Discriminate Crystallinities of Tin Doped Indium Oxide Films on Self

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Discriminate Crystallinities of Tin Doped Indium Oxide Films on Self-Assembled Monolayers Modified Glass Substrates Jadab Sharma, Hsuan-chun Chang, and Yian Tai* Department of Chemical Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Taipei 106, Taiwan Received December 10, 2009. Revised Manuscript Received March 4, 2010 Tin doped indium oxide (ITO) films have generated tremendous research interest and received widespread applications in optoelectronic devices due to a good combination of desired optical and electrical properties. Their electrical properties vary depending on the crystallinity of the film. A good quality ITO film should have low resistivity, which can be achieved with highly crystalline films deposited at very high temperature. Thus, film quality is sensitive to the deposition conditions. Generally, low-temperature deposition of ITO results in poor quality films due to amorphous growth. In this study, we have demonstrated that crystallinity of the ITO films can be improved even at room temperature (RT) using self-assembled monolayers (SAMs) modified glass substrates. The present study demonstrates that SAM with -SH terminal group is necessary for the high-quality ITO growth, while SAMs with other terminal groups (-NH2 and -CH3) generate ITO films with moderate crystallinity. Various properties of such films were investigated using X-ray diffraction, X-ray photoelectron depth profile, four-point probe, and Hall measurements. It is confirmed from such measurements that ITO film deposited on -SH terminated SAM substrate has excellent crystallinity, conductivity, and optical transmission.

Introduction Tin doped indium oxide (ITO) is the most commonly used transparent conducting oxide (TCO) in many optical and optoelectronic devices, which include solar cell electrodes, resistive heaters, antireflective coatings, heat reflecting mirrors, electromagnetic shield coatings and antistatic coatings for instrumental panels, liquid crystal displays (or flat panel displays), organic light-emitting diodes (OLED), and photodetectors.1-5 A good quality ITO film requires high electrical conductivity and high optical transmission, which can be achieved with highly crystalline ITO films.6,7 Accordingly, several methods have been developed for the fabrication of ITO films, and a good quality material is obtained when the temperature of the substrate is raised to around 220 °C.3,7-9 This temperature requirement is not suitable for many of the recent devices due to the heat sensitivity of substrates used in the fabrication. To avoid the use of high temperature, plasma sputtering techniques have been developed for the low-temperature deposition, enabling fabrication of ITO films on a variety of substrates.10,11 *Corresponding author: Ph þ886-2-2737-6620, Fax þ886-2-2737-6644, e-mail [email protected]. (1) Kobayashi, H.; Ishida, Y.; Nakato, Y.; Tsubomura, H. J. Appl. Phys. 1991, 69, 1736. (2) Costellano, J. E. Handbook of Display Technology; Academic Press: New York, 1992. (3) Tahar, R. B. H.; Ban, T.; Ohya, Y.; Takahashi, Y. J. Appl. Phys. 1998, 83, 2631. (4) Odaka, H.; Iwata, S.; Taga, N.; Ohnishi, S.; Kaneta, Y.; Shigesato, Y. J. Appl. Phys. 1997, 36, 5551. (5) Granqvist, C. G. Sol. Energy Mater. Sol. Cells 2007, 91, 1592. (6) Kulkarni, A. K.; Schulz, K. H.; Lim, T. S.; Khan, M. Thin Solid Films 1999, 345, 273. (7) Lee, H.-C. Appl. Surf. Sci. 2006, 252, 2647. (8) Behrisch, R. Sputtering by Particle Bombardment I, Physics & Applications; Springer: Berlin, 1981; p 32. (9) Cracium, V.; Cracium, D.; Chen, Z.; Hwang, J.; Singh, R. K. Mater. Res. Soc. Symp. Proc. 2000, 617, 42. (10) Laux, S.; Kaiser, N.; Z€oller, A.; G€otzelmann, R.; Lauth, H.; Benitzki, H. Thin Solid Films 1998, 335, 1. (11) Anguita, J.; Twaites, M.; Holton, B.; Hockley, P.; Rand, S.; Haughton, S. Plasma Process. Polym. 2007, 4, 48.

