Is there any characterization technique or any tool(method) available?
HI, UV-Visible spectroscopy is easiest way; though it has some drawbacks but for preliminary experiments is convenient. Plz read this: http://en.wikipedia.org/wiki/Tauc_plot
I hope it will help you. Best luck.
To avoid any complication, I'd refer you directly to the well-known Tauc relation that has been vastly raised in papers. There are some criteria for interband transitions that you can test those for your specific case and then plot the diagram. You can use transmission data for that. Sure, you need some data about the thickness if you work on thin films. Otherwise, being aware of the width of the cell that has been used for spectrophotometery will be adequate. By extrapolation of the plot you can assign your result to one of the criteria and conclude the nature of transition. More details have been come in " Optical properties of condensed matter and applications" by J. Singh
If you measure absorption coefficient \alpha as a function of frequency \nu of the light and then plot (1) \alpha^2 vs. \nu and also plot (2) \alpha^{1/2} against \nu and check which plot is a straight line then you will be able to say if your material has direct or indirect band. If the plot (1) is a straight line then the band gap is direct and if the plot (2) is not a straight line then the band gap is indirect. This is because of the different frequency dependence of these two processes. Read text books on semiconductor physics and you will find the relevant equations. Also remember that the transition in the direct band gap does not need the assistance of phonons (crystal momentum) whereas in indirect band gap the absorption of light needs the assistance of the phonons.
I agree with the above-mentioned recommendations. However, I would point out that it is very ambiguous score, if there are no theoretical calculations concerning the energy structure of the investigated material. As an example, the approximation of the absorption edge to determine the energy of direct and indirect optical transitions can give work regarding the study of gallium selenide crystals: https://www.researchgate.net/publication/234027213_Exciton_Absorption_of_GaSe_Crystals_in_the_Indirect_Transition_Region?ev=prf_pub
In this case, fact the presence of direct and indirect optical transitions was known. However, the energy of indirect transitions at that time was not known.
Article Exciton Absorption of GaSe Crystals in the Indirect Transition Region
thank you all for your useful suggestions. However I have already determined Bandgap from the Tauc plot (using formula same as mentioned above) using Transmission and Reflection data of Silicon thin film in Past. In this case I was aware that amorphous Si act as indirect bandgap material so I have used m = 1/2.
In above mentioned suggestions, in order to plot Tauc curve prior knowledge of material to be of direct/indirect bandgap is essential to substitute the value of m.
But what will be the case if I don't know whether the material is direct or indirect!! i.e. what value should I substitute for m = 2 or 1/2.
Is there any standard technique/tool or perhaps any theory from which i can predict the nature of material (direct/indirect bandgap)??
Take d{ln(αhν)} vs. d(hν). Considering Tauc's relation this would be equal to
= m/(hν − Eg). So, there should be a discontinuity in the d{ln(αhν)}/d(hν)
versus hν plot at the band gap energy (Eg), i.e. at hν = Eg. The discontinuity at
a particular energy value gives the band gap, Eg. Using the
value of Eg thus obtained, one can determine the value of m
and hence the nature of the optical transition. Again, from Tauc's relation we have:
ln(αhν) = lnA + mln(hν − Eg)
thus, the value of m is determined from the slope of the plot
of ln(αhν) versus ln(hν − Eg)
You need not know before hand the value of m. If the plot of (alpha.h.v)^1/2 vs. hv gives a straight line, it is indirect bandgap material, while if a plot of (alpha.h.v)^2 vs. hv yields a straight line, its a direct bandgap material.
@Atharva: do you mean to say that in the case i have to check plotting both for m=2 and m=1/2 ? which ever leads straight line?
@atharva sahasrabudhe, In one of my doped titania material I have got straight line for both the graphs. Then which plot I can choose?
Photo-luminescence could be another technique to determine if it is a direct bandgap material or indirect. if it is a direct bandgap, the PL spectrum can be sharp and strong; for indirect bandgap material in contrast, the spectrum can be broad due to the photon-phonon intereaction.
If the values of absorption coefficient is much less than 10^4 cm^-1 , then we can conclude that the band gap is indirect, otherwise the band gap is direct.
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