Caso “Competencias de las municipalidades en materia de Planes

There are two opposing mechanisms purporting to explain IBAD biaxial texture development, anisotropy of sputtering rates and anisotropy of ion-induced surface damage for different grain orientations35. The first researchers to observe IBAD biaxial texturing attributed it to anisotropy of sputtering rates for different grain orientations28,36,37. This model supposes that faster growing grains eventually occlude slower growing grains. This is a reasonable picture for the biaxial texture development seen in IBAD YSZ, which exhibits a gradual decrease in the out-of-plane (∆ω) and in- plane orientation distribution (∆φ) as the film grows, not reaching a minimum until the film is about 600 nm thick38. Coupled with the increased sputtering yield experienced by misaligned grains, there is also a shadowing effect that aggravates the growth disparity between grains. Using a 2D molecular dynamics simulation, Ying et al.39 showed that shadowing effects alone could cause grains to overgrow adjacent grains. Taller grains (less sputtered because of correct alignment with incoming ions) were observed to

62 incorporate border adatoms into their crystal structure, effectively growing laterally over the shorter grains.

More recent experiments and simulations have cast doubt on the selective sputtering mechanism. Ressler et al.35 used a 150 eV and 300 eV Ar+ ion beam to etch three samples of YSZ, half of each sample having the (111) and the other half having the (110) oriented to the ion beam. Using typical IBAD deposition conditions, the etch rates for the 150 eV Ar+ ion beam were two orders of magnitude smaller (~ .02 A/s) than the typical deposition rates (1.2 or 2.4 A/s). The difference in etch rates for (111) and (110) surfaces using the 300 eV Ar+ ion beam were only about 0.03A/s, with some samples having higher etch rates for the (111) oriented halves and the others having higher etch rates for the (110) oriented halves. A similar experiment by Iijima et al.40 was performed by etching a single-crystal of YSZ with a 300 eV Ar+ ion beam at a 55o from normal incident angle. They found no statistically different etch rate as they rotated the sample around the normal axis. A selective sputtering mechanism predicts that the etch rate will be lower when the (111) is lined up with the ion beam then when it is misaligned. However, no evidence of increased etch rate for the misaligned crystal was observed.

3.1.2.1 Anisotropic ion damage

Anisotropic ion damage for different grain orientations has also been proposed as the dominant biaxial texturing mechanism27,41,35. This model proposes that biaxially textured films do not develop by having the aligned grains grow over the misaligned grains, but evolve via lateral grain growth. Grains with stable lattice planes oriented directly into the ion beam are assumed to sustain less damage than misaligned grains. After an ion impact there is local heating that allows for reordering of the local surface

atoms. During this local thermal spike the grain with the least damage grows into the more highly damaged grain through a recrystallization process. The recrystallization driven grain boundary migration rate is proportional to the difference in energy density of adjacent grains. The grain boundary acts as a sink for surface defects as it migrates into the more damaged grain, leaving a more perfect crystal behind it. The recrystallized sections take on the orientation of the less damaged grain and increase the size of the well-aligned grain at the expense of the misaligned one27.

Recent 3-D molecular dynamics simulations by Dong and Srolovitz27,42 support the anisotropic ion damage model. A simulated bi-crystal fcc film (nominally Al) was created with one fiber axis in the (111) and the other in the (110). The ion damage sustained by each crystal was examined by bombarding each crystal orientation with twenty 100 eV ions. Figure 3.3 shows that the (111) oriented crystal sustained significantly more damage than the (110) oriented crystal27. The classical boundary migration theory was validated by taking a damaged (111) oriented crystal, like in Figure 3.3, turning it 90o, attaching the damaged portion to the side of a perfect (110) oriented

Figure 3.3 Molecular dynamics simulation of FCC crystals after twenty 100 eV, perpendicular Ar ion impacts. The top crystal has a (110) c-axis orientation and the bottom crystal has a (111) c-axis orientation27. Crystal damage depends on the crystal direction oriented toward the ion flux.

