Epitaxy is a very special case of thin-film deposition: the deposited film will be single crystalline. This can only take place when special conditions are met. The deposited layer registers the crystalline information from the substrate. In order to do so properly, the crystal lattices of the film and the substrate must be identical or closely matching. The simplest case is homoepitaxy: film and substrate are the same material, for example silicon deposition on silicon. Because crystal information is "transmitted" across the substrate/film interface, the surface quality of the starting wafer is of paramount importance. A residual film a few atomic layers thick can prevent epitaxy. Epitaxy reactors are therefore designed with extreme cleanliness in mind, use the highest purity chemicals and are very delicate and expensive pieces of equipment.
Epitaxy is a demanding process and high-quality epitaxial films are difficult to make. Epitaxial deposition can fail partially and result in defective single crystalline material, or it can fail completely and result in polycrystalline or even amorphous film. Whether the defective material is usable for devices depends on the density and location of those defects: if defects are confined to substrate/film interface, and the deposited layer is mostly defect-free, the material may be usable, but this depends on the device operating principle and engineering judgment is needed to decide on acceptable defect levels.