Nuclear Instruments and Methods in Physics Research B 242 (2006)

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1 Nuclear Instruments and Methods in Physics Research B 242 (2006) NIM B Beam Interactions with Materials & Atoms Ion beam synthesis of Te and Bi nanoclusters in silicon: The effect of post-implantation high frequency electromagnetic field M. Kalitzova a, *, A. Peeva a, V. Ignatova b, O.I. Lebedev c, G. Zollo d, G. Vitali d a Institute of Solid State Physics BAS, Boulevard Tzarigradsko Chaussee 72, BG-1784 Sofia, Bulgaria b SCK.CEN, Reactor Materials Research, Boeretang 200, 2400 Mol, Belgium c EMAT, RUCA, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium d Dipartimento di Enerdetica, Univ. La Sapienza, Via A. Scarpa 14, Roma, Italy Available online 13 September 2005 Abstract The post-implantation effect of high frequency electromagnetic field (HFEMF) on the microstructure and electrical properties of high dose Te + and Bi + implanted (100) Si was investigated by cross-sectional high resolution transmission electron microscopy and fourpoint probe electrical measurements. Te and Bi nanoclusters (NCs) embedded in amorphized Si have been formed by ion implantation. Post-implantation treatment with HFEMF reorganizes the cluster shape and distribution by stimulation of spinodal decomposition and ordering of Te NCs to a percolation system. The effect of HFEMF on Bi NCs is assumed to be connected with the formation of electrical microcurrents causing local heating of their interfaces with the a-si matrix. The results of electrical measurements show that the HFEMF application reduces the sheet resistance by a factor of about 6 for Te + and about 3 for Bi + irradiation. Ó 2005 Elsevier B.V. All rights reserved. PACS: 61.80; A; D Keywords: Nanoclusters; Ion implantation; High frequency electromagnetic field annealing; HRTEM; Sheet resistance 1. Introduction High dose implantation may result in the formation of supersaturated solid solutions as a result of the non-equilibrium nature of ion solid interaction [1]. For instance, it has been shown that ion implantation of insoluble elements, when their concentration exceeds the solubility limit in silicon, may lead to nucleation and growth of nanoscale inclusions embedded in the implanted layer [2 4]. In our previous work, the behaviour of Te, Bi and Pb, which exhibit low solubility in Si, has been investigated after ion implantation [5,6]. It has been shown that implantation of these species in Si at fluences higher than cm 2 produces a fully amorphised silicon surface * Corresponding author. Tel.: ; fax: address: markaliz@issp.bas.bg (M. Kalitzova). layer (a-si). Moreover, with the fluence increasing further, ion beam assisted formation of Te, Bi and Pb nanoclusters (NCs) has been observed. In our recent studies on twophase structures (a-si/te or Bi NCs), produced by high frequency Te + or Bi + ion implantation in Si, the effect of high frequency electromagnetic field (HFEMF) was analyzed [7,8]. Both types of two-phase structures exhibit significant decrease of their electrical resistance, after applying the HFEMF [7]. On the basis of results from high resolution transmission electron microscopy (HRTEM) a conclusion was made that HFEMF treatment influences the dynamics of nanoclustering in Te + implanted Si [8]. HFEMF stimulates the formation of nanocluster extended structures, laterally connected in a planar network. These structural properties lead to lowering of the sheet resistance. So far, no clear explanation of the electrophysical properties of Bi + implanted Si after HFEMF treatment is proposed X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi: /j.nimb

