Surface chemical reactions during atomic layer deposition of ZnO, ZnS, and Zn(O,S)

Thin films of oxide, sulfide, and oxysulfide of zinc can be deposited with atomic layer deposition (ALD) using diethylzinc with H2O and H2S. While the atomic ratio between O and S in zinc oxysulfide can be arbitrarily tuned by varying the supercycles of oxide and sulfide ALD, it is known that the fraction of S incorporated in the film is higher than its ratio in the reactant pulses. To understand the mechanism controlling the composition of ALD zinc oxysulfide thin films, its surface chemical reactions are investigated using density functional theory calculations. It is observed that all half reactions of both oxide and sulfide ALD would be facile and spontaneous, while the reactions for deposition of sulfide are more favorable over those for oxide. Furthermore, an *OH group on the surface can be easily substituted into *SH by reaction with H2S, but the inverse reaction is less likely. As a result, excess S can be contained in the Zn(O,S) ALD thin films due to the exchange reactions between surface moieti...