Determination of orientation of small crystallites in thin nickel films on UHV-cleaved mica

Abstract

Because of the small size of thin film crystallites (<100 A) in the initial stages of growth, the application of electron diffraction methods such as small area diffraction to determine crystallographic orientation isnot possible. Direct lattice imaging techniques are limited to large lattice spacings and require improved electron-optical operation conditions, Conventional dark field imaging shows poor azimuthal angle resolution ~10° ) . A powerful but simple method can overcome these difficulties. It allows the determination of lattice planes according to Bragg image contrast 1 and with further improvement the evaluation of the exact and unique orientation of small crystals 2 is possible without the need to use very high resolution microscopes. The technique is based on the fact that the dark field image of a diffracting crystal is displaced by defocusing the objective lens (defocused dark field imaging : DDF). The amount of the shift depends on the defocus and the diffraction angle, and hence on the imaging lattice plane. Combining the amount and the direction of the shift (which is perpendicular to the lattice plane (hkl)) results in a vector proportional to the corresponding reciprocal lattice vector (h, k,/). The shift can be measured by doubly exposing an electron micrograph with a defocus difference between the two exposures. An azimuthal angle resolution of about 1 ° is obtainable.