Dear Dr. Kasim Rawanduzy

Peace be upon you

Along the absorption coefficient curve and near the optical band edge there is an exponential part called Urbach tail. This exponential tail appears in the low crystalline, poor crystalline, the disordered and amorphous materials because these materials have localized states which extended in the band gap. In the low photon energy range, the spectral dependence of the absorption coefficient (α) and photon energy (hν) is known as Urbach empirical rule, which is given by the following equation:

                                                     α  = αo exp (hν/EU) 

where αo is a constant and EU denotes the energy of the band tail or sometimes called Urbach energy, which is weakly dependent upon temperature and is often interpreted as the width of the band tail due to localized states in the normally band gap that is associated with the disordered or low crystalline materials. Taking the logarithm of the two sides of the last equation, hence one can get a straight line equation. It is given as follows:

                                        ln α = ln αo + (hν/EU)                                 

Therefore, the band tail energy or Urbach energy (EU) can be obtained from the slope of the straight line of plotting ln (α) against the incident photon energy (hν).

For more details, you can go to these links:

https://www.researchgate.net/publication/281495242_Influence_of_composition_on_optical_and_dispersion_parameters_of_thermally_evaporated_non-crystalline_Cd50_S50-xSex_thin_films?ev=prf_pub

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

Good luck and my best wishes

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

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

Dear Dr. Kasim Rawanduzy

Herein find two papers where the calculation method is mentioned.

1- http://www.sciencedirect.com/science/article/pii/S0022309311000937#

2- http://www.sciencedirect.com/science/article/pii/S0921452605008884

Firstly Urbach energy is a function of disorder or localized states so the crystallization will have many defects, the degree of defects depends on the value of  Urbach energy.  To calculate it  you should have both T and R of the material, bulk or film and according to the equation that mentioned through those two papers you will get it. both T and R you can use as 0.00 (decimal).

In amorphous materials, due to absence of long range order, valence band and conduction band do no have sharp cut off but have tails of localized states. Due to these band tails, optical absorption edge is not sharp and has tail at lower energies. When we make Tauc plot we do not get straight line in the whole energy range. Urbach tail is related to localized states at band edges. The method suggested above is good enough for the calculation of Urbach energy.

Good information...thanks a lot to Adel Eldenglawey and Dr A. Kumar.

Dear Dr. Kasim Rawanduzy

Peace be upon you

Along the absorption coefficient curve and near the optical band edge there is an exponential part called Urbach tail. This exponential tail appears in the low crystalline, poor crystalline, the disordered and amorphous materials because these materials have localized states which extended in the band gap. In the low photon energy range, the spectral dependence of the absorption coefficient (α) and photon energy (hν) is known as Urbach empirical rule, which is given by the following equation:

                                                     α  = αo exp (hν/EU) 

where αo is a constant and EU denotes the energy of the band tail or sometimes called Urbach energy, which is weakly dependent upon temperature and is often interpreted as the width of the band tail due to localized states in the normally band gap that is associated with the disordered or low crystalline materials. Taking the logarithm of the two sides of the last equation, hence one can get a straight line equation. It is given as follows:

                                        ln α = ln αo + (hν/EU)                                 

Therefore, the band tail energy or Urbach energy (EU) can be obtained from the slope of the straight line of plotting ln (α) against the incident photon energy (hν).

For more details, you can go to these links:

https://www.researchgate.net/publication/281495242_Influence_of_composition_on_optical_and_dispersion_parameters_of_thermally_evaporated_non-crystalline_Cd50_S50-xSex_thin_films?ev=prf_pub

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

Good luck and my best wishes

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

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

Dear Ahmed Saeed Hassanien ,

Thank you for excellent and perfect clarification of Urbach tail.

Best Regards

Ghaleb Al-Dahash

University of Babylon-College of Science for women

Thanks a lot dear Ahmed Saeed Hassanien

Dear Dr. Ghaleb AL-Dahash

@Ghaleb AL-Dahashy

Alsalam Alikom

Thank you very much for your nice words. I hope my answer help any researcher.

Good luck and all my best Wishes

Dear Kasim Rawanduzy

@Kasim Rawanduzy

Alsalam Alikom

You are welcome, I hope my answer helped you

Good luck

All my best Wishes

Urbach effect applies only to the crystals, and glass is has always blurred edge. So, why do you need this energy?

Please  go to my references

How necessary to calculate the urbach energy of semiconductor after doping ? the method still seem not clear,.after the slop and intercept value how to get the urbach energy in meV?

you can follow this paper: Article Investigation of structure and physical properties of cobalt...

Hi, Can we calculate the Urbach energy through PL curve of material instead of absorption curve ?

I have absorption data in two columns, X-axis is wavelength, Y-axis is absorbance as usual.

I multiplied the absorbance column with 2.303 and divided it with film thickness in cm. This is alpha.

I took Log of alpha in excel as I dont know how to do it in origin, for this I used the formula =LN(column name).

The problem is that the natural log values have negative sign. Is it normal? what do I need to do next?

which part is Urbach tail?

also what will be the units of Urbach energy values obtained? eV or meV?

It is normal to have negative signs as the values of alpha are lower than 1.