Hideshi Yamaguchi

Characterization of (Bi3.25Nd0.75)Ti3O12 Thin Films with a- and b-Axis Orientations Deposited on Nb:TiO2 Substrates by High-Temperature Sputtering

a- and b-axis-oriented (Bi3.25Nd0.75)Ti3O12 (BNT-0.75) films, 3.0 µm thick, were fabricated on conductive Nb:TiO2(101) substrates with 0.001–0.79 mass % Nb at 650 °C by high-temperature sputtering. All the films had a mostly single-phase orthorhombic structure and a- and b-axis orientations. The degree of a- and b-axis orientations was high, with values of ≥96%. BNT-0.75 films grown heteroepitaxially on Nb:TiO2(101) substrates containing 0.79 mass % Nb were comprised of nanoplate-like crystals and exhibited the best hysteresis loop shapes, with a remanent polarization (2Pr) of 29 µC/cm2 and a coercive field (2Ec) of 297 kV/cm.

Fabrication of High-Density (Bi,La)(Zn,Mg,Ti)O3–PbTiO3 Solid Solutions with Ferroelectric and Piezoelectric Functionalities by Microstructural Control

The Bi(Zn0.5Ti0.5)O3–PbTiO3 solid solution was modified by the replacement of A- and B-site cations in the perovskite crystal with lanthanum and magnesium so as to realize ferroelectric and piezoelectric functionalities. This substitution reduces the covalency and increases the ionic bonding in the crystal. The tetragonal perovskite solid solution with composition 0.33(Bi0.7La0.3)(Zn0.15Mg0.35Ti0.5)O30.67PbTiO3 was demonstrated to exhibit ferroelectric and piezoelectric functionalities with high Curie temperature (Tc = 300 °C) and parameters of relative permittivity (e33T/e0 = 410), remanent polarization (Pr = 30 µC/cm2), coercive field (Ec = 47 kV/cm), piezoelectric coefficient (d33 = 94 pC/N), and tolerance factor (t = 0.993). This material is thus suitable for use as a high-temperature piezoelectric with performance comparable to that of Pb(Zr,Ti)O3 (PZT).

Development of ferroelectric RAM (FRAM) for mass production

We have developed ferroelectric capacitor fabrication technique and a new sensing amplifier circuit to realize low-voltage and high-density FRAM. Improvement of IrO, top electrode near the ferroelectric interface successively lowers operation voltage. And our developed “Dual Reference Sensing Amplifier” enables to commercialize highly-reliable FRAM with memory density of 4Mb or larger.

Growth of ferroelectric bismuth lanthanum nickel titanate thin films by rf magnetron sputtering

The epitaxial growth, structural properties, and ferroelectric properties of bismuth lanthanum nickel titanate (Bi1−xLax) (Ni0.5Ti0.5)O3 (BLNT) thin films deposited on Pt(100)∕MgO(100) substrates by rf magnetron sputtering have been investigated using x-ray diffraction, transmission electron microscope, and polarization–electric field hysteresis loop measurements. The ferroelectric BLNT(00l) phase with c-axis orientation and a single-phase tetragonal perovskite structure appeared at x⩾0.3. The tetragonality (c∕a) increased significantly from 1.004 to 1.028 with increasing La content. The fabricated BLNT films of x⩾0.3 indicated the apparent fourfold rotational symmetry observed for a MgO(202) substrate, a bottom Pt(202) electrode, and a BLNT(101) ferroelectric film, based on ϕ scan measurements. These results imply that the present La-substituted BLNT films are grown heteroepitaxially at x⩾0.3. It was confirmed that Bi in the BLNT films is in a trivalent state at A sites in the perovskite crystal, based o...

Growth of Perovskite (Bi, Ln)(Ni0.5Ti0.5)O3 Thin Films by RF Magnetron Sputtering

The epitaxial growth and formation mechanism of the partially La- and Nd-substituted perovskite structures, and the ferroelectric properties of bismuth lanthanoid nickel titanate [(Bi1-xLax)(Ni0.5Ti0.5)O3; BLNT and (Bi1-xNdx)(Ni0.5Ti0.5)O3; BNNT] based solid solution films deposited on Pt(100)/MgO(100) substrates by rf sputtering have been investigated using X-ray diffraction, transmission electron microscope, and polarization–electric field hysteresis loop measurements. BLNT samples at x≥0.3 and BNNT samples at x≥0.4 were confirmed to have a single-phase perovskite structure. This small difference is speculated that it is related to metal–oxygen bond dissociation energy. The sample substituted with La exhibited the best hysteresis loop at x = 0.5 with a remanent polarization of Pr = 12 µC/cm2 and the sample substituted by Nd exhibited the best hysteresis loop at x = 0.4 with a remanent polarization of Pr = 2 µC/cm2.

