Single-crystal silicon coimplanted by helium and hydrogen: Evolution of decorated vacancylike defects with thermal treatments

Si $p$-type (100) samples were coimplanted at room temperature with ${\mathrm{He}}^{+}$ ions at $30\mathrm{keV}$ with a dose of $1\times {10}^{16}\text{ions}∕{\mathrm{cm}}^2$ and successively with ${\mathrm{H}}^{+}$ ions at $24\mathrm{keV}$ with a dose of $1\times {10}^{16}\text{ions}∕{\mathrm{cm}}^2$. A series of samples was thermally treated for $2\mathrm{h}$ from $100\text{to}900°\mathrm{C}$ at $100°\mathrm{C}$ steps to study the evolution of pointlike and extended defects by two complementary techniques: positron Doppler broadening spectroscopy and transmission electron microscopy. Depth profiling the samples with a positron beam led to the identification of five different traps and the evolution of their profile distributions with thermal treatments. The positron traps were identified as decorated vacancy clusters of different sizes. Their decoration by implanted ions and in some case by oxygen was probed by coincidence Doppler broadening spectroscopy. Up to $300°\mathrm{C}$ annealing temperature positrons probe three distributions of different decorated defects covering regions of the sample down to $400–450\mathrm{nm}$. Starting from $300°\mathrm{C}$ annealing temperature no defects were revealed by positrons in the region next to the peak of the implanted ions distributions positioned around $280\mathrm{nm}$, where extended defects are expected; this indicates complete filling of the defects by H and He. From $300\text{to}600°\mathrm{C}$ decorated vacancy clusters of different sizes appear progressively in the region below $280\mathrm{nm}$, with a distribution moving deeper into the sample. Comparison with previous measurements on He-implanted samples points out the chemical action of H. Hydrogen atoms interact with the previous damage by He, producing more stabilized vacancylike defects distributed through the damage region of the sample. Electron microscopy shows the transformation of the extended defects from platelets to blisters and cavities.

Figure 1

(a) Cross section image of the as implanted sample. (b) Cross section image of the sample thermal treated at $500°\mathrm{C}$. (c) Cross section image of the sample thermal treated at $800°\mathrm{C}$.

Figure 2

Plan view of the sample thermal treated at $700°\mathrm{C}$.

Figure 3

Doppler broadening measurements of the positron annihilation line. $S_n$ parameter vs positron implantation energy (lower scale) and mean positron implantation depth (upper scale). (a) as-implanted and samples heat treated up to $300°\mathrm{C}$. (b) Samples heat treated up to $600°\mathrm{C}$. (c) Samples heat treated up to $900°\mathrm{C}$. (d) Reference c-Si virgin sample and c-Si virgin sample annealed at $900°\mathrm{C}$. The curve of the as-implanted samples is reported in (a), (b), and (c) for clarity. The continuous lines are best fit to the data with the positron diffusion model.

Figure 4

Block diagram showing the different group of defects and their distribution in depth (full width at 10%) as found by analysis of Doppler broadening measurements data. Details about defect types and their distribution are reported in Table . Black boxes: defect distribution $d1$. Grey boxes defect distribution $d2$. Dashed boxes: defect distribution $d3$. White boxes: defect distribution $d4$. Squared boxes: defect distribution $d5$. The two dashed lines mark the layer in which TEM analysis reveals extended defects and then nanocavities.

Figure 5

Ratio of the coincidence Doppler broadening spectrum measured in the as-implanted sample at different positron implantation energies to the coincidence Doppler broadening spectrum of bulk Si.

Figure 6

Ratio of the characteristic coincidence Doppler broadening spectrum to the coincidence Doppler broadening spectrum of bulk Si for positron annihilating at the surface and into the different defects of the three defect distributions $d1$, $d2$, $d3$ present in the as-implanted sample. These characteristic curves are extracted from the measured curves of Fig. 5. The vertical lines help in the identification of the position of the peaks.

Figure 7

Characteristic ratio curves of the defects of distribution $d1$. Evolution of the decoration with the thermal treatment.

Figure 8

Characteristic ratio curves of defects of distribution $d4$. Evolution of the decoration with the thermal treatment.