Abstract
Simple thin-film capacitor stacks were fabricated from sputter-deposited doped barium titanate dielectric films with sputtered Pt and/or Ni electrodes and characterized electrically. Here, we report small signal, low frequency capacitance and parallel resistance data measured as a function of applied DC bias, polarization versus applied electric field strength and DC load/unload experiments. These capacitors exhibited significant leakage (in the range 8–210 μA/cm2) and dielectric loss. Measured breakdown strength for the sputtered doped barium titanate films was in the range 200 kV/cm −2 MV/cm. For all devices tested, we observed clear evidence for dielectric saturation at applied electric field strengths above 100 kV/cm: saturated polarization was in the range 8–15 μC/cm2. When cycled under DC conditions, the maximum energy density measured for any of the capacitors tested here was ~4.7 × 10−2 W-h/liter based on the volume of the dielectric material only. This corresponds to a specific energy of ~8 × 10−3 W-h/kg, again calculated on a dielectric-only basis. These results are compared to those reported by other authors and a simple theoretical treatment provided that quantifies the maximum energy that can be stored in these and similar devices as a function of dielectric strength and saturation polarization. Finally, a predictive model is developed to provide guidance on how to tailor the relative permittivities of high-k dielectrics in order to optimize their energy storage capacities.
Highlights
Barium titanate and its relatives with the perovskite structure have been called the “most significant electroceramic dielectric phase in industry” [1]
The data clearly indicate the onset of dielectric saturation at higher electric fields, with a concomitant increase in loss
Applied DC bias for a test capacitor fabricated with a Pt bottom electrode, a Ni top electrode and a modified barium titanate dielectric deposited at 900 °C, 10 mtorr pressure and a sputter gas mixture of 10% oxygen/ 90% argon
Summary
Barium titanate and its relatives with the perovskite structure have been called the “most significant electroceramic dielectric phase in industry” [1]. These materials are widely used as capacitor dielectrics on account of their high relative permittivity; they are used as piezoelectrics, pyroelectrics and in electro-optics. In 2000, a United States (US) patent was issued to Peter Hansen that claimed capacitors with dielectrics based on doped barium titanate with reported maximum relative permittivities up to 33,500 [2]. The ability to store such large amounts of energy in what is essentially a very large multi-layer ceramic capacitor (MLCC) depends critically on the ability of the dielectric to maintain a very high relative permittivity at high electric fields.
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