2 210 M. Kalitzova et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) The aim of this study is to clarify the dynamics of the nanoclustering in Bi + implanted Si after applying of HFEMF. For this purpose transmission electron microscopy of cross-sectional specimens in phase contrast (HRTEM) and diffraction contrast (TEM) modes, and four-point probe electrical measurements of sheet resistance, R s, were used in characterizing the system. Some of the previously obtained results for Te + implanted Si are presented here for comparison with the results for Bi + implanted Si in order to systemize the physical processes connected with HFEMF treatment on both systems. 2. Experimental P-type (1 0 0) oriented silicon wafers (17 X cm) were implanted at room temperature with 50 kev Te + or Bi + ions at an angle of 7 with respect to the surface normal. The implantation fluence was cm 2 and the ion beam current 10 lacm 2. The sample temperature was not measured directly during the irradiation but it was estimated to be 240 ± 40 C for the given implantation fluence and ion beam current. HFEMF treatment was performed with a high frequency (HF) generator (up to 24 kw at a frequency of 0.45 MHz). Fig. 1. HRTEM in (110) projection of Te + implanted (100) Si, D = cm 2, E = 50 kev: (a) as-implanted, the arrows indicate the NCs dispersed in a-si layer; (b) furnace annealed at 200 C for 30 min; (c) HFEMF annealed, 0.45 MHz, 30 min at 200 C, the arrows indicate the planar network of NCs. Fig. 2. TEM in (110) projection of Te + implanted (100) Si, E = 50 kev: (a) D = cm 2 and (b) D = cm 2.

3 M. Kalitzova et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) In the process of HFEMF annealing the specimens were mechanically attached to a graphite susceptor covered by SiC. The HF power was adjusted to deliver a susceptor temperature of 200 C. This temperature was chosen to be well below the melting temperatures of the implants (450 C and 272 C for Te and Bi, respectively). In order to distinguish the influence of the high frequency from the influence of the thermal treatment, part of the samples were furnace annealed at the same temperature of 200 C and for the same duration of 30 min as for the HFEMF annealing. HRTEM observations were performed in a JEOL 4000 EX electron microscope, operating at an accelerating voltage of 400 kv, and TEM observations in a JEOL 200 CX electron microscope, operating at 200 kv. Sheet resistance measurements were performed using the four-point probe technique (instrument FPP-100). Fig. 3. HRTEM in (110) projection of Bi + implanted (100) Si, D = cm 2, E = 50 kev: (a) as-implanted; (b) furnace annealed specimen at 200 C for 30 min; (c) highly magnified image of the area selected with the square S in (b), which is the a-si matrix surrounding the NCs. The insert in (c) is the power spectrum obtained by digital FFT of the image; (d) HFEMF treated at 200 C, 0.45 MHz for 30 min; (e) highly magnified image of the area selected with S square in (d). The insert is the FFT spectrum of the image, the interfringe distance is calculated to be 3.13 Å, corresponding to (111) Si interplanar distance. The surface is indicated by s ; (f) highly magnified image of the cluster labeled with 1 in (b); (g) highly magnified image of the cluster labeled with 2 in (d); (h) highly magnified image of the cluster labeled with 3 in (d). The inserts in (f), (g) and (h) are the power spectra obtained by digital FFT of the corresponding images. The interfringe distance in (f) and (g) is calculated to be 3.13 Å (corresponding to (111) Si lattice planes). The interfringe distances in (h) are calculated to be 2.2 Å and 3.8 Å (corresponding to (110) and (012) interplanar distances of Bi crystal.

