We studied the influence of vanadium content and different annealing conditions on crystalline orientation and ferroelectric properties of bismuth titanate (BTO) thin film.
In this study, BTO and Bi$_{4-x}/3$Ti$_{3-x}$V$_x$O$_{12}$ (BTV) (x=0.01, 0.03, 0.05, 0.07 and 0.09) thin films were deposited on Pt/TiO$_2$/SiO$_2$/Si substrate by Liquid Source Misted Chemical Deposition (LSMCD). The LSMCD uses ultrafine mist (diameter : 1～5 μm) of complex oxide precursor in liquid mixture. The size of mist is much smaller than that of droplets (20～100 μm) used in sol-gel method. So, the mist can provide better uniformity, higher film-density and better interface adhesion between films and substrates.
To fabricate BTO and BTV thin films, bismuth nitrate, titanium isopropoxide and vanadium oxytriisopropoxide were used as metal precursors. Each precursor was dissolved in 2-methoxyethanol and mixed together. The mixed solution was converted to mist, which was deposited on substrate in LSMCD system. Finally, the deposited films were annealed at 650~750 ℃ for 1~2 hr under oxygen atmosphere.
The ferroelectric properties, crystalline orientation and surface morphology of prepared films were characterized by RT66A ferroelectric tester, XRD and SEM, respectively. In addition, XPS was used for the investigation of vanadium oxidation state in BTV film and it was founded that only V$^{5+}$ existed in the film.
XRD result showed that crystalline orientation of BTV films were heavily influenced by vanadium content. In BTO film, the degree of c-axis orientation was suppressed gradually with increasing substituted vanadium content, x, up to 0.03. However, additional vanadium content promoted c-axis orientation when x was greater than 0.03. SEM images were also consistent with XRD result. The different crystalline orientations had an effect on remanent polarization (Pr) and in particular, c-axis orientation deteriorated the value of Pr. According to the results, the BTV film at x=0.03 had the largest 2Pr, 21.5 μC/cm$^2$ at applied voltage=12V. This value was larger than that (16～20 μC/cm$^2$) of BTV films fabricated by sol-gel and chemical solution deposition methods.
Annealing temperature and time also influenced on crystalline orientation. For the BTV (x=0.03) film, higher annealing temperature (750 ℃) or longer annealing time (2 hr) promoted preferred c-axis orientation and produced small values of $P_r$ compared with the BTV (x=0.03) film annealed at 700 ℃ for 1hr. In addition, higher annealing temperature and longer annealing time generated the formation of impurity phases related to bismuth volatility.
