Zone-axis critical voltages and large-angle electron diffraction at 200–400 kV

Abstract

This paper reviews aspects of electron diffraction with electron microscopes operating at voltages in the range 200–400 kV. each of which appears rich with possibilities for future development. Critical voltage measurements have mostly been made under conditions of one-dimensional diffraction. Recent developments have revealed a wide variety of two-dimensional critical voltages which occur within the operating range of modern instruments and offer the prospect of accurate and useful crystallographic information. Three recent examples include distinguishing titanium carbide from titanium nitride, measurement of the local composition of lead tin telluride and the stoichiometry of high-temperature supercondctors. Large-angle convergent-beam electron diffraction is becoming an invaluable tool in revealing how intensity arrives in a particular Bragg reflection, and hence in solving crystallographic problems. It is also proving extremely effective in studying crystal defects and interfaces. In particular, its application in plan-view samples to the detailed study of quantum well structures and to local variations which occur offers the prospect of routine investigation of epi-layers on a macroscopic scale. The challenge involved in electron diffraction studies is often that of finding a way of bringing physical insight to the interpretation of experimental results. The means of doing this may vary greatly, depending on whether a question of symmetry is involved, or an interpretation is required for a high resolution image or for some higher-order Laue zone (HOLZ) lines in a convergent-beam electron diffraction pattern. In this review I will concentrate on some recent results of electron diffraction obtained using accelerating voltages of up to 300 kV, which depend for their interpretation on the Bloch wave formulation of dynamical theory or its coexistence with kinematical diffraction theory.