Dear Dr. L. Herojit Singh 

Have a nice time and good day

You can determine the band gap energy by using Tauc's Relation, which is give as:

alpha*h*neo = αo(hυ-Eg)n                     or                    (αhυ)^(1/n)= αo(hυ-Eg )             (*)

where αo is a constant and sometimes called the band tailing parameter and it is an energy independent constant, and Eg is the optical energy gap, which situated between the localized states near the mobility edges according to the density of states model proposed by Mott and Davis. In the equation (*), there is another constant (n), which is called the power factor of the transition mode. It is dependent upon the nature of the material, whether it is crystalline or amorphous and the photon transition. The values of (n) for direct allowed, indirect allowed, direct forbidden and indirect forbidden transitions are n = 1/2, 2, 3/2 and 3, respectively. After the plotting of all power probabilities of 1/2, 2, 3/2 or 3 versus the photon energy (hν) for your samples, i.e.  plotting (αhν)^(1/n) versus the photon energy (hν) you can find the most suitable and adequate plot. Accordingly, you will know the transition type.

Good luck,

Yours,

Ahmed Saeed Hassanien