Gasphasenverarmung in horizontalen MOCVD-Reaktoren

Abstract
Growth rates of GaAs in the low pressure MOCVD process have been studied as a function of both lateral and axial position in a horizontal reactor with rectangular cross-section (AIXTRON system, see D. Schmitz et al., J. Crystal Growth 93 (1988) 312). For all experiments a steady decrease of the growth rate along the flow direction has been found. In lateral direction the growth rate decreases towards the periphery of the wafer. Influences of reactor pressure and gas velocity on growth rate and uniformity are investigated. It could be shown that the axial distribution of the growth rate becomes more homogeneous when the gas velocity is increased whereas the lateral homogeneity decreases since there is less time to equalize lateral inhomogeneities by diffusion. In the investigated pressure range of 20 - 100 mbar no significant influence of the reactor pressure on the uniformity of the GaAs layers was found. A three-dimensional numerical model to describe growth rates in laminar flow syst ems on the basis of concentration profiles under diffusion and/or reaction controlled conditions has been developed. According to the results of Holstein et al. (J. Crystal Growth 94 (1989) 131) the velocity and temperature profiles establish fairly rapidly and therefore are taken to be fully developed. Consequently the approximative solutions of Holmes and Vermeullen (Chem. Eng. Sci. 23 (1968) 717) are taken to describe the axial velocity profile in reactors of rectangular cross-section. In addition to the convective and diffusive transport of chemical species the effect of thermal diffusion is taken into consideration. Since it is necessary to respect the temperature dependences of the diffusion coefficients, thermal diffusion factors and kinematic viscosity in the numerical model a finite-volume discretisation of the transport equation is applied perpendicular to the flow direction. In axial direction the transport equation is solved by Gear's method (C.W. Gear (1971), Numerical In i