A critical appraisal of growth mechanisms in MOVPE

Abstract

The metalorganic vapor phase epitaxial (MOVPE) growth process will be considered by first examining, individually, each of the four fundamental aspects of the overall reaction: (1) thermodynamics, (2) homogeneous gas phase reactions, (3) mass transport, and (4) surface kinetic processes. These will first be examined in general, and then an attempt will be made to understand the MOVPE process in terms of these fundamental concepts using data for specific III/V systems as examples. Thermodynamics yields information concerning the basic driving force for both the crystal growth process itself and undesirable parasitic reactions occurring homogeneously in the vapor phase and on the reactor walls upstream from the substrate. Thermodynamic factors often control the solid composition of III/V alloys. The primary homogeneous gas phase reactions, pyrolysis of the reactants and adduct formation, possibly accompanied by elimination reactions which lead to polymer formation on the walls, will be examined. The vast difference in behavior between systems using trimethyl-versus triethyl-group III sources will be explored for the growth of Ga and In containing III/V compounds and alloys. In addition, the pyrolysis of both MO source molecules and the group V hydrides will be examined, especially relative to the determination of solid composition. For many systems, at ordinary growth temperatures, the MOVPE growth rate is controlled by diffusion through the gas phase. In this case, for V/III ⪢ 1, the temperature dependence of growth rate and the group III distribution coefficients for mixing on the III sublattice (A x B 17minus; x C) are particularly simple; thermodynamics controls solid composition, i.e., at the interface, the solid is nearly in equilibrium with the vapor. The results obtained for the alloy system GaAsSb with V/III ⪡ 1 will be explored in detail. In this regime, under certain conditions, the MOVPE growth process is found capable of producing metastable alloys. Surface kinetics may control the MOVPE process at relatively low growth temperatures. The major factors observed to date seem to relate to AsH 3 and PH 3 pyrolysis at the growing interface. The detailed surface reaction mechanisms are nearly totally unknown.