Quantum well
[4] They suggested that a heterostructure made up of alternating thin layers of semiconductors with different band-gaps should exhibit interesting and useful properties.
The development of quantum well devices is greatly attributed to the advancements in crystal growth techniques.
Therefore, having great control over the growth of these heterostructures allows for the development of semiconductor devices that can have very fine-tuned properties.
[6] The theory surrounding quantum well devices has led to significant advancements in the production and efficiency of many modern components such as light-emitting diodes, transistors for example.
These structures can be grown by molecular beam epitaxy or chemical vapor deposition with control of the layer thickness down to monolayers.
For example, an electron in the conduction band can have lower energy within the well than it could have in the AlGaAs region of this structure.
The solution wave functions cannot exist in the barrier region of the well, due to the infinitely high potential.
The model also neglects the fact that in reality, the wave functions do not go to zero at the boundary of the well but 'bleed' into the wall (due to quantum tunneling) and decay exponentially to zero.
The quantized energy eigenstates inside the well, which depend on the wave vector and the quantum number (
[4] Solutions for the allowed energy states in superlattices is similar to that for finite quantum wells with a change in the boundary conditions that arise due to the periodicity of the structures.
Because of their quasi-two-dimensional nature, electrons in quantum wells have a density of states as a function of energy that has distinct steps, versus a smooth square root dependence that is found in bulk materials.
These two factors, together with the reduced amount of active material in quantum wells, leads to better performance in optical devices such as laser diodes.
They were initially used in a resonant pulse modelocking (RPM) scheme as starting mechanisms for Ti:sapphire lasers which employed KLM as a fast saturable absorber.
For example, saturation fluence can be controlled by varying the reflectivity of the top reflector while modulation depth and recovery time can be tailored by changing the low-temperature growing conditions for the absorber layers.
This freedom of design has further extended the application of SESAMs into mode-locking of fibre lasers where a relatively high modulation depth is needed to ensure self-starting and operation stability.
In general, greater temperature differences between the cavity and the reservoirs increases electron flow and output power.
The theoretical maximum efficiency of traditional single-junction cells is about 34%, due in large part to their inability to capture many different wavelengths of light.
Multi-junction solar cells, which consist of multiple p-n junctions of different bandgaps connected in series, increase the theoretical efficiency by broadening the range of absorbed wavelengths, but their complexity and manufacturing cost limit their use to niche applications.
In room temperature conditions, these photo-generated carriers have sufficient thermal energy to escape the well faster than the recombination rate.
It has also been shown that metal or dielectric nanoparticles added above the cell lead to further increases in photo-absorption by scattering incident light into lateral propagation paths confined within the multiple-quantum-well intrinsic layer.
[14] With conventional single-junction photovoltaic solar cells, the power it generates is the product of the photocurrent and voltage across the diode.
[15] With the introduction of quantum wells (QWs), the efficiency limit of single-junction strained QW silicon devices have increased to 28.3%.
[15] With their experiments on p–i–n junction photodiodes, Barnham's group showed that placing QWs in the depleted region increases the efficiency of a device.
Multi-junction solar cells are created by stacking multiple p-i-n junctions of different bandgaps.
As more quantum wells (QWs) are grown together, the material grows with dislocations due to the varying lattice constants.
In a heavily compressed material, the heavy holes (hh) move to a higher energy state.
[7][19] One can calculate the difference in energy due to the splitting of hh and lh from the shear deformation potential,
[21] With the effective use of carriers in the QWs, researchers can increase the efficiency of quantum well solar cells (QWSCs).
[7] This result provides some evidence that there is a struggle of extending the QWs' absorption thresholds to longer wavelengths due to strain balance and carrier transport issues.
A viable option such as QWSCs provides the public with an opportunity to move away from greenhouse gas inducing methods to a greener alternative, solar energy.
