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Langmuir 2009, 25, 2140-2147
Study on the Formation of Self-Assembled Monolayers on Sol-Gel Processed Hafnium Oxide as Dielectric Layers Guy G. Ting II,† Orb Acton,‡ Hong Ma,‡ Jae Won Ka,‡ and Alex K.-Y. Jen*,†,‡ Department of Chemistry, UniVersity of Washington, Box 351700, Seattle, Washington 98195-1700, and Department of Materials Science and Engineering, Box 352120, UniVersity of Washington, Seattle, Washington 98195-2120 ReceiVed September 8, 2008. ReVised Manuscript ReceiVed December 11, 2008 High dielectric constant (k) metal oxides such as hafnium oxide (HfO2) have gained significant interest due to their applications in microelectronics. In order to study and control the surface properties of hafnium oxide, self-assembled monolayers (SAMs) of four different long aliphatic molecules with binding groups of phosphonic acid, carboxylic acid, and catechol were formed and characterized. Surface modification was performed to improve the interface between metal oxide and top deposited materials as well as to create suitable dielectric properties, that is, leakage current and capacitance densities, which are important in organic thin film transistors. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, contact angle goniometry, atomic force microscopy (AFM), and simple metal-HfO2-SAM-metal devices were used to characterize the surfaces before and after SAM modification on sol-gel processed hafnium oxide. The alkylphosphonic acid provided the best monolayer formation on sol-gel processed hafnium oxide to generate a well-packed, ultrathin dielectric exhibiting a low leakage current density of 2 × 10-8 A/cm2 at an applied voltage of -2.0 V and high capacitance density of 0.55 µF/cm2 at 10 kHz. Dialkylcatechol showed similar characteristics and the potential for using the catechol SAMs to modify HfO2 surfaces. In addition, the integration of this alkylphosphonic acid SAM/hafnium oxide hybrid dielectric into pentacene-based thin film transistors yields low-voltage operation within 1.5 V and improved performance over bare hafnium oxide.
Introduction High dielectric constant (k) metal oxides such as hafnium oxide have gained significant interest due to their applications in microelectronics. HfO2-based high-k dielectrics have emerged as leading candidates to replace SiO2 for scaling below the 65 nm node in advanced metal-oxide-semiconductor applications. HfO2 has a dielectric constant of 16-29 and a wide band gap of 5.8 eV, which is suitable for dielectrics with high capacitance and low leakage current even using films only a few nanometers in thickness.1 Typical processing routes for HfO2 include atomic layer deposition,2 chemical vapor deposition,3 and physical vapor deposition. Although these techniques could achieve high-quality films with nanometer precision, they each have drawbacks and all require expensive, high vacuum equipment. In contrast, sol-gel processing has been widely used to fabricate high-quality metal oxides with advantages of low cost, relative simplicity, and good control of chemical composition.4 The ability to chemically modify the HfO2 surface in a wellcontrolled manner provides the possibility to tune the interfacial and dielectric properties of the metal oxide which are important to be controlled in organic electronics. More specifically, a key challenge for realizing practical applications lies in developing gate dielectrics with low leakage current, low interface trap density, high breakdown strength, and high capacitance for low* Corresponding author. E-mail:
[email protected]. Telephone: (206) 543-2626. Fax: (206) 543-3100. † Department of Chemistry. ‡ Department of Materials Science and Engineering.
