Chemical bonding, adatom-adatom interaction, and replacement reaction of column-3 metals on GaAs(110)

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Abstract
Studies of the electronic states associated with column-3 (Al, Ga, and In) overlayers on cleaved GaAs(110) surfaces by photoemission electron spectroscopy (PES) and low-energy electron diffraction are reported. Deposition of In or Ga leaves the valence-band spectra devoid of structure other than that characteristic of clean GaAs(110). Comparison with theory shows that the models assumed previously in calculations are in error and that a fundamentally new understanding of the metal-semiconductor bonding is needed. The most significant result is that a strong interaction between the column-3 adatoms leads to cluster formation on the surface. It is shown that at submonolayer coverage Ga most likely forms flat (one- or two-atom-thick) clusters on the surface, which are not in registry with the GaAs surface lattice, while In shows a greater tendency toward three-dimensional cluster formation. As a result, the electronic structure of the overlayer is free-electron-like even at submonolayer coverage. As a contrasting example, Sb (a column-5 element), which forms an ordered overlayer, produces quite significant changes in the surface valence band. The room-temperature reaction between Al adsorbate atoms and a GaAs(110) surface is observed by means of soft-x-ray photoemission techniques using core-level spectroscopy. Evidence of two states of Al which are distinct from the bulk Al metal is seen at submonolayer coverages. The sequential appearance of these states suggests that Al, as deposited at room temperature, tends to replace Ga below the surface layer first; but, as monolayer coverage of Al is reached, a large degree of replacement in the top or surface layer may take place. This interpretation is consistent with the finding by Kahn, Duke, and co-workers that (in thermodynamic equilibrium) Al tends to replace Ga below the GaAs(110) surface layer before much replacement in the first layer occurs. It is concluded that the differences between the room-temperature data of various workers in both the type (surface versus subsurface) and extent of the Al-Ga replacement reaction (for a given Al coverage without annealing) is due to the situation that kinetically, at room temperature, nonequilibrium states of the Al adatoms can dominate, and that the relative proportion of Al in nonequilibrium states to Al in the equilibrium state is a sensitive function of experimental conditions. Smearing of the upper-valence-band features (as seen in photoemission data) occurs as a result of the replacement reaction, suggesting that the replacement reaction is too random at room temperature to produce sharp well-defined new valence-band structure. The Al overlayers do not have structure as free-electron-like as those for Ga and In; this is most likely due to a partial replacement of Ga by Al atoms at the surface and a different type and/or size of clusters formed by Al overlayers compared to Ga or In.

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