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Study of nitrogen-related defects by means of gold and hydrogen diffusion investigations
Microelectronic Engineering 66 (2003) 379–384 www.elsevier.com / locate / mee
Study of nitrogen-related defects by means of gold and hydrogen diffusion investigations A.L. Parakhonsky a , *, E.B. Yakimov a , D. Yang b a
b
Institute of Microelectronics Technology RAS, Chernogolovka 142432, Russia State Key Laboratory of Silicon Material, Zhejiang University, Hangzhou 310027, PR China
Abstract Gold and hydrogen diffusion in Czochralski grown Si with and without nitrogen, has been studied. It was found that the Au concentration after diffusion at 700, 750 and 800 8C in nitrogen-doped Si is always less than that in Si without nitrogen. A decrease of Au concentration in nitrogen-doped crystals is explained under the assumption that nitrogen stimulates oxygen precipitation. It is shown that the hydrogen penetration depth under wet chemical etching into nitrogen-doped samples is larger than that into Si without nitrogen that is explained by the nitrogen effect on the formation of oxygen-related traps for hydrogen. 2002 Published by Elsevier Science B.V. Keywords: Silicon; Nitrogen; Gold; Hydrogen
1. Introduction Nitrogen codoping is known to have beneficial effects on the properties of crystalline silicon. Thus it was reported that it can increase the v /G (v is the pull rate and G is the crystal temperature gradient at the solid–liquid interface) tolerance for grown-in defects free region [1], increase the void-free denuded zone depth after annealing in hydrogen atmosphere [2], reduce the crystal originated particle size after annealing [3], increase mechanical strength [4,5], etc. Nitrogen was found to essentially interact with some impurities; thus, it enhances the oxygen precipitation [6–8] and forms the nitrogen–oxygen-related shallow donors [9–11]. However, the knowledge concerning the detail mechanisms of nitrogen effects is rather poor. The main reason for this is associated with the rather low solubility limit of nitrogen in Si (about 10 15 cm 23 ). Most of the nitrogen exists in silicon in an electrically inactive state; therefore, highly sensitive electrical methods cannot be used for its characterization and only a limited number of characterization techniques can be used to reveal the * Corresponding author. E-mail address: [email protected] (A.L. Parakhonsky). 0167-9317 / 02 / $ – see front matter 2002 Published by Elsevier Science B.V. doi:10.1016/S0167-9317(02)00912-7
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nitrogen-related defects and to study their effect on Si properties. One of such methods could be associated with using gold diffusion experiments. Au diffusion could be considered as a rather sensitive method for the characterization of defect structure in Si [12–15]. It is known to diffuse via the kick-out mechanism [16] that is it diffuses very quickly over interstitial sites Au i and then Au i are transformed into electrically active substitution sites Au s via the kick-out reaction. This reaction is very sensitive to a change in self-interstitial generation or annihilation rates. For example, the defects playing a role of effective sinks for self-interstitials could lead to an essential increase in the Au concentration [12,14] while a decrease in Au concentration is usually associated with an enhanced self-interstitial generation [13,15] that could lead to a reversion of kick-out reaction. Thus, the analysis of Au diffusion allows to reveal the sinks for self-interstitials, vacancy-related complexes and to detect the self-interstitial generation processes taking place during Au diffusion. As shown in Refs. [17,18], the investigations of Pt and Au diffusion in nitrogen-doped Si could provide useful information concerning nitrogen behavior in Si. Thus, it was found [17] that Pt concentration after its diffusion in nitrogen-doped floating zone (FZ) Si is essentially higher than that after diffusion in nitrogen-free Si. Such increase was explained by the capture of interstitial Pt on the nitrogen–vacancy complexes that allows the estimation of concentration of these complexes. Contrary, in Czochralski grown (Cz) Si the Au concentration was found to be lower in the crystals with nitrogen than that in nitrogen-free Si [18] this was considered as an indication of oxygen precipitation stimulated by nitrogen. Hydrogen diffusion in Si is mainly used for the study of defect passivation [19] and of formation of new defects containing hydrogen [20–22]. The application of hydrogen profile study for the defect characterization in Si is not so wide. Nevertheless, such experiments also could provide important information concerning the hydrogen interaction with defects existing in the samples under study and the properties of defects playing the role of traps for hydrogen. For example, oxygen-related traps for hydrogen were detected in Ref. [23] by the comparable analysis of hydrogen profiles in Cz and FZ Si. In the present paper some new results concerning the gold and hydrogen diffusion experiments in nitrogen-doped Si are presented. The analysis carried out shows that nitrogen enhances oxygen precipitation even during rather short annealing and suppresses the formation of oxygen-related traps for hydrogen in Cz Si.
