quantized electronic structure
Quantized electronic structure (QUEST)
A material that confines electrons in such a small space that their wavelike behavior becomes important and their properties are strongly modified by quantum-mechanical effects. Such structures occur in nature, as in the case of atoms, but can be synthesized artificially with great flexibility of design and applications. They have been fabricated most frequently with layered semiconductor materials. Generally, the confinement regions for electrons in these structures are 1–100 nanometers in size. The allowable energy levels, motion, and optical properties of the electrons are strongly affected by the quantum-mechanical effects. The structures are referred to as quantum wells, wires, and dots, depending on whether electrons are confined with respect to motion in one, two, or three dimensions. Multiple closely spaced wells between which electrons can move by quantum-mechanical tunneling through intervening thin barrier-material layers are referred to as superlattices. See Quantum mechanics
The most frequently used fabrication technique for quantized electronic structures is epitaxial growth of thin single-crystal semiconductor layers by molecular-beam epitaxy or by chemical vapor growth techniques. These artificially synthesized quantum structures find major application in high-performance transistors such as the microwave high-electron-mobility transistor (HEMT), and in high-performance solid-state lasers such as the semiconductor quantum-well laser. They also have important scientific applications for the study of fundamental two-dimensional, one-dimensional, and zero-dimensional physics problems in which particles are confined so that they have free motion in only two, one, or zero directions. Chemically formed nanocrystals, carbon nanotubes, zeolite cage compounds, and carbon buckyball C60 molecules are also important quantized electronic structures.
The optical applications are based on the interactions between light and electrons in the quantum structures. The absorption of a photon by an electron in a quantum well raises the electron from occupied quantum states to unoccupied quantum states. Electrons and holes in quantum wells may also recombine, with the resultant emission of photons from the quantized electronic structure as the electron drops from a higher state to a lower state. See Electron-hole recombination
The photon emission is the basis for quantum-well semiconductor lasers, which have widespread applications in optical fiber communications and compact disk and laser disk optical recording. Quantum-well lasers operate by electrically injecting or pumping electrons into the lowest-conduction-band (n = 1) quantum-well state, where they recombine with holes in the highest-valence-band (n = 1) quantum-well state (that is, the electrons drop to an empty n = 1 valence-band state; illus. a), producing the emission of photons. These photons stimulate further photon emission and produce high-efficiency lasing. See Laser
The photon absorption is the basis for quantum-well photodetectors and light modulators. In the quantum-well infrared photodetector an electron is promoted from lower (say, n = 1) to higher (say, n = 2) conduction band quantum-well states (illus. b) by absorption of an infrared photon. An electron in the higher state can travel more freely across the barriers, enabling it to escape from the well and be collected in a detector circuit. Changes in quantum-well shapes produced by externally applied electric fields can change the absorption wavelengths for light in a quantized electronic structure. The shift in optical absorption wavelength with electric field is known as the quantum-confined Stark effect. It forms the basis for semiconductor light modulators and semiconductor optical logic devices. See Optical modulators, Stark effect
Modulation doping is a special way of introducing electrons into quantum wells for electrical applications. The electrons come from donor atoms lying in adjacent barrier layers (illus. c). Modulation doping is distinguished from conventional uniform doping in that it produces carriers in the quantum well without introducing impurity dopant atoms into the well. Since there are no impurity atoms to collide with in the well, electrons there are free to move with high mobility along the quantum-well layer. Resistance to electric current flow is thus much reduced relative to electrical resistance in conventional semiconductors. This enhances the low-noise and high-speed applications of quantum wells and is the basis of the high-electron-mobility transistor (HEMT), which is also known as the modulation-doped field-effect transistor (MODFET). HEMTs are widely used in microwave receivers for direct reception of satellite television broadcasts. See Transistor
Electrical conductivity in carbon nanotubes occurs without doping and results from the absence of any energy gap in the electronic energy band structure of the nanotubes and the presence of allowed states at the Fermi energy. Individual nanotubes can be electrically contacted. Simple quantum wire transistors displaying quantized electron motion have been formed from single nanotubes.
Quantum-mechanical tunneling is another important property of quantized electronic structures. Tunneling of electrons through thin barrier layers between quantum wells is a purely quantum-mechanical effect without any real analog in classical physics or classical mechanics. It results from the fact that electrons have wavelike properties and that the particle waves can penetrate into the barrier layers. This produces a substantial probability that the particle wave can penetrate entirely through a barrier layer and emerge as a propagating particle on the opposite side of the barrier. The penetration probability has an exponential drop-off with barrier thickness. The tunneling is greatest for low barriers and thin barriers.