4 212 M. Kalitzova et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) Results and discussion Table 1 Four-point probe electrical measurements of sheet resistance, R s Ion type Impl. fluence, cm 2 Typical HRTEM micrographs of Te + implanted samples before and after furnace annealing, and HFEMF annealing are demonstrated in Fig. 1. The lattice image of as-implanted Si (see Fig. 1(a)) indicates that a continuous a-si layer was formed during implantation. The faint dark spots, homogeneously dispersed across the a-si layer (some of which are indicated by arrows) are attributed to Te NCs. Their mean dimension is about 0.5 nm. These NCs are better visualized in diffraction contrast mode, as illustrated in Fig. 2. The difference in the imaging contrast of the a-si layer before the formation of NCs, for Te + implanted at a fluence of cm 2 (see Fig. 2(a)), and the a-si layer with embedded NCs, obtained at an ion fluence of cm 2 (Fig. 2(b)), is clearly visible. Returning again to Fig. 1, we can conclude that furnace annealing at 200 C does not produce any visible change in the microstructure of the a-si layer, see Fig. 1(b). In contrast, postimplantation treatment with HFEMF, as seen in Fig. 1(c), causes NC growth to a mean size of 7 nm and migration towards the mean projected Te ion range (R pte =32nm as calculated using SRIM 2000 [9]). Extended structures of the NCs, which are laterally connected in a planar network are indicated by the arrows in Fig. 1(c). Moreover, a zone, which is depleted of NCs, is formed at the a-si/c-si interface. The computer processing of the HRTEM images shows an amorphous structure. We assume that the HFEMF influences the dynamics of Te clustering by stimulating spinodal decomposition [10] and evolution to a percolated system [4,8]. In the case of Bi + implantation the morphology of NCs is different as seen in Fig. 3(a). A pronounced band NCs with a mean size of about 10 nm is formed around R pbi, corresponding to a depth of 29 nm as calculated by SRIM This band extends to the surface. In our opinion, the observed morphology corresponds to the final stage of the phase transition and Ostwald ripening [11] where smaller clusters are driven to dissolve and transfer their monomers to larger ones. We have shown before [5] that bismuth NCs crystallize in the hexagonal phase. As a result of sputtering during high dose implantation the specimen surface becomes rough. The application of conventional thermal annealing (see Fig. 3(b)) leads to some shrinkage of the NC band, so that the zone at the a-si/c-si interface which is depleted of NCs is broadened (compare Fig. 3(b) with Fig. 3(a)). Post-implantation HFEMF treatment causes as well a shrinkage of the NC band and an enlargement of the zone which is depleted of NCs (compare Fig. 3(d) with Fig. 3(a)). A new effect has been observed, provoked by the HFEMF treatment only, the a-si matrix surrounding the bismuth NCs is recrystallized. This result has been verified by Fast Fourier Transformation (FFT) of the corresponding area of the image (see Fig. 3(e)). The insert in Fig. 3(e) reveals the presence of two spots related to the Si(1 1 1) interfringe distance. The insert in Fig. 3(c) verifies the absence of crystallization in the a-si matrix surrounding bismuth NCs in the case of conventional thermal annealing. We relate the effect of high frequency on the crystallization of the a-si matrix to electrical microcurrents at the surface of the metallic NCs under the influence of applied HFEMF. As a result of the skin effect, additional local heating at NCs boundaries stimulates the a-si surroundings to crystallize. Fig. 3(f) (h) present some of the experimental NCs images observed in Fig. 3(b) and (d) at higher magnification, together with their power spectra obtained by FFT. A correlation between the NCs evolution and the sheet resistance, R s, change under the influence of HFEMF is found. The results of the R s measurements are summarized in Table 1. The data show that the two types of annealing change R s of the implanted layers only. Implantation with Te + or Bi + at a fluence of cm 2 creates a surface layer with a resistivity higher than that of the Si substrate. After HFEMF treatment the R s value of Te + implanted Si is decreased by more than a factor of 6 (from 39 kx/h to 6 kx/h), while for Bi + it is decreased by a factor of about 3 (from 33 to 12 kx/h). For furnace annealed specimens this effect is much smaller. 4. Conclusion As shown by microstructure evolution and by electrical measurements, HFEMF treatment causes configurational changes of Te and Bi NCs generated in Si by ion implantation. HFEMF is proposed to influence the dynamics of nanoclustering by stimulation of spinodal decomposition and rearranging the Te NCs to a percolation system. For the case of Bi + implantation it is assumed that the high frequency stimulates electrical microcurrents through the NCs causing local heating the NCs boundaries and recrystallization of the surrounding a-si matrix. The processes of percolation in Te + implanted Si and crystallization of a-si matrix in Bi + implanted Si influence in different degree reduction of the sheet resistivity under the action of the high frequency electromagnetic field. Acknowledgements R s of as-impl., kx/h R s of furnace annealed, kx/h Non-implanted Te Bi R s of HFEMF treated, kx/h The authors would like gratefully to thank Dr. K. Gesheva for HFEMF annealing and sheet resistance measurements of the specimens. Also, we would like to acknowledge stimulating discussions with Prof. N. Pashov. This work was partially supported by Bulgarian NSF research project No. F 1310.

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