The Effect of the Asymmetric Vortex in Vertical VPE Reactors on Deposition Nonuniformity (1989)

Recently it has been shown that in vertical VPE reactors with top inlet which operate at atmospheric pressure, the interaction of buoyancy and forced convection generally gives rise to a stable asymmetric vortex.The presence of this type of vortex had not been recognized before.In the present paper it is demonstrated that--for a nonrotating susceptor--the asymmetry of the flow pattern leads to severe variations of the epilayer thickness over the diameter of the susceptor.On the other hand, it is exactly this asymmetry of flow pattern that is responsible for the substantial improvement that can be achieved by susceptor rotation.At sufficiently low pressures buoyancy becomes negligible and the asymmetric vortex disappears; the resulting epilayer thickness is much more uniform than that obtained at atmospheric pressure.The deposition data reported ~in this paper concern A10.1Ga0.gAslayers grown by organo-metallic vapor-phase epitaxy.For many applications of semiconductor epilayers, sufficiently uniform thickness over a given area is a prerequisite.Here we shall consider the regime in which epilayer deposition rates are limited t~y diffusional transport of reactants through the slowly streaming boundary layer on the surface of the substrate.Variations of the thickness of this layer with varying position on the substrate depend on the gas flow pattern in the reactor.Therefore knowledge of this pattern is important.In a number of papers (1-4) gas flow patterns in vertical reactors with top inlet and cylindrical horizontal susceptor, as considered in this work, were studied with the aid of, among other things, numerical simulations.From this it appears that, above a certain pressure, buoyancy plays a preponderant role under the usual growth conditions [except in those vertical reactors that have a very special, small volume shape ( 5)].It was demonstrated in Ref. ( 6), however, that the way in which this effect has been taken into account in the simulations is erroneous.The essential point is that the procedure employed eliminates the possibility of obtaining asymmetric solutions.Yet of all solutions for the afore-mentioned pressures and for the usual values of the other growth parameters, it is an asymmetric solution (exhibiting an asymmetric vortex) that generally corresponds with the stable pattern met with in reality.Strong asymmetry of the actual flow patterns is concluded from the following three experimental observations.1.When using AsH3 as one of the growth components, the deposition of an arsenic mirror on the reactor wall is strongly asymmetric.2. Visualization of steady-state flow by TiO2 particles provides a distinct illustration of the asymmetry of the flow pattern, see Fig. 2 of Ref. (6).3. Applying susceptor rotation results in a large improvement of the deposition uniformity.It is surprising that for the geometry considered here there seem to be no other publications presenting the deposition rate in the absence of susceptor rotation.Still it is obvious that there would be no improvement if--in accordance with the general assumption--the growth rate were already axisymmetric without rotation!In this paper we report on the variations in deposition rate over the susceptor, observed in A10.1Ga0.,Aslayers grown by organo-metallic vapor-phase epitaxy (OMVPE).These variations turn out to be severe and nonaxisymmetric over a fixed susceptor.The dramatic improvement achieved by rotating the susceptor can be ascribed for the greater part to the averaging of growth rates over each concentric circle.We shall start with presenting a brief recapitulation of the evidence for asymmetric flow patterns produced by simulation and visualization (6).