A review has been given from a theorist point of view on the study of double oxide surfaces. Among wide classes of the related materials, focusing was made on the transition-metal oxides (TMO) particularly with the perovskite structure. The materials covered in this review are BaTiO3 (ferroelectric), SrTiO3 (quantum paraelectric), Sr2RuO4 (p-wave superconductor), La1-xSrxMnO3 (colossal magnetoresistance) and TiO2 (photocatalyst). The last one, which is neither double oxides nor perovskite, was included because it is the system that has been most intensively studied among TMO. It is pointed out that TMO is generally regarded as a strongly correlated system and that the symmetry lowering and dimension lowering by creating the surface can cause dramatic changes in the electronic and magnetic properties near the surfaces.
Nano-scale surface structures of single crystal metal oxides, especially the perovskite oxides were examined by coaxial impact collision ion scattering spectroscopy (CAICISS) and atomic force microscopy (AFM). CAICISS measurements enabled us to determine the terminating atomic species and their arrangements of single-crystal perovskite oxide substrates and epitaxial oxide films. Through thermal-annealing of the single crystal oxide substrates, atomically flat terrace and stepped structures were developed on the surface. The atomic-scale substrate engineering made it possible to attain the novel heteroepitaxial growth such as step-decoration epitaxy resulting in the nanowire structures and diamond epitaxy on the ultrasmooth sapphire substrate. The novel application of the ultrasmooth oxide substrate to the AFM observation stage for DNA molecules was also presented.
Artificial construction of atomically defined metal oxide layers is important in making electronics devices including high temperature superconducting oxide films, magnetic and optical devices. Hence, the atomistic understanding of the epitaxial growth process of metal oxide surfaces is desired to fabricate atomically controlled structure that exhibits unexplored and interesting physical properties. Recently, computational chemistry has played an important role to clarify the nature and property of various materials. However its targets are limited to the materials of which structures are already known experimentally. In order to realize the theoretical design of novel materials with unexplored and interesting properties, the prediction of completely new structures by using the computational chemistry is essential. However, no theoretical simulations have been devoted to the prediction of new structures. Hence, we developed a new crystal growth simulator MOMODY based on molecular dynamics approach and applied it to the investigation on various epitaxial growth processes of metal oxide thin films in order to design completely new structures.
This report describes our recent progress in the surface/interface study of high-Tc superconductors, where MBE-grown Nd1.85Ce0.15CuO4(NCCO) surfaces and metal/NCCO interfaces have been investigated by photoemission spectroscopies. Experiments were performed focusing on the evolution of the surface or interface electronic structure with oxygen nonstoichiometry at the surface or interface region. The results indicate that the surface and interface electronic structures of NCCO are strongly influenced by the oxygen nonstoichiometry, which is easily caused by an inappropriate reduction process or a redox reaction at metal/NCCO interfaces due to inherently weak nature of Cu-O bonds. This problem may be universal in cuprates. The correct oxygen stoichiometry is the most crucial issue for cuprate superconductors in preparing the bulk-representative surface (interface), which is indispensable in obtaining reliable data using surface sensitive experiments and in fabricating tunnel junctions and superlattices with desirable characteristics.
We examined the surface of a YBa2Cu3Ox (YBCO) single crystal by an atomic force microscope (AFM) in order to use the surface as a substrate for homoepitaxial growth. A step-and-terrace feature was revealed on the YBCO (001) surface after mechanical polishing, however YBCO at the topmost layer showed an unusual c-axis length (appeared in step height) much greater than normal one in precise AFM observations. Instability of the surface at high temperatures and poor reproducibility of homoepitaxial films obtained were caused by this degraded YBCO layer. The damaged layer was removed by the chemical etching using HCl/methanol solution. This treatment brought a topmost surface of YBCO single crystal, which made it possible to grow homoepitaxial YBCO films reproducibly.
The diffusion coefficient of sulfur in the p(1×1) phase of a S/Ni(111) system has been measured in a temperature region from 486 K to 577 K by micro-probe Auger electron spectroscopy, in which the initial coverage profile of sulfur is prepared by depositing sulfur molecules in a localized circular area. The diffusion coefficient measured is in agreement with that reported in our previous paper, in which H2S gas was used as a sulfur source and the initial coverage profile of sulfur was prepared by argon ion sputtering. It has been concluded that surface roughness of nickel, supposedly brought about by argon ion sputtering used in our previous work, does not exert appreciable influence upon the diffusion behavior of sulfur.
Uniform InGaAs Quantum dots were fabricated by modified droplet epitaxy method termed Separated-Phase Enhanced Epitaxy with Droplets (SPEED). Due to the surface diffusion length difference between In and Ga, highly dense Ga droplets were formed around InGa alloy droplets. During the crystallization process of the droplets, these highly dense Ga droplets effectively prevented the two-dimensional growth of InGaAs. Moreover, phase separation effect was enhanced during the annealing process. As the result, InGaAs quantum dots, whose size was smaller than that of droplets, were formed in the upper part of the sample with a flat surface. These quantum dots provide narrow (21.6 meV) photoluminescence spectra. From the magneto-photoluminescence measurements, the sizes of quantum dots were estimated to be 10 nm and 3.7 nm in lateral and vertical directions, respectively.