This effect finds application in resonant tunnel devices, which can show strong negative resistance in their electrical properties. In such a device (illus. d), electrons from an n-type doped region penetrate the barrier layers of a quantum well by tunneling. The tunneling current is greatest when the tunneling electrons are at the same energy as the quantum-well energy. The tunneling current actually drops at higher applied voltages, where the incident electrons are no longer at the same energy as the quantum-state energy, thus producing the negative resistance characteristic of the resonant tunneling diode. See Artificially layered structures, Resonance (quantum mechanics), Tunneling in solids
quantized electronic structure
[¦kwän‚tīzd i·lek·¦trän·ik ′strək·chər]Quantized electronic structure (QUEST)
A material that confines electrons in such a small space that their wavelike behavior becomes important and their properties are strongly modified by quantum-mechanical effects. Such structures occur in nature, as in the case of atoms, but can be synthesized artificially with great flexibility of design and applications. They have been fabricated most frequently with layered semiconductor materials. Generally, the confinement regions for electrons in these structures are 1–100 nanometers in size. The allowable energy levels, motion, and optical properties of the electrons are strongly affected by the quantum-mechanical effects. The structures are referred to as quantum wells, wires, and dots, depending on whether electrons are confined with respect to motion in one, two, or three dimensions. Multiple closely spaced wells between which electrons can move by quantum-mechanical tunneling through intervening thin barrier-material layers are referred to as superlattices.
The most frequently used fabrication technique for quantized electronic structures is epitaxial growth of thin single-crystal semiconductor layers by molecular-beam epitaxy or by chemical vapor growth techniques. These artificially synthesized quantum structures find major application in high-performance transistors such as the microwave high-electron-mobility transistor (HEMT), and in high-performance solid-state lasers such as the semiconductor quantum-well laser. They also have important scientific applications for the study of fundamental two-dimensional, one-dimensional, and zero-dimensional physics problems in which particles are confined so that they have free motion in only two, one, or zero directions. Chemically formed nanocrystals, carbon nanotubes, zeolite cage compounds, and carbon buckyball C60 molecules are also important quantized electronic structures.
The optical applications are based on the interactions between light and electrons in the quantum structures. The absorption of a photon by an electron in a quantum well raises the electron from occupied quantum states to unoccupied quantum states. Electrons and holes in quantum wells may also recombine, with the resultant emission of photons from the quantized electronic structure as the electron drops from a higher state to a lower state.
The photon emission is the basis for quantum-well semiconductor lasers, which have widespread applications in optical fiber communications and compact disk and laser disk optical recording. Quantum-well lasers operate by electrically injecting or pumping electrons into the lowest-conduction-band (n = 1) quantum-well state, where they recombine with holes in the highest-valence-band (n = 1) quantum-well state (that is, the electrons drop to an empty n = 1 valence-band state; illus. a), producing the emission of photons. These photons stimulate further photon emission and produce high-efficiency lasing. See Laser, Optical communications, Optical recording
The photon absorption is the basis for quantum-well photodetectors and light modulators. In the quantum-well infrared photodetector an electron is promoted from lower (say, n = 1) to higher (say, n = 2) conduction band quantum-well states (illus. b) by absorption of an infrared photon. An electron in the higher state can travel more freely across the barriers, enabling it to escape from the well and be collected in a detector circuit. Changes in quantum-well shapes produced by externally applied electric fields can change the absorption wavelengths for light in a quantized electronic structure. The shift in optical absorption wavelength with electric field is known as the quantum-confined Stark effect. It forms the basis for semiconductor light modulators and semiconductor optical logic devices. See Optical detectors, Optical modulators
Modulation doping is a special way of introducing electrons into quantum wells for electrical applications. The electrons come from donor atoms lying in adjacent barrier layers (illus. c). Modulation doping is distinguished from conventional uniform doping in that it produces carriers in the quantum well without introducing impurity dopant atoms into the well. Since there are no impurity atoms to collide with in the well, electrons there are free to move with high mobility along the quantum-well layer. Resistance to electric current flow is thus much reduced relative to electrical resistance in conventional semiconductors. This enhances the low-noise and high-speed applications of quantum wells and is the basis of the high-electron-mobility transistor (HEMT), which is also known as the modulation-doped field-effect transistor (MODFET). HEMTs are widely used in microwave receivers for direct reception of satellite television broadcasts. See Transistor
Electrical conductivity in carbon nanotubes occurs without doping and results from the absence of any energy gap in the electronic energy band structure of the nanotubes and the presence of allowed states at the Fermi energy. Individual nanotubes can be electrically contacted. Simple quantum wire transistors displaying quantized electron motion have been formed from single nanotubes.
Quantum-mechanical tunneling is another important property of quantized electronic structures. Tunneling of electrons through thin barrier layers between quantum wells is a purely quantum-mechanical effect without any real analog in classical physics or classical mechanics. It results from the fact that electrons have wavelike properties and that the particle waves can penetrate into the barrier layers. This produces a substantial probability that the particle wave can penetrate entirely through a barrier layer and emerge as a propagating particle on the opposite side of the barrier. The penetration probability has an exponential drop-off with barrier thickness. The tunneling is greatest for low barriers and thin barriers.
This effect finds application in resonant tunnel devices, which can show strong negative resistance in their electrical properties. In such a device (illus. d), electrons from an n-type doped region penetrate the barrier layers of a quantum well by tunneling. The tunneling current is greatest when the tunneling electrons are at the same energy as the quantum-well energy. The tunneling current actually drops at higher applied voltages, where the incident electrons are no longer at the same energy as the quantum-state energy, thus producing the negative resistance characteristic of the resonant tunneling diode. See Negative-resistance circuits