Why we prefer the evaluation of bandgap energy of semiconducting materials from tauc's plot instead of measuring it directly from the absorption spectrum i.e Absorption (Y-axis) vs Energy (x-axis)?
You must examine the Tauc theory, refined by Mott and Davis in the early 1970's. They show that the contribution to the absorption stemming from Gap-thru transitions is described by a sort of power law : (A * hv)^s = Const*(Eg-hv) where s is a parameter that takes the value of 2 for direct-band-gap semiconductors and 1/2 for indiirect-band-gap ones. That is the first member of the eq. is linear to the right member, which allows making an extrapolation of the corresponding ramp (in the )A*hv)^s vs hv plot) to the x-axis thus providing a direct measure of Eg. This is not possible with a pure absorption spectrum because A is not proportional to hv in the ramp... no linear extrapolation to the x-axis is possible or could be considered correct.
Dear Muhammad Mubeen ,
Ramadan Kareem!
I will not repeat the satisfactory answer of Giuseppe Curro but I would like to stress the fact that the more precise and correct method is that of the Tauc plot.
The energy gap obtained from the absorption curve is just an approximate value.
The problem is that one observes first an appreciable absorption when the photoenergy becomes larger than the energy gap. The absorption at the bandgap it self is practically zero. The bandgap edges may be also not sharp because of the presence of the dense allowed electronic sates near the band edges.
Tauc method helps overcoming these problems and locating more accurate value to energy gap.
Best wishes
I will not add much to what esteemed colleagues have said to help you understand this point, except that I will suggest that you read these two papers to gain a comprehensive understanding of the subject.
The Main Role of Thermal Annealing in Controlling the Structural and Optical Properties of ITO Thin Film Layer
The precise role of UV exposure time in controlling the orbital transition energies, optical and electrical parameters of thermally vacuum evaporated Se 50 Te 50 thin film
You can calculate the optical bandgap directly from optical absorption plot but it will not be accurate because you can not say the electron transfer in semiconductor is direct or indirect. If there is direct allowed transition Y-axis will be (αhν)1/2 and if it's indirect transition it will be (αhν)2. For the same Photocatalyst if you will calculate optical bandgap using direct and indirect transition the optical bandgap will change. Therefore you have to calculate the optical bandgap using tauc plot not by absorption Vs energy.
Yes it is correct , you can calculate it from the curve which involved band gap energy at X-axis
There is always a reason for which a vast scientific and technical community accepts and commonly uses a model or procedure developed by some collegue. In the case of Tauc's method, the main reason is in the ability to select the spectrum region where to fit the model equation. In the Tauc's proposal such a region is linear, which greatly helps in identifying where the theoretiical linear segment is at best aligned to the spectrum ramp. Of course it is really difficult with a non-linear curve fit and errors are much more probable in that case, specially when intra-band contributions start to be relevant.
Muhammad Mubeen 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
You can calculate the optical band gap energy by firstly calculate the optical absorption coefficient
- Badges
- Science topic