Direct Imaging of Epitaxial Layers by Auger Electrons
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sequently, ADAM is an exciting new technique for characterizing materials surfaces and epitaxially grown surfaces. Auger électrons hâve been employed for surface elemental analysis and depth- profiling for many years.2 The limited escape-depth of Auger électrons makes Auger spectroscopy inherently sensitive to the surface région. Depth profiling of materials, often performed in combination with ion etching, is widely used. ADAM offers the possibility of nondestructively obtaining profiles and structural information concerning buried interfaces3 and can also yield images of atomic structure. We will illustrate the technique by
*See note about this article on page 1, Table of Contents.
Figure 1. ADAM angular distributions are measured vs. the angular coordinates of the detector (a and j8). Auger électron émission is stimulated by a fixed grazing incidence électron gun. Auger électrons emitted from the sample (also fixed) along trajectories selected by the moving analyzer must pass through angle- and energy-resolving optics before being amplified, counted, and recorded.
MRS BULLETIN/MAY 1990
describing work which explored its usefulness for investigating the structure of epitaxially deposited atomic layers. Because our laboratory specializes in electrochemical research, we chose the electrodeposition of silver monolayers onto a platinum[lll] single-crystal surface from aqueous electrolyte solutions. Electrodeposition produces atomic layers and films4 analogous to those prepared by molecular beam epitaxy, chemical vapor déposition, and other methods. The samples were shown by Auger spectroscopy and electrochemical coulometry to contain submonolayer, monolayer and two-monolayer deposits of silver on platinum[lll]. High-quality ADAM images were obtained at each stage of déposition. Interprétation of the images was straightforward in terms of atoms behaving as point emitters and spherical scatterers. The ADAM Experiment ADAM measurements are performed as illustrated in Figure 1. To stimulate Auger émission, the sample is irradiated with an energetic électron beam. The sample and électron beam are not moved throughout the ADAM experiment, so the angle of incidence remains constant. Electron émission along trajectories selected by the analyzer is angle-resolved with collimating apertures, energy-resolved and modulated by means of electrostatic lenses, and synchronously detected in order to distinguish the Auger signal (which is sharply peaked in energy) from background électrons. The Auger signal is then digitized and transferred to computer memory. The collimating analyzer is scanned over ail angles in the hémisphère above the sample surface (a and j8 in Figure 1). For example, in the work described hère an incident beam of 15 /iA at 2,000 eV irradiated a 20 mm2 area of the sample at an angle of 11° from the surface plane. The modulation amplitude was 10 eV peak-to-peak at a frequency of 1 kHz, angular resolution was ±0.7°, and the final complète distribution contained 18,471 data points (derived from an initial data set
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