COMPOSITIONAL AND STRUCTURAL CHARACTERIZATION OF SIXGE1-X ALLOYS AND HETEROSTRUCTURES BY HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPY

A method is described for the quantitative characterization of coheren t interfaces of strained Si/SixGe1-x alloy multilayers by high-resolut ion transmission electron microscopy (HRTEM) in [110] and [100] crysta l projections. The method uses systematic variations of the image cont rast patterns with the local composition x for certain ranges of objec tive lens defoci DELTAf and specimen thicknesses t. From a detailed an alysis of linear and non-linear beam interference contributions to the image intensity of 5- and 9-beam images in [110] projection and of 5- beam images in [100] projection, ranges of DELTAf and t were identifie d by Bloch-wave and multi-slice image simulations at 400 keV for which a quasi-linear functional relationship between the composition x and the first-order Fourier coefficients of the image intensity exists. Un der such optimized conditions, the lattice images show a systematic re versal of the image contrast when x varies from 0 to 1. This contrast behaviour is found to be only weakly dependent on tetragonal lattice d istortions and on Fresnel contrast contributions near the interfaces. For quantitative composition determination, a novel three-step algorit hm is described, especially designed for the application to strained h eterostructures. By this algorithm, compositions x can be determined l ocally with an accuracy of DELTAx less-than-or-equal-to +/-0.1. Positi ons of sharp Si/SixGe1-x interfaces can be determined with monolayer a ccuracy. Applications of the method to interface characterization of s hort-period Si(m)Ge(n) strained-layer superlattices and to Si/SixGe1-x quantum wells are presented.