An overlayer is a layer of adatoms adsorbed onto a surface, for instance onto the surface of a single crystal.
Overlayers on single crystals
Adsorbed species on single crystal surfaces are frequently found to exhibit long-range ordering; that is to say that the adsorbed species form a well-defined overlayer structure. Each particular structure may only exist over a limited coverage range of the adsorbate, and in some adsorbate/substrate systems a whole progression of adsorbate structure are formed as the surface coverage is gradually increased.
References
Overlayer Wikipedia(Text) CC BY-SA