Substrate temperature, growth reactions

As most organometallic precursors, V(NEt2)4 pyrolysis involves a complicated mechanism highly dependent on the experimental conditions. For this reason, the CVD experiments were conducted at reduced pressure (Table 15.4) in order to improve the diffusivity of the species, reduce their interactions in the gas phase and disfavor subsequent reactions. Two CVD units (hot-wall and cold-wall) of the same geometry were used in this study. Since the reactions in the gas phase are likely to be different in these two types of reactors, we could use them to study the influence of the gas phase chemistry on the growth rate. The composition of the deposits was studied as a function of the substrate temperature under He gas and as a function of the nature of the carrier gas when H2 and NH3 were added in various amounts. 

The adsorbed species, which are considered to be adatoms, can diffuse to favorable low-energy sites and react, or they can be emitted into the gas phase. At sufficiently low temperatures, adatoms may have insufficient energy to diffuse and react or to be emitted into the gas phase. These adatoms will be codeposited with the compound film as crystal defects or as a second solid phase. As a result of these competing processes in the surface reaction zone, the growth rate and film composition depend on the flux and energy of the incident species and on the substrate temperature. 

In the CVD process, the substrate temperature is a key parameter by heating the substrate, one supplies, directly to the growing surface, additional energy that favorably influences the specific chemical reactions that lead to ZnO growth. 

As the speed of the reactions at the growing surface is increased when the temperature is raised, a temperature-dependent growth rate should basically be observed for LP-CVD process. In Fig. 6.19, the deposition rate and the consumption of DEZ (defined as the percentage of DEZ effectively used for ZnO growth, the rest of DEZ being pumped out of the chamber) are shown as a function of the substrate temperature for the LP-CVD process developed at IMT Neuchatel, which operates at 0.5mbar and uses DEZ and water vapors as growth precursors. 

In general, during CVD growth, the rate of polymer chain propagation exceeds those of initiation and termination and is a strong function of the substrate temperature. This rate varies inversely with substrate temperature, i.e., decreases with increasing substrate temperature and vice versa. As mentioned earlier, chemical reactions on the surface, lateral diffusion of the incoming species, vaporization, are all characterized by activation processes and obey an Arrhenius temperature dependence of the form ... 

The value for Kk, the saturation coefiicient or growth-limiting substrate concentration at which organism growth rate is one-half its maximum value, varies considerably with the reaction, the species mediating the reaction, and environmental conditions. In anaerobic methane fermentations, Kk tends to be much larger, at least with acetate and propionate, than for aerobic oxidations. Values of 10" moles/liter seem appropriate for the former and 10" for the latter. Kk also tends to increase with decrease in temperature. 

Figures 5 and 8 show the influence of substrate temperature on growth rates of diamond in CVD. Clearly, a small change in the substrate temperature can alter the growth rates markedly, especially at higher gas pressures. This is attributable to the Arrhenius behavior of the deposition reactions. Moreover, in both low-temperature and high-temperature regimes, the growth rates are lower.

Due to lack of research in this area, only limited information on film properties-CVD growth conditions relationships is available. The highest reported for a CVD-de-rived NbN film is 15.7 K (Table 2-4) [76]. The dependence of phase development on substrate temperature and CI2 flow rate will next be discussed. The reaction of NbCls... 

---