Clustering and reaction for Cr/GaAs(110): Scanning tunneling microscopy and photoemission studies

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Abstract
Scanning tunneling microscopy (STM) and photoelectron spectroscopy have been used to study the nucleation and growth of Cr overlayers on GaAs(110). Initial Cr-atom deposition onto GaAs(110) at 300 K produces surface defects that appear to be Ga-As divacancies. These defects do not trap the weakly chemisorbed adatoms. Instead, the Cr-adatom effective surface mobility on GaAs(110) is high and planar clusters form. Preferential clustering along [2¯11] step edges is observed when steps occur. The planar arrays do not exhibit any distinct growth direction or show any tendency to form epitaxial layers. Instead, the planar Cr clusters act as the precursors to Cr-GaAs reactive intermixing. Intermixing is evident from the photoemission results but the STM images show little surface modification except in the areas beneath the clusters themselves. The STM images of the clusters show that they are irregular and appear to be disordered. We attribute this to the incorporation of Ga and As atoms, i.e., mixed Cr-Ga, Cr-As, and Cr-Cr bonding configurations. Overlayers grown by ∼3-Å deposition cover the surface with Cr-derived structures with a roughness of 2.5 Å. This roughness is reduced by continued deposition or by mild annealing. Photoemission studies of growth at 60 and 300 K show equivalent amounts of surface disruption, implying that Cr clustering is also possible at 60 K. This is consistent with the idea that surface diffusion immediately after the condensation of weakly chemisorbed atoms is largely athermal. Although reaction on the 2–3-monolayer scale is observed at 60 and 300 K, quite different tendencies are observed with respect to Ga and As segregation to the surface of thicker films. Thus, kinetic trapping yields an overlayer with excess Ga and As at the buried interface. Finally, Cr overlayers grown by direct deposition of preformed Cr clusters containing hundreds to thousands of atoms are shown to be free of disruption and intermixing. This can be understood on the basis of kinetic constraints imposed by cluster-solid reactions.