2. Experimental Si wafers of n-type conductivity with a phosphorus concentration of about 10 14 cm 23 grown by the Czochralsky method with a thickness of 300 mm were used in our experiments. Nitrogen-doped Si ingots (NCz) were grown in a nitrogen atmosphere at reduced pressure [24], while the Cz ingots without nitrogen were grown in similar conditions but in an argon atmosphere. Pairs of Cz and NCz Si wafers cut from almost the same tail or head parts of ingots, which contain very similar oxygen concentrations of about 1.0310 18 cm 23 and similar phosphorus concentrations, were compared. The carbon concentration in the samples was less than 5310 15 cm 23 . The oxygen and carbon concentrations in the samples were measured by the IR absorption at room temperature using the calibration factors 3.14310 17 cm 22 for oxygen and 1.0310 17 cm 22 for carbon, respectively. Part of samples was annealed at 650 8C for 0.5 h to annihilate oxygen thermal donors. After such annealing nitrogen–oxygen shallow donors were generated in nitrogen-doped NCz crystals [9–11]. To estimate
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the concentration of nitrogen–oxygen donors the samples were annealed at 950 8C to destroy these centers and their concentration was obtained as a difference between the total donor concentration measured in the samples annealed at 650 and 950 8C. Au films with a thickness of about 30 nm were evaporated on one side of the samples. Then Au diffusion annealing was carried out at 700, 750 or 800 8C for 1 h. After diffusion annealing a layer of about 50 mm thickness was removed by mechanical polishing from one side of all gold diffused samples. On a few samples Au depth profile was measured, which was found to be similar to that predicted by the kick-out model. The substitutional gold concentration Au s and its depth profile were measured by deep level transient spectroscopy (DLTS). The total Au s concentration was calculated as a sum of the concentrations of centers with the deep levels Ec 20.54 eV and Ec 20.20 eV, associated with Au s and gold–hydrogen complexes formed under chemical etching [20,21], respectively. Hydrogen in the samples was introduced during wet chemical etching (WCE) in the acid solution HF:HNO 3 (1:7) at ambient temperature, the etching rate was about 2 mm / min. The charge carrier depth distribution in the samples was reconstructed from the C–V measurements. Under a reasonable assumption that before hydrogen passivation the donor distribution was independent of depth, the depth distribution of neutral hydrogen-donor centers was calculated as a difference between the charge carrier concentration at a large enough depth, where it is independent of depth and is equal to the initial value of total donor concentration, and the carrier concentration at any particular depth, obtained from the C–V curves. No new defects were observed by the DLTS in the region saturated with hydrogen during WCE. The Schottky contacts of 1.5-mm diameter for the DLTS and C–V measurements were fabricated by vacuum evaporation of Au on the etched surfaces. Ohmic contacts were produced by scratching the back side of the samples with the eutectic Al–Ga alloy.
3. Results and discussion
3.1. Gold diffusion experiments The results of Au s concentration measurements at a depth of about 50 mm in the Cz Si samples codoped with nitrogen and without nitrogen are presented in Table 1. The Au s concentration extrapolated from the high temperature measurements [12,25,26] are also presented for comparison. It is seen that at all diffusion temperatures used, the Au s concentration in NCz samples is lower than that Table 1 Au s concentration measured by the DLTS in Cz and NCz silicon (head and tail parts of ingots) after diffusion annealing at 700, 750, and 800 8C for 1 h, the Au s concentration extrapolated from the high temperature diffusion experiments was shown in the right column T dif
Au s (NCz) (cm 23 )
Au s (Cz) (cm 23 )
(8C)
Head
Tail
Head
Tail
(cm 23 )
700 750 800
(1.0360.15)310 11 (6.0562.65)310 11 (5.1460.10)310 12
(1.0360.11)310 11 (1.1960.06)310 12 (3.5660.43)310 12
(8.1860.74)310 11 (2.8960.35)310 12 (1.4760.11)310 13
(5.8560.19)310 11 (5.2960.31)310 12 (1.4860.25)310 13
2.48310 11 1.79310 12 9.98310 12
Au s (extrapol.)
