Cu gettering to nanovoids in soi materials

V ol. 46 No. 1SCIENCE IN CHINA(Series E)February 2003Cu gettering to nanovoids in SOI materialsZHANG Miao (张苗)1,WU Yanjun (吴雁军)1, LIU Weili (刘卫丽)1,AN Zhenghua (安正华)1, LIN Chenlu (林成鲁) & 朱剑豪(Paul K. Chu)21. State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and informationTechnology, Chinese Academy of Sciences, Shanghai 200050, China;2. Department of Physics and Materials, City University of Hong Kong, ChinaCorrespondence should be addressed to Zhang Miao (email: mzhang@)Received July 20, 2002Abstract In this paper, the gettering of Cu impurities in silicon-on-insulator (SOI) materials is studied. Nanovoids are formed in the substrate of SOI beneath the buried oxide (BOX) by room temper ature H+ (3.5×1016 /cm2 ) or He+ (9×1016 /cm2) implantation and subsequent annealing at 700o C. The gettering of different doses of Cu (5×1013/cm2, 5×1014 /cm2, 5×1015/cm2), which are introduced in the top Si layer by ion implantation, to the nanovoids are investigated by cross-section transmission electron microscopy (XTEM) and secondary ion mass spectroscopy (SIMS). The results demonstrat e that Cu impurities in the top Si layer can diffuse through the bur-ied oxide (BOX) layer of SIMOX and Smart-Cut SOI at temperature above 700o C and be trapped by the nanovoids. Some of Cu impurities can be captured by the intrinsic defects at the BOX inter-face of SIMOX, but will be released out at high temperatures. The gettering effect of SIMOX intrin-sic defects at BOX is much lower than that of the nanovoids. No Cu impurities are trapped at the perfect BOX interfaces of Smart-Cut SOI. After 1000℃ annealing, high dose of Cu (3.6×1015 /cm2) was gettered by the nanovoids. The Cu gettering efficiency to the nanovoids increased with the decreasing of Cu doses. When the Cu doses in the top Si layer were lower than 4×1015 /cm2, the nanovoids could getter more than 90% of the Cu impurities and reduce the Cu concentration in the top Si layer to less than 4%. The results indicate that nanovoids gettering is a promising method for removing the impurities in SOI materials.Keywords:SOI, gettering,ion implantation,nanovoids.Silicon-on-insulator (SOI) materials have a number of inherent advantages over bulk silicon substrates owing to their high speed, low power complementary metal-oxide-semiconductor (CMOS) integrated circuits, such as immunity to radiation hardness, high speed, and high tem-perature tolerance [1]. Separation by implantation of oxygen (SIMOX), bonding and etch back (BESOI) and Smart-Cut are the leading technologies for SOI material fabrication. Transitional metal impurities such as Cu, Fe and Ni will contaminate the SOI materials and devices during the fabrication process. These impurities will degrade the devices by decreasing the minority carrier’s lifetime, increasing the pn-junction leakage and reducing the radiation hardness[2,3]. Cu is a com-mon impurity in SOI materials and devices, and its high diffusivity in Si makes it extremely dele-terious [4]. It is necessary to remove the harmful impurities from the top Si of SOI on which the active devices will be fabricated.No. 1 CU GETTERING TO NANOVOIDS IN SOI MA TERIALS 61Traditional gettering methods such as internal gettering and implantation damage gettering are through forming precipitate at the gettering sites and can only reduce the metal level to their solubility in Si. This cannot meet the requirements of ULSI to reduce the metal impurity below 2×109 /cm3 [5]. Although the solubility of transition metals vary with temperature, they are higher than 109/cm3 [6]. H+ or He+ implantation and subsequent annealing can form nanovoids in silicon[7—9]. Recent studies indicated that these nanovoids can effectively capture metal impurities such as Cu and Ni in bulk silicon material[7—10]. Since the metal impurities are trapped strongly to the dangling bonds on the voids wall by chemical absorption, their concentration can be reduced below the solubility in silicon. It was reported that nanovoids can reduce Cu impurities from 1014 to 107/cm3 [11]. Therefore nanovoids gettering meets the requirement of ULSI. Since there is a BOX layer between the top silicon layer and the Si substrate, traditional gettering methods can hardly