Superlattice

A superlattice is a material with periodically alternating layers of several substances. Such structures possess periodicity both on the scale of each layer's crystal lattice and on the scale of the alternating layers. Semiconductor superlattices were first grown in the 70's and metallic superlattices in 1980.

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Superlattices can be produced in a number of ways, but the most common are Molecular-beam epitaxy and Sputtering. With these methods lattices as thin as a few atomic layers can be produced. One nomenclature used to describe the produced superlattice is [Fe₂₀V₃₀]₂₀ signifying a bi-layer consisting of 20Å of Iron (Fe) followed by 30Å of Vanadium (V), the whole structure repeated 20 times yielding a total thickness of 1000Å or 100nm.

The structural quality of the produced superlattices can be verified by means of X-ray diffraction which give rise to characteristic satellite peaks. The same applies to Neutron diffraction. Other effects of the alternating layering are: Giant magnetoresistance, tunable reflectivity for X-ray and Neutron mirrors, neutron spin polarization, and changes in the elastic and acoustic properties. Depending on the nature of components, a superlattice may be called magnetic, optical or semiconductor.

The motion of charge carriers in a superlattice is modified with respect to each individual lattice. This can lead to significant increase of carrier mobility (used in microwave devices) or special optical features (such as a semiconductor laser).

There also exists a class of quasiperiodic superlattices named after Fibonacci. The Fibonacci superlattices are usually studied as a single-dimensional model of quasicrystal, where either electron hopping transfer interactions or on-site energies take two values arranged in a Fibonacci sequence.

References

• Ivan K. Schuller, "A New Class of Layered Materials", Phys. Rev. Lett. 44, 1597 (1980).
• Morten Jagd Christensen, "Epitaxy, Thin Films and Superlattices", Risø National Laboratory, (1997).