HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPY OF HETEROSTRUCTURES

Chemical information on a near-atomic scale may be obtained from high-resolution transmission electron microscope (HREM) images. This requires first a distinct contrast difference in the adjacent semiconductor materials in a particular projection direction for a wide range of objective lens defoci and specimen thicknesses for the given microscope parameter (acceleration voltage, spherical aberration constant). Second, the image must contain Fourier components which behave almost linear with layer composition. The contrast formation in the systems Al/GaAs and Si/Ge, where such conditions may be found and which are representative for amplitude and phase contrast respectively, is discussed. The method of chemical lattice imaging is applied to AlxGa1-xAs structures and reveals differences in the abruptness of GaAs on AlxGa1-xAs and AlxGa1-xAs on GaAs interfaces. Selected area diffraction is shown to yield the period length of short period superlattices in unstrained Al/GaAs and strained Si/Ge systems with high accuracy.