Why is high exciton binding energy in ZnO important?

[hadoque]

The title says it all. I'm doing a presentation on ZnO nanoflowers, and one article says that the bandgap of 3.3eV in combination with the high exciton binding energy of 60meV makes ZnO promising for optoelectronic applications. The bandgap I understand, what property does the exciton binding energy translate to?

 

[Cthugha]

Roughly speaking the exciton binding energy gives the stability against thermal dissociation of excitons. Theat means that there will still be a significant number of carriers way beyond 90% present as excitons in ZnO, while the excitons will have a tendency to dissociate due to the thermal energy becoming comparable to the exciton binding energy already in more "clean" materials like GaAs.

This means that room temperature operation of anything exciton-based requires a material with huge exciton binding energy like ZnO or GaN.

 

[hadoque]

Ok, what does it mean that a material is exciton based? Are all semiconductors exciton based?

 

[Cthugha]

Ok, it is not really the material that is exciton-based, but rather applications.

Excitons are mainly important for the optical properties of a material. Usually optical recombination causing the emission of photons from a semiconductor occurs between free electrons and holes. For both there is a continuum of possible energies and therefore you will also get a rather broad range of photon emission energies. Now if excitons are formed, this means that there is a bound electron-hole state at an energy slightly lower than the band gap energy - it is in fact reduced by the amount of the exciton binding energy. Although also excitons can in principle have kinetic energy and therefore a broader spectrum, it is not as broad as the free-carrier spectrum. Also you have the possibility to trap excitons and reduce their motion by using low-dimensional nanostructures like quantum wells or quantum dots which further narrows the energy spread.

Such a narrow energy spread is highly attractive for optical devices. It is much easier to couple to or build lasers on a material which has a rather discrete density of states than on one which shows a broad continuum.

 

[hadoque]

Yes, I understand. Thanks a lot for the explanation, I was looking all over the web, and everywhere it said that high exciton binding energy is good, but not why...