Phase transitions and optical characterization of lead-free piezoelectric (K0.5Na0.5)0.96Li0.04(Nb0.8Ta0.2)O3 thin films

Lead-free piezoelectric thin films, (K0.5Na0.5)0.96Li0.04(Nb0.8Ta0.2)O3, were epitaxially grown on MgO(001) and Nb-doped SrTiO3(001) substrates using pulsed laser deposition. The optimum deposition temperature was found to be 600 °C. Two types of in-plane orientations were observed in the films depending on the substrates used. The transmittance and photoluminescence spectra as well as the dielectric and ferroelectric properties of the films were measured. The measured band-gap energy was found to be decreased with the deposition temperature. The dielectric constant decreased from 550 to 300 as the frequency increased from 100 Hz to 1 MHz. The measured remnant polarization and coercive field were 4 μC/cm2 and 68 kV/cm, respectively. The phase transitions of the films were studied by Raman spectroscopy. Two distinct anomalies originating from the cubic-to-tetragonal (TC-T ~ 300 °C) and tetragonal-to-orthorhombic (TT-O ~ 120 °C) phase transitions were observed. Our results show that Raman spectroscopy is a powerful tool in identifying the phase transitions in ferroelectric thin films.