possible answer may addition of Zn creates new levels in conduction band and valance band leads to increase in band gap. pl spectra should also be taken.
https://www.researchgate.net/publication/253340293_Stoichiometry_dependence_of_the_optical_properties_of_amorphous-like_InGaZn1xO10.5x_thin_films?ev=prf_pub
Article Stoichiometry dependence of the optical properties of amorph...
CdS energy-gap is 2.42 eV. ZnS energy-gap is 3.54/3.91. For bulk crystals. When the thickness of a film of these materials comes down to the the nanometre regime the gap increases, but in very similar manner for both materials.
The two semiconductors are soluble one in the other ending as a definite alloy. CdS gap is smaller than the one of ZnS ; going from CdS to ZnS by adding Zn to the CdZnS alloy you move the gap from the one of CdS to the one of ZnS, therefore the gap of the alloy increases as a growing function of the zinc content.
ZnS and CdS have different band gap (3.6 around for ZnS, Above 2 eV for CdS), Usually, such as III-V and II-VI materials, they can form alloys. The band bap for the new alloy will stand between these two material. There are many examples, like BeMgZnSe, InGaAs, to show the band gap dependence on the composition ratio in the alloy
Ayan,
For the origin of the change in bandgap due to the creation of more states, take a look at some Ab-initio calculations, such as those on Materials Project:
ZnS: https://materialsproject.org/materials/mp-672/
CdS: https://materialsproject.org/materials/mp-10695/
Phase Diagram: https://materialsproject.org/apps/phase_diagram/?open_element=None&elements=S%20Cd%20Zn
You can do an orbital projected density of states. With ternary alloys, you need to do some hand-waiving, but you can assume the alloy is somewhat ordered/disordered giving rise to some combination of these states within the band diagram. You can also think of the alloy as a pseudo-potential of a mystical Zn-Cd ion hybrid sitting on each lattice site with some composite core electron behavior.
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