What is the unidentified XX peak in my temperature variation PL spectra?

[sttan]

I had done a temperature variation PL (4 to 200 K) on my samples. I have found out that there are three emission line in the PL spectra, namely free exciton (FE), exciton bound to neutral donors(DX) and an unknown peak (let's called it XX).

As the temperature was increased from 4 K to 200 K, as expected, the DX was quenched at a temperature of around 80 K. The FE is then the dominant peak from temperature 80 K onwards. Of course, there is redshift of emission for the FE due to the temperature effect. What confused me is the the XX peak.

The XX peak doesn't even quench at high temperature that up to 200 K. This puzzled me because it seems that the XX is exciton-related emission. I had eliminated out the few possible emission that could be attributed to the XX peak:

(1) it could not be the defect-related emission since it is not being quenched at high temperature.

(2) it could not be the FE-LO since the energy difference between the FE and XX is smaller than the LO energy.

(3) it is not biexciton because the energy separation is too large in this case.

Is there anyone knows what is this XX peak?

 

[Gokul43201]

1. What is the sample ?

2. What are the energies/wavelengths of the 3 peaks ?

3. What are their relative intensities ?

4. What shifts, if any, occur in the XX peak ?

 

[sttan]

1. What is the sample ?

2. What are the energies/wavelengths of the 3 peaks ?

3. What are their relative intensities ?

4. What shifts, if any, occur in the XX peak ?

1. ZnO films grown on sapphire.

2. @ 4K, FE (3.373 eV), BE1 (3.359 eV) & BE2 (3.353 eV), XX (3.335 eV)
@ 200K, FE (3.348 eV), BE1 & BE2 (quenched), XX (3.300 eV)

3. @ 4K, BE1:BE2:FE:XX = 1:0.86:0.34:0.26; FE:XX = 1:0.77
@ 200K, FE:XX = 1:0.59

 

[Gokul43201]

Sorry for not looking at this earlier, it must have slipped by me !

I'm almost positive that XX is emission from an exciton bound to a neutral acceptor.

Looking at the donor bound exciton peaks, they are about 10-20 meV from the FE peak. That tells you that their binding energies are of order 10-20 meV (or about 100-200K). Naturally, at temperatures above 100K, these peaks will be significantly quenched.

Similarly, you find that the binding energy (assuming this is a bound exciton) of XX is of order 500K. So, there is no reason to expect it to quench by 200K.

 

[sttan]

Dear Gokul43201,

Thank you for your information. I have overlooked this possible origin.