films. Conversely, properties of films unaffected by the substrate-film interaction may significantly differ from their substrate-affected counterparts.1b,j Zinc oxide (ZnO) adopts the wurtzite structure (space group P63mc) pictorially described in terms of hexagonal close packing (hcp) of oxygen ions stacked along the [001] direction with cations occupying one-half of the tetrahedral sites.1b,e,2 It is an intrinsic n-type semiconductor having lattice defects consisting of oxygen vacancies and involving interstitial (Zn2+-2e-) pairs and cation and anion vacancies (0M, 0O).2 The band gap width of ∼3.3 eV,3 in polycrystalline ZnO, shows anomalous behavior because of the existence of potential barriers at the grain boundaries4 while the resistivities depend on the synthetic procedure and/or on the presence of dopants within the ZnO lattice.5 Both its direct wide band gap and large exciton binding energy (60 meV)1b,h make ZnO one of the most important functional materials with almost unique properties in terms of near-UV emission, optical transparency, electric conductivity, piezoelectricity, and sensing characteristics.6 For instance, double-side ZnO films can show chemisorption on both surfaces, thus improving sensing detection limits. Besides ZnO represents an important emerging 1D nanomaterial.1b,f,h,7 Moreover, the most common varistor is represented by bulk ZnO material sandwiched between two electrodes.8 Varistors show nonohmic current-voltage behaviors and are usually incorporated in electronic circuits as protection devices. They show high resistances at low voltages, and low resistances at high voltages; hence, they divert a large current, due to excessive transient voltages, from the sensitive components. It turns out that a varistor remains nonconductive under voltages well below * Corresponding author. E-mail:
[email protected], fax: +39-095580138.
Figure 1. Arrhenius log-plot of the evaporation rates versus 1/T of present source precursors. (1) R ) -0.99832, SD ) 0.06421, P < 0.0001. (2) R ) -0.99604, SD ) 0.08348, P < 0.0001. (3) R ) -0.9966, SD ) 0.0842, P < 0.0001.
the “clamping value” while, upon excessively high transient pulses, they melt, burn, vaporize, or otherwise are damaged or destroyed, thus protecting the whole circuit. In this perspective, substrate-free thin ZnO films could have varistor applications in very sensible electronic circuits operating at low currents. Nanostructured ZnO has been obtained using a large variety of techniques.1h,6,9,10 We already reported on the metal organic chemical vapor deposition (MOCVD) of ZnO thin films using the low-melting (37-51 °C) zinc hexafluoroacetylacetonate dihydrate polyether, Zn(C5F6HO2)2 · 2H2O · CH3(OCH2CH2)nOCH3 (n ) 2, 3, 4) complexes (hexafluoroacetylacetonate ) 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate) as precursors.11 MOCVD from liquids has revealed a choice of election since it allows elevated reproducibility associated with constant precursor evaporation (hence constant mass-transport) rates for given source temperatures. The Zn(C5F6HO2)2 · 2H2O · polyether complexes are liquid (at MOCVD conditions) and allow MOCVD of ZnO from liquid sources.11 Therefore, in the present investigation we report on the first ever case of substrate-free ZnO thin films showing a thickness of a few hundred nanometers. Experimental Details The Zn(C5F6HO2)2 · 2H2O · CH3(OCH2CH2)2OCH3 (1), Zn(C5F6HO2)2 · 2H2O · CH3(OCH2CH2)3OCH3 (2), and Zn(C5F6-
10.1021/jp8039466 CCC: $40.75 2008 American Chemical Society Published on Web 08/19/2008
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Gulino et al.
Figure 2. Representative as-exfoliated ZnO film fragment.
Figure 3. Cooling temperature of the quartz reactor tube vs time (s).
