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Register a new userNanoscale dynamics during self-organized ion beam patterning of Si: I. Ar$^+$ Bombardment
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Coherent grazing-incidence small-angle X-ray scattering is used to investigate the average kinetics and the fluctuation dynamics during self-organized nanopatterning of silicon by Ar$^+$ bombardment at 65$^{circ}$ polar angle. At early times, the surface behavior can be understood within the framework of linear theory. The transition away from the linear theory behavior is observed in the dynamics through the intensity correlation function. It quickly evolves to exhibit stretched exponential decay on short length scales and compressed exponential decay on length scales corresponding the dominant structural length scale - the ripple wavelength. The correlation times also peak strongly at the ripple length scale. This behavior has notable similarities but also significant differences with the phenomenon of de Gennes narrowing. Overall, this dynamics behavior is found to be consistent with simulations of a nonlinear growth model.
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Despite extensive study, fundamental understanding of self-organized patterning by broad-beam ion bombardment is still incomplete and controversial. Understanding the nanopatterning of elemental semiconductors, particularly silicon, is both foundational for the broader field and of intrinsic scientific and technological interest itself. This is the second component of a two-part investigation of the kinetics and fluctuation dynamics of self-organized nanoscale ripple development on silicon during 1 keV Ar$^+$ (Part I) and Kr$^+$ bombardment. Here, its found that the ion-enhanced viscous flow relaxation is essentially equal for Kr$^+$-induced patterning as previously found for Ar$^+$ patterning. The magnitude of the surface curvature dependent roughening rate in the early stage kinetics is larger for Kr$^+$ than for Ar$^+$, qualitatively consistent with expectations for erosive and mass redistributive contributions to the initial surface instability. As with the Ar$^+$ case, fluctuation dynamics in the late stage show a peak in correlation time at the length scale corresponding to the dominant structural feature on the surface -- the ripples. Analogy is made to the phenomenon of de Gennes narrowing in liquids, but significant differences are also pointed out. Finally, its shown that speckle motion during the surface evolution can be analyzed to determine spatial inhomogeneities in erosion rate and ripple velocity on the surface. This allows the direction and speed of ripple motion to be measured in real time, a unique capability for measuring these fundamental parameters outside the specialized environment of FIB/SEM systems.
We report formation of self organized InP nano dots using 3 keV Ar+ ion sputtering, at $15^circ$ incidence from surface normal, on InP(111) surface. Morphology and optical properties of the sputtered surface, as a function of sputtering time, have been investigated by Scanning Probe Microscopy and Raman Scattering techniques. Uniform patterns of nano dots are observed for different durations of sputtering. The sizes and the heights of these nano dots vary between 10 to 100 nm and 20 to 40 nm, respectively. With increasing of sputtering time, t, the size and height of these nano dots increases up to a certain sputtering time $t_c$. However beyond $t_c$, the dots break down into smaller nanostructures, and as a result, the size and height of these nanostructures decrease. The uniformity and regularity of these structures are also lost for sputtering beyond $t_c$. The crossover behavior is also observed in the rms surface roughness. Raman investigations of InP nano dots reveal optical phonon softening due to phonon confinement in the surface nano dots.
Ion beam irradiation has recently emerged as a versatile approach to functional materials design. We show in this work that patterned defective regions generated by ion beam irradiation of silicon can create a phonon glass electron crystal (PGEC), a longstanding goal of thermoelectrics. By controlling the effective diameter of and spacing between the defective regions, molecular dynamics simulations suggest a reduction of the thermal conductivity by a factor of $approx$20 is achievable. Boltzmann theory shows that the thermoelectric power factor remains largely intact in the damaged material. To facilitate the Boltzmann theory, we derive an analytical model for electron scattering with cylindrical defective regions based on partial wave analysis. Together we predict a figure of merit of ZT$approx$0.5 or more at room temperature for optimally patterned geometries of these silicon metamaterials. These findings indicate that nanostructuring of patterned defective regions in crystalline materials is a viable approach to realize a PGEC, and ion beam irradiation could be a promising fabrication strategy.
Developing a comprehensive understanding of the modification of material properties by neutron irradiation is important for the design of future fission and fusion power reactors. Self-ion implantation is commonly used to mimic neutron irradiation damage, however an interesting question concerns the effect of ion energy on the resulting damage structures. The reduction in the thickness of the implanted layer as the implantation energy is reduced results in the significant quandary: Does one attempt to match the primary knock-on atom energy produced during neutron irradiation or implant at a much higher energy, such that a thicker damage layer is produced? Here we address this question by measuring the full strain tensor for two ion implantation energies, 2 MeV and 20 MeV in self-ion implanted tungsten, a critical material for the first wall and divertor of fusion reactors. A comparison of 2 MeV and 20 MeV implanted samples is shown to result in similar lattice swelling. Multi-reflection Bragg coherent diffractive imaging (MBCDI) shows that implantation induced strain is in fact heterogeneous at the nanoscale, suggesting that there is a non-uniform distribution of defects, an observation that is not fully captured by micro-beam Laue diffraction. At the surface, MBCDI and high-resolution electron back-scattered diffraction (HR-EBSD) strain measurements agree quite well in terms of this clustering/non-uniformity of the strain distribution. However, MBCDI reveals that the heterogeneity at greater depths in the sample is much larger than at the surface. This combination of techniques provides a powerful method for detailed investigation of the microstructural damage caused by ion bombardment, and more generally of strain related phenomena in microvolumes that are inaccessible via any other technique.
Investigating the relationship between structure and dynamical processes is a central goal in condensed matter physics. Perhaps the most noted relationship between the two is the phenomenon of de Gennes narrowing, in which relaxation times in liquids are proportional to the scattering structure factor. Here a similar relationship is discovered during the self-organized ion beam nanopatterning of silicon using coherent x-ray scattering. However, in contrast to the exponential relaxation of fluctuations in classic de Gennes narrowing, the dynamic surface exhibits a wide range of behaviors as a function of length scale, with a compressed exponential relaxation at lengths corresponding to the dominant structural motif - self-organized nanoscale ripples. These behaviors are reproduced in simulations of a nonlinear model describing the surface evolution. We suggest that the compressed exponential behavior observed here is due to the morphological persistence of the self-organized surface ripple patterns which form and evolve during ion beam nanopatterning.
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