Koichiro Saiki

Epitaxial growth of transition metal dichalcogenides on cleaved faces of mica

We have grown ultrathin films of layered transition metal dichalcogenides (MoSe2,NbSe2) heteroepitaxially on cleaved faces of mica (muscovite). This is the first success in the heteroepitaxial growth between highly heterogeneous layered materials having different crystal structures and lattice constants that differ by as much as 58%. The lattice matching condition is greatly loosened in those cases because the growth proceeds with weak van der Waals forces between the substrate and the grown layer. This opens a new way to fabricate a heterostructure composed of many kinds of layered materials having various physical and chemical properties.

Heteroepitaxial growth of layered transition metal dichalcogenides on sulfur‐terminated GaAs{111} surfaces

Layered transition metal dichalcogenides (MoSe2, NbSe2) have been heteroepitaxially grown on (NH4)2 Sx (x≂2) treated GaAs(111)Ga, GaAs(∼(111))As surfaces in spite of the large difference in their crystal structures. The in situ observation of reflection high‐energy electron diffraction has shown that the grown film has its own lattice constant even from the first layer. The lattice matching condition, which is severely restricting in the usual heteroepitaxial growth case, is greatly relaxed in the present system because only weak van der Waals forces exist between the grown film and the substrate. This results from the fact that sulfur atoms regularly terminate dangling bonds on the GaAs surface after the (NH4)2Sx treatment.

Heteroepitaxial growth by Van der Waals interaction in one-, two- and three-dimensional materials

Abstract The lattice matching condition usually encountered in heteroepitaxial growth has been found to be relaxed greatly when the interface between constituent materials has Van der Waals nature and forms no direct chemical bonds. Layered transition metal dichalcogenides are the typical materials having that nature, and a variety of heterostructures can be grown by using them. This kind of approach has been proved to be applied also to heteroepitaxial growth between such quasi-one-dimensional materials as tellurium and selenium, that onto dangling-bond terminated three-dimensional material substrates, and that of organic materials forming Van der Waals type crystals.

Heteroepitaxy of Layered Semiconductor GaSe on a GaAs(111)B Surface

Growth of a III-VI compound semiconductor GaSe on a GaAs(111)B substrate has been tried by the molecular beam epitaxy technique. Although GaSe and GaAs have completely different lattice structures, it has been found that a good GaSe film having its own lattice constant grows with its c-axis normal to the GaAs substrate surface. The growth proceeds via van der Waals-like weak forces between each layer of GaSe, relaxing the lattice-matching condition drastically. The heteroepitaxial growth of GaSe will be applied to effective surface passivation of GaAs.

Heteroepitaxy of a two-dimensional material on a three-dimensional material

Abstract An ultra-thin single-crystal film of MoSe2 has been proved to grow heteroepitaxially on a clean CaF2(111) substrate, regardless of the large difference in the crystal structures and of the lattice mismatch which is as large as 17%. It results from the fact that the CaF2(111) surface is very inert because of the termination of active bonds by regularly arrayed F atoms on the top layer, onto which heteroepitaxy proceeds by van der Waals forces in the same way as the van der Waals epitaxy between layered materials. It has opened a new way to grow a variety of heterostructures made of various layered materials on a three-dimensional material substrate.

Heteroepitaxy of CuCl on GaAs and Si substrates

Heteroepitaxial growth of CuCl is investigated on GaAs and Si substrates with (001) and (111) faces. It is found that single-crystalline films grow heteroepitaxially on those substrates. Although island growth occurs at the initial stage, the surface becomes flat as the growth proceeds. For the growth on (111) substrates, the CuCl orientation is uniquely determined in case of GaAs, while two domains rotating by 180° are formed in the case of Si. Formation of GaCl bonds at the interface of CuClGaAs is suggested from the core level photoemission spectra.

Heteroepitaxy of alkali halide on Si and GaAs substrates

Abstract Heteroepitaxial growth of alkali halide was examined on Si and GaAs substrates with (001) and (111) crystal faces. Single-crystalline film growth was observed only for combinations with small lattice misfit: LiBr/Si and NaCl/GaAs systems. A flat surface was observed for growth on (001) faces, while pyramids facetted with {001} faces were observed for growth on (111) faces. The importance of the atomic bond at the interface between the alkali halides and the tetrahedrally bonded semiconductors is discussed and compared with the interface of alkali halide heterostructures.

Heteroepitaxial Growth of Layered Semiconductor GaSe on a Hydrogen-Terminated Si(111) Surface*

Layered III-VI semiconductor GaSe has been heteroepitaxially grown on HF-treated Si(111) surfaces. The HF-treated Si surface is chemically inactive because of the hydrogen termination of active dangling bonds. GaSe can be grown on such a surface through weak van der Waals interaction, which relaxes the lattice matching requirement. With careful control of the substrate temperature, thermal desorption of surface hydrogen atoms was prevented so that a single-domain film of GaSe could be grown with good crystallinity. Auger electron spectra and high-resolution electron energy loss spectra revealed high quality of the grown GaSe film.

NANOSTRUCTURE FABRICATION BY SELECTIVE GROWTH OF MOLECULAR CRYSTALS ON LAYERED MATERIAL SUBSTRATES

Nanostructures consisting of C60 molecules were fabricated on a GaSe/MoS2 heterostructure. A submonolayer film of a lamellar compound semiconductor GaSe was grown on a MoS2 substrate to form nanoscale holes or grooves surrounded by monolayer steps. Atomic force microscope (AFM) observation indicates that C60 molecules grow only in the bare MoS2 nanoregions at a substrate temperature of 180 °C. C60 molecules fill up those holes and grooves, and nanoscale C60 domains with polygonal shapes can be formed. This selective growth method can be combined with nanoscale patterning made by a scanning tunneling microscope or AFM to produce nanostructures of molecular crystals with designed shapes.

Strontium and SrO epitaxy on hydrogen‐terminated Si(111)

Epitaxy of Sr and SrO films onto hydrogen‐terminated Si (111) has been demonstrated. The use of hydrogen‐terminated Si (111) has enabled the epitaxy of metallic Sr films, whereas direct epitaxy on a clean 7×7 Si (111) surface has not been successful. The alternate supply of Sr and O during growth has made it possible to grow good epitaxial SrO films. It has been revealed by electron energy loss spectroscopy that stoichiometric SrO films are obtained by this method.