Direct Sub-Lattice Imaging of Interface Dislocation Structures in CdTe/GaAs(001)
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not only to enhance the understanding of growth process but also to yield important information on the atomic structure at dislocation cores. DIRECT SUB-LATTICE IMAGING IN STEM Z-contrast imaging (7,8) in STEM is an incoherent imaging technique in which, when observing crystalline specimens oriented along principal zone axes, the recorded image can be interpreted simply as a convolution between an object function (the Z-sensitive columnar scattering intensity into the high-angle electron detector) and a point spread function (the incident electron probe). In this way, the recent development of a 300kV STEM (1.3K probe) has facilitated the direct observation of structural polarity in the sub-lattice in III-V semiconductor materials oriented along the [110] direction (9). In contrast to HREM in which image contrast can change as a function of beam defocus, Z-contrast imaging provides immediately intuitive data at atomic resolution. Furthermore, as a result of image incoherence it is possible, by applying the technique of maximum entropy, to retrieve the Z-contrast object function from each image, providing detailed information on both atomic column positions and intensities (10). Shown in Figure l(a) is an atomic resolution Z-contrast image of a region of GaAs oriented along the [110] direction. It is clear from this image that, not only is it possible to discern individual atomic columns, relative column intensities (As brighter than Ga) reveal the structural polarity of the material. By application of maximum entropy, the corresponding ,most likely' Z-contrast object function, an array of delta functions of varying intensity located at column sites, is shown in Figure 1(b). The most readily interpretable evidence of material structure is however shown in the 'maximum entropy image' in Figure 3(c) in which the object 625 Mat. Res. Soc. Symp. Proc. Vol. 355 01995 Materials Research Society
Figure 1. (a) As-acquired atomic resolution Z-contrast image of the sublattice in GaAs. (b) 'Most likely' Z-contrast object function of (a) obtained by maximum entropy. (c) Convolution of (b) with a small Gaussian point spread function. Scale: nearest neighbor separation in the GaAs [110] projection is 1.41A. function is convoluted with a small Gaussian probe. In this image, quantitative object function information is preserved, while it is still possible to observe structures intuitively as in Figure 1(a), but in the absence of shot noise. It is in this form that all subsequent images in this paper will be shown. In the analysis of compound semiconductors by Z-contrast imaging, GaAs is the most demanding both in lattice dimensions and relative image contrast between different atomic species. Having established that it is indeed possible to observe structural polarity in the most stringent 'perfect lattice' case, we now proceed to the investigation of more complicated and less well characterized dislocation structures. ANALYSIS OF CdTe (111)/GaAs (001) CdTe can be grown epitaxially on GaAs(001) in two orientations; (001) or (111)
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