Epitaxy is the deposition technique allowing a thin mono
crystalline layer to grow over a mono crystalline substrate of the
same type. It is used for example in the fabrication of
semiconductor lasers, where the alloy InGaAsP is growth over a
substrate of InP to constitute the laser active region.
Naturally in order to perform epitaxy the film crystallographic
structure has to be very similar (ideally identical) to that of the
substrate. For this reason often epitaxy is performed using very
similar materials or event the same material with different types
of dopants.