Kentaro Morito

Electric field induced piezoelectric resonance in the micrometer to millimeter waveband in a thin film SrTiO3 capacitor

Thin film SrTiO3 metal–insulator–metal (MIM) capacitors were fabricated in order to characterize the piezoelectric resonance in the micrometer to millimeter waveband arising from electric field induced ferroelectricity in the SrTiO3 film. The specimens showed a second-order phase transition, and the piezoelectric resonance appeared when an electric field of 250 kV/cm was applied at room temperature. Finite element method (FEM) analysis was used to interpret the piezoelectric resonance observed in the capacitors. The FEM analysis data basically agrees well with the experimental data, and the few differences between the theoretical and experimental data are interpreted mainly as artifacts caused by overlapping of resonant/antiresonant peaks during the measurement. The piezoelectric resonance of thin film MIM capacitors is strongly influenced by the longitudinal stacked structure of the MIM.

Effect of internal residual stress on the dielectric properties and microstructure of sputter-deposited polycrystalline (Ba,Sr)TiO3 thin films

The role of internal residual stress in the dielectric properties of sputter-deposited polycrystalline (Ba,Sr)TiO3 thin films was studied experimentally using test samples in the form of parallel-plate capacitors on platinized silicon substrate. Different deposition-pressure conditions provided a systematic change in the film stress and we found that a reduction in the in-plane tensile stress of the films (primarily originating from the thermal expansion mismatch) was effective for increasing the dielectric constant in the out-of-plane direction. On the other hand, a large in-plane tensile stress (exceeding 1GPa) cannot be elastically accommodated in a 250-nm-thick (Ba,Sr)TiO3 thin film and the film relaxes through the optically visible microcrack formation. The relationship between the internal residual stress and the dielectric constant of the crack-free (Ba,Sr)TiO3 thin films could be well explained by the phenomenological thermodynamic theory of Landau–Ginsburg–Devonshire, without a transition to the ...

Electrical Characterization of SrTiO3 Thin Films Grown on Nb-Doped SrTiO3 Single Crystals

SrTiO3(100) films homoepitaxially grown on Nb-doped SrTiO3(100) substrates with various Nb concentrations and SrTiO3(111) films epitaxialy grown on the [111]-oriented Pt thin-film electrode formed on SiO2/Si substrates with Ta buffer layers were prepared by the helicon sputtering method. After the top Pt electrode formation, the obtained Pt/SrTiO3/SrTiO3:Nb metal-insulator-semiconductor (MIS) capacitors and Pt/SrTiO3/Pt/Ta/SiO2/Si metal-insulator-metal (MIM) capacitors were electrically evaluated by current versus bias voltage characteristic (I–V) and capacitance versus bias voltage characteristic (C–V) measurements to clarify the specific features of the SrTiO3/SrTiO3:Nb interface compared with the SrTiO3/Pt interface. The existence of a space charge layer at the SrTiO3/SrTiO3:Nb interface was clearly characterized.

Microstructure and Electrical Properties of Nonstoichiometric Strontium Titanate Thin Films Grown on Platinum Electrodes

Nonstoichiometric strontium titanate films with the Sr/Ti ratio ranging from 0.9 to 1.4 were grown on Pt electrodes fabricated on Si substrates (Pt/Ta/SiO2/Si) by the RF-sputtering method with low and high deposition temperatures (250°C and 600°C, respectively). The insulating and dielectric properties of nonstoichiometric SrTiO3 films have been studied as a function of Sr/Ti ratio. The highest dielectric constant was obtained by a SrTiO3 film with a slight excess of SrO independent of the deposition temperature; the film showed better insulating property in comparison with the stoichiometric specimen, while the insulating property of TiO2-excess SrTiO3 films, consisting of SrTiO3 crystalline phase and Ti excess amorphous phase, notably depended on the deposition temperature. The high-temperature deposition resulted in better insulative resistance.

Diffusion properties of point defects in barium strontium titanate thin films

The relation between the diffusion behavior of hydrogen and the electrical properties of (Ba, Sr)TiO3 thin-film capacitors was investigated using thermal desorption spectroscopy and secondary ion mass spectroscopy analysis.