reduce the metal impurities in the top silicon layer to a certain level. In this work, Cu im-purity gettering to nanovoids in SIMOX and Smart-Cut SOI materials are investigated and the results demonstrate that nanovoids can reduce Cu impurity and in the top silicon layer of SOI to 4% of its original concentration.1 Experiment#1 SIMOX wafer was fabricated by implanting 3.3×1017 /cm2, 70 keV O+ into n-type (100) CZ silicon wafers at 500o C. Then the oxygen-implanted silicon wafer was annealed at 1300o C for 6 h in the ambient of Ar + 0.5% O2 to form the sharp BOX layer. The thickness of the top silicon layer and the BOX layer were 110 and 80 nm, respectively. The fabrication processes of #2 and #3 SIMOX were similar to that of #1 SIMOX. By choosing different O+ implantation doses and en-ergies SIMOX wafers with different thickness of top silicon and BOX were formed. The Smart-Cut SOI wafer was provided by SOITEC Company.#1a reference sample: In order to study the distribution of Cu in SIMOX material without nanovoid gettering sites, 5×1015 /cm2 of Cu+ was implanted into the top Si of #1 SIMOX wafer with the energy of 70 keV at room temperature and then annealed at 1000℃ for 2 h in flowing N2. The Cu distribution in the sample was analyzed using SIMS.Cu gettering to nanovoids induced by H+ implantation in SIMOX material: 3.5×1016/cm2 H+ was implanted into the Si substrate of #1 and #2 SIMOX with the energy of 50 keV (R p=550 nm) and 70 keV (R p=660 nm), respectively. After thermal treatment at 700o C for 30 min, a nano-void layer was formed in the Si substrate beneath the BOX layer. Then with an energy of 70 keV, 5×1015/cm2 of Cu+ was implanted into the top Si layer of the #1 SIMOX (#1b), 5×1014/cm2 (#2a) and 5×1013/cm2 (#2b) of Cu+ was implanted into #2 SIMOX. Then samples #1b, #2a and #2b were annealed at 700o C and 1000℃ for 2 h in N2.Cu gettering to nanovoids induced by He+ implantation in SIMOX and Smart-Cut SOI mate-rials: 9×1016/cm2 He+was implanted into #3 SIMOX and the #4 Smart-Cut SOI wafer with the62 SCIENCE IN CHINA (Series E) V ol. 46 energy of 60keV and 170keV , respectively. These two wafers were subsequently annealed at 700o C for 30 min to form nanovoids. Then with the same energy of 70 keV , 5×1015/cm 2 of Cu + was implanted into the top Si layer of #3 SIMOX and 1×1014/cm 2 of Cu + was implanted into Smart-Cut SOI wafer. Finally the samples were annealed at 700, 850 and 1000℃ for 90 min or 3 h in N 2. The Cu distribution and the microstructures of the specimen were characterized by SIMS and XTEM, respectively.The samples used in this study are summarized in table 1.Table 1 Samples used in this studySampleThickness of top Si/nm Thickness of BOX/nm Cu dose/cm 2 Note #1a SIMOX5×1015 without nanovoids #1b SIMOX110 80 5×1015 #2a SIMOX5×10 #2b SIMOX70 210 5×1013 with nanovoids im-planted by H + #3 SIMOX80 110 5×1015 #4 Smart-Cut200 400 1×1014 with nanovoids im-planted by He +2 Results and discussion2.1 Cu distribution in the SIMOX without nanovoid gettering sitesFirstly, high dose of Cu (5×1015/cm 2) was introduced into the top silicon layer of #1 SIMOX wafer and annealed at 1000o C. The distribution of Cu in SIMOX without nanovoid gettering sites was studied. Fig. 1 shows the SIMS depth profile of #1a before and after annealing. The O and Siprofiles are shown mainly for depth comparison. The O and Si profiles indicate a clear region ofconstant oxygen concentration. The oxygenconcentration at the upper Si/SiO 2 interfaceincreases very rapidly, but the lower SiO 2/Siinterface is not so sharp. This result indicatesthat the oxygen concentration at the lower in-terface of the BOX in #1 SIMOX is not highenough to form stoichiometric SiO 2. From fig.1 we can see that the Cu in the as-implantedsample shows a Gaussian distribution in the topsilicon layer, and the location of the peak is inagreement with the projected range (R p =50 nm)of 70 keV Cu ions simulated by TRIM94. Afterthe 1000°C annealing, Cu has redistributed andexhibits two peaks. Peak A is still located in thetop silicon layer but nearer to the surface andnarrower than the un-annealed sample. The Cu Fig. 1. SIMS depth profile of #1a SIMOX only implanted with 5×1015Cu +/cm 2 and annealed at 1000o C for 2h (without nanovoids).No. 1 CU