In solid-state physics, a band gap/energy gap is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in eV) between the top of the VB and the bottom of the CB in insulators and semiconductors. It is the energy required to promote a valence electron bound to an atom to become a conduction electron, which is free to move within the crystal lattice and serve as a charge carrier to conduct electric current.
A semiconductor will not absorb photons of energy less than the band gap and the energy of the electron-hole pair produced by a photon is equal to the band gap energy.
The band gap is a major factor determining the electrical conductivity of a solid. Band-gap engineering is the process of controlling/tuning the band gap of a material by controlling the composition of certain semiconductor alloys, such as GaAlAs, InGaAs, and InAlAs. Band gap depends on doping, size, temperature, pressure etc. It is also possible to construct layered materials with alternating compositions by techniques like molecular-beam epitaxy. These methods are exploited in the design of heterojunction bipolar transistors (HBTs), laser diodes and solar cells.
In a quantum dot crystal, the band gap is size dependent and can be altered to produce a range of energies between the valence band and conduction band. Band gap increases with decrease in size due to electron confinement at Nano-scale. It is also known as Quantum confinement effect.
However, I am working on inorganic semiconducting silicide materials for potential applications in solar cells. Beta phase Iron di-silicide (β-FeSi2) has a band gap of about 0.87 eV which can be tuned from 0.8 eV to less than 0.9 eV.
How can we increase its band gap further, beyond 0.9 eV? In particular, can we tune its band gap near to optimum band gap of 1.5 eV for solar energy harvesting?
Thank you, Rüdiger Mitdank sir for your precious reply. This is actually what I was waiting for. Your thought is extraordinary. Now, I can discuss about it with my Supervisor.
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Bests.
It may be helpful.
https://www.researchgate.net/post/What_is_the_reason_behind_increasing_bandgap_on_doping
Dear Md. Mahabub,
it is an interesting question and a challenge, because the enhancement of the gap is to be nearly a factor 2. From your list of possibilities I would select the quantum size effects. If a very thin layer of your material (I call it active ,absorbing material) is arranged between 2 materials with sufficient high gap (passive material with a gap > 2*active gap), than the active gap increases. This is a quantum well and the parameter can be calculated if you know the selected materials. Because such a well is to be used to separate electron hole pairs, the thickness of this material is too low. Furthermore, you need a conductivity between the different layers. This problem can be solved to grow a superlattice which consists of sufficient wells. In this superlattice form minibands which can transport the carrier. So more quantum wells, so thicker the superlattice and therefore the absorbing layer. Then, you must construct a p-n-junction in the superlattice by doping or a shottky-barrier, so that e-h-pairs generated by the light, can be separated.
This is a proposal and I believe it could work.
Regards
R. Mitdank
Thank you, Rüdiger Mitdank sir for your precious reply. This is actually what I was waiting for. Your thought is extraordinary. Now, I can discuss about it with my Supervisor.
Md. Mahabub Alam Moon
Interesting question....
β-FeSi2 has a Eg of ~0.85 eV that can be tuned 0.8-0.9 eV.
Increasing its Eg up to 1.5 eV is definitely a challenging task.
Depending on the preparation method, it could be possible to prepare the material with enhanced degree of amorphicity, which should boost the band gap to higher values.
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