is it accurate to calculate the band gap of semiconductors by using the peak wavelength of UV spectrum? How to determine the tangent point in Tauc plot?
greetings. Hope you are doing well.
To evaluate the band gap using Tauc plot we must be careful about the axes and the portion of the curve choosing to extrapolate. In the Tauc plots used to evaluate the optical band gap (either direct or indirect), the scales of both X-axis and Y-axis should be started from zero. Then only the true optical band gaps can be obtained. Further to obtain these values, the linear portions of these curves should be extrapolated but not the tangents that cut the X-axis represent these values.
step 1: From the absorption spectrum, we get the value of absorption A (from x axis) and wavelength λ (from y axis) .
Find the hν, from the well known relation, ie.,
ν=c/λ
Substitute the value of λ from the absorption spectrum to obtain the hν in terms of eV values.
step 2: Find the absorption coefficient value
step 3: Find the value of (hν)1/n
n= is index number
(tauc’s plot) if n= 1/2 refer to direct allowed transitions
=1/3 direct forbidden transitions
= 2 indirect allowed transitions
= 3 indirect forbidden transitions
step 4: Plot the graph between the step 1 and step 3.
step 5: Extrapolate the linear portion of the curve at (hν)1/2 =0 (for indirect allowed transition) and (hν)2 =0 (for direct allowed transition), the band gap values of the prepared samples were obtained from the X axis scale value ( the point where it touches on the X axis) with respect to direct and indirect allowed transitions.
All the very best.
There are multiple ways to extract the bandgap from a spectrum. Recommended to get it from the Im. part of the dielectric function ε2 (λ)
1) Extract n(λ) & k(λ) from the spectrum of the material in question
2) Compute the Im. part of the dielectric function ε2 (λ) = 2xn(λ)xk(λ) and plot it vs. λ
3) Find Et energy (Urbach tail) where δ2 ε2 (λ)/δλ2 = 0 (Second derivative = 0)
4) Plot the tangent at Et
5) The tangent intersection with the λ (Or E) axis is the bandgap
Another simpler way is to find the λ where ε2 (λ) = 0
Thank you, Mr. Houssam Chouaib ,Madam Luma M. Ahmed , Mr. Omar M S , and Mr. Moussaab Belarbi .
Thank you so much.
See a previous discussion on a similar question:
https://www.researchgate.net/post/When_building_Tauc_plots_I_got_suspiciously_small_X-axis_Energy_values_Can_it_actually_be_Is_it_valid_to_use_Tauc_plots_to_find_Eg_in_this_case
You can use Kubelka Munk or Tauc plot for the calculation of semiconductor's bandgap from UV-Vis spectrum. For more details, follow the attached link.
https://www.sciencedirect.com/science/article/pii/S0921883117304260
greetings. Hope you are doing well.
To evaluate the band gap using Tauc plot we must be careful about the axes and the portion of the curve choosing to extrapolate. In the Tauc plots used to evaluate the optical band gap (either direct or indirect), the scales of both X-axis and Y-axis should be started from zero. Then only the true optical band gaps can be obtained. Further to obtain these values, the linear portions of these curves should be extrapolated but not the tangents that cut the X-axis represent these values.
step 1: From the absorption spectrum, we get the value of absorption A (from x axis) and wavelength λ (from y axis) .
Find the hν, from the well known relation, ie.,
ν=c/λ
Substitute the value of λ from the absorption spectrum to obtain the hν in terms of eV values.
step 2: Find the absorption coefficient value
step 3: Find the value of (hν)1/n
n= is index number
(tauc’s plot) if n= 1/2 refer to direct allowed transitions
=1/3 direct forbidden transitions
= 2 indirect allowed transitions
= 3 indirect forbidden transitions
step 4: Plot the graph between the step 1 and step 3.
step 5: Extrapolate the linear portion of the curve at (hν)1/2 =0 (for indirect allowed transition) and (hν)2 =0 (for direct allowed transition), the band gap values of the prepared samples were obtained from the X axis scale value ( the point where it touches on the X axis) with respect to direct and indirect allowed transitions.
All the very best.
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