Superlattice

In solid solutions, a superlattice is a crystal structure in which the component species are arranged in a regular and ordered manner. Formation of superlattice is possible if the elements comprising the solid solution have simple, integer ratios. Superlattices were discovered early in 1925 by Johansson and Linde[1] after the studies on gold-copper and palladium-copper systems through their special X-ray diffraction patterns. Further experimental observations and theoretical modifications on the field were done by Bradley and Jay,[2] Gorsky,[3] Borelius,[4] Dehlinger and Graf,[5] Bragg and Williams[6] and Bethe[7]. Theories were based on the transition of arrangement of atoms in crystal lattices from disordered state to an ordered state.

A superlattice is a structure composed of alternating layers of different materials. These layers are typically measured in nanometers and the typical superlattice is extremely small. These structures are being used in the creation of new forms of semiconductors that exhibit different properties than the included materials. As this technology enters the mainstream, it is believed that it will allow scientists to create materials with very different properties without any change in its appearance.

The structure is made by stacking layers of different materials on top of each other. These layers are very thin, even thinner than a human hair. By stacking such thin materials together, the properties of the individual materials blend together in unexpected ways. This combination of properties allows scientists to create substances that have properties that are rare or unknown among natural materials.

There are two common reasons for creating a superlattice structure. The first is to increase the material’s resistance to shear effects. The process of creating a superlattice increases the shear resistance well beyond the resistance possessed by any of the constituent materials. This strength allows the material to maintain its structure under greater stresses than traditional materials.

The other common reason for building a superlattice is to produce new varieties of semiconductors. These materials transmit electricity better than an insulator, but not as much as a conductor. They are used in almost every form of modern electronics, often in the form of an integrated circuit or microchip. Current semiconductors are generally made of silicon, but superlattice semiconductors can be made of many different things.

Superlattice semiconductors have a handful of advantages over typical semiconductors. These fabricated materials can conduct electricity faster or slower than a typical silicon semiconductor, simply by altering the amounts of substances in the lattice. This will allow for custom construction of a semiconductor with very specific tolerances.

Another advantage is keeping some of the properties of the cross-linked materials separate. By creating a layered conductor, currents of varying power can be sent through the semiconductor. In effect, each layer transmits power at its natural rate. This will allow a single material to operate at two different frequencies simultaneously, improving the material’s response time.
Few artifacts use superlattices. Some companies are experimenting with batteries and light bulbs that use superlattice-based cathodes, but they are very rare. Ongoing research in the field is likely to change that. Superlattice structures have many properties that, when added to common consumer goods, increase their lifespan and reduce energy consumption.

The thermal properties associated with superlattices are fundamental to semiconductor laser development. The heat conduction of superlattices is less understood than that of homogeneous thin films. It has been theorized that superlattices have lower thermal conductivity due to impurities from lattice discrepancies and heterojunctions. In this case, phonon-interface scattering at the heterojunctions must be considered; fully elastic scattering underestimates heat conduction, while fully inelastic scattering overestimates it.

References

  1. Johansson; Linde (1925). “The X-ray determination of the atomic arrangement in the mixed-crystal series gold-copper and palladium-copper”. Annalen der Physik.
  2. Bradley; Jay (1932). “The formation of Superlattices in Alloys if Iron and Aluminium”.
  3. Gorsky (1928). “X-ray investigations of transformations in the CuAu alloy”. Z. Phys.
  4. Borelius (1934). “The theory of transformations of metallic mixed phases”. Annalen der Physik.
  5. Dehlinger; Graf (1934). “Transformation of solid metal phases I. The tetragonal gold-copper alloy CuAu”. Z. Phys. Chem. 26: 343.
  6. Bragg, W.L.; Williams, E.J. (1934). “The effect of thermal agitation on atomic arrangement in alloys I”.
  7. Bethe (1935). “Statistical theory of superlattices”.
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