HO2)2 · 2H2O · CH3(OCH2CH2)4OCH3 (3) adducts (hereafter Zn(hfa)2 · 2H2O · diglyme, Zn(hfa)2 · 2H2O · triglyme and Zn(hfa)2 · 2H2O · tetraglyme, respectively; hfa ) 1,1,1,5,5,5-hexafluoro2,4-pentanedionate C5F6HO2 ligand, diglyme ) bis(2-methoxyethyl)ether; triglyme ) 2,5,8,11-tetraoxadodecane; tetraglyme ) 2,5,8,11,14-pentaoxapentadecane), were synthesized and characterized as already reported.11 MOCVD of ZnO were performed using an horizontal hotwall reactor, under reduced pressure.11 The reactor system mainly consists of a gas-handling facility, a tubular furnace, a quartz reactor tube (total length ) 80 cm and inner diameter ) 2.4 cm), two separate, parallel quartz inlet tubes for Ar and O2, and a vacuum system. The precursors were contained in alumina boats. All the crystallized Zn(hfa)2 · 2H2O · polyether complexes were used. Optical grade fused silica substrates, after being cleaned in an ultrasonic bath with isopropyl alcohol, were placed within the reactor quartz tube, exactly in the middle of the
Figure 4. X-ray diffraction patterns, over a 30° < 2θ < 70° angular range, for a representative as-exfoliated ZnO film fragment.
furnace. Pure Ar (200 sccm) and O2 (100 sccm) were used as carrier and reaction gases, respectively. The O2 gas was allowed to bubble within H2O at room temperature prior to reaching the quartz reactor tube. The substrate temperature was 500 °C. The adopted precursor evaporation temperature was 75-90 °C, and the deposition time was 60-180 min. The total pressure was kept in the 2-5 torr range. The thermal behavior of the precursors was investigated by thermal gravimetric analyses (TGA) under 1 atm of prepurified nitrogen, using a 2 °C/min heating rate. A Mettler Toledo TGA/ SDTA 851 system was used. Samples of 5-8 mg were accurately weighed and examined in the 20-300 °C range. X-ray diffraction (XRD) patterns were obtained by Philips D-5000 diffractometer (Cu KR radiation, 30 mA, and 40 kV). Surface chemical characterization was carried out with X-ray photoelectron spectroscopy using a PHI 5600 Multi Technique
ZnO Thin Films System (base pressure of the main chamber 2 × 10-10 Torr).11 Spectra were excited with Al KR radiation. The nominal analyzer resolution was set to 400 meV, and experimental uncertainties in binding energies lie within (0.5 eV. Freshly prepared samples were quickly transferred to the XPS main chamber. The detection limit for atomic percentage analysis is ∼0.1%. Secondary electron microscopy (SEM) images of ZnO films were obtained using a JEOL-JSM 6300F operating at energies in the 2-5 kV range. High resolution transmission electron microscopy (HRTEM) analyses were performed with a JEOL JEM4000 EX operating at 400 kV. Results and Discussion Zn(hfa)2 · 2H2O · polyether complexes cleanly and quantitatively evaporate in single steps within a very well-defined temperature range.11 Figure 1 shows the Arrhenius log-plots of evaporation rates vs 1/T of the Zn(hfa)2 · polyether complexes.8 Linear correlations are always observed, and related slopes provide the activation energies values (53.2, 54.5, and 59.5 kJ/ mol for 1, 2, and 3, respectively). All these values are entirely consistent with vaporization activation energies already reported for others β-diketonate complexes.12 No relevant differences were found on the obtained ZnO films depending on the particular precursor. Therefore, hereafter we refer to MOCVD experiments involving the precursor 1 with an evaporation temperature of 75 °C. Obviously, ZnO films were deposited both on the silica substrate as well as on the internal walls of the quartz reactor tube. ZnO films deposited on the internal walls of the quartz reactor tube, left to cool within the reactor furnace, remained always well adherent to the tube. In contrast, rapid quenching to room temperature of the quartz reactor tube caused selfexfoliation of the film deposited on its internal walls ((4 cm with respect to the middle of the furnace), thus giving flexible film fragments having sizes in the millimeter range (Figure 2). In particular the quartz tube removed from the furnace was at 300 °C after 4 min only, and exfoliation was observed at this temperature. The temperature was below 100 °C (Figure 3) in 20 min time. Eye-guided evaluation of size distribution and range was performed on 50 randomly chosen film fragments. Results indicated that 40% of them fall in the 2-3 mm range, 50% fall in the 4-6 mm range, and finally, 10% show sizes up to 8 mm. The flexibility of these film fragments was inferred because of their tendency to corrugate (Figure 2) and on the basis of the possibility to render them rather planar using a spatula tip. Film exfoliation was not observed for MOCVD experiments shorter than 90 min or longer than 150 min, under the adopted conditions. No exfoliation at all was observed from films grown on flat silica substrates, even in the case of rapid quenching to room temperature. This observation suggests that the lattice strain induced by the high curvature radius of the quartz reactor tube represents the driving force for their exfoliation when suddenly cooled. These experiments proved to be highly repeatable, and ZnO films with thickness of 300-500 nm (vide infra) were always obtained for deposition experiments in the 90-150 min range. X-ray Diffraction (XRD) data obtained for these film fragments provide evidence of hexagonal ZnO crystallites (Figure 4). The presence of (002), (102), and (103) reflections (2θ ) 34.49, d ) 2.598 Å; 2θ ) 47.78, d ) 1.902 Å; 2θ ) 63.05, d ) 1.473 Å; respectively) points to some texturing.11,13 These films are transparent, and their UV-visible transmittance has a minimum at λ ) 375 nm (the absorption edge) and reaches the 90% in the visible and near-infrared range.
