Difference between Super lattice and quantum well

2. Superlattice

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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

10. 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

12. 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.