RF-Magnetron Sputtered Strontium Titanate: Structure, Processing and Property Relationships
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ABSTRACT The low frequency dielectric properties of epitaxial SrTiO 3 thin films deposited on LaAIO 3 are presented. The films were deposited using radio-frequency magnetron sputtering from stoichiometric targets in an Ar/0 2 atmosphere. For the first time, the effects of in situ ozone annealing during the early stages of deposition were explored. X-ray diffraction results indicated that the ozone treatment resulted in more symmetric and sharper diffraction peaks (2 0FWHM decreased from 0.170 to 0.100). In addition, the peaks for the ozone treated samples were shifted in 2 9towards values approaching the bulk value. Rutherford backscattering measurements showed Sr/Ti ratios of 1: 1 for these samples, indicating these peak shifts are not due to compositional variations. The dielectric constant of the ozone treated samples increased from 275 at room temperature to 1175 at 22 K (measured at 100 kHz). The effective loss tangent of the device remained between 1 x 1 0 -4and 1 x 10-3 down to 100 K, where it began to increase. The tunability was also measured. The ozone treated sample showed tunability of 46%, 43% and 38% at 22 K, 40 K and 60 K, respectively. Finally, similar measurements were completed at 1 MHz, indicating a minimal dependence of these properties on frequencies in this range. INTRODUCTION Electrically tunable microwave devices based on SrTiO 3 (STO) have been extensively investigated recently. In these designs, one takes advantage of the dc electric field tunability of the nonlinear dielectric SrTiO 3 film at cryogenic temperatures. For these devices to be practical, it is desirable to have the largest dielectric tunability in combination with the lowest dielectric loss achievable. These dielectric losses originate from various sources, including losses in the bulk of the STO film, losses at the interface between the substrate (typically LaA10 3) and the STO film, and losses at the interface between the STO film and the electrode. Within the film, such defects as misorientation, grain boundaries, and oxygen vacancies can contribute to the loss. Ideally, elimination of as many of these factors as possible should enhance the dielectric properties of STO thin films. Of the majority of the reported work on STO thin films for high frequency applications, pulsed laser deposition (PLD) is the most commonly used deposition technique [ 1-4]. Other methods, including chemical routes [5], metalorganic chemical vapor deposition (MOCVD) [6], molecular beam epitaxy (MBE) [7], and rf-sputtering (both on and off-axis) have been utilized [8-11], as well. In terms of PLD, it is well known that the process inherently results in the presence of macroparticles on the film surface. Such features may affect the dielectric properties of the final device. Thus, a technique which results in smoother surfaces/interfaces should beneficially influence the dielectric properties. Therefore, in this work rf-magnetron sputtering is used to deposit the STO-based heterostructures. In addition, in order to address the issue of oxygen non-st
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