GETTERING TO NANOVOIDS IN SOI MA TERIALS 63concentration decays rapidly with depth and reaches its lowest value at the upper BOX interface. Peak B is situated at the lower BOX interface and can be attributed to metal segregation at a re-gion of lower stoichiometry. No Cu peak was observed at the sharp upper BOX interface. The amounts of Cu in peaks A and B are 54% and 15% of the total implanted Cu. In addition to these two peaks, Cu yield in the Si substrate beneath the BOX layer is also high (31% of the Cu). The segregation of Cu in this region may be related to the strain in the presence of BOX layer. The gettering of Cu to the interfaces of the BOX layer and to the thin Si substrate beneath the BOX layer has been reported by Kamins[12].These results suggest that some Cu impurities can be gettered in the regions around and be-neath the BOX layer, but there are still 55% of the implanted Cu impurities left in the top Si layer after the 1000o C annealing, indicating that the gettering efficiency of the intrinsic defects in SI-MOX is low. On the other hand, with the improvement of SIMOX fabrication technology, the BOX interfaces are becoming sharper, and the defects at the BOX interfaces will greatly decrease. The intrinsic gettering sites in SIMOX will decrease or even disappear. Stronger gettering sites should be introduced to reduce the impurity concentration in the top Si layer.2.2 Cu gettering to nanovoids induced by H+ implantation in SIMOXStudies demonstrated that when moderate dose of H+ ions (3×1016—1×1017/cm2) are im-planted in Si wafer, Si-H centers and vacancy complexes displaced Si atoms, and other complex defects are generated in the damaged region. The vacancies and vacancy clusters can congregate and trap H to form stable vacancy-H complexes (VmHn). During annealing, H atoms released from other defects may diffuse to the vacancy-H complexes and developed into small bubbles. The density of the bubbles increases with the increasing H+ implantation dose[13]. The pressure in the bubbles increases rapidly with the congregation of H. If the H+ implantation dose is higher than 4×1016/cm2 and the temperature is higher than 400o C, the pressure in the bubble will be high enough to deform the silicon layer that cover these bubbles plastically: the surface layer will be pushed outward to form blisters, leading to the distortion of surface lattice; sometimes the cover-ing layer even flakes off. This phenomenon is the key factor of Smart-Cut SOI fabrication tech-nology[14]. If the H+ implantation dose is in the range of 3—4×1016/cm2, the density of bubbles is too small to deform the covering silicon layer during annealing and the H will release out from the bubbles and diffuse out of the silicon wafer rapidly, leaving empty small voids in the silicon. Above 700o C, the small voids migrate and aggregate to form nanovoids. The dangling bonds at the void wall are high reactive and can trap metal impurities strongly.In order to study the gettering ability of nanovoids in SIMOX, after the nanovoids were formed by H+ implantation and annealing, high dose of Cu impurities (5×1015/cm2) was implanted into the top silicon layer of #1 SIMOX. Fig. 2(a) shows the XTEM image of #1b which had been annealed at 1000°C for 2 h. It can be seen that the thickness of the BOX is homogenous and the upper and lower SiO2/Si interface are sharp and straight. Some precipitates have formed in the top64 SCIENCE IN CHINA (Series E) Vol. 46silicon layer by the Cu+ implantation and subsequent annealing. No intrinsic defects are found beneath the BOX. A void band has formed in the silicon substrate at a depth of about 550 nm from the surface. The diameter of the voids varies from 20 to 100 nm. Most of the voids have evolved into a shape of polyhedron to lower the surface energy. SIMS analysis detected no H in the void layer, indicating that the implanted H have diffused out from the void layer (formerly bubble layer) after the 1000o C annealing, leaving empty voids as observed by XTEM (fig. 2(a)).Fig. 2 #1b SIMOX implanted with 3.5×1016H+/cm2, 5×1015Cu+/cm2 and annealed at 1000o C for 2 h. (a) XTEM photo; (b) SIMS depth profile.Fig. 2(b) gives the SIMS results of #1b annealed at 1000°C. Obviously, the Cu profile in this figure is