Gouji Asano

Large remanent polarization of 100% polar-axis-oriented epitaxial tetragonal Pb(Zr0.35Ti0.65)O3 thin films

100% polar-axis (c-axis)-oriented epitaxial Pb(Zr0.35Ti0.65)O3 (PZT) thin films were grown and their large remanent polarization (Pr) was directly measured. Perfectly c-axis-oriented epitaxial PZT thin films were obtained on (100)cSrRuO3//(100)SrTiO3 substrates when the deposition temperature increased to 540 °C together with a decrease in the film thickness down to 50 nm. Polarization–electric-field hysteresis loops were well saturated and had a square shape. The Pr of a 50 nm thick film saturated at 0.9 V, and its value was over 90 μC/cm2; almost independent of the measurement frequency within the range from 20 Hz to 1 kHz. This value was in good agreement with the estimated one from the a- and c-axes mixture-oriented epitaxial PZT film having the same composition taking into account the fact that only the c-axis-oriented domain contributed to the polarization. On the other hand, the coercive field value of a perfectly c-axis-oriented film was 140 kV/cm and almost the same as that of the mixture-oriente...

Fatigue-free RuO2/Pb(Zr,Ti)O3/RuO2 capacitor prepared by metalorganic chemical vapor deposition at 395 °C

We deposited an RuO2/Pb(Zr0.40Ti0.60)O3/RuO2 capacitor by metalorganic chemical vapor deposition. RuO2 and Pb(Zr0.40Ti0.60)O3 films were prepared at 350, 395, and 445 °C from respective Ru(C7H11)(C7H9)–O2 and Pb(C11H19O2)2–Zr(O⋅t-C4H9)4–Ti(O⋅i-C3H7)4–O2 systems. Good ferroelectricity was observed for PZT films deposited at 445 °C but not at 395 °C. However, we obtained ferroelectricity with a remanent polarization above 30 μC/cm2 by inserting a 10-nm-thick sputtered-Pt layer between the PZT and RuO2 bottom electrodes, which improved the crystallinity of PZT films even those deposited at 395 °C. This capacitor had hardly any fatigue after 1×1010 switching cycles. This demonstrates the possibility of preparing fatigue-free capacitor all deposited below 400 °C for high-density ferroelectric random-access memory applications.

Compositional Dependence of Electrical Properties of Highly (100)-/(001)-Oriented Pb(Zr,Ti)O3 Thick Films Prepared on Si Substrates by Metalorganic Chemical Vapor Deposition

Pb(Zr,Ti)O3 (PZT) films having 2.0 µm in thickness with the Zr/(Zr+Ti) ratio ranging from 0.20 to 0.75 were prepared on (111)Pt/TiO2/SiO2/(100)Si substrates at 600°C by metalorganic chemical vapor deposition (MOCVD). Highly (100)-/(001)-oriented PZT films were successfully prepared with a deposition rate of 1.5 µm/h. Effects of the film composition on the crystal structure, ferroelectricity and electric field-induced strain of these films were investigated. It was ascertained by θ-2θ scans of X-ray diffraction that a tetragonal single phase and a rhombohedral single phase were obtained for the films with the Zr/(Zr+Ti) ratio below 0.40 and above 0.60, respectively. On the other hand, the mixed phases with tetragonal and rhombohedral phases were observed for the Zr/(Zr+Ti) ratio ranging from 0.43 to 0.57. This composition is similar to the morphotropic phase boundary (MPB) for the bulk ceramics. Remanent polarization (Pr) and coercive field (Ec) were almost constant and monotonically decreased, respectively, when the Zr/(Zr+Ti) ratio increased. This change of Ec value with the Zr/(Zr+Ti) ratio was in good agreement with the reported one for the sintered body, but not for the Pr value. On the other hand, the relative dielectric constant and the field-induced strain took maximum values around this composition which was in good agreement with the reported data for the sintered body. Field-induced strain was in good agreement with the estimation from the phenomenological equation, and the large change of polarizations is found to cause large field-induced strain. These results show that the highly (100)-/(001)-oriented films showed behavior similar to that of a PZT sintered body in terms of field-induced strain, coercive field and the dielectric constant together with the coexistence of the tetragonal and rhombohedral phases around the MPB composition.

