I have prepared thin film of SnO2 from SnCl2.2H2O and Ethanol. Its band gap is constantly falling in the value of around 3.8eV when drawn from the direct band gap equation of the Tauc Plot. However when indirect band gap equation was used, the band gap was found to be around 3.6eV. So is there any difference or does the value is lying correctly in both cases?
This value is different from direct to indirect so this different is natural
Dear Muhammad Salik Qureshi,
The optical transition nature (direct or indirect) can be deduced from the Tauc plot:
αhv = A(hv - Eg)n
where hv is the photon energy, A and n are constants. A is a complex parameter that depends on the transition probability. The n values can be 1/2, 3/2, 2, and 3 for allowed direct, forbidden direct, allowed indirect and forbidden indirect transitions, respectively.
If the absorption edge is associated with an allowed direct transition (n = 1/2), then the plot (αhv)2 vs hv will present a lineal behavior at the neighborhood of this edge. On the other hand, if it is an allowed indirect transition, then the plot (αhv)1/2 vs hv will present a lineal behavior at the neighborhood of this edge.
You must pay attention to what of these two plots satisfy the lineal condition at the absorption edge. The plot that fits better can give you an insight into the nature of the optical transition and will help you to decide the most adequate bandgap value (Eg). You can check a current article I published for more details.
I hope my comments can help you. Do not doubt to ask for any questions you may have.
Kind regards,
Simón Roa.
Article Chemical bath deposition of high structural and morphologica...
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