significantly different from that in fig.1. There are 4 Cu peaks in fig. 2(b) (A, B, C, and D). The near surface peak A and lower SiO2/Si interface peak B, which are the only Cu peaks in fig.1, are also observed in fig. 2(b). Peak C lies between the lower BOX interface and the void layer. Peak D, which is the highest, is situated at the position of the void band. Similar to the phe-nomenon observed in fig. 1, the movement of peak A to the surface is found in fig. 2(b). This movement is attributed to the following reason: high dose of Cu implantation almost changes the thin surface region of top crystal Si into amorphous Si. The α-Si will undergo recrystallization at high annealing temperature. The Cu peak in the α-Si moves toward the surface along with the movement of α-Si/c-Si interface. 13% of Cu is trapped in peak A. It should be noted that the highest peak B in fig. 1 becomes the lowest peak in fig. 2(b). Only 3% of the implanted Cu r e-mains at peak B in fig. 2(b). The great decrease of peak B is obviously related to the existence of voids. The dangling Si bonds on void walls are highly reactive and trap the Cu impurities diffus-ing through the BOX strongly by chemisorption, resulting in a decrease in Cu at the lower Si/SiO2 interface. 10.4% of the implanted Cu is located at Peak C that may result from the lattice deforma-No. 1 CU GETTERING TO NANOVOIDS IN SOI MA TERIALS 65 tion induced by the BOX and voids. 73.6% (corresponding to a dose of 3.6×1015/cm 2) of Cu is trapped in peak D. It is obvious that in the presence of nanovoids, the Cu concentration in the top silicon layer and at the interfaces of BOX are greatly decreased from 54% and 15% to 13% and 3%, respectively. This result demonstrates that the gettering effect of nanovoids is much stronger than that of the BOX interface. And large amount of Cu can be gettered in the void layer.In order to demonstrate the gettering effect of the voids more clearly, lower doses of Cu + (5×1014/cm 2 (#2a) and 5×1013/cm 2 (#2b)) were implanted in the top silicon of #2 SIMOX. The gettering effect of different doses of Cu to the voids was compared. Figs. 3 and 4 illustrate the SIMS depth profiles of these two samples after annealing at 700°C and 1000°C. Following the 700°C annealing, 67.7% of the Cu in #2a sample was left in the top silicon layer; only 25.8% of Cu was found in the void band (fig. 3), while 66% of the implanted Cu in #2b had been captured by the voids with 14% of Cu remaining in the near surface peak and 17% of Cu in the BOX (fig.4). By increasing the annealing temperature to 1000°C, the Cu in the top silicon of #2a was re-duced to 3.6% and the Cu gettered by voids increased to 86.4% with 9.6% of Cu in the BOX layer. For sample, in #2b, the amount of Cu trapped in the void band increased to 92% and only 1% of Cu was left in the top Si. This result suggests that the gettering efficiency of nanovoids increases with the decreasing Cu doses and with the increasing annealing temperatures. This feature of nanovoids gettering meets the requirement of ULSI technology.Fig. 3. SIMS depth profile of#2a SIMOX, which is im-planted with H + (3.5×1016/cm 2), Cu + (5×1014/cm 2) and annealedat 700o C and 1000℃ for 2 h. Fig. 4. SIMS depth profile of #2b SIMOX implanted with H + (3.5×1016/cm 2), Cu + (5×1013/cm2) and annealed at 700o Cand 1000o C for 2 h.66 SCIENCE IN CHINA (Series E) Vol. 462.3 Cu gettering to nanovoids induced by He+ implantation in SIMOXHe+ implantation can produce more nanovoids with a larger dose range in silicon without flaking off surface than H+ implantation. From the viewpoint of impurity gettering, He+ implanta-tion is more favorable. In this work, nanovoids were introduced in the silicon substrate of #3 SI-MOX by 9×1016 /cm2 He+ implantation and 700o C annealing. The gettering of high dose Cu to the voids was studied. The microstructure of #3 S IMOX annealed at 1000o C for 90 min is shown in fig. 5. Dense nanovoids were formed in a band located at a depth of 330—560 nm from the sam-ple surface and the void density in the lower part of the void band was bigger than that in the up-per part. SIMS was used to study the gettering efficiency of high dose Cu to the He+-induced voids in S IMOX at different temperatures (fig. 6). After 700o C annealing, Cu diffused from the surface and redistributed in different gettering sites: about 28% of Cu remained in the top Si layer; 32% of Cu was captured in the voids and formed a wide Cu peak. Cu also observed at the two BOX interfaces. 