Oxygen vacancies in barium strontium titanate thin films

Due to its high dielectric constant, barium strontium titanate (BST) is a promising candidate material for use in high charge storage density capacitors. However, the phenomenon of resistance degradation can limit the lifetime of the device, therefore knowledge of the mechanism involved is key in developing science-based predictive models of device performance. Resistance degradation in bulk perovskite oxides is known to involve migration of oxygen vacancies. This study utilizes observation of 18O tracer incorporation using secondary ion mass spectrometry and subsequent modeling to determine an average vacancy concentration of approximately 1 ppm in 500Å thick MOCVD BST films. The mechanism of charge conduction across Pt/BST/Pt capacitors is examined through detailed observation of the voltage and temperature dependence of current leaking though the device. Thermionic emission and field emission across the Pt/BST Schottky barrier are both observed when current is electrode-limited, and a bulk, space-charge-limited regime is also observed at high temperatures and fields. Holding the capacitor under do bias results in changes in the conduction mechanism across the device and Schottky barrier height at the interface. Using SIMS, changes in 18O tracer profiles are seen as a result of do biasing the Pt/BST/Pt capacitors. This suggests that charged oxygen vacancies move in response to an applied electric field and may be responsible for the changes in electronic conduction observed. Using these observations combined with others, an explanation of the mechanism of resistance degradation in BST thin film capacitors is offered.