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Scanning tunneling microscopy has been used to study the transition in surface structure between the As-terminated 2ϫ4 and c͑4ϫ4͒ reconstructions on both GaAs͑001͒ and InAs͑001͒, as a function of surface temperature under an As 2 flux. For both materials, two-phase surface reconstructions exist through the transition regime. On GaAs, the two-phase surface consists of disordered ͑2ϫ4͒-like structures plus a c͑4ϫ4͒-like phase terminating one monolayer below the 2ϫ4 surface. On InAs, a disordered asymmetric 1ϫ3 phase occurs ͕a͑1ϫ3͖͒ in addition to the main phases, one monolayer below each main phase. In both cases, simple addition of As via As-on-As chemisorption cannot account for the formation of the c͑4ϫ4͒ reconstruction from the 2ϫ4 surface. The c͑4ϫ4͒ phase is inherently multilayered, which explains how the structure can form without the addition or removal of the group III component and still maintain its layering registry with the residual 2ϫ4 or a͑1ϫ3͒ phase. One result of this formation process is the necessary intermixing of group III and group V species in the second layer of the c͑4ϫ4͒ reconstruction. Direct evidence of species intermixing on the top layer of the InAs͑001͒-a͑1ϫ3͒ structure is also shown and models for all of these reconstructions are proposed.

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The growth modes of InAs on the three low index orientations of GaAs during molecular beam epitaxy (MBE) are very different, despite a constant lattice mismatch of ≈7%. Coherent three-dimensional (3D) growth occurs only on (001) surfaces; on the other two orientations strain relaxation involves misfit dislocation formation and a continuous two dimensional growth mode. Strain is therefore not a sufficient condition to induce 3D growth. Reflection high-energy electron-diffraction and scanning tunnelling microscopy observations confirm that an intermediate `wetting layer' is formed on (001)-oriented substrates prior to the formation of quantum dots. The thickness and composition of this layer is dependent on both growth temperature and the amount of InAs deposited, but it is always an (In, Ga)As alloy. We have also confirmed that substantial mass transport occurs during quantum dot formation and that the dots themselves have an alloy composition. A model to account for at least some of these effects, based on rate equations, is introduced.