In the presence process, arsine and trimethylgallium vapors are diluted in hydrogen gas and fed to the horizontal CVD reactor shown in Figure 30.2. The composition of arsine and trimethylgallium in the feed gas are both 0.1 mol %, which is very dilute. You may assume that the amount of arsine and trimethylgallium delivered with the feed gas is much higher than the amount of arsine and trimethylgallium consumed by the reaction, so that the concentration of these reactants in the bulk gas phase is essentially constant down the length of the reactor. You may also assume that the surface-reaction rates are instantaneous relative to the rates of mass transfer, so that the gas-phase concentrations of both arsine vapor and trimethylgallium vapor at the surface of the wafer are equal to zero.
a. What are the local molar fluxes of arsine and trimethylgallium along the surface of the 10-cm Si wafer at x = 4 cm (left edge), x = 9 cm (center), and x =14 cm (right edge), assuming that the surface reactions are controlled by the rate of convective mass transfer? From this result, what is the composition of the GaAs composite thin film; e.g., the molar composition of gallium (Ga) and arsenic (As) in the solid?
b. How could the feed-gas composition be adjusted so that the molar ratio of Ga to As within the solid thin film is 1:1?
c. Based on your results in part (a), calculate the thickness of the GaAs thin film (in units of μm) after 2 min at each position x, assuming that the density of the GaAs composite thin film is 5:8 g/cm3.
