Local traps as nanoscale reaction-diffusion probes: B clustering in c-Si
Supplementary Online Material
Estimation of a/c interface depths after Ge implantation
The a/c interface depth generated by 12 keV, 1

10
15
/cm
2 , 0° tilt Ge implantation, used as the starting point for the experiments reported in Figs. 1 and 2, was measured by channeling RBS-c
(uncertainty ~ ± 1 nm) and was confirmed by X-TEM. Two RBS-c measurements gave values of
22.1 nm and 22.5 nm. Taking into account a shift of 0.7 nm relative to the SIMS measurements owing to the presence of a native oxide layer of thickness ~ 1.5 nm in the samples measured by
SIMS, we estimate that the a/c interface is located at a depth of ~ 23 nm in the SIMS profile shown in Fig. 2A of the paper.
The a/c interface depth produced by 75 keV, 1

10
15
/cm
2 , 0° tilt Ge implantation, used as the starting point for the experiments reported in Fig. 3 of the paper, was not measured by RBS or X-
TEM in this study. However, it can be estimated from two independent sources of information obtained in this work. First, as noted in the paper, an extrapolation of the temperature-dependent results in Fig. 3A, using an Arrhenius fit, suggests that the a/c interface depth after Ge implantation was approximately 95 nm. Second, the a/c interface depth after an additional 77 K B implantation can be estimated from the slight discontinuity in the 77 K B profile (black curve in Fig. 3A of the paper), which can be seen at a depth of 110 nm on the black curve by comparing to the corresponding 293 K B profile (blue curve). Taking into account the 15 nm shift in the 77K profile due to the presence of an ice layer on the sample, this result places the a/c interface depth after 77
K B implantation at 95 nm. Since 77 K B implantation is not expected to generate any beaminduced regrowth, this provides a second, independent source of information that the depth of the a/c interface after Ge implantation is 95 nm. The results described above are shown in Fig. S1.
1

85
80
75
70
100
95
90
65
0 20 40 60 80 100 120 140 160
Fig. S1: A/c interface depths corresponding to the data shown in Fig. 3A of the paper, plotted as a function of inverse temperature. Symbols are experimental data and the curve is an Arrhenius fit to the data at room temperature and above (symbols at left side of figure).
The curve passes through the data at 77 K (symbol at right) inferred from the discontinuity in the 77 K B profile.
Ge implant profiles, displacement damage, and their effects
This section presents Ge implant profiles measured by SIMS, together with corresponding profiles and damage distributions predicted by the binary collision Monte Carlo code TRIM, for the conditions studied in the paper. Similar data are included for the 20 keV B implant used for the experiment in Fig. 3 of the paper. Also presented are a/c interface depths estimated from experiment.
Fig. S2 shows results for the 12 keV, 1

10
15
/cm
2 , 0° tilt Ge implant profiles used for the experiments presented in Figs. 1 and 2, as predicted by TRIM (thick blue line). Also included is a simulation of the corresponding Frenkel pair distribution (thin blue line).
2

1x10
24
1x10
23
1x10
22
1x10
21
1x10
20
1x10
19
1x10
18
0 5 10 15 20 25 30 35 40 45 50
Fig. S2: TRIM prediction of the Ge profile after 12 keV, 1 × 10
15
Ge/cm
2 , 0° tilt implantation at RT (thick blue line), as used in the experiments in Figs. 1 and 2. Also included is a TRIM simulation of the corresponding Frenkel pair distribution (thin blue line). The as-implanted a/c interface depth, measured by channeling RBS-c, is also shown
(vertical black dashed line).
Fig. S3 shows results for the 75 keV, 1

10
15
Ge/cm
2 , 0° tilt implant profile (thick blue line) used for the experiments presented in Fig. 3A of the paper, as predicted by TRIM. Also shown is the simulated Frenkel pair distribution (thin blue line).
Fig. S3 also shows results for the B 20 keV, 3

10
15
/cm
2
implant profile used for the experiments presented in Fig. 3A of the paper, as measured by SIMS (red square symbols) and simulated (thick red line). Also shown is the simulated Frenkel pair distribution (thin red line). Both B and Frenkel pair distributions were depth-scaled by a factor of 0.84 to match experimental profile. It should be noted that the calculated Frenkel pair profile does not account for the strong self-annealing effects in B implantation damage which take place at RT and above, but it may give a rough representation of the damage profile at low temperature. From this it appears that the 77 K B implant does not significantly deepen the amorphous region produced by the prior Ge implant at RT. This conclusion is supported by the results discussed in the previous section, which show the same a/c interface depth of 95 nm after Ge implantation and after additional 77 K B implantation.
3

