Artificially layered structures

Artificially layered structures

Manufactured, reproducibly layered structures with layer thicknesses approaching interatomic distances. Modern thin-film techniques are at a stage at which it is possible to fabricate these structures, also known as artificial crystals or superlattices, opening up the possibility of engineering new desirable properties into materials. In addition, a variety of solid-state physics problems can be studied which are otherwise inaccessible. The various possibilities include: the application of negative pressure, that is, stretching of the crystalline lattice; the study of dimensional crossover, that is, the transition from a situation in which the layers are isolated and two-dimensional in character to where the layers couple together to form a three-dimensional material; the study of collective behavior, that is, properties which depend on the cooperative behavior of the whole superlattice; and the effect and physics of multiple interfaces and surfaces. For a discussion of semiconductor superlattices See Crystal structure, Semiconductor heterostructures

The preparation techniques can be conveniently classified into two groups: evaporation and sputtering. In the evaporation system, two or more particle sources (thermal or electron beam gun) are aimed at a heated substrate where the artificially layered structure is grown. The sputtering method relies on bombarding targets of the proper materials with an inert gas, such as argon, thus producing the beams of the various elements. See Crystal growth, Molecular beams

Once the artificially layered structure is prepared, it is necessary to characterize whether the layer structure is stable at the growth temperature. This is of considerable importance, since the interdiffusion of the constituents in many cases eliminates the layered growth. One of the most successful methods of characterizing layered growth has been x-ray diffraction.

Artificially layered structures are especially useful for the construction of mirrors for soft x-rays since there are no suitable, naturally occurring crystals for this purpose. Superlattices with zero temperature coefficient of resistivity are useful as resistor material, and high-critical-field-magnet tapes using superconducting-insulator superlattices have been proposed. See X-rays

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.