Semiconductor devices which approximate a two dimensional structure (films), a one dimensional structure (wires) or a zero dimensional strucure (dots) have very interesting and important properties:

• The theory of the behavior of electrons in structures of dimensions lower than three is well known. Although true zero, one and two dimensional physical structures are not possible, if one or more dimensions are less than the de Broglie wave length of an electron the structure has the quantum properties of a lower dimension structure.
• The technology for producing structures with one or more dimension sufficiently small to achieve low-dimension quantum effects (approximately 10 nm) has recently advanced enough to allow fabrication of devices based upon low-dimension quantum effects.
• Devices operating on quantum confinement are more efficient in energy so there is less wasted energy to be dissipated as heat.
• Quasi-two dimensional devices, such as quantum well semiconductor lasers, are now economically practial, but quasi-one and zero dimensional devices such as quantum wires and quantum dots are not now economically practical, perhaps because of the additional processing required to create wires and dots. IBM, Bellcore and Phillips dropped research and development programs for quantum wires and dots in the mid-1990s.
• Electrons can be confined to one semiconductor material by sandwiching the semiconductor material between two layers of higher energy-band gap materials. Such a structure is called a heterojunction.
• There are allowed and forbidden energy levels for a electrons in a material. The conductivity of a material is determined by the occupancy of the allowed energy bands. Energy bands which are filled are called valence bands and the ones that are sparcely occupied are called conduction bands. The conductivity of a material is determined by the occupancy of its energy bands.
• The availability of electrons to fill the energy bands depends upon the valence electrons of the material, but can be altered by doping, the introduction of chemically-related material into the crustal lattice of the material. Also photo-electric photons can change the occupancy of a material.
• The development of Molecular-Beam Epitaxy (MBE) in the laste 1960's made quasi-two dimensional structures feasible. Not only did this allow the creation of ultrathin films but multiple layers of such films. However it was not until 1974 that quantum well devices were produced.
• The typical laser energy arrangement involves four states:
  1. E₃: The state to which electrons are pumped
  2. E₂: The upper state of two states involved in the productions of photons
  3. E₁: The lower state to which electrons drop from the upper state after emitting a photon
  4. E₀: The ground state to which electrons fall from E₁
• The key to the operation of the laser is a population inversion, a higher population in the upper state E₂ than in the lower state E₁. This is achieved by pumping to E₃and the rapid exit of electrons from the lower state E₁.
• In conventional lasers the energy states are natural energy level of the lasing material such as ruby crytal or helium=neon gases. In the quantum cascade laser the energy states are determined by the physical characteristics of the quantum wells and can be adjusted to any desired levels. The quantum cascade laser relies upon cascades of 25 quantum well configurations.
• Quantum wires and quantum dots may be more efficient and faster than quantum films.
• It is much more difficult to fabricate quantum wires than quantum films. One promising technology deposits semiconductor material at the bottom of V-shaped lines, such might be found in a diffraction grating.
• Laser wires generate photons through the self annihilation of exciton, pairings of electrons and holes. Conventional lasers, by contrast, emit photons from the annihilation of free electrons and free holes. When the current is increased a conventional laser's emission frequency may derease, whereas for wire or dot lasers the frequencies are stable when current is increased.
• Variations in the width of quantum wires may result in their functioning as a chain of quantum dots. This may occur at low temperatures.
• Variations in the thickness of quantum films result in clumps that function as quantum dots. This approach offers the possibility of creating arrays of quantum dots.
• There is a possibility of using molecules as quantum dots. The problem is creating contacts and linkages.

Sources:

Elizabeth Corcoran and Glenn Zorpette, "Diminishing Dimensions," Scientific American,