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Figure 5. Tauc plot for a representative ZnO film fragment. R ) 0.99323, SD ) 3.04567 × 10-5, P < 0.0001. Inset: transmittance UV-vis spectrum.
X-ray photoelectron spectra (XPS) indicated Zn 2p3/2, 2p1/2 spin-orbit components at 1021.3 and 1044.3 eV, respectively, as well as the absence of any fluorine contamination. The carbon atomic percentage was below 0.5%, thus of the same order of the adventitious carbon, omnipresent on air-exposed samples. Figure 5 shows the plot (Rhν)2 vs hν for a representative, as-exfoliated ZnO film fragment.14 It was well established that for a large number of semiconductors the dependence of the absorption coefficient R, for the high frequency region, upon the photon energy hν, for optically induced transitions, is given by the following expression:
Rhν ) k(hν - Eg)n where Eg represents the optical band gap, hν is the photon energy, k is a constant, and n depends on the nature of the transition. In fact, n assumes values of 1/2, 3/2, 2, and 3 for allowed direct, forbidden direct, allowed indirect, and forbidden indirect transitions, respectively.14 In the present case, the best fit of (Rhν)1/n vs the photon energy was obtained for n ) 1/2. A straight segment with an intercept at 3.29 eV was observed, thus suggesting that these ZnO films are semiconducting with an allowed direct transition at this energy.1b,e SEM images of ZnO film fragments are shown in Figure 6. All films show different features on the two different sides. A rather flat film surface was always obtained for the film side in contact with the quartz reactor tube (Figure 6a) while a crystalline surface, almost identical to that observed for the notexfoliated film obtained on flat silica substrates, was evident for the opposite side (Figure 6b), thus showing a well-defined crystalline habit with big agglomerates of 200-300 nm. Film thicknesses of 380 nm were observed for 120 min experiments. HRTEM analysis shows rather uniform nanocrystals whose sizes are in the 5-10 nm range. Finally, d-spacing data obtained by measurements on lattice fringes in Figure 7, in agreement with XRD results, show evidence of the most intense (103) reflection.13 Conclusion In conclusion, substrate-free ZnO thin films have been obtained by rapid quenching from 500 °C to room temperature of the MOCVD reactor quartz tube. The (103) reflection predominates in the XRD pattern as well as in the HRTEM analysis. The films are 90% transparent in the visible and nearinfrared range and semiconducting with a 3.29 eV band gap. In the fast evolving area of semiconductor-based electronic devices, the present ZnO films are, no doubt, interesting for many different applications where thin, transparent, and substratefree inorganic semiconductors are required. Furthermore, the present very simple procedure, consisting of a rapid quenching
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Figure 6. SEM images for representative ZnO film fragments.
