Yasuto Yonezawa

Preparation of Pb(Zn0.52Ti0.48)O3 Films by Laser Ablation

Pb(Zr0.52Ti0.48)O3 films were prepared on an r-plane sapphire substrate by laser ablation using the ArF excimer laser. This is the first report on the preparation of Pb(Zr0.52Ti0.48)O3 films by laser ablation. The composition of the deposited films was found to be fairly close to the composition of the target material for a wide range of the substrate temperature, 400–750°C. The substrate temperature greatly influences the crystal structure; a low substrate temperature produces a pyrochlore phase and a high substrate temperature (750°C) a perovskite phase.

Highly Oriented Pb(Zr, Ti)O3 Thin Films Prepared by Pulsed Laser Ablation on GaAs and Si Substrates with MgO Buffer Layer

Highly [100]-oriented Pb(Zr, Ti)O 3 (PZT) films were prepared on (100) GaAs and (100) Si substrates with MgO buffer layer by pulsed laser ablation (PLA). The depth profile of the constituent elements observed by X-ray photoelectron spectroscopy (XPS) shows that there are no remarkable interdiffusion and/or no formation of an alloying layer at both interfaces between PZT and MgO and between MgO and GaAs substrate. The [100]-oriented perovskite PZT films which exhibit the ferroelectric hysteresis loop were obtained on (100) Si substrate with MgO buffer layer with the thickness of only 50 A, showing that this technique promises the realization of metal-ferroelectric-semiconductor field-effect transistors (MFS-FETs).

Thermal Analysis of Target Surface in the Ba-Y-Cu-O Film Preparation by Laser Ablation Method

For clarifying the mechanism of material removal from the surface of Ba-Y-Cu-O by a high-intensity pulsed laser beam, a transient temperature distribution was calculated, and the obtained results were compared with the experimental ones. The calculated values well explain the experimental results such as a threshold of the Ba-Y-Cu-O evaporation. It was found that the subsurface temperature exceeds the front surface temperature. Under such conditions, an explosive removal of material can occur resulting in very rapid material removal with no compositional deviation between the target and the deposited film.

Annealing Temperature Dependence of MgO Substrates on the Quality of YBa2Cu3Ox Films Prepared by Pulsed Laser Ablation

The effect of annealing on a MgO substrate was systematically investigated, varying the annealing temperature, for preparation of high-quality YBa2Cu3Ox (YBCO) films by pulsed laser ablation. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were carried out for characterization of the MgO substrate, and X-ray diffraction (XRD) measurement, including θ-2θ scan, ω-scan (rocking curve) and -scan, and electrical measurement were carried out for characterization of YBCO films. Thermal annealing of MgO above 1000° C in oxygen ambient was found to give rise to a great improvement of the crystallinity and the reproducibility of crystal orientation of YBCO films, while it was also found to induce Ca segregation on the surface of MgO and reduce the oxygen content of YBCO films. Annealing below 1000° C causes no outstanding improvement in the crystal structure of YBCO. On the MgO substrate annealed at 1200° C c-axis-oriented YBCO film was found to reproducibly show a full width at half-maximum (FWHM) of the rocking curve of 0.3°, an FWHM of (005) diffraction of 0.1°, an epitaxial relation without in-plane misorientation, a critical zero temperature of 89 K, and a critical current density of 106 A/cm2 at 77.4 K and zero field.

NH3-Plasma-Nitridation Process of (100) GaAs Surface Observed by Angle-Dependent X-Ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy with varying the photoelectron take-off angle reveals the surface-nitridation process of (100) GaAs by an rf NH3 plasma with a magnetic field. The plasma treatment for shorter time or at lower temperature leads to the formation of a Ga-As-N ternary-compound layer on the GaAs surface. Increasing the treatment time or treatment temperature changes the main part of the surface layer into GaN due to the desorption of As. The oxidation resistance is also examined, showing that this plasma-nitridation method is one of the promising technologies for the passivation of (100) CaAs surface.

