Modeling evaporation, ion-beam assist, and magnetron sputtering of TiO 2 thin films over realistic timescales
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John M. Walls Department of Electronic and Electrical Engineering, Loughborough University, Loughborough, Leicestershire, LE11 3TU, United Kingdom (Received 15 July 2011; accepted 30 September 2011)
Results are presented for modeling the growth of TiO2 on the rutile (110) surface. We illustrate how long time scale dynamics techniques can be used to model thin film growth at realistic growth rates. The system evolution between deposition events is achieved through an on-the-fly Kinetic Monte Carlo method, which finds diffusion pathways and barriers without prior knowledge of transitions. We examine effects of various experimental parameters, such as substrate bias, plasma density, and stoichiometry of the deposited species. Growth of TiO2 via three deposition methods has been investigated: evaporation (thermal and electron beam), ion-beam assist, and reactive magnetron sputtering. We conclude that the evaporation process produces a void filled, incomplete structure even with the low-energy ion-beam assist, but that the sputtering process produces crystalline growth. The energy of the deposition method plays an important role in the film quality. I. INTRODUCTION
Titania (TiO2) in its rutile form is an important material in industry, with industrial scale uses from pigmentation in paint to sunscreen. This work, however, is focused on its use within photovoltaic devices. High quality, dense films of TiO2 are used extensively in multilayer optical coatings because of their high refractive index and ability to absorb ultraviolet radiation.1 Porous TiO2 is used in dyesensitized solar cells.2 The porous TiO2 is immersed in a ruthenium-polypyridine dye,3 where a thin film of the dye bonds to the TiO2, which acts as an anode. Metal oxide thin films can be deposited using a variety of industrial scale processes, including evaporation (thermal and electron beam), ion-beam-assisted evaporation,4–6 and reactive magnetron sputtering.5,7–9 In all three processes, we assume that TiO2 reaches the surface at the correct stoichiometry, via the deposition of the species, Ti, TiO, TiO2, O, and O2. Typical industrial deposition rates lie between ½ and 2 monolayers per second. For simplicity, we chose ½ monolayer per second. The evaporation process involves stoichiometric deposition of TiOx ad-clusters onto the substrate, with kinetic energy typically ,1 eV. An ion source (usually argon) may be used to densify the film; this ion-beam introduces energy into the growing film5,6 to enhance mixing. For the model of ion assist, the same arrival rate of argon was used as for a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.380 J. Mater. Res., Vol. 27, No. 5, Mar 14, 2012
the TiO2 species (equivalent to ;1.6 1015 atoms/cm2/s), but we expect the majority of argon to leave the surface. Magnetron sputtering is also used to deposit thin films of TiO2 using radio frequency (RF), direct current (DC), or pulsed DC power. Targets can be TiO2 (RF), Ti (DC), or TiOx (pulsed DC). Sputtering in the presence of a r
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