Tunability of Bi-rich BZN Cubic Pyrochlore Thin Films by Reactive Sputtering
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Tunability of Bi-rich BZN Cubic Pyrochlore Thin Films by Reactive Sputtering Dong Hyuk Back, Yoon Seop Lee, Young Pyo Hong, Joong Ho Moon, Kyung Hyun Ko* Department of Materials Science and Engineering, Ajou University, Suwon 442-749, Korea Corresponding author, (Fax: +82-31-219-2534, E-mail:[email protected])
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ABSTRACT (Bi3xZn2-3x)(ZnxNb2-x)O7 thin films (x=1/2 and 2/3) have potential great for tunable RF and microwave devices due to medium dielectric constant and low dielectric loss. The tunable dielectric properties of Bi-rich, (Bi1.5 Zn0.5)(Zn0.5 Nb1.5)O7 thin films were investigated. To make Bi-rich cubic pyrochlore thin films, Bi2O3-ZnO-Nb2O5 monoclinic pyrochlore ceramic targets were used in reactive RF magnetron sputtering process. Substrate heating was employed to improve surface morphology and tunability. As-deposited films were crystallized or amorphous state depending on substrate temperature. All films were annealed at 600˚C ~ 800˚C for 3 hours in the air. There were no zinc niobate secondary phases in the films before and after postannealing, while quite significant amount BZN thin films were found in sputtered using cubic pyrochlore ceramic targets, especially after post-annealing. It was found that Bi-rich BZN films have much larger tunability when as-deposited phase are amorphous. The maximum tunability 38% was obtained when substrate is heated to 350˚C and composition of films is close to exact stoichiometric cubic BZN.
INTRODUCTION For many years, the tunable dielectric materials for high frequency applications such as broadband mobile communication have developed successfully. Among them, there are many efforts to investigate ferroelectric materials such as SrTiO3, Ba1-xSrxTiO3 for voltage tunable applications [1-4]. However, ferroelectric thin films inherently have larger dielectric losses than paraelectric materials due to capacitive hysteresis in the microwave region. As a paraelectric, Bi2O3-ZnO-Nb2O5 pyrochlore system is one of candidates owing to its high dielectric constant, low loss and low co-firing temperature. It has been reported that BZN films prepared by metal organic deposition (MOD) process [5-7], pulsed laser deposition (PLD) [8, 9] and RF magnetron sputtering [10-12] showed noticeable voltage tunable dielectric properties.
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There are two main phases in the BZN system depending on composition; (Bi3xZn23x)(ZnxNb2-x)O7
, where x=1/2 for cubic and x=2/3 for monoclinic structures, respectively [13,14]. Recently, it was found that one of the most important factors for tunability is nonstoichiometry of BZN films (especially Bi contents) when cubic BZN target is used[5, 6]. In this work, the dielectric tunable properties of BZN thin films by RF magnetron sputtering using monoclinic BZN target were investigated in terms of nonstoichiometry.
EXPERIMENTAL PROCEDURE The BZN thin films were deposited on Pt(111)/TiO2/SiO2/Si (1500Å/200Å/3000Å/550㎛) substrates. The target materials ( Bi2(Zn1/3Nb2/3)2O7 )were fabricated using the conventional mixed-oxide process. Bi2
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