Z-contrast Imaging and EELS of Dislocation Cores at the Si/GaAs Interface
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Z-contrast Imaging and EELS of Dislocation Cores at the Si/GaAs Interface S. Lopatin1, G. Duscher1, 2, S. J. Pennycook2 and M. F. Chisholm2 1
Department of Materials Science and Engineering, NCSU, Raleigh NC 27695
2
Oak Ridge National Laboratory, Solid State Division, Oak Ridge, TN 37831
ABSTRACT The interface between silicon and epitaxial GaAs thin film grown by metalorganic chemical vapor deposition was studied using atomic-resolution Z-contrast imaging. Zcontrast imaging provides chemical composition information and allows direct interpretation of micrographs. Electron energy loss spectroscopy (EELS) yields correlation between structure and chemistry. Different types of dislocation were identified at the Si/GaAs interface. Atomic structure of non-reconstructed 90° dislocation (exhibiting a dangling bond) is refined by means of computer simulations based on functional density theory. EELS form planar Si/GaAs interface and dislocation cores obtained and analyzed.
INTRODUCTION One of the technological goals of the semiconductor industry is the integration of various crystalline and amorphous materials (metal, dielectrics, and semiconductors) into a single superstructure. In principle, a wide variety of devices could be manufactured on silicon substrates by the introduction of new materials into high-speed silicon integrated circuits, including majority carrier devices, field effect transistors, heterojunction bipolar transistors, and light emitting diodes [1]. The crystal lattices mismatch is the major problems in manufacturing of semiconductor layers on Si substrates. This mismatch causes interfacial dislocations, which affect the electronic and optical properties of the interface and the resulting devices [2]. The presence of dislocations at the Si/ GaAs interfaces was shown experimentally in recent years. Detailed atomic structure 60° and 90° dislocations obtained by Z-contrast imaging was described in [3]. Non-reconstructed type of 90° dislocation was reported. However the exact structure of the observed non-reconstructed 90° dislocation was left for a further clarification.
EXPERIMENTAL DETAILS In this study, we used a dedicated scanning transmission electron microscope (STEM) for high-resolution Z-contrast imaging [4, 5] and simultaneous electron energy loss spectroscopy (EELS) of the 90° dislocations at the GaAs/Si interface. The GaAs-
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thin films were grown by MOCVD at 650° C on a Si (001) substrate. Cross-section [1 1 0] samples were prepared by standard mechanical polishing and ion milling [6]. To obtain Z-contrast images, a crystalline sample is oriented to a low order zone axis and scanned by the focused electron probe of the STEM. While the sample is scanned, an annular detector collects electrons scattered to high-angles. Only 1s Bloch states reach the detector and contribute to the image [7], achieving, in effect, column-by-column illumination of the crystal. The image is acquired serially, and represents a map showing the location of the projected atomic columns and their relat
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