A horizontal chemical vapor deposition (CVD) reactor for growth of gallium arsenide (GaAs) thin films is shown in the figure. In this process, arsine (AsH₃), trimethyl gallium, Ga(CH₃)₃, and H₂gases are fed into the reactor. Inside the reactor, the silicon wafer rests on a heated plate. The reactant gases flow parallel to the surface of the wafer and deposit a GaAs thin film according to the simplified CVD reactions:

2 AsH₃ (g) †’ 2 As (s) + 3 H₂ (g)

2 Ga(CH₃)₃ + 3H₂ (g) †’ 2 Ga(s) + 6 CH₄ (g)

If the process is considerably diluted in H₂ gas, then the mass transfer of each species in the H₂ carrier gas can be treated separately. The surface reaction is very rapid, and so the mass transfer of the gaseous reactants to the surface of the wafer limits the rate of GaAs thin film formation. In the present process, the edge of a 10.0 cm silicon wafer is positioned 4.0 cm downstream of the leading edge of the susceptor plate. The wafer is inset within this plate so that a contiguous flat surface is maintained.

The process temperature is 800 K, and the total system pressure 101.3 kPa (1.0 atm). Consider a limiting case where the flow rate of the H₂-rich feed gas to the reactor results in a bulk linear velocity of 100 cm/s, where trimethylgallium is present in dilute concentration. Determine the local mass-transfer coefficient (k_c) for trimethylgallium in H₂ gas at the center of the wafer using boundary-layer theory. The binary gas-phase diffusion coefficient of trimethylgallium in H₂ is 1.55 cm²/s at 800 K and 1.0 atm.

Transcribed Image Text:

Feed gas Horizontal CVD Reactor (cross-section) Feed gas H2 + Ga(CH3)3 + AsH3 10-cm silicon wafer Heated plate (susceptor) X = 0 cm X = 4 cm X= 9 cm Gas distributor