Anomalous Plastic Deformation and Sputtering of Ion Irradiated Silicon Nanowires

Silicon nanowires of various diameters were irradiated with 100 keV and 300 keV Ar+ ions on a rotatable and heatable stage. Irradiation at elevated temperatures above 300 °C retains the geometry of the nanostructure and sputtering can be gauged accurately. The diameter dependence of the sputtering shows a maximum if the ion range matches the nanowire diameter, which is in good agreement with Monte Carlo simulations based on binary collisions. Nanowires irradiated at room temperature, however, amorphize and deform plastically. So far, plastic deformation has not been observed in bulk silicon at such low ion energies. The magnitude and direction of the deformation is independent of the ion-beam direction and cannot be explained with mass-transport in a binary collision cascade but only by collective movement of atoms in the collision cascade with the given boundary conditions of a high surface to volume ratio.

: Image analysis protocol. (a) The SEM images are cleaned up with a 3 × 3 median filter and a gaussian unsharp mask filter with σ = 1 px weighted with 60 % [1]. Next the Otsu thresholding [2] algorithm is applied to extract the wire area from the background (b). The wire was separated from non-contiguous pixel clusters by a particle analysis algorithm (c) [3,4]. The particle analysis algorithm is used to turn the wire upright (d). Finally, the gray values of the rows were summed to get a diameter profile shown in graph (e). To compare the wires after different fluences, the maximum at the base is aligned. Only the area in between the indicated lines is evaluated, since thresholding at the tip and bottom of the wire is inaccurate.
A large effort was made to minimize the influence of slightly different focal planes and contrast settings in the SEM analysis, as this is crucial to the evaluation of the sputter yield. The image analysis shown in figure 1 produces reliable profiles of the diameter of the nanowires over their height. It was performed after each irradiation step. If these profiles are well aligned, the change in diameter per ion fluence can be used to calculate the sputter yield (SY) from d 1 and d 2 , the local diameters before and after irradiation, using: with ρ Si the atomic density of silicon, ∆V = πh(d 2 1 − d 2 2 )/4 the sputtered volume, ∆A = h · sin(45 • ) · (d 1 + d 2 )/2 the corresponding irradiated area and Φ 12 the fluence. All results are plotted over the average diameter (d 1 +d 2 )/2. As the difference in diameters after irradiation of 1 × 10 16 cm −2 was only sightly larger than the SEM resolution, only the two subsequent fluence steps of 2 × 10 16 cm −2 each were evaluated.
Sputtering at 100 keV and 300 keV The respective ion ranges of in Si-bulk at 45 • are calculated with SRIM. The measured data-points (black/red triangles) correspond to the average of hundreds of individual measurements grouped together every 10 nm, the 'error bars' indicate the standard deviation. The outliers and discontinuities in the measured data curves correlate with a low number of evaluated nanowires for those diameters and the change from one nominal diameter on the samples to another.

Evaluation of the mass-transport-rate (MTR)
The number of atoms per height can be calculated from the local radius assuming constant density. Weighting the local number of atoms with the height gives the center of mass. The effective mass-transport rate (in atoms · nm/ion) required to account for deformation seen in the SEM images is equal to the movement of the center of mass per fluence. As in the sputter yield evaluation, the difference of the sum of all atoms in a nanowire was divided by the number of ions hitting the wire after the set fluence to determine the sputter yield. The sputtered atoms have to be discounted in the evaluation of the movement of the center of mass. The effective mass-transport rate (M T R) is thus calculated to: The height of the center of mass z c can be calculated by first summing up the height weighted by the number of atoms z c · N = i πr 2 i h · ρ · z i and dividing this by the number of atoms N = i πr 2 i h · ρ in the nanowire. The sums are over all slices i of height h = 1 pixel  . Moving 1 N atoms from their center of gravity 1 z c to 2 z c is equivalent to movingN atoms from their center of gravitŷ z to 2 z c , subtracting sputtered atoms 1 N − 2 N . b) Histogram of all results obtained by evaluating the plastic deformation as a mass-transport rate. The average mass-transport rate for all fluence-steps Φ 12 (see main text) is plotted. Due to the large spread, there is neither a significant correlation between the mass-transport rate and the average diameter nor the ion energy (not shown).
Focussed ion beam dual-beam processing. Figure 6: a) Schematic of picking up a nanowire of a growth sample with a micro-manipulator in a focused ion beam (FIB) System. The nanowire is glued to the micro-manipulator with e-beam deposited P t and cut from the substrate with the focused Ga + ion beam. To be able to orient and rotate the single nanowire in the ion irradiation chamber, the nanowire is glued to a gold microwire, also with e-beam deposited P t. Finally the wire is cut from the micro-manipulator with the FIB. The SEM images in c) and d) show the respective situations sketched in a) and b). During this procedure there may be some P t deposited on the wire, but as the temperatures are low, there will be very little intermixing of P t and Si