Effect of heat on the nonlinearity of plane strontium titanate film capacitors on sapphire in a microwave field

Abstract

Nonlinear characteristics of plane strontium titanate film capacitors operating at microwave frequencies are investigated by measuring the power of a capacitor signal generated at the third-order intermodulation distortion (IMD) product frequency when the capacitor is excited by a two-tone microwave signal. Measurements are performed at 4 GHz at temperatures of 78 and 300 K. At T=300 K, the nonlinear response of the capacitor corresponds to the nonlinearity determined from the low-signal capacitance-voltage characteristic (CVC). At T=78 K, the nonlinear response to a two-tone microwave signal is greatly amplified when the signal components have equal amplitudes and close frequencies. It is demonstrated that this effect is due to the beat-frequency modulation of the strontium titanate film temperature, because the thermal time constant of a SrTiO3 film on sapphire is small (∼10−8 s). An analytical expression for third-order IMD product power generated by a SrTiO3 capacitor is obtained with regard for the heat-induced nonlinearity.