We discuss the first real‐time optical spectroscopic measurements of (001) GaAs and AlAs surfaces during crystal growth by molecular‐beam epitaxy. In these experiments, reflectance–difference spectroscopy (RDS) is used to enhance the typically low sensitivity of optical probes to surface phenomena. Comparison of reflectance–difference and reflection high‐energy electron diffraction (RHEED) signals upon interruption of the As flux during otherwise normal growth of GaAs and AlAs shows that RD signals are sensitive to either surface chemistry or surface structure according to wavelength. The spectral dependence of the chemical component is sufficiently pronounced so that Al‐terminated surfaces can be distinguished from Ga‐terminated surfaces. We use this spectral capability to assess the competition between codeposited Al and Ga for the same surface‐bonding sites. We also report the first observation of the RD analog of RHEED oscillations, that is, oscillations in the RD response upon initiation of growth. These are seen to occur under both chemically and structurally sensitive conditions. The initial heteroepitaxial growth of GaAs on AlAs shows very complicated energy and time dependences that extend over a number of monolayers. These results suggest that systematic investigations of the optical properties of growth surfaces will lead to a new understanding of the fundamental mechanisms of crystal growth.

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