Surface vs Gasphase Processes in the MOCVD of GaAs

Abstract

In its early years MOCVD (metal organic chemical vapor deposition) was considered a simple technique based on a straight forward mechanism leading to the production of compound semiconductor films of potentially high purity. Characteristic for this technique is the use of gaseous starting materials (usually metal organic group III compounds and group V hydrides) in a carrier gas, commonly hydrogen. It soon became apparent that the kinetics of the process contain a number of details that remain to be clarified. One of these is the relative importance of homogeneous (gas phase) and heterogeneous (surface) reactions in the overall process. There are a number of observations pointing to the catalytic role of the substrate on the dissociation of the starting materials, particularly of the group III MO compound. On the other hand, the experience from MOMBE (metal organic molecular beam epitaxy), using the same starting materials pure, i.e. without carrier, indicates that the reaction of the absorbed group III compound with the group V hydride at a substrate surface is at best a very slow process in the absence of a gas phase.