Dear Dr. Ayed N Saleh

Peace be upon you

The optical energy gap cab be estimated from the measurements of

(1) the absorbance alone or

(2) using the transmittance and reflectance measurements together.

Once you have (1) or (2) you can determine the optical energy gap by using Tuac's Equation and plotting   ( αhν)^n   versus (hν), where n is called the order or transition parameter.

Regards 

Dear Ayed N Saleh

Peace be upon you

Have a nice day and good times

Experimentally, the absorption coefficient (α) can be calculated  from this simple relation:

α = 1/t  ln [(1-R)2 / T]

where t  is the sample thickness, T and R are the transmission and  reflection.But if you don't have T and R and you have Absorbance, then:

absorption coefficient  (α) = 2.303 A / t 

where (A) is absorbance  and (t) is thickness of thin film.

Moreover, You can get  the value of Eg by usually use the Tauc relation, which is given by this equation:                                

 αhν = A (hν - Eg )n

while  (hν) is the photon energy, where:

hν(eV) = 1240 / [incident wavelength (nm)]

Now, If you plot a graph between (αhν)1/n  versus (hν), then you 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 Influence of composition on optical and dispersion parameter...

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

Dear Ayed N Saleh

Peace be upon you

You can also use the swanepoel method for semiconducting films by using either the transmittance or the reflectance in case of obtaining interference fringes. The appended paper discuss in details the estimation of Eg by this method.

Greeting

https://www.researchgate.net/publication/304150361_Study_of_optical_properties_of_thermally_evaporated_ZnSe_thin_films_annealed_at_different_pulsed_laser_powers

Article Study of optical properties of thermally evaporated ZnSe thi...