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43651-AC5
Probing a New Form of Surface Chemistry: Interactions with Point Defects
In the same way that gases react with surfaces from above, point defects within a solid can react from below. Such defects include vacancies and interstitial atoms. Little attention has been paid to this form of surface chemistry, even though the rates govern many aspects of important phenomena such as solid-state diffusion, photostimulated power production and catalysis in semiconductors, solid electrolyte sensors, and metal oxide catalysis. To better understand such chemistry, we employ measurements of solid-state diffusion in semiconductors, together with electron microscopy of extended defects.
Our basic idea is that saturating dangling bonds at a semiconductor surface controllably interferes with both the annihilation and generation of defects. A surface having many dangling bonds can annihilate interstitial atoms by simple addition of the interstitials to dangling bonds. However, if the same surface becomes saturated with a strongly bonded adsorbate, annihilation requires the insertion of interstitials into existing bonds. Such insertion should have a higher activation barrier and a correspondingly reduced probability of occurrence. By analogy, surfaces with many dangling bonds should be capable of generating self-interstitials more rapidly than saturated surfaces. The atoms on atomically clean surfaces have lower coordination than atoms on saturated surfaces, so that interstitial creation from a clean surface should require less bond breaking and exhibit a lower activation energy. Higher generation rates should ensue.
This past year we showed that such principles can be employed in the technologically relevant application of integrated circuit manufacture. The continual downscaling of silicon devices for integrated circuits requires the formation of pn junctions that are progressively shallower, incorporate increasing levels of electrically active dopant, and sustain minimal implantation damage. In the case of boron implanted into silicon, we showed that all these goals can be accomplished simultaneously through the use of surface with many dangling bonds. During annealing, the surface acts as a large sink that removes Si interstitials selectively over dopant interstitials.
Diffusion of dopants in crystalline silicon is controlled by a network of elementary steps whose activation energies are important to know for experimental design and interpretation. We employed maximum a posteriori (MAP) estimation to improve existing values for these activation energies, based on self-diffusion data collected at different values of the loss rates for interstitial atoms to the surface. Parameter sensitivity analysis shows that for high surface loss fluxes, the energy for exchange between an interstitial and the lattice plays the leading role in determining the shape of diffusion profiles. At low surface loss fluxes, the dissociation energy of large-atom clusters plays a more important role.
We have started to apply similar experimental and computational approaches to examine the role of surface effects in controlling defect populations in metal oxides. For the case of titanium dioxide, preliminary results suggest that the effects mimic those in silicon.
