Structural and thermal effects of ion-irradiation induced defect configurations in silicon

Classical molecular dynamics calculations were used to investigate the formation of defects produced during irradiation of energetic ions on silicon. The aim of this study was to characterize the nature of defects and defective regions formed through ion irradiation and to establish a connection between the ion irradiation parameters, lattice defect configurations, and the resulting modified lattice thermal conductivity of silicon. The defective regions were characterized according to the total number of defects generated, the size and the density of the defective region, and the longitudinal and radial distribution of defects along the ion impact path. In addition, the clustering of the defects into amorphous pockets is analyzed and the effect of these processing parameters on the properties of the clusters is also studied. Further, the lattice defect configurations produced during continuous bombardment of multiple ions are directly investigated and compared to the single-ion impact results. A range of irradiation parameters including ion species, ion energies, fluence, and beam width have been explored to elucidate the dependence of the resulting defect configurations on these experimental design parameters. High density defective regions are found to be produced by low-energy ions with high atomic number. Analysis of the defects produced under varying beam diameters indicates that the beam diameter, rather than the beam energy, is the more prominent factor in determining the extent of the defective region. We demonstrate that the thermal conductivity of the material is most significantly influenced by the effective diameter of the defective region, making the beam diameter the most influential experimental parameter for tuning the lattice thermal conductivity. A reduction in thermal conductivity of up to 80% from pristine silicon was achieved with the processing parameters used in this work. This study indicates that ion beam irradiation can be a realizable manufacturing process with high tunability and control to achieve desired material properties.

Figure 1

Schematic illustration of (a) side view and (b) perspective top view of the 3D simulation setup. Silicon atoms at the bottom are fixed. Thermostat atoms are present at the bottom and at the four sides. Ions are introduced above the lattice at the specified polar angle $\theta$ and azimuthal angle $\varphi$ relative to the basal plane surface normal.

Figure 2

Calculated thermal conductivity for pristine silicon system at 300 K.

Figure 3

Schematic illustration of the top view of the 3D simulation setup for (a) single-ion impact and (b) multi-ion impact events. Thermostat atoms are present on the four sides. The ions impact the center of the lattice within the minimum irreducible area as shown in the case of single-ion impacts. During continuous bombardment, successive ions impact the center of the lattice within the given circular area based on a Gaussian-distributed ion beam.

Figure 4

Perspective view of a representative defective silicon lattice produced by irradiation of (a) Pt(78) 10-keV single-ion and (b) Pt(78) 10-keV multi-ion (five ions) impact events. The pristine regions are transparent, while the defective and surface atoms are indicated by red and cyan, respectively.

Figure 5

Perspective view of a representative defective silicon lattice produced by irradiation of (a) Ga(31) 5-keV single-ion and (b) Pt(78) 5-keV single-ion impact events. The pristine regions are transparent, while the defective and surface atoms are indicated by red and cyan, respectively.

Figure 6

Perspective view of a representative defective silicon lattice produced by irradiation of (a) Xe(54) 4 keV single-ion and (b) Xe(54) 6 keV single-ion impact events. The pristine regions are transparent, while the defective and surface atoms are indicated by red and cyan respectively.

Figure 7

Average values for number of defects (row 1), diameter (row 2), height (row 3), area density (row 4), and volumetric density (row 5) of the defective region for single-ion impact (red) and multi-ion impacts (green) irradiation of silicon with (a) Ne(10), (b) Ga(31), (c) Xe(54), and (d) Pt(78) ions of different energies. The multi-ion impact case corresponds to a fluence of $2.54\times {10}^{13}{\mathrm{ions}\mathrm{cm}}^{-2}$ and a beam diameter of 5 nm. The ratio of each of the properties (blue) are also plotted on the secondary axis on the right for comparison.

Figure 8

(a) Defect radial distribution function (DRDF) and (b) Defect depth distribution function (DDDF) for irradiation of Silicon with Ne(10), Ga(31), Xe(54), and Pt(78) ions of multiple energies at a fluence of $2.54\times {10}^{13}{\mathrm{ions}\mathrm{cm}}^{-2}$ and a beam diameter of 5 nm.

Figure 9

Perspective view of a representative defective silicon lattice produced by irradiation of Ne(10) 1.5-keV ions at a fluence of (a) $2.54\times {10}^{13}$ and (b) $7.64\times {10}^{13}{\mathrm{ions}\mathrm{cm}}^{-2}$. The pristine regions are transparent, while the defective and surface atoms are indicated by red and cyan, respectively.

Figure 10

Average values for: number of defects (row 1), diameter (row 2), height (row 3), area density (row 4), volumetric density (row 5), and thermal conductivity (row 6) of the defective region for continuous irradiation of silicon at multiple fluences with (a) Ne(10), (b) Ga(31), (c) Xe(54), and (d) Pt(78) ions of different energies.

Figure 11

Defect radial distribution function (DRDF) for irradiation of silicon with (a) Ne(10), (b) Ga(31), (c) Xe(54), and (d) Pt(78) ions of different energies and fluences with a constant beam diameter of 5 nm.

Figure 12

Defect depth distribution function (DDDF) for irradiation of silicon with (a) Ne(10), (b) Ga(31), (c) Xe(54), and (d) Pt(78) ions of different energies and fluences with a constant beam diameter of 5 nm.

Figure 13

Thermal conductivity values for representative defective lattice obtained from continuous irradiation of silicon at multiple fluences with (a) Ne(10), (b) Ga(31), (c) Xe(54), and (d) Pt(78) ions of different energies.

Figure 14

Perspective view of a representative defective silicon lattice produced by irradiation of Xe(54) 5-keV ions at a constant fluence of $2.54\times {10}^{13}{\mathrm{ions}\mathrm{cm}}^{-2}$ for a beam diameter of (a) $d=50$ and (b) 100 Å . The pristine regions are transparent, while the defective and surface atoms are indicated by red and cyan respectively.

Figure 15

(a) Average number of defects, (b) diameter, (c) height, (d) area density, (e) volumetric density, and (f) thermal conductivity of the defective region for irradiation of silicon with Xe(54) 5-keV ions at a fluence of $2.54\times {10}^{13}{\mathrm{ions}\mathrm{cm}}^{-2}$ and varying beam diameters.

Figure 16

(a) Defect radial distribution function (DRDF) and (b) defect depth distribution function (DDDF) for irradiation of silicon with Xe(54) 5-keV ions at a fluence of $2.54\times {10}^{13}{\mathrm{ions}\mathrm{cm}}^{-2}$ and varying beam diameters.