INFLUENCE OF GAS MIXING AND EXPANSION IN HORIZONTAL MOVPE REACTORS

The ability to grow highly uniform compound semiconductors by MOVPE is influenced by a number of parameters in the gas handling manifold and the growth cell. The head height of the cell above the susceptor must be small in order that forced convection can overcome the thermal convection induced by the temperature gradient above the susceptor. Other areas which can influence the uniformity are the gas manifold, the mixing region and the inlet area. The manner in which the gas is mixed either within or after the manifold and the way it is expanded after being mixed are crucial to the uniformity of the grown layers. We have tried a number of different mixing arrangements including perforated plates (both stationary and rotating), meshes, spray tubes, silica frits and orifices. Data will be presented to show the difference in uniformity achieved when the ractant gases are forced through an orifice of sufficiently small diameter to ensure turbulent mixing. These data will be compared with identical growth conditions using a diameter of orifice which is insufficient to ensure adequate mixing by turbulence. Once the reactant gases have been adequately mixed they must be expanded to the width of the susceptor and this expansion must be achieved without the gas breaking away from the cell walls and leading to recirculation problems and upstream deposition. The criteria that we have established to achieve this in a horizontal atmospheric pressure reactor will be presented.