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However, low-temperature deposition yields amorphous material with high resistivity and hence with limited performance. To improve the crystallinity and electrical properties of ITO films, high-power plasma, which uses a mixture of argon and oxygen, is generally used.12 In the present work, however, we have demonstrated that by surface modification of glass substrates with different selfassembled monolayers (SAMs)13-17 and using argon plasma with very low power (10 W), ITO films with discriminate crystallinities could be created. SAM technique provides a unique opportunity to manipulate the physical and chemical properties of surfaces on a variety of substrates.18-22 The consequent Fermi level pinning affects the electrical and electronic properties of underlying substrate materials.23,24 SAM can also alter other physical properties as well, such as surface energy (and, therefore, tuning the surface from hydrophobic to hydrophilic or vice versa) and chemical reactivity of substrates.25 The ability to precisely control the properties of surfaces is the key factor that draws increasing interest in SAM (12) Wakeham, S. J.; Thwaites, M. J.; Holton, B. W.; Tsakona, C.; Cranton, W. M.; Koutsogeorgis, D. C.; Ranson, R. Thin Solid Films 2009, 518, 1355. (13) Flink, S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Phys. Org. Chem. 2001, 14, 407. (14) Crego-Calama, M.; Reinhoudt, D. N. Adv. Mater. 2001, 13, 1171. (15) Halliwell, C. M.; Cass, A. E. G. Anal. Chem. 2001, 73, 2476. (16) Onclin, S.; Ravoo, B. J.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2005, 44, 6282. (17) Pallavicini, P.; Dacarro, G.; Galli, M.; Patrini, M. J. Colloid Interface Sci. 2009, 332, 432. (18) Ulman, A. An Introduction to Ultrathin Organic Films: From LangmuirBlodgett to Selfassembly: Academic Press: New York, 1991. (19) Ulman, A. Chem. Rev. 1996, 96, 1533. (20) Chaki, N. K.; Aslam, M.; Sharma, J.; Vijayamohanan, K. Proc. Indian Acad. Sci. (Chem. Sci.) 2001, 113, 659. (21) Gooding, J. J.; Mearns, F.; Yang, W.; Liu, J. Electroanalysis 2003, 15, 81. (22) Frank, S. J. Phys.: Condens. Matter 2004, 16, R881. (23) Lenfant, S.; Guerin, D.; Van, F. T.; Chevrot, C.; Palacin, S.; Bourgoin, J. P.; Bouloussa, O.; Rodelez, F.; Vuillaume, D. J. Phys. Chem. B 2006, 110, 13947. (24) Heimel, G.; Romaner, L.; Zojer, E.; Bredas, J.-L. Acc. Chem. Res. 2008, 41, 721. (25) Chechik, V.; Crooks, R. M.; Stirling, C. J. Adv. Mater. 2000, 12, 1161.

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technique and its various applications in molecular electronics. For example, control over charge carrier tunneling has been demonstrated for a pseudo metal-oxide-semiconductor fieldeffect transistors (MOSFET), where modulation of electrical properties is accomplished by the monolayers of grafted π-acceptor or π-donor molecules atop silicon surfaces.26 Similarly, electrical properties of metal-semiconductor junctions can be manipulated depending on the dipole moment of the SAM on the substrate.27 As a result of change in surface energy, crystal growth trend on SAM functionalized surfaces can also be controlled, exhibiting excellent nucleation site for different crystals. Accordingly, wet chemical growth of different crystals has been well studied and understood on several SAM modified substrates. For example, several studies have been successfully carried out on ω-functionalized SAMs in wet chemical environment at low temperature to grow ceramic thin films with controlled crystalline structure.28-31 Similar to several other substrates, SAM formation on glass substrates has been well-studied, and extensive spectroscopic chemical analysis has been carried out to understand the nature of SAMs on glass substrates.14,15,17 Herein, we extend this concept to the growth of ITO films on functionalized glass substrates by plasma sputtering, which exhibits variations in crystallinity depending on terminal functional groups. Highly crystalline ITO films can be grown on -SH terminated SAMs at room temperature (RT), while other SAMs with -CH3 and -NH2 terminal groups show the growth of ITO films with moderate crystallinity with significant implication on their electrical properties.

