Visable-Deep UV Dielectric Functions and Electronic Band Structure of Lead Zirconate Titanate Thin Films

We measure pseudodielectric functions in the visible-deep ultraviolet spectral range of Pb(Zr$_{x}$Ti$_{1-x})$O$_{3}$ (x=0.2, 0.56, 0.82) (PZT), Pb$_{0.98}$Nb$_{0.04}$(Zr$_{0.2}$Ti$_{0.8})_{0.96}$O$_{3}$,$_{ }$Pb$_{0.91}$La$_{0.09}$(Zr$_{0.65}$Ti$_{0.35})_{0.98}$O$_{3}$, and$_{ }$Pb$_{0.85}$La$_{0.15}$Ti$_{0.96}$O$_{3}$ films. Using a parametric optical constant model, we estimate the dielectric functions ($\varepsilon )$ of the perovskite oxide thin films. Taking the second derivative of the fitted layer dielectric functions and using the standard critical point model, we determine the parameters of the critical points. In the second derivative spectra, the lowest band gap energy peak near 4 eV is fitted as a double peak for annealed PZTs due to perovskite phase. As-grown PZTs have mainly pyrochlore phase and the lowest band gap peak is fitted as a single peak. We also examine the effect of dopants La and Nb, which substitute at Pb and Zr (Ti) sites, respectively. We found three band gaps E$_{a}$ ($\sim $3.9eV), E$_{b}$ ($\sim $4.5 eV)and E$_{c}$ ($\sim $6.5$_{ }$eV) in the order$_{ }$of increasing energy. The E$_{a}$ and E$_{b}$ band gap energies were not sensitive to Zr composition. We discuss the change of critical point (CP) parameters for PZTs in comparison to the band structure calculations based on local-density-approximation. The near-constancy of the lowest band gap energy independent of Zr composition is consistent with the band calculations.