(100)-/(001)-oriented epitaxial Pb(Zr,Ti)O3(PZT) films with various Zr/(Zr + Ti) ratios and film thicknesses were grown on (100)cSrRuO3//(100)SrTiO3substrates at 600 °C by metalorganic chemical vapor deposition. Their crystal structure and microstructure consisting of particularly mixed phases with tetragonal and rhombohedral symmetries were investigated in detail. Films with the mixed phases with tetragonal and rhombohedral symmetries were ascertained at a Zr/(Zr + Ti) ratio of around 0.50 at a thickness of more than 200 nm from x-ray diffraction (XRD) analysis, while no mixed phase region was observed for 50 nm thick film. The relative volume fraction of the tetragonal cdomain decreased with increasing film thickness for 2-μm-thick films with mixed phases, while the tetragonal adomain and rhombohedral domain increased. These mixed phases were ascertained to have originated from the same cubic PZT grown above Curie temperature from results obtained by temperature-dependent high-resolution XRD reciprocal space mapping. The transmission electron microscopy images clearly revealed that the microstructure of the film gradually changed along the film thickness, despite the fact that the film was very uniform. Tetragonal phases were dominant in the bottom region (near the interface with the substrate) of the film, while rhombohedral phase was dominant in the upper region (near the surface), which was also ascertained by cross-sectional Raman analysis. In addition, the tetragonal a/cdomain was wider in the bottom region than that in the upper region of film with mixed phases. These data clearly demonstrate that both the crystal structure and microstructure changed along the film thickness.

Crystal structure of epitaxially grown (100)/(100)-oriented PbTiO3 films on (100)SrTiO3 substrates with 3.8 μm in thickness was investigated by the XRD reciprocal space mapping techniques. The observed poles of PbTiO3 004, 400, 204, 402 and 420 scans can be successfully explained using the model consisting with the one type of (001) domain and three types of (100) domains in good agreement with the estimation from the scanning probe microscopy in conjunction with the in-plane and out-of plain XRD analysis.

We grew the epitaxial Pb(Zr1-xTix)O3 [PZT] and (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 [PMN-PT] films, above 2 μm in thickness, on (100)cSrRuO3//(100)SrTiO3 substrates by metalorganic chemical vapor deposition (MOCVD). PbTiO3 content (x) dependencies of the crystal structure, dielectric and piezoelectric properties were systematically investigated for these films. The constituent phase changed from a rhombohedral (pseudocubic) single phase, a mixture phase of rhombohedral (pseudocubic) and tetragonal phases, and a tetragonal single phase with increasing x for both of PZT and PMN-PT films. The mixture phase region was observed when x=0.40−0.60 for PZT films and x=0.40−0.55 for PMN-PT films, which became wider than that reported ones for PZT sintered bodies and PMN-PT single crystals. In addition, x value moves to the higher one than that reported for the single crystal and/or the sintered body in case of PMN-PT films, while was almost the same in case of PZT films. The dependence of relative dielectric constant εr was maximum at the mixed phase region for both films, which were similar to the case of their bulk materials. The higher value of εr was ascertained for the PMN-PT films compared with PZT films, however, the magnitude was lower than the reported one for bulk materials. The longitudinal electric-field-induced strain Δx33 and transverse piezoelectric coefficient e31,f for PZT films were also maximum at the mixed phase region, on the other hand, that for PMN-PT films were maximum at larger x edge of rhombohedral (pseudocubic) region. Almost the same order of Δx33 was observed under applied electric fields up to 100 kV/cm, while larger e31,f was observed in PMN-PT films compared with the case of PZT films. e31,f coefficients of ∼−8.9 C/m2 and ∼−11.0 C/m2 were calculated for the PZT film with x=0.46 and for the PMN-PT film with x=0.39, respectively.