Effects of Mn Dopant on the Leakage Current Properties in SrTiO3 Thin Films

With increasing Mn dopant concentration, polycrystalline Sr(Ti1-xMnx)Oy (x=0, 0.002, 0.01, and 0.05) thin films grown on Pt electrodes show a notable improvement in leakage characteristics in the high electrical field region, due to Mn dopant substitution on Ti sites as a low-valency acceptor. The addition of Mn also induces a distinct transition of the conduction mechanism, which can be clearly classified into two models; one is a Schottky-emission current mode limited by interface through thermionic electron emission, appearing at low Mn dopant concentrations up to x=0.01, and the other is a space-charge-limited current mode strongly dependent upon the inherent conductivity in thin-film materials, observed only in highly doped thin films with an Mn concentration of x=0.05. This mode transition is attributable to the improvement in the insulating property of Sr(Ti1-xMnx)Oy thin films, possibily through the oxygen-vacancy compensation of Mn acceptor dopant.

THE LATEST ADVANCES IN HIGH-DIELECTRIC THIN-FILM CAPACITOR TECHNOLOGY FOR GHz-RF DEVICES

ABSTRACT We present parallel-plate (Ba,Sr)TiO3 and SrTiO3 thin-film capacitor technology on silicon for RF devices. The in-plane thermal stress that is critical to low expansive Si substrates can be controlled by tuning the sputtering deposition pressure, and a corresponding change in the permittivity due to the electrostrictive effect is observed. The high-frequency dielectric response analysis combined with the equivalent circuit model and electromagnetic simulation enables accurate estimation of the intrinsic dielectric properties, thus demonstrating the linear increase of dielectric loss with frequency. With the bias applied, an electromechanical resonance appears in the GHz band due to piezoelectric effect, which is reproduced using the finite element method.

Diffusion Properties of Point Defects in Barium Strontium Titanate Thin Films

The relationship between the diffusion behavior of hydrogen and the electrical properties of (Ba, Sr)TiO3 (BST) thin-film capacitors was investigated using thermal desorption spectroscopy and secondary ion mass spectroscopy analyses. It has been clearly shown that the frequency dependence of the complex impedance profile of the BST thin-film capacitors could be successfully represented by two parallel resistor-capacitor (RC) electrical equivalent networks in series correlated with the distribution of the hydrogen, namely, the Pt/BST interface region with the influence of hydrogen and the BST bulk region without the influence of hydrogen. However, the I-V properties of the BST thin-film capacitors could be determined almost from the hydrogen atoms existing at the Pt/BST interface.

INFLUENCE OF HYDROGEN ON ELECTRICAL PROPERTIES OF Pt/(Ba,Sr)TiO3/Pt THIN FILM CAPACITORS

ABSTRACT The influence of hydrogen on the electrical properties of Pt/(Ba,Sr)TiO3/Pt thin film capacitors have been studied by secondary ion mass spectroscopy (SIMS) and thermal desorption spectroscopy (TDS) methods. From the analyses, it is suggested that in a BSTO thin film grown by RF sputtering, there exist two types of hydrogen with different energy states. One stays thermally stable in BSTO even at 600°C, and the other is thermally unstable and is released from BSTO above 450°C. When the Pt/BSTO/Pt capacitors were exposed to hydrogen-containing plasma, a pronounced hydrogen accumulation was observed at the Pt/BSTO interface in the SIMS profiles. The concentration of accumulated hydrogen at the interface is identical to that of the thermally unstable hydrogen that is released above 450°C. It is experimentally confirmed that thermally unstable hydrogen degrades the electrical properties of Pt/BSTO/Pt thin film capacitors, and its degradation mechanism is proposed.

Hysteresis Behavior of Capacitance–Voltage Curve in (Ba0.6Sr0.4)TiO3 Thick Films Caused by Strained Heterostructure

(Ba0.6Sr0.4)TiO3 (BST) films with 300–1700 nm thicknesses were deposited on SrTiO3 (STO), 0.5 wt % Nb-doped STO and (La0.5Sr0.5)CoO3-coated STO substrates by pulsed laser deposition. X-ray diffraction analysis results indicate that the deposited BST thick films were epitaxially grown on the substrates and c-lattice parameter of BST was increased compared with that of the bulk material owing to stress in the film. The fabricated film capacitors showed a hysteresis curve in the capacitance–voltage curve at room temperature. Hysteresis width linearly increased with increasing BST c-lattice parameter.