(1) Robertson, J. Rep. Prog. Phys. 2006, 69, 327. (2) Moahammad, S. A.; Jack, C. L.; Naim, M.; Jeff, P. Appl. Phys. Lett. 2006, 88, 082901. (3) Maunoury, C.; Dabertrand, K.; Martinez, E.; Saadoune, M.; Lafond, D.; Pierre, F.; Renault, O.; Lhostis, S.; Bailey, P.; Noakes, T. C. Q.; Jalabert, D. Appl. Phys. Lett. 2007, 101. (4) Aoki, Y.; Kunitake, T.; Nakao, A. Chem. Mater. 2005, 17, 450.
voltage organic thin film transistors (OTFTs).5 Molecular selfassembled monolayers (SAMs) have been proven to be excellent candidates for gate dielectrics in low-voltage OTFTs. SAM dielectrics are composed of densely packed organic molecular monolayers that suppress carrier tunneling via highly ordered aliphatic chains even though they are only a few nanometers in thickness.6 In addition, by tuning the surface terminal group of the SAM, it is possible to modify the interface between the organic semiconductoranddielectricbyexploitingcompatibleorganic-organic interactions resulting in improved device performances.7 SAMs are densely packed molecular films formed by the adsorption of an organic surfactant on a solid substrate from the solution or vapor phase.8 These monolayers can be prepared using different types of molecules for different substrates. SAMs from the adsorption of carboxylic acids9-16 and hydroxamic acids17 on metal oxides, the chemisorption of alkanethiols onto gold and silver,17-21 and the hydrolysis and reaction of (5) Facchetti, A.; Yoon, M. H.; Marks, T. J. AdV. Mater. 2005, 17, 1705. (6) Boulas, C.; Davidovitus, J. V.; Rondelez, F.; Vuillaume, D. Phys. ReV. Lett. 1996, 74, 4797. (7) McDowell, M.; Hill, I. G.; McDermott, J. E.; Bernasek, S. L.; Schwartz, J. Appl. Phys. Lett. 2006, 88, 073505. (8) Ulman, A. An Introduction to Ultrathin Organic Films: from LangmuirBlodgett to Self-Assembly; Academic Press Inc.: Boston, 1991. (9) Tao, Y.-T. J. Am. Chem. Soc. 1993, 115, 4350. (10) Samant, M. G.; Brown, C. A.; Gordon, J. G., II Langmuir 1993, 9, 1082. (11) Allara, D. L.; Atre, S. V.; Ellinger, C. A.; Snyder, R. G. J. Am. Chem. Soc. 1991, 113, 1852. (12) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (13) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (14) Chen, S. H.; Frank, C. W. Langmuir 1989, 5, 978. (15) Smith, E. L.; Porter, M. D. J. Phys. Chem. 1993, 97, 8032. (16) Chauhan, A. K.; Aswal, D. K.; Koiry, S. P.; Gupta, S. K.; Yakhmi, J. V.; Surgers, C.; Guerin, D.; Lenfant, S.; Vuillaume, D. Appl. Phys. A: Mater. Sci. Process. 2008, 90, 581. (17) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M.; Nuzzo, R. G. Langmuir 1995, 11, 813. (18) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (19) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.
10.1021/la802944n CCC: $40.75 2009 American Chemical Society Published on Web 01/07/2009
Formation of SAMs on Sol-Gel Processed HfO2
alkyltrichlorosilanes onto hydroxylated surfaces22-24 are the most thoroughly studied. However, in the latter context of siloxane chemistry, such monolayer formation often competes with lateral cross-linking due to low surface OH group content on the metal surface oxide and is critically dependent on the water content of the deposition solvent and environment.25-27 Attaining structural order in such films is not straightforward. Although organophosphonic acid can only condense with surface silanols on SiO2 at high temperatures, they chemisorb readily with a wide range of metal oxides at room temperature without being limited by surface OH group content.28 In fact, monolayers of long-chain alkylphosphonic acids self-assemble in an epitaxial fashion on metal oxides without homocondensation which is similar to alkanethiols on noble metals.28 High-quality monolayers formed by phosphonic acids have been employed as corrosion