20% of Cu was detected in narrow peak at the upper Si/BOX interface and 21% of Cu was trapped at the lower BOX/Si interface. This result shows again that the intrinsic defects at the BOX interface are gettering site for Cu impurity. But the gettering effect varied with the different structure and quality of SIMOX wafers. I ncreasing the annealing temperature to 850℃,Fig. 5. XTEM of #3 SIMOX, implanted with He+(9×1016/cm2) and Cu+ (5×1015/cm2) and annealed at 1000℃for 90 min. Fig. 6. SIMS results of #3 SIMOX, implanted with He+(9×1016/cm2) and Cu+(5×1015/cm2) and annealed at dif-ferent temperatures.No. 1 CU GETTERING TO NANOVOIDS IN SOI MA TERIALS 67Cu gettered by the nanovoids increased to 61% and Cu concentration at the lower part of the voids band was higher than that in the front part. This distribution was in agreement with the observa-tion of voids distribution in the band. There was still 31% of Cu in the top silicon layer. But no Cu accumulated at the BOX interfaces. After the 1000o C annealing, the Cu amount at the void band increased to 80% (4×1015 /cm2) and only 15% of Cu was left in the top silicon. Compared with the H+ implanted SIMOX wafer, He+ implantation induced denser voids and thus produced more getter-ing sites, but the amount of Cu gettered in the void band in #3 SIMOX did not increase obviously. High dose of Cu implantation at room temperature induced heavy damage, which could getter Cu and compete with nanovoids, in the top silicon. These damages were very stable and difficult to be annealed out, preventing the releasing of Cu from the top silicon. Therefore even in the presence of much more nanovoids by He+ implantation, the gettered Cu did not increase significantly.2.4 Cu gettering to nanovoids induced by He+ implantation in Smart-Cut SOIDifferent from the BOX of SIMOX, which are formed by high dose of oxygen implantation and high temperature annealing, the BOX layer of Smart-Cut SOI is grown by thermal oxidation and is therefore denser. The gettering in Smart-Cut SOI will be different from that in SIMOX. Nanovoids were introduced in Smart-Cut SOI by He+ implantation and the effect of Cu gettering was studied. After nanovoids were formed in the substrate, 1×1014/cm2 Cu+ was implanted into the surface of the top Si layer and annealed at 1000℃ for 3 h. The microstructure and Cu in-depth distribution are given in fig. 7. After the 1000℃ annealing, almost all of the originally implantedFig. 7. Smart-Cut SOI sample implanted with He+(170keV, 9×1016/cm2 ), Cu+(70 keV, 1×1014/cm2) and annealed at 1000o C for 3 h. (a)XTEM photo; (b)SIMS depth profile.68 SCIENCE IN CHINA (Series E) V ol. 46Cu (96%) was found at the nanovoid band, and only 3% of Cu remained in the top silicon. No Cu pileup was observed at the interfaces of the BOX, demonstrating that Cu can diffuse through the buried thermal oxide layer at elevated temperature and the perfect BOX interfaces in Smart-Cut SOI did not trap Cu impurity. During the annealing at 1000℃, Cu was released out from the damaged top Si layer and diffused through the dense BOX to the void layer and was finally get-tered by the voids. The gettering of Cu in all the samples used in this study after 1000o C annealing are summarized in table 2.Table 2 Cu distribution in the top Si and void band of the SOI samples after 1000℃annealing Sample Cu dose/cm2Cu in top Si(%) Cu in void band(%) Note#1a SIMOX 5×101554 0 without nanovoids#1b SIMOX 5×1013 73.6 #2a SIMOX 5×1014 3.6 86.4 #2b SIMOX 5×1013 1 92 with nanovoids im-planted by H+#3 SIMOX 5×1015 80 #4 Smart-Cut 1×1014 3 96 with nanovoids im-planted by He+2.5 Principle of nanovoids gettering in SOI materialsIt is known that each gettering process should consist of three steps[15]: impurity release from the impurity source, diffusion to the gettering sites and trapping or precipitating at the gettering sites. The gettering process cannot be completed if any one of the three steps fails. Nanovoids can getter transition metal strongly by chemical absorption and the gettering