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Excitons can be observed when we excite electrons to the conduction band.

I don't know about excitons being observed when we excite the electrons to an electronic level that would eventually be in the middle of the bandgap.

Why? Is it related with the binding energy, density of states of the level, or is it a fundamental aspect that I am not aware of?

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In the middle of the bandgap there could only be deep doping (or other defect) levels, electrons on them are bound, and when an electron is put to that level, there's no hole counterpart which to combine with to form an exciton. – Ruslan Jan 10 '14 at 12:57
@Ruslan I understand that such levels are possible by defects such as vacancies or a doping. But I do not understand why "there's no hole counterpart" when we excite to these electronic states. – cinico Jan 10 '14 at 14:29
I'd guess that a hole is created if an electron is excited to some level in the band gap. It might not be a big deal as there probably aren't enough states in the bandgap to make much of a difference. – lnmaurer Jan 10 '14 at 23:30
@Inmaurer What do you mean by "to make much of a difference"? Are you trying to say that you believe that the exciton does exist in this case but that the emission intensity is low? – cinico Jan 11 '14 at 0:38

To form an exciton you need at least two conditions:

  1. an electron and hole.
  2. overlap of wavefunction (proximity).

In bulk materials you also need low temperatures, however in semiconductor nanostructures it is possible to have excitons at room temperature because of the quantum confinement of both carrier types (in a type-I quantum well or quantum dot for example).

Trap centres

As Ruslan pointed out above, if you photo-excite into an electron accepting state in the bandgap you have satisfied the first criteria, however not the second because the hole the photogenerated hole remains is the valence band. Traps can accept either electrons or holes depending on the type of defect or impurity, so although you pin one carrier type but the carrier with opposite charge will always remain free.

Recombination centres

It is possible to get defects/impurities which accept both carriers types, but these are not trap centres, they are recombination centres. For example, if the site pins a hole it may also attract an electron resulting in rapid recombination. We don't normally think of these sites forming excitions because, presumably, the lifetime of the state is extremely short. However, in principle I don't see why it isn't possible, but it might just be an insignificant effect.

Maybe some more insight could be gained by consider the quantum mechanics of exitonic wavefunction vs. the wavefunction for recombination centres.

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Thank you for your answer. However, I'm not convinced yet. I can have a recombination centre, such as energy levels of a rare-earth, which can have lifetimes of the order of miliseconds. Why wouldn't we have an exciton in this case? – cinico Jan 14 '14 at 14:05
Just a small note, not really important for the question: there are materials, such as ZnO with binding energies of the exciton high enough to be observable at room temperature. In the type of materials I work with, I usually go down to 10K. I admit that it can still be too high to observe an exciton created by exciting a recombination centre, but the question of "can it be created or not?" still remains – cinico Jan 14 '14 at 14:06
Remember that lifetime is deeply tired up with the quantum mechanics of the system such as wavefunction overlap, parity (direct/indirect transitions etc.) and material parameters so one cannot generalise too much. In the above I was specifically thinking of semiconductors (III-V etc.) Rare-earths are not a bimolecular system and so you don't form excitons! They have monomolecular atomic transitions. As for ZnO, yes of-course there are exceptions to the rule, like most things in physics. Try and do a literature search for "exciton on impurity/defects" might show something interesting results. – boyfarrell Jan 14 '14 at 15:20
I should know this but I don't: I do not understand, in this case, the difference between a bimolecular and a monomolecular system and why the latter does not form an exciton. Maybe what I misunderstood is that an electron cannot in fact be excited from the valence band of the host to an energy level of the RE dopant. Is this true? Thank you once again :) – cinico Jan 14 '14 at 15:35
Search Luminescence Spectroscopy of Semiconductors By Ivan Pelant, Jan Valenta for "monomolecular". I would encourage you to ask another question on this topic rather than cluttering up this comment thread. – boyfarrell Jan 14 '14 at 15:43

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