Experimental Section All glass substrates used for the deposition were purchased from Corning and were cut into 1  1 cm2 pieces. n-Propyltriethoxysilane (H3C-SAM, 97%, Aldrich), 3-aminopropyltriethoxysilane (H2N-SAM, 95%, Aldrich), and 3-mercaptopropyltriethoxysilane (HS-SAM, 95%, Acros Organics) were used as received. Acetone, 2-propanol, and decane were purchased from Acros Organics and were of either semiconductor or reagent grade (99%). The glass substrates were degreased in a dilute detergent followed by sonication in an ultrasonic bath using different solvents in the sequence of deionized water, acetone, and 2-propanol (IPA). Afterward, the glass substrates were immersed in 0.2 mM solutions of respective SAM molecules (H3C-SAM, H2N-SAM, and HS-SAM) in decane for 24 h.13-17 On removing from the solutions, all glass substrates were rinsed with decane and blown dry by constant N2 flow. The qualities of SAMs were verified by contact angle and XPS measurements (see Supporting Information). ITO films were deposited onto different substrates using a deposition unit with RF magnetron sputtering from a ceramic target of 90 wt % In2O3 (99.99%, Cathay Advanced Materials Limited) and 10 wt % SnO2 (99.99%, Cathay Advanced Materials Limited), using a plasma power of 10 W. A shutter was placed immediately above the sample to ensure the deposition started only after the equilibrium was reached. Prior to the deposition, the deposition chamber was pumped to a base pressure lower than 10-7 Torr and then backfilled with Ar to 10-2 Torr. The glass (26) He, T.; He, J.; Lu, M.; Chen, B.; Pang, H.; Reus, W. F.; Nolte, W. M.; Nackashi, D. P.; Franzon, P. D.; Tour, J. M. J. Am. Chem. Soc. 2006, 128, 14537. (27) Hiremath, R. K.; Rabinal, M. K.; Mulimani, B. G.; Khazi, I. M. Langmuir 2008, 24, 11300. (28) Bunker, B. C.; Rieke, P. C.; Tarasevich, B. J.; Campbell, A. A.; Fryxell, G. E.; Graff, G. L.; Song, L.; Liu, J.; Virden, J. W.; McVay, G. L. Science 1994, 264, 48. (29) Kυ.ther, J.; Termel, W. Chem. Commun. 1997, 2029. (30) Kυ.ther, J.; Nelles, G.; Seshadri, R.; Schaub, M.; Butt, H. J.; Termel, W. Chem.;Eur. J. 1998, 4, 1834. (31) Aslam, M.; Pethkar, S.; Bandopadhya, K.; Mulla, I. S.; Sainkar, S. R.; Mandal, A. B.; Vijayamohanan, K. J. Mater. Chem. 2000, 10, 1737.

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Figure 1. XRD patterns of ITO films deposited on different substrates at room temperature. ITO films were deposited simultaneously on various substrates using a deposition unit with RF magnetron sputtering from a ceramic target of 90 wt % In2O3 and 10 wt % SnO2 using a plasma power of 10 W. A shutter was placed immediately above the sample to ensure the deposition started only after the equilibrium was reached. XRD data were normalized by a standard. substrates were maintained at the desired temperature during the entire deposition using a remote temperature controller. The target diameter was 5 cm while substrate size was 1  1 cm2, and the sputter-down deposition approach was employed. The angle at which the targets were mounted was 90°, and the deposition rate was 10 nm/min. Sampling or deposition time was adjusted according to the requirements for various films. For example, deposition time for a 150 nm ITO film was ∼15 min with less than 8% deviation. The samples were analyzed by X-ray photoelectron (XP) depth profile measurements with a VG-Thermo theta probe X-ray photoelectron spectrometer (XPS). Monochromated Al KR was used as the X-ray source, while samples were sputtered by Arþ ions with energy of 0.7 keV. The electrical properties were measured using Ecopia HMS-3000 Hall measurement and fourpoint probe instruments. The optical measurements were performed with a JASCO V-670 UV-vis spectrometer. The morphology and thickness of the films were determined using a field emission scanning electron microscope (SEM, JEOL JSM-6500F), and the crystallinities were investigated by subjecting the samples to X-ray diffraction (XRD, PANalytical X’Pert PRO). All XRD data were normalized by a standard.

