  As a system with both profound physics and promising application potentials,
the strain-induced self-assembled semiconductor nanostructures have been
investigated for decades of years. The optical and electrical properties of
this system are mainly determined by the nanostructures, but can be greatly
affected by the generally existent two dimensional structures from which the
nanostructures evolve, the so called wetting layer (WL). The WL configurations
can be varied with different growth conditions, and further influence the
nanostructure morphology and properties. This is a reviewing article
introducing some recent progresses in investigating the evolution of WLs in
InAs/GaAs system with reflectance difference spectroscopy. Different kinds of
WL evolution processes in Stranski-Krastanov growth mode are introduced. The
segregation and desorption of indium atoms and the effect of growth
interruption are mentioned. The existence and the evolution process of wetting
layers in droplet epitaxy method are also discussed.