We performed x-ray diffraction measurements by using highly brilliant synchrotron radiation on epitaxial Pb(Zr0.35Ti0.65)O3 film capacitor structures. Small regions of 300-nm-thick epitaxial Pb(Zr,Ti)O3 thin films with Pt and SrRuO3 top electrodes were measured after applying various numbers of switching cycles of the electric field. Epitaxial Pb(Zr,Ti)O3 thin films were prepared on epitaxial (100)cSrRuO3/(100)SrTiO3 substrates by pulsed-metalorganic chemical vapor deposition. The volume faction of c-domain and remanent polarization was plotted against the number of switching cycles. In the both capacitors, the Vc increased as the switching cycle increased independent of fatigue behavior.

Room-temperature epitaxy of AlN thin films on sapphire (0001) substrates was achieved by pulsed laser deposition using an epitaxial NiO ultrathin buffer layer (approximately 6 nm thick). Four-circle x-ray diffraction analysis indicates a double heteroepitaxial structure of AlN (0001)/NiO(111)/sapphire (0001) with the epitaxial relationship of AlN [10-10] ‖ NiO [11-2] ‖ sapphire [11-20]. The surface morphology of room-temperature grown AlN thin films was found to be atomically smooth and nanostepped, reflecting the surface of the ultrasmooth sapphire substrate with 0.2-nm-high steps.

BaTiO3/SrTiO3, BaTiO3/BaZrO3, and SrZrO3/SrTiO3 artificial superlattices were fabricated on SrTiO3 substrates by the molecular beam epitaxy (MBE) process. The stacking periodicity of each layer was varied from 1 unit cell to 40 unit cells, and the total thickness was fixed at 80 unit cells. In-situ reflection high-energy electron diffraction (RHEED) and x-ray diffraction (XRD) clearly shows the formation of the superlattice structure. Their lattice parameters obtained from the reciprocal space mapping measurement indicated that the lattice distortion was dependent on the stacking periodicity. The capacitance and the complex admittance of the superlattices were measured with interdigital electrodes by an impedance analyzer up to 110 MHz. It was found that the dielectric permittivity changed with the superlattice periodicity. The Q-V measurement results showed clear hysteresis curves and this suggested that ferroelectricity was induced into the SrZrO3/SrTiO3 superlattices despite a combination between paraelectrics/paraelectrics.

Pb(ZrxTi1-x)O3 (PZT) thin films with the various Zr/(Zr+Ti) ratios and the film thickness of 50 and 250 nm were epitaxiallly grown on (100)cSrRuO3//(100)SrTiO3 substrates by metalorganic chemical vapor deposition at 540 °C. 50 nm-thick tetragonal PZT films consistd of 100% c-axis-orientation for the Zr/(Zr+Ti) ratio ranging from 0.13 to 0.56, while 250 nm-thick tetragonal ones consistd of about 70% c-axis-orientation for the range from 0.19 to 0.45. On the other hand, 50 and 250 nm-thick films composed rhombohedral single phase for the Zr/(Zr+Ti) ratio above 0.56 and 0.60, respectively. Mixture region of tetragonal and rhombohedral phases was observed from 0.45 to 0.60 for 250 nm-thick films, but was not for 50 nm-thick ones. Remanent polarization monotonously decreased with increasing the Zr/(Zr+Ti) ratio for 50 nm-thick films in tetragonal PZT films. These results show that Ti rich tetragonal PZT thin films have large spontaneous polarization with good square-shape hysteresis loops and are applicable for the high-density capacitor-type ferroelectric random access memories.

a-/b-axis-oriented epitaxial bismuth layer-structured ferroelectric thin films were epitaxially grown on (101)-oriented oxide with rutile structure. The long-range lattice matching between the ferroelectric layer and the bottom rutile layer, particularly the number of rutile units facing one ferroelectric unit and the surface orientation, were discussed for (100)(010)Bi4Ti3O12//(101)TiO2 structure. Cross sectional transmission electron microscope analysis suggests that seven rutile units lie under one a-/b-axis-oriented Bi4Ti3O12 unit with lower misfit dislocation density comparing to eight rutile units by one Bi4Ti3O12 model. Based on this result, the surface orientation at the interface was simulated to give us an appropriate ion alignment model. The titanium layer in the (101)TiO2 structure is most likely to match with the oxygen layer in the a-/b-axis-oriented Bi4Ti3O12 film.

