It is demonstrated here that for the study of epitaxial semiconductor crystal growth from vapour-phase precursors, photon-based methods may be applied under true ‘in situ’ conditions and can, where appropriate, reveal a great deal regarding the nature of the growing surface. While several techniques exist which may probe the gaseous phase, the study of the surface phase under conditions of high pressures (up to 1000 mbar) and high temperatures (up to 1000 K) is far from trivial. This paper sets out to emphasise recent developments in technique which permit the study of the surface phase during epitaxial crystal growth. Data are presented for several semiconductor/substrate systems obtained using the linear technique of reflectance anisotropy and the non-linear technique of optical second-harmonic generation (SHG). In particular, the technique of reflectance anisotropy is shown to be a very powerful probe of surface stoichiometry, real-time monitoring of layer-by-layer growth processes, adsorption and desorption kinetics and also the detection of surface reconstructions that occur under high vacuum and near-atmospheric pressure conditions. For the SHG technique it is shown that the application of this method to centroasymmetric substrates such as GaAs is not trivial, yet under certain conditions surface SHG responses can be observed which are directly related to the structure of the surface layers. While the data presented are for special epitaxial semiconductor systems, throughout the paper emphasis is placed upon the applicability of the experimental approach towards the study of crystal growth systems in general.
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