It seems as a common knowledge, but I cannot understand:
Why does non-radiative decay (electron-hole recombination) in direct-gap semiconductors occur faster than radiative decay?
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$\begingroup$ Somehow that sounds counterintuitive. If non-radiative decay would be fast and radiative decay slow, then there would be almost no light generated by these materials. $\endgroup$– CuriousOneCommented Aug 1, 2016 at 6:51
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$\begingroup$ I think, there are optical selection rules, which, when satisfied, produce light, and, when not satisfied, do not produce light. When the optical selection rules are not satisfied, then recombination can still take place, however, non-radiatively. So, it is sort of competition, but not the absolute one. $\endgroup$– Capo MestreCommented Aug 1, 2016 at 8:23
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I think there is some problem in understanding. In direct band gap materials most of the radiation is through radiative recombination.
The process of radiative recombination is slow in the indirect band gap materials compared to the radiative recombination in direct band gap materials.
Also in the case of indirect band gap materials the radiative recombination is slower than non radiative recombination. In indirect band gap materials the de-excitation is mainly through non radiative process.
The non radiative decay is a result of the de-excitation of electron from conduction band to valance band in the presence of a phonon. The excessive energy and momentum is then carried out by the phonon.
As you know that when the electron is excited from the valance band to the conduction band in indirect band gap materials, first the electrons tend to come to ground state of the conduction band. The conduction band minima is not matching with the top state of the valance band in $\omega-k$ space. Since all the vacancies in the valance band are in the top state, the transition from the conduction band to valance band becomes difficult/forbidden as momentum can not remain conserved i.e. the lifetime of the excited state increases quite a bit. Same can be seen in this Wikipedia link.
Involvement of the phonon helps in conserving the momentum and results in faster non-radiative transition in indirect band gap materials.
I hope this will help
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$\begingroup$ Thank you very much for your reply. However, I was interested mostly only in the direct bandgap case. Even if there is a direct gap, we can also talk about radiative and nonradiative recombinations. And it seems to be a common knowledge that nonradiative decay occurs faster than radiative one, resulting in the photoluminescence quantum yield lower than 100%. $\endgroup$ Commented Aug 2, 2016 at 1:43
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$\begingroup$ I think if the non radiative process is stronger than radiative one the photoluminescence yield is <50%. In any case there is a contribution from both the channels and hence the photoluminescence yield will be less than 100% $\endgroup$– hsinghalCommented Aug 2, 2016 at 2:12
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$\begingroup$ Yes, if the photoluminescence (PL) quantum yield (QY) in direct-gap semiconductors is less than 100%, I guess, it implies that non-radiative processes are faster than radiative ones. So, non-radiative recombinations had already taken place before radiative ones have been accomplished. And my question was why non-radiative recombinations were occurring faster than radiative relaxations, which is still open. $\endgroup$ Commented Aug 2, 2016 at 2:57
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$\begingroup$ I think there is some confusion. First the quantum yield is always less than unity. The maximum quantum yield is the ratio of the emitted photon energy and absorbed photon energy. Secondly both the processrs i.e. radiative and non radiative are occuring simultaneously. Hence both channels are open and the relative strength of these channels will give you final quantum efficiency. $\endgroup$– hsinghalCommented Aug 2, 2016 at 3:33
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$\begingroup$ Yes, that's essentially what I've said. Both non-radiative and radiative processes compete with each other occuring simultaneously. But they, obviously, occur on different time scales. Thus, the competition takes place only on the timescale of the fastest process, right? $\endgroup$ Commented Aug 3, 2016 at 4:03