Crystallographic orientation dependence of the dielectric constant in polymorphic BaNb₂O₆ thin films deposited by laser ablation

Well-crystallized barium metaniobate (BaNb₂O₆) thin films were fabricated on fused quartz substrates by pulsed laser deposition. The influence of substrate temperature and oxygen pressure on the crystal structure and preferred orientation were studied to understand the growth mechanism of BaNb₂O₆ thin films. The films formed at 600°C at an oxygen pressure of 100 mTorr exhibited predominantly the orthorhombic (040) orientation, and turned to the orthorhombic (230) orientation at 800°C. It was found that (220)-oriented hexagonal thin films were formed at 600°C at an oxygen pressure less than 50 mTorr. The dielectric constant of the BaNb ₂O₆ thin films was measured by scanning microwave microscopy (SMM). Preferentially (230)-oriented orthorhombic and (220)-oriented hexagonal BaNb₂O₆ thin films were shown to have significantly enhanced dielectric constants of 47.8 and 56.7, respectively. This could be attributed to the dependence of the dielectric constant on crystallographic orientation.