Results and Discussion Figure 1 shows the XRD profiles of ITO films grown at RT. It is evident from the XRD profiles that growth of ITO films differs significantly depending on the surface properties of substrates. Notably, the ITO film grown on the glass substrate modified with -SH terminal SAM shows a predominant orientation of (222) crystal plane, while (440) crystal plane is also evident, albeit with a very low intensity peak. Similar crystallographic patterns were reported previously for microwave plasma-assisted sputtering of ITO films.32 In sharp contrast, ITO films grown on substrates with -NH2 and -CH3 terminal groups have no well-defined XRD peaks. The (222) peak is significantly broadened, and the intensity is also very low. It is worthwhile to mention that the (32) Latz, R.; Daube, C.; Haranou, T.; Ocker, B.; Suzuki, K. J. Non-Cryst. Solids 1997, 218, 329.

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Article Table 1. Electrical Properties Measured by Four-Point Probe Technique of ITO Films (Thickness 150 nm, Deposited at RT and 150 °C) on Bare and SAM Modified Glass Substratesa ITO films

sheet resistance (Ω/0) (RT)

sheet resistance (Ω/0) (150 °C)

ITO/glass 26.7 ( 1.3 23.2 ( 1.3 ITO/HS-SAM/glass 20.4 ( 0.9 15.3 ( 0.8 25.1 ( 1.5 22.9 ( 1.2 ITO/H3C-SAM/glass 22.4 ( 1.2 21.9 ( 1.1 ITO/H2N-SAM/glass ITO (commercial) 15.1 ( 0.8 a The sheet resistance of a commercial ITO film (thickness 150 nm, deposition temperature 200 °C, and annealing temperature 400 °C) is also given for comparison.

Figure 2. XRD patterns of ITO films deposited on different substrates at 150 °C. ITO films were deposited simultaneously on various substrates using a deposition unit with RF magnetron sputtering from a ceramic target of 90 wt % In2O3 and 10 wt % SnO2 using a plasma power of 10 W. A shutter was placed immediately above the sample to ensure the deposition started only after the equilibrium was reached. XRD data were normalized by a standard.

XRD pattern of ITO film grown on a bare glass substrate at RT is significantly different, which shows weak peaks corresponding to (411) and (431) crystal planes. To ascertain the effect of SAM on growth, similar experiments were carried out at high temperature, high enough to see the temperature effect but much lower than the temperature (usually >200 °C) used for the growth of regular crystalline ITO films on glass substrates. Figure 2 shows the XRD profiles of ITO films grown at 150 °C. The discrimination of crystal growth became less prominent and all the ITO films exhibited similar XRD patterns. The plausible reason is that at 150 °C the temperature effect overrides other factors involving SAMs. Again, the most significant orientation is the (222) plane, while other planes such as (440) and (622) are less significant. However, a notable difference of crystallinity of ITO film grown on a substrate with -SH terminated SAM is still persisted. For instance, it shows a minor peak of the (400) plane, which is absent for all other films. This signifies that nature of monolayers on glass substrates plays a critical role on the growth of ITO films, possibly due to the variation of surface energy of substrates resulting from different functional groups present in SAMs. In general, ITO films deposited at low temperature are mostly amorphous or with very low crystallinity.12 However, a good quality crystalline ITO film grown at high temperature exhibits several peaks corresponding to (211), (222), (400), (440), and (622) crystalline planes. The intensity of different peaks, and hence the predominance of specific crystalline planes, may vary depending on existing conditions during the growth, which, in turn, vastly affect their electrical properties.33 A clear distinction of crystal growth on various substrates is also evident from their SEM images (Supporting Information). Another important factor that plays a significant role is defects in the SAM. To understand the role of defects, we deposited ITO films on -SH terminated SAM modified glass substrates which were deliberately irradiated with UV light at various time intervals to induce defect/damage on SAM. XRD profiles of ITO films (150 nm) grown on such substrates at RT (33) Jung, Y. S.; Lee, S. S. J. Cryst. Growth 2003, 259, 343.