Crystallization Induced by Laser Irradiation in Ba-Y-Cu-O Superconducting Films Prepared by Laser Ablation

Ba2YCu3Ox superconducting films were prepared by laser ablation with some newly developed laser processing techniques. First, we revealed that a rapid postannealing by laser irradiation converts an amorphous film to a crystallized superconducting one. Second, we tried the in situ preparation without a high-temperature postannealing. To reduce the substrate temperatures Ts for the film preparation, we examined the effect of the laser irradiation on the growing film surface. We found that in situ excimer laser irradiation of the growing film surface improves the superconducting properties and the surface morphology of the films prepared at a relatively low Ts. This improvement is brought about mainly by the enhancement of the film crystallization, rather than by providing additional oxygen atoms to the film.

Spectroscopic Study on N2O-Plasma Oxidation of Hydrogenated Amorphous Silicon and Behavior of Nitrogen

A novel oxidation process in hydrogenated amorphous silicon (a-Si:H) utilizing nitrous oxide (N2O) plasma was established and studied in detail. The interfacial neutral defect density for the sample prepared in this process at 300°C is greatly reduced compared with that obtained using O2 plasma. The distribution and behavior of N incorporated in this oxide was examined in detail by means of X-ray photoelectron spectroscopy (XPS). As a result, the accumulation of N near the SiO2/a-Si:H interface was confirmed. At low oxidation temperature, the N bonded to O exists near the oxide surface. The reasons for the reduction of the interfacial neutral defect density and for the accumulation of N near the interface are also shown. It is also clarified that this process utilizes the advantages of both the effect of atomic O and the accumulation of N near the interface.

Highly Moisture-Resistive SiNx Films Prepared by Catalytic Chemical Vapor Deposition

Silicon nitride (SiNx) films on Si and poly(ethylene terephthalate) (PET) substrates were prepared at approximately 150°C by catalytic chemical vapor deposition (Cat-CVD), using a SiH4/NH3 gas mixture. A water vapor transmission rate as low as 0.2 g/m2day and an O2 gas transmission rate of 0.6 cm3/m2day were achieved for a stoichiometric Si3N4 film of 77 nm thickness. Although these transmission rates depended on N/Si ratio, no optical absorption was observed under preferable deposition conditions.

Preparation of SiNx passivation films for PZT ferroelectric capacitors at low substrate temperatures by catalytic CVD

The feasibility of SiNx films prepared by catalytic chemical vapor deposition (Cat-CVD) at low substrate temperatures was studied for passivation of ferroelectric non-volatile random access memories (FRAMs). First, the influence of exposure to active NH3 gas generated by the heated catalyzer on ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) capacitors was examined. Second, SiNx films were prepared by Cat-CVD at low substrate temperature, at which the ferroelectricity of PZT is not degraded. The ferroelectric degradation of PZT capacitors due to exposure to active NH3 gas strongly depended on the sample temperature and the ambient. However, no degradation occurred when keeping the sample temperature below 200°C at an ambient of 1.3 Pa by controlling the heat flow from the catalyzer. By adjusting the flow rate ratio of SiH4/NH3, the refractive index of SiNx films measured by ellipsometry was controlled to be 2.0 for various substrate temperatures. The dense SiNx films, which were resistive to oxidation in air exposure, were prepared at 200°C at an ambient of 1.3 Pa. The ferroelectric PZT capacitors were not degraded during SiNx film deposition using the Cat-CVD method. The results appeared to demonstrate the feasibility of application of Cat-CVD films to passivation of ferroelectric devices.

Novel oxidation process of hydrogenated amorphous silicon utilizing nitrous oxide plasma

A novel oxidation process in hydrogenated amorphous silicon (a‐Si:H) using nitrous oxide (N2O) plasma was studied in detail for the first time. The N2O‐plasma oxidized a‐Si:H has an excellent interface whose interfacial defect density is largely reduced compared with the O2‐plasma oxidized a‐Si:H. It was elucidated that this oxide layer has almost stoichiometric composition and contains a small amount of N piling up at the interface between the oxide layer and a‐Si:H layer. It also turned out that this process has less ion damage than the O2‐plasma oxidation process. The reason for the reduction of the interfacial defect density is attributed to the presence of N at the interface and/or less ion damage in this process.