Dear Anand Pandey,
The difference between Band gap and Urbach Energy.
The fundamental absorption edge shifted toward a lower photon energy with respect to concentration for NiO and Co3O4 thin films whereas it shifted to higher photon energy for ZnO thin films. An inverse relation between band gap energy and Urbach energy was found.
(Ref: http://www.chalcogen.ro/847_AugustineC.pdf)
I hope I have answered your question.
With Best Wishes,
Samir G. Pandya
Urbach energy is governed by the structural disorder, imperfection in stoichiometric and passivation at the surface . Urbach energy also indicates the disorder of phonon states in the film. Urbach energy Eu is found below absorption band edge of compound. It has been observed that the absorption of photons in a large number of amorphous materials, the Tauc relation holds true. The Tauc relation is of the form; alpha hv = A(hv-Eg)n
The optical band energy gap is the value of optical energy gap between the valance band the conduction band. The optical band gap of the samples is determined from the absorption spectra near the absorption edges.
Yes as said by fellow respectful researchers. One further point, the optical energy gap Eopt is a function of disorder (thus inversely proportional to the Urbach energy in this respect),in addition Eopt is a function of the separation between the conduction and valence bands edges as you know , i.e. Eopt may also change with alloying for example independent of disorder. When the well known model due to Cody of inverse relation between Urbach energy and gap energy is somewhat violated in some experimental situations, this is an indication of significant changes in the locations of the band edges in addition to disorder effects.
maybe the references mentioned in https://www.researchgate.net/post/What_is_the_best_experimental_method_for_calculating_of_the_band_gap_of_a_semiconductor_material_what_is_it_based_on
can be of your help.
Anand Pandey
You can read this papers:
Article Investigation of structural, morphological and optical chara...
Article Consequence of Mn and Ni doping on structural, optical and m...
Good luck and my best wishes
Dear Anand Pandey
Please read the article:
https://www.ias.ac.in/public/Volumes/sadh/044/01/0026.pdf.
https://www.degruyter.com › epoly.2010.10.1.87 › pdf
Anand Pandey The Urbach energy is given by the following equation
α=α0exp(hν-Eg/Eu)
where
α is the absorption coefficient
α0 is a constant absorption
Eg is the bad gap energy
Eu is the Urbach energy
There are different mechanisms of light absorption by semiconductors; which are interband absorption, absorption by free charge carriers, absorption by the crystalline lattice, dopant absorption, excitonic absorption and weak tail absorption.
The fundamental absorption (interband absorption), which leads to the generation of electron-hole pairs as a result of optical excitation of electrons from the valence band to the conduction band, which is equal to the band gap of the semiconductor. In this description, semiconductors are described as being able to absorb photons above but are transparent to photons below the band gap energy.
Urbach energy or Urbach edge (EU) is below the band gap energy governed by the structural disorder, imperfection and passivation at the surface. Any modification or deviation from the ideal structure results curvature in the band shape. Thus, the analysis of the optical absorption in the spectral range corresponding to the tails of the density of states allows to determine the Urbach energy and thereby characterize the degree of material imperfection. It is to be noted that the density of states in three-dimensional semiconductors increases further from the band gap. For this reason, the absorption coefficient, increases with energy.
The Urbach energy quantifies the steepness of the onset of absorption near the band edge, and hence the broadness of the density of states. A sharper onset of absorption represents a lower Urbach energy.
Franz Urbach evaluated this property systematically in crystals. He used silver bromide for his study while working at the Kodak Company in 1953. Regarding the Urbach energy, the following points are to be noted that;
· It is temperature-dependent
· It is from few meV to hundreds of meV
· It also indicates the disorder of phonon states.
· It varies inversely to the band gap energy Eg
The last point states that there is an inverse relation between Urbach energy and optical band gap, which means that a sample having a narrower band gap is expected to have a wider band tail. The Urbach energy is not to be confused with the activation energy since activation energy describes the disorder associated with one band, not both bands.
By re-arranging the Urbach equation and comparing with the straight-line equation, the Urbach energy is the inverse slope, if lnα is plotted along the y-axis and hv along the x-axis.
In the following video tutorial, I have explained the Urbach energy with diagrams and calculated its value from the UV-Vis absorbance data using OriginLab. Link of the video is
https://youtu.be/Klh3LwUgZoU
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