Quantum dot: Crown jewel of nanoscience

RAPID PROGRESS in the fabrication of semiconductor structures in the last couple of decades has allowed researchers to create structures with dimensions in the nanometer (billionth of a meter) scale. One major step in these developments has been the reduction of the effective dimension of devices from three- dimensional bulk systems to two-dimensional to one dimensional, and finally to zero- dimensional systems. When the size of all these devices becomes comparable with the de Broglie wavelength, a consequence of the wave nature of electrons, electrons confined in these devices display quantum effects.

The novel electronic and optical properties of these reduced dimensional systems, that can be controlled to a certain extent through clever design, make these systems promising candidates for a variety of future applications, that include improved semiconductor lasers and microelectronics. Quantum dots (QDs) represent the ultimate reduction in the dimensionality of a semiconductor device. In these systems, electrons are confined in all directions. Therefore they have no kinetic energy and as a result, they occupy spectrally-sharp energy levels like those found in atoms. This is the reason that quantum dots are often referred to as artificial atoms. The interest in QD stems in part from its potential to operate at the level of a single electron, certainly the ultimate limit for an electronic device. In recent years, quantum dots have emerged as a unique nano laboratory where fundamental ideas of quantum mechanics can be tested and eventually put to novel applications, such as quantum dot lasers, QD memory devices, QD photodetectors, and even quantum cryptography.Quantum planes, lines and dots

The process of quantum confinement begins with electrons confined to a ultra-thin plane between two different materials, typically gallium arsenide (GaAs) and an alloy of gallium aresenide and aluminium (ALGaAs). In the epitaxial growth technique, for example the molecular beam epitaxy (MBE), the layers are grown by spraying down one layer of atoms at a time, allowing control to sub nanometer dimensions. At the interface electrons are trapped in a ultra-thin layer and experience the quantum confinement, whereby their allowed energy levels become discrete, instead of a continuous range of energy that is available to free electrons.

Parallel to the interface, electrons move freely as in a bulk system and form a two-dimensional electron gas (2DEG). On this 2DEG, researchers then use techniques similar to that used to ``write'' integrated circuits at a scale of one-millionth of a meter, to further confine the electron motion. For example, by confining the electron motion in one other dimension, electrons are forced to move on a line, and the system is called a quantum wire. When the electrons are also confined in the remaining free direction, one gets the quantum dot or a zero-dimensional electron system. Here the rules of quantum mechanics restricts the electron motion in all three dimensions. In the early days of QD research, QDs were made by lithographic patterning on the 2DEG. The dots that were created were however quite big, a few hundred nanometers in dimension. These days, they are created by ``self-assembly'', i.e., the dots appear spontaneously during the epitaxial growth of layers with materials of different lattice parameters. These self-organized QDs are very small, usually in the 10 mm range and have become an essential component in the QD devices described below.

Quantum dot lasers

Semiconductor lasers are the key components in a large number of technological products used in every day life. They are most commonly used in optical-data storage, optical communications, in CD players, as bar-code readers, laser printers, laser pointers, and in many other devices. QD lasers are of considerable contemporary interest because they offer the potential to yield superior properties compared to conventional bulk or quantum well lasers. They emit light at wavelengths that are not determined by the band gap energy, a material property and therefore fixed for a device, but rather by the energy levels of the dots.

This offers flexibility to tune the wavelength of the QD-laser. Further, the atom-like sharp energy levels of QDs associated with strong confinement have led to expectations that there will be a great reduction of threshold current which should also have a weak temperature dependence.

In the QD laser, the active region consists of layers of QDs. Stimulated recombination of electron-hole pairs takes place in the active region, the GaAs well.

The population inversion (nonequilibrium population of electrns and holes) works better in lower dimensions. Red QD laser has already been demonstrated in many laboratories. The quest for blue QD laser, crucial for optical data storage and transfer has led to a modern- day gold rush.

T.Chakraborthy

Institute of Mathematical Sciences Chennai 600113

(To be concluded)

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