-Notur mssenschaften Imaging Surface Atomic Layers by Means of-Auger Electrons Nikola Batina, Oliver M. R. Chyan, Douglas G. Frank, Teresa Golden and Arthur T. Hubbard Department of Chemistry, University of Cincinnati, Cincinnati, OH 45221-0172, USA Auger electron angular distributions yield images of surface atomic and molecular struc- ture. Angular distribution Auger microscopy (ADAM) produces silhouettes of near-surface atoms by illuminating them with Auger electrons originating from atoms located deeper in the solid. The silhouettes are then observed by mea- surement and display of the complete angular dis- tribution of Auger electrons. The locations and shapes of these silhouettes directly reveal the re- lative positions of atoms near the surface, and also provide important new information regard- ing the interaction of electrons with matter. Interpretation of ADAM images is straightfor- ward in terms of atoms behaving as point emit- ters and spherical scatterers of Auger electrons. Consequently, ADAM is an exciting new tech- nique for structural characterization of materials surfaces, thin films, molecular layers and crys- tals, and also for fundamental studies of electron physics, provided that certain important experi- mental criteria are met. We will illustrate the ADAM technique by describing its recent ap- plication to determination of the structure of metal single-crystal surfaces and epitaxial atomic layers. A uger electrons are emitted when a fast-moving electron or X-ray removes a core electron, leading to a relaxation process in which an ou- ter electron fills the core vacancy and a third electron, the "Auger" electron, is ejected from the atom. This process was first identified by Pierre Auger in cloud- chamber experiments [1] in which Auger electrons were found to have discrete energies characteristic of the emitting elements, and thus have found wide ap- plication for elemental identification and analysis [21. Auger signals from solid samples vary significantly with direction of emission [3]. In order to more clearly understand the nature of Auger electron angular dis- tributions, we constructed instrumentation capable of measuring Auger electron emission over the full range of angles with an accuracy better than _+ 1 °, while maintaining a constant direction of the incident beam with respect to the sample [4]. Low-energy Auger electrons are preferable for surface imaging because they produce simpler, more surface-sensitive images. Kinetic energy modulation (+ 5 eV) with synchronous amplification was used to obtain the derivati;~e spectrum with respect to energy, allowing separation of the image-containing Auger electrons from the more numerous background electrons. Experimental Aspects ADAM angular distributions are measured [6] versus the angular coordinates o~ and t3 shown in Fig. 1, and displayed in spherical coordinates (0, qS) as shown in Fig. 2C. Auger electron emission is stimulated by an incident electron beam impinging on the sample at a fixed angle of 11 ° from the surface plane. It is import- ant that the direction of the incident beam relative to the sample be held constant throughout the data col- lection process because even minor variations can lead to dramatic changes in intensity [3, 5]. In the present study a 20-mm 2 portion of the sample was irradiated with a 15-/xA beam at 2000 eV; smaller beam currents should be used with samples sensitive to beam dam- age. An emitted Auger electron having the selected energy (E) and direction (e~, t3) passes through the col- limator and energy analyzer before being amplified and counted. A complete angular distribution was gathered during a 2-h time period. Data were obtained by scanning the/3 angle from 20 ° to 160 ° for each c~ setting, which was stepped in 1 ° increments (Fig. 1), displayed in real time, and also stored for later exami- nation. Thus, the data set contains one value per de- gree in o~ and/3 (18471 data points). Single crystals were oriented by means of X-ray reflec- tion photography such that all six faces were crystallo- graphically equivalent, allowing their use as immersed Naturwissenschaften 77, 557- 560 (1990) © Springer-Verlag 1990 557