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Dissolution of oxygen precipitates in germanium-doped Czochralski silicon during rapid thermal annealing
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Journal of Crystal Growth 308 (2007) 247–251 www.elsevier.com/locate/jcrysgro
Dissolution of oxygen precipitates in germanium-doped Czochralski silicon during rapid thermal annealing Jiahe Chen, Deren Yang, Xiangyang Ma, Hong Li, Liming Fu, Ming Li, Duanlin Que State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China Received 16 March 2007; received in revised form 5 June 2007; accepted 5 June 2007 Communicated by K.W. Benz Available online 11 August 2007
Abstract The thermal stability of the oxygen precipitates in Czochralski silicon (Cz-Si) crystal with germanium doping has been investigated with rapid thermal annealing. It was found that the grown-in oxygen precipitates could be dissolved easily in germanium-doped Cz-Si (GCz-Si) than in conventional Cz-Si. After prolonged high-temperature thermal cycle, it was found that the germanium doping inclined to dramatically reduce the thermal stability of oxygen precipitates in Cz-Si crystal, either generated at low temperature (800 1C) or formed at high temperature (1000 1C). It is proposed that the germanium doping in Cz-Si could result in the oxygen precipitates with small size and plate shape, which reduce their thermal stability. r 2007 Elsevier B.V. All rights reserved. PACS: 61.72.Tt; 61.72.Cc; 71.55.Cn Keywords: A1. Doping; A1. Germanium; A1. Point defects; A2. Czochralski method; A2. Single crystal growth; B2. Semiconductor silicon
1. Introduction The continuous scaling of device feature size has made clear that microelectronics may be running out of steam in a couple of years, since certain fundamental limits of silicon cannot be overcome. Doping silicon with group IV impurities like germanium could modify the silicon properties such that performance is pushed to higher limits. This explains the present strong interest in SiGe based technologies. Besides the enhanced mobility and the possibility of band gap engineering, group IV impurities offer some additional features, which may be beneficial for certain applications. It has, for instance, been noted that doping with germanium improves the radiation hardness of electron-irradiated silicon [1]. Recently, properties of Czochralski silicon (Cz-Si) with the germanium doping have been investigated intensively. It has been proposed Corresponding author. Tel.: +86 571 87951667; fax: +86 571 87952322. E-mail address: [email protected] (D. Yang).
0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.06.037
that the germanium doping could suppress thermal donors [2,3] thus could stabilize the electrical properties, could retard movements of dislocation [4,5] so as to improve the mechanical strength, and could suppress the formation of voids [6,7] to offer high-quality polished surface for the growth of epi-films. Meanwhile, the enhanced oxygen precipitation in germanium-doped Cz-Si (GCz-Si) has been reported, while the intrinsic gettering capability for metallic contamination has also been believed to be promoted [8,9]. It is therefore believed that GCz-Si is of technological importance in microelectronics industry [10,11]. However, the behavior of oxygen precipitates in GCz-Si has not been substantially clear, especially under rapid thermal annealing (RTA), which has been applied widely in modern device fabrication. In this paper, it is demonstrated that the thermal stability of oxygen precipitates in GCz-Si may be poorer than that of Cz-Si. Generation of small-size oxygen precipitate with platelet shape in GCz-Si is also suggested here. Based on the experimental facts, the mechanism of germanium doping on oxygen precipitates is discussed.
