Three-Dimensional Carrier Confinement in Strain-Induced Self-Assembled Quantum Dots

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Abstract
Recent technological and materials advances in semiconductors have brought about the possibility of producing heterostructures within which carriers are confined to an ultrasmall region of space (a few thousand atoms) by a potential barrier. When the dimensions of the confining potential are smaller than the electron wavelength (a few tens of nanometers), the semiconductor electronic and optical properties are drastically altered. In these so-called quantum structures, carrier energy levels are quantized and their energy depends on the confining-potential dimensions and magnitude. Some of these quantum structures have already found technological applications. For example the quantum-well (QW) semiconductor laser is part of every CD player. It is also widely used as the light source for intercontinental optical communications. The carrier confining potential in this case is provided by two wider bandgap semiconductor layers sandwiching a thin (3–20 nm) smaller bandgap semiconductor film. The carriers have two degrees of freedom within the QW. The QWs are grown by epitaxial deposition on a crystalline substrate. The substrate may or may not be lattice-matched with the epitaxial film. In some instances, a small lattice mismatch may be required to obtain the desired band-gap value for the QW material. These are the so-called pseudomorphically strained QW structures and devices.