Thin films of BIT, La-substituted BIT (BLT) and La- and V-cosubstituted BIT(BLTV) were epitaxially grown on SrRuO3//SrTiO3 substrates at 850°C by metalorganic chemical vapor deposition (MOCVD), and their electrical properties were systematically compared. All films on (100), (110) and (111)-oriented substrates were epitaxially grown with (001)-, (104)-/(014)-and (118) –preferred orientations, respectively. The leakage current density of the BLTV film was almost the same with that of the BLT film, but was smaller than that of BIT film, suggesting that the La substitution contributed to the decrease of the leakage current density especially in pseudoperovskite layer. Spontaneous polarization of the BLTV film was estimated to be almost the same with the BLT film but was smaller that that of the BIT film. This is explained by the decrease of Tc with the La substitution, while V did not contribute to the change of the Curie temperature (Tc ). On the other hand, the coercive field (Ec) value of the BLTV was smaller than that of the BIT and the BLT films. As a result, La substitution contributed to the decrease of the leakage current density together with the decrease of the spontaneous polarization due to the decrease of the Tc. On the other hand, V substitution contributes to the decrease of the defects that suppress the domain motion and increases the Ec value. Therefore, each substitution of La and V plays different roles and this contribution is remarkable for the films deposited at lower temperature.

200-nm-thick Pb(Zrx,Ti1-x)O3 (PZT) thin films with zirconium composition in the range from 0% to 65% were epitaxially grown on (001)c SrRuO3 (SRO)//SrTiO3 (STO) single crystal substrates by pulsed metalorganic chemical vapor deposition (pulsed MOCVD). Constituent crystallographic phases were characterized by high-resolution X-ray diffraction reciprocal space mapping. It was found that PZT thin films having zirconium composition from 45% to 60% show mixed tetragonal and pseudocubic phases and their lattice parameters remained constant in this composition range.

The orientation dependence of the ferroelectricity of epitaxially grown Bi4Ti3O12 thin films was investigated. The (001)-, (118)-, and (104)-oriented Bi4Ti3O12 films were epitaxially grown on (100)cCaRuO3//(100)SrTiO3, (110)cSrRuO3//(110)SrTiO3, and (111)cSrRuO3//(111)SrTiO3 substrates, respectively, by metalorganic chemical vapor deposition. Ferroelectric property with different magnitude was observed for (001)- and (118)-oriented films but for (104)-oriented film due to its large leakage current. The remanent polarization and the coercive field were 1.5 mC/cm2 and 15 kV/cm, 16.5 νC/cm2 and 132 kV/cm for the (001)- and (118)-oriented thin films, respectively. The spontaneous polarization (PS) was 4.0 νC/cm2 and 27.0 νC/cm2 for (001)- and (118)-oriented films, respectively. This was different from the result of SrBi2Ta2O9 in that the ferroelectricity was not observed for (001)-oriented one, and was in good agreement with the estimation from the crystal structure. The estimated PS values along the c and a axes of Bi4Ti3O12 were 4.0 and 48.4 νC/cm2, respectively, and agreed well with the reported values for the single crystal. Furthermore, both films showed good fatigue endurance after 7.8 × 1010 switching cycles measured with 500 kHz rectangular pulses.

Pb(Zr,Ti)O3 (PZT) is one of the most promising materials for ferroelectric random access memory (FeRAM) application. Among the various preparation methods, metalorganic chemical vapor deposition (MOCVD) has been recognized as a most important one to realize high density FeRAM because of its potential of high-step-coverage and large-area-uniformity of the film quality.

In the present study, pulsed-MOCVD was developed in which a mixture of the source gases was pulsed introduced into reaction chamber with interval. By using this deposition technique, simultaneous improvements of the crystallinity, surface smoothness, and electrical property of the film have been reached by comparing to the conventional continuous gas-supplied MOCVD. Moreover, this film had larger remanent polarization (Pr) and lower leakage current density. This is owing to reevaporation of excess Pb element from the film and increase of migration on the surface of substrate during the interval time.

