Microwave Properties of Strontium Barium Niobate Thin Films Grown by Pulsed Laser Deposition
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Microwave Properties of Strontium Barium Niobate Thin Films Grown by Pulsed Laser Deposition Víctor Rodríguez-Santiago, Yelitza González, and Félix E. Fernández Department of Physics, University of Puerto Rico Mayagüez, PR 00681-9016 Carl H. Mueller, Fred W. Van Keuls, and Félix A. Miranda Communications Technology Division, NASA Glenn Research Center Cleveland, OH 44135-3191 ABSTRACT Strontium barium niobate ( SrxBa1-xNb2O6 - SBN) with 0.25#x#0.75 is a ferroelectric material of interest for diverse optoelectronic applications. Dielectric properties of bulk SBN crystals were comprehensively studied over 30 years ago for a range of compositions and at frequencies up to 30 MHz, but there is little information on properties at higher frequencies. In particular, and up to the best of our knowledge, there are no published results about SBN thin film dielectric properties at high frequencies. For the study reported here, SBN thin films with x = 0.61 were grown on MgO and LaAlO3 substrates by Pulsed Laser Deposition (PLD). Films with good crystallinity and oriented with c-axis normal to the substrate surface were obtained on both types of substrates, while films on MgO had much better texture due to better lattice matching. Interdigital electrode (IDE) capacitors and coupled microstrip phase shifters (CMPS) were fabricated with both types of samples in order to study dielectric response. Capacitance of the IDE capacitors was measured at 1 MHz as a function of temperature and bias voltage, revealing very low losses but poor capacitance tunability, particularly for samples on MgO. Response of the CMPS structures was measured at room temperature and at high frequencies, up to 21 GHz. Insertion losses were measured up to 28 GHz. INTRODUCTION Strontium barium niobate (SrxBa1-xNb2O6 - SBN) with 0.25#x#0.75 is the solid solution of the SrNb2O6 - BaNb2O6 binary system. It is of interest for optoelectronic applications such as electro-optic (E-O) modulators (its transverse linear E-O coefficient can reach 1400 pm/V), realtime holography and information storage (due to the fact that it is photorefractive), and has very high pyroelectric coefficient (p up to 310 nC/cm2 K).[1] SBN is ferroelectric at room temperature, and has low-frequency 033 / 0o values from ~ 100 to over 3000, depending on composition. Its crystal structure is tetragonal (4mm point group) and typified as tungstenbronze. The Sr2+ and Ba2+ cations in SBN occupy only 5 out of 6 possible sites in the unit cell, which introduces some disorder in their locations.[2] Dielectric properties for SBN bulk crystals with various compositions were studied over three decades ago by Glass,[3] who measured them as functions of temperature (through the ferroelectric transition) and of frequency (up to tens of MHz). The occupancy disorder mentioned above causes a gradual ferroelectric-paraelectric transition for SBN, typical of tungsten-bronze ferroelectrics. This “relaxed” ferroelectric transition is broader and with lower C7.12.1
permittivity peak value for Sr0.75Ba0.25Nb2O6 , in
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