& Tornado Alley
Lower Dimension Quantum Devices
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
- 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:
- E3: The state to which electrons are pumped
- E2: The upper state of two states involved in the productions
- E1: The lower state to which electrons drop from the upper
state after emitting a photon
- 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
- 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
- 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.
Elizabeth Corcoran and Glenn Zorpette, "Diminishing Dimensions,"