Lead zirconium titanate(Pb(Zr,Ti)$O_3$(PZT)) thin films have been grown on Si and Pt substrate by reactive sputtering using multi-target. The substrate and buffer layer effect on the phase formation and its electrical properties such as dielectric constant, polarization-field (P-E), current-voltage(Ⅰ-Ⅴ) and fatigue have been studied. Deposition characteristics and electrical properties of PZT thin films with Zr/Ti ratio were also investigated.
The crystal orientation and crystallinity of Pb(Zr,Ti)$O_3$ thin films are strongly affected by the deposition temperature, buffer layer and substrate type. The crystal structure of PZT films deposited on Si substrate changed with Pb target power. At sufficient PbO flux, polycrystalline PZT films without second phase were obtained and crystallization temperature of PZT film in tetragonal region is lower than that of the MPB composition. Broad interface layer regarded as lead silicate layer were existed between PZT film due to reaction of Pb and Si.
The films deposited on Pt/$SiO_2$/Si substrate at 600℃ had strong (100) oriented regardless of Zr/Ti ratio, whereas polycrystalline PZT films were obtained at 550℃. To increase the crystallinity of PZT films at the deposition temperature of 550℃, buffer layer between PZT films and Pt substrate had been investigated and $PT(PbTiO_3)$ and $PZ(PbZrO_3)$ layer were selected as a buffer layer. By using the PT buffer layer, polycrystalline film without buffer layer changed to highly (100) oriented films at the deposition temperature of 550℃. These crystalline structure change can be resulted from PT buffer preferred orientation at the initial growing stage in which PT buffer can be easily grown (001)(100) orientation. Whereas, phase formation of PZT films were suppressed by using the PZ buffer, which arisen from the lack of $PbZrO_3$ phase formation itself at the deposition temperature of 550℃. Dielectric constant(Er) was increased by using 35Å and 70Å PT buffer layer, whereas decreased above 70Å due to increased low Er PT buffer layer. Hence, we can find out the optimum thickness of PT buffer layer for PZT thin films is 35-75Å.
Crystallographic orientation of PZT thin films were varied with PT buffer orientation which changed from (001)(100) orientation to (111) orientation as the Pb target current change 65mA to 35mA. PZT crystalline structure were also affected by the substrate type( Pt/Ti/$SiO_2$/Si, Pt/Ta/$SiO_2$/Si, Pt/MgO). Polycrystalline films and epitaxial films were grown on polycrystalline Pt/Ti substrate and epitaxial Pt/MgO substrate respectively. These substrate type affected the electrical properties of PZT films. From the C-V and P-E hysteresis result, asymmetric Ec were found in the tetragonal PZT thin films and increase with tetragonality(C/A ratio), whereas symmetric shape of P-E loops and C-V curves(symmetric Ec) were observed in the rhombohedral region. These asymmetric Ec in tetragonal are resulted from the internal bias field which created from the defect association of oxygen vacancy and acceptor site. Dielectric constant and Pr(remanent polarization) of PZT thin films deposited on 70Å PT buffer layer had maximum value at the MPB composition and its value is 1250 and 26μC/㎠. And $ε_r$ value of PZT(55/45) films deposited at 600℃ which had (100) strong orientation is 1600.
I-V characteristics of PZT MFM structure showed difference in switching and non-switching current, and leakage current level decreased 1.1 order from 0 to 10 second delay time during Ⅰ-Ⅴ measurement. From theⅠ-Ⅴ characteristics, leakage current mechanism can be explained with metal-semiconductor-metal structure having p-type semiconducting PZT films.
From the fatigue test of PZT films with Zr/Ti ratio, rhombohedral PZT films had high fatigue resistance compared to tetragonal films due to low initial defect concentration in rhombohedral composition.