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2. Experiment One p-type boron-doped /1 0 0S GCz-Si crystal with germanium concentration of 1018 cm 3 at seed-end and a conventional Cz-Si crystal without the germanium doping were pulled under almost the same growth conditions. Sample wafers were sliced from the homological portion of the tail segments of the main body from these crystal ingots and were polished to a thickness of 800 mm. Following the norm of ASTM F 121-89 standard and after wafer thickness calibration, the initial interstitial oxygen concentrations ([Oi]0s) for the selected sample wafers were in the range (1.0–1.1) 1018 cm 3 on the basis of Fourier transform infrared spectroscopy (FTIR) using a calibration factor of 3.14 1017 cm 2. Their initial carbon concentrations were below the FTIR detection limit of 1016 cm 3 and their resistivities were in the range 10–20 O cm. Sample wafers were divided into two groups and subjected to different thermal cycles. Firstly, both the CzSi and GCz-Si wafers in group I were annealed in an argon ambient in two different conditions to investigate the dissolution behavior of their grown-in oxygen precipitates: a conventional furnace annealing (CFA) at 1270 1C for 1 h and an RTA at 1280 1C for 60 s. For clarifying the influence of the absolute value of oxygen concentration on the dissolution behavior, an additional lower-[Oi]0 (in the range (6–7) 1017 cm 3) group of Cz-Si and GCz-Si samples with the similar thermal history and comparable resistivity to group I samples were taken into account. Secondly, both the Cz-Si and GCz-Si wafers in group II were pre-annealed at 800 1C and/or 1000 1C for 225 h and then subjected to a prolonged RTA treatment performed at 1260 1C for 300 s in argon atmosphere to investigate the thermal stability of oxygen precipitates by the germanium doping. After the whole heat treatments, [Oi]s in all of the samples were checked by the FTIR method mentioned above and the oxygen recoveries (d[Oi]s) in the annealed wafers were calculated following the norm of ASTM F 1239-89 standard. All the samples in group II were cleaved in cross-section and etched in Sirtl etchant preferentially for 3 min to observe the representative bulk micro-defects with an Olympus MX50 optical microscope, characterizing the oxygen precipitates in both the Cz-Si and GCz-Si wafers. 3. Results and discussion Fig. 1 indicates the oxygen recovery percentages in both the Cz-Si and GCz-Si wafers with a similar thermal history but varied [Oi]0s after RTA treatment at 1280 1C for 60 s. It could be noted that the [Oi]s in both the Cz-Si and GCz-Si increased after such treatment while the heightening of [Oi]0 benefited the [Oi] increasing capacity slightly. Generally, the thermal history of wafer is understood as an in situ annealing of Cz-Si crystals during the solidification in chamber, in which the oxygen interstitials incline to form small grown-in oxygen precipitates [12]. Majority of these
Fig. 1. Oxygen recovery percentage (d[Oi]/[Oi]0) in both Cz-Si and GCz-Si wafers with the similar thermal history but different initial interstitial oxygen concentrations ([Oi]0s) after RTA 1280 1C for 60 s.
precipitates can be dissolved and revert to oxygen interstitials by subjecting to high temperature CFA [13] or by performing at high-temperature RTA [14]. It has also been established that more oxygen precipitation could be generated when the absolute [Oi], actually the oxygen super-saturation, was higher in Cz-Si crystal [15]. Here, it could be found in Fig. 1 that the precipitation and dissolution behaviors of oxygen seemed to be quite similar when the wafers possessed the similar thermal history even if their [Oi]0s were varied. However, the germanium doping in Cz-Si could change the dissolution behavior of oxygen precipitate obviously both in the higher and lower [Oi]0 cases, and reasons for this are discussed below. Fig. 2 shows the [Oi]s for both the Cz-Si and GCz-Si wafers in the statuses of as-grown, after the 1270 1C/1 h-CFA treatment and after the 1280 1C/60 s-RTA treatment. As can be seen, the [Oi]s after CFA or RTA treatment for both the Cz-Si and GCz-Si were higher than that in the as-grown ones, and this is ascribed to the dissolution of the grown-in oxygen precipitation in crystals. It has also been suggested that the [Oi]s would be slightly higher in GCz-Si than in Cz-Si, indicating the easier dissolution of oxygen precipitates by the germanium doping. It has been proposed that the large-sized high-density oxygen precipitates could be generated in grown-in GCz-Si crystals at elevated temperatures. Meanwhile, such oxygen precipitates were believed to be of the mixed morphology consisting of platelets and polyhedrons shapes [10]. It is considered that the germanium doping in Cz-Si would probably decline the thermal stability of grown-in oxygen precipitates by generating the platelet shape precipitates. For understanding completely the dissolution behavior of oxygen precipitates in GCz-Si, the annealed for long time wafers with large amount of oxygen precipitates were
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Fig. 2. Interstitial oxygen concentrations ([Oi]s) in the Cz-Si and GCz-Si wafers with as-grown status, CFA treatment at 1270 1C/h and RTA treatment at 1280 1C/60 s.
