When I calculate the optical gap of my thin films deposited on Corning glass via transmittance UV-Vis-NIR measurements , I find a different value than that I calculated via reflectance UV-Vis-NIR measurement.
I wonder what in the structure or the surface of the film has to be taken in consideration to make Reflectance measurements ????
Thank You
Optical gap measurement by electronic spectroscopy can be affected by nanoparticle-to-matrix interface bonding, as well as by Rayleigh Scattering. So, differences on transmission and reflectance measurements may be related to Rayleigh Scattering contribution. Of course, this effect should be related to sintering degree of your film, I would measure the optical Gap as a function of sintering degree.
Good luck
First, thank you Dr Ubirajara
How would you do this: measure the optical Gap as a function of sintering degree ?
Kind regards
I agree with Ubirajara Rodrigues Filhothat that scattering contribute more pronouncely on reflectance measurements. However for me the difference comes from the application of different approximation models for calculating Eg.
I agree with the above view point. Including some scattering model in you calculation,you may get a more reasonable value.however,if the model don't match the practice surface morphology,the results will not precise. I encountered the same problem in calculating optical gap of thin film silicon prepared with PECVD. the roughness increase with growth of thin film thickness. So if you reduce the deposition periodic to get thinner sample，it may results in a preciser value.
if the difference is not so large, it is obvious
you should also take into account the film thickness
For band gap calculation, you require transmittance as well as reflectance along with the thickness of the films. You can use Tomlin's equations for determining extinction coefficient and then Tauc's plot will give you reasonable perfect value of band gap.
Dr. Manish Kumar,
Please kindly suggest or attach an elaborate journal article to buttress your point on this.
Thank you in anticipation for your kind gesture.
Hi Mr. Balogun,
Thanks for your interest. You may look details into following paper,
[1] J.Tauc, R. Grigorovici and A. Vancu, Phys. Stat. Sol. 15 (1966) 627
An easy way to use Tauc's plot for reliable bandgap
[2] M Kumar, GB Reddy, AIP Advances 1 (2011), 022111
and for best calculations of extinction coefficients..
[3] S.G. Tomlin, J.Phys. D 1 (1968) 1667
Here you need to take care of normalization of measured reflectance values as well because in spectrophotometers reflectance measurement also depends on the beam optics and have inherent losses by multiple reflecing from imperfect mirrors, thats why most of the time the measured reflectance values are more than the actuals.
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