Imaging of Defects in Cadmium Telluride using High Resolution Transmission Electron Microscopy
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IMAGING OF DEFECTS IN CADMIUM TELLURIDE USING HIGH RESOLUTION TRANSMISSION ELECTRON MICROSCOPY
F. A. Ponce, T. Yamashita, R. H. Bube, R. Sinclair Department of Materials Science and Engineering, Stanford University Stanford, California USA
ABSTRACT The defect structure of cadmium telluride has been investigated using high resolution transmission electron microscopy. The variation of the TEM images with the defocus value is discussed, and defect symmetry considerations are used to correlate the image contrast characteristics with the lattice structure. Experimental micrographs of stacking faults and dislocations in the structure are analyzed.
INTRODUCTION Most semiconductor materials have tetrahedrally coordinated structures. The structure of elemental semiconductors (silicon and germanium) is diamond cubic, whereas most binary compounds have either the sphalerite or the wurtzite structure. The sphalerite structure (also known as zinc blende) is a derivative of the diamond cubic where the two elements occupy alternate {lll} layers each. Faults in the structure are likely to adversely affect the electronic characteristics of the material [1] and so it is useful to obtain fundamental information concerning the structure. The defect structure in these materials can be observed in sharp detail using multibeam, high resolution transmission electron microscopy (HRTEM). Point resolutions of 3.0 A or less allow the interpretation of some features of the image in terms of direct structural considerations. This technique has been applied to the study of defects in silicon [2-5] and germanium [6,7]. We have carried out a study of the defect structure in cadmium telluride, a compound semiconductor with the sphalerite structure which is a candidate for solar cell applications. In this paper we discuss the observation of several types of faults in this material, the behavior of the image with defocus, and use defect symmetry to correlate image contrast characteristics with the lattice structure [5]. EXPERIMENTAL The specimens were prepared from a single crystal by cutting a thin disc with a flat oriented along a 110 direction. The disc was mechanically thinned and polished and then sputtered with an Ar+ ion beam to obtain electron transparency. The observations reported here were made with a Philips EM400 electron microscope equipped with a high resolution stage with a spherical aberration coefficient of 1.1 mm. The images were recorded under axial illumination with a beam divergence of less than 2.1 mrad. Under these conditions the resolution limit of the instrument is about 2.9 A. The images shown in this article are from very thin areas of the specimen, probably less than a few hundred Angstrom units in thickness.
504 IMAGING OF THE SPHALERITE STRUCTURE The projection is best suited for the study of defects in the sphalSince two of the four {11} slip planes are seen edge-on erite structure. along this projection, there is a 50% probability for observing edge-on defects In cadmium telluride, the projection exhibits pr
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