Fig. 3. Variation of oxygen recoveries (d[Oi]s) in both Cz-Si (square points) and GCz-Si (circle points) wafers as a function of duration at RTA preformed at 1260 1C with pre-annealing at 800 1C (full points) or 1000 1C (open points) for 225 h.
subjected to RTA treatment at 1260 1C. Fig. 3 shows the variations of d[Oi]s in both the Cz-Si and GCz-Si with the pre-annealing, respectively, at 800 1C/225 h and 1000 1C/225 h as a function of duration in RTA at 1260 1C. The [Oi]s of both types of samples before or after the 1260 1C/300 s RTA treatment are given in Fig. 4. It has been demonstrated that, for both of the Cz-Si and GCz-Si wafers, the oxygen precipitates tended to revert to the oxygen interstitials with increase in the duration at high temperature RTA. It should be ascribed to the increase of the cracked bonding amount for Si–O and Si–Si bonding in oxygen precipitates during RTA treatments. Besides, it could be seen that the d[Oi]s in GCz-Si pre-annealed either at 800 1C or at 1000 1C were much greater than that in the Cz-Si in the same duration of RTA, i.e., the germanium doping could reduce the thermal stability of
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Fig. 4. [Oi]s in both Cz-Si and GCz-Si wafers subjected to 1260 1C/300 s-RTA treatment with 225 h-CFA pre-treatments at 800 or 1000 1C.
oxygen precipitates dramatically. However, it seemed that the dissolution capabilities of oxygen precipitates generated by low-temperature annealing and by high-temperature annealing were different. The cross-sectional optical microphotographs of the 800 1C/225 h CFA pre-treated Cz-Si and GCz-Si wafers with or without 1260 1C/300 s RTA treatment are given in Fig. 5. It could be seen that, in comparison with Cz-Si, the oxygen precipitates with higher density and smaller size were generated in the GCz-Si after the 800 1C/225 h preannealing, corresponding with the enhancement effect of germanium on oxygen precipitation shown in Fig. 4. After the 1260 1C/300 s RTA treatment, the majority of oxygen precipitates could be dissolved in both of the Cz-Si and GCz-Si wafers and the dissolved percentage of oxygen precipitates in GCz-Si seemed to be greater than that in CzSi: oxygen precipitates almost dissolved fully in the GCz-Si wafer whereas the oxygen precipitates with lower density also presented in the Cz-Si wafer. It has been indicated by the TEM detection in our previous publication [8] that the oxygen precipitates in 800 1C/225 h pre-annealed Cz-Si were mainly in platelet shapes. It has been well known that the platelet shape oxygen precipitates could be easily dissolved by the high-temperature annealing due to their high surface energy to silicon matrix [16]. Therefore, possessing the distribution of smaller size and platelet shape, the oxygen precipitates in GCz-Si could be dissolved much greater than those in Cz-Si after high-temperature RTA. Concerning the 1000 1C/225 h case, the cross-sectional optical microphotographs of the CFA pre-treated Cz-Si and GCz-Si wafers before or after the 1260 1C/300 s RTA treatment are illustrated in Fig. 6. As can be seen, the oxygen precipitates with higher density and smaller sizes could also be formed in the GCz-Si wafers after such CFA treatment, which was believed to be based on the
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Fig. 5. Optical images of the cleaved cross-section in the 800 1C/225 h-CFA pre-treated Cz-Si and GCz-Si wafers with or without the 1260 1C/300 s-RTA treatment: (a) Cz-Si without RTA; (b) GCz-Si without RTA; (c) Cz-Si with RTA; and (d) GCz-Si with RTA.
Fig. 6. Optical images of the cleaved cross-section in the 1000 1C/225 h-CFA-pre-treated Cz-Si and GCz-Si wafers with or without the 1260 1C/300 s-RTA treatment: (a) Cz-Si without RTA; (b) GCz-Si without RTA; (c) Cz-Si with RTA; and (d) GCz-Si with RTA.
