What are the differences between supperlattices and Multiple Quantum Wells Semiconductor Structures? Semiconductor Optoelectronics:
What are the differences between multiple Quantum Wells and supperlattices.
 A: Quantum wells (or more precisely multi-quantum wells, MQWs) are nanometer wide layers of a lower band gap semiconductor grown on a higher band gap barrier material. For example GaAs/AlAs layers. 
The defining feature of MQWs is that the barriers are wide enough such that wavefunctions in adjacent quantum wells do not overlap. This means that the tunnelling probability from well to  well is essentially zero.
The defining feature of a superlattices is that the barriers are very thin such that the wavefunctions of adjacent wells overlap strongly. This means that electrons in superlattices are delocalised because they can easily tunnel out.
The semiconductor material has a lattice, which is the periodic arrangement of atoms which gives rise to the semiconductor bands. Similarly, the periodic arrangement of quantum wells superimposes a different periodicity on on top of the physical lattice; hence a super lattice. This gives rise to the formation of mini-bands within the superlattice which is another unique feature.
A: Normally, you call a system either MQW or superlattice based on a width of the band formed by interaction of the levels in adjoint QWs. If the bandwidth is small enough (in comparison with temperature, characteristic times, etc., whatever is important for your problem) and you may consider carriers as localized within definite QW and jumping between them, you call this MQW. If the bandwidth is large (same remark) and this is more electron distributed between many QWs (you may not call it belonging to one of a wells at a time) then you call this superlattice. 
E.g., the same structure can be MQW at low temperatures and superlattice @ room temperature.
