Photo-assisted crystallization of zirconia thin films prepared using chelate compounds
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A highly crystallized and smooth-surfaced zirconia (ZrO2) thin film was prepared using a precursor solution with 2,4-pentanedione addition (molar ratio of 1:1 for zirconium alkoxide); the film was irradiated with ultraviolet (UV) light using an ultrahigh-pressure mercury lamp. This thin film was compared with another thin film, which prepared using a precursor solution without additives and UV irradiation. The crystallinity of ZrO2 thin films improved with increasing 2,4-pentanedione addition and UV irradiation time and changed according to the type of organic additives. These results occurred presumably because the orientation of polymerization in zirconium alkoxide units and stability of chelate compounds for UV irradiation were controlled under these conditions. In addition, the method was effective for lower-temperature crystallization of highly crystallized, smooth-surfaced thin films.
I. INTRODUCTION
Zirconia (ZrO2) is an attractive material with its high-dielectric constant (15–21),1,2 large band gap (5.3– 6.4 eV),3–5 and good thermal stability against silicate formation on silicon.6–8 Thus, ZrO2 thin films are well suited for many applications, e.g., as buffer layers for high-Tc superconductors or integration of ferroelectric thin films in nonvolatile ferroelectric memories,9,10 as the gate dielectric for complementary metal-oxide semiconductors,1,2 or as oxygen sensors.11 For the various applications of ZrO2 thin films, it is important to control crystallinity, orientation, and surface morphology. Controlling the physical and structural properties of metaloxide thin films has received extensive coverage in the literature.12–19 Photochemical reactions differ from thermochemical reactions in that they occur at lower temperatures and show various specific reactivities via excited states. Thus, photochemical reactions are expected to be extremely effective in controlling the crystal structure, microstructure, and surface morphology of inorganic thin films. This has led to many studies on the photo-assisted processing of metal-oxide thin films.20–26 In our previous study, we found that the crystallinity, orientation, and surface roughness of alkoxy-derived SrBi2Ta2O9 thin films were controlled by ultraviolet light
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0335 2608 J. Mater. Res., Vol. 22, No. 9, Sep 2007 http://journals.cambridge.org Downloaded: 18 Mar 2015
(UV) irradiation.27,28 In addition, we also reported that wavelength dependence was observed in the photoassisted crystallization of alkoxy-derived ZrO2 thin films and that UV irradiation conditions for alkoxy-derived ZrO2 thin films affected the crystallinity and electrical properties of annealed ZrO2 thin films.29,30 As the next approach, we focused on the molecular structure of the precursor for control of crystallinity of ZrO2 thin films. Various methods for the preparation of ZrO2 thin films have been reported.31–36 Among these methods, the solgel method has attracted considerable attention
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