inhibitors,29 for metal extraction,30 in biomedical applications27 such as the prevention of calcification, and for sensors and thin film transistors.31,32 Phosphonic acids have also been used to form robust and ordered films on relatively inert oxide surfaces such as titanium and stainless steel.33-35 The reason for the increased reactivity of phosphonic acids over other organic acids has been speculated upon. The reactivity may be due to the phosphonic acid’s low pKa and thus its ability to participate in acid-base reactions with surface hydroxyl groups and µ-oxo groups or they may be physisorbed onto the surface in the form of salts.33-36 SAMs can be arranged as diffuse (disordered) or dense (ordered) monolayers. The latter will be preferred when using SAMs as a dielectric layer. For diffuse monolayers, the carbon chains contain gauche conformations, resulting in gaps between the molecules, and thus, low-density monolayers are formed.12,37-39On the other hand, for dense monolayers, the carbon-carbon chains have an all-trans conformation. These monolayers contain rigid molecules due to additive van der Waals forces between alkyl chains of each molecule, resulting in a closely packed monolayer.12 When a long-chain SAM is densely packed and highly ordered, it can be effectively used as a dielectric material that suppresses charge carrier tunneling even though it is only a few nanometers in thickness.6 In this study, we report on the surface bonding, conformational order, and dielectric properties of long-chain phosphonic acids, carboxylic acids, and catechols adsorbed onto sol-gel processed (20) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (21) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370. (22) Hoeppener, S.; Maoz, R.; Sagiv, J. AdV. Mater. 2006, 18, 1286. (23) Netzer, L.; Sagiv, J. J. Am. Chem. Soc. 1983, 105, 674. (24) Moaz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. (25) Wang, Y.; Lieberman, M. Langmuir 2003, 19, 1159. (26) Rye, R.; Nelson, G. C.; Dugger, M. T. Langmuir 1997, 13, 2965. (27) Nie, H. Y.; Miller, D. J.; Francis, J. T.; Walzak, M. J.; McIntyre, N. S. Langmuir 2005, 21, 2773. (28) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1996, 12, 6429. (29) Duprat, M.; Shiri, A.; Derbali, Y.; Pebere, N. Electrochem. Methods Corros. Res., Proc. Int. Symp., 1985 1986, 8, 267. (30) Aguilar, M.; Miralles, N.; Sastre, A. M. ReV. Inorg. Chem. 1989, 10, 93. (31) Acton, O.; Ting, G.; Ma, H.; Ka, J. W.; Yip, H.-L.; Tucker, N. M.; Jen, A. K.-Y. AdV. Mater. 2008, 20, 3697. (32) Ma, H.; Acton, O.; Ting, G.; Ka, J. W.; Yip, H.-L.; Tucker, N. M.; Schofield, R.; Jen, A. K.-Y. Appl. Phys. Lett. 2008, 92, 113303. (33) Gawalt, E. S.; Avaltroni, M. J.; Koch, N.; Schwartz, J. Langmuir 2001, 17, 5736. (34) Raman, A.; Dubey, M.; Gouzman, I.; Gawalt, E. S. Langmuir 2006, 22, 6469. (35) Pellerite, M. J.; Dunbar, T. D.; Boardman, L. D.; Wood, E. J. J. Phys. Chem. B 2003, 107, 11726. (36) Quinones, R.; Gawalt, E. S. Langmuir 2007, 23, 10123. (37) Van Alsten, J. G. Langmuir 1999, 15, 7605. (38) Badia, A.; Lennox, R. N.; Reven, L. Acc. Chem. Res. 2000, 33, 475. (39) Chechik, V.; Crooks, R. M.; Stirling, C. J. M. AdV. Mater. 2000, 12, 1161.
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HfO2. SAMs were characterized by attenuated total reflectance infrared (ATR-FTIR) spectroscopy, contact angle goniometry, atomic force microscopy (AFM), and simple metal-SAM-HfO2-metal devices. Octadecyphosphonic acid (ODPA) has the densest assembly on the sol-gel processed HfO2, resulting in the best dielectric properties compared to the other SAMs studied. However, 4,5-dioctadecyl-benzene-1,2-diol (C36C) showed similar characteristics and the potential for using the catechol SAMs to modify HfO2 surfaces.
