Amorphization of Ge and Si nanocrystals embedded in amorphous ${\text{SiO}}_2$ by ion irradiation

Finite-size effects become significant in nanoscale materials. When a nanocrystal is surrounded by a host matrix of a different nature, the perfection of the crystal structure is distorted by the interface formed between the nanocrystal and the matrix. The larger the surface-to-volume ratio of the nanocrystal, the higher the influence of the interface defect states on its properties. The presence of defect states in the interface can also explain the different responses of the nanocrystals (NCs) on external influences. By the combination of molecular-dynamics simulations and x-ray absorption spectroscopy measurements, we show that the amorphization of Si and Ge nanocrystals is reached at doses roughly one order of magnitude lower than those for the bulk crystals. Examining nanocrystals in the size range from 2.4 to 9 nm, we also show that the susceptibility to the amorphization decreases with increasing nanocrystal size. The finite-size effect remains significant also for the largest nanocrystals of 9 nm.

Figure 1

Full primary recoil spectrum of Si, O, and Ge atoms present in silica with a thin Ge layer in between after ${\text{Si}}^{+}$ irradiation with $E_0=5\text{MeV}$. The dashed lines select the range of recoils used in the simulation.

Figure 2

Simulation time for every cascade as a function of primary recoil energy.

Figure 3

XANES region of the XAS spectra for (a) bulk standards, (b) 9.0 nm Ge NCs, (c) 6.0 nm Ge NCs, and (d) 5.0 nm NCs. The arrows indicate regions of the spectra, where a progressive change with irradiation fluence is evident due to the reasons commented on the text. Not all irradiation fluences are shown and the bulk spectra have been offset for clarity.

Figure 4

Fractions of c-Ge, a-Ge, and Ge-O environments for unirradiated and irradiated Ge NC distributions plotted as a function of Si irradiation fluence. Triangles, squares, and circles correspond to 9.0, 6.0, and 5.0 nm mean diameter Ge NCs, respectively. Full lines and open symbols correspond to the c-Ge fraction, dashed lines and filled symbols correspond to the a-Ge fraction, while dotted lines and half-filled symbols correspond to the Ge-O fraction.

Figure 5

(a)–(d) $k^2$-weighted EXAFS oscillations (left column) and (e)–(h) corresponding Fourier transforms (right column) obtained from the range delimited by the dashed vertical lines in (d). Spectra on the left column were shifted for clarity. Panels (e) to (h) were plotted in the same scale to enable comparison between the amplitudes of the peaks for all samples.

Figure 6

(a) Variation in the Debye-Waller factors and (b) the mean coordination number of the Ge NC ensembles as a function of Si irradiation fluence. The dashed and dash-dotted lines indicate the values for bulk c-Ge and a-Ge, respectively. Symbols correspond to values measured from experimental data and solid lines are just guides to the eyes.

Figure 7

Evolution of the EXAFS Debye-Waller factor with measurement temperature for the sample containing 9.0 nm Ge NCs irradiated with Si to a fluence of $2\times {10}^{14}{\text{cm}}^{-2}$. Symbols are experimental data and the line is the fit with the Einstein model described in Refs. 11, 37.

Figure 8

Cross section of a 4 nm NC before and after irradiation

Figure 9

Comparison of simulated and experimental amorphization rates for Ge NCs and bulk Ge. (a) Comparison as a function of fluence and (b) comparison as a function of dose for the recoil spectrum case. Also shown in plot (b) is the amorphization rate for 0.1 and 1 keV recoils.

Figure 10

Comparison of degree of amorphization of the Ge NCs of different sizes obtained by simulation of ion irradiation (1 keV monoenergetic recoil case) as a function of the absorbed irradiation dose per atom. Also shown are the simulated and experimental bulk results.

Figure 11

Amorphization of Si and Ge NCs and bulk of different sizes as a function of absorbed irradiation dose per atom. The Ge curves are thinner than the Si ones.

Figure 12

Potential energy of atoms as a function of distance to the center in arbitrary units.