3.1.1. Ag2O

The concept of the supercycle can be effective when two individual processes are compatible with each other and each is in saturation. Therefore, before the introduction of Ag as a dopant into the CeO2 structure, silver oxide thin films were deposited first on silicon substrates to find the self-limiting growth regime of the ALD process. This could be assessed by measuring the growth rate as a function of the amount of precursor delivered into the reactor. The ALD saturation growth study was carried out at deposition temperature of 200 ◦C. This temperature was chosen to match the reaction temperature of CeO2 and it has been shown that the nucleation period of Ag is significantly shortened at this temperature [29]. Figure 2 shows the saturation curves for (hfac)Ag(PMe3) and O3. The pulsing time of (hfac)Ag(PMe3) or O3 was adjusted within the range of 0.5–4 s while the other precursor pulse was fixed at 2.5 s. The saturation of (hfac)Ag(PMe3) and O3 precursors can be seen at 2.5 s with constant Ag2O growth rate of 0.28 Å/cycle. No further increase of the growth rate was noticed after increasing the precursor pulse time up to 4 s.

**Figure 2.** The effect of ozone and (hfac)Ag(PMe3) pulse time on the growth rate of silver oxide at 200 ◦C reactor temperature.

Figure 3 shows the Ag2O film thickness versus the number of ALD cycles from 10 to 750 deposited at 200 ◦C. From Figure 3 it can be seen that the thickness increases linearly after 100 ALD cycles. The incorporation of silver atoms is directly related to the density of hydroxyl groups on the substrate surface that act as adsorption sites for (hfac)Ag(PMe3) molecules. As illustrated in Figure 3, the initial growth rate of the films per cycle (GPC) is substrate dependent at the start of the ALD process and it takes about 100 cycles to obtain a stable GPC of 0.28 Å/cycle. The film growth can be separated into two regimes: an island-like growth for the first 100 cycles and layer-by-layer growth as is expected from the proceeding atomic layer deposition. If the bare Si has a higher density of reactive sites compared to the deposited Ag2O or these sites have a higher reactivity than the reactive sites on Ag2O, then the growth rate will initially be higher until a complete film is formed. Alternatively, some hfac ligands may not be completely removed by ozone and may remain bound to the surface. These comments are expanded on in Section 3.1.2.

**Figure 3.** Thickness of the silver oxide film as a function of the number of deposition cycles at the deposition temperature of 200 ◦C.