Experimental Section Methods and Materials. All solvents and reagents were purchased from Aldrich and used as received unless otherwise stated. Octadecylphosphonic acid (ODPA, 99.9% purity) was purchased from PCI Synthesis. Absolute (200 proof) ethanol (Aaper Alcohol and Chemical Company) was used for making solutions and rinsing. 4-Octadecyl-benzene-1,2-diol (C18C) and 4,5-dioctadecyl-benzene1,2-diol (C36C) were designed and synthesized in our laboratories. Ether was distilled under nitrogen from sodium with benzophenone as the indicator. Methylene chloride was distilled over P2O5. 1H NMR spectra (300 MHz) were taken on a Bruker-300 FT NMR spectrometer with tetramethylsilane (TMS) as internal reference. Elemental analysis was determined at QTI (Whitehouse, NJ). Electrospray ionization mass spectrometry (ESI-MS) spectra were obtained on a Bruker Daltonics Esquire Ion Trap mass spectrometer. Synthesis of 4-Octadecyl-benzene-1,2-diol (C18C).
Stearoyl Chloride. A solution of stearic acid (30.0 g, 105.5 mmol) in thionyl chloride (100 mL) was refluxed for 12 h. After that, an excess amount of thionyl chloride was distilled off to yield colorless oil (27.0 g, 85%). 4-Octadecanoyl-1,2-dimethoxybenzene. To a mixture of 1,2dimethoxybenzene (5.0 g, 36.2 mmol) and aluminum chloride (5.3 g, 39.8 mmol) in dichloromethane (250 mL) was added stearoyl chloride (11.0 g, 36.2 mmol) in dichloromethane (50 mL) at 0 °C. The reaction mixture was refluxed for 24 h and then cooled at 0 °C. The excess aluminum chloride was quenched by slow addition of 6N HCl (100 mL). The aqueous layer was extracted with dichloromethane. The solvent was removed under reduced pressure to yield a light brown solid. The mixture was recrystallized from toluene to give a white solid (12.5 g, 85%). 1H NMR (300 MHz, CDCl3): δ 0.83-0.93 (t, 3H), 1.18-1.43 (m, 28H), 1.64-1.80 (m, 2H), 2.88-2.95 (m, 2H), 3.94 (s, 3H), 3.95 (s, 3H), 6.88 (d, 1H, J ) 8.3 Hz), 7.54 (d, 1H, J ) 1.9 Hz), 7.59 (dd, 1H, J ) 8.3 Hz, 1.9 Hz). Calcd for C26H44O3: C, 77.18; H, 10.96. Found: C, 76.97; H, 11.07. ESI-MS (m/z): calcd, 404.3; found, 403.2. 4-Octadecyl-1,2-dimethoxybenzene. To a suspension of lithium aluminum hydride (2.1 g, 55.6 mmol) in dry ether (50 mL) was added aluminum chloride (2.5 g, 18.5 mmol) in dry ether via a double tipped needle at 0 °C. After stirring for 15 min, a solution of 4-octadecanoyl-1,2-dimethoxybenzene (5.0 g, 12.4 mmol) was added dropwise to the suspension maintained at 0 °C. The mixture was stirred at room temperature for 4.5 h. The reaction was quenched by 6N HCl solution at 0 °C. The organic layer was washed with water, and solvent was removed under reduced pressure. The mixture was purified by column chromatography (silica gel, CH2Cl2/hexane
2142 Langmuir, Vol. 25, No. 4, 2009 ) 1/1) to give a white solid (3.4 g, 70%). 1H NMR (300 MHz, CDCl3): δ 0.83-0.92 (t, 3H), 1.20-1.38 (m, 30H), 1.52-1.65 (m, 2H), 2.50-2.58 (m, 2H), 3.86 (s, 3H), 3.88 (s, 3H), 6.68-6.82 (m, 3H). Calcd for C26H46O2: C, 79.94; H, 11.87. Found: C, 79.82; H, 11.98. ESI-MS (m/z): calcd, 390.4; found, 390.4. 4-Octadecyl-benzene-1,2-diol (4-Octadecylcatechol, C18C). To a solution of 4-octadecyl-1,2-dimethoxybenzene (1.35 g, 3.5 mmol) in dry dichloromethane (200 mL) was slowly added BBr3 (1.3 mL, 13.8 mmol). The reaction mixture was stirred for 12 h at room temperature, and then the mixture was poured into ice water. The aqueous layer was extracted with dichloromethane, and the solvent was evaporated under reduced pressure. The mixture was purified by column chromatography (silica gel, CH2Cl2 to EtOAc) to give a brown solid (0.82 g, 65%). 1H NMR (300 MHz, CDCl3): δ 0.83-0.92 (t, 3H), 1.20-1.38 (m, 30H), 1.52-1.65 (m, 2H), 2.50-2.58 (m, 2H), 6.68-6.82 (m, 3H). Calcd for C24H42O2: C, 79.50; H, 11.68. Found: C, 79.36; H, 11.79. ESI-MS (m/z): calcd, 362.3; found, 362.2. Synthesis of 4,5-Dioctadecyl-benzene-1,2-diol (C36C).
