Effect of crystallinity on the dielectric loss of sputter-deposited (Ba,Sr)TiO 3 thin films in the microwave range

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Young-Taek Lee and Sangwook Nam School of Electrical Engineering and Computer Science, Seoul National University, Seoul 151-744, Korea (Received 20 September 2002; accepted 16 December 2002)

The crystallinity dependence of the microwave dielectric losses in (Ba,Sr)TiO3 thin films was investigated. The sputter-deposition temperatures were altered to vary the level of thin-film crystallinity on a Pt/Si substrate. The dielectric losses (tan ␦) were measured up to 6 GHz without parasitic (stray) effects by using a circular-patch capacitor geometry and an equivalent-circuit model. The microwave dielectric losses increased from 0.0024 ± 0.0018 to 0.0102 ± 0.0017 with increasing crystallinity. These deteriorated dielectric losses showed a good correlation with the symmetry-breaking defects, as confirmed by Raman spectra at approximately 760 cm−1, inducing microscopic polar regions above the Curie temperature of the bulk (Ba0.43Sr0.57)TiO3.

I. INTRODUCTION

(Ba,Sr)TiO3 (BST) thin films have been studied intensively for use in microwave devices, such as phase shifters, band-pass filters, and frequency triplers.1–3 This is due to its high-dielectric constant, relatively low loss, and high dielectric nonlinearity (tunability) in the microwave-frequency range.1–6 In particular, dielectric loss is one of the most important factors for highfrequency applications when considering the efficiency of devices. Dielectric losses depend on the microstructures of the thin films, such as the phase, grain size, composition, strain, defects, etc.1–9 Generally, pure paraelectric thin films, for example ZrTiO4, show high quality factors (Q ⳱ 1/tan ␦) in the crystalline phase, and Q is low in the amorphous phase.9 However, there are reports showing that crystalline (BaxSr1−x)TiO3 thin films (paraelectric phase with 0 艋 x 艋 0.6 at room temperature2) have higher dielectric losses than the amorphous phase. In contrast, the dielectric constants and tunabilities show reverse behaviors (i.e., high in the crystalline phase and low in the amorphous phase).7,10 Despite these reports, there have been few systematic studies on the correlations between the microstructures and the dielectric-loss behaviors of BST thin films.

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Address all correspondence to this author. e-mail: [email protected]

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J. Mater. Res., Vol. 18, No. 3, Mar 2003 Downloaded: 14 Mar 2015

Since dielectric losses have frequency-dependent behavior, it is necessary to investigate the dielectric losses in the GHz frequency range where the dielectric devices are operated. However, in spite of the microwave applications, many studies have been performed around the sub-MHz range.1–3,5–7 In this paper, the correlations between the order of crystallinity and the dielectric losses of BST thin films were investigated systematically in the microwave-frequency range. II. EXPERIMENTAL

Using radio frequency magnetron sputtering with a (Ba0.5Sr0.5)TiO3 target sintered at 1350 °C, BST thin films (approximately 250-nm thick) were deposited on 100-nm-thick Pt

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