Control of bismuth volatility in SBT by vanadium doping

The volatility of bismuth (Bi) presents difficulty in the integration of ferroelectric strontium bismuth tantalate (SrBi2Ta2O9 or SBT) thin films for FeRAM applications. This paper presents preliminary results indicating that the Bi volatility problem may be controlled by the addition of vanadium (V) to SBT. A series of V-doped SBT films (150 nm), of a nominal composition SrBi2.2(Ta1-xVx)2O9 with x = 0, 0.01, and 0.07, were deposited on Pt/SiO2/Si wafers by sol-gel processing. XRD and SEM revealed a polycrystalline morphology and a vanadium solubility in SBT of at least 7%. SBT with 7% V-doping exhibited little structural damage from a forming gas anneal. 7% V-doping also limited Bi diffusion into Pt bottom electrodes and increased the surface Bi 4f7/2 electron binding energies. The ferroelectric and polarization fatigue properties did not improve with V-doping, which was attributed to oxygen vacancy formation due to acceptor doping. Vanadium stabilized Bi in the SBT matrix by reducing from the pentavalent to the tetravalent state, which provided the free energy to either oxidize bismuth or prevent the reduction of bismuth.