Electron microscope contrast of misfit dislocations in epitaxial thin films

Abstract

The electron microscope contrast of misfit dislocations in epitaxial thin films was studied theoretically and experimentally. We used the systems PbSe/PbS, PbTe/PbSe and PbTe/PbS, grown by the monolayer-by-monolayer process, and the system Sn/SnTe, grown by the nucleation and growth process. For these combinations of chalcogenide compounds, calculations (including many-beam diffraction) and observation agreed fairly well on the following aspects. The increase of misfit from 3.2% in PbSe/PbS to 8.7% in PbTe/PbS, and the consequent narrowing of the misfit dislocation spacing, reduce and disperse the localized lattice distortion around the misfit dislocations and thus make the intrinsic contrast of the misfit dislocations less distinct from the moiré contrast due to the undistorted lattices of the overgrowth and substrate crystals. The misfit dislocation contrast can be enhanced by reducing the moiré contrast by using weak beam conditions. The misfit dislocations can also be manifested by the excitation of an accidental reflection from their distorted lattices. The misfit in Sn/SnTe is as large as 8.5% in the [100]Sn direction and as small as 0.6% in the [001]Sn direction. Because of this and because of the special extinction rule applicable to Sn reflections, it is more difficult to distinguish misfit dislocations, if they exist. Nevertheless, calculation shows that appreciable enhancement and reduction of alternate moiré fringes should be caused by misfit dislocations when the specimen is tilted well away from the Bragg positions in higher order dark field images. However, we did not observe this alternate enhancement and reduction of the fringes under weak beam conditions. This contradicts Vincent's and Jesser and van der Merwe's arguments about the growth process in terms of misfit dislocations following Cabrera's suggestion.