Structural imaging of beta-Si3N4 by spherical aberration-corrected high-resolution transmission electron microscopy.

Modern transmission electron microscopes (TEM) allow utilizing the spherical aberration coefficient as an additional free parameter for optimizing resolution and contrast. By tuning the spherical aberration coefficient of the objective lens, isolated nitrogen atom columns as well as the Si-N dumbbells within the six-membered ring were imaged in beta-Si3N4 along [0001] and [0001 ] projections with a dumbbell spacing of 0.94 A in white atom contrast. This has been obtained with negative or positive spherical aberration coefficient. We clarify contrast details in beta-Si3N4 by means of extended image calculations. A simple procedure has been shown for pure phase imaging, which is restricted to linear imaging conditions.