| AVS 65th International Symposium & Exhibition | |
| Thin Films Division | Tuesday Sessions |
| Session TF-TuM |
| Session: | Emerging Applications for ALD |
| Presenter: | Jason Avila, U.S. Naval Research Laboratory |
| Authors: | J. Avila, U.S. Naval Research Laboratory D.R. Boris, U.S. Naval Research Laboratory S.B. Qadri, U.S. Naval Research Laboratory J.A. Freitas, U.S. Naval Research Laboratory S.G. Walton, U.S. Naval Research Laboratory C.R. Eddy, Jr., U. S. Naval Research Laboratory V.D. Wheeler, U.S. Naval Research Laboratory |
| Correspondent: | Click to Email |
Atomic layer deposition (ALD) of TiO2 has been widely explored recently due to its promise in non-volatile resistive switch, high-k gate dielectric, solar cell, and photocatalytic applications. This growth method has become increasingly useful as device dimensions are reduced and non-planar complexity is increased. To facilitate epitaxial films at low growth temperatures (Tg), many have investigated plasma, laser or photon, or electron enhanced ALD processes. Specifically for TiO2, it would be beneficial to selectively grow epitaxial anatase or rutile phases in order to tailor optical, catalytic and electrical properties for the required application. Typically, TiO2 phase selectively is attained by varying the underlying substrate, Ti and/or oxidation precursor, or Tg. However, here we demonstrate phase selectivity of high quality epitaxial TiO2 films simply by adjusting plasma gas composition, pressure and Tg.
A Veeco Fiji G2 reactor was used to deposit TiO2 films on different sapphire orientations (c-, m-, a-) with tetrakis(dimethylamido)titanium (TDMAT) and either Ar/O2 or pure O2 plasma at 100-350° C. Previous reports indicate that tuning the ion energy, specifically through substrate biasing, can influence TiO2 film crystallinity and phase [1]. The high pumping speed and large gas flow range available in this specific ALD system provides a wide variation in operating pressures (7-100’s mTorr), which effectively allows tuning of plasma characteristics. Operating at relatively low pressures (9-21mTorr) resulted in a significant flux (0.5-1.5x1019 m-2s-1) of energetic ions (30-50eV), with both the flux and energy decreasing as the pressure is increased. The low pressure conditions yield high-quality epitaxial films at all temperatures, which differs from previous reports using these specific precursors [2,3] likely due to these unique plasma conditions.
Gas composition during the plasma step also had a substantial effect on growth rate, TiO2 phase, and strain. At Tg < 300° C, the growth rate was increased from 0.5 to 0.7 Å /cycle by switching from Ar/O2 to pure O2. Moreover, an O2 plasma produced only rutile TiO2 films, with less strain, independent of growth temperature or underlying substrate orientation. In contrast, films deposited with an Ar/O2 plasma show a phase dependence on temperature and substrate. Films on c-plane Al2O3 go from anatase at Tg below 200° C to rutile above 300° C. The films on m-plane Al2O3 are rutile independent of temperature.
Profijt et al. Electrochem. Sol. Stat. Lett. 15(2) G1 (2012).
Xie et al. J. Electrochem. Soc. 155(9) H688 (2008).
Maeng and Kim. Electrochem. Sol. Stat. Lett. 9(6) G191 (2006).