The Quantum Well
What is a Quantum Well
Optical Integraded devices are normally built with the consideration that the device size will be large compared to the wavelength of the beams in the system. When however, the device size is reduced to a size of the same order of magnitude as the wavelength of light in the system, unique properties can be observed. The class of device that operates under the unique properties of this arrangement is the “quantum well.”
Uses of Quantum Wells
Quantum wells may be integrated to other optical and opt-electronic integrated circuits. Uses of quantum wells include improved lasers, photodiodes, modulators and switches.
Building a Quantum Well
A quantum well structure features one or more very thin layers of narrow bandgap semiconductor material, interleaved with layers of wider bandgap semiconductors. The thickness of the layers in a quantum well are typically 100 Angstroms or smaller. Quantum wells with many layers are termed a “Multiple Quantum Well” (MQW) structure and quantum wells with only one layer are termed a “Single Quantum Well (SQW) structure. A typical MQW structure may have around 100 layers. The GaAs-AlAs material system or GaInAsP are common choices for materials in quantum well structures.
Superlattice Structure
A superlattice structure is a term for a case in whic a multiple quantum well structure is built with barrier wals that are thin enough that electrons are able to tunnel through the structure.
The Quantum Well and Quantum Dot
The quantum well reduces the separation between an electron and hole in a semiconductor, altering the wavefunction and allowing a strong exciton bonding effect at room temperature. The semiconductor laser results from this process. Wave functions in the well are shown to the right.
When a field is applied across the well, this can result in the tilting of the wells. This can reduce the effective band gap of the material. The process of tilting the wells the alter the band gap is called the Quantum Confined Stark Effect.
Quantum wells are generally understood in two dimensions. The conduction band is forced to be closer the valence band. When this is done in three dimensions to create a small box, where this squeezing effect can be emulated in all dimensions, this is termed a Quantum Dot. A Quantum Dot it turns out is highly effective at producing a high level of energy and as a result there is a high probability that it works as a coherent light source (laser). Quantum dots are readily used today, however since the process of fabrication employs the use of defects in a material to create a quantum dot, the coherency of the light produced is not perfect. Quantum dots are used in data centers for light transmission at a distance of meters. Quantum dots remain a low cost and reasonably efficient light transmission source for small distances. One reason for the low cost of quantum dots is that they can be grown on silicon wafers. A quantum well is not easily (highly unreliably, but perhaps not impossible) grown on Silicon wafers. The issue that arises with quantum wells when being grown on silicon wafers is that the size of atoms in the wafers and thereby the lattice constant is not readily compatible.
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