This process is also very effective to decrease the deposition temperature of the film having high quality. In fact, the Pr and the leakage current density of polycrystalline Pb(Zr0.35Ti0.65)O3 film deposited at 415 °C were 41.4 μC/cm2 and on the order of 10−7 A/cm2 at 200 kV/cm. This Pr value was almost the same as that of the epitaxially grown film deposited at 415 °C with the same composition corrected for the orientation difference. This suggests that the polycrystalline PZT film prepared by pulsed-MOCVD had the epitaxial-grade ferroelectric properties even through the deposition temperature was as low as 415 °C. Moreover, large “process window” comparable to the process window at 580 °C, above 150 °C higher temperature and was widely used condition, was achieved even at 395°C by the optimization of the deposition condition.

a- and b-axis-oriented epitaxial Bi4Ti3O12 films were prepared on (101)TiO2 substrates (rutile) at 600°C by metalorganic chemical vapor deposition (MOCVD). Subsequently, the same films were similarly prepared on (101)-oriented conductive materials with the same rutile structure and similar lattice parameters as TiO2, such as RuO2 and IrO2. Their perfect epitaxial growth was confirmed by several X-ray diffraction measurements. RuO2 and IrO2 were deposited on not only structurally equivalent (101)RuO2//(101)TiO2 and (101)IrO2//(101)TiO2 structures, but were also successfully deposited on the corundum (012)Al2O3 and (110)Al2O3 single crystals by the MOCVD and rf-sputtering method, respectively. A well-saturated P-E hysteresis with a remanent polarization above 20 μC/cm2 was observed for a- and b-axis-oriented Bi4Ti3O12-based materials, (Bi4−xNdx)(Ti3−yVy)O12 (BNTV), that were epitaxially grown on rutile-rutile and rutile-corundum stacking structures. These results should enable broader application of the bismuth layer-structured ferroelectrics.

Epitaxial thin films of bismuth layer-structured ferroelectrics (BLSF) with different m-numbers, i.e., Bi2VO5.5 (BVO) (m=1), SrBi2Ta2O9 (SBT) (m=2), and Bi4Ti3O12 (BIT) (m=3), were grown by metalorganic chemical vapor deposition (MOCVD). (00l)-oriented films were deposited on (100)SrTiO3. (114)-oriented BVO, (116)-oriented SBT, and (118)-oriented BIT films were deposited on (110)SrTiO3. Moreover, (102)-oriented BVO, (103)-oriented SBT, and (104)-oriented BIT films were deposited on (111)SrTiO3. On (100), (110), and (111)SrTiO3 substrates, c-axis of the deposited films was tilted about 0°, 45°, and 56°, respectively, against perpendicular to the surface of the substrates irrespective of m-number. This suggests the growth of crystallographic equivalent orientation. The distinctive surface morphology originated to the feature of the film orientation was observed. The dielectric constant and the leakage current of c-axis-oriented film was smaller than that of non-c-axis-oriented one, indicating smaller dielectric constant and leakage current along c-axis than a- or b-axes. A larger ferroelectric anisotropy was ascertained for SBT and BIT films. Furthermore, the evaluated spontaneous polarization along a- and c- axes of BIT from the data of the epitaxially grown BIT films well agreed with the reported one for the single crystal. This suggests the ferroelectric property was not strongly affected by the strain in the films.

Epitaxial (001)-, (116)- and pseudo (103)-oriented Sr0.35Bi2.2Ta2O9 (SBT (0.35/2.2/2.0)) films were successfully grown on (001), (110) and (111) SrTiO3 substrates, respectively. High-resolution X-ray diffraction reciprocal space mapping (HRXRD-RSM) measurements and pole figure measurements clearly indicated that the (116)-oriented SBT (0.35/2.2/2.0) film consisted of two growth domains those c-axis are separated 180° apart in in-plane and pseudo (103)-oriented SBT film consisted of three growth domains those c-axis are separated 120° apart in in-plane. Moreover, lattice parameter measurements indicated that SBT films grew in fully relaxed state.