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2009, 113, 12962–12965 Published on Web 07/01/2009
Surface Reaction Mechanisms during Plasma-Assisted Atomic Layer Deposition of Titanium Dioxide Vikrant R. Rai and Sumit Agarwal* Department of Chemical Engineering, Colorado School of Mines, Golden, Colorado 80401 ReceiVed: April 21, 2009; ReVised Manuscript ReceiVed: June 8, 2009
We have investigated the surface reaction mechanisms during the O2-plasma-assisted atomic layer deposition (ALD) of TiO2 from titanium tetraisopropoxide using in situ attenuated total reflection Fourier transform infrared spectroscopy. We show that the reaction mechanism involves a combination of O3- and H2O-based ALD chemistries, where both metal carbonates and surface hydroxyl groups are the reactive site for chemisorption of the metal precursor. The TiO2 films have anatase crystal structure at 150 °C with a growth per cycle of ∼0.83 Å, which is much higher than that for amorphous films. Titanium dioxide is a technologically relevant material due to its high dielectric constant and refractive index, good transparency in the visible and infrared, and high photocorrosion resistance.1 Ultrathin conformal TiO2 thin films can be deposited using thermal and plasma-assisted atomic layer deposition (ALD).2 Various metal organic3,4 and metal halide5 precursors have been used in conjunction with H2O as an oxidizing agent for the ALD of TiO2.5 However, due to the insufficient reactivity of H2O at temperatures 150 °C.3,39 Niskanen et al.37 have reported anatse during the O-radical-enhanced ALD of TiO2 from TTIP at 250 °C. The mechanism for crystalline TiO2 deposition at such low temperatures is unclear. We speculate that it may be due to the high flux of ions and radicals in the O2-plasma bombarding the substrate, which may lead to local heating. Acknowledgment. We gratefully acknowledge support from the American Chemical Society Petroleum Research Fund (Grant No. 44934-G5) and the Renewble Energy MRSEC program at the Colorado School of Mines (NSF Grant No. DMR-0820518). References and Notes (1) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (2) George, S. M.; Ott, A. W.; Klaus, J. W. J. Phys. Chem. 1996, 100, 13121. (3) Aarik, J.; Aidla, A.; Uustare, T.; Ritala, M.; Leskela, M. Appl. Surf. Sci. 2000, 161, 385. (4) Ritala, M.; Leskela, M.; Niinisto, L.; Haussalo, P. Chem. Mater. 1993, 5, 1174. (5) Ritala, M.; Leskela, M.; Nykanen, E.; Soininen, P.; Niinisto, L. Thin Solid Films 1993, 225, 288. (6) Matero, R.; Rahtu, A.; Ritala, M.; Leskela, M.; Sajavaara, T. Thin Solid Films 2000, 368, 1. (7) Lim, J. W.; Yun, S. J.; Lee, J. H. Electrochem. Solid State Lett. 2004, 7, F73. (8) Kim, S. K.; Kim, W. D.; Kim, K. M.; Hwang, C. S.; Jeong, J. Appl. Phys. Lett. 2004, 85, 4112. (9) Rai, V. R.; Agarwal, S. J. Phys. Chem. C 2008, 112, 9552. (10) Goldstein, D. N.; McCormick, J. A.; George, S. M. J. Phys. Chem. C 2008, 112, 19530. (11) Matero, R.; Rahtu, A.; Ritala, M. Chem. Mater. 2001, 13, 4506. (12) Rahtu, A.; Ritala, M. Chem. Vapor Depos. 2002, 8, 21. (13) Heil, S. B. S.; Kudlacek, P.; Langereis, E.; Engeln, R.; van de Sanden, M. C. M.; Kessels, W. M. M. Appl. Phys. Lett. 2006, 89, 3.
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