This section deals with the influence of the some main SPT parameters on proper- ties of the deposited films.
1.4.2.1 Influence of the temperature
The deposition temperature is involved in all mentioned processes, except in the aerosol generation. Consequently, the substrate surface temperature is the main pa- rameter that determines the shape and properties of the film [69]. By increasing the temperature, the film morphology can change from a cracked to a porous microstruc- ture. In many studies the deposition temperature was reported indeed as the most im- portant spray pyrolysis parameter. The properties of deposited films can be varied and thus controlled by changing the deposition temperature; it influences structural, opti- cal and electrical properties of thin films [93].
The influence of substrate temperature on the structural, optical and electrical properties of ZnO films prepared by the spray pyrolysis method using aqueous solu- tion of zinc acetate has been investigated by Afify et al [94]. The films are polycrystal- line and X-ray diffraction measurements show a strong preferred orientation along the (002) plane which is strongly dependent on the substrate temperature. Optical absorp- tion spectra, show high transparency of the film (90–95% transmission) in the visible range, with a sharp absorption edge around 375 nm wavelength of light which closely corresponds to the intrinsic band gap of ZnO (3.3 eV). ZnO films with the lowest resis-
tivity, which is due to the increased mobility resulting from the improvement of the crystallinity of the films, can be prepared at a substrate temperature of 490°C.
1.4.2.2 Influence of precursor solution
The precursor solution is the second important process variable. The solvent, salt type, salt concentration and additives affect the physical and chemical properties of the precursor solution. Thus, many properties of the deposited film can be changed by changing the composition of the precursor solution. Such as film thickness, morpholo- gy, chemical structure, and electrical and optical properties [89]. Lehraki et al. [95] they deposited thin films of zinc oxide (ZnO) by pyrolysis technique using different liquids . Three starting solutions salts namely: zinc acetate, zinc chloride and zinc ni- trate were used. The properties of these solutions and their influence upon ZnO films growth rate are investigated. The obtained results indicate that the dissociation ener- gy of the starting solution plays an important role on films growth rate. A linear rela- tionship between the solution dissociation energy and the growth rate activation ener- gy was found. However, the surface tension of the used solution controls the droplet shape impact. Both solution surface tension and dissociation enthalpy alter the micro- structure of the formed film. Films deposited with zinc acetate are characterized by a smooth surface, dense network and high transparency, while films deposited with zinc chloride have a better crystallinity and low optical transmittance [95].
1.4.2.3 Influence of atomizer (nozzle)-to-substrate distance
In the case of changing atomizer (nozzle)-to-substrate distance there are four types of processes that may occur during deposition are shown in figure 1.5. In process 1, the droplet splashes on the substrate, vaporizes, and leaves a dry precipitate in which decomposition occurs. In process 2, the solvent evaporates before the droplet reaches the surface and the precipitate impinges upon the surface where decomposition occurs. In process 3, the solvent vaporizes as the droplet approaches the substrate, then the solid melts and vaporizes (or sublimes) and the vapor diffuses to the substrate to undergo a heterogeneous reaction there. This is true CVD [96]. In process 4, at the highest temperatures, the metallic compound vaporizes before it reaches the substrate and the chemical reaction takes place in the vapor phase.
Figure 1.5:Schematic depicting different deposition processes that occur as the nozzle-to-substrate distance and deposition temperature change [96].
1.4.3 Growth kinetics of thin films
Thin film is prepared by deposition of the film materials (metals, semiconductors, insulators, dielectric, etc.) on a substrate through a phase transformation. Sufficient time interval between the two successive deposition of atoms or molecules and also layers are required so that these can occupy the minimum potential energy configura- tion. In thermodynamically stable films, all atoms (or molecules) will take up positions and orientations energetically compatible with the neighboring atoms of the substrate or to the previously deposited layers, and then the effect substrate or the initial layers will diminish gradually [97].
The deposition process of a film can be divided into three basic phases: 1. Preparation of the film forming particles (atoms, molecules, cluster); 2. Transport of the particles from the source to the substrate;
These phases can depend the specific deposition process on the choice of the depo- sition parameters be considered as either independent or as influencing one another. The former is desirable since it allows controlling the basic steps independently and therefore yields a high flexibility in the deposition process.
The importance of coatings and the synthesis of new materials for industry have resulted in a tremendous increase of innovative thin film processing technologies. The properties of thin film strongly depend on their structure. So it is important to know the factors that govern the structure of the film.
The theory of nucleation and growth of thin films is reviewed. The basic phenom- ena on the substrate have been described. The ideal condition of the film formation in- volves the deposition of the material atom by atom, molecule by molecule and layer by layer from the vapor phase of a material. Atomistic condensation takes place at the ear- liest stage of observation, in the form of a three dimensional nuclei which then grow to form a continuous film by diffusion-controlled processes. This condensation is the net result of equilibrium between the absorption and desorption processes taking place in the vicinity of the substrate surface. Many theoretical models of condensation are pro- posed by various researchers that are consistent with the experimental observations [97]. The sequential growth stages in the formation of thin films are schematically shown in figure 1.6. There are several stages in the growth process from the initial nu- cleation of the deposits to the final continuous three dimensional film formation stages. These stages are as given below [98- 102].
1. In the nucleation stage, randomly distributed, three-dimensional nuclei are first formed if the supersaturation condition is fulfilled. These nuclei then grow to form observable islands, whose shapes are determined by interfacial energies and deposition conditions.
2. Further deposition increases the size of the islands and often has tendencies to develop crystallographic facets during the early stage of their growth. Island density rate can be controlled by the deposition conditions.
3. When the island distribution reaches a critical state, a rapid large-scale coales- cence of islands results. After reaching saturation, the island density decreases with increasing substrate temperature and with subsequent film growth. The rate of decrease of island density is more rapid also with increasing smoothness of the substrate [99].
4. Continuous coalescence results film channels in between. These channels need not remain void and soon some secondary nuclei starts to grow within this void
space in the channel. Sometimes these channels may not be completely filled up even with increasing film thickness thus leaving some holes or gaps in the ag- gregate mass.
5. The final stage of growth is a slow process of filling the empty channels that requires a considerable amount of deposit. In an ideal film, there should not be any gap and this stage can be attained when the film has certain average thick- ness. The minimum film thickness for the continuous stage is also dependent on the nature of the deposits, deposition parameters etc.
The above sequence is qualitatively common to all types of vapor deposited films; however, the kinetics of each stage may vary markedly depending on the deposi- tion parameter and the deposited film-substrate combinations.
Figure 1.6: Various stages of nucleation and growth of thin films.