Hi,

Though I am by no means an expert in this area, I had a hunch that there should be a measurably different temperature dependence for UV-Vis transitions in indirect and direct semiconductors. 

After a quick search I came up with these:

https://photon.libretexts.org/The_Science_of_Solar/Solar_Basics/C._Semiconductors_and_Solar_Interactions/III._Absorption_of_Light_and_Generation/2._Direct_Semiconductors

and

https://photon.libretexts.org/The_Science_of_Solar/Solar_Basics/C._Semiconductors_and_Solar_Interactions/III._Absorption_of_Light_and_Generation/3._Indirect_Semiconductors

Notably the equation for absorption coefficient (alpha) of an indirect semiconductor has a squared relationship with photon energy, and a 1/e^T relationship with temperature. In contrast, the absorption coefficient in a direct semiconductor has a square-root relationship with photon energy and is independent of temperature.

I would therefore suggest recording your absorption spectra at different temperatures and look for this inverse exponential relationship (indicating an indirect semiconductor). Alternatively, plot absorption intensity against the square or square-root of photon energy and look for a linear relationship. 

I hope I understood your question correctly and that this helps.

Dear  Dr. S. A. Hejazi

Al Salamu Alekum

Have a nice day and good times

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

 αhν = A (hν - Eg )n

where α is the absorption coefficient, it is given in terms of T and R by  this Eq.:               

α = 1/t  ln [(1-R)2 / T]

Or, in terms of A it is given as:

α = (1/d) ln (Io/I) = (1/d) ln A = (2.303*A)/d

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

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

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=3/2  for direct forbidden transition and

n=3 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

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

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

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

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

Dear Dr. A. S. Hassanien

Wishing good times for you , And thanks for your attention

It is true that you say, but my sample is powder.I'm used to measure the absorbance of the sample solution in  ethanol ,  So I'm not thick

Do you think it's true that I have crystallite size instead d in equation?

α = (1/d) ln (Io/I) = (1/d) ln A = (2.303*A)/d

I hope to help you

azadeh hejazi

Dear Dr. Azadeh Hejazi

Wishing to you good times, too

You can use that equation as it is, as if you substitute about (d) by the unity. Then compete your calculations normally.

αd= ln A = (2.303*A).

Good Luck

Your,

Ahmed 

Dear Professor Hassanien,

Could you explain more about eliminating the d factor in solution?

what is your mean about getting d by the unity?

Is the equation a=(2.303*A)/d acceptable for the solution? what is the substitute for d in solution?

Regard.

Amin

Dear Amin Imani

Peace be upon you

This law is a general law. It can be applied in any case.

About (d), I have told you that you can substitute about its value by the unity, as if you find the required as a function of d, not more.

Good Luck

Ahmed