What is behind logic in the doping effect of Sn-doped ZnO thin film to work on red shift effect in its band gap? Can it be lower than 3.18 eV?
what does it mean in the following article?
link:
http://iopscience.iop.org/article/10.1088/0022-3727/42/18/185002/pdf
"The redshift of the optical band gap is due to the deep states in the band gap, and a change of ~0.2 eV can only be observed when a substrate is not used."
Hi Reefaz,
I can understand your concern. In my opinion, the mentioned article does not describe the effect of clssical doping. Raman spectroscopy identified rutile SnO2 related lines, i.e. the film represents a mixture of ZnO with SnO2. The authors do not provide any quantitative analysis of the transmission data for obtaining band gaps. Why do we see in inset of Fig. 3 a double-step decrease of transmission and why do the authors attribute the long-wavelength onr to the band gap? It would mean that one should find already for ZnO a sharp second onset of absorption about 300 meV above the fundamental absorption edge. It contradicts to all previous studies of high quality ZnO.
See for example the attached first file and the cited work therein of Müller or Shokhovets. Also for SnO2, a quantitative analysis is possible.
Sorry to say it, but the reference you mentioned, does not provide reliable band gaps.
Best regards, Ruediger
Article Optical properties of MgZnO alloys: Excitons and exciton-pho...
Article Ordinary and extraordinary dielectric functions of rutile Sn...
Hello,
Thank you for your valuable comment. I am going thoroughly into your provided papers.
Thanking you,
Reefaz
Dear Reefaz,
welcome!,
I agree with Rudiger and want to add that The addition of Sn to ZO would build a compound of SnO2 and ZO. According to the principle of band gap of mixed compouns
the band gap of the compound= ( 1-X) Eg of ZO = X Eg of SnO2.
As the bandgap of SnO2 is greater than the enrgy gap of ZO, one expects basically the increase of the energy gap by doping with Sn.
However, deviations from this rule is due to the deviation from the Stoichiometry. In addition there is appreciable effects of the grain size energy band structure of the material. So, the red shift is not possibly due to doping but due to the large traps at the grain boundaries.
Best wishes
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