10
24
10
23
10
22
10
21
10
20
10
19
10
18
0 20 40 60 80 100 120 140 160
Fig. S3: TRIM simulation of Ge profile after 75 keV, 10
RT (thick blue), and B profile after 20 keV, 3 × 10
15
15 Ge/cm 2 , 0° tilt implantation at
B/cm
2
RT implantation – simulated
(thick red), experimental (red symbols), as used in the experiments in Fig. 3. Also shown are TRIM simulations of the corresponding Frenkel pair distribution for Ge implant (thin blue line) and for B implant (thin red line). B and FP profiles were depth-scaled by 0.84 to match experimental profile. The a/c interface depth after Ge implantation (vertical dashed black line) is indicated as explained in the next section.
Finally, it should be noted that the Ge concentrations at depths beyond the a/c interface are very small in both Fig. S2 and Fig. S3. Higher concentrations, up to ~ 1 at.%, occur after SPER in Figs.
2B, 2C and the higher temperature implants in Fig. 3A, however, in all cases the Ge content is too low to produce significant SiGe alloy effects on diffusion or clustering (which typically start to emerge at Ge composition > 5%). It is therefore sufficient to consider Ge implantation in our experiment solely as a source of Si amorphization and EOR defect formation.
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Distinguishing between the effects of implantation and SIMS on B diffusion
There are two potentially valid explanations for the diffusion of B at RT and above which is observed in the paper. First, it may be a result of the B implantation process. Second, it might be
(at least partially) a result of B diffusion during the SIMS measurements, as these were carried out at RT. This section discusses in detail the evidence that the observed diffusion occurred during B implantation and not during SIMS.
We first consider the known properties of B diffusion at RT. Around this temperature B can migrate for substantial distances as a BI complex, its dissociation energy of ~ 1 eV preventing a rapid return to a substitutional site. In theory this should lead to BI migration over large distances at RT, however, in this situation BI is most likely to be captured first by a trap – either another B atom or a point defect that reacts with BI to form an immobile species. Ref [20] of the paper reported experimental studies in which ion-implanted BI migrate and are trapped in a background of B doping, and in Ref [19] Napolitani et al . reported experiments where BI are displaced by self- interstitials during SIMS profiling at RT, becoming trapped over a characteristic distance of ~ 100 nm by defects in the host lattice. Both of these processes lead to the removal of an interstitial by the capture of a BI . Thus, at RT, an interstitial can only cause one B atom to diffuse, after which it is ‘out of play’.
The experiments of Napolitani et al . showed the migration of ~ 10
12
B atoms/cm
2
, corresponding to ~ 10% of the B atoms in a lightly doped marker layer, as a result of the release of self-interstitials from the deep tail portion of the SIMS profiling damage. If more self-interstitials had been released during SIMS profiling, a larger number of B atoms would have been displaced (this is directly evidenced in a later paper by Napolitani et al. [J. Vac. Sci. B 24, 394 (2006)], where the role of oxygen flooding is considered). In contrast to those studies, in our present paper most of the B profiles in the c-Si region contain ~ 10
15
B/cm
2
. Assuming the same number of self-interstitials generated by the SIMS beam in our experiment as in Napolitani’s study, only ~ 1:1000 B atoms could potentially be displaced by SIMS profiling. This figure is even likely to be an over-estimate, as in our experiments the SIMS ion beam reaches the B atoms of interest as soon as it enters the c-Si region, leaving little time for the injection of self-interstitials. Nevertheless, extensive B migration is observed in all of the B profiles discussed in our paper, apart from the one implanted at 77 K. The exception at 77 K is actually expected, based on Napolitani’s observation that BI migration during ion beam irradiation (whether by SIMS or ion implantation) can be suppressed by reducing the ambient temperature below -50 °C. Thus we conclude that SIMS effects are close to negligible in our experiment, and that the effects we have observed take place during B implantation. A similar argument based on the absence of significant dose-dependent changes in the SIMS profiles in Fig. 4, already discussed in the paper, clearly consolidates the above evidence.
However, as pointed out in our paper, a residual effect of SIMS at RT does seem to be needed to explain the very slight discontinuity in the 77 K B profile in our Fig. 3A. This might be an effect of energetic ion-solid collisions displacing B atoms into BI positions during sputtering.
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Interpretation of the absence of B diffusion in a-Si in this study
In the present temperature range of interest, boron diffusion in a-Si is negligible and diffusion in c-Si only occurs because part of the implanted B stops on interstitial sites, while excess selfinterstitials generated directly by the implant can also displace B that arrives on substitutional sites.
Since at low temperature B migrates for long distances in c-Si, this leads to significant redistribution in the c-Si region but not in the a-Si region where diffusion-mediating defects
(dangling bonds, etc) are much less mobile in the temperature range we discuss. This can be confirmed by referring to a paper by Mirabella et al . [Phys. Rev. Lett. 100, 155901 (2008)], which shows significant diffusion of B at 500°C (773 K) in amorphized Si after an
8-hour anneal. This is a much higher temperature and much longer time than the range of interest in our study, where events occur in seconds (or less) during implantation at RT.
It should also be noted that we see no redistribution of B anywhere within the amorphous layer in our experiment, even close to the a/c interface which provides such a sensitive probe of diffusion in the crystalline region, and where B piles up to a significantly higher concentration than in the adjacent amorphous layer. This seems to be evidence in agreement with the literature that B is not moving significantly within the amorphous layer in our experiment – in fact, this result, though negative, is a further piece of information obtained by using our a/c interface as diffusion probe.
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