4.1.4 Precursor Decomposition

The precursor, as it moves through the heated ambient undergoes various changes, which are characterized in Figure 4.4. Evaporation, precipitate formation, and vaporization all occur depending on the droplet size and ambient temperature.

The four processes (A-D) from Figure 4.4 represent the four potential cases which the droplet can undergo during spray pyrolysis deposition as it is traveling towards the substrate. All processes occur during a spray pyrolysis process, however, it is Process C, the CVD-like deposition which is desired to yield a dense high quality film.

4.1.4.1 Process A: low temperature - large initial droplet

When the large droplets approach a heated substrate and the temperature is not sufficiently high to fully evaporate the solution, they will impact with the substrate and decompose. Upon contact, the droplet is entirely vaporized and a dry precipitate is left behind. Because droplet vaporization requires some heat, the substrate temperature is slightly decreased at the impact point, adversely affecting the reaction kinetics [187]. This process has a weak sticking probability.

4.1.4.2 Process B: lower/intermediate temperature - larger/medium droplet size

When medium-sized droplets are initially formed, some evaporation occurs. Just as the droplet reaches the surface, however, it forms a precipitate as an amorphous salt and a dry precipitate hits the surface, where decomposition occurs. Some particles evaporate and condense into gaps between particles, where surface reaction occurs. However, this process has a medium sticking probability.

4.1.4.3 Process C: intermediate/higher temperature - medium/smaller droplet size

When the processing environment causes droplets to evaporate prior to reaching the substrate vicinity, a precipitate will form early. As the precipitate reaches the immediate vicinity of the substrate, it is converted into a vapor state and it undergoes a heterogeneous reaction through the following steps [187]:

1. Reactant molecules diffuse to the surface.
2. Adsorption of some molecules at the surface.
3. Surface diffusion and a chemical reaction, incorporating the reactant into the lattice.
4. Desorption and diffusion of the product molecules from the surface.

This is a classical CVD reaction, which results in a high quality film deposition and a high sticking probability.

4.1.4.4 Process D: high temperature - small droplet size

When small initial droplets are formed, or the temperature is high enough the droplet quickly forms a precipitate. As the precipitate approaches the substrate, it is vaporized and a chemical reaction subsequently occurs in the vapor phase. This homogeneous reaction leads to the condensation of molecules into crystallites in the form of a powder precipitate. The powder falls to the substrate surface, but without a deposition reaction.