Plasma-enhanced atomic layer deposition of highly transparent zinc oxy-sulfide thin films

The potential of Plasma Enhanced Atomic Layer Deposition (PEALD) for the synthesis of zinc oxy-sulfide Zn(O,S) thin films was explored for the first time, using a supercycle strategy and DEZ, Ar/O₂ plasma and H₂S as precursors. The growth and the properties of the material were studied by varying the pulse ratio on the full range of composition and the process temperature from T_dep = 120 °C to 220 °C. PEALD-Zn(O,S) films could be grown from pure ZnO to pure ZnS compositions by varying the H₂S/(O₂ plasma + H₂S) pulse ratio. Three distinct growth modes were identified depending on the nature of exchange mechanisms at the film surface during the growth. Films globally have an amorphous structure, except for the extremely sulfur-rich or sulfur-poor ones. High transmission values (up to 85% for Zn(O,S) for 500 < λ < 2500 nm) and optical band gaps (3.3-3.8 eV) have been obtained. The PEALD-Zn(O,S) process and the thin film properties were compared with ALD-Zn(O,S) to highlight the specificities, disadvantages and benefits of plasma enhancement for the synthesis of multi-element materials.