Full Paper Atomic Layer Deposition of Titanium Oxide from TiI 4 and H 2 O 2 ** By Kaupo Kukli,* Mikko Ritala, Mikael Schuisky , Markku Leskelä, Timo Sajavaara, Juhani Keinonen, Teet Uustare, and Anders Hårsta TiO 2 thin films have been grown on amorphous soda lime glass and polycrystalline silicon substrates from TiI 4 and H 2 O 2 by atomic layer deposition (ALD) in the temperature range 250±490 C. The film growth rate and refractive index increased lin- early with growth temperature up to 300 C. Between 300 C and 400 C, the film growth rate tended to stabilize. Above 400 C, there was a further rapid increase in the growth rate, together with a corresponding increase in the thickness profile. The refractive index reached 2.70±2.75 in the films grown above 300 C. The films contained low amounts of residual hydro- gen and were virtually iodine-free. When deposited below 300 C and above 325 C, the films contained weakly crystallized, but still distinct, anatase and rutile phases, respectively. Reflective high-energy electron diffraction (RHEED) studies re- vealed that the uppermost layers of the films grown on silicon at 275 C, 325 C, and 425 C contained anatase phase, regard- less of deposition temperature. In the temperature range 300±325 C, a transition region was established where either the films became less ordered, or they contained rather strongly ordered, but unstable, suboxide phases. The suboxide phase, when present, was transformed into a mixture of anatase and rutile when stored in air at room temperature. Keywords: TiO 2 , TiI 4 , Atomic layer deposition 1. Introduction Various properties of the extensively studied metal ox- ide TiO 2 have been of interest, including deposition pro- cess control, [1] crystal structure, [2] phase stability, [3±5] opti- cal quality, [6,7] (electro)luminescence, [8] photosensitivity, [9] microhardness, [6] epitaxial growth, [10] permittivity, [11,12] and leakage current. [5,12,13] With reference to factors affecting the physical properties, an interesting aspect in TiO 2 growth has been the stability of various crystallographic phases such as anatase and rutile. These phases possess dif- ferent electro-optical properties. For instance, although uniform anatase films can be grown with a quite high per- mittivity value (70), [4] the anatase is still characterized by a lower dielectric constant, [12] higher leakage current, [5] and lower breakdown field strength, [12] than rutile. At the same time, pure anatase is optically less absorbent than rutile. [7] As a metastable polymorph of TiO 2 , anatase is often first to form at relatively low processing temperatures, espe- cially in fine-grained form. [2,3,14] Possibly the lowest reported in-situ processing temperature of pure rutile has been 300 C in laser ablation, [15] or rutile mixed with ana- tase in films evaporated at 300 C and above. [5,16] The stabilization of rutile in CVD has usually been possible at temperatures above 500 C, or after post-deposition heat treatment. [1,4,12,14,17] The composition of the substrate may also have an influenceÐit has been noted that, at rather low temperatures (around 300 C), high Al 2 O 3 and Na 2 O contents in the substrate material favor the formation of rutile and anatase phases, respectively. [7] Recently, rutile TiO 2 has been produced by low-pressure CVD from titanium-tert-butoxide on SnO 2 (of rutile structure) substrates at temperatures as low as 290 C and on glass at 322 C. [4] Anatase and rutile thin films have been obtained by physical methods such as reactive sputtering [6,13] and elec- tron beam evaporation, [5] or by various chemical methods such as ambient air hydrolysis of metal precursors, [9] hydro- lysis in aqueous medium, [2,8] chemical beam epitaxy (CBE), [10] and CVD. [1,4,11,12,17,18] Various titanium precur- Chem. Vap. Deposition 2000, 6, No. 6 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim,2000 0948-1907/00/0611-0303 $ 17.50+.50/0 303 ± [*] Dr. K. Kukli, Dr. M. Ritala, Prof. M. Leskelä Department of Chemistry, University of Helsinki PO Box 55, FIN-00014 Helsinki (Finland) Dr. K. Kukli Institute of Experimental Physics and Technology University of Tartu Tähe 4, EE-51010 Tartu (Estonia) E-mail: kaupok@ut.ee Dr. M. Schuisky, Dr. A. Hårsta The ngstro È m Laboratory, Inorganic Chemistry Uppsala University PO Box 538, SE-75121 Uppsala (Sweden) Dr. T. Sajavaara, Prof. J. Keinonen Accelerator Laboratory, University of Helsinki PO Box 43, FIN-00014 Helsinki (Finland) Dr. T. Uustare Institute of Materials Science, University of Tartu Tähe 4, EE-51010 Tartu (Estonia) [**] The authors are indebted to Mr. Antti Niskanen for performing the grazing incidence diffraction studies. The authors appreciate the access to the EDX measurement facilities at the Electron Microscope Unit at the University of Helsinki. This work has been partially supported by the Academy of Finland (Projects Nos. 16385 and 43329), Finnish National Technology Agency TEKES (Project No. 40365/98), the Swedish Institute ,and Estonian Science Foundation (Grant No.1878).