Damage evolution in low-energy ion implanted silicon

The annealing of damage generated by low-energy ion implantation in polycrystalline silicon (poly-Si) and amorphous silicon (a-Si) is compared. The rate of heat release between implantation temperature and $350–500°\mathrm{C}$ for Si implanted in both materials and for different ions implanted in poly-Si shows a very similar shape, namely, a featureless signal that is characteristic of a series of processes continuously distributed in terms of activation energy. Nanocalorimetry signals differ only by their amplitude, a smaller amount of heat being released after light ion implantation compared to heavier ones for the same nominal number of displaced atoms. This shows the importance of dynamic annealing of the damage generated by light ions. A smaller amount of heat is released by implanted poly-Si compared to a-Si, underlining the effect of the surrounding crystal on the dynamic annealing and the relaxation of the defects. Damage accumulation after $30\text{−}\mathrm{keV}$ Si implantation is also characterized by Raman scattering and reflectometry, featuring a similar trend in a-Si, poly-Si, and monocrystalline silicon (c-Si) with a saturation around $4\mathrm{Si}∕{\mathrm{nm}}^2$. Considering these results together with other recent experiments in c-Si and molecular dynamic simulations, it is concluded that the damage generated by low-energy ion implantation that survives dynamic annealing is structurally very similar if not identical in both crystalline and amorphous silicon, giving rise to the same kind of processes during a thermal anneal. However, the damage peak obtained by channeling saturates only above $10\mathrm{Si}∕{\mathrm{nm}}^2$. This suggests that between 4 and $10\mathrm{Si}∕{\mathrm{nm}}^2$, further damage occurs by structural transformation without the addition of more stored energy.

Figure 1

Planar and cross-sectional views of a nanocalorimeter sensor with poly-Si on the back side (not to scale).

Figure 2

Dark-field plan-view TEM image of a $140\text{−}\mathrm{nm}$-thick poly-Si layer deposited on a $\mathrm{Si}{\mathrm{N}}_x$ membrane at $300°\mathrm{C}$ and annealed at $900°\mathrm{C}$ for $100\mathrm{s}$. The diffraction pattern appears in the inset.

Figure 3

Example of a complete heat rate measurement (dashed line) and temperature scan rate (solid line) after a $30\mathrm{keV}$ Si implant in poly-Si at a fluence of $4\mathrm{Si}∕{\mathrm{nm}}^2$, showing the significant noise appearing in the measurement above $500°\mathrm{C}$ and the decreasing scan rate, both associated with radiative losses that become significant at high temperature.

Figure 4

Heat rate versus temperature for poly-Si implanted with indicated ions and fluences.

Figure 5

Heat rate per atom in the implanted zone for poly-Si implanted with $4.0{\mathrm{He}}^{+}∕{\mathrm{nm}}^2$ at $4\mathrm{keV}$ and $1.0{\mathrm{C}}^{-}∕{\mathrm{nm}}^2$ at $15\mathrm{keV}$ (0.059 DPA), and with $0.04{\mathrm{Si}}^{-}∕{\mathrm{nm}}^2$ at $15\mathrm{keV}$ and $0.4{\mathrm{Si}}^{-}∕{\mathrm{nm}}^2$ at $30\mathrm{keV}$ (0.067 DPA).

Figure 6

Raman intensity decrease $(1\text{−}{I}_{\mathit{imp}}∕I_{\text{virgin}})$ in poly-Si (triangles) and c-Si (stars) (left scale). Normalized heat release between RT and $500°\mathrm{C}$ (circles) measured by nanocalorimetry (left scale). Estimated damage in c-Si as obtained from c-RBS (plus) (right scale).

Figure 7

Rate of heat release by poly-Si (solid lines) and a-Si (dashed-dot lines) implanted at low temperature with $30\mathrm{keV}$ ${\mathrm{Si}}^{-}$ at different fluences ranging from $0.1\mathrm{ions}∕{\mathrm{nm}}^2$ to $10\mathrm{ions}∕{\mathrm{nm}}^2$.

Figure 8

Raman spectra obtained from poly-Si (top) and c-Si (bottom) unimplanted and implanted with $30\mathrm{keV}$ ${\mathrm{Si}}^{-}$ at fluence ranging from ${10}^{14}$ to ${10}^{15}\mathrm{ions}∕{\mathrm{cm}}^2$. On poly-Si, the incident power was reduced to avoid heating.

Figure 9

Normalized fractional change in optical reflectivity $(\Delta R∕R_o)$ of c-Si (stars) and poly-Si (plus). Normalized heat release between RT and $500°\mathrm{C}$ (circles) as measured by nanocalorimetry for $30\mathrm{keV}$ ${\mathrm{Si}}^{-}$ implanted at room temperature.

Figure 10

Channeling spectra from c-Si implanted with $30\mathrm{keV}$ Si at room temperature at indicated fluences (symbols). The random (solid line) and aligned (dashed line) spectra of unimplanted c-Si are shown for comparison. Spectra were obtained along the ⟨100⟩ axis using $2\mathrm{MeV}$ ${}^4{\mathrm{He}}^{+}$ ions detected at a scattering angle of 170° to the incident beam axis.