Improved SrTiO3 multilayers for microwave application: Growth and properties

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Abstract
Au/SrTiO 3 (STO), Au/YBa 2 Cu 3 O 7−x (YBCO)/STO, Au/STO/CeO 2 /STO , and Au/STO/MgO/STO multilayer structures on LaAlO 3 substrates have been fabricated. Their properties are investigated at 1 MHz and at microwave frequencies (40 MHz–40 GHz) as a function of applied voltage, and in the temperature range between 20 and 300 K. The STO thin films showed comparatively high values of dielectric constant (up to 1800 at 50 K) with low measuredfilm losses, tan δ<0.005, (limit of the measurement setup), at 20 GHz and 20 K, indicating their applicability for microwavedevices. The dielectric constant is shown to be independent of frequency to at least 40 GHz, while the losses exhibit no or weak frequency dependence. Relaxation of the STO film in oxygen atmosphere before deposition of YBCO is shown to reduce interactions between the YBCO and STO films. After an “ex situ” high-temperature (900 ° C ) treatment in flowing oxygen a reduction of the STO dielectric constant as well as tan δ has been observed. Using cerium dioxide as a sublayer in a multilayer STO structure gives rise to an additional (110) STO peak. Substituting the CeO 2 layer with a MgO layer drastically reduces this peak, but the lattice distortion increases, with increased dielectric constant and higher controllability as a result. Multilayers with eight intermediate oxygen relaxations during the STO film deposition showed the lowest losses and highest dielectric constant with 25% tunability of the effective dielectric constant at 20 K and with an applied electric field of 19 kV/cm.