Sputter target stoichiometry dependent dielectric properties of Bi₂O₃-ZnO-Nb₂O₅ pyrochlore system

Tunability of Bi₂O₃-ZnO-Nb₂O₅ (BZN) pyrochlore systems can be varied widely, even among isostructural phases (e.g., α-BZN crystals), because of the nonstoichiometry. Therefore, the tunability of BZN pyrochlore paraelectric thin films was reinvestigated. Using nonstoichiometric BZN targets, films with either cubic or monoclinic structure and a second phase devoid of Zn were obtained. Owing to the low sputtering yield of Bi, (Bi_1.5Zn_0.5)(Zn_0.5Nb_1.5)O₇ (α-BZN) was obtained by sputtering α-BZN targets and also targets with Bi-enriched monoclinic composition, i.e., Bi₂(Zn_0.33Nb_0.66)₂O₇ (β-BZN) targets. However, α-BZN films sputtered from β-BZN targets exhibited far better tunability than do those sputtered from α-BZN targets, proving the significance of Bi enrichment of targets. Furthermore, α-BZN films were continuously deposited even when the target composition, Bi:Zn:Nb, increased from the stoichiometry ratio of 3:1:2 to nonstoichiometric 4:1:2. At a target composition of 3.2:1:2, the dielectric properties maximized with a dielectric constant of 184; loss tangent of 0.008; and maximum tunability of 42 % at 1.2 MV/cm of dc-bias field under 1 MHz. The properties minimized at the target composition of 4:1:2, at which β-BZN films formed. For the composition mid-range, Bi₅Nb₃O₁₅ phases occurred along with the α-BZN phase and showed synergetic effects on the properties of the α-BZN films.