Role of ozone in reactive coevaporation of lead zirconate titanate thin films
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Role of ozone in reactive coevaporation of lead zirconate titanate thin films Kazuyoshi Torii, Fumiko Yano, and Yoshihisa Fujisaki Central Research Laboratory, Hitachi Ltd., 1-280 Higashi koigakubo, Kokubunji, Tokyo 185, Japan (Received 3 April 1997; accepted 29 May 1997)
The role of ozone in the reactive coevaporation of the lead zirconate titanate thin film was investigated by depositing films at various growth rates with various ozone fluxes or molecular oxygen fluxes on unheated substrates and then crystallizing them using rapid thermal annealing. The oxidation state of lead in the as-deposited film was determined from the ratio of the ozone to the total metal fluxes. The amount of atomic oxygen supplied to the surface of the film was at least 103 times larger when the deposition was done using ozone rather than molecular oxygen. When the ozone flux was more than one-third of the total metal flux, well-oxidized films were obtained. To ensure obtaining well-oxidized film, the ozone flax should be more than twice as much.
Oxygen vacancies and their related defects in thin films of oxide perovskite compounds are reported to cause such degradations as fatigue,1 imprint,2 and resistibility degradation.3 This means that a highly activated oxidizing agent is needed to prepare high-quality films and to suppress film degradation. We recently demonstrated that lead zirconate titanate (PZT) thin film having the highest charge storage density (280 fCymm2 at 1.5 V for 75 nm film) yet reported can be fabricated by reactive coevaporation using high-concentration ozone.4 This PZT film shows no switching polarization loss after 1011 polarization-switching cycles. However, the effects of ozone in the reactive coevaporation of PZT thin films have not yet been reported. Although a high ozone (or oxygen) pressure helps to achieve sufficient film oxidation, the pressure during film growth should be sufficiently low that the source metal in the crucible is not oxidized. Otherwise, the evaporation rate may decrease during film growth, changing the film composition. We have investigated flux ratio of ozone to cations required to obtain well-oxidized PZT film without source-metal oxidation. We explored the role of ozone in PZT thin film deposition by reactive coevaporation and clarified the oxidizing activity of ozone compared with that of molecular oxygen. We also determined the film orientation dependence on the lead oxidation state.
were evaporated from electron beam guns. An ozoneoxygen mixture gas was slowly fed from an ozonizer into a cryostat cooled to 90 K, where the ozone liquefied. The liquefied ozone was stored in the cryostat. The flow of ozone to the substrate was regulated by controlling the temperature of the liquid ozone.6 We also prepared films deposited using molecular oxygen for comparison. The oxygen was liquefied at 47 K from oxygen gas; the oxygen flux was controlled by changing the temperature of the liquid oxygen in the range from 42 K to 50 K. The PZT films were de
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