Figure 7. HR-TEM micrographs for a representative ZnO film fragment. Nanoparticle sizes are in the 5-10 nm range.
to room temperature of a small diameter MOCVD reactor, certainly represents a viable low cost route to probe the fabrication of other systems. Acknowledgment. The authors thank NATO (SfP project 981964) and the Ministero Universita` e Ricerca (MUR, Roma) forfinancialsupport(PRIN2005andFIRB2003-RBNE033KMA). References and Notes (1) (a) Nadarajah, A.; Word, R. C.; Meiss, J.; Koˇnenkamp, R. Nano Lett. 2008, 8, 534. (b) Yang, Z.; Liu, Q. H.; Yu, H. C.; Zou, B.; Wang,
Gulino et al. Y. G.; Wang, T. H. Nanotechnology 2008, 19, 035704. (c) Shen, L.; Bao, N.; Yanagisawa, K.; Zheng, Y.; Domen, K.; Gupta, A.; Grimes, C. A. J. Solid State Chem. 2007, 180, 213. (d) Park, S. Y.; Kim, P. J.; Lee, Y. P.; Shin, S. W.; Kim, T. H.; Kang, J. H.; Rhee, J. Y. AdV. Mater. 2007, 19, 3496. (e) Steinmiller, E. M. P.; Choi, K-S. Langmuir 2007, 23, 12710. (f) Wang, C. H.; Wong, A. S. W.; Ho, G. W. Langmuir 2007, 23, 11960. (g) Ju, S.; Facchetti, A.; Xuan, Y.; Liu, J.; Ishikawa, F.; Ye, P.; Zhou, C.; Marks, T. J.; Janes, D. B. Nature Nanotechnol. 2007, 2, 378. (h) Fauteux, C.; Longtin, R.; Pegna, J.; Therriault, D. Inorg. Chem. 2007, 46, 11036. (i) Wang, L.; Yoon, M.-H.; Lu, G.; Yang, Y.; Facchetti, A.; Marks, T. J. Nat. Mater. 2006, 5, 893. (j) Fu, M.; Zhou, J.; Xiao, Q.; Li, B.; Zong, R.; Chen, W.; Zhang, J. AdV. Mater. 2006, 18, 1001. (k) Nair, J. P.; Stavitski, N.; Lyahovitskaya, V.; Zon, I.; Lubomirsky, I. Mater. Sci. Semicond. Process. 2002, 5, 195. (2) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, 1994. (3) Studenikin, S. A.; Golego, N.; Cocivera, M. J. Appl. Phys. 1998, 84, 5001. (4) (a) Roth, A. P.; Webb, J. B.; Williams, D. F. Phys. ReV. B 1982, 25, 7836. (b) Srikant, V.; Clark, D. R. J. Mater. Res. 1997, 12, 1425. (5) (a) Wang, R.; Sleight, A. W.; Cleary, D. Chem. Mater. 1996, 8, 433. (b) Agarwal, G.; Speyer, R. F. J. Mater. Res. 1997, 12, 2447. (c) Palmer, G. B.; Poeppelmeier, K. R.; Mason, T. O. Chem. Mater. 1997, 9, 3121. (d) Djembo-Taty, K.; Plaindoux, L.; Kossanyi, J.; Ronfard-Haret, J. C. J. Chim. Phys. 1998, 95, 595. (e) Kazeoka, M.; Hiramatsu, H.; Seo, W.; Koumoto, K. J. Mater. Res. 1998, 13, 523. (f) Tsubota, T.; Ohtaki, M.; Eguchi, K.; Arai, H. J. Mater. Chem. 1998, 8, 409. (6) (a) Newton, M. C.; Firth, S.; Warburton, P. A. Appl. Phys. Lett. 2006, 89, 072104. (b) Wang, T. H.; Gao, T. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 1451. (c) Yi, G. C.; Wang, C.; Park, W. I. Semicond. Sci. Technol. 2005, 20, S22. (d) Wang, Z. L. J. Phys.: Condens. Matter. 