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exhibit gradual decrease in (222) peak intensity with increasing UV irradiation time (Supporting Information). It can be concluded that with the increase in the population of defects in the SAM, crystallinity of the ITO film decreases significantly. As several properties of ITO films depend on the crystallinity, this result also suggests that electrical properties will be affected by defects in the SAMs. In general, electrical properties can be controlled by varying the doping level and oxygen vacancy.34,35 Higher oxygen vacancy and doping level would increase the electrical conductivity of ITO films. We employed a different experimental condition during ITO growth using argon plasma with no oxygen supply. As a result, the doping level of SnO2 was high with high oxygen vacancy necessary for good electrical conductivities.35 Understandably, the present ITO films show high carrier densities. For better understanding of the effect of SAMs on ITO films, electrical properties were measured using the four-point probe conductivity technique. Table 1 summarizes the electrical properties of ITO films grown on different substrates at RT and 150 °C. It is evident that electrical properties of ITO films also vary depending on the nature of substrates. At room temperature, the ITO film grown on a -SH terminated SAM modified glass substrate shows the lowest sheet resistance. A similar result of electrical properties of a ITO film grown at 150 °C is also observed. The value of sheet resistance of the ITO film grown at 150 °C on a -SH terminated SAM modified glass substrate is comparable to that of a commercial ITO film. Apparently, SAM plays a crucial role in determining the electrical properties. However, it should be noted that electrical properties of ITO films depend on a variety of factors including grain size and crystallographic orientations.3,6 For further understanding of electrical properties, Hall measurements were carried out for all the ITO films discussed above. Table 2 summarizes the results for different ITO films grown at RT and at 150 °C. Specifically, higher carrier concentrations were obtained for ITO films on different SAM modified glass substrates. It is expected that ITO films grown on various SAM modified glass substrates will have higher carrier densities due to the better chemical reactivity of functionalized substrates. However, the chemical reactivity of substrates will vary according to the nature of terminal groups. Indeed, the data in Table 2 confirm that ITO film grown on a -SH terminated SAM modified glass substrate has higher carrier concentration than the rest of the substrates. This indicates better chemical reactivity of dopants with -SH terminated SAMs. Table 2 also clearly shows that the ITO film grown on a -SH terminated SAM modified glass substrate at RT has relatively lower mobility than the commercial ITO. However, since the film (34) Lee, H.-C.; Park, O. O. Vacuum 2004, 77, 69. (35) Luo, S. N.; Kono, A.; Nouchi, N.; Shoji, F. J. Appl. Phys. 2006, 100, 113701.

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Table 2. Electrical Properties (from Hall Effect Measurement) of ITO Films (Thickness 150 nm, Deposited at RT and 150 °C) on Bare and SAM Modified Glass Substratesa carrier density, n (1021 cm-3) ITO on different substrates (deposited at RT and 150 °C)

RT

150 °C

Hall mobility, μ (cm2 V-1 s-1) RT

150 °C

resistivity, F (10-4 Ω cm) RT

150 °C

bare glass 0.53 ( 0.07 0.69 ( 0.05 15.58 ( 0.8 18.09 ( 0.7 4.22 ( 0.28 3.22 ( 1.3 HS-SAM/glass 1.64 ( 0.03 1.89 ( 0.03 17.91 ( 0.6 16.09 ( 0.5 2.77 ( 0.12 2.11 ( 1.3 1.55 ( 0.04 1.30 ( 0.05 13.39 ( 0.6 17.38 ( 0.4 3.54 ( 0.15 3.56 ( 1.3 H2N-SAM/glass 1.38 ( 0.02 1.26 ( 0.04 13.30 ( 0.5 18.01 ( 0.6 3.30 ( 0.17 3.01 ( 1.3 H3C-SAM/glass ITO commercial 0.94 ( 0.03 34.97 ( 0.4 1.22 ( 0.17 a The electrical data of a commercial ITO film (thickness 150 nm, deposition temperature 200 °C, and annealing temperature 400 °C) are also given for easy comparison.

