Yukio Sakashita

Dependence of electrical properties on film thickness in Pb(ZrxTi1−x)O3 thin films produced by metalorganic chemical vapor deposition

Strongly c‐axis oriented Pb(ZrxTi1−x)O3 (PZT) thin films with tetragonal perovskite structure (0.45≤x≤0.52) were epitaxially grown on (100)Pt/(100)MgO substrates using metalorganic chemical vapor deposition. Film thickness could be varied by altering the growth time. The electrical properties of PZT thin films sharply change below a thickness of 0.5 μm: the dielectric constant and remanent polarization decrease while the coercive field increases. These phenomena are explained by a model in which a layer with low dielectric constant exists in series with the normal PZT layer. The origins of this layer are systematically studied and found to be the intrinsic stress produced by the coalescence of crystal grains.

Preparation and electrical properties of MOCVD‐deposited PZT thin films

Strongly [001] oriented lead zirconate‐titanate [Pb(ZrxTi1−x)O3] thin films with tetragonal structure (0<x<0.52) have been successfully grown on (100)Pt/(100)MgO substrates by using metalorganic chemical vapor deposition (MOCVD). The metalorganic precursors were Pb(C2H5)4, Zr(DPM)4 and Ti(i‐OC3H7)4. Scanning electron micrographs showed dense and noncolumnar growth with good surface morphology. The relative dielectric constants at room temperature were 200–350, and appeared to have less dependence on composition x than for PZT bulk ceramics. Typical D‐E hysteresis loops which occur with PZT bulk ceramics were observed. Remanent polarizations were 30–40 μC/cm2. The coercive field decreased from 65 to 40 kV/cm with increases in Zr content. The pyroelectric coefficients without poling treatment were about 3×10−8 C/cm2 K, showing almost the same value as that of poled PZT bulk ceramics.

Metalorganic Chemical Vapor Deposition of c-Axis Oriented PZT Thin Films

Ferroelectric PZT thin films were prepared by the simultaneous deposition of PbO, ZrO2 and TiO2 on heated substrates under a reduced pressure of 6 Torr. Both tetraethyl lead-zirconium tetraisopropoxide and tetradipivaloylmethane-titanium tetraisopropoxide systems were examined as source materials. The films obtained at 500° to 650°C with those systems were constituted of PZT of the single perovskite phase, and highly c-axis-oriented films were grown on MgO(100) with the latter system. The c-axis orientation suggests an epitaxial growth of PZT on the substrates. With increasing molar fraction of ZrO2 in the PZT, the crystals were transformed from a tetragonal to rhom-bohedral structure. The film deposition rates were 100 to 1000 A/min, i.e., more than ten times those obtained by the conventional sputtering method.

Preparation and characterization of LiNbO3 thin films produced by chemical‐vapor deposition

Lithium niobate [LiNbO3 (LN)] thin films with good surface morphology and crystallinity have been epitaxially grown on lithium tantalate [LiTaO3 (LT)] substrates by chemical‐vapor deposition using lithium dipivaloylmethane [Li(C11H19O2)] and niobium pentaethoxide [Nb(OC2H5)5] without postannealing. The Li/Nb ratio of LN thin films was easily controlled by adjusting the Nb source temperature, and it is possible to prepare LN thin films with stoichiometric composition (Li/Nb=1). The crystallinity of LN thin films greatly depends on substrate temperature and surface orientation of the LT substrate. It becomes better with increasing substrate temperature and is equivalent to a single crystal at 700 °C. It also becomes better in the order of (012), (110), and (001) planes of the LT substrate. The properties of surface acoustic wave propagation can be improved by preparing the LN thin film on the LT substrate. The electromechanical coupling factor k2 of LN/(012)LT is 0.52%, and is greater than that of the (012)...

Thickness dependence of dielectric properties in bismuth layer-structured dielectrics

c-axis-oriented epitaxial SrBi4Ti4O15 and CaBi4Ti4O15 films having natural superlattice structure were grown on (001)cSrRuO3‖(001)SrTiO3 substrates by metal organic chemical vapor deposition. SrBi4Ti4O15 films suffer no degradation with a dielectric constant of 200 down to a film thickness of 15nm, which corresponds to four unit cells. Temperature coefficients of capacitance were low enough despite their high dielectric constant. They exhibited stable capacitance and superior insulating properties against applied electric field, irrespective of film thickness. These results open the door to designable size-effect-free materials with high dielectric constant having bias- and temperature-independent characteristics together with superior electrical insulation for high-density capacitor applications.

