Heteroepitaxy Between Lattice-Mismatched Materials with Van Der Waals Interactions
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HETEROEPITAXY BETWEEN LATTICE-MISMATCHED MATERIALS WITH VAN DER WAALS INTERACTIONS ATSUSHI KOMA University of Tokyo, Department of Chemistry Bunkyo-ku, Tokyo 113, Japan
ABSTRACT The lattice matching condition encountered usually in the heteroepitaxial growth has been proved to be relaxed drastically, if one uses the interface having van der Waals nature. Such interface can be realized on a cleaved face of a layered material or a quasi-one dimensional material and on a surface of a dangling-bond-terminated three dimensional material. Various kinds of heterostructures, which cannot be made by conventional growth methods, can be fabricated by using a variety of layered transition metal dichalcogenides, in which there exist superconducting, metallic, semiconducting or insulating layered materials. Moreover those heterostructures have been found to be grown on such an ordinary three-dimensional material as GaAs, if the dangling bonds on its surface are terminated by suitable atoms.
INTRODUCTION Recently heterostructures with atomic order thicknesses have been successfully fabricated by molecular beam epitaxy (MBE) and other epitaxial growth techniques. The full use of them has made it possible to realize such new electronic devices as superlattice devices and electronic wave devices. But so far good heteroepitaxial growth was possible only for very limited combinations of materials, which restrict the freedom in making various kinds of heterostructures. The restriction comes from the severe lattice matching condition to be satisfied by constituent materials. Usually there appear dangling bonds on a clean surface of a substrate as is seen in Fig. 1(a), and this makes it difficult to grow good hetero-epitaxial film without good lattice match in the constituent materials. There are, however, materials having no dangling bonds on their clean surfaces as is seen in Fig. 1(b), on which epitaxial growth proceeds via van der Waals forces [14]. We call that kind of epitaxy'van der Waals epitaxy'. It is expected that good heterostructures can be grown even between materials having large lattice mismatch in the van der Waals epitaxy. It is also expected in the van der Waals epitaxy that a very
(a) bd
dangling bond
(b) van der Waals gap
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(G) quasi van der Waals gap
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Fig. 1 Interfaces connected by (a) active bonds, (b) van der Waals gap and (c) quasi van der Waals gap.
Mat. Res. Soc. Symp. Proc. Vol. 198. 01990 Materials Research Society
106
abrupt interface with small amounts of defects can be fabricated because of the nonexistence of dangling bonds. Thus the van der Waals epitaxy seems to be one of the most powerful technique to prepare a good quality of heterostructure with atomic order thickness. The heteroepitaxial growth was carried out by a three-chamber MBE system, and grown thin films were characterized, 'in- situ, by reflection high energy electron diffraction (RHEED), Auger electron spectroscopy (AES) and low-energy electron energy loss spectroscopy (LEELS). Some specimens were investigated als
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