efficiency will be deter-mined by the impurity releasing and diffusion. The special structure of SOI material makes the gettering process more complicated.The damage caused by the Cu implantation becomes more serious with an increase in doses, and the Cu-implantation-induced damage is one kind of gettering sites for Cu impurities. Fur-thermore, Cu will react with silicon and form silicide during annealing. All these factors prevent the releasing of Cu. By increasing annealing temperature, Cu can be released out gradually from the damages and silicide. As a result, the gettering efficiency increases with the increasing an-nealing temperature. The BOX layer of SOI does not prevent the diffusion of Cu, but it brings about some obstacles to the nanovoids gettering. The intrinsic defects at the BOX interfaces act as gettering sites for the Cu impurities in the top silicon but they will release the impurities out at high temperatures and act as an impurity source for the nanovoids.The dangling bonds on the void walls are highly reactive and the Cu diffusing into the void band will be strongly captured. Thus the Cu concentration in solution near the voids is decreased and a negative Cu concentration gradient in the silicon near the voids is formed. This negative gradient is the force driving Cu to diffuse to the voids. If the temperature is high enough, Cu will be released out from the impurity sources, namely the Cu-implantation induced damage, the Cu silicide or the defects at the BOX interfaces. This process will not stop until the voids are satu-rated or no more Cu impurities are released from the Cu impurity source. The impurity level in theNo. 1 CU GETTERING TO NANOVOIDS IN SOI MA TERIALS 69top silicon is greatly decreased in this way.It should be pointed out that although the nanovoids are produced by ion implantation, its gettering mechanism is quite different from the gettering of traditional ion implantation induced damage. This can be demonstrated by the following two facts. Firstly, the nanovoids induced by H+ or He+ implantation can getter Cu impurity from the Cu-implantation induced heavy damages, suggesting that the nanovoid gettering is more effective. Secondly, the gettering effect of the nanovoids is still very high after the crystal defects induced by H+ or He+ implantation in the nanovoids band are recovered after annealing at 1000℃.3 ConclusionIn this study, nanovoids are introduced in the silicon substrate of SIMOX and Smart-Cut SOI by H+ and He+ implantation and annealing. The gettering of the nanovoids is studied and the fol-lowing results are obtained:1) The intrinsic defects in SIMOX can trap some of Cu impurity, but the efficiency is low and the gettering is not stable above 850℃. The gettering efficiency of the intrinsic BOX defects changes with the sample structure and quality. The BOX interfaces of Smart-Cut are very sharp and cannot getter Cu.2) The nanovoids introduced in SIMOX by H+ and He+ implantation have strong gettering effect on the high dose of Cu implanted in the top silicon layer. As high as 3.6×1015 /cm2 can be trapped by the voids. Reducing the Cu implantation doses to lower than 1×1014/cm2, the gettering efficiency can be higher than 90%. The gettering efficiency increases with the decreasing Cu im-plantation dose.3) Cu can diffuse through dense thermal oxide BOX of Smart-Cut SOI. For a Cu implanta-tion dose of 1×1014/cm2, the gettering efficiency of nanovoids can be higher than 96%.Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 69906005) and Shanghai Youth Foundation under grant No. 01QMH1403. The authors would like to thank Prof. P. F. P. Fichtner at Universidade do Rio Grande do Sul, Brazil for the TEM analysis of Smart-Cut SOI.References1. Collinge, J. P., Silicon-on-Insulator Technology: Materials to VLSI, Boston: Kluwer Academic Pulishers, 1991.2. Honda, K., Ohsawa, A., Tokokura, N., Breakdown in silicon oxides-correlation with Cu precipitates, Appl. Phys. Lett.,1984，45(3): 270—271.3. Honda, K., Ohsawa, A., Tokokura, N., Breakdown in silicon oxides(II)-correlation with Fe precipitates, Appl. Phys. Lett.,1985,46(6):582—584.4. Weber, E., Transition metals in silicon, Appl. Phys. A,1983, 30(1): 1—22.5. 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