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in the corresponding Cz samples and this difference is remarkably larger than the difference in the Au s concentration between the samples cut from the head and tail parts of ingots. The difference in the Au s concentration between NCz and Cz samples was found to increase with decreasing diffusion temperature. It is seen that the Au s concentration in Cz samples after 800 8C diffusion is close to the extrapolated value but after 700 8C diffusion it is about two to three times larger. This could be explained under an assumption that some grown-in sinks for self-interstitials are present in the Cz samples without nitrogen. An increase in diffusion temperature leads to a corresponding increase of self-interstitial generation rate that can lead to the disappearance or reconstruction of these grown-in defects. The Au s concentration in NCz samples at all temperatures used is lower than that predicted for defect-free Si. This decrease can be explained as the result of an enhanced oxygen precipitation in such crystals [6–8]. Self-interstitials generated under oxygen precipitation can reverse the kick-out reaction and decrease the Au s concentration. No effect of grown-in defects on the Au s concentration was observed in NCz Si at all diffusion temperatures used. This result contradicts those obtained on FZ Si [17] where the pronounced increase in Pt concentration was observed after diffusion at similar temperatures. The increased in Pt concentration was explained [17] under the assumption that the simple nitrogen–vacancy complexes were formed during the ingot cooling after growth that decreased vacancy concentration and suppressed the formation of large vacancy-type clusters. In NCz Si such defects could present in a configuration which is not such an efficient sink for the self-interstitials. Besides, self-interstitials created during oxygen precipitation could annihilate or reconstruct such vacancy-type defects.
3.2. Hydrogen diffusion experiments The distribution of hydrogen-donor complexes in Si after WCE can be reconstructed from the measurements of charge carrier distribution. As shown in Refs. [27,28], the decay of depth distribution of these complexes can be described by the exponential law with the characteristic depth L 5 (4p rNd )21 / 2 , where Nd is the dopant concentration and r is the hydrogen capture radius. In most Si crystals hydrogen capture is mainly determined by the dopant atoms but if some additional traps are present in the sample under study this should lead to a decrease of L. The hydrogen penetration in Si during WCE is characterized also by the second independent parameter z 0 , which was measured as the depth, at which 10% of shallow donors were passivated with hydrogen, and is approximately proportional to (Nd )21 / 2 . The examples of charge carrier profiles measured in Cz and NCz Si with very close dopant concentration are presented in Fig. 1. It is seen that near the surface the charge carrier concentration decreases due to the hydrogen passivation of donors. Analysis of such profiles allows to obtain z 0 and L values for the NCz and Cz samples studied. The L values measured in this study were found to correlate rather well with the L(Nd ) dependence obtained in [29] and is practically the same for the both types of crystals. That means that no effective traps for hydrogen are associated with nitrogen. As shown in Ref. [29], nitrogen–oxygen donors are passivated by hydrogen and their capture radius is close to that of phosphorus. However, the concentration of such donors in our samples did not exceed 20% of phosphorus concentration therefore their effect on L is rather small. On the contrary, the z 0 value, was found to be larger in NCz crystals than in Cz ones (Fig. 2). Just this difference determines the difference in charge carrier profiles shown in Fig. 1. It should be noted
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Fig. 1. Charge carrier depth profiles in Cz and NCz silicon after WCE.
that the difference in the hydrogen penetration in Cz and FZ samples was observed in Refs. [23,28], where it was explained under the assumption that some oxygen-related traps for hydrogen are formed in Cz Si. The capture radius and / or concentration of these traps are too low to effect hydrogen trapping in the region where shallow donors are not passivated, therefore they do not effect on L. But close to the surface, where the hydrogen concentration increases and the concentration of nonpassivated shallow donors decreases, their relative effect on hydrogen trapping increases and for this reason these defects contribute to z 0 . In n-type Cz Si, hydrogen is trapped by both the shallow donors and these oxygen-related traps; therefore hydrogen penetrates deeper into FZ Si than into Cz one. In NCz Si hydrogen penetration is larger than that in Cz Si and smaller or comparable with hydrogen penetration in FZ Si. Thus, it could be assumed that nitrogen effects the formation of oxygen-related traps for hydrogen decreasing their effect on hydrogen penetration in Cz Si.
Fig. 2. The dependence of the penetration depth z 0 on the shallow donor concentration for Cz (j) and NCz (s) n-Si. The corresponding (Nd )21 / 2 dependencies are presented by solid lines. For the comparison the dependence obtained for FZ n-Si (3) is presented.
