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.
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(Click here to see large image, 25kB)
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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.
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(Click here to see large image, 25kB)
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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.
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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.
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| II | III | IV |
V | VI |
| . | B | C |
N | O |
| . | Al | Si |
P | S |
| Zn | Ga | Ge |
As | Se |
| Cd | In | Sn |
Sb | Te |
| Hg | . | . |
. | . |
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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.
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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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