Shape Modification of Germanium Nanowires during Ion Irradiation and Subsequent Solid‐Phase Epitaxial Growth

Abstract

During ion irradiation which is often used for the purposes of band gap engineering, nanostructures can experience a phenomenon known as ion-induced bending (IIB). The mechanisms behind this permanent deformation are the subject of debate. In this work, germanium nanowires are irradiated with 30 or 70 keV xenon ions to induce bending either away from or towards the ion beam. By comparing experimental results with Monte-Carlo calculations, the direction of the bending is found to depend on the damage profile over the cross-section of the nanowire. After irradiation, the nanowires are annealed at temperatures up to 440°C triggering solid-phase epitaxial growth (SPEG) causing further modification to the deformed nanowires. After IIB, it is observed that nanowires which had bent away from the ion beam then straighten during SPEG whilst those which had bent towards the ion beam bend even more. This is attributed to differences in the mechanisms responsible for the ion-beam-induced bending in opposite directions. Thus, the results reported here give insights into the mechanisms causing the IIB of nanowires and demonstrate how to predict the evolution of nanowires under irradiation and annealing. Finally, they show that, under certain conditions, the bending can even be removed via SPEG.

Nanostructuring Germanium Nanowires by In Situ TEM Ion Irradiation

“…It might be argued that the deformation of the nanowire's shape is due to the ion‐induced bending (IIB) effect. [ 37–39 ] However, here, the bending seems to occur as a result of the nanostructuring: it is observed only when the sample becomes nanoporous and only if the fluence is relatively high (i.e., at 3.3 × 10 15 ions cm −2 ). On the other hand, when IIB is observed on germanium nanowires, the shape modification occurs almost immediately after the irradiation starts and leads to a bending either toward or away from the ion beam and not to the weave‐like shape displayed in Figure 3.…”

“…On the other hand, when IIB is observed on germanium nanowires, the shape modification occurs almost immediately after the irradiation starts and leads to a bending either toward or away from the ion beam and not to the weave‐like shape displayed in Figure 3. [ 37,40 ] It must be noted that in the previous work, where the target material was thicker, the bending was not reported during nanostructuring. [ 41–43 ] This is probably due to the fact that the irradiated layer was supported by a non‐porous layer which limited the deformation of the film during nanostructuring.…”

“…It is worth noting that mass transport and morphological changes are facilitated during crystallization and it was observed in a previous work that the shape of irradiated germanium nanowires can evolve during recrystallization. [ 37 ] In fact as the amorphous phase of germanium is less dense than its crystalline phase, the aforementioned shrinking is to be expected as the density of germanium will change during recrystallization. [ 37 ] In the literature, it was for instance shown that after annealing of amorphous nanoporous germanium, the thickness of the wall between nanopores decreased of approximately 20%.…”

Low-temperature investigations of ion-induced amorphisation in silicon carbide nanowhiskers under helium irradiation

“…The Stopping and Range of Ions in Matter (SRIM) Monte Carlo code [29] was used to calculate the damage distribution for 6 keV He ions in SiC. Since SRIM estimates the damage in a planar target, an algorithm using MATLAB known as Ion Damage and RAnge in the Geometry Of Nanowires (IDRAGON) [30] was designed to account for the circular cross-sectional shape of a nanowire. The IDRAGON calculations were performed for 3000 He ions at an energy of 6 keV, with a displacement energy of 35 eV for Si and 21 eV for C [31], and the target density to 3.21 g/cm 3 [32].…”

Understanding amorphization mechanisms using ion irradiation in situ a TEM and 3D damage reconstruction

“…In order to model the ion-induced atomic collision cascades in 3D, a modified version of the opensource Ion Damage and RAnge in the Geometry Of Nanowires (IDRAGON) code which implements SRIM within MATLAB has been used. [33] Version 2013 of SRIM was run in the "Detailed calculation with full damage cascades" for 1 ion at a time using a displacement energy of 21 eV. [34] The collision cascade calculated by SRIM (in the collision.txt file) was then imported into MATLAB where the number of recoils was calculated within successive cylindrical volumes of radius rx whose axes were centred along the ion beam direction.…”