Sputter-Model

To describe the nano-cBN deposition four models have been proposed the compressive stress model [190, 191], the sub-plantation model [192, 193], the selective sputter model [194], and the momentum transfer model [195],... 

The essential mechanism in the sputter-model is that h-BN can be removed more easily by selective sputtering than c-BN (if the BN mixtures are deposited simultaneously and the h-BN is selectively etched, the c-BN layer remains) [187, 196]. 

Defect-mediated sputtering model. This model developed to explain emissions from insulating surfaces (Si02, LiF, NaCl, etc.) asserts that the defects are introduced in the form of self-trapped excitons and/or holes in response to the valence band excitation induced on ion neutralization/impact. Electron-phonon conpling then results in desorption. 

Except for the interaction of highly charged primary ions, the present consensus tends to favor the defect-mediated sputtering model (Rabalais 1994). For highly charged ions, the intense ultra-fast excitation model appears likely (Aumayr and Winter 1994, 2003). As the probability of inelastic energy loss depends on the overlap of the respective orbital wave functions, simulations of a pure potential sputtering process requires quantum mechanics. As mentioned in Section 3.2.1.3, this is considered outside the scope of this text. 

Defect mediated sputtering model A model for describing sputtering... 

To examine the soUd as it approaches equUibrium (atom energies of 0.025 eV) requires molecular dynamic simulations. Molecular dynamic (MD) simulations foUow the spatial and temporal evolution of atoms in a cascade as the atoms regain thermal equiUbrium in about 10 ps. By use of MD, one can foUow the physical and chemical effects that induence the final cascade state. Molecular dynamics have been used to study a variety of cascade phenomena. These include defect evolution, recombination dynamics, Hquid-like core effects, and final defect states. MD programs have also been used to model sputtering processes. 

Electron-tunneling Model. Several models based on quantum mechanics have been introduced. One describes how an electron of the conducting band tunnels to the leaving atom, or vice versa. The probability of tunneling depends on the ionization potential of the sputtered element, the velocity of the atom (time available for the tunneling process) and on the work function of the metal (adiabatic surface ionization, Schroeer model [3.46]). 

Local Thermodynamic Equilibrium (LTE). This LTE model is of historical importance only. The idea was that under ion bombardment a near-surface plasma is generated, in which the sputtered atoms are ionized [3.48]. The plasma should be under local equilibrium, so that the Saha-Eggert equation for determination of the ionization probability can be used. The important condition was the plasma temperature, and this could be determined from a knowledge of the concentration of one of the elements present. The theoretical background of the model is not applicable. The reason why it gives semi-quantitative results is that the exponential term of the Saha-Eggert equation also fits quantum-mechanical expressions. 

GP 4] [R 11] For methanol conversion over sputtered silver catalyst, reaction rates and an activation energy (Figure 3.36) of 14.3 kcal moh were reported (8.5 vol.-% methanol balance oxygen 10 ms slightly > 1 atm) [72]. Since the latter is much lower than literature values (about 22.5-27 kcal moh ), different kinetics may occur or limitations of the reactor model may become evident. 

In order to understand the observed shift in oxidation potentials and the stabilization mechanism two possible explanations were forwarded by Kotz and Stucki [83], Either a direct electronic interaction of the two oxide components via formation of a common 4-band, involving possible charge transfer, gives rise to an electrode with new homogeneous properties or an indirect interaction between Ru and Ir sites and the electrolyte phase via surface dipoles creates improved surface properties. These two models will certainly be difficult to distinguish. As is demonstrated in Fig. 25, XPS valence band spectroscopy could give some evidence for the formation of a common 4-band in the mixed oxides prepared by reactive sputtering [83],... 

Figure 8.6 summarizes our current knowledge of the appearance of point defects in STM images. The most prevalent point defects on sputtered/annealed Ti02(l 1 0) lxl surfaces have been identified as Ob-vacs, OHb, and OHb pairs and these are shown in a ball model together with an STM image decorated with a number of all three types of defects. 

As said before, the ionization probability, which accompanies sputtering, is at best qualitatively understood. There have been several attempts to develop models for secondary ion formation. The interested reader may consult the literature for reviews [2,4]. Here we will briefly describe one model that accounts quantitatively for a number of observations on metals, namely the perturbation model of Nprskov and Lundquist [11]. This assumes that the formation of a secondary ion occurs just above the surface, immediately after emission. Then ... 

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