What are Quantum Dots

Interest in quantum dots (QDs) stems from the work of Esaki and Tsu [1], who in 1970, reported the direct observation of quantization effects in Alx Ga1-XAs/GaAs/ Alx Ga1-X As quantum well structures, opening a new chapter of solid-state electronics. A quantum well is a thin layer (film) of one material sandwiched between pieces of another material, where the bandgap of the film is less than the bandgap of the surrounding material (e.g., a thin film of GaAs) (bandgap Eg = 1.42 eV) sandwiched between AlxGa1-XAs (e.g., Al03Ga07As, Eg = 2.0 eV). In this het-erostructure, any carriers (electrons and holes) that move into the middle (e.g., GaAs) layer are effectively trapped there. In this case, the sandwich structure is analogous to a "well," where the carriers easily enter but are very difficult to remove. If the thickness of the middle (e.g., GaAs) layer is equal to or smaller than the de Broglie wavelength of the carriers, which is typically about a few tens of nanometers [2] for an electron in a semiconductor, then quantization of the energy levels of the carriers in the well occurs. Only certain energy levels are allowed (i.e., those for which the well width corresponds to an integral number of half de Broglie wavelengths), so that carriers in the well occupy discrete energy levels.

This quantization of the levels in the well leads to the structure being called a "quantum well." Obviously, the energy levels in the quantum well can be tuned by changing the dimensions (the width and depth) of the well. For example, by narrowing the well width, the longest allowed wavelength of an electron in the well is shortened and, as a result, the lowest electron energy level becomes higher. The depth of the well is adjustable by changing the composition of the material of the well. It follows that energy levels can be controlled by adjusting the width and composition of the quantum well and the work of engineering the energy levels is called bandgap engineering. In this way, devices based on semiconductor quantum well structures, such as lasers, have been successfully manufactured in many key areas of modern technology such as optical communications [3], magneto-optical recording disc drive memory systems [4], and optical printing systems [5].

In the quantum well structure, carriers are confined only in one dimension. By the end of the 1980's, research had been started to further reduce the dimension of semiconductor structures from three (bulk materials) and two (quantum wells) to one (quantum wire) [6] and "zero" (QDs) [7, 8]. So a quantum wire comprises a material of a smaller bandgap geometrically confined in two dimensions within material of a larger bandgap, with the dimensions in two directions being sufficiently small resulting in quantization of the energy levels corresponding to those two directions. Similarly, a QD has the same form but with the dimensions constrained in three dimensions. Carriers in quantum wires and QDs are confined in two and three dimensions,

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Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 10: Pages (175-189)

respectively. The effects of the dimensionality of the confinement on the density of electronic states are illustrated in Figure 1.

The size of semiconductor QDs is usually in the range of a few nanometers to tens of nanometers, containing up to a few hundred thousand atoms. Because carriers in QDs are confined three dimensionally, a QD behaves in many ways like a single giant atom [9-11]. Evidence for the atom-like electronic states of QDs is shown by the narrow photoluminescence linewidth from excitons in a single QD [12-15]. The unusual behavior of optical and transport properties seen in the QDs provides the basis for novel device concepts including large-gain, low-threshold quantum lasers [16, 17], light-emitting diodes [18-20], single-electron logic devices [21], and optical computing quantum units [22, 23].

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