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

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Discussion Overview

The discussion revolves around the identification of an unidentified peak (XX) observed in temperature variation photoluminescence (PL) spectra of samples ranging from 4 K to 200 K. Participants explore the characteristics of the XX peak in relation to known emission lines, including free exciton (FE) and exciton bound to neutral donors (DX), while considering various hypotheses regarding its origin.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • The XX peak does not quench at high temperatures, which suggests it may be exciton-related, but its exact nature is uncertain.
  • Some participants propose that the XX peak could be emission from an exciton bound to a neutral acceptor, based on the binding energy considerations.
  • Participants eliminate several possibilities for the XX peak's origin, including defect-related emissions, FE-LO coupling, and biexciton emissions, based on energy differences and quenching behavior.
  • Details about the sample, energy levels of the peaks, and their relative intensities are provided to aid in the discussion.
  • Questions are raised regarding the specific characteristics of the XX peak, including shifts and relative intensities compared to other peaks.

Areas of Agreement / Disagreement

Participants express differing views on the origin of the XX peak, with some supporting the idea of it being related to a neutral acceptor while others remain uncertain. The discussion does not reach a consensus on the identification of the XX peak.

Contextual Notes

Participants note the importance of temperature effects on emission peaks and the binding energies of excitons, which may influence the behavior of the XX peak. There are unresolved aspects regarding the specific nature of the XX peak and its relationship to other emissions.

sttan
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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?
 
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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 ?
 
Gokul43201 said:
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
 
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.
 
Dear Gokul43201,

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