Channeling impurity atoms

One of the most fascinating applications of channeling RBS is the study of lattice locations of impurity atoms. By measuring the angular dependence of the back-scattering yield of the impurity and host atoms around three independent channeling axes it is possible to calculate the position of the impurity. Details can be found elsewhere [3.122]. 

The dip resulting from the implanted sample is shallower compared with that of the pure sample, indicating a small fraction of the impurity atoms not being shadowed. In the <10> direction we get even an increased yield in comparison with the random orientation, this being a sign for a position almost in the middle of the channel. Details of this technique and its application may be found in. ... 

The reduction in scattering yield associated with channeling can be applied to determine the lattice site position of impurity atoms and defects in the crystal (Fig. 8.3). An impurity on a lattice site has a reduction in scattering yield equal to that of the bulk crystal interstitial impurities or atoms located more than 0.1 A from a lattice site are exposed to the flux of channeled ions. Consequently, the backscattering yield from such nonsubstitutional atoms does not exhibit the same decrease as that of the host crystal. 

Channelling effects can provide two types of information in RBS experiments. If a detector is adjusted to have an energy window corresponding to a chosen atomic species, a specimen tilt-through over a channelled direction brings information on the perfection of crystallinity of the target and also on the lattice location of dopants or impurities. The yields vs. tilt-through curve has a minimum in the channelled direction, and the smaller this minimum yield, the more perfect is the crystal. 

Several types of ion-channeling experiments (see Chapter 9) also give useful information on atomic positions at impurities or impurity complexes. These include both scattering of channeled ions by atoms that disrupt the uniformity of a channel path and the production of nuclear reactions by collision of a channeled ion with an impurity nucleus (e.g., incident 3He colliding with dissolved 2H to give 4He plus a proton, which can be detected). Here again, one can study lattice positions of solute atoms and changes in populations of different sites. 

The detailed interactions between the SiH radicals and silicon surfaces can be captured by MD simulations of radical impingement at a grid of locations on the surface with varying radical molecular orientation with respect to the surface. Tlie reactions of SiH with the pristine Si(001)-(2 x 1) surface can be classified broadly into two classes (Ramahngam et al., 1998b). The radical either adsorbs dissociatively onto the surface or penetrates below the top surface layer into the substrate also resulting in dissociation. The H atom that is released upon radical dissociation becomes an interstitial impurity of the substrate Si lattice and it can migrate rapidly or channel deeper into the substrate. These reactions can be represented as... 

In crystals, impurities can take simple configurations. But depending on their concentration, diffusion coefficient, or chemical properties and also on the presence of different kind of impurities or of lattice defects, more complex situations can be found. Apart from indirect information like electrical measurements or X-ray diffraction, methods such as optical spectroscopy under uniaxial stress, electron spin resonance, channelling, positron annihilation or Extended X-ray Absorption Fine Structure (EXAFS) can provide more detailed results on the location and atomic structure of impurities and defects in crystals. Here, we describe the simplest atomic structures more complicated structures are discussed in other chapters. To explain the locations of the impurities and defects whose optical properties are discussed in this book, an account of the most common crystal structures mentioned is given in Appendix B. 

Fig. 8.3. Schematic of channeling trajectories and the interaction of the channeled particles with surface atoms and impurities. Scattering from the first monolayers of the solid, surface scattering, reveals details of the surface structure. The channeled ions, typically 98% of the incident beam, do not make close impact collisions with the host atoms or substitutional impurities. The dashed line, for substitutional scattering, indicates the small yield from substitutional impurities. The channeled particles can interact strongly with displaced host atoms or interstitial impurities...

We conclude that the thermal decomposition of O3 is a useful source of 0-atoms providing that O3 does not react appreciably with the substrate. In the case of the reaction with CO traces of carbonyl impurity complicate the kinetics and chemiluminescence, especially at the lower temperatures. These complexities can be overcome and our results indicate that there are simultaneous biraolecular and third body channels for the reaction of 0-atoms with CO. This may account for the discordant literature for this reaction. 

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