Crystal Orientation Dependence on Electrical Properties of Pb(Zr,Ti)O3 Thick Films Grown on Si Substrates by Metalorganic Chemical Vapor Deposition

(111)c- and (100)c-oriented SrRuO3 films were successfully grown on (111)Pt/TiO2/SiO2/(100)Si and (100)LaNiO3/(111)Pt/TiO2/SiO2/(100)Si substrates, respectively, by RF-magnetron sputtering method. On these (111)c- and (100)c-oriented SuRuO3 films, (111)- and (001)/(100)-oriented fiber-textured Pb(Zr0.35Ti0.65)O3 films with 2.0 µm in thickness were grown by metalorganic chemical vapor deposition (MOCVD). Well-saturated polarization-electric field (P-E) hysteresis loops were observed for both films. The remanent polarization (Pr) values of (111)- and (001)/(100)-oriented 2.0 µm-thick Pb(Zr,Ti)O3 (PZT) films were almost the same at approximately 45 µC/cm2 at 200 kV/cm, while the coercive field (Ec) values of these films were slight different at 61 kV/cm and 71 kV/cm, respectively. Moreover, the field-induced strains measured by scanning probe microscopy were also almost the same at approximately 0.2% up to 100 kV/cm. These data show the crystal orientation independence of the remanent polarization and field-induced strain of Pb(Zr0.35Ti0.65)O3 films.

Highly-reproducible preparation of Pb(Zr, Ti)O3 films at low deposition temperature by metal organic chemical vapor deposition

Polycrystalline Pb(Zr, Ti)O 3 (PZT) films were prepared on (111)Pt/TiO 2 /SiO 2 /Si substrates at 395°C by source-gas-pulsed-introduced metal-organic chemical vapor deposition (pulsed-MOCVD). The process window for obtaining the PZT single phase relative to the input source gas of lead was observed even at this low deposition temperature and became wider when the pressure of the reactor decreased from 670 to 67 Pa. This can be explained by the acceleration of the reevaporation of excess element of lead from the surface of the film. The width of the process window at 395 C and 67 Pa by pulsed-MOCVD was almost the same at 580 C and 670 Pa by conventional-source-gas-introduced MOCVD. The film deposited at 395 C showed good ferroelectricity with the remanent polarization value of 29 μC/cm 2 . As a result, a highly-reproducible PZT film deposition compatible with the deposition at 580 C and 670 Pa was obtained even for the 395 C and 67 Pa deposition by pulsed-MOCVD.

Conformality of Pb ( Zr , Ti ) O3 Films Deposited on Trench Structures Having Submicrometer Diameter and Various Aspect Ratios

Pb(Zr,Ti)O 3 films were deposited at 540°C on trenched substrates with submicrometer diameters by pulsed-metallorganic chemical vapor deposition. The respective fluctuations of Pb/(Zr + Ti) and Zr/(Zr + Ti) molar ratios were in a range of ′19 and ′17% from each average value at the sidewalls of trenches with a diameter of 200 nm and a depth of 685 nm (aspect ratio of 3.4). The sidewall-bottom step coverage was successfully maintained above 70% up to a depth of 685 nm. The present results show the possibility of uniform Pb(Zr,Ti)O 3 film deposition on the trench substrates with high aspect ratio.

Effect of Solvent on MOCVD of Pb ( Zr , Ti ) O 3 Films with Liquid-Delivery Source Supply Method

We investigated the effect of a solvent on the metallorganic chemical vapor deposition (MOCVD) of Pb(Zr, Ti)O 3 (PZT) films by comparing conventional bubbling and sublimation delivery methods with one using liquid delivery where the source materials were dissolved in a solvent and vaporized in separate vaporizers. A Pb(C 11 H 19 O 2 ) 2 -Zr(O.t-C 4 H 9 )-Ti(O.i-C 3 H 7 ) 4 -O 2 system was used as the source, while C 8 H 18 was used as the solvent together with C 4 H 8 O. The deposition rates of all constituent oxides decreased with the liquid-delivery method. Moreover, the process window to obtain stoichiometric PZT films [Pb/(Pb + Zr + Ti) = 0.5], irrespective of the supply rate of the Pb source, was diminished with the liquid delivery method, suggesting the contribution of the solvent on the deposition mechanism of PZT film. These phenomena were observed not only with C 8 H 18 but also when C 4 H 8 O was used as a solvent.

