9.8 QUANTUM WELLS AND DOTS

Conventional semiconductor physics does not consider the quantum- mechanical properties of atoms and electrons, but quantum properties become significant on very small scales. A critical threshold is 50 nanometers, roughly the "wavelength" of an electron in a semiconductor. (Just as light has a dual existence as a wave and a particle, electrons can behave as waves, and thus have a wavelength of their own.)

Earlier you learned that double heterostructures can confine light and electrons so they can move in only two dimensions. When the double heterostructure is made thinner than 50 nanometers, its internal energy levels change, altering its confinement of electrons and making it a quantum well. Quantum wells confine electrons more tightly than bulk semiconductor layers. Stacking two or more quantum wells inside the active layer in a multiple quantum-well laser increases the quantum-well volume in which stimulated emission can occur. Photons have wavelengths of hundreds of nanometers in a semiconductor, so they are confined on a larger scale than electrons.

It's possible to make quantum wires that confine electrons within a linear structure smaller than an electron wavelength, so they can only move in one dimension, along the quantum wire. However, there is more interest in quantum dots, which confine electrons in all three dimensions on a scale smaller than the electron wavelength.

Quantum dots can be made in sizes from 2 to 50 nm, and have become a hot area in nanotechnology. They come in many forms, some of which can be embedded in the junction layers of diode lasers or LEDs. Recombination then occurs in the quantum dot, on sharply defined transitions that depend on the size and composition of the quantum dot. Quantum dot lasers have low thresholds and offer a new way to control the emission wavelength, but they have yet to achieve the high efficiency sought by developers.