We have grown epitaxial strontium titanate films on lanthanum aluminate substrates at a range of oxygen pressures and substrate temperatures. The complex dielectric function was measured versus frequency (10 kHz to 1 MHz) and temperature (room temperature to 4.2 K) on coplanar capacitors patterned on the films. Preliminary data from 1.5 to 2.5 GHz is also presented. The dielectric constant epsilon _r was as high as 4600 at 65 K, a factor of two greater than previously reported for strontium titanate thin films and a factor of 1.8 greater than the bulk value at the same temperature. Tuning the capacitor by applying a dc bias of +-15 V across the 3 um m coplanar gap at 4.2 K yielded a ratio of maximum to minimum dielectric constant of 2.8 At 101 K this ratio was 2.2 A bulk capacitor does not show tuning over such a large temperature range.
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