Figure 3. Curves showing the variation of mobility vs carrier concentration for ITO films deposited at various temperatures on bare glass and -SH terminated SAM modified glass substrates.

has higher charged carriers, it has low resistance value compared to rest of the ITO films. Further, a distinct effect of monolayer properties on the ITO growth is also evident from the conductivities of different ITO films. It is evident from the Table 2 that ITO film grown on a -SH terminated SAM modified glass substrate shows better electrical conductivity (low resistance) than the ITO films grown on bare glass substrates and -NH2 and -CH3 terminated SAMs modified glass substrates. Figure 3 shows the plots of mobility vs carrier concentration for ITO films deposited at various temperatures on glass and -SH terminated SAM modified glass substrates. The curve depicting the behavior of ITO films deposited on bare glass substrates shows that mobility increases with increasing temperature up to 180 °C and then starts to decrease on further increase of temperature. Clearly, two borderline features are observed: (i) lowtemperature zone where grain boundary scattering is predominant and (ii) high-temperature zone where scattering from charged carriers is predominant.36,37 In sharp contrast, ITO films deposited on -SH terminated SAM modified glass substrates do not exhibit this behavior. Figure 3 clearly shows that mobility of such ITO films decreases with increasing temperature and carrier concentration. For the ITO film grown on a bare glass substrate, it is grain boundary which controls the mobility when deposited at low temperature. The low-temperature deposition results in amorphous ITO film, and therefore, mobility increases with the increase in carrier concentration and temperature (Figure 3). However, at sufficiently high temperature (above 180 °C), crystalline film will grow, and mobility starts to decrease as grain boundary effect plays a less significant role and scattering from charged carriers becomes dominant.37 It is clear from Figure 3 that the ITO film grown on a -SH terminated SAM modified glass substrate does not exhibit the above feature because of the (36) Lee, H.-C.; Park, O. Vacuum 2004, 75, 275. (37) Lee, H.-C. Appl. Surf. Sci. 2006, 252, 3428.

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growth of highly crystalline ITO films at all deposition temperatures.36,37 This further confirms our conclusion that high carrier densities due to better dopant concentration is responsible for the better electrical conductivity of ITO films on -SH terminated SAM modified glass substrates.38 Atomic ratio of different elements as calculated from XPS further manifests that atomic percentage of Sn is higher for ITO film grown on a -SH terminated SAM modified glass substrate than the ITO film grown on a bare glass substrate (Supporting Information). XPS has been widely used to study the nature of SAMs on various substrates. Recent studies have unambiguously established the SAM formation on ITO substrates by various molecules including alkanethiols.39-42 Especially, XPS has been widely used to understand the S-ITO interaction by analyzing the S 2p peak.39-41 Accordingly, XPS signals of the S 2p peak at varied thickness of ITO were recorded to understand the S-ITO interaction (Supporting Information). A single S 2p signal at 163.4 eV was obtained for the pristine SAM on a glass substrate, which could be attributed to the terminal thiol group. This peak was subsequently shifted to low energy at 162.0 eV at the initial stage of ITO deposition, presumably due to the formation of thiolate species. With increasing ITO thickness, this peak was split into two (164.6 and 162.1 eV) and finally became well-separated at 168.8 and 162.1 eV at the ITO thickness of 2.5 nm. A similar behavior has been reported for SAMs of alkanethiols on ITO substrate.39-41 The high-energy peak of S 2p in the range of 163165 eV is understood to be due to the formation of disulfide or interaction with metal oxide through the oxygen linkage, while the peak beyond 165 eV has been assigned to the formation of sulfonate species.40,43 XPS studies further helped us to understand the behavior of different SAMs after the deposition of ITO films. The XPS depth profile can distinctly determine the distribution of various elements and detect any diffusion of SAM species into the ITO matrix.44 Figure 4 is the XPS depth profile carried out for various elements on a ca. 7.5 nm ITO film grown on a HS-SAM modified glass substrate (deposited at RT), while inset shows the enlarged view of the profiles at interfacial region for Sn, Si, C, and S. As expected, profile shows that concentrations of In 3d and Sn 3d are constant up to the interface region. Significantly, depth profile at interface region clearly shows a higher concentration of Sn, (38) Kono, A.; Feng, Z.; Nouchi, N.; Shoji, F. Vacuum 2009, 83, 548. (39) Yan, C.; Zharnikov, M.; G€olzh€auser, A.; Grunze, M. Langmuir 2000, 16, 6208. (40) Brewer, S. H.; Brown, D. A.; Franzen, S. Langmuir 2002, 18, 6857. (41) Karsi, N.; Lang, L.; Chehimi, M.; Delamar, M.; Horowitz, G. Langmuir 2006, 22, 3118. (42) Pulsipher, A.; Westcott, N. P.; Luo, W.; Yousaf, M. N. Adv. Mater. 2009, 21, 3082. (43) Rieley, H.; Kendall, G. K.; Zemicael, F. W.; Smith, T. L.; Yang, S. Langmuir 1998, 14, 5147. (44) Song, W.; So, S. K.; Moulder, J.; Qiu, Y.; Zhu, Y.; Cao, L. Surf. Interface Anal. 2001, 32, 70.