4-Octadecanoyl-5-octadecyl-1,2-dimethoxybenzene. To a mixture of 4-octadecyl-1,2-dimethoxybenzene (2.30 g, 6.0 mmol) and aluminum chloride (0.88 g, 6.6 mmol) in dichloromethane (150 mL) was added stearoyl chloride (1.80 g, 6.0 mmol) in dichloromethane (20 mL) at 0 °C. The reaction mixture was refluxed for 24 h and then cooled at 0 °C. The excess aluminum chloride was quenched by slow addition of 6N HCl (50 mL). The aqueous layer was extracted with dichloromethane. The solvent was removed under reduced pressure to yield a brown solid. The mixture was purified by column chromatography (silica gel, hexanes) to give a white solid (1.60 g, 42%). 1H NMR (300 MHz, CDCl3): δ 0.82-0.96 (t, 6H), 1.18-1.42 (m, 58H), 1.45-1.60 (m, 2H), 1.62-1.77 (m, 2H), 2.74-2.9 (m, 4H), 3.88 (s, 3H), 3.92 (s, 3H), 6.72 (s, 1H), 7.12 (s, 1H). Calcd for C44H80O3: C, 80.42; H, 12.27. Found: C, 80.30; H, 12.35. ESI-MS (m/z): calcd, 656.6; found, 656.5. 4,5-Dioctadecyl-1,2-dimethoxybenzene. In a three-neck flask equipped with a nitrogen inlet was dissolved lithium aluminum hydride (0.12 g, 3.1 mmol, 4.5 equiv) in dry diethyl ether (25 mL). A solution of aluminum chloride (0.14 mg, 1.0 mmol, 1.5 equiv) in dry ether (25 mL) was added via cannula to the lithium aluminum hydride solution, cooled at 0 °C. After 15 min, a solution of 4-octadecanoyl-5-octadecyl-1,2-dimethoxybenzene (0.45 g, 0.68 mmol) in ether (20 mL) was added dropwise to the reaction flask maintained at 0 °C. Upon completion of the addition, the mixture was allowed to warm up to room temperature. After 4 h, the reaction was cooled to 0 °C and quenched slowly by addition of 100 mL of 6 M HCl. The aqueous phase was extracted with dichloromethane, and the combined organic layers were washed with water. The organic solvent was removed under vacuum to yield a yellow oil purified by column chromatography (silica gel, CH2Cl2/hexane ) 3/7). This yields the product as a light-yellow oil (0.31 mg, 70%). 1H NMR (300 MHz, CDCl3): δ 0.84-0.93 (t, 6H), 1.18-1.42 (m, 60H), 1.44-1.60 (m, 4H), 2.47-2.57 (m, 4H), 3.84 (s, 6H), 6.65 (s, 2H). Calcd for C44H82O2: C, 82.17; H, 12.85. Found: C, 82.03; H, 12.96. ESI-MS (m/z): calcd, 642.6; found, 642.6. 4,5-Dioctadecyl-benzene-1,2-diol (4,5-Dioctadecylcatechol, C36C). To a solution of 4,5-dioctadecyl-1,2-dimethoxybenzene (0.20 mg,
Ting et al. 0.31 mmol) in dry dichloromethane (20 mL) was added dropwise BBr3 (0.12 mL, 1.24 mmol). The reaction mixture was stirred for 12 h at room temperature, and then the mixture was poured into ice water. The aqueous layer was extracted with dichloromethane, and the solvent was evaporated under reduced pressure. The mixture was purified by column chromatography (silica gel, CH2Cl2 to EtOAc) to give a brown solid (0.17 g, 90%). 