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Definition: microscopic structures confining carriers in three dimensions
A quantum dot is a small structure, e.g. a small piece of semiconductor material embedded in another semiconductor material, which can confine particles (usually electrons) in all three dimensions.
The confinement is a quantum effect. It totally changes the density of states for the confined particles, compared with the density of states for particles in a larger piece of the material. For an ideal quantum dot, there are discrete energy levels, corresponding to a delta-shaped density of states with no states in between the delta peaks. Such a behavior is known from atoms; in this sense, quantum dots can be considered as a kind of artificial atoms where the energy levels can be adjusted by design, e.g. by controlling the quantum dot dimensions or the material composition. In reality, large ensembles of quantum dots are normally used, and their size distribution leads to a somewhat smeared-out density of states, i.e. to inhomogeneous broadening.
Quantum dots can be fabricated from semiconductors. This can even accidentally happen when thin quantum wells are fabricated with molecular beam epitaxy (MBE) or metal–organic chemical vapor deposition (MOCVD), where some fluctuations of the thickness can occur. Under certain conditions (Stranski–Krastanov growth of a lattice-mismatched material on some substrate, e.g. InAs on GaAs), quantum dots can be grown in a self-organized fashion, so that they acquire more consistent sizes and distribution. Subsequently, they are often overgrown with the substrate material. Typical dimensions of the often pyramidally shaped quantum dots are of the order of 5–20 nm; quantum dots are therefore considered to be a topic of nanotechnology. Their density can be of the order of 109 dots per square centimeter. In most cases, a large number of dots are simultaneously used in a device, although some quantum optics experiments require single quantum dots.
Applications
Quantum dots are useful for a variety of applications:
- The perhaps oldest application of quantum dots is the formation of beautiful colors in stained glasses, e.g. in church windows.
- Quantum dots make possible the fabrication of laser diodes with very low threshold pump power and/or low temperature sensitivity.
- Quantum dots can be used in white light-emitting diodes (LEDs): they are excited with a blue or near-ultraviolet LED and emit e.g. red and green light, so that overall a white color tone is achieved.
- In semiconductor saturable absorber mirrors (SESAMs), quantum dots can serve as absorbers with a very low saturation fluence. Such quantum dot absorbers can also be contained in a glass matrix.
- Quantum dots can be parts of very sensitive photodetectors, and in the future they may function in efficient photovoltaic cells.
- In the context of quantum cryptography, quantum dots can serve as single-photon emitters.
- Quantum dots might also be suitable for performing quantum computations.
The so far technologically most important applications are probably those in LEDs and laser diodes; quantum dot semiconductor lasers are expected to replace some of the currently used quantum well laser diodes.
See also: quantum wells
Categories: photonic devices, quantum optics
