Naoki Kanda

Vapor–liquid–solid tri-phase pulsed-laser epitaxy of RBa2Cu3O7−y single-crystal films

We report on the fabrication of single-crystalline thin films of RBa2Cu3O7−y (R: rare-earth element, R123) using an approach of vapor–liquid–solid tri-phase epitaxy. This method is based on application of pulsed-laser deposition under appropriate compositions and conditions predetermined from the relevant thermodynamic phase diagram. The laser-ablated gases of R, Ba, and Cu, and their oxides dissolve into a liquid Ba3Cu5Ox (3BaCuO2+2CuO) layer placed on the film/substrate surface, penetrate to reach the liquid–solid interface with a seed R123, and are condensed into the solid R123 phase under a quasiequilibrium state. The uniform single-crystalline nature of the film was verified by x-ray diffraction, atomic-force microscopy, and transmission electron microscopy by the observation of giant grain size and atomic-scale surface smoothness.

Atomic force microscopy on homoepitaxial SrTiO3 films grown under monitoring of intensity oscillation in reflection high energy electron diffraction

Homoepitaxial SrTiO3 film growth by laser molecular beam epitaxy (MBE) was tuned up to a level as to give not only fine streak in the reflection high energy electron diffraction (RHEED) pattern but also the intensity oscillation persisting more than 70 unit cell lengths. The films thus produced were verified to have atomically flat surfaces (root‐mean‐square roughness of 0.12 nm) and very high crystal quality (χmin of 2%) by the analyses of atomic force microscopy and the Rutherford backscattering, respectively.

Two-Dimensional Epitaxial Growth of SrTiO3 Films on Carbon-Free Clean Surface of Nb-Doped SrTiO3 Substrate by Laser Molecular Beam Epitaxy

A carbon-free clean SrTiO3(100) surface was obtained by rinsing with acetone and ethanol and subsequent oxidation under O2 flushing of 1×10-6 Torr at 650°C. A unit cell layer-by-layer growth was achieved in the laser molecular beam epitaxy growth of insulative SrTiO3 thin films on conductive Nb-doped SrTiO3 substrates thus cleaned, as verified by the observation of persisting RHEED intensity oscillation.

Laser MBE of ceramic thin films for future electronics

The advantages of pulsed laser deposition for oxide film growth and of molecular beam epitaxy for two-dimensional film growth were combined to develop a method for atomically controlled layer-by-layer epitaxy of complex oxides and other ceramic thin films. This laser MBE method has been proved to be particularly useful for the purpose. Two-dimensional molecular layer epitaxy has been verified by the clear observation of RHEED intensity oscillations for the growth of various ceramics including perovskites, infinite-layer cuprates, rock salt oxides, corundum, and fluorites on oxide substrates. Epitaxial growth of BaTiO3, BaO and sapphire films was achieved even at such a low temperature as 20°C. Some phenomena indicating novel quantum effects were observed in high quality ZnO film and SrTiO3/SrVO3 superlattice, both fabricated by laser MBE. In view of the versatility and structure-sensitive properties of oxides, lattice engineering by laser MBE is expected to be a promising technology for opening a new field of oxide electronics.

Fabrication of High-Quality Perovskite Oxide Films by Lateral Epitaxy Verified with RHEED Oscillation

Epitaxial growth of SrTiO3-x and SrVO3-y films by laser molecular beam epitaxy (laser MBE) has been controlled on an atomic scale. Reflection-high-energy-electron-diffraction (RHEED) intensity oscillation persisting for more than 50 periods was observed during the homoepitaxial growth of SrTiO3-x film, indicating the lateral epitaxy in a unit cell layer-by-layer mode. The conductivity of SrTiO3-x ultrathin films was sensitive to the oxygen pressure at the deposition to a far greater extent than was that of SrVO3-y films.