1H NMR (300 MHz, CDCl3): δ 0.84-0.93 (t, 6H), 1.27-1.40 (m, 60H), 1.42-1.58 (m, 4H), 2.42-2.50 (m, 4H), 6.65 (s, 2H). Calcd for C42H78O2: C, 82.02; H, 12.78. Found: C, 81.90; H, 12.87. ESI-MS (m/z): calcd, 614.6; found, 614.5. Preparation of Substrates. Heavily doped p-type silicon wafers Si(100) (from Montco Semiconductors) were diced and cleaned in 3:1 by volume sulfuric acid/hydrogen peroxide (piranha solution) at 70 °C for 30 min, rinsed thoroughly with deionized (DI) water, sonicated for 15 min in DI water/ammonium hydroxide/hydrogen peroxide (5:1:1 by volume), rinsed and dried under nitrogen, and used immediately. The silicon substrates were characterized to have a root-mean-square (rms) roughness of 0.18 ( 0.02 nm. Preparation of Sol-Gel Hafnium Oxide. Hafnium oxide sol-gel was prepared by dissolving hafnium(IV) chloride (HfCl4) (98% Aldrich) in 200 proof ethanol under a nitrogen atmosphere, followed by adding a mixture of nitric acid (HNO3) and DI water in air (molar ratio of components HfCl4/EtOH/HNO3/H2O ) 1:410:5:5). The solution was filtered using a 0.2 µm poly(tetrafluoroethylene) (PTFE) filter and heated at 50 °C for 3 h to promote hydrolysis and polymerization of the HfOx sol-gel. Hafnium oxide films were prepared by spin-coating the HfOx sol-gel onto cleaned native oxide Si substrates at 6000 rpm for 30 s. After leaving to sit for 1 h, the films were crystallized in an oven at 600 °C for 30 min, and then removed and cooled to room temperature. The hafnium oxide substrates were characterized to have a rms roughness of 0.21 ( 0.02 nm. Formation of Monolayers. Self-assembled monolayers were immediately prepared following hafnium oxide preparation by immersing substrates into solutions of 0.1 mM ODPA, stearic acid (SA), C18C, or C36C in nitrogen-purged dry tetrahydrofuran (THF)/ EtOH (1:1) at 70 °C. The substrates were kept in solution at 70 °C in the dark for 16 h followed by drying with nitrogen, thermal annealing at 110 °C for 10 min in an inert argon environment, sonication in THF, rinsing with THF and EtOH, and blow drying with nitrogen. Characterization of Monolayers. ATR-FTIR. The surfacemodified substrates were studied using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Polarized FTIR spectra were obtained using a Bruker Tensor spectrometer (Ettlingen, Germany) equipped with a nitrogen-cooled Harrick GATR single angle reflection accessory (Ossining, NY). Each spectrum was run with a minimum of 1056 scans at resolution of 4 cm-1 while being purged with dry air in between each data collection to eliminate water vapor from the sample compartment. Contact Angle Goniometry. Contact angle measurements were used to analyze the hydrophobicity of the surface. Contact angles were measured by the sessile drop technique using a Rame-Hart 100 goniometer under laboratory conditions (∼40% humidity). A 2 µL drop of deionized water (Millipore) was brought into contact with the sample to analyze the wettability of the SAMs formed on the sol-gel processed hafnium oxide. Three to five measurements were taken on different areas on multiple samples, and the average and standard deviations were calculated. AFM. Imaging measurements were made in the tapping mode using a Multimode Nanoscope III scanning probe microscope (Digital Instruments) operating in ambient conditions at a scan rate of 0.5-1.0 Hz. Silicon cantilevers with spring constants ranging from 12 to 103 N/m were used. Tips were etched silicon tips with a typical resonant frequency of 300-350 kHz. Image resolution was 512 × 512 pixels. Roughness measurements and cross-sectional analysis were performed using the algorithms contained in the AFM software. Si-HfO2-SAM-Au Dielectric DeVices. An Agilent 4192 impedance meter was used for capacitance characterization for frequencies from 100 Hz to 100 kHz. Gold dot contacts (1 mm in diameter) were
