After creating the defects we use to get the defect state between the host band gap of the materials and depending on the there position they may be deep states or shallow states. These state are affected by the polaron and trion etc. But I want to understand how?
Dear Akash,
perhaps the best way to help you is to give an example :
In pure Silicon crystal, you can add some impurities like Phosphorous (P). P is pentavalent (5 electrons in the outer shell) and has therefore 5 electrons but there are only 4 bonds. It sits in Silicon lattice and the energy state of the 5th electron is just below Silicon conduction band. The level is therefore named as "shallow". Now, in the manufacturing process, there might be introduced also some point defects like vacancies and these can can form a defect complex: P-vacancy complex. The energy level of the p 5th electron can now drop deeper into the forbidden band gap, particularly because of the large lattice distortion around the defect (polaron effect - selftrapping). In fact, there are defects in Silicon, like Se and the vacancy itself , where the lattice distortion is so large that it can can compensate for the Coulomb repulsion and the defect in question (vacancy) can be occupied by no electron, one electron , two electrons and all the energy levels are still in the Silicon band gap.
So, in short , a localised electron leads to lattice distortion which in turn leads to depening of the potential well into which electron drops even further (its energy decreasing) and so on. The process is often called self-trapping and the final quasi-partice - electron and the strong lattice distortion around it, is polaron.
I do not know what the trion is ?!
I hope this will help
With best regards
Petr
some refs on trions.
Trion fine structure and coupled spin–valley dynamics in monolayer tungsten disulfide http://www.nature.com/articles/ncomms12715.pdf
Stability of singlet and triplet trions in carbon nanotubes http://www.sciencedirect.com/science/article/pii/S037596010900231X
Three-Particle Complexes in Two-Dimensional Semiconductors http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.107401
Theory of neutral and charged excitons in monolayer transition metal dichalcogenides http://journals.aps.org/prb/abstract/10.1103/PhysRevB.88.045318
Dear Akash and Ioannis,
this is just to thank Ioannis for the references to trions. Interesting excitations, one learns every day !
With best regards
Petr
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