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References [1] M. Iida, W. Kusaki, M. Tamatsuka, E. Iino, M. Kimura, S. Muraoka, ECS Extended Abstracts, in: 195th Meetings of the Electrochemical Society, The Electrochemical Society, Pennington, NJ, 1999, No. 357. [2] M. Tamatsuka, N. Kobayashi, S. Tobe, T. Masui, ECS Extended Abstracts, in: 195th Meetings of the Electrochemical Society, The Electrochemical Society, Pennington, NJ, 1999, No. 353. [3] A. Ikari, K. Nakai, Y. Tachikawa, H. Deai, Y. Hideki, Y. Ohta, N. Masahashi, S. Hayashi, T. Hoshino, W. Ohashi, Solid State Phenomena 69–70 (1991) 161–166. [4] K. Sumino, I. Yonenaga, M. Imai, T. Abe, J. Appl. Phys. 54 (1983) 5016. [5] D. Li, D. Yang, D. Que, Physica B 273–274 (1999) 553. [6] F. Shimura, R.S. Hockett, Appl. Phys. Lett. 48 (1986) 224. [7] D. Yang, R. Fan, L. Li, D. Que, K. Sumino, J. Appl. Phys. 80 (1996) 1493–1498. [8] D. Yang, X. Ma, R. Fan, J. Zhang, L. Li, D. Que, Physica B 273–274 (1999) 308–311. [9] A. Hara, T. Fukuda, T. Miyabo, I. Hirai, Appl. Phys. Lett. 54 (1989) 626. [10] M. Suezawa, K. Sumino, H. Harada, T. Abe, Jpn. J. Appl. Phys. 25 (1986) L859–L861. [11] D. Yang, D. Que, K. Sumino, J. Appl. Phys. 77 (1995) 943–944. [12] E. Yakimov, G. Mariani, B. Pichaud, J. Appl. Phys. 78 (1995) 1495–1499. [13] E. Yakimov, I. Perichaud, Appl. Phys. Lett. 67 (14) (1995) 2054–2056. [14] V.C. Venezia, D.J. Eaglesham, T.E. Haynes, A. Agarwal, D.C. Jacobson, H.-J. Gossmann, F.H. Baumann, Appl. Phys. Lett. 73 (1998) 2980–2982. [15] I. Perichaud, E. Yakimov, S. Martinuzzi, C. Dubois, J. Appl. Phys. 90 (2001) 2806–2812. ¨ [16] W. Frank, U. Gosele, H. Mehrer, A. Seeger, in: G.E. Murch, A.S. Nowick (Eds.), Diffusion in Crystalline Solids, Academic Press, Orlando, FL, 1984, pp. 64–142. [17] M. Jacob, P. Pichler, H. Russel, R. Falster, J. Appl. Phys. 82 (1997) 182–191. [18] A.L. Parakhonsky, E.B. Yakimov, D. Yang, J. Appl. Phys. 90 (2001) 3642–3644. [19] S.J. Pearton, J.W. Corbett, M. Stavola, in: Hydrogen in Crystalline Semiconductors, Springer, Heidelberg, 1992, p. 200. ¨ ¨ Appl. Phys. Lett. 61 (1992) 2323–2325. [20] E.O. Sveinbjornsson, O. Engstrom, [21] A.L. Parakhonsky, O.V. Feklisova, S.S. Karelin, N.A. Yarykin, Semiconductors 30 (1996) 362–367. [22] O.V. Feklisova, N.A. Yarykin, Semiconduct. Sci. Techn. 12 (1997) 742–749. [23] O.V. Feklisova, E.B. Yakimov, N.A. Yarykin, in: O. Bonnaud, T. Mohammed-Brahim, H.P. Strunk, J.H. Werner (Eds.), Polycrystalline semiconductors VI, Scitec, Uetticon, Switzerland, 2001, pp. 121–126. [24] Q. Duanlin, L. Liben, Z. Jinhin, C. Xiuzhi, L. Yupin, Z. Xiao, Y. Jiansong, Sci. China A 34 (1991) 1017. ¨ [25] N.A. Stolwijk, B.J. Holzl, W. Frank, E.R. Weber, H. Mehrer, Appl. Phys. A 39 (1986) 37–48. [26] H. Zimmermann, H. Ryssel, Appl. Phys. A 55 (1992) 121–134. [27] O. Feklisova, S. Knack, E. Yakimov, N. Yarykin, J. Weber, Physica B 308–310 (2001) 213–215. [28] O. Feklisova, E.B. Yakimov, N. Yarykin, Semiconductors 36 (2002) 282–285. [29] A.L. Parakhonsky, E.B. Yakimov, D. Yang, in: V. Raineri, F. Priolo, M. Kittler, H. Richter (Eds.), Solid State Phenomena 82–84, Scitec, Uetikon, Switzerland, 2002, pp. 145–149.
