Mamoru Yoshimoto

Atomic Control of the SrTiO3 Crystal Surface

The atomically smooth SrTiO3 (100) with steps one unit cell in height was obtained by treating the crystal surface with a pH-controlled NH4F-HF solution. The homoepitaxy of SrTiO3 film on the crystal surface proceeds in a perfect layer-by-layer mode as verified by reflection high-energy electron diffraction and atomic force microscopy. Ion scattering spectroscopy revealed that the TiO2 atomic plane terminated the as-treated clean surface and that the terminating atomic layer could be tuned to the SrO atomic plane by homooepitaxial growth. This technology provides a well-defined substrate surface for atomically regulated epitaxial growth of such perovskite oxide films as YBa2Cu3O7-δ.

Atomic‐scale formation of ultrasmooth surfaces on sapphire substrates for high‐quality thin‐film fabrication

The atomically ultrasmooth surfaces with atomic steps of sapphire substrates were obtained by annealing in air at temperatures between 1000 and 1400 °C. The terrace width and atomic step height of the ultrasmooth surfaces were controlled on an atomic scale by changing the annealing conditions and the crystallographic surface of substrates. The obtained ultrasmooth surface was stable in air. The topmost atomic structure of the terrace was examined quantitatively by atomic force microscopy and ion scattering spectroscopy as well as a theoretical approach using molecular dynamics simulations.

In situ RHEED observation of CeO2 film growth on Si by laser ablation deposition in ultrahigh-vacuum

In a newly developed ultrahigh-vacuum (UHV) film preparation system equipped with in situ reflection high-energy electron diffraction (RHEED) and X-ray photoelectron spectrometer (XPS) analyzer, a sintered CeO2 target was ablated by ArF excimer laser to deposit films on Si(001), (111) and (110) substrates at temperatures ranging from 600 to 700°C. Tetravalence of the Ce in the films was confirmed by XPS, indicating that stoichiometric CeO2 was formed even in the UHV (≤10-6 Torr) condition. The orientation of deposited CeO2 films was strongly dependent on the surface state of Si substrates. The results are well explained in terms of preferential arrangement of the first plane of growing CeO2 by its interaction with substrate surface.

Determination of surface polarity of c-axis oriented ZnO films by coaxial impact-collision ion scattering spectroscopy

We have identified the surface polar structure of wurtzite-type ZnO films by coaxial impact-collision ion scattering spectroscopy. High-quality ZnO epitaxial films were prepared on sapphire (α-Al2O3) (0001) substrates by laser molecular beam epitaxy using a ZnO ceramic target. The (0001) crystallographic plane (the O face) was found to terminate the top surface of the ZnO film by comparing spectra of the films with those of well-defined (0001) and (0001) surfaces of bulk single crystals. The preferential [0001] growth direction of ZnO films is discussed from the viewpoints of the chemical interaction at the interface and surface stability against sublimation.

Growth mode and surface morphology of a GaN film deposited along the N-face polar direction on c-plane sapphire substrate

The dependence of polar direction of GaN film on growth conditions has been investigated by changing either the group-V/group-III ratio (V/III ratio) in supplying the source gas or the deposition rate. GaN films were deposited on a nitrided sapphire by two-step metalorganic chemical vapor deposition. The surface morphology changed from flat hexagonal to pyramidal hexagonal facet with the increase of V/III ratio. However, the polar direction of GaN on an optimized buffer layer of 20 nm thickness was N-face (−c) polarity, independent of both the V/III ratio and the deposition rate. The polarity of the GaN epitaxtial layer can be determined by that of an interface (nitrided sapphire, annealed buffer layer or GaN substrate) at the deposition of GaN epitaxial layer. The higher V/III ratio enhanced the nucleation density, and reduced the size of hexagonal facets. The nuclei, forming the favorable hexagonal facets of wurtzite GaN, should grow laterally along the {1010} directions to cover a room among the facet...

A-site layer terminated perovskite substrate: NdGaO3

A perovskite single-crystal substrate, NdGaO3 (001), was thermally annealed in air to give an atomically defined surface structure. From analysis with coaxial impact-collision ion scattering spectroscopy, the terminating atomic layer was identified to be NdO1+δ , i.e., the A-site oxide monolayer in perovskite ABO3. This result is contrary to the B-site oxide (BO2−δ) termination observed in other perovskite surfaces, such as wet etched SrTiO3 and LaAlO3 or annealed (LaAlO3)0.3–(Sr2AlTaO6)0.7 (LSAT).

Analysis of the polar direction of GaN film growth by coaxial impact collision ion scattering spectroscopy

Nondestructive determination of the polarity of GaN has been achieved by the use of coaxial impact-collision ion scattering spectroscopy analysis. The polarity of a GaN film with a smooth surface on non-nitrided c-plane sapphire was identified (0001) (Ga face; +c). GaN films with a 20 nm buffer layer on nitrided sapphire had (0001) (N face; −c) polarity and a hexagonal faceted surface. The influence of both the buffer layer and of substrate nitridation on the polarity of wurtzite {0001} GaN films deposited by two-step metal organic chemical vapor deposition (MOCVD) has been investigated. The polarity of the buffer layer on a nitrided sapphire substrate was altered by varying its thickness or the annealing time. It was found that the polarity of the GaN film is determined by the polarity of the annealed buffer layer; MOCVD-GaN films on buffer layers with +c and −c polarity have either +c (smooth surface) or −c (hexagonal facet) polarity, respectively.

Heteroepitaxial Growth of CeO2(001) Films on Si(001) Substrates by Pulsed Laser Deposition in Ultrahigh Vacuum

Pulsed laser deposition in ultrahigh vacuum (UHV) was applied to the epitaxial growth of CeO2 film on Si(001). Although the direct deposition of CeO2(001) on Si(001) was unsuccessful, the desired epitaxy was achieved by inserting the growth of a SrTiO3 layer. The formation of a CeO2(001)//SrTiO3(001)//Si(001) layered structure was verified by reflection high-energy electron diffraction analysis of the growing surface at a temperature between 650 and 700°C in UHV. In addition to lattice matching, chemical interaction at the growing surface had a decisive effect on the epitaxy and orientation of growing ceramic lattices.

CERAMIC LAYER EPITAXY BY PULSED LASER DEPOSITION IN AN ULTRAHIGH VACUUM SYSTEM

Ceramic layer epitaxy, defined as the epitaxial growth of ceramic thin films with thicknesses regulated on an atomic scale, was verified by a new method using pulsed laser deposition in ultrahigh vacuum (UHV). The intensity oscillation of reflection high‐energy electron diffraction (RHEED) was observed in an ArF excimer laser deposition of CeO2 and SrTiO3 films on Si(111) and SrTiO3(001) substrates, respectively, at 650–750 °C under 5×10−7 Pa. The oscillation periodicities corresponded well to interplane distances of CeO2(111) and SrTiO3(001). This first observation of RHEED oscillation in laser deposition of ceramic films suggests that UHV laser deposition is a promising method for producing atomically regulated ceramic layers inclusive of possible new high Tc superconductors.

Open air plasma chemical vapor deposition of highly dielectric amorphous TiO2 films

By using the cold plasma torch we developed, TiO2 films were deposited on substrates exposed to air by feeding Ti(OEt)4 into the plasma. XPS and x‐ray analyses revealed that all films were stoichiometric and amorphous TiO2, but Raman spectra indicated the existence of short‐range crystallinity in films deposited above 350 °C. The short range structure changed from anatase to rutile by admixing of hydrogen to the plasma. The rutile containing TiO2 film had a higher breakdown electric field as well as a higher dielectric constant than the anatase containing film.