assumption of germanium-related complexes [10]. For CzSi case, after such prolonged RTA cycle, the size of oxygen precipitate could be dissolved partly while the density of oxygen precipitate remained. For GCz-Si case, the smallsized oxygen precipitate can be dissolved fully and the large-sized ones shrank in size. It could be furthermore found that much more oxygen precipitates were dissolved
by the germanium doping due to the larger oxygen recovery in GCz-Si than in Cz-Si shown in Figs. 3 and 4. It has been established that the thermal stability of platelet shape precipitate was poorer than the polyhedral oxygen precipitate in Cz-Si wafer [17]. Therefore, the easier dissolution of oxygen precipitates should be probably ascribed to the following two reasons: (1) formation of
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small-sized oxygen precipitate, and (2) generation of platelet-shaped oxygen precipitate. And the oxygen precipitates with polyhedral shape could not be dissolved easily, oxygen recovery in Cz-Si was slightly more than that in GCz-Si. Indeed, the oxygen precipitate with mixed morphology consisted of platelets and polyhedral shapes were considered to be present in GCz-Si wafer after 1000 1C/225 h pre-annealing by the previous TEM detection [8]. Actually, germanium atom located at the substitutional site in Cz-Si crystal and it could induce the distortion and local stresses in silicon lattice due to the larger atom radius. So, the lattice sites where germanium atom located are provided with potential activities and inclined to interact with other structural defect and/or impurity. Ge–Vm and Ge–Vm–On (m, nX1) complexes, in the large amounts, are supposed to relieve the lattice stresses and they could further act as the heterogeneous precipitation nuclei to accumulate the oxygen interstitials in GCz-Si. Due to the limit of oxygen content, the oxygen precipitates in GCz-Si inclines to present in much smaller size than Cz-Si. Actually, it is reasonable that such phenomenon depends on the fact, but not the extent, of oxygen super-saturation, therefore, the oxygen precipitation behavior seem to be similar even through [Oi]0 varied. When dissolved by RTA treatment, the Si–O and Si–Si bonding could be easily cracked and then the oxygen atoms situated in the precipitate originally could revert to oxygen interstitials and finally diffuse out of the precipitate. Due to the distribution of smaller size and higher density, the total surface area of oxygen precipitates in GCz-Si could be dramatically heightened. The net oxygen flux out of precipitates could be enhanced and the precipitates could, therefore, be dissolved more easily by germanium doping. Meanwhile, surface energy of oxygen precipitate in GCz-Si seems to be higher due to the accumulation of other structural defect and/or impurity. Hence, oxygen precipitates with platelet shape can be present in the GCz-Si wafer. The oxygen atoms incline to out-diffuse from the precipitates so as to lower total system energy. Therefore, oxygen precipitates with poor thermal stability could result in for GCz-Si crystal. 4. Conclusion We have investigated the thermal stability of oxygen precipitates in GCz-Si crystal. It was suggested that the
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germanium doping could enhance the dissolution of grown-in oxygen precipitates. Compared with the oxygen precipitates in the conventional Cz-Si, the oxygen precipitates whether formed by low-temperature annealing or generated by high-temperature treatment were less stable thermally in GCz-Si. The smaller-sized and platelet-shaped precipitates in Cz-Si induced by the germanium doping are considered to charge for the easy dissolution of the oxygen precipitates.
Acknowledgments The authors would like to thank the Natural Science Foundation of China (no. 50572094), PCSIRT project and 973 project (no. 2007CB6130403) for financial support. Mr. Bin Huang of QL semiconductor Co. Ltd. in Ningbo, China, is thanked for his kind assistance in sample preparation.
