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?