Formation of SAMs on Sol-Gel Processed HfO2
Langmuir, Vol. 25, No. 4, 2009 2143
Figure 1. Surface characterization of unmodified sol-gel processed HfO2. (A) AFM topographical image showing the smoothness and uniformity of the substrate with a rms roughness of 0.21 nm. (B) ATR-FTIR spectrum containing no peaks attributable to organic deposition.
prepared by thermally evaporating gold through a shadow mask directly onto the SAMs and were used as the top electrode with heavily doped silicon used as the bottom electrode. Capacitance values were taken at 10 kHz. DeVice Fabrication and Testing. Pentacene (from Aldrich, used without further purification) was deposited at 0.5 Å/s from a resistively heated quartz crucible at 2 × 10-6 Torr, with the room-temperature substrates. Interdigitated source (S) and drain (D) electrodes (W ) 9000 µm, L ) 90 µm, W/L ) 100) were defined on top of the pentacene by evaporating a 50 nm thick gold film at 0.5 Å/s through a shadow mask from a resistively heated Mo boat at 2 × 10-6 Torr. All OTFT and J-V characterization was performed under ambient conditions using an Agilent 4155B semiconductor parameter analyzer. The field-effect mobility was calculated in the saturation regime from the linear fit of (-Ids)1/2 versus Vgs. The threshold voltage (Vt) was estimated as the x intercept of the linear section of the plot of (-Ids)1/2 versus Vgs. The subthreshold slope was calculated by taking the inverse of the slope of -Ids versus Vgs in the region of exponential current increase. All values are an average of over 10 transistors tested from two different batches.
Results and Discussion Metal oxides such as HfO2 are of interest in microelectronics because they have dielectric constants that are larger than that of SiO2, making them potential replacements for SiO2 as a gate dielectric.40 Well-controlled modification of the SiO2/Si surface can also be difficult and problematic, often resorting to techniques such as radical-induced hydrosilylation of unsaturated organics that are inappropriate for bonding to SiO2 because they involve H-terminated Si which requires the use of HF to remove the oxide layer, as well as other multicycle processes and high temperature bonding reactions. For instance, Hanson et al. have demonstrated that phosphonic acids do not react with SiO2 at room temperature and only under high temperatures will react with hydroxyl species of SiO2 to form phosphonates.41 On the other hand, it has been previously demonstrated that HfO2 films can be easily prepared through sol-gel processing.4,31,42 Therefore, it would be very useful if we can develop protocols to functionalize SAMs onto HfO2 for studying their assembling characteristics. In this study, HfO2 films were spun coat from a HfCl4 precursor sol-gel solution onto heavily doped Si substrates and annealed (40) Honnon, J. B.; Afzali, A.; Klinke, C.; Avouris, P. Langmuir 2005, 21, 8569. (41) Hanson, E. L.; Schwartz, J.; Nickel, B.; Koch, N.; Danisman, M. F. J. Am. Chem. Soc. 2003, 125, 16074. (42) Nishide, T.; Honda, S.; Matsuura, M.; Ide, M. Thin Solid Films 2000, 371, 61.