References [1] A. Brelot, J. Charlemagne, Radiat. Eff. 9 (1971) 65. [2] M.Ya. Dashevskii, A.A. Dokuchaeva, S.I. Sadilov, Inorg. Mater. 25 (1989) 580. [3] H. Li, D. Yang, X. Yu, X. Ma, D. Tian, L. Li, D. Que, J. Phys.: Condens. Matter 16 (2004) 5745. [4] T. Taishi, X. Huang, I. Yonenaga, K. Hoshikawa, Mater. Sci. Semicond. Process 5 (2003) 409. [5] D. Li, Y. Zhao, D. Yang, J. Rare Earths 24 (2006) 83. [6] C.M. Carbonaro, V. Fiorentini, F. Bernardini, Phys. Rev. B 66 (2002) 233201. [7] D. Yang, J. Chen, H. Li, X. Ma, D. Tian, L. Li, D. Que, J. Crystal Growth 292 (2006) 266. [8] H. Li, D. Yang, X. Ma, X. Yu, D. Que, J. Appl. Phys. 96 (2004) 4161. [9] J. Chen, D. Yang, H. Li, X. Ma, D. Que, J. Appl. Phys. 99 (2006) 073509. [10] J. Chen, D. Yang, H. Li, X. Ma, D. Que, J. Crystal Growth 291 (2006) 66. [11] J. Chen, D. Yang, X. Ma, H. Li, D. Que, J. Appl. Phys. 101 (2007) 033526. [12] S. Kishino, Y. Matsushita, M. Kanamori, T. Iizuka, Jpn. J. Appl. Phys. 21 (1982) 1. [13] G. Fraundorf, P. Fraundorf, R.A. Craven, R.A. Frederick, J. Moody, R.W. Shaw, J. Electrochem. Soc. 132 (1985) 1701. [14] H.D. Chiou, J. Electrochem. Soc. 139 (1992) 1680. [15] R.C. Newman, J. Phys.: Condens. Matter 12 (2000) R335. [16] A. Borghesi, B. Pivac, A. Sassella, A. Stella, J. Appl. Phys. 77 (1995) 4169. [17] F. Shimura, Semicond. Semimetals 42 (1994) 602.
Journal of Crystal Growth 308 (2007) 247–251 www.elsevier.com/locate/jcrysgro
Dissolution of oxygen precipitates in germanium-doped Czochralski silicon during rapid thermal annealing Jiahe Chen, Deren Yang, Xiangyang Ma, Hong Li, Liming Fu, Ming Li, Duanlin Que State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China Received 16 March 2007; received in revised form 5 June 2007; accepted 5 June 2007 Communicated by K.W. Benz Available online 11 August 2007
Abstract The thermal stability of the oxygen precipitates in Czochralski silicon (Cz-Si) crystal with germanium doping has been investigated with rapid thermal annealing. It was found that the grown-in oxygen precipitates could be dissolved easily in germanium-doped Cz-Si (GCz-Si) than in conventional Cz-Si. After prolonged high-temperature thermal cycle, it was found that the germanium doping inclined to dramatically reduce the thermal stability of oxygen precipitates in Cz-Si crystal, either generated at low temperature (800 1C) or formed at high temperature (1000 1C). It is proposed that the germanium doping in Cz-Si could result in the oxygen precipitates with small size and plate shape, which reduce their thermal stability. r 2007 Elsevier B.V. All rights reserved. PACS: 61.72.Tt; 61.72.Cc; 71.55.Cn Keywords: A1. Doping; A1. Germanium; A1. Point defects; A2. Czochralski method; A2. Single crystal growth; B2. Semiconductor silicon
1. Introduction The continuous scaling of device feature size has made clear that microelectronics may be running out of steam in a couple of years, since certain fundamental limits of silicon cannot be overcome. Doping silicon with group IV impurities like germanium could modify the silicon properties such that performance is pushed to higher limits. This explains the present strong interest in SiGe based technologies. Besides the enhanced mobility and the possibility of band gap engineering, group IV impurities offer some additional features, which may be beneficial for certain applications. It has, for instance, been noted that doping with germanium improves the radiation hardness of electron-irradiated silicon [1]. Recently, properties of Czochralski silicon (Cz-Si) with the germanium doping have been investigated intensively. It has been proposed Corresponding author. Tel.: +86 571 87951667; fax: +86 571 87952322. E-mail address: [email protected] (D. Yang).
0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.06.037
that the germanium doping could suppress thermal donors [2,3] thus could stabilize the electrical properties, could retard movements of dislocation [4,5] so as to improve the mechanical strength, and could suppress the formation of voids [6,7] to offer high-quality polished surface for the growth of epi-films. Meanwhile, the enhanced oxygen precipitation in germanium-doped Cz-Si (GCz-Si) has been reported, while the intrinsic gettering capability for metallic contamination has also been believed to be promoted [8,9]. It is therefore believed that GCz-Si is of technological importance in microelectronics industry [10,11]. However, the behavior of oxygen precipitates in GCz-Si has not been substantially clear, especially under rapid thermal annealing (RTA), which has been applied widely in modern device fabrication. In this paper, it is demonstrated that the thermal stability of oxygen precipitates in GCz-Si may be poorer than that of Cz-Si. Generation of small-size oxygen precipitate with platelet shape in GCz-Si is also suggested here. Based on the experimental facts, the mechanism of germanium doping on oxygen precipitates is discussed.
