Editing Superlattice (section)

== Semiconductor materials ==

[[File:GaAs-AlAs SL.svg|300px|left|thumb|GaAs/AlAs superlattice and potential profile of conduction and valence bands along the growth direction (z).]]Semiconductor materials, which are used to fabricate the superlattice structures, may be divided by the element groups, IV, III-V and II-VI. While group III-V semiconductors (especially GaAs/Al<sub>x</sub>Ga<sub>1−x</sub>As) have been extensively studied, group IV heterostructures such as the Si<sub>x</sub>Ge<sub>1−x</sub> system are much more difficult to realize because of the large lattice mismatch. Nevertheless, the strain modification of the subband structures is interesting in these quantum structures and has attracted much attention.

In the GaAs/AlAs system both the difference in lattice constant between GaAs and AlAs and the difference of their thermal expansion coefficient are small. Thus, the remaining strain at room temperature can be minimized after cooling from [[epitaxial growth]] temperatures. The first compositional superlattice was realized using the GaAs/Al<sub>x</sub>Ga<sub>1−x</sub>As material system.

A [[graphene]]/[[boron nitride]] system forms a semiconductor&nbsp;superlattice&nbsp;once the two crystals are aligned. Its charge carriers move perpendicular to the electric field, with little energy dissipation. h-BN has a [[hexagonal]] structure similar to graphene's. The superlattice has broken [[inversion symmetry]]. Locally, topological currents are comparable in strength to the applied current, indicating large valley-Hall angles.<ref>{{Cite journal | doi = 10.1126/science.1254966| title = Detecting topological currents in graphene superlattices| journal = Science| year = 2014| last1 = Gorbachev | first1 = R. V.| last2 = Song | first2 = J. C. W.| last3 = Yu | first3 = G. L.| last4 = Kretinin | first4 = A. V.| last5 = Withers | first5 = F.| last6 = Cao | first6 = Y.| last7 = Mishchenko | first7 = A.| last8 = Grigorieva | first8 = I. V.| last9 = Novoselov | first9 = K. S.| last10 = Levitov | first10 = L. S.| last11 = Geim | first11 = A. K.|arxiv = 1409.0113 |bibcode = 2014Sci...346..448G | volume=346 | issue = 6208| pages=448–451 | pmid=25342798| s2cid = 2795431}}</ref>

=== Production ===

Superlattices can be produced using various techniques, but the most common are [[molecular-beam epitaxy]] (MBE) and [[sputtering]]. With these methods, layers can be produced with thicknesses of only a few atomic spacings. An example of specifying a superlattice is [{{chem|Fe|20|V|30}}]<sub>20</sub>. It describes a bi-layer of 20Å of Iron (Fe) and 30Å of Vanadium (V) repeated 20 times, thus yielding a total thickness of 1000Å or 100&nbsp;nm. The MBE technology as a means of fabricating semiconductor superlattices is of primary importance. In addition to the MBE technology, [[Metal organic chemical vapor deposition|metal-organic chemical vapor deposition]] (MO-CVD) has contributed to the development of superconductor superlattices, which are composed of quaternary III-V compound semiconductors like InGaAsP alloys. Newer techniques include a combination of gas source handling with ultrahigh vacuum (UHV) technologies such as metal-organic molecules as source materials and gas-source MBE using hybrid gases such as arsine ({{chem|AsH|3}}) and phosphine ({{chem|PH|3}}) have been developed.

Generally speaking MBE is a method of using three temperatures in binary systems, e.g., the substrate temperature, the source material temperature of the group III and the group V elements in the case of III-V compounds.

The structural quality of the produced superlattices can be verified by means of [[X-ray diffraction]] or [[neutron diffraction]] spectra which contain characteristic satellite peaks. Other effects associated with the alternating layering are: [[giant magnetoresistance]], tunable reflectivity for X-ray and neutron mirrors, neutron [[spin polarization]], and changes in elastic and acoustic properties. Depending on the nature of its components, a superlattice may be called ''magnetic'', ''optical'' or ''semiconducting''.

[[File:Fe20v30.png|300px|right|thumb|X-ray and neutron scattering from the  [Fe<sub>20</sub>V<sub>30</sub>]<sub>20</sub> superlattice.]]