Prospects of atomic resolution imaging with an aberration-corrected STEM

We investigated high-resolution scanning transmission electron microscope ( STEM) images obtained from a microscope equipped with a spherical aberratio n corrector. The probe size (full-width at half-maximum) is reduced to 0.76 Angstrom at 200 kV by assuming the fifth-order spherical aberration coeffi cient C-5 = 100 mm. For the simulation we have used the recently developed scheme for a STEM image simulation based on the Fast Fourier Transform (FFT ) multislice algorithm. The peak-to-background (P/B) ratio of the high-angl e annular dark-field (HAADF) image is significantly improved at a thin spec imen region. Although the P/B ratio becomes worse at a thicker region, the resolution is kept high even at such a region. An almost true HAADF signal will be obtained even from a weak-scattering phosphorous column in InP [001 ] when the background is subtracted. in the bright-field image the coherent character of elastic scattering is suppressed by averaging over a large co nvergence angle, making the specimen effectively self-luminous. The claim t hat HAADF imaging is relatively insensitive to a defocus as well as a speci men thickness is valid only qualitatively, and a detailed image simulation will be required for a quantitative analysis as in the case of the conventi onal transmission electron microscope. It was noted that the delta function approximation for the object function may not be applicable for a very fin e probe, and that the achievable resolution of the HAADF imaging will be li mited by the widths of the high-angle thermal diffuse scattering potential.