The answer of the question has been already there https://www.researchgate.net/post/What_is_the_difference_between_direct_and_indirect_band_gap

The optical band gap (Eg) value can be easily determined from absorption spectra and from Tauc Plot.

• For the direct band gap material, you have to plot (αhv)2 vs. hv (energy in eV). Where, α is absorption coefficient (cm-1), hv is energy in eV. Then, the direct band gap energy, Eg, can be obtained by extrapolating the linear portion to (αhv)2 = 0 in that graph.
• For the indirect band gap material, you have to plot (αhv)1/2 vs. hv (energy in eV). Where, α is absorption coefficient (cm-1), hv is energy in eV. Then, the indirect band gap energy, Eg, can be obtained by extrapolating the linear portion to (αhv)1/2 = 0 in that graph.

In a direct band gap semiconductor, the top of the valence band and the bottom of the conduction band occur at the same value of momentum.

In an indirect band gap semiconductor, the maximum energy of the valence band occurs at a different value of momentum to the minimum in the conduction band energy.

For direct or indirect identification you must first calculate the band gap with the formula 1240/λ (λ is absorption edge that obtained from the DRS analysis) and then compare with each of the numbers achieved with the method that Dipayan Pal has mentioned.

The main difference between direct and indirect band gap are the position of momentum value, k and the phonon absorption or emission. The attached image may be helpful to understand more easily.

Respected Modugu Deepthi Moshe ,

I agree with the post of M N H Liton and Dipayan Pal . I hope the following link will very useful to you .

https://www.youtube.com/watch?v=dc2EHBMlyBA

thanks and Regards

Gaurav Gupta

Md. Aminul Islam Ebrahim Abbasi Asl kindly tell me from the figure below, what will be the band gap

??? is it direct or indirect???

hpow can we differentiate what type of trasition(direct or Indirect )need to be calculated????

Modugu Deepthi Moshe Tauc used an equation in 1968 to calculate the absorption edges (bandgaps) of amorphous Ge and Si from their absorption data.

(αhν)γ=A(hν-Eg)

In this equation, α is the absorption coefficient, h is Planck’s constant, υ is the frequency of the incident photon, A is a proportionality constant (which is determined by the index of refraction, electron, and hole effective masses; however, it is usually taken as 1 for amorphous materials), and Eg is the bandgap energy. The important term is the exponent γ, which denotes the nature of the electronic transition, that is, when γ=2 it is a direct allowed transition, and when it is equal to 1/2, it is an indirect allowed transition. For γ=2/3 it is a direct forbidden transition, and for γ=1/3 an indirect forbidden transition. Typically, the allowed transitions dominate the basic absorption processes, giving either direct or indirect transitions.

Thus, the basic procedure for a Tauc analysis is to acquire optical absorbance data for a sample in question that spans a range of energies from below the bandgap transition to above it.

Plotting the (αhν)γ versus (hν) is a matter of testing γ=2 or γ=1/2 to compare which provides the better fit and thus identifies the correct transition type.

I have provided the Template file (Origin file) in the video description. Thanks

https://youtu.be/LLESVSsZmlg

If the k-vectors are different, the material has an "indirect gap".

The band gap is called "direct" if the crystal momentum of electrons and holes is the same in both the conduction band and the valence band

thus an electron can directly emit a photon.