at 600 °C for 30 min. AFM images indicate that the films are smooth and pinhole free with a rms roughness of 0.21 nm (Figure 1A). The HfO2 films were relatively smooth in comparison to the underlying silicon which had a rms roughness of 0.18 nm. Small angle X-ray reflectivity (XRR) and ellipsometry thickness measurements indicate a layer of 3.1 nm thick HfO2 on 1.9 nm thick SiO2.31 Immediately after HfO2 film formation, SAMs were prepared in nitrogen purged THF/EtOH (1:1) solutions containing 0.1 mM of the given molecules at 70 °C for 16 h followed by thermal annealing at 110 °C for 10 min, rinsing with THF, EtOH, and drying with a stream of nitrogen. Figure 2A shows the chemical structures of the self-assembling molecules used in this work. These molecules were chosen because of several distinct characteristics including (1) better stability to moisture, (2) less tendency for homocondensation between neighboring molecules, (3) same number of carbon units in the aliphatic tails, and (4) varying pKa values of the binding groups. It has been reported that chain length can substantially alter the ability of molecules to form a high-quality monolayer.43 Shorter chain lengths of four carbon units or less are not long enough to provide enthalpic gain of the van der Waals interactions between the chains to offset the entropic loss when bonds are held in the all-trans configuration present in ordered films.34 In the literature, an eight-carbon chain is one of the shortest molecules reported to form ordered monolayers on silicon and other oxides44 while a length between 13 and 20 carbon units usually forms the best order.8 In this study, SAMs with 18 carbon units were chosen in order to exploit the formation of well-ordered SAMs and take advantage of the alkyl chain as a dielectric material. Advancing contact angles were measured to determine the wettability and to estimate the monolayer quality of methylterminated adsorbates. Densely packed and well-ordered monolayers predominantly expose methyl groups at the surface, decreasing the surface wettability. In contrast, loosely packed monolayers expose a substantial fraction of methylene groups in addition to methyl groups at the surface, thereby increasing the wettability and decreasing the contact angle.45 These measurements were made by forming a 2 µL drop of deionized water (Millipore) at the end of a microliter syringe which was (43) Meyers, D. Surfaces, Interfaces, and Colloids: Principles and Applications, 2nd ed.; John Wiley and Sons: New York, 1999. (44) Gandhi, D. D.; Lane, M.; Zhou, Y.; Singh, A. P.; Nayak, S.; Tisch, U.; Eizenberg, M.; Ramanath, G. Nature 2007, 447, 299-U2. (45) Park, J. S.; Smith, A. C.; Lee, T. R. Langmuir 2004, 20, 5829.
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Figure 2. (A) Chemical structures of different aliphatic organic molecules used in this study with varying binding groups used to form SAMs on the sol-gel processed HfO2. (B) AFM topographical image of ODPA SAM on sol-gel processed HfO2 with a rms roughness of 0.20 nm. This image is also representative of the substrate smoothness of other SAM modifications. Table 1. Summary of Surface Characterization Results SAM bare HfO2 ODPA/HfO2 C36C/HfO2 C18C/HfO2 SA/HfO2
-1
νa(CH2) (cm ) 2918 2920 2922 2925
νs(CH2) (cm-1)
advancing contact angle (deg)
rms roughness (nm)
2851 2851 2852 2855
