Quantum Structures

QUANTUM STRUCTURES

Volume is a three-dimensional quantity. To reduce the volume of the box, we can shorten its length, its width or its height. The same is true for the region occupied by the electrons in a solid.

There are three dimensions to confine and achieving quantum confinement typically requires confining at least one of these dimensions to less than 100nm or just a few nanometers.

A quantum confined structure is one in which the motion of the electrons or holes are confined in one or more directions by potential barriers.

Based on the confinement direction, a quantum confined structure will be classified into three categories as quantum well, quantum wire and quantum dot. The classification of quantum confined structures is shown in Table 5.1

Table 5.1 classification of quantum confined structures

i) Quantum well (2 dimension)

Definition

When we constrain electrons inside a region of minimal width, we create a quanum well.

In other words, if one dimension is reduced to the nano range while the other two dimensions remain large, then we get a structure known as quantum well.

Fig. 5.3 shows a 2-D structure or quantum well.

Construction

Quantum wells are made from alternative layers of different semiconductors or by deposition of very thin metal films.

Explanation

The well is like a cage in which the carrier particles (the excitons) are trapped. These trapped particles can be considered to be quantum confinement. Due to this quantum confinement, the motion of carriers is reduced. In a quantum well, the excitons can move freely sideways in the plane of a thin layers, but they might like to move in the forward and backward directions as well. Due to the confinement of carriers, the structure quantum well has important applications in making useful devices.

Use

Quantum wells are now widely used to make semiconductor layers and other important devices.

ii) Quantum wire (1 dimension)

Definition

If we constrain width and depth of electron's domain, we create a quantum wire.

In other words, if two dimensions are so reduced and one remains large, the resulting structure is quantum wire.

Fig. 5.4 shows a 1-D structure or quantum wire. Explanation

The carriers trapped in such structures can be considered to be in 1-D quantum confinement. In this case, an exciton is only free to choose its trajectory along the wire. However, for each motion of its movement, the exciton can have various ways of being confined.

Example

Examples of quantum wire structures are nanowires, nanorod and nanotube.

iii) Quantum Dots (0-dimension)

Definition

When all three dimensions are minimized the resulting structure is quantum dot.

The dot can can be particle located inside a larger structure or on its surface. can also be a place where electrons have been trapped using electric fields.

Fig. 5.5 shows a 0-D structure or quantum dot.

Explanation

Hence, in this situation, the exciton only has confined states i.e., there are no freely moving excitons. Although a quantum dot has many thousands of atoms, but due to its peculiar properties, it is considered more like a single atom rather than many atoms.

Use

Quantom dot may be used as a basic building block in making a quantum

computer.

Fig. 5.6 and Fig. 5.7 illustrates the processes of diminishing the size for the case of rectilinear geometry and curvilinear geometry, respectively.

Fig. 5.6 Progressive generation of rectangular nanostructures.

Fig. 5.7 Progressive generation of curvilinear nanostructures.

Fig. 5.8 Three quantum structures

Fig 5.8 shows a comparison of three quantum confined structures with bulk material.