Quantum well laser
Encyclopedia
A quantum well laser is a laser diode
Laser diode
The laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current...

 in which the active region of the device is so narrow that quantum confinement occurs. The wavelength of the light emitted by a quantum well laser is determined by the width of the active region rather than just the bandgap of the material from which it is constructed. This means that much shorter wavelengths can be obtained from quantum well lasers than from conventional laser diodes using a particular semiconductor material. The efficiency of a quantum well laser is also greater than a conventional laser diode due to the stepwise form of its density of states
Density of states
In solid-state and condensed matter physics, the density of states of a system describes the number of states per interval of energy at each energy level that are available to be occupied by electrons. Unlike isolated systems, like atoms or molecules in gas phase, the density distributions are not...

 function.

Origin of the concept of quantum wells

In 1972, Charles H. Henry
Charles H. Henry
Charles H. Henry was born in Chicago, Illinois, USA, on May 6, 1937. He received an MS. degree in physics in 1959 from the University of Chicago, and a PhD degree in physics in 1965 from the University of Illinois, under the direction of Charlie Slichter...

, a physicist and newly-appointed Head of the Semiconductor Electronics Research Department at
Bell Laboratories, had a keen interest in the subject of integrated optics, the fabrication of optical circuits in which the light
travels in waveguides.

In late 1972, while pondering the problems associated with waveguides, he had a sudden insight, a realization that a
double heterostructure
Double heterostructure
A double heterostructure is formed when two semiconductor materials are grown in to a "sandwich". One material is used for the outer layers , and another of smaller band gap is used for the inner layer. In our example, there are two AlGaAs-GaAs junctions at each side of the inner layer...

 is a waveguide for electron waves, not just lightwaves. On further reflection, he saw that there is a complete
analogy between the confinement of light by a slab waveguide and the confinement of electrons by the potential well that is formed from the difference in bandgaps in a double heterostructure
Double heterostructure
A double heterostructure is formed when two semiconductor materials are grown in to a "sandwich". One material is used for the outer layers , and another of smaller band gap is used for the inner layer. In our example, there are two AlGaAs-GaAs junctions at each side of the inner layer...

.

Henry realized that there should be discrete modes (levels) in the potential well, and a simple estimate showed that if the active layer of the heterostructure is as thin as several tens of nanometres, the electron levels would be split apart by tens of milli-electron volts, which should be observable. This structure is now called a quantum well
Quantum well
A quantum well is a potential well with only discrete energy values.One technology to create quantization is to confine particles, which were originally free to move in three dimensions, to two dimensions, forcing them to occupy a planar region...

.

Henry then calculated how this quantization would alter the optical absorption edge of the semiconductor. His conclusion was
that instead of the optical absorption increasing smoothly, the absorption edge of a thin heterostructure would appear as a series
of steps.

In addition to Henry's contributions, the quantum well (or double-heterostructure laser, as it was originally known) was actually first proposed in 1963 by Herber Kroemer in Proceedings of the IEEE and simultaneously (in 1963) in the U.S.S.R by Zh. I. Alferov and R.F. Kazarinov . Alferov and Kroemer shared a Nobel Prize in 2000 for their work in semiconductor heterostructures.

Experimental verification of quantum wells

In early 1973, Henry proposed to R. Dingle, a physicist in his department, that he look for these predicted steps. The very thin
heterostructures were made by W. Wiegmann using molecular beam epitaxy
Molecular beam epitaxy
Molecular beam epitaxy is one of several methods of depositing single crystals. It was invented in the late 1960s at Bell Telephone Laboratories by J. R. Arthur and Alfred Y. Cho.-Method:...

. The dramatic effect of the steps was observed in the ensuing
experiment, published in 1974 .

Invention of the quantum well laser

After this experiment showed the reality of the predicted quantum well energy levels, Henry tried to think of an application.
He realized that the quantum well structure would alter the density of states of the semiconductor, and result in an improved
semiconductor laser requiring fewer electrons and electron hole
Electron hole
An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics, chemistry, and electrical engineering. The concept describes the lack of an electron at a position where one could exist in an atom or atomic lattice...

s to reach laser threshold. Also, he realized that the laser wavelength
could be changed merely by changing the thickness of the thin quantum well
Quantum well
A quantum well is a potential well with only discrete energy values.One technology to create quantization is to confine particles, which were originally free to move in three dimensions, to two dimensions, forcing them to occupy a planar region...

 layers, whereas in the conventional laser a change in wavelength
requires a change in layer composition. Such a laser, he reasoned, would have superior performance characteristics compared to the
standard double heterostructure lasers being made at that time.

Dingle and Henry received a patent on this new type of semiconductor laser comprising a pair of wide bandgap layers having an active region sandwiched between them, in which "the active layers are thin enough (e.g., about 1 to 50 nanometres) to separate the quantum levels of electrons confined therein. These lasers exhibit wavelength tunability by changing the thickness of the active layers. Also described is the possibility of threshold reductions resulting from modification of the density of electron states." The patent was issued on September 21, 1976, entitled "Quantum Effects in Heterostructure Lasers," U.S. Patent No. 3,982,297 .

Quantum well lasers require fewer electrons and holes to reach threshold than conventional double heterostructure
Double heterostructure
A double heterostructure is formed when two semiconductor materials are grown in to a "sandwich". One material is used for the outer layers , and another of smaller band gap is used for the inner layer. In our example, there are two AlGaAs-GaAs junctions at each side of the inner layer...


lasers. A well-designed quantum well laser can have an exceedingly low threshold current.

Moreover, since quantum efficiency (photons-out per electrons-in) is largely limited by optical absorption by the
electrons and holes, very high quantum efficiencies can be achieved with the quantum well laser.

To compensate for the reduction in active layer thickness,
a small number of identical quantum wells are often used. This
is called a multi-quantum well laser.

Early demonstrations

The term "quantum well laser" was coined in the late 1970s by Nick Holonyak
Nick Holonyak
Nick Holonyak, Jr. invented the first practically useful visible LED in 1962 while working as a consulting scientist at a General Electric Company laboratory in Syracuse, New York and has been called "the father of the light-emitting diode"...

 and his students at the University of Illinois at Urbana Champaign.
They demonstrated the first quantum well laser in 1977. W.T. Tsang at Bell Laboratories in the late 1970s and early 1980s showed the
promise of quantum well lasers by demonstrating that when the quantum well parameters are optimized, they have exceedingly low threshold
current and very high efficiency in converting current-in to light-out, making them ideal for widespread use.

The story of the origin of the quantum well concept, its
experimental verification, and the invention of the quantum well
laser is told by Henry in more detail in the Foreword to "Quantum Well
Lasers," ed. by Peter S. Zory, Jr.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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