Study of nitrogen-related defects by means of gold and hydrogen diffusion investigations A.L. Parakhonsky a , *, E.B. Yakimov a , D. Yang b a
b
Institute of Microelectronics Technology RAS, Chernogolovka 142432, Russia State Key Laboratory of Silicon Material, Zhejiang University, Hangzhou 310027, PR China
Abstract Gold and hydrogen diffusion in Czochralski grown Si with and without nitrogen, has been studied. It was found that the Au concentration after diffusion at 700, 750 and 800 8C in nitrogen-doped Si is always less than that in Si without nitrogen. A decrease of Au concentration in nitrogen-doped crystals is explained under the assumption that nitrogen stimulates oxygen precipitation. It is shown that the hydrogen penetration depth under wet chemical etching into nitrogen-doped samples is larger than that into Si without nitrogen that is explained by the nitrogen effect on the formation of oxygen-related traps for hydrogen. 2002 Published by Elsevier Science B.V. Keywords: Silicon; Nitrogen; Gold; Hydrogen
1. Introduction Nitrogen codoping is known to have beneficial effects on the properties of crystalline silicon. Thus it was reported that it can increase the v /G (v is the pull rate and G is the crystal temperature gradient at the solid–liquid interface) tolerance for grown-in defects free region [1], increase the void-free denuded zone depth after annealing in hydrogen atmosphere [2], reduce the crystal originated particle size after annealing [3], increase mechanical strength [4,5], etc. Nitrogen was found to essentially interact with some impurities; thus, it enhances the oxygen precipitation [6–8] and forms the nitrogen–oxygen-related shallow donors [9–11]. However, the knowledge concerning the detail mechanisms of nitrogen effects is rather poor. The main reason for this is associated with the rather low solubility limit of nitrogen in Si (about 10 15 cm 23 ). Most of the nitrogen exists in silicon in an electrically inactive state; therefore, highly sensitive electrical methods cannot be used for its characterization and only a limited number of characterization techniques can be used to reveal the * Corresponding author. E-mail address: [email protected] (A.L. Parakhonsky). 0167-9317 / 02 / $ – see front matter 2002 Published by Elsevier Science B.V. doi:10.1016/S0167-9317(02)00912-7
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nitrogen-related defects and to study their effect on Si properties. One of such methods could be associated with using gold diffusion experiments. Au diffusion could be considered as a rather sensitive method for the characterization of defect structure in Si [12–15]. It is known to diffuse via the kick-out mechanism [16] that is it diffuses very quickly over interstitial sites Au i and then Au i are transformed into electrically active substitution sites Au s via the kick-out reaction. This reaction is very sensitive to a change in self-interstitial generation or annihilation rates. For example, the defects playing a role of effective sinks for self-interstitials could lead to an essential increase in the Au concentration [12,14] while a decrease in Au concentration is usually associated with an enhanced self-interstitial generation [13,15] that could lead to a reversion of kick-out reaction. Thus, the analysis of Au diffusion allows to reveal the sinks for self-interstitials, vacancy-related complexes and to detect the self-interstitial generation processes taking place during Au diffusion. As shown in Refs. [17,18], the investigations of Pt and Au diffusion in nitrogen-doped Si could provide useful information concerning nitrogen behavior in Si. Thus, it was found [17] that Pt concentration after its diffusion in nitrogen-doped floating zone (FZ) Si is essentially higher than that after diffusion in nitrogen-free Si. Such increase was explained by the capture of interstitial Pt on the nitrogen–vacancy complexes that allows the estimation of concentration of these complexes. Contrary, in Czochralski grown (Cz) Si the Au concentration was found to be lower in the crystals with nitrogen than that in nitrogen-free Si [18] this was considered as an indication of oxygen precipitation stimulated by nitrogen. Hydrogen diffusion in Si is mainly used for the study of defect passivation [19] and of formation of new defects containing hydrogen [20–22]. The application of hydrogen profile study for the defect characterization in Si is not so wide. Nevertheless, such experiments also could provide important information concerning the hydrogen interaction with defects existing in the samples under study and the properties of defects playing the role of traps for hydrogen. For example, oxygen-related traps for hydrogen were detected in Ref. [23] by the comparable analysis of hydrogen profiles in Cz and FZ Si. In the present paper some new results concerning the gold and hydrogen diffusion experiments in nitrogen-doped Si are presented. The analysis carried out shows that nitrogen enhances oxygen precipitation even during rather short annealing and suppresses the formation of oxygen-related traps for hydrogen in Cz Si.
