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A general aim when growing a device is to avoid lattice mismatch between the
substrate and the epitaxial layers. A main concern is that the strain
originating from such mismatch can relax and lead to misfit dislocations and
even to amorphous structures. Therefore, only materials which lattice constants
match the ones of GaAs or InP, typical substrate materials, gained
attention. Such materials are AlGaAs in the whole composition range and
Ga
In
P grown on GaAs, and In
Ga
As and
In
Al
As grown on InP. The quaternary GaInAsP can also match
either GaAs or InP for certain mole fractions. However, if
is sufficiently
low or the layer is sufficiently thin, it can preserve the substrate lattice
constant, respectively, the strain. The result can be alteration of the
bandgap, which can be beneficial sometimes, e.g. for SiGe grown on Si.
Vassil Palankovski
2001-02-28