Atsushi Koma

Van der Waals epitaxy—a new epitaxial growth method for a highly lattice-mismatched system

One of the major obstacles to realizing various kinds of epitaxial heterostructure is the lattice matching between constituent materials. The lattice-matching condition, however, has been found to be drastically relaxed when the epitaxial growth proceeds with van der Waals interactions. Many kinds of layered material are suitable for that purpose. They are easily cleaved along the layers without producing dangling bonds on their cleaved surfaces. The epitaxial growth of a layered material onto those surfaces proceeds with a van der Waals force, resulting in good heteroepitaxial growth even with a large lattice mismatch between the grown and the substrate materials. It has also been found that van der Waals epitaxy can be applied to the epitaxial growth of a layered material on an ordinary three-dimensional material substrate, if regular termination of the surface dangling bonds is accomplished. Sulphur or selenium-atom-terminated GaAs(111), H-atom-terminated Si(111) and F-atom-terminated CaF2(111) surfaces can be used for that purpose. van der Waals epitaxy has also been successfully applied to grow such organic molecular crystal films as metal phthalocyanines, higher aromatics and C60.

Van der Waals epitaxy for highly lattice-mismatched systems

One of the major obstacles to realize various kinds of heterostructures is the lattice matching between constituent materials. This difficulty has been proved to be overcome if one uses the interface having van der Waals nature. This kind of interface can be formed when a layered material is grown on a cleaved face of another layered material. Moreover, it has been found that these heterostructures can be grown on a three-dimensional material substrate such as Si or GaAs, if the dangling bond on its surface is terminated by proper atoms. This idea has also been successfully applied to grow epitaxial films of organic molecular crystals such as metal phthalocyanines and fullerenes.

van der Waals epitaxial growth and characterization of MoSe2 thin films on SnS2

A variation on molecular beam epitaxy (MBE), called van der Waals epitaxy, is described where a material with primarily two‐dimensional (2D) bonding is grown on a substrate which also has a 2D structure. Lattice matching difficulties, which limit the choice of materials in MBE of 3D systems, are circumvented since the interlayer bonding is from weak van der Waals interactions. The title system shows a lattice mismatch of 10% yet high quality epitaxial films can be grown. The films were characterized in situ with reflection high energy electron diffraction, Auger electron spectroscopy, and low energy electron loss spectroscopy. Additional characterization after exposure to ambient by x‐ray photoelectron spectroscopy, low energy electron diffraction, transmission electron microscopy confirmed the highly ordered nature of the films. Scanning tunneling microscopy provided real space images of the morphology of the epitaxial layer and showed unusual structures attributed to lattice mismatch.

Studies on an (NH4)2Sx-Treated GaAs Surface Using AES, LEELS and RHEED

Surface properties of (NH4)2Sx-treated GaAs (100), (111)Ga and ()As planes were studied by means of Auger electron spectroscopy (AES), low-energy electron energy loss spectroscopy (LEELS) and reflection high-energy electron diffraction (RHEED). We found that oxide-free and sulfur-terminated GaAs surfaces produced by the (NH4)2Sx treatment provided the reduction of interface state density. Furthermore, comparison of various planes revealed that (i) sulfur atoms could combine with both Ga and As and (ii) bonding between Ga and S was stronger than that between As and S.

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.

Molecular beam epitaxial growth of organic thin films

Good quality ultrathin organic films have been successfully grown on various substrates using molecular beam epitaxy. Many kinds of organic films have been epitaxially grown on such substrates with van der Waals surfaces as cleaved faces of layered materials regardless of the large lattice mismatch between the grown organic crystals and the substrates. It has also been found that the epitaxial growth of organic films is possible on such technologically important materials as silicon and GaAs, if the surfaces of those substrates are changed into quasi van der Waals ones by the proper termination of surface dangling bonds with suitable atoms. As for organic molecules with asymmetric charge distribution, such ionic crystals as alkali halides can also be used as good substrates; epitaxial growth is achieved in this situation with the aid of electrostatic interaction.

Van der Waals epitaxial growth of C60 film on a cleaved face of MoS2

C60 film has been grown heteroepitaxially on a cleaved face of MoS2 by means of van der Waals epitaxy. The C60 film forms a close-packed structure with its principal crystal axes parallel to those of the substrate. Although the lattice constant of the C60 crystal is much larger than that of MoS2, good heteroepitaxial growth becomes possible because of the van der Waals-type interaction between the grown film and the substrate.

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.

Periodic lattice distortions as a result of lattice mismatch in epitaxial films of two-dimensional materials

Epilayers of transition metal dichalcogenides (TMDs) with two‐dimensional structures can be grown with molecular beam epitaxy onto other TMDs substrates without regard to lattice matching. Although there is no strong bonding between the epilayer and the substrate, the van der Waals interaction between the two hexagonally closest packed lattices results in a periodic distortion which, due to electronic effects, is prominently imaged with the scanning tunneling microscope.

Epitaxial growth of vanadyl‐phthalocyanine ultrathin films on hydrogen‐terminated Si(111) surfaces

Ultrathin films of vanadyl phthalocyanine (VOPc) have been grown on hydrogen‐terminated Si(111) surfaces by molecular beam epitaxy. Epitaxial growth was examined on the two types of substrates. Reflection high energy electron diffraction studies have revealed that VOPc molecules form commensurate lattices [−3[3 1]4], [−1[4 3]3], and [1[4 4]1] on the surface terminated homogeneously with monohydride. On the other hand, epitaxial growth did not occur on the surface terminated with the mixture of polyhydride. Homogeneity and microscopic flatness of the substrate surface seem to be important factors for the epitaxial growth of VOPc on the hydrogen‐terminated Si substrate.