Surface Structural Techniques Applied to Interfaces

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omic level. This is most apparent in the form of interface roughness. The rôles of strain and misfit dislocations in interface formation, also studied by thèse techniques, are outside the scope of this article. Low Energy Electron Diffraction (LEED) LEED is a classic example of a technique that has been adapted and optimized for surfaces. First demonstrated by Davisson and Germer,1 it utilizes the wave properties of the électron to undergo diffraction from a crystal; matching the wavelength to the atomic spacing requires using électrons in the 5-500 eV energy range. Since électrons of this energy interact very strongly with matter, the pénétration is limited to about 10 Â, and so the measurements are intrinsically surface sensitive. Otherwise, the structural and morphological information is contained in the angular distribution of the diffraction intensity, which can be measured accurately and modeled reasonably with a dynamical calculation. 2 The limited pénétration is, however, a problem for an interface that is more than 10 Â inside a material. Henzler and colleagues3 developed a method of chemically processing Si/Si02 interfaces to remove the oxide and expose the bare Si crystal morphology to the vacuum, thus rendering them suitable for analysis by LEED. What is learned about thèse interfaces concerns not so much the atomic arrangement, which is probably perturbed by the HF etching,3 but the distribution of steps on the surface. The présence of steps implies that an interface is rough, and the roughness is quantified by the spatial distribution of steps. This information is contained in the reciprocal

space distribution of intensity around each of the diffraction peaks. For this reason, the technique is called spotprofile analysis, or SPA-LEED. To do a good job, well-collimated électron beams must be used to obtain high resolution. An example is shown in Figure 1, taken from the published work of Henzler's group in Hannover.4 Because only 10 Â of crystal is penetrated, the shape of the électron diffraction pattern in reciprocal space is elongated into continuous rods oriented perpendicular to the surface (interface). Along thèse rods at regular intervais lie the Bragg peaks of the bulk crystal, at which every atom scatters in-phase to give a maximum in the intensity (ignoring dynamical effects for now). Exactly halfway between thèse positions, alternate layers of the crystal scatter 180° out-of-phase with each other, so if a step is présent the terraces on either side destructively interfère with each other and lead to broadening of the peak in the plane perpendicular to the rod. The width of the peak is then inversely related to the size of the terrace (i.e., to the spacing of the steps.) Furthermore, the distribution of intensity across the rod is related (via Fourier transformation) to the distribution of steps. The scans of Figure 1 were made through the reciprocal space position (11/2, 11/2, 11/2), exactly satisfying this out-of-phase condition along the specular rod of the (lll)-oriented substrate. The sol