Epitaxy


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epitaxy

[′ep·ə‚tak·sē]
(crystallography)
Growth of one crystal on the surface of another crystal in which the growth of the deposited crystal is oriented by the lattice structure of the substrate.

Epitaxy

 

the oriented growth of one crystal on the surface of another crystal, which is known as the substrate.

A distinction is made between what may be called heterogenous epitaxy, in which the substance of the substrate differs from that of the growing crystal, and what may be called homogeneous epitaxy (or autoepitaxy), in which the substances are the same. The oriented growth of a crystal inside another crystal may be called endotaxy.

Epitaxy is observed in corrosion and in the crystallization of a vapor, a solution, or a melt. It is governed by the conditions under which the lattice of the growing crystal is bonded to that of the substrate; a structural and geometric correspondence between the lattices is essential. Substances that crystallize into structure types that are identical or similar to each other bond most readily; the face-centered cubic lattice of Ag, for example, bonds easily with the NaCl-type lattice, and the sphalerite lattice, with the diamond-type lattice. Epitaxy, however, can also be achieved with structures that differ from each other markedly, such as corundum- and diamond-type lattices.

In a description of epitaxy, growth planes and their crystallographic axes are expressed as, for example, [112] (111) Si//[1100] (0001) A12O3. Such a designation means that the (111) face of the Si crystal (which has a diamond-type lattice) is growing parallel to the (0001) face of the A12O3 crystal (which has a corundum-type lattice) and that the [112] crystallographic axis in the growing crystal is parallel to the [1100] axis of the substrate (see CRYSTALS).

Epitaxy may be achieved rather easily if the difference between lattice constants does not exceed 10 percent. Only the most densely packed planes and axes bond when the difference is large, in which case some of the planes of one lattice do not extend to the edges of those of the other. The edges of such abbreviated planes constitute what may be called a misfit dislocation. The dislocation usually forms a grid in which the dislocation plane can be controlled by varying the periodicity of the two lattices—for example, by changing the composition of the substance of the substrate. The number of dislocations in the growing layer can be controlled in the same way.

Epitaxy occurs in such a way that the total energy at a boundary consisting of a substrate-crystal, crystal-medium, or substrate-medium interface is minimal. In substances with structures that are similar to each other—for example, when Au is deposited on Ag—the formation of an interlinking boundary is energetically unfavorable, and the growing layer acquires precisely the same structure as that of the substrate. This phenomenon is called pseudomorphism. The energy stored in a compressively stressed pseudomorphic film increases with film thickness, even when the critical thickness is exceeded (for Au on Ag the critical thickness is ~600 Å). As a result, a film with its own distinctive structure grows.

Apart from structural and geometric correspondence, the bonding of two substances in epitaxy depends on such factors as the temperature of the process, the degree of supersaturation (or supercooling) of the crystallizing substance in the medium, the degree of perfection of the substrate, and the cleanliness of the substrate’s surface. The “epitaxial temperature”—that is, the temperature below which only a nonoriented film can grow—varies according to the substances involved in the epitaxy and the growth conditions.

The process of epitaxy usually begins with the formation of nuclei, which, on coalescing, form a continuous film. Different types of growth are possible on the same substrate, as indicated by [100] (100) Au// [100] (100) NaCl and [100] (111) Au// [110] (100)NaCl.

Epitaxy is used extensively in microelectronics in the fabrication of transistors, integrated circuits, and light-emitting diodes, as well as in quantum electronics in the preparation of multilayer semiconductor heterostructures (seeSEMICONDUCTOR HETERO-JUNCTION) and injection lasers. Epitaxy is also widely used in the manufacture of such integrated optics devices as the magnetic bubble memories of computers.

REFERENCES

Palatnik, L. S., and I. I. Papirov. Orientirovannaia kristallizatsiia. Moscow, 1964.
Palatnik, L. S., and I. I. Papirov. Epitaksial’nye plenki. Moscow, 1971.

A. A. CHERNOV and E. I. GIVAROIZOV

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