Composition and Structure Control during Zinc Phosphate Atomic Layer Deposition Supercycles: Mechanistic Origins of Initial Growth Effects

Abstract Atomic layer deposition (ALD) supercycles, formed by combining distinct binary ALD cycles in different ratios offer a versatile route to deposit multicomponent materials. However, predictive composition control is often limited by initial growth effects that arise when switching between ALD chemistries. These effects are particularly relevant for the deposition of metal phosphate thin films but remain insufficiently understood. Here, we investigate supercycles combining one zinc oxide (ZnOx: diethylzinc (DEZ)–O2 plasma) and varied (n = 1–5) phosphorus oxide (POy: trimethylphosphate (TMP)–O2 plasma) ALD subcycles to deposit zinc phosphate thin films. The resulting films exhibit tunable morphology and stoichiometry, spanning compositions from P-doped ZnOx over P-rich zinc orthophosphate to Zn-rich zinc metaphosphates as a function of the supercycle definition. The film growth also shows a strong dependence on the supercycle definition and is quenched beyond a critical number of POy subcycles. Experimental data complemented by density functional theory (DFT) demonstrate that the surface reactions of both DEZ and TMP are highly sensitive to the growth surface established by the preceding subcycles, particularly the balance between surface groups that promote facile precursor chemisorption and those which demonstrate only weak interactions with precursors. As the phosphorus content increases, the film growth is suppressed by the depletion of sites promoting chemisorption, limiting the achievable stoichiometry control for practical growth rates of the films. These findings highlight how initial growth effects govern supercycle ALD and underscore their importance for achieving controlled synthesis of metal phosphate materials.