Yes, you have to use the Kubelka-Munk function to convert your reflection data.

Determination of band gap from diffuse reflectance spectra is a standard technique; sample thickness is an important parameter.

See the attached papers.

… Read more

Effect of the thermal treatment on the morphology and optica...

• 710.39 KB2013_Solid State Ionics_LaFeO3-magnetism-band-gap.pdf
• 402.65 KBKubelka-Munk-band-gap-sulfides_Mat.Res.Bull.1983.pdf
• 1.64 MBBand-gap-Tauc-plot-manuel_Shimadzu.pdf

Dear Kavin Dave,

You may look at the following sources:

https://www.sciencedirect.com/science/article/pii/S2590147819300270

https://www.youtube.com/watch?v=YYVoG0O30cI

https://www.google.com/search?sxsrf=AOaemvLl9CajB9NCl9BpXzS4nYId1tQZtQ:1641193904746&q=Is+there+a+method+to+obtain+refractive+index+or+bandgap+from+specular+reflection+data?&nfpr=1&sa=X&ved=2ahUKEwi4o8zdg5X1AhX-jIkEHegnAFcQvgUoAXoECAEQMg&biw=1506&bih=738&dpr=2.55

https://pubs.rsc.org/en/content/articlehtml/2019/cp/c9cp02291k

Dear @Len Leonid Mizrah and @Hadi Jabbar Alagealy

Thank you for your response.

I did use the Kubleka Munc function. However my plot did not have a steep rise as shown in tutorials. The curve was almost growing linearly. I should feel like i should mention this that i carried out specular reflectance of GaAs wafer.

Still in my early days of research so if I've done some naive mistake, please forgive.

Kavin Dave can you show the spectrum? K-M function is the last resort for the polycrystalline or powder materials where specular reflectance or transmission is not possible. Otherwise, specular reflectance can give you much more accurate bandgap than the K-M function fitting.

BTW, for a single crystal GaAs wafer, you can simply look up the literature to get the accurate bandgap (~1.425 eV at 300K) and refractive index for different wavelengths.

@Yong Zhang, please find attached GaAs reflectance and Kubleka Munk function plot

You should be able to see a small feature very close to the bandgap. However, you need to have enough spectral resolution to see it. I randomly found this paper: Article Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/GaAs...

In my view ellipsometry is a better way than reflectance for the determination of refractive indices. From reflectance data alone, there could be a correlation with thickness and/or absorption, leading to inaccuracies. I you know the thickness, then it will help.