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2. Experiment One p-type boron-doped /1 0 0S GCz-Si crystal with germanium concentration of 1018 cm 3 at seed-end and a conventional Cz-Si crystal without the germanium doping were pulled under almost the same growth conditions. Sample wafers were sliced from the homological portion of the tail segments of the main body from these crystal ingots and were polished to a thickness of 800 mm. Following the norm of ASTM F 121-89 standard and after wafer thickness calibration, the initial interstitial oxygen concentrations ([Oi]0s) for the selected sample wafers were in the range (1.0–1.1) 1018 cm 3 on the basis of Fourier transform infrared spectroscopy (FTIR) using a calibration factor of 3.14 1017 cm 2. Their initial carbon concentrations were below the FTIR detection limit of 1016 cm 3 and their resistivities were in the range 10–20 O cm. Sample wafers were divided into two groups and subjected to different thermal cycles. Firstly, both the CzSi and GCz-Si wafers in group I were annealed in an argon ambient in two different conditions to investigate the dissolution behavior of their grown-in oxygen precipitates: a conventional furnace annealing (CFA) at 1270 1C for 1 h and an RTA at 1280 1C for 60 s. For clarifying the influence of the absolute value of oxygen concentration on the dissolution behavior, an additional lower-[Oi]0 (in the range (6–7) 1017 cm 3) group of Cz-Si and GCz-Si samples with the similar thermal history and comparable resistivity to group I samples were taken into account. Secondly, both the Cz-Si and GCz-Si wafers in group II were pre-annealed at 800 1C and/or 1000 1C for 225 h and then subjected to a prolonged RTA treatment performed at 1260 1C for 300 s in argon atmosphere to investigate the thermal stability of oxygen precipitates by the germanium doping. After the whole heat treatments, [Oi]s in all of the samples were checked by the FTIR method mentioned above and the oxygen recoveries (d[Oi]s) in the annealed wafers were calculated following the norm of ASTM F 1239-89 standard. All the samples in group II were cleaved in cross-section and etched in Sirtl etchant preferentially for 3 min to observe the representative bulk micro-defects with an Olympus MX50 optical microscope, characterizing the oxygen precipitates in both the Cz-Si and GCz-Si wafers. 3. Results and discussion Fig. 1 indicates the oxygen recovery percentages in both the Cz-Si and GCz-Si wafers with a similar thermal history but varied [Oi]0s after RTA treatment at 1280 1C for 60 s. It could be noted that the [Oi]s in both the Cz-Si and GCz-Si increased after such treatment while the heightening of [Oi]0 benefited the [Oi] increasing capacity slightly. Generally, the thermal history of wafer is understood as an in situ annealing of Cz-Si crystals during the solidification in chamber, in which the oxygen interstitials incline to form small grown-in oxygen precipitates [12]. Majority of these
Fig. 1. Oxygen recovery percentage (d[Oi]/[Oi]0) in both Cz-Si and GCz-Si wafers with the similar thermal history but different initial interstitial oxygen concentrations ([Oi]0s) after RTA 1280 1C for 60 s.
precipitates can be dissolved and revert to oxygen interstitials by subjecting to high temperature CFA [13] or by performing at high-temperature RTA [14]. It has also been established that more oxygen precipitation could be generated when the absolute [Oi], actually the oxygen super-saturation, was higher in Cz-Si crystal [15]. Here, it could be found in Fig. 1 that the precipitation and dissolution behaviors of oxygen seemed to be quite similar when the wafers possessed the similar thermal history even if their [Oi]0s were varied. However, the germanium doping in Cz-Si could change the dissolution behavior of oxygen precipitate obviously both in the higher and lower [Oi]0 cases, and reasons for this are discussed below. Fig. 2 shows the [Oi]s for both the Cz-Si and GCz-Si wafers in the statuses of as-grown, after the 1270 1C/1 h-CFA treatment and after the 1280 1C/60 s-RTA treatment. As can be seen, the [Oi]s after CFA or RTA treatment for both the Cz-Si and GCz-Si were higher than that in the as-grown ones, and this is ascribed to the dissolution of the grown-in oxygen precipitation in crystals. It has also been suggested that the [Oi]s would be slightly higher in GCz-Si than in Cz-Si, indicating the easier dissolution of oxygen precipitates by the germanium doping. It has been proposed that the large-sized high-density oxygen precipitates could be generated in grown-in GCz-Si crystals at elevated temperatures. Meanwhile, such oxygen precipitates were believed to be of the mixed morphology consisting of platelets and polyhedrons shapes [10]. It is considered that the germanium doping in Cz-Si would probably decline the thermal stability of grown-in oxygen precipitates by generating the platelet shape precipitates. For understanding completely the dissolution behavior of oxygen precipitates in GCz-Si, the annealed for long time wafers with large amount of oxygen precipitates were
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Fig. 2. Interstitial oxygen concentrations ([Oi]s) in the Cz-Si and GCz-Si wafers with as-grown status, CFA treatment at 1270 1C/h and RTA treatment at 1280 1C/60 s.
