My Fluorescence spectra peak of ZnO thin films is decreasing with the increased thickness. Please suggest the probable reason or paper to explain this phenomenon.
ZnO has large number of different defects originated from Zn interstitial vacancy and oxygen vacancy. The origin of fluorescence spectra is due to the radiative recombination between Zn interstitial defects states and oxygen vacancy states. Also some non- raditative recombination between conduction band surface states which is basically due to the oxygen vacancy. As the thickness of ZnO increases concentration of surface states increase so as non- radiative recombination. This might be the reason of decreasing fluorescence with increasing thickness of ZnO. If you'll the samplein presence oxygen, you may get reverse action.
Have you performed Absorption or Excitation measurements? Have you seen if your emission peak is shifted towards higher or lower energies? This extra experiments can help you with that question too.
In most of the literature PL intensity has increased with the thickness whereas it is opposite in my case and i didn't found any explanation for decrease in PL peak intensity with increased thickness in literature. Yes i have absorption spectra curve which is minimum for lower thickness. Fluorescence peak is shifting towards higher wavelength with increased thickness.
It is generally known the ZnO reveals two luminescence bands at least. One band is due to excitons (band located at ~3.3 eV) other band is due to defects (the "green" luminescence). The reasons for these bands intensity change are different. Therefore the questions -(I) was detected the both luminescence bands? (ii) Did the fluorescence intensity decreases in both bands?
The second luminescence band is "Red" . In this region also intensity is decreasing with thickness.
What is the first band? Is it the 3,3 eV ? The red band in ZnO could be associated with impurities.
I do not know anything about the experiment and the method and formation of the ZnO films. However a very similar effects are often observed in low temperature matrix studies, and in studies of thin layers at low temperatures. As long as the layer is thin, the deposited layer has the temperature of the cold substrate. When its thickness grows, the solidified gas has a poor thermal conductivity, and the surface is being warmed up by the deposition of the additional layers from the surface, and they are therefore better annealed, heated by the impact of the deposited particles, bringing in their kinetic energy. It is quite possible that a similar effect may be responsible in your ZnO observations.
Yes , first band is nearby 3.3eV, deposition technique was RF sputtering followed by annealing at 400 degree.
Despite of material properties variation you may have a variation of emission variation from a thin film which is attributed to extraction variation with thickness. I am modeling PL intensity from thin film currently for a paper and what I observed is that for an emission band around 750nm the PL extraction may be constant for thin film with thickness bellow 100nm, for higher thickness the PL intensity is oscillating due to extraction oscillating (extraction is ratio of what is emit in the air over what is emit in the active layer). So I don’t know on which range of thickness of ZnO film you work but you may keep in mind that the PL intensity is not only material dependent but also geometrical dependent.
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