SURFACE STOICHIOMETRY AND THE ROLE OF ADSORBATES DURING GAAS ATOMIC LAYER EPITAXY

Several questions regarding the stoichiometry of atomic layer epitaxy (ALE) arise because most polar compound semiconductor surfaces reconstruct into structures terminated with less than monolayer coverage at the outermost layer. In this paper we briefly discuss how to self-consistently account for stoichiometry changes on surfaces that are not ideally terminated. The key aspect of the methodology is that surface steps are allowed to act as a reservoir where atoms may be added or removed. The methodology shows that ideal ALE of GaAs(100) cannot occur by cycling between the known adsorbate-free Ga-rich and As-rich surface reconstructions, because no such transition would yield the observed 1 monolayer (ML) per cycle growth rate. In fact, ideal ALE (1 ML/cycle) must involve at least one adsorbate induced surface reconstruction. Adsorbates may stabilize ideally terminated (i.e. vacancy free) III-V surfaces because of their ability to passivate dangling bond states. For example, methyl groups adsorbed on GaAs(100) exhibit a (1 x 2) LEED pattern, which is not seen for the clean GaAs(100) surface reconstructions. By using the electron counting model we interpret this structure as 1/2 ML CH3 adsorbed on a complete layer (1 ML) of dimerized Ga atoms. The GaAs(100)-(1 x 2)-CH3 surface was also examined using surface infrared spectroscopy (SIRS) using a multiple internal reflection geometry. This surface exhibits relatively sharp infrared linewidths suggestive of a well ordered structure. The polarization dependence of the symmetric stretching and bending CH3 modes also supports our proposed structure of the GaAs(100)-(1 x 2)-CH3 surface. The ideal termination of the Ga-rich GaAs(100)-(1 x 2)-CH3 surface allows for a plausible ALE mechanism which yields 1 ML deposition per cycle.