Super lattice and quantum well

Difference between Super lattice and quantum well

1. Superlattice

2. DEFINITION A  superlattice  is a periodic structure of layers of two (or more) materials. Typically, the thickness of one layer is several  nanometers. It can also refer to a lower-dimensional structure such as an array of quantum dot or quantum wires.

3. INTRODUCTION A semiconductor superlattice is a periodic structure of two or more semiconductors of significantly different band gaps such that multiple quantum wells are formed in the low band gap layers. The layers have to be thin enough to allow carrier transport by tunnelling  to take place.The semiconductor superlattice will have multiple hetero-junctions. 

4. Superlattice heterostructure   Growing multiple heterojunctions of two different materials with periodic repetition. But here thickness of the each layer is very thin (generally 1-5 nm, depending upon lattice mismatch). Due to these thin layers carriers can easily tunnel through these though it provides multiple quantum wells. Growing superlattice is one of the common techniques to reduce strain in the (top) epitaxial layer. 

5. Conditions for  Semiconductor super lattice   A semiconductor hetero-structure can become a super-lattice when the thicknesses of the constituting layers fulfill the following conditions : They are smaller than the De-Broglie wavelengths corresponding to confined electrons and holes in wells and barriers, in order to obtain quantum confinement of electrons and holes. They are sufficiently small to give enough overlap of adjacent electron and hole wave functions so that Quantum Tunneling Effect can hold through the hetero-structure. These thicknesses of semiconductors 1 and 2 are periodically repeated in space so that the resulting superstructure of these two different materials forms a kind of periodic lattice called in this way a super-lattice.

6. GaAs/ AlAs superlattice and potential profile of conduction and valence bands along the growth direction (z ). In the GaAs/ AlAs system both the difference in lattice constant between GaAs and AlAs and the difference of their thermal expansion coefficient are small. Thus, the remaining strain at room temperature can be minimized after cooling from  epitaxial growth  temperatures .

7. Production Superlattices can be produced using various techniques, but the most common are  molecular-beam epitaxy  (MBE) and  sputtering . With these methods, layers can be produced with thicknesses of only a few atomic spacings. An example of specifying a superlattice is [Fe 20V 30] 20 . It describes a bi-layer of 20Å of Iron (Fe) and 30Å of Vanadium (V) repeated 20 times, thus yielding a total thickness of 1000Å or 100 nm. MBE is a method of using three temperatures in binary systems, e.g., the substrate temperature, the source material temperature of the group III and the group V elements in the case of III-V compounds.

8. Differences between multiple Quantum Wells and supperlattices . 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. In MQWs 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. In superlattices 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 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 

9. Figure . Difference in electronic states between multiple quantum well structures (barriers >40 A) and superlattices (barriers <40 A); miniband formation occurs in the superlattice structure, which permits carrier delocalization 

10. Two typical examples of superlattice materials and their multi-level structural features. ( a ) A cubic cellular material composed of a material with a hexagonal lattice at level 2. All the ligaments have the same material type and orientation. ( b ) Self-assembled nanocrystal superlattice with FCC structures at both levels. However, the orientations of the lattices are different at the two levels.