Al on GaAs(110) interface: Possibility of adatom cluster formation

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Abstract
A reexamination of the experimental data and previous electronic-structure calculations on the prototype Schottky system Al/GaAs(110), together with new calculations, indicates that at low coverages and temperatures neither a covalent bond nor a metallic bond is likely to be formed between Al and the substrate. Instead, the predominant species is likely to be Al clusters which interact only weakly and largely nondirectionally with the substrate. In contrast with all previous theoretical models which assume an epitaxially ordered array of chemisorption bonds even at submonolayer coverage, it then appears that the formation of a Schottky barrier as well as other physical and chemical characteristics of the interface (e.g., core level and exciton shifts, valence-band photoemission spectra, gap states, surface atomic relaxation) are not explainable in terms of strong and ordered chemisorption bonds. This weakly interacting cluster model leads to several interesting predictions regarding the atomic structure and spectroscopy of this metal-semiconductor interface at the initial stages of its formation. The properties of the interface at higher temperatures (i.e., after annealing) are discussed in terms of an Al-Ga exchange reaction.

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