Fig. 3. Variation of oxygen recoveries (d[Oi]s) in both Cz-Si (square points) and GCz-Si (circle points) wafers as a function of duration at RTA preformed at 1260 1C with pre-annealing at 800 1C (full points) or 1000 1C (open points) for 225 h.
subjected to RTA treatment at 1260 1C. Fig. 3 shows the variations of d[Oi]s in both the Cz-Si and GCz-Si with the pre-annealing, respectively, at 800 1C/225 h and 1000 1C/225 h as a function of duration in RTA at 1260 1C. The [Oi]s of both types of samples before or after the 1260 1C/300 s RTA treatment are given in Fig. 4. It has been demonstrated that, for both of the Cz-Si and GCz-Si wafers, the oxygen precipitates tended to revert to the oxygen interstitials with increase in the duration at high temperature RTA. It should be ascribed to the increase of the cracked bonding amount for Si–O and Si–Si bonding in oxygen precipitates during RTA treatments. Besides, it could be seen that the d[Oi]s in GCz-Si pre-annealed either at 800 1C or at 1000 1C were much greater than that in the Cz-Si in the same duration of RTA, i.e., the germanium doping could reduce the thermal stability of
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Fig. 4. [Oi]s in both Cz-Si and GCz-Si wafers subjected to 1260 1C/300 s-RTA treatment with 225 h-CFA pre-treatments at 800 or 1000 1C.
oxygen precipitates dramatically. However, it seemed that the dissolution capabilities of oxygen precipitates generated by low-temperature annealing and by high-temperature annealing were different. The cross-sectional optical microphotographs of the 800 1C/225 h CFA pre-treated Cz-Si and GCz-Si wafers with or without 1260 1C/300 s RTA treatment are given in Fig. 5. It could be seen that, in comparison with Cz-Si, the oxygen precipitates with higher density and smaller size were generated in the GCz-Si after the 800 1C/225 h preannealing, corresponding with the enhancement effect of germanium on oxygen precipitation shown in Fig. 4. After the 1260 1C/300 s RTA treatment, the majority of oxygen precipitates could be dissolved in both of the Cz-Si and GCz-Si wafers and the dissolved percentage of oxygen precipitates in GCz-Si seemed to be greater than that in CzSi: oxygen precipitates almost dissolved fully in the GCz-Si wafer whereas the oxygen precipitates with lower density also presented in the Cz-Si wafer. It has been indicated by the TEM detection in our previous publication [8] that the oxygen precipitates in 800 1C/225 h pre-annealed Cz-Si were mainly in platelet shapes. It has been well known that the platelet shape oxygen precipitates could be easily dissolved by the high-temperature annealing due to their high surface energy to silicon matrix [16]. Therefore, possessing the distribution of smaller size and platelet shape, the oxygen precipitates in GCz-Si could be dissolved much greater than those in Cz-Si after high-temperature RTA. Concerning the 1000 1C/225 h case, the cross-sectional optical microphotographs of the CFA pre-treated Cz-Si and GCz-Si wafers before or after the 1260 1C/300 s RTA treatment are illustrated in Fig. 6. As can be seen, the oxygen precipitates with higher density and smaller sizes could also be formed in the GCz-Si wafers after such CFA treatment, which was believed to be based on the
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Fig. 5. Optical images of the cleaved cross-section in the 800 1C/225 h-CFA pre-treated Cz-Si and GCz-Si wafers with or without the 1260 1C/300 s-RTA treatment: (a) Cz-Si without RTA; (b) GCz-Si without RTA; (c) Cz-Si with RTA; and (d) GCz-Si with RTA.
