Imaging Surface Atomic Structure by Means of Auger Electrons
EXCITATION OF AN ATOM, SUCH AS BY A FAST-MOVING electron or x-ray, can result in the removal of a core electron, followed by a relaxation process in which an outer electron fills the core vacancy and a third electron, an "Auger electron," is ejected from the atom. Auger electrons were first recognized by Pierre Auger in cloud chamber experiments , and were found to have discrete energies characteristic of the emitting elements. Auger electron spectroscopy has since found wide application for elemental identification and analysis . In the course of that work, Auger signals from solid samples were found to vary significant with the direction of emission from the surface [3, 4]. Based upon relatively limited data, these variations have been mistakenly attributed to anisotropic emission from individual atoms, to diffraction, to multiple scattering or to a combination of these effects [5-20]. In an effort to more clearly understand the nature of Auger electron angular distributions, we designed and constructed instrumentation capable of measuring Auger emission over the full range of angles above a solid surface. The resulting observations reveal that the measured angular distribution contains the "silhouettes" of near-surface atoms "back lit" by Auger emission originating from atoms deeper in the solid.
Theoretical simulations based upon isotropic Auger electron emission from atomic point-emitters and scattering by spherical atomic scatters of uniform cross section are in close agreement with the measured angular distributions. Best agreement occurred when the radii of the scatterers were taken to be 60 to 90 percent of their atomic radii, and the scatterers were 40 percent transparent.
Other mechanisms, such as anisotropic emission, diffraction, or multiple scattering, are not needed to explain the observed results. These experimental and theoretical findings reveal the potential usefulness of such measurements, which we have termed "angular distribution Auger microscopy" (ADAM), as a tool for imaging atomic and molecular structure at interfaces, as well as a means by which to study the interaction of electrons with matter.
Measurement of Auger electron angular distributions. The experimental apparatus employed for ADAM is illustrated in Fig. 1A . The measurements were performed in an ultrahigh vacuum chamber operated at a pressure below [10.sup.-7] pascal ([10.sup.-12] atm) to preserve sample cleanliness and to permit the unobstructed travel of electrons. To stimulate Auger emission, a [3-mm.sup.2] portion of the sample was irradiated with a 4[microamp]' electron beam at 2000-eV kinetic energy, impinging on the  plane at 79[degrees] from the surface normal (toward the  plane). (Smaller beam currents should be used with samples sensitive to beam damage.) The resulting Auger emission (65 eV) was angle-resolved ([+ or -] 0.7[degrees]) with the use of collimating apertures. Energy resolution was accomplished by means of an electrostatic analyzer; the electrons passing through the energy analyzer were modulated with an amplitude equivalent to [+ or -]10 eV at a frequency of 1 kHz, amplified, counted, synchronously detected by means of a lock-in amplifier  and digitized, and then transferred to...