Norifumi Fujimura

Control of preferred orientation for ZnOx films: control of self-texture

Abstract We tried to control preferred orientation of ZnOx films deposited by radio frequency (RF) magnetron sputtering, and to make the growth mechanisms clear. Zinc oxide has tetrahedral coordinates caused by sp3 hybridized orbits, and the (0001) plane has the lowest surface free energy. Therefore, the film grows with strong (0001) preferred orientation even on glass. Considering the formation of tetrahedral coordination in the vapor phase and the deposition rate, however, we succeeded to control the preferred orientation of ZnOx films on glass. The (1120) textured film was obtained under sputtering gas composition which deteriorates the formation of tetrahedral coordination in the vapor phase and the high deposition rate. Formation of each texture is strongly related to the formation of tetrahedral coordination in the vapor phase and on the substrate during sputtering. Therefore, (1120) textured film had higher carrier concentration than that of the (0001) textured film caused by existing excess Zn atoms. Moreover, the growth mechanism with considering the density of surface energy, and the applications of the control for the epitaxial growth, are discussed.

EPITAXIALLY GROWN YMNO3 FILM : NEW CANDIDATE FOR NONVOLATILE MEMORY DEVICES

We have proposed ReMnO3 (Re:rare earth) thin films as a new candidate for nonvolatile memory devices. In this letter, we report on fabrication of (0001) YMnO3 films on (111)MgO, (0001)ZnO:Al/(0001) sapphire, and (111)Pt/(111)MgO using rf magnetron sputtering. We succeeded in obtaining (0001) epitaxial YMnO3 films on (111) MgO and (0001)ZnO:Al/(0001) sapphire substrate, and polycrystalline films on (111)Pt/(111)MgO. The dielectric properties of the epitaxial and polycrystalline YMnO3 films are almost the same. The dielectric permittivities of both films are smaller than those reported for YMnO3 single crystal.

Magnetic and magneto-transport properties of ZnO:Ni films

Abstract Magnetic semiconductor (MS), Ni doped ZnO film was fabricated by pulsed laser deposition method on sapphire (0 0 0 1) substrates. Ni dissolves until 25 at % into ZnO by low temperature growth technique. Lattice constant c once increases with increasing Ni content and has maximum point at the Ni content of 5 at % , and then it suddenly decreases. In all the Ni content range, the films exhibits n-type conduction. With increasing the Ni content, the carrier density and mobility decrease. ZnO films with the Ni content range from 3 to 25 at % exhibit ferromagnetic behavior at 2 K . At 30 K , the magnetization against applied magnetic field shows superparamagnetic behavior and it maintains at least up to 300 K . To study the effect of carrier density, Al was additionally doped. The effect of carrier density and Ni content on the structure, magnetic and magneto-transport behaviors are described.

Ferroelectric properties of YMnO3 epitaxial films for ferroelectric-gate field-effect transistors

Ferroelectric properties of YMnO3 epitaxial films were studied. The ferroelectric properties of epitaxially grown (0001) YMnO3 films on (111)Pt/(0001)sapphire (epi-YMO/Pt) with an excellent crystallinity were compared to (0001)-oriented poly crystalline films on (111)Pt/ZrO2/SiO2/Si. The epi-YMO/Pt had saturated polarization–electric-field (P–E) hysteresis loops, with a remanent polarization (Pr) of 1.7 μC/cm2 and a coercive field (Ec) of 80 kV/cm. The fatigue property showed no degradation up to 1010 measured cycles. These results suggested that the YMnO3 epitaxial films were suitable ferroelectric material for the ferroelectric-gate field-effect transistors. Consequently, epitaxially grown (0001)YMnO3 films on epitaxial Y2O3/Si (epi-YMO/Si) were fabricated. The epi-YMO/Si capacitor had almost equivalent crystallinity compared to epi-YMO/Pt. It was recognized that the epi-YMO/Si capacitor exhibited the ferroelectric type C–V hysteresis loop with the width of the memory window of 4.8 V, which was almost i...

Effect of Ce doping on the growth of ZnO thin films

Recently, ZnO films doped by trivalent elements have attracted much attention for use in transparent conductive films. The impurity doping to control the carrier concentration generally causes degradation of crystallinity and preferred orientation which leads to the deterioration of electrical characteristics. In this paper, the effects of various doping trivalent elements on the growth process and change in carrier concentration in thin films are discussed. We find that the deteriorated crystallinity of ZnO film by Al doping is improved by Ce doping.

