Dielectric properties of BaMg1∕3Nb2∕3O3 doped Ba0.45Sr0.55Tio3 thin films for tunable microwave applications

Abstract

Ba(Mg[Formula: see text]Nb[Formula: see text]O3 (BMN) doped and undoped Ba[Formula: see text]Sr[Formula: see text]TiO3 (BST) thin films were deposited via radio frequency magnetron sputtering on Pt/TiO2/SiO2/Al2O3 substrates. The surface morphology and chemical state analyses of the films have shown that the BMN doped BST film has a smoother surface with reduced oxygen vacancy, resulting in an improved insulating properties of the BST film. Dielectric tunability, loss, and leakage current (LC) of the undoped and BMN doped BST thin films were studied. The BMN dopant has remarkably reduced the dielectric loss ([Formula: see text]38%) with no significant effect on the tunability of the BST film, leading to an increase in figure of merit (FOM). This is attributed to the opposing behavior of large Mg[Formula: see text] whose detrimental effect on tunability is partially compensated by small Nb[Formula: see text] as the two substitute Ti[Formula: see text] in the BST. The coupling between [Formula: see text] and V[Formula: see text] charged defects suppresses the dielectric loss in the film by cutting electrons from hopping between Ti ions. The LC of the films was investigated in the temperature range of 300–450[Formula: see text]K. A reduced LC measured for the BMN doped BST film was correlated to the formation of defect dipoles from [Formula: see text], V[Formula: see text] and Nb[Formula: see text] charged defects. The carrier transport properties of the films were analyzed in light of Schottky thermionic emission (SE) and Poole–Frenkel (PF) emission mechanisms. The result indicated that while the carrier transport mechanism in the undoped film is interface limited (SE), the conduction in the BMN doped film was dominated by bulk processes (PF). The change of the conduction mechanism from SE to PF as a result of BMN doping is attributed to the presence of uncoupled Nb[Formula: see text] sitting as a positive trap center at the shallow donor level of the BST.