The Surface Chemistry of GaAs Atomic Layer Epitaxy

Abstract

ABSTRACTIn this paper we review three proposed mechanisms for GaAs ALE and review or present data in support or contradiction of these mechanisms. Surface chemistry results clearly demonstrate that TMGa irreversibly chemisorbs on the Ga-rich GaAs(100) surface. The reactive sticking coefficient (RSC) of TMGa on the adsorbate-free Ga-rich GaAs(100) surface was measured to be ∼0.5, conclusively demonstrating that the “selective adsorption” mechanism of ALE is not valid. We describe kinetic evidence for methyl radical desorption in support of the “adsorbate inhibition” mechanism. The methyl radical desorption rates determined by temperature programmed desorption (TPD) demonstrate that desorption is at least a factor of ∼10 faster from the As-rich c(2 × 8)/(2 × 4) surface than from the Garich surface. It is this disparity in CHs desorption rates between the As-rich and Ga-rich surfaces that is largely responsible for GaAs ALE behavior. A gallium alkyl radical (e.g. MMGa) is also observed during TPD and molecular beam experiments, in partial support of the “flux balance” mechanism. Stoichiometry issues of ALE are also discussed. We have discovered that arsine exposures typical of atmospheric pressure and reduced pressure ALE lead to As coverages ≥ 1 ML, which provides the likely solution to the stoichiometry question regarding the arsine cycle.