A wetting layer is an monolayer of atoms that is epitaxially grown on a flat surface.
The atoms forming the wetting layer can be semimetallic elements/compounds or metallic alloys (for thin films).
This article refers to the wetting layer connected to the growth of self-assembled quantum dots (e.g. InAs on GaAs).
The temperatures required for wetting layer growth typically range from 400-500 degrees Celsius.
When the thickness of layer A increases further, it becomes energetically unfavorable for material A to keep the lattice constant of B.