In Situ Optical Diagnosis of Pulsed Laser Deposition and Oxidation of YBa2Cu3O7-δ Thin Films

Laser light (670 nm) was polarized and impinged on YBa2Cu3O7-δ (YBCO) films during their growth by pulsed laser deposition and subsequent oxidation. By simulation of the light reflectance variation, the refractive index (n) and extinction coefficient (k) of the growing films were determined together with the deposition rate. A reflectance change was also observed during cooling the deposited films and was attributed to the increase in oxygen content in the film by comparison with ex situ X-ray diffraction data. Furthermore, the ortho-tetra phase transition could be detected in situ by this method. Thus, the reflectance measurement of p-polarized light was verified to enable in situ characterization of optical properties and oxygen nonstoichiometry of oxide films during their pulsed laser deposition.

Pulsed Laser Deposition and Atomic Scale Characterization of Perovskite Oxide Films

Perovskite oxide films inclusive of high Tc cuprates were fabricated by pulsedlaser deposition (PLD) under both conventional Po 2 (0.01∼1Torr) and high vacuum(≤10 −6 Torr) conditions and their surface morphology, crystal quality, and conductivity were evaluated. Under optimized conventional PLD conditions, we obtained high quality YBa 2 Cu 3 O 7−δ (YBCO) films that showed clear energy gaps and atomically flat surface image in the small area (4x4nm) STM/STS measurements. AFM and SEM ana!yscs on wider film surfaces (∼1x1μm), however, revealed granular structure of about 250nm grains with their root mean square roughness of 5nm. Under high vacuum PLD, i.e. laser MBE conditions, not only epitaxial but also two dimensional growth has been verified for such films as SrTiO 3−x , SrVO 3−y , and (AE)CuO 2−z (AE=alkaline earth metal) by the RHEED observation of fine streak patterns and intensity oscillations. AFM demonstrated atomically flat surfaces of these films. Oxgen or nitric oxide pressure at the laser MBE growth gave crucial effects on the electrical conductivity of SrTiO 3−x . and predominant crystal phase of SrCuO 2−z . Insulating BaTiO 3 films with atomically flat surfaces could be fabricated by both conventional and high vacuum PLD conditions. RBS channeling measurements revealed very high epitaxial quality (χ min −2%) of our SrTiO 3 and BaTiO 3 films. The factors controlling the surface morphology and electrical properties of perovskite oxide epitaxial films,are discussed.

Dimension controlled epitaxy of quantum functional oxide thin films by pulsed-laser processes

Abstract Laser MBE has been demonstrated to be a process especially useful for epitaxial layer-by-layer growth of ceramics thin films directly from sintered ceramics targets. Using the laser MBE system designed for oxide film growth, we have convincingly proved the atomically controlled epitaxy of various oxide films with such lattice structures as perovskite, rock salt, fluorite, and corundum, by the in situ observation of clear RHEED intensity oscillations. Due to the non-steady state nature of film growth by the laser MBE, as well as by the conventional pulsed-laser deposition, we could find a further advantage of the method; the independent optimization of nucleation and growth processes by pulse sequence and laser energy density. This facilitates the control of dimensionality of films. Some of our recent results on the fabrication of dimension controlled epitaxial oxide films are presented and discussed in relation to the atomic scale characterization and exploration of quantum structures and properties in oxides.

Magnetization measurements on Nd1−xBa2+xCu3O7 thin films

Abstract We have measured the magnetization of Nd 1−x Ba 2+x Cu 3 O 7 (x≈0.1) thin films by SQUID at different applied magnetic fields and temperatures. Both the field cooled and zero field cooled of the magnetization were measured. We found that rather than a diamagnetic response, a paramagnetic behavior which resembles those observed by other groups on BSCCO and YBCO single crystals was observed in the field cooled measurements. The origin of such a paramagnetic response is being discussed.

Thin Film Technology Directed towards Josephson Tunnel Junction

An overview is given on the high Tc and related oxide epitaxy directed towards Josephson tunnel junction with emphasis on atomic scale control of film deposition by the laser MBE process. We show how the control of the topmost atomic layer is important for ideal epitaxy of complex oxide. The development of wet process for obtaining atomically flat SrTiO3 surface remarkably improved the controllability of oxide epitaxy. Persistent RHEED oscillation demonstrated that the epitaxy of oxide reached such a level as perfect as that of semiconductors.