2004, 16, R829. (e) Cheng, J.; Guo, R.; Wang, Q. M. Appl. Phys. Lett. 2004, 85, 5140. (f) Wang, Y. G.; Yuen, C.; Lau, S. P.; Yu, S. F.; Tay, B. K. Chem. Phys. Lett. 2003, 377, 329. (g) Vayssieres, L.; Keis, K.; Hagfeldt, A.; Lindquist, S. E. Chem. Mater. 2001, 13, 4395. (7) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (8) (a) Quang, L. H.; Jin, C. S. AdV. Mater. Res. 2008, 31, 192. (b) Nahm, C.-W. J. Mater. Sci. 2007, 42, 8370. (c) Ramirez, M. A.; Simoes, A. Z.; Marquez, M. A.; Maniette, Y.; Cavalheiro, A. A.; Varela, J. A. Mater. Res. Bull. 2007, 42, 1159. (d) Gulino, A.; Fragala`, I. Chem. Mater. 2002, 14, 116. (9) (a) Zhao, Y. N.; Cao, M. S.; Jin, H. B.; Zhang, L.; Qiu, C. Scr. Mater. 2006, 54, 2057. (b) Zhang, X. L.; Kang, Y. S. Inorg. Chem. 2006, 45, 4186. (c) Greene, L. E.; Yuhas, B. D.; Law, M.; Zitoun, D.; Yang, P. Inorg. Chem. 2006, 45, 7535. (d) Xing, Y. J.; Xi, Z. H.; Zhang, X. D.; Song, J. H.; Wang, R. M.; Xu, J.; Xue, Z. Q.; Yu, D. P. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 1527. (e) Jie, J.; Wang, G.; Chen, Y.; Han, X.; Wang, Q.; Xu, B. Appl. Phys. Lett. 2005, 86, 031909. (f) Ma, X.; Zhang, H.; Ji, Y.; Xu, J.; Yang, D. Mater. Lett. 2005, 59, 3393. (g) Su, X.; Zhang, Z.; Wang, Y.; Zhu, M. J. Phys. D: Appl. Phys. 2005, 38, 3934. (h) Ahn, S. E.; Lee, J. S.; Kim, H.; Kim, S.; Kang, B. H.; Kim, K. H.; Kim, G. T. Appl. Phys. Lett. 2004, 84, 5022. (i) Liu, B.; Zeng, H. C. Langmuir 2004, 20, 4196. (j) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, 4430. (k) Li, Z.; Xiong, Y.; Xie, Y. Inorg. Chem. 2003, 42, 8105. (l) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, 4430. (m) Dem’yanets, L. N.; Kostomarov, D. V.; Kuz’mina, I. P. Inorg. Mater. 2002, 38, 124. (n) Lyu, S. C.; Zhang, Y.; Ruh, H.; Lee, H. J.; Shim, H. W.; Suh, E. K.; Lee, C. J. Chem. Phys. Lett. 2002, 363, 134. (10) (a) Liu, J. F.; Bei, Y. Y.; Wu, H. P.; Shen, D.; Gong, J. Z.; Li, X. G.; Wang, Y. W.; Jiang, N. P.; Jiang, J. Z. Mater. Lett. 2007, 61, 283. (b) Ethayaraja, M.; Bandyopadhyaya, R. Langmuir 2007, 23, 6418. (c) Yubuta, K.; Sato, T.; Nomura, A.; Haga, K.; Shishido, T. J. Alloys Compd. 2007, 436, 396. (d) Yuan, H.; Zhang, Y. J. Cryst. Growth 2004, 263, 119. (e) Shen, X.-P.; Yuan, A.-H.; Hu, Y.-M.; Jiang, Y.; Xu, Z.; Hu, Z. Nanotechnology 2005, 16, 2039. (f) Wu, J. J.; Liu, S. C. AdV. Mater. 2002, 14, 215. (11) Gulino, A.; Castelli, F.; Dapporto, P.; Rossi, P.; Fragala`, I. Chem. Mater. 2000, 12, 548. (12) Mehrotra, R. C.; Bohra, R.; Gaur D. P. Metal β-Diketonates and Allied DeriVatiVes; Academic Press: London, 1978. (13) JCPDS card 36-1451, ZnO. (14) (a) Gulino, A.; Compagnini, G.; Scalisi, A. A. Chem. Mater. 2003, 15, 3332. (b) Tauc, J.; Menth, A. J. Non-Cryst. Solids 1972, 8-11, 569.
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