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SUMMARY OF SEMICONDUCTOR CRYSTAL STRUCTURES

Author: d.w.palmer@semiconductors.co.uk

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The picture on the right shows the "diamond structure" that is the arrangement of the Group IV atoms in diamond, silicon and germanium. Each atom has four equidistant, tetrahedrally arranged, nearest neighbours, the atom-atom bond direction is <111>
and the interbond angle is 109deg 28min.


The next picture on the right shows the arrangement of the Group III and Group V atoms in the cubic-crystal form ("zinc-blende" or "sphalerite") that is the stable structure of many III-V compounds, including GaAs. This atomic arrangement is identical to that of diamond described above, EXCEPT that here alternate atoms are respectively of Group III and of Group V of the Periodic Table of Elements. Many semiconducting II-VI compounds also have the "zinc-blende" atomic arrangement, ZnS being the original compound of that name.

(Click here to see large image, 25kB)

Although GaAs has inter-atomic bonding that is strongly covalent, there is a small amount of electron charge transfer from the gallium atoms to the arsenic atoms and so the material can be considered as very slightly ionic; the red and blue balls of the picture may therefore be usefully seen as indicating the slightly positive gallium atoms and the slightly negative arsenic atoms respectively.


It is clear from the part of the Periodic Table of Elements shown below, that GaAs is one of a great number of possible binary III-V compounds, such as also GaP, InP, AlAs, GaN etc formed by combination of Group III elements and Group V elements at equal atomic concentrations. Ternary and quaternary III-V compounds, such as Al(x)Ga(1-x)As, InAs(1-y)P(y), In(x)Ga(1-x)As(1-y)P(y) can also be formed and many of them also have valuable properties for semiconductor technology. The II-VI semiconductors comprise the compounds containing Zn, Cd and Hg as the cations and O, S, Se and Te as the anions.


II III IV V VI
. B C N O
. Al Si P S
Zn Ga Ge As Se
Cd In Sn Sb Te
Hg . . . .

The III-V and II-VI compounds comprising elements of lower atomic numbers are more ionic (ie, less covalent) than are the III-V and II-VI compounds of elements of higher atomic numbers. The usual result is that the lower-atomic-number semiconducting compounds have larger valence-band to conduction-band energy gaps Eg.


In some III-V semi-conductors, such as GaN, the cubic structure described above is only meta-stable, the stable form being an atomic arrangement (the "wurzite" structure) in which the atoms are slightly displaced from their cubic-structure positions and form a crystal of hexagonal symmetry. Detailed information about GaN and other III-Nitride semiconductors can be found on the III-Nitrides Page of this Web-Site.
 

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