SrBi2Ta2O9 thin films made by liquid source metal-organic chemical vapor deposition

Abstract

A liquid source metal-organic chemical vapor deposition system was installed to deposit SrBi₂Ta₂O₉ (SBT) thin films on sapphire and Pt/Ti/SiO₂/Si substrates. The process parameters such as deposition temperature and pressure, and ratio of Sr: Bi: Ta in the precursor solutions were optimized to achieve stoichiometric films with good reproducible ferroelectric properties. It was found that the nucleation of SBT started at a deposition temperature close to 500 °C and grain growth dominated at 700 °C and higher temperatures. With increasing deposition temperatures, the grain size of SBT thin films increased from 0.01 μm to 0.2 μm; however, the surface roughness and porosity of the films also increased. To fabricate specular SBT films, the films had to be deposited at lower temperature and annealed at higher temperature for grain growth. A two-step deposition process was developed which resulted in high quality films in terms of uniformity, surface morphology, and ferroelectric properties. The key to the success of this process was the homogeneous nucleation sites at lower deposition temperature during the first step and subsequent dense film growth at higher temperature. The two-step deposition process resulted in dense, homogeneous films with less surface roughness and improved ferroelectric properties. SBT thin films with a grain size of about 0.1 μm exhibited the following properties: thickness: 0.16–0.19 μm; 2P_r: 7.8–11.4 μC/cm² at 5 V; E_c: 50–65 kV/cm; I_leakage: 8.0–9.5 × 10⁻⁹ Acm⁻² at 150 kV/cm; dielectric constant: 100–200; and fatigue rate: 0.94–0.98 after 10¹⁰ cycles at 5 V.