Suppression of gate leakage current in in-situ grown AlN/InAlN/AlN/GaN heterostructures based on the control of internal polarization fields

This paper investigates the gate leakage characteristics of in-situ AlN capped InAlN/AlN/GaN heterostructures grown by metal-organic vapor phase epitaxy. It was revealed that the leakage characteristics of AlN capped InAlN/AlN/GaN heterostructures are strongly dependent on the growth temperature of the AlN cap. For an AlN capped structure with an AlN growth temperature of 740 °C, the leakage current even increased although there exists a large bandgap material on InAlN/AlN/GaN heterostructures. On the other hand, a large reduction of the gate leakage current by 4–5 orders of magnitudes was achieved with a very low AlN growth temperature of 430 °C. X-ray diffraction analysis of the AlN cap grown at 740 °C indicated that the AlN layer is tensile-strained. In contrast to this result, the amorphous structure was confirmed for the AlN cap grown at 430 °C by transmission electron microscopy. Furthermore, theoretical analysis based on one-dimensional band simulation was carried out, and the large increase in two...

Ferro- and piezoelectric properties of (Bi 3.25 Nd 0.75 )Ti 3 O 12 nanoplates epitaxially grown on Nb:TiO 2 (101) substrates by sputtering

Epitaxially a- and b-axis-oriented (Bi<inf>3.25</inf>Nd<inf>0.75</inf>)Ti<inf>3</inf>O<inf>12</inf> (BNT-0.75) films and nanoplates have been fabricated on four Nb:TiO<inf>2</inf>(101) substrates with 0–0.79 mass% Nb by high-temperature sputtering. It is shown from piezoresponse scanning force microscopy (PFM) measurements that the effective piezoelectric coefficients (d<inf>33</inf>) at room temperature derived from the two linear gradients for the asymmetric displacement-voltage hysteresis loop are 11–13 pm/V.

Fabrication and Characterization of Binary Piezoelectric (Bi1/2Na1/2)TiO3–Ba(Cu1/3Nb2/3)O3 Solid Solutions

Perovskite-structured solid solutions with compositions of 0.9963(1-x)(Bi1/2Na1/2)TiO3–xBa(Cu1.1/3Nb2/3)O3 + 0.0037MnO2, with x = 0–0.06 [abbreviated as (1-x)BNT–xBCN)], were fabricated by normal sintering, and their structural, and piezo- and ferroelectric properties were investigated in detail. The X-ray diffraction profiles of the (1-x)BNT–xBCN solid solutions with x = 0–0.06 suggested that the rhombohedral-tetragonal morphotropic phase boundary (MPB) in this material system is in the compositional region x = 0.0425–0.0460. The cross-sectional transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) patterns of the (1-x)BNT–xBCN samples with x = 0.0475 suggested that the present materials have a modulation structure arranged by a double period along the [111] direction. It is shown that the tetragonal BNT–BCN (x = 0.0475) solid solution with a composition of 0.9490BNT0.0473BCN0.0037MnO2 near the MPB has a piezoelectric coefficient (d33), a relative permittivity (e33T/e0), and a remanent polarization (Pr) of approximately 1.8, 3.2, and 2.8 times larger, respectively, than those of the BNT solid solution without BCN substitution.

Control of La-doped Pb(Zr,Ti)O3 crystalline orientation and its influence on the properties of ferroelectric random access memory

We investigated the crystallization mechanisms of sputter-deposited La-doped Pb(Zr,Ti)O3 (PLZT) on a Pt/Ti metal stack in the postdeposition annealing (PDA) at 600 °C in O2-mixed Ar ambient. As-deposited amorphous PLZT generally transforms to a perovskite phase over 550 °C through a metastable pyrochlore phase during the PDA. We found that the O2 content of the PDA ambient crucially affects the pyrochlore-perovskite transformation (PPT) speed. While an O2 content much higher than 2% of the PDA ambient suppresses PPT, an O2 content much lower than 2% enhances PPT. An O2 content around of 2% of the PDA suppresses PPT near the surface of PLZT and simultaneously keeps PPT fast in the inner regions of PLZT in the pyrochlore phase because of the O2 diffusion limit from the PLZT surface, eventually resulting in almost only the growth of highly {111} oriented columnar PLZT on Pt, which reveals better electric properties than those obtained by the PDA with the ambient of O2 contents much higher or lower than 2%.

Fabrication and Characterization of Binary Piezoelectric (Bi

Perovskite-structured solid solutions with compositions of 0.9963(1-x)(Bi1/2Na1/2)TiO3–xBa(Cu1.1/3Nb2/3)O3 + 0.0037MnO2, with x = 0–0.06 [abbreviated as (1-x)BNT–xBCN)], were fabricated by normal sintering, and their structural, and piezo- and ferroelectric properties were investigated in detail. The X-ray diffraction profiles of the (1-x)BNT–xBCN solid solutions with x = 0–0.06 suggested that the rhombohedral-tetragonal morphotropic phase boundary (MPB) in this material system is in the compositional region x = 0.0425–0.0460. The cross-sectional transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) patterns of the (1-x)BNT–xBCN samples with x = 0.0475 suggested that the present materials have a modulation structure arranged by a double period along the [111] direction. It is shown that the tetragonal BNT–BCN (x = 0.0475) solid solution with a composition of 0.9490BNT0.0473BCN0.0037MnO2 near the MPB has a piezoelectric coefficient (d33), a relative permittivity (e33T/e0), and a remanent polarization (Pr) of approximately 1.8, 3.2, and 2.8 times larger, respectively, than those of the BNT solid solution without BCN substitution.