Metastable deep defect in semiconductors

[mendes]

how can we understand it please ? Thanks.

 

[berkeman]

how can we understand it please ? Thanks.

Can you please provide some context and background for your question? What do you know so far?

http://www.google.com/search?source...02&q=metastable+deep+defect+in+semiconductors

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[caffenta]

A DX center is a typical example of a metastable deep defect. There's a lot of info on them available online.

 

[mendes]

Can you please provide some context and background for your question? What do you know so far?

http://www.google.com/search?source...02&q=metastable+deep+defect+in+semiconductors

.

What I know so far is that a defect whose electronic configuration changes upon changes in its electronic charge (loss or capture of carriers) and the defect becomes in a way a different defect is said to have a metastable state. Electrons need energy to overcome a barrier before getting captured by the "new" defect, and hence, at low temperature, only few electrons are captured and the defect is still in its metastbale state. This leads for example to persistent photoconductivity.

My question is: what is the connection between the changes in the electronic configuration of the defect, that is the relaxation of the lattice, and the energy capture barrier for carriers ? Is the fact when a carrier manages to overcome the capture barrier and get captured by the defect a transfer of energy to the lattice to be able to relax ? That is, where does the lattice get the energy necessary for its relaxation ? Or is the lattice relaxation and the carrier capture barriers 2 independant things ?

 

[caffenta]

When the deep level is excited, it changes the local electronic structure of the defect; bonds break and there is a shift in the local lattice structure (relaxation). That gives rise to a new shallow defect (for example). There is an energy barrier that prevents the return to the original deep level configuration. If the temperature is too low, the energy barrier is not overcome, so the new shallow defect is "stable". This is why photoconductivity persists even after the light is turned off. When the sample is warmed up, the barrier is overcome and the defect returns to its original deep level configuration, trapping the electron and persistent photoconductivity stops.

In my earlier post, I had linked to a paper that had a good explanation of the mechanism, but after consulting with mentors, I decided to remove it because it was a copyrighted paper. If you have access to the journal, this is the reference: D. J. Chadi, K. J. Chang, Phys. Rev. B, 39(14), 10063 (1989).

Do a search on DX centers or EL2 defects on Google. You can find lots of info on the mechanism.

 

[mendes]

When the deep level is excited, it changes the local electronic structure of the defect; bonds break and there is a shift in the local lattice structure (relaxation). That gives rise to a new shallow defect (for example). There is an energy barrier that prevents the return to the original deep level configuration. If the temperature is too low, the energy barrier is not overcome, so the new shallow defect is "stable". This is why photoconductivity persists even after the light is turned off. When the sample is warmed up, the barrier is overcome and the defect returns to its original deep level configuration, trapping the electron and persistent photoconductivity stops.

In my earlier post, I had linked to a paper that had a good explanation of the mechanism, but after consulting with mentors, I decided to remove it because it was a copyrighted paper. If you have access to the journal, this is the reference: D. J. Chadi, K. J. Chang, Phys. Rev. B, 39(14), 10063 (1989).

Do a search on DX centers or EL2 defects on Google. You can find lots of info on the mechanism.

Thanks for the answer.

As I said in my last post what is not clear for me is the connection between the capture barrier for electrons to be recaptured and the required relaxation of the defect to recover its steady state. Are these two things connected or they are 2 independent things ?

Also I have another question : why is the optical ionization energy higher than the thermal ionization energy for these "metastable" defects ?

Thanks again.

 

[caffenta]

The barrier is related to the structural shift that needs to occur to go from metastable to original configuration. If an electron drops into the metastable shallow state, it will not be trapped into the deep level unless the structural shift happens. Otherwise, it will just be re-excited thermally into the conduction band eventually.

The attached picture should explain a lot (including the second question).

 

[mendes]

The barrier is related to the structural shift that needs to occur to go from metastable to original configuration. If an electron drops into the metastable shallow state, it will not be trapped into the deep level unless the structural shift happens. Otherwise, it will just be re-excited thermally into the conduction band eventually.

The attached picture should explain a lot (including the second question).

I must be missing something here. I am trying to see if there is a connection between the capture barrier energy for carriers and the restructuration of the lattice around the defect. is is a "real" potential barrier for carriers to be trapped by the defect or it is just a thermal activation energy for the restructuration of the lattice ? In other words, does the capture of a carrier with higher energy than the capture barrier lead to the restructuration of the lattice, or it is the restructuration of the lattice that leads to the possibility of recapturing carriers ? What is the cause and what is the effect ? :)

 

[caffenta]

It's both a potential barrier and the energy required to shift the donor atom from its normal substitutional site to the deep level site. The capture energy is the kinetics part of the process.

You can’t really separate cause and effect. The electron must drop into the shallow donor level and the donor atom must shift to trap the electron. If the level is empty, the deep level state is not stable so an electron can’t get trapped in it. But if the electron drops into the shallow level and the atom does not shift position, the electron will not get trapped.

Maybe someone else can explain it better than I do.