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A quantum well is used to confine electrons at specific energy levels. Quantum wells consist of an extremely thin semiconductor with a small band gap, resting between material with a larger band gap. They are extremely small, usually between 1 and 20 nanometers. They are most often used in laser diodes and infrared imaging.
This well uses the properties of electron behavior and band gaps to work. Band gaps are areas in an electron's orbital between the ground state, where electrons rest normally, and the conduction band, higher energy orbitals electrons move to when they are excited. The gaps are barriers between the ground state bands and conduction bands, which prevent the electrons from reaching the conduction band without gaining more energy than they have in their ground states. The larger the band gap, the more energy is necessary for the electrons to jump this gap and reach the conduction band.
Once the electron reaches the conduction band, it releases its excess energy and falls back into its ground state. By placing a microscopically thin semiconductor between material with band gaps too wide for electrons to jump easily, scientists can force the electrons to remain in the two-dimensional area of the thin semiconductor. Trapping electrons in this manner allows for specific energy manipulation.
Since the electrons can only move in two directions, they can only produce the type of energy the scientist or manufacturer wishes. This energy is also focused in an extremely narrow stream. Due to this focus, quantum wells create accurate lasers for optical devices. A well-known example of a quantum well is in the read lasers in compact disc (CD) players.
Quantum wells are named "wells" not only because of their behavior of trapping electrons like a well would trap water, but also because of their appearance when graphed. When quantum wells are pictured on energy vs. position graphs, they look like deep valleys, or wells, often in a rectangular shape. A quantum well is a type of potential well, meaning there is a potential for a minimum, fixed amount of energy to be produced.
Grown, rather than created, a quantum well is usually made of material like gallium arsenide surrounded by aluminum arsenide. Wells are grown, most often, by a process called molecular beam epitaxy, which uses an effusion cell to shoot molecules of the substance at a base substance. This method creates a single atomic layer of the well material with each firing of the cell.