Fig. 6. Optical images of the cleaved cross-section in the 1000 1C/225 h-CFA-pre-treated Cz-Si and GCz-Si wafers with or without the 1260 1C/300 s-RTA treatment: (a) Cz-Si without RTA; (b) GCz-Si without RTA; (c) Cz-Si with RTA; and (d) GCz-Si with RTA.
assumption of germanium-related complexes [10]. For CzSi case, after such prolonged RTA cycle, the size of oxygen precipitate could be dissolved partly while the density of oxygen precipitate remained. For GCz-Si case, the smallsized oxygen precipitate can be dissolved fully and the large-sized ones shrank in size. It could be furthermore found that much more oxygen precipitates were dissolved
by the germanium doping due to the larger oxygen recovery in GCz-Si than in Cz-Si shown in Figs. 3 and 4. It has been established that the thermal stability of platelet shape precipitate was poorer than the polyhedral oxygen precipitate in Cz-Si wafer [17]. Therefore, the easier dissolution of oxygen precipitates should be probably ascribed to the following two reasons: (1) formation of
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small-sized oxygen precipitate, and (2) generation of platelet-shaped oxygen precipitate. And the oxygen precipitates with polyhedral shape could not be dissolved easily, oxygen recovery in Cz-Si was slightly more than that in GCz-Si. Indeed, the oxygen precipitate with mixed morphology consisted of platelets and polyhedral shapes were considered to be present in GCz-Si wafer after 1000 1C/225 h pre-annealing by the previous TEM detection [8]. Actually, germanium atom located at the substitutional site in Cz-Si crystal and it could induce the distortion and local stresses in silicon lattice due to the larger atom radius. So, the lattice sites where germanium atom located are provided with potential activities and inclined to interact with other structural defect and/or impurity. Ge–Vm and Ge–Vm–On (m, nX1) complexes, in the large amounts, are supposed to relieve the lattice stresses and they could further act as the heterogeneous precipitation nuclei to accumulate the oxygen interstitials in GCz-Si. Due to the limit of oxygen content, the oxygen precipitates in GCz-Si inclines to present in much smaller size than Cz-Si. Actually, it is reasonable that such phenomenon depends on the fact, but not the extent, of oxygen super-saturation, therefore, the oxygen precipitation behavior seem to be similar even through [Oi]0 varied. When dissolved by RTA treatment, the Si–O and Si–Si bonding could be easily cracked and then the oxygen atoms situated in the precipitate originally could revert to oxygen interstitials and finally diffuse out of the precipitate. Due to the distribution of smaller size and higher density, the total surface area of oxygen precipitates in GCz-Si could be dramatically heightened. The net oxygen flux out of precipitates could be enhanced and the precipitates could, therefore, be dissolved more easily by germanium doping. Meanwhile, surface energy of oxygen precipitate in GCz-Si seems to be higher due to the accumulation of other structural defect and/or impurity. Hence, oxygen precipitates with platelet shape can be present in the GCz-Si wafer. The oxygen atoms incline to out-diffuse from the precipitates so as to lower total system energy. Therefore, oxygen precipitates with poor thermal stability could result in for GCz-Si crystal. 4. Conclusion We have investigated the thermal stability of oxygen precipitates in GCz-Si crystal. It was suggested that the
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germanium doping could enhance the dissolution of grown-in oxygen precipitates. Compared with the oxygen precipitates in the conventional Cz-Si, the oxygen precipitates whether formed by low-temperature annealing or generated by high-temperature treatment were less stable thermally in GCz-Si. The smaller-sized and platelet-shaped precipitates in Cz-Si induced by the germanium doping are considered to charge for the easy dissolution of the oxygen precipitates.
Acknowledgments The authors would like to thank the Natural Science Foundation of China (no. 50572094), PCSIRT project and 973 project (no. 2007CB6130403) for financial support. Mr. Bin Huang of QL semiconductor Co. Ltd. in Ningbo, China, is thanked for his kind assistance in sample preparation.
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