Return flows in horizontal MOCVD reactors studied with the use of TiO2 particle injection and numerical calculations

Flow visualization experiments with the use of TiO2 particles were performed for a horizontal MOCVD reactor which was resistance-heated from below and water-cooled on top. It is shown that, due to the heating of the incoming gas, very pronounced return flows can occur at the leading edge of the heated susceptor. The formation of these return flows was systematically studied as a function of pressure, flow velocity and temperature. We show that for typical flow rates, carrier gases and reactor design used in MOCVD growth of GaAs and AlGaAs, it is possible to avoid return flows by decreasing the pressure to about 0.2×105 Pa. Besides these experimental studies also numerical calculations on gas flows based on the differential equations for the conservation of mass, energy and momentum were performed, using a two-dimensional finite difference procedure. Parameters in this study were again pressure, flow velocity and temperature, and in addition also the height of the reactor and the type of carrier gas were varied. In comparing the results obtained by the flow simulation experiments and the numerical calculations, it is found that the occurrence of return flows is predicted accurately by the numerical flow calculations. Small differences can be ascribed to three dimensional effects which are not included in the 2D numerical calculations. Both flow visualization experiments and numerical calculations show that the occurrence of return flows is mainly determined by only two dimensionless hydrodynamic numbers: the Grashoff number Gr and the Reynolds number Re. It is possible to define a number αcrit such that no return flow occurs for GrReκ<αcrit. Here κ equals 1 for small Reynolds numbers (Re ≤ 4) and goes to 2 for larger Reynolds numbers (Re ≥ 8), being independent of all other hydrodynamic parameters. The critical value αcrit was found to depend slightly on the temperature difference and is independent of all other parameters. The criterion is confirmed by both the TiO2 experiments and the numerical calculations for 0.1 < Re < 10 and 10 < Gr < 10 000.

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