Preparation of C-Axis-Oriented PLT Thin Films by the Metalorganic Chemical Vapor Deposition Method

La-modified lead titanate (PLT) thin films are prepared by the metalorganic chemical vapor deposition method. Tetraethyl lead, titanium tetraisopropoxide and trisdipivaloylmethanato lanthanum are used as source materials. The films obtained at 600°C under reduced pressure of 6 Torr constitute PLT of the single perovskite phase, and highly c-axis-oriented films are grown on MgO(100). The crystal structures are transferred from the tetragonal to the cubic structure with increasing molar fraction of La2O3. The dielectric constant and tan δ are around 800 to 2700 and 0.04 to 0.1.

Born Effective Charges and Piezoelectric Coefficients of BiXO3

We have calculated the Born effective charges and piezoelectric coefficients of BiXO3 in various crystals by first-principles methods. Results suggest that there is a difference between the tetragonal and the rhombohedral phases in the electric state in BiXO3, and the covalency between the bismuth and surrounding oxygen of the rhombohedral phase is stronger than that of the tetragonal phase. The calculated Born effective charges and piezoelectric coefficients suggest that the covalency between bismuth and oxygen is important for the piezoelectric response in the rhomboherdal phase, but in the tetragonal phase, the covalency between the B-site cation and oxygen is important for the piezoelectric response. In the R3c symmetry, the piezoelectric coefficients are largest, suggesting that the counter rotation of oxygen around the [111] direction in the R3c symmetry is effective for the hybridization between bismuth and oxygen.

Growth of Epitaxial 100-Oriented KNbO3?NaNbO3 Solid Solution Films on (100)cSrRuO3?(100)SrTiO3 by Hydrothermal Method and Their Characterization

Films of solid solution in KNbO3–NaNbO3 (KNN) were deposited at 240 °C on (100)cSrRuO3∥(100)SrTiO3 substrates by the hydrothermal method. (KxNa1-x)NbO3 films with x = 0–1.0 were synthesized by changing the fraction of KOH in a solution of KOH and NaOH. The x in (KxNa1-x)NbO3 continuously changed with the volume fraction of KOH, while the deposition amount strongly depended on x. Epitaxial films with 100 orientation were obtained in the entire composition range and their out-of-plane lattice spacing changed with x. All the films showed ferroelectricity and their remanent polarization became larger than what above x = 0.58.

Novel Candidate of c-axis-oriented BLSF Thin Films for High-Capacitance Condenser

c-axis-oriented SrBi 2 Ta 2 O 9 thin films with various thickness ranging from 20–170 nm were epitaxially grown on (100)SrRuO 3 (SrRuO 3 is give for the pseudo-cubic unit cell)//(100)SrTiO 3 substrates by metalorganic chemical vapor deposition (MOCVD). The relative dielectric constants of these films kept a constant value of about 55 with decreasing film thickness down to 20 nm. The capacitances of these films were almost independent of the applied electric field; change of capacitance for these films on applied electric field from 0 kV/cm to 100 kV/cm was within 0.017 % and tano value was within 1.3 %. The leakage current densities were constant against the film thickness on the order of 10- 8 A/cm 2 at 150 kV/cm. Surface flatness of these films were also almost the same irrespective of the film thickness. These characteristics are very attractive for high-capacitance condenser application. Therefore, c-axis-oriented BLSF thin films are novel candidates for high-capacitance condenser application having both bias-free and thickness independent characteristics together with the good surface smoothness.

Effect of Dopant on Piezoelectric Properties of Lead-Free BiFeO3–BaTiO3 Film

To realize high-performance lead-free piezoelectric materials, we have selected BiFeO3 as the rhomboheral phase material and BaTiO3 as the tetragonal phase material, following the MPB mechanism of PZT. To further improve the piezoelectric properties of BiFeO3–BaTiO3 (BFBT) thin films, we investigated the effect of dopant on the piezoelectric properties of BFBT. Considering both Shanon effective ionic radii and valence number, we have selected Sr2+ (BFBT–SrTiO3, abbreviated as BFBT-ST) and Nb5+ (abbreviated as BFBT-Nb) substitutions. BFBT-ST and BFBT-Nb films were epitaxially grown by pulsed laser deposition on an epitaxial SrRuO3/MgO/Si(100) substrate. The piezoelectric constant d33AFM is estimated from the slope of the maximum displacement and the maximum voltage strength by atomic force microscopy. Compared with 0.8BF0.2BT films with d33AFM = 65 pm/V, the 0.8BF0.2BT–Nb2.5% films achieved d33AFM = 78 pm/V. The displacement curve of 0.8BF0.2BT–Nb2.5% became soft with the occurrence of a hysteresis shape, compared with that of 0.8BF0.2BT. Moreover, the d33AFM of 0.77BF0.13BT–0.1ST (BF/BT ratio= 0.85/0.15) was 83 pm/V, compared with the d33AFM (58 pm/V) of 0.85BF0.15BT. According to these results, we confirmed that the Sr2+ and Nb5+ substitutions improve the piezoelectric properties of BFBT.