Frequency dependent negative capacitance of (Ba0.6Sr0.4)(ZrxTi1-x)O3 thin films grown on La0.9Sr1.1NiO4 buffered SrTiO3 substrate

Ba0.6Sr0.4TiO3 and (Ba0.6Sr0.4)(Zr0.3Ti0.7)O3 thin films were deposited on La0.9Sr1.1NiO4 buffered SrTiO3 substrates. (Ba0.6Sr0.4)(Zr0.3Ti0.7)O3 film (2.77 nF) showed one order large capacitance compared to that of Ba0.6Sr0.4TiO3 film (270 pF) at 100 kHz. (Ba0.6Sr0.4)(Zr0.3Ti0.7)O3 film showed negative capacitance at f >3 MHz except for f=5.05 to 7.36 MHz, and 10.4 to 13.4 MHz, where it showed positive capacitance. Tunability of the Ba0.6Sr0.4TiO3 film (~15%) is much lower than that of the (Ba0.6Sr0.4)(Zr0.3Ti0.7)O3 film (30 to 65%, both normal and inverse). A significant change of the tunability was observed at frequencies f>500 kHz for the (Ba0.6Sr0.4)(Zr0.3Ti0.7)O3 film showing inverse tunability, this can be attributed to the negative capacitance effect, where current lags behind the voltage. (Ba0.6Sr0.4)(Zr0.3Ti0.7)O3 film (6.87x10-6 A/cm2) showing one order high leakage current density than BST film (1.32x10-7 A/cm2). Ba0.6Sr0.4TiO3 film showed large grain size (140 nm) and surface roughness (11.5 nm) and (Ba0.6Sr0.4)(Zr0.3Ti0.7)O3 film showed small grain size (80 nm) and roughness (2.3 nm).