Preparing Pb(Zr,Ti)O3 films less than 100nm thick by low-temperature metalorganic chemical vapor deposition

Polycrystalline Pb(Zr,Ti)O3 (PZT) films 70–80nm thick on (111)Ir∕TiO2∕SiO2∕Si substrates were prepared at 415°C by metalorganic chemical vapor deposition (MOCVD). At 3V, the remanent polarization (Pr) of the as-deposited films was approximately 22μC∕cm2. Inserting PbTiO3 seeds between the PZT films and Ir bottom electrodes improved the crystallinity of the films markedly but improved their ferroelectric properties only slightly. Low-temperature postannealing, on the other hand, even at 400°C (i.e., below the deposition temperature), improved Pr values and hysteresis loop shapes without obviously improving the crystallinity of the films. The electrical properties were improved even more when the films were annealed at 500°C. These results suggest that the low-temperature processing and sub-100-nm film thickness needed for making three-dimensionally structured ferroelectric capacitors can be obtained by using low-temperature MOCVD to deposit PZT films, and then annealing those films at a temperature no grea...

Good Ferroelectricity of Pb(Zr,Ti)O3 Thin Films Fabricated by Highly Reproducible Deposition on Bottom Ir Electrode at 395°C

Polycrystalline Pb(Zr,Ti)O3 (PZT) films were prepared on (111)Ir/TiO2/SiO2/Si and (111)Pt/TiO2/SiO2/Si substrates at 395°C by source-gas-pulsed-introduced metal organic chemical vapor deposition (pulsed-MOCVD). By decreasing the deposition pressure from 670 to 67 Pa, a highly reproducible process was obtained irrespective of the type of substrate, used by accelerationing of the reevaporation of excess Pb from the surface of the film. It was found by X-ray diffraction reciprocal space mapping measurement that a reaction product of Pb and Pt, PbPtx, is formed at a deposition pressure of 67 Pa for the films deposited on the (111)Pt/TiO2/SiO2/Si substrate. However, this reaction product was not detected for those deposited on the (111)Ir/TiO2/SiO2/Si substrate. PZT films with ferroelectricity and a remanent polarization value of 40 µC/cm2 were fabricated on the (111)Ir/TiO2/SiO2/Si substrates with high reproducibility even at 395°C. This shows that Ir is a candidate bottom electrode for the highly reproducible preparation of MOCVD-PZT films having a good interface with the bottom electrode, good ferroelectricity and a remanent polarization value of 40 µC/cm2.

Comparison of the Ferroelectricity for 70–80 nm Thick Pb(Zr,Ti)O3 Films Deposited on (111)Ir Bottom Electrodes at Different Temperatures by MOCVD

70–80 nm-thick Pb(Zr,Ti)O3 (PZT) films were deposited on (111)Ir/TiO2/SiO2/Si substrates at 415 and 540°C by metalorganic chemical vapor deposition (MOCVD). The remanent polarization (P r) value showed 22 μ C/cm2 at an applied voltage of 3 V, even for the film deposited at 415°C. However, this value was almost 40% of that for the film deposited at the higher temperature. P r value was enhanced up to 40 μ C/cm2 and the squareness of the polarization-electric field (P-E) hysteresis loop was also improved by the post-annealing at 500°C, even though a noticeable change was not detected on the x-ray diffraction pattern. Moreover, both of the P r and coercive field (E c) values of the film annealed at 500°C after 415°C-deposition were almost saturated below 2 V, as same as the as-deposited films at 540°C.