If the plot (1/c)^2 vs. E gives a positive slope, we can say that the elaborated film is N semiconductor type. if the slope is negative, so your material is P semiconductor type.

Cordially.    

Dear Oualid; 

 Mott-Schottky plots was used to confirm the type of the deposited thin films. 

•  but i'm looking for the origin of ZnS 

best regards 

Dears scientists; 

 I found a paper it might help people to explain the origin of ZnS  n-type

https://www.researchgate.net/publication/256683140_Native_point_defects_in_ZnS_First-principles_studies_based_on_LDA_LDA_U_and_an_extrapolation_scheme

http://www.sciencedirect.com/science/article/pii/S0009261412001807

Best regards 

Article Native point defects in ZnS: First-principles studies based ...