Off Axis Growth of Strontium Titanate Films with High Dielectric Constant Tuning and Low Loss
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Off Axis Growth of Strontium Titanate Films with High Dielectric Constant Tuning and Low Loss Satreerat Kampangkeaw and Charles T. Rogers Department of Physics, University of Colorado at Boulder, Boulder, Colorado 80309, U.S.A. ABSTRACT We have measured the nonlinear dielectric properties of strontium titanate (STO) thin films grown on neodymium gallate (NGO) and lanthanum aluminate (LAO) substrates. The films prepared by off-axis pulsed laser deposition were characterized by their dielectric constant and loss tangent at 1 MHz and 2 GHz, and from room temperature down to 4 K. The resulting films show significant variations of dielectric properties with position of the substrates with respect to the plume axis. STO films on LAO substrates show low loss and high dielectric constant in regions near the plume axis. On the other hand, STO on NGO shows this effect only on the films grown far from the plume axis. We also obtained a figure of merit from the relative variation of the dielectric constant divided by the loss tangent in the presence of a DC electric field up to +/4 V/µm. Careful mapping of the plume crossection allowed us to improve the quality and reproducibility of the dielectric films, obtaining a best figure of merit at 2 GHz and 4 K close to 100 for NGO substrate but only well off axis. INTRODUCTION The dielectric constant of strontium titanate (STO) is a strong function of the applied electric field [1]. Thus, it is a promising material for fabricating RF and microwave devices such as phase shifters, tunable filters and dielectric varactors [2]. Also, the lattice mismatch between STO and the high temperature superconductors such as YBa2Cu3O7-x is on the order of 2% or better. This property as well as its good chemical compatibility make STO a good candidate for superconducting microwave electronics. The dielectric response of thin films shows a significantly different behavior than is seen for bulk STO. Unstressed bulk strontium titanate is an incipient ferroelectric material and it remains paraelectric down to the lowest temperature due to quantum fluctuations [3]. The dielectric constant of bulk STO increases nonlinearly from 300 at room temperature to 30000 at 4 K. On the other hand, thin films of STO, while showing a dielectric constant close to 300 at room temperature, typically reach a maximum between 1000 and 10000 in the 30 K to 100 K range depending on the film. Thin STO films also show a loss tangent which is an order of magnitude higher than the 10 -3-10 -4 of bulk STO. The microwave applications are hindered by the high dielectric loss in the material. Defects arising from lattice mismatch between the substrate and the film are believed to cause some of the loss, but other film defects could be involved [4]. Recently, approaches to produce films with reduced loss involved epitaxial lift off to relieve the strain in the films [5,6,8,9]. This method requires the growth of a sacrificial intermediate layer that needs to be etched away in order to release the film, hence involving a number of steps
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