Ternary cadmium and zinc sulfides and selenides: electrodeposition by ECALE and electrochemical characterization

Ternary compound semiconductors such as CdxZn1−xS (with 0.75>x>0.31) and CdxZn1−xSe (with 0.74>x>0.34) were grown on Ag (1 1 1) by the electrochemical atomic layer epitaxy (ECALE) method. The interest in ternary compounds arises from the possibility of varying the bandgap with composition. The elements forming the ternary cadmium and zinc sulfide were, at first, deposited in the sequence: S/Cd/S/Zn…, the order being one cycle of CdS and one cycle of ZnS. However, the quantitative analysis of the compound obtained revealed a very high percentage of Cd, roughly 75%. This means that the presence of cadmium somehow hinders Zn deposition. The most effective way to increase the amount of Zn is to deposit more cycles of ZnS. As an example, the compound with 50% of Cd and 50% of Zn is obtained by performing 3 deposition cycles of ZnS per deposition cycle of CdS. Therefore, the stoichiometry of the ternary sulfides depends on the ZnS/CdS deposition sequence. An analogous result was obtained for CdxZn1−xSe. The stoichiometry of the compounds obtained with different deposition sequences was determined by classical quantitative analysis after dissolving the deposits. The composition measurements were confirmed by ex situ X-ray photoelectron spectroscopy (XPS) in ultra-high vacuum. The right 1:1 ratio between cations and anions was found for all compounds examined, whatever the stoichiometry. A systematic electrochemical investigation was carried out with ZnS/CdS deposition sequences equal to 1, 2, 5.