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crystal, and allowing the combined structure to equilibrate over time at a typical substrate temperature. The grain boundary was observed to grow into the (111) oriented crystal leaving relatively damage free (110) oriented crystal material behind it, just as predicted by classical boundary migration theory27. The final simulation evidence comes from IBAD growth simulations. The full IBAD simulation produced efficient occlusion of the (111) crystal by the (110) crystal. Turning off the selective sputtering during a second simulation produced no noticeable effect on the grain boundary migration rate into the (111) crystal. However, when the anisotropic ion damage was excluded from the simulation, leaving only the selective sputtering grain boundary migration mechanism,

Figure 3.4 In-plane alignment direction for IBAD YSZ as a function of r (ion/atom flux ratio) and ion bombardment angle. The different symbols represent that the films were grown by different deposition methods, e.g., sputter deposition and e-beam evaporation, and substrate temperatures, e.g., room temperature to 600o C35.

the (110) boundary showed only slight migration into the more slowly growing (111) grain42. These simulations indicate that anisotropic ion damage dominates over selective sputtering in IBAD texture formation.

Recent experiments have also shown strong support for the anisotropic ion damage mechanism. The highest density lattice planes are typically most resistant to ion- induced damage. The lattice density, as seen by an ion, is a function of the ion energy. At 200 eV the CeO2 (110) plane has a higher density than the (111) plane. At 300 eV the relative densities are reversed. Following this trend, the in-plane orientation for IBAD grown CeO2 (using a 55o from normal ion incidence) switched from (110) to (111) when, under otherwise identical conditions, the ion energy was changed from 200 eV to 300 eV43. Taken as a whole, yttrium stabilized zirconia (YSZ) IBAD data also support the anisotropic ion damage mechanism35. YSZ IBAD films grown at high ion/atom ratios typically show (111) in-plane orientation, while low ion/atom ratios produce (110) in- plane orientations. Atomic binding calculations show that Zr4+ ions on the (111) and (110) surfaces have very similar free energies, ~80 eV. However, the O2- ions on the (111) and (110) surfaces have been calculated to have –16 eV and –12 eV free energies, respectively35. At low ion/atom flux ratios the (110) surface is the most damage-resistant because it has a higher density than the (111) surface. However, the O2-/Zr4+ ratio on the (110) surface is two, while the O2-/Zr4+ ratio on the (111) surface is only one. At high ion/atom flux ratios the O2- is preferentially sputtered from the surface and can not be replaced due to a high ion flux. This leaves the (110) surface more susceptible to ion- induced damage than the (111) surface, effectively allowing the (111) in-plane orientation to develop.

66 Even among the anisotropic ion damage mechanism proponents there is disagreement about the cause of the damage anisotropy. One view is that ion damage is reduced by increased ion channeling for the selected grain orientation. Ensinger41 argued for this mechanism based on experiments with IBAD TiN fiber texture development. He suggested that as ion energies increased, with ion/atom flux equal to one, the (100) fiber texture began to dominate the non IBAD preferred (111) fiber texture because the three fold more open (100) preferentially escaped damage by allowing more ions to channel. The other main view is that the ability of lattice planes to withstand damage is a function of its ability to disperse the energy of ion impacts, as opposed to its ability to avoid high- energy collisions. Ressler et al.35 attempted to demonstrate this by showing that the (111) or (110) oriented toward the ion beam based on the ion flux and not the ion incident angle. They argued that if channeling was responsible for the in-plane orientation, then using a 45o ion beam incidence should produce a (110) oriented film, while using a 55o ion beam incidence should produce a (111) oriented film, these being the respective channeling angles for a (001) oriented film. Figure 3.435 shows that both (111) and (110) oriented films were grown at either ion beam incidence angle and the authors conclude that in-plane alignment can not be a result of ion channeling. Ressler et al. further supported their claim by growing (111) in-plane oriented LCMO, even though at the ion energy used channeling was calculated to be impossible for the (111), but possible for (100) and (110)35.