Growth mechanism of YMnO3 film as a new candidate for nonvolatile memory devices

We have proposed ReMnO3 (Re: rare earth) thin films for nonvolatile memory devices. We examine the growth mechanism of YMnO3 films on (0001)ZnO:Al/(0001) sapphire substrate using rf magnetron sputtering and pulsed laser deposition methods with oxide compound target. We have succeeded in obtaining (0001) epitaxial YMnO3 films on (111) MgO and (0001)ZnO:Al/(0001) sapphire substrate, and polycrystalline films on (111)Pt/(111)MgO. For an optimal structure, the film needed much less oxygen from the gas phase compared to other oxide films. The composition (Y/Mn ratio) of the YMnO3 films changed drastically by varying the partial oxygen pressure in the sputtering gas. In addition the Y/Mn ratio slinfled with sputter time due to target surface modification, probably caused by Y segregation. An extremely small amount of oxygen is required to form the YMnO3 crystal. This was confirmed by pulsed laser deposition experiments.

Structural, dielectric, and magnetic properties of epitaxially grown BaFeO3 thin films on (100) SrTiO3 single-crystal substrates

We have prepared single-crystalline films of BaFeO3, which may contain high valent Fe4+ ions, on (100) SrTiO3 substrates by pulsed laser-beam deposition. The reflection high-energy electron diffraction patterns for the films apparently show a C4 symmetry operation, suggesting that the sample had a pseudocubic perovskite structure rather than hexagonal unit cell. The films were found to be highly insulating, and showed a high value of dielectric constant of e=59. At 300 K, the magnetization loop of the film apparently shows hysteresis, as well as small remanent magnetization. These characteristics are totally different from those of bulk BaFeO3.

Characterization of ferroelectricity in metal/ferroelectric/insulator/semiconductor structure by pulsed C–V measurement; Ferroelectricity in YMnO3/Y2O3/Si structure

Ferroelectric properties of the metal/ferroelectric/insulator/semiconductor (MFIS) structure were investigated using the Pt/YMnO3/Y2O3/Si structure. The ferroelectric C–V hysteresis was observed for not only the Pt/YMnO3(0001)/Y2O3/Si capacitor but also the Pt/amorphousYMnO3/Y2O3/Si capacitor. The polarization evaluated by conventional C–V measurement should include interfacial polarization and rearrangement of the space charge together with the spontaneous polarization by ferroelectricity. To eliminate the generation of the interfacial polarization and the rearrangement of the space charge, the shorter charging time should be used to evaluate the ferroelectricity of the MFIS capacitor. Therefore, we propose the pulsed C–V measurement as a new method for evaluating the MF(I)S capacitor.

FERROELECTRIC PROPERTIES OF C-ORIENTED YMNO3 FILMS DEPOSITED ON SI SUBSTRATES

We have proposed the use of RMnO3 (R: rare earth elements) films for metal–ferroelectric–semiconductor field effect transistor (MFSFET)-type ferroelectric random access memories (ferroelectric RAMs). This reports the production of YMnO3 films on Si substrates for MFSFET with confirmation of the distinct ferroelectricity by P–E hysteresis and capacitance–voltage (C–V) measurement. (0001)-oriented YMnO3 films were obtained on a (111)Si substrate using a pulsed-laser deposition method. Although the Pt/YMnO3/Si structure exhibits a very small remnant polarization of 1.2 nC/cm2, it has clear ferroelectric polarization switching type C–V characteristics with a memory window of 1.1 V. The dielectric constant and the dissipation factor were 27.8 and 0.035, respectively. The polarization switching characteristics are discussed.

Electro-optic property of ZnO:X (X=Li,Mg) thin films

Abstract We have proposed an application of ZnO:X (X=Li,Mg,Ni,Al etc.) films for monolithic optical integrated circuits (OICs) (Mat. Res. Soc. Symp. Proc. 574 (1999) 317). Although non-doped ZnO has an electro-optic effect, it is only a Pockel’s effect. The electro-optic effect of Pb(Zr,Ti)O 3 (Jpn. J. Appl. Phys. 34 (1995) 5091) is superior to ZnO, because that is caused by a non-linear Kerr effect. Our group demonstrated that Li-doped ZnO (ZnO:Li) films exhibited ferroelectric behavior (Appl. Phys. A, in press). ZnO with ferroelectricity should have a non-linear electro-optic effect against the applied voltage. In this paper, to design the ZnO monolithic slab waveguide for electro-optical switch, the refractive indices of top and bottom electrode layers and core layer were investigated. Then, electro-optical property of ZnO:Li,Mg films was evaluated, and the possibility of applying to an optical switch was also discussed.