M. Tanamura

Crystallographic and Electrical Properties of Epitaxial BaTiO3 Film Grown on Conductive and Insulating Perovskite Oxides

BaTiO3 films of various thicknesses were grown on doped and undoped SrTiO3(100) substrates and on (La, Sr)2CuO4 films by a pulse laser deposition method. Both BaTiO3 and (La, Sr)2CuO4 films were smooth and free of particulates. X-ray diffractometry showed that all materials were three-dimensionally aligned with the cube-on-cube epitaxial relationship, and that the films obtained on SrTiO3(100) were of a c-axis-oriented tetragonal phase contrary to previous studies using metal bottom electrodes. As the film thickness decreased, the tetragonality of these films increased and the dielectric constant along the c-axis decreased rapidly, despite reduction of disorder at the interface by use of perovskite electrodes. The 2000-A-thick BaTiO3 grown on (La, Sr)2CuO4 film had the remnant polarization of about 1.5 µ C/cm2. The results were discussed using a surface layer model.

Memory retention and switching speed of ferroelectric field effect in (Pb,La)(Ti,Zr)O3/La2CuO4:Sr heterostructure

We report long period memory retention characteristics and switching speed of (Pb,La)(Ti,Zr)O 3 /La 2 CuO 4 field effect devices previously proposed by one of the authors. Most of the devices retained one-half of their initial conductance modulations induced by the ferroelectric field effect for about two weeks. Some retained memory for over 10 months, which markedly surpasses previously reported performances for the memory retention of ferroelectric field effect devices. The switching time of devices having effectively a 200-μm-long and 40-μm-wide gate area reduced to 10 μs, as the conductance between the source and drain increased. Such a short switching time is favorable for interpretation of the conductance modulation based on the field effect. Approaches to high density integration are discussed.

A Preliminary Experimental Study on Dielectric Properties of Metal/ (Pb, La)(Ti, Zr)O3/Semiconductive Perovskite Heterostructures

(Pb, La)(Ti, Zr)O 3 films were epitaxially grown on various semiconductive perovskites including (La, Sr) 2 CuO 4 , (La, Sr)MnO 3 , (La, Sr)FeO 3 , (La, Sr)CoO 3 , LaNiO 3 , (Nd, Ce) 2 CuO 4 films, and Sr(Ti, Nb)O 3 . Excellent alignments of the materials in the heterostructures were confirmed by X-ray diffractometry. Using the heterostructures, effects of the carrier type, the carrier concentration, and the mobility on the ferroelectric and dielectric properties were examined. In these heterostructures, the depolarization instability was so weak that it could not be detected in their polarization hysteresis loops.

FERROELECTRIC/(LA,SR)2CUO4 EPITAXIAL HETEROSTRUCTURE WITH HIGH THERMAL STABILITY

Three‐dimensionally aligned epitaxial (Pb,La)(Zr,Ti)O3/(La,Sr)2CuO4 multilayers were grown on SrTiO3 (100) single crystals by pulse laser deposition. A cube‐on‐cube epitaxial relationship of these multilayers was confirmed by the θ‐2θ diffraction profile and the pole figure. The stability of their interfaces was confirmed by depth profiles and x‐ray diffractometry of the as‐deposited and the annealed multilayers having 100–300‐A‐thick (La,Sr)2CuO4 layers. The results suggest that the multilayers can be applied to the ferroelectric field‐effect transistor.

LASER POWER DEPENDENCE OF PARTICULATE FORMATION ON PULSE LASER DEPOSITED FILMS

The density and the size of particulates in films laser‐deposited at room temperature using various target materials were observed to depend strongly on the target material and the laser power density. However, loose universal relations between the deposition rates and the particulate density as well as the particulate size were found, where the latter corresponds approximately to the ratio of the laser power density to the ablation threshold. Furthermore, particulates consisting of only some of the target elements such as CuOx were found. Additionally, an acceptably high deposition rate was obtained by using halide and sulfide targets. These materials offer a possibility of deposition using a low power laser.

Ferroelectrics / (La,Sr)2CuO4 epitaxial hetero-structure and hysteretic diode property

Abstract Three dimensionally aligned epitaxial (Pb,La)(Zr,Ti)O 3 / (La,Sr) 2 CuO 4 multi-layers were grown on SrTiO 3 (100) single crystal by a pulse laser deposition. Reproducible current-voltage hystereses and memmory effect were observed in the diodes characteristics.

Epitaxial Stress Effect on Crystallographic Properties of Bi4Ti3O12 Heterostructures and Their Leakage Current Behaviors

Three dimensionally aligned epitaxial Bi4Ti3O12 films were grown on SrTiO3 (001) and LaAlO3 (001) substrates with and without a semiconductive La2CuO4 bottom layer. The leakage current and crystallographic properties of Bi4Ti3O12 were found to be markedly different from ABO3-type ferroelectric such as Pb(Zr, Ti)O3, BaTiO3, and SrTiO3. The d-spacings of Bi4Ti3O12 depend markedly on the substrate material, suggesting that the amount of misoriented domains formed to relieve epitaxial stress was less than that in ABO3-type ferroelectrics. This epitaxial stress effect was observed even in relatively thick films 1500 A thick. Furthermore, the time dependence of the leakage current in Bi4Ti3O12 was substantially reduced compared to those in the ABO3-type ferroelectrics.

Compositional distribution of laser‐deposited films and rapid sequential pulsed laser deposition

Compositional distributions of films laser deposited in vacuo at energy densities <1 J/cm2 were found to be partly different from previous observations and theories. Analyzing them, we have inferred that evaporation processes at low energy densities contain decomposition of the target materials and evaporation of the decomposed materials. Based on these analyses, we have concluded that pulsed laser codeposition was one of best pulsed laser deposition methods. To realize this using one laser source, a modified version of pulsed‐laser‐deposition rapid‐sequential‐pulsed‐laser deposition, is introduced. The dependence of YBa2Cu3O7 films properties on deposition conditions is discussed. Elimination of particulates is demonstrated and good electrical and crystallographic properties as well as suppression of precipitates were achieved in films having the correct stoichiometric composition.

Macroscopically and atomically smooth YBa2Cu3O7 film grown by reactive rapid sequential pulse laser deposition

Abstract A modified version of pulsed laser deposition : R apid S equential P ulsed L aser D eposition (RS-PLD) is proposed. The 500 A thick YBa 2 Cu 3 O 7 (YBCO) films with Tc =80∼90 K were grown in-situ by sequentially irradiating metal targets. Typically, no particulates were observed in the surface area of 100 μm by 100 μm. Fittings to the Grazing Incidence X-ray reflectance (GIXR) gave the surface roughness of ±1.5 A for those on MgO and ±9A for those on SrTiO 3 .

Element sensitive atom beam etching based on projectile mass

Abstract A new approach of ion/atom beam etching is proposed, which utilizes the projectile mass dependence of the sputtering process. As a model experiment, we etch YBa2Cu3O7−δ films using inert gases having different masses. Various etching characteristics are found to depend evidently on the mass of the gases. Substantial improvements such as 2.5-fold increase of the etching selectivity and the reduction of the etching damage of the etched area are demonstrated by using Kr. A primitive semi-empirical extension of Sigmund theory is proposed, which explains the results indeed as a mass effect and suggests the directions of further improvements. The results show a new important factor in plasma and ion/atom beam etching, which can be useful for realizing artificial structures of metal oxides.