Heterostructure

Heterostructure - the combination of multiple heterojunctions together in a device is called a heterostructure although the two terms are commonly used interchangeably. A heterojunction is the interface that occurs between two layers or regions of dissimilar crystalline semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction. It is often advantageous to engineer the electronic energy bands in many solid state device applications including semiconductor lasers, solar cells and transistors to name a few. The combination of multiple heterojunctions together in a device is called a heterostructure although the two terms are commonly used interchangeably. The requirement that each material be a semiconductor with unequal band gaps is somewhat loose especially on small length scales where electronic properties depend on spatial properties. A more modern definition may be to say that a heterojunction is the interface between any two solid state materials including crystalline and amorphous structures of metallic, insulating, fast ion conductor and semiconducting material. In 2000, the Nobel Prize was awarded with one half jointly to Herbert Kroemer (University of California at Santa Barbara, California, USA) and Zhores I. Alferov (A.F. Ioffe Physico-Technical Institute, St. Petersburg, Russia) for "developing semiconductor heterostructures used in high-speed- and opto-electronics". In quantum dots the band energies are dependent on crystal size due to the quantum size effect. This enables band offset engineering in nanoscale heterostructures. It is possible4 to use the same materials but change the type of junction, say from straddling (type I) to staggered (type II), by changing the size or thickness of the crystals involved. The most common nanoscale heterostructure system is coreshell CdSeZnS which is a straddling gap (type I) offset. In this system the much larger band gap ZnS passivates the surface of the fluorescent CdSe core thereby increasing the quantum efficiency of the luminescence.There is an added bonus of increased thermal stability due to the stronger bonds in the ZnS shell as suggested by its larger band gap. Since CdSe and ZnS both grow in the zincblende crystal phase and are closely lattice matched, core shell growth is preferred. In other systems or under different growth conditions it may be possible to grow anisotropic structures such as the one seen in the image on the right.

Image of a nanoscale heterojunction between iron oxide (Fe3O4 — sphere) and cadmium sulfide (CdS — rod) taken with a TEM. This staggered gap (type II) offset junction was synthesized by Hunter McDaniel and Dr. Moonsub Shim at the University of Illinois in Urbana-Champaign in 2007

Further information:

1. Pallab, Bhattacharya (1997), Semiconductor Optoelectronic Devices, Prentice Hall
2. J. Tersoff, Physical Review, B30, 4874, 1984
3. N. Debbar et al., Physical Review, B40, 1058, 1989
4. Sergei A. Ivanov, Andrei Piryatinski, Jagjit Nanda, Sergei Tretiak, Kevin R. Zavadil, William O. Wallace, Don Werder and Victor I. Klimov, J. Am. Chem. Soc., 129, 11708-11719, 2007
5. Istvan Robel, Masaru Kuno and Prashant V. Kamat, J. Am. Chem. Soc., 129, 4136-4137, 2007
6. H. Kroemer, Proceedings of the IEEE, Vol. 51, pp. 1782-1783, 1963
7. Feucht, D. Lion & A.G. Milnes (1970), written at New York and London, Heterojunctions and metal-semiconductor junctions, Academic Press
8. Article  Heterostructure from Wikipedia, the Free Enciclopedia. Available under the license Creative Commons Attribution-Share Alike.

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