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Figure 4. XPS depth profile carried out for various elements on a ca. 7.5 nm ITO film grown on a HS-SAM modified glass substrate (deposited at RT), while the inset shows the enlarged view of the profiles at interfacial region for Sn, Si, C, and S.

indicating better chemical interaction with the terminal group of SAM layer. On crossing the interface region at ca. 7.5 nm, the concentrations of In and Sn suddenly drop down to zero. Similarly, O 1s concentration increases abruptly at the interface region due to the large pool of oxygen in glass matrix, while Si 2p signals appear only after the interface region. Notably, S 2s and C 1s signals appear only at the interface region. No signals of S 2s and C 1s were detected before and after the interface region. This signifies that SAM is highly confined on the surface of glass substrate. No cleavage of SAM molecules occurred, and no surface diffusion of S took place during the deposition of ITO. A comparison of XPS depth profiles for ITO grown on SAM modified glass and bare glass substrate will further help to understand the interactions of different species at the interface regions. In Figure 5, the XPS depth profile for a ca. 7.5 nm ITO film grown on a bare glass substrate is given (deposited at RT), while the inset shows the enlarged view of the profiles at interfacial region for Sn, Si, and C. Most of the above features were also recorded, however, with few distinct exceptions. For example, Si 2p and O 1s signals exhibit identical behavior throughout the complete scan. Similarly, the In 3d signal remains constant up to the interface region and decreases down to zero on crossing the interface, while no significant C 1s signal was detected throughout the scan. A noticeable difference is observed for Sn 3d signals. The abrupt increase of Sn 3d signals observed for the ITO film on a HS-SAM modified glass substrate is absent for the ITO on a bare glass substrate. This is due to the absence of any chemical interactions of Sn species at the interface. This is the origin of the variation in different properties of ITO films grown on different substrates including the discriminate crystal growth. Thus, XPS depth profiles provide conclusive evidence and illustrate the origin of the discrimination of different properties of ITO films grown on SAM modified and bare glass substrates. Apart from the electrical measurements, all the ITO films discussed above were tested for optical transmission properties. They exhibited good optical characteristics as well with >85% optical

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Figure 5. XPS depth profile carried out for various elements on a ca. 7.5 nm ITO film grown on a bare glass substrate (deposited at RT), while the inset shows the enlarged view of the profiles at interfacial region for Sn, Si, and C.

transmission (Supporting Information). This value complies with the requirements for optoelectronic applications. However, optical transmission is marginally lower than the value expected (>90%) from the highly crystalline ITO films. This is reasonable since the large scattering effect experienced by such ITO films (due to the high doping level) lowers the optical transmittance.37 In essence, ITO films grown on different SAM modified glass substrates have good electrical and optical properties suitable for their various applications.

Conclusions In conclusion, a clear effect of SAMs on the crystal growth trend is demonstrated for ITO thin films. Apart from several other applications of SAM, the present study has demonstrated that crystallinity of ITO films can be controlled using selective SAMs under mild conditions. Since many of the physical properties, such as electrical properties, depend on the nature of ITO thin films, the current approach offers an alternative route to control such properties. This study will find applications in many other thin films, where delicate alteration of crystalline structure is necessary for the desired physical properties while avoiding adverse treatments. Acknowledgment. The authors thank Prof. Lu-Sheng Hong for the use and support of sputtering system and Prof. Thomas C.-K. Yang for generously providing us with XRD facilities. This work was supported by Academia Sinica (nano-2394) and National Science Council (98-2113-M-011-002-MY2). Supporting Information Available: Data of water contact angle measurements, XRD data of ITO films deposited on UV treated HS-SAM functionalized glass substrates, additional X-ray photoelectron spectra (XPS), scanning electron microscopy images, optical transmittance spectra, atomic % ratio of various elements of ITO films deposited on different substrates as determined by XPS. This material is available free of charge via the Internet at http://pubs.acs.org.

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