Hello,

I would say that for direct bandgap materials, ellipsometry will be accurate and coherent with other technique.

For other materials, there exists several conventions (Tauc plots, dispertion parameters...) for bandgap determinations and it is possible that different conventions will give different values.

After that, comparing ellipsometry and UVIS absorption can be tried as long as the investigation is done over a similar spectral range and of course on same samples.

Dear Dr. PV Raghavendra Karanth

Experimentally, to get  the value of Eg , we usually use of the Tauc relation, which is given by this equation:                                

 αhν = A (hν - Eg )n

where α is absorption coefficient given by                

α = 1/t  ln [(1-R)2 / T ]

Noting that in the UV region, R may equal zero, so

α = 1/t ln (1 / T)

where t  is the sample thickness, T and R are the transmission and  reflection, while  (hν) is the photon energy, where:

hν = 1240 / incident wavelength

If any one plot a graph between (αhν)1/n  versus (hν), he can get a straight line. This line intersects the X-axis at (αhν)1/n = 0 . The values of Eg have been estimated from this intercept.  The value of n is dependent on the electronic transition type. Where:

n=1/2 for direct allowed transition,

 n= 2 for indirect allowed transition, 

n=3  for direct forbidden transition and

 n=3/2 indirect forbidden transition. You should try to select the suitable n according to your samples and their preparations.

The appended papers may help you; they are examples of direct and indirect allowed transitions.

Good luck

Your,

Ahmed Saeed Hassanien

https://www.researchgate.net/publication/283338024_Effect_of_Se_addition_on_optical_and_electrical_properties_of_chalcogenide_CdSSe_thin_films

https://www.researchgate.net/publication/281495242_Influence_of_composition_on_optical_and_dispersion_parameters_of_thermally_evaporated_non-crystalline_Cd50_S50-xSex_thin_films

Article Effect of Se addition on optical and electrical properties o...

Article Influence of composition on optical and dispersion parameter...

How one can know that the sample have four optical transitions or not in order to apply Tauc method where m=0.5,, 2, 1/3 and 2/3?

Hi Dr. Mohammed

You should try the four probabilities. The case which gives you the longest straight line it will be considered and discussed.

Regards

Thank you very much Dr. Ahmed for your reply. please why the case which gives me the longest straight line will be considered? do you have papers for this? and if you agree we can do collaboration together to do a good paper , because I need you for the discussion, what is your opinion?

In the high absorption region one should try the four possibilities of the exponent n that appears in Tauc equation. The one which gives the best fit is then adopted. However, and in some cases, more than on exponent give good fit and thus one should resort to any additional measurements, or alternatively additional information, to resolve this.

The explanation by Dr. Hassanien is very helpful.

Hi Dr. Mohammed RASHEED

You can connect me by e-mail

[email protected]

Phone me at +966502001295

Or by the Face Book messenger

Ahmed Saeed Hassanien

Regards

The optical transitions that occur in the materials are always one of two types, a direct or an indirect transition. The valence band maximum (VBM) and the conduction band minimum (CBM) are each characterized by a certain crystal momentum (k-vector) in the Brillouin zone (BZ). If the VBM and CBM occur at the same value of momentum kmax=kmin it is called a "direct gap". If they are different kmax≠kmin, it is called an indirect gap. Electronic band structures and various possibilities of energy gap in thin film (the electronic transition at Brillouin zone, point) have been theoretically investigated through the use of several modeling techniques based on density functional theory [1-7]. Also, angle-resolved photoemission spectroscopic (ARPES) study provides the direct experimental evidence of the distinct transition in the band structure for thin film samples [8].

1- M. Han, J. Wang, Q. Deng, J. Wang, W. Li, P. Zhang, C. Li, Z. Hu, Journal of Alloys and Compounds 647 (2015) 1028.

2- X. Hu, Z. Zhong , G.A. Fiete, Sci. Rep. 5, 11072; doi: 10.1038/srep11072 (2015).

3- E. Salmani, O. Mounkachi, H. Ez-Zahraouy , A. Benyoussef , M. Hamedoun, E.K. Hlil, Journal of Alloys and Compounds 578 (2013) 77.

4- L. Lin, Z. Li, J. Feng, Z. Zhang, Phys.Chem. Chem. Phys15 (2013) 6063.

5- M. B Johansson, G. Baldissera, I. Valyukh, C. Persson, H. Arwin, G. A. Niklasson, L. O sterlund , J. Phys.: Condens. Matter 25 (2013) 205502.

6- L.S. Cavalcante, M.F.C. Gurgel, E.C. Paris, A.Z. Simoes, M.R. Joya, J.A. Varela, P.S. Pizani, E. Longo, Acta Materialia 55 (2007) 6416.

7- B. Saha, G. Naik, V.P. Drachev, A.Boltasseva,E. E. Marinero, T. D. Sands, J. Appl. Phys. 114 (2013)063519.

8- Y. Zhang, T. R.Chang B.Zhou, Y.T.Cui, H.Yan, Z.Liu, F.Schmitt, J. Lee, R. Moore, Y. Chen, H. Lin, H.T.Jeng, S.K Mo, Z. Hussain, A. Bansil, Z. X. Shen, NATURE NANOTECHNOLOGY, doi:10.1038/nnano.2013.277.

By method of UV-VIS spectroscopic measurements:A material may give both direct and indirect transitions. Is it possible?

Dear Dr. J.D. Desai , yes it is possible. You can get both direct and indirect transitions for some materials.

Regards

Thanks Dr. Ahmed

it is correctly when we  use the Tauc's relation, we should say "most probable transition" is occurred or considered because sometimes we can get two identical longest straight line for two different values of n for the same onset or transition edge

The person is confused which one he chooses?

He select one value of n and the type of transition should be refereed as "most probable transition"

Dear Dr. M M Makhlouf

Peace be upon you

We should not confused.

The energy gap or the electron transition is dependent upon the composition itself, its constitutes and its crystal structure. In general, in the high cystalline materials or single crystal materials, the probable transition is a direct one, while if the materials which have amorphous structures, the probable transition is the indirect transition. But if the materials are polycrystalline, semicrystalline or have low crysallinity structure then the transition may be direct or indirect. In all case you should take the transition which has the longest straight line.

Good luck

I found the same thing, to avoid this problem I propose to use the method of second derivative.

The derivative spectroscopy is a fast and intuitive method for estimation of the number and the positions of the bands in various branches of molecular spectroscopy (UV-Vis-NIR, IR/Raman, CD). In addition - it has substantial importance in the pharmaceutical spectral analysis and it is an integral part of the treatment of NIR data.

Sometime a material composition shows two or three straight line part in tauc plot. Please let me know it's faulty or it's possible?

read the reference

M. B Johansson, G. Baldissera, I. Valyukh, C. Persson, H. Arwin, G. A. Niklasson, L. O sterlund , J. Phys.: Condens. Matter 25 (2013) 205502