Dislocations in lattice-mismatched wide-gap II-VI/GaAs heterostructures as laser light scatterers: Experiment and theory

It is considered in this work that the dislocations that form upon attainment of the critical thickness in lattice-mismatched wide-gap II-VI/GaAs heterostructures can act as laser light scatterers. Experimentally, a near-normal-incidence HeNe laser probe has been employed during epitaxial growth, which generates both a specularly reflected laser light signal as well as a nonspecularly reflected, or scattered, light signal for epilayer thicknesses beyond the critical thickness. It has been determined that the scattered light originates within the bulk of the II-VI epilayer, as opposed to the free surface, based on observation of a π-phase shift between the specular and nonspecular reflections which were monitored simultaneously. A strong correlation has also been observed between the dislocation density as determined postgrowth by transmission electron microscopy analysis of ZnSe/GaAs heterostructures and the optical data (scattered light intensity) recorded in situ during the growth of such heterostructures as a function of epilayer thickness. Theoretically the refractive index perturbations necessary for such scattering have been considered to be the result of strong microelectric fields which surround the dislocations evolving during plastic deformation. Specifically, the field distribution around a dislocation is considered with regard to three different potentials, namely, the deformation potential, the charged dislocation potential, and the piezoelectric potential. The magnitudes of these fields are considered with reference to ZnSe and subsequently the electro-optic effect is evoked in order to argue that a refractive index perturbation (approximately 10^-5-10^-4) sufficient in magnitude to scatter light could result in the ZnSe/GaAs system as a consequence of such fields.