Effects of titanium and tantalum adhesion layers on the properties of sol-gel derived SrBi ₂Ta ₂O ₉ thin films

The effects of titanium (Ti) and tantalum (Ta) adhesion layers on the ferroelectric and microstructural properties of sol-gel-derived SrBi ₂Ta ₂O ₉ (SBT) films are investigated. It is found that the atoms from the adhesion layer play a significant role in the resultant microstructures and physical properties of SBT films. The electron spectroscopy for chemical analysis clearly indicates that both the Ti and Ta atoms of adhesion layers have out-diffused behavior onto the surface of bottom-electrode Pt films after a thermal treatment of 750°C, 1 min. Various out-diffused species do cause the distinct properties of SBT films, which are confirmed by the results of surface analysis and polarization-electric field (P-E) measurements. The formation of undesirable second phase compounds near the SBT/PT interface has been observed in specimens with Ti layer, and it is speculated to be the significant degradation of spontaneous polarization. On the contrary, Ta species are found to exhibit the pure bismuth-layered structure (BLS) phase and the good ferroelectric properties of SBT films, which have been identified by x-ray diffraction spectra and P-E measurements. The remnant polarization values of SBT films on top of Pt/Ti/SiO ₂/Si and Pt/Ta/Ti/SiO ₂/Si substrates are 11.1 and 14.2 μC/cm ² at 5 V, respectively. The same phenomenon was also observed at specimens with an increased annealing temperature of 800°C. Consistently, the SBT film with Ta adhesion layer displays a better polarization value of 16.2 μC/cm ² than that with Ti adhesion layer (12.6 μC/cm ²). The fatigue tests of all samples depict the near-fatigue-free characteristics except for the specimens with Ti adhesion layer crystallized at 800°C. It is suggested that the employment of the Ta element for the adhesion layer in the SBT process is more favorable to obtain a desirable BLS phase and to improve the ferroelectric properties.