calculating band gap

[parham6183]

How can I calculate Bandgap & Refractive index of a semiconductor from its Uv-Vis spetrum? can anybody help or introduce a reference?

[Anders Blom]

You can't. The band gap can be computed from the band structure, from which the spectrum can be calculated, but other way around doesn't work. The dielectric function is a bit trickier.

[zh]

The first-principles calculation for the band gap and refractive index of a semiconductor has been well established. If you search "Refractive index band structure optical first-principles" by Google, you will find many references. The problem is that the band gap of semiconductor or insulator is usually underestimated in the LDA or GGA calculation.

The formula for computing optical properties can be found in the following reference:
1). C. Ambrosch-Draxl and J.O. Sofo. Linear optical properties of solids within the full-potential
linearized augmented planewave method, arXiv:cond-mat/0402523 v1, http://arxiv.org/abs/cond-mat/0402523 Computer Physics Communications
Volume 175, Issue 1, 1 July 2006, Pages 1-14, http://dx.doi.org/10.1016/j.cpc.2006.03.005
2). Hiroyuki Kageshima and Kenji Shiraishi, Momentum-matrix-element calculation using pseudopotentials, Phys. Rev. B 56, 14985 - 14992 (1997), http://prola.aps.org/abstract/PRB/v56/i23/p14985_1
3). B. Adolph,  J. Furthmüller, and F. Bechstedt, Optical properties of semiconductors using projector-augmented waves, Phys. Rev. B 63, 125108 (2001), http://prola.aps.org/abstract/PRB/v63/i12/e125108
 

[Nordland]

Calculatating he refractive index of a semi-conductor is possible through scripting in ATK, however it is far from straight forward.
I have implemented it some time ago using the Kubo-Greenwood approach, combined integration using tethrahedrones.

It worked quite nicely, however you needed to included a scissors correct in order to get really good values, but if this is done, then the result are close to perfect compared with experiments. I can see if I can find the scripts for doing it.