A bit of a simplistic answer: defects can create states in the bandgap. These states can potentially absorb sub-bandgap photons and affect the absorption spectra, causing artifacts and features that can be interpreted as lower bandgap than the actual, non-defected value.

I borrowed the image from the following website, which explains the problem further and in a clear, didactic manner

http://aziz.seas.harvard.edu/novel-semiconductors

A bit of a simplistic answer: defects can create states in the bandgap. These states can potentially absorb sub-bandgap photons and affect the absorption spectra, causing artifacts and features that can be interpreted as lower bandgap than the actual, non-defected value.

I borrowed the image from the following website, which explains the problem further and in a clear, didactic manner

http://aziz.seas.harvard.edu/novel-semiconductors

Thank you for the answer.

The presence of the defects in the semiconductor influences the optical band gap which is explained  by Burstein–Moss effect. Owing to the defects, the absorption edge in the conduction band is pushed to the higher energies. As a result, all the states near the conduction band edge will be populated and the value of the optical bandgap increases. I am attaching an image from the following article, which may provide some more insight. Y. Liu, Y. Li, H. Zeng, Journal of Nanomaterials, 2013 (2013) 9.

 I see that there is a contradiction between the approach of Mr. Rajesh and Mr.Lior. one says that the defects caused a decrease in the band gap. however, Mr. Rajesh explained that the defects causes an increase in the band gap energy. May i get more clarifications please.

There is no contradiction. The B-M effect is relevant for extreme (degenerate) doping levels and it basically describes a highly populated band with no free states to which you could excite carriers with photons possessing  energies corresponding to the bandgap.

Not only is there no contradiction, but in theory nothing prevents both cases from coexisting

Dear Abdelmounaim Chetoui

Peace be upon you 

Usually, the defects play an important role in the electronic transitions and affect the values of the the energy gap. There are many defect types like dangling bonds, lone pair electrons, localized energy states,...etc.

These defects lead to decreasing the optical energy gap and  shifting the absorption edge towards the higher wavelength of the incident photons.

The attached papers may help you in your study.

Happy Ramadan and Good luck

Yours

Ahmed Saeed Hassanien 

https://www.researchgate.net/publication/281495242_Influence_of_composition_on_optical_and_dispersion_parameters_of_thermally_evaporated_non-crystalline_Cd50_S50-xSex_thin_films

https://www.researchgate.net/publication/281495010_Estimation_of_some_physical_characteristics_of_chalcogenide_bulk_Cd_50_S50-x_Se_x_glassy_systems

https://www.researchgate.net/publication/294426798_Studies_on_dielectric_properties_opto-electrical_parameters_and_electronic_polarizability_of_thermally_evaporated_amorphous_Cd50S50-xSex_thin_films

Article Influence of composition on optical and dispersion parameter...

Article Estimation of some physical characteristics of chalcogenide ...

Article Studies on dielectric properties, opto-electrical parameters...

Dear Ahmed Saeed Hassanien;

your contribution is very helpful and i would like to present my scincere thanks for your efforts.

Ramadan Moubarak.

When defects are present, their electronic occupancy depends on its energy levels with respect to the Fermi level. The energy levels in the band gap can supply carriers to the bands, being the occupancy of the bands governed by the balance between the ionized donors and receptors