Transparent, ferroelectric Ca:HfO2 and Sr:HfO2 films grown from chemical solution on ITO-coated glass

The discovery of ferroelectricity in HfO2 thin films has revived the interest on ferroelectric-based memories. Yet, other applications like transparent micro sensors and actuators are also potentially possible. By using a simple chemical solution method, we achieved the growth of transparent Ca:HfO2 and Sr:HfO2 films on ITO coated glass and reached polarization values of 12 and 6 µC/cm2, respectively, after deposition of top Pt electrodes. The selection of Ca or Sr as doping element brought about large variations in thermal conditions at which ferroelectricity in doped-HfO2 films was achieved. Sr:HfO2 required significantly shorter annealing times than Ca:HfO2 and caused deformation of the glass substrate despite otherwise equal processing conditions. Moreover, although, both Ca and Sr induced the crystallization of hafnia into high symmetry phases, the latter also enhanced the appearance of the monoclinic structure. Interestingly, high resolution transmission microscopy revealed a semi-epitaxial “cube on cube” growth of doped-HfO2 on individual ITO crystals. This predicts that accomplishing fully epitaxial HfO2 films by a chemical solution method should be possible. Technological innovation might come from achieving fully transparent ferroelectric/piezoelectric HfO2-based devices.