2. Experimental Si wafers of n-type conductivity with a phosphorus concentration of about 10 14 cm 23 grown by the Czochralsky method with a thickness of 300 mm were used in our experiments. Nitrogen-doped Si ingots (NCz) were grown in a nitrogen atmosphere at reduced pressure [24], while the Cz ingots without nitrogen were grown in similar conditions but in an argon atmosphere. Pairs of Cz and NCz Si wafers cut from almost the same tail or head parts of ingots, which contain very similar oxygen concentrations of about 1.0310 18 cm 23 and similar phosphorus concentrations, were compared. The carbon concentration in the samples was less than 5310 15 cm 23 . The oxygen and carbon concentrations in the samples were measured by the IR absorption at room temperature using the calibration factors 3.14310 17 cm 22 for oxygen and 1.0310 17 cm 22 for carbon, respectively. Part of samples was annealed at 650 8C for 0.5 h to annihilate oxygen thermal donors. After such annealing nitrogen–oxygen shallow donors were generated in nitrogen-doped NCz crystals [9–11]. To estimate
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the concentration of nitrogen–oxygen donors the samples were annealed at 950 8C to destroy these centers and their concentration was obtained as a difference between the total donor concentration measured in the samples annealed at 650 and 950 8C. Au films with a thickness of about 30 nm were evaporated on one side of the samples. Then Au diffusion annealing was carried out at 700, 750 or 800 8C for 1 h. After diffusion annealing a layer of about 50 mm thickness was removed by mechanical polishing from one side of all gold diffused samples. On a few samples Au depth profile was measured, which was found to be similar to that predicted by the kick-out model. The substitutional gold concentration Au s and its depth profile were measured by deep level transient spectroscopy (DLTS). The total Au s concentration was calculated as a sum of the concentrations of centers with the deep levels Ec 20.54 eV and Ec 20.20 eV, associated with Au s and gold–hydrogen complexes formed under chemical etching [20,21], respectively. Hydrogen in the samples was introduced during wet chemical etching (WCE) in the acid solution HF:HNO 3 (1:7) at ambient temperature, the etching rate was about 2 mm / min. The charge carrier depth distribution in the samples was reconstructed from the C–V measurements. Under a reasonable assumption that before hydrogen passivation the donor distribution was independent of depth, the depth distribution of neutral hydrogen-donor centers was calculated as a difference between the charge carrier concentration at a large enough depth, where it is independent of depth and is equal to the initial value of total donor concentration, and the carrier concentration at any particular depth, obtained from the C–V curves. No new defects were observed by the DLTS in the region saturated with hydrogen during WCE. The Schottky contacts of 1.5-mm diameter for the DLTS and C–V measurements were fabricated by vacuum evaporation of Au on the etched surfaces. Ohmic contacts were produced by scratching the back side of the samples with the eutectic Al–Ga alloy.
3. Results and discussion
3.1. Gold diffusion experiments The results of Au s concentration measurements at a depth of about 50 mm in the Cz Si samples codoped with nitrogen and without nitrogen are presented in Table 1. The Au s concentration extrapolated from the high temperature measurements [12,25,26] are also presented for comparison. It is seen that at all diffusion temperatures used, the Au s concentration in NCz samples is lower than that Table 1 Au s concentration measured by the DLTS in Cz and NCz silicon (head and tail parts of ingots) after diffusion annealing at 700, 750, and 800 8C for 1 h, the Au s concentration extrapolated from the high temperature diffusion experiments was shown in the right column T dif
Au s (NCz) (cm 23 )
Au s (Cz) (cm 23 )
(8C)
Head
Tail
Head
Tail
(cm 23 )
700 750 800
(1.0360.15)310 11 (6.0562.65)310 11 (5.1460.10)310 12
(1.0360.11)310 11 (1.1960.06)310 12 (3.5660.43)310 12
(8.1860.74)310 11 (2.8960.35)310 12 (1.4760.11)310 13
(5.8560.19)310 11 (5.2960.31)310 12 (1.4860.25)310 13
2.48310 11 1.79310 12 9.98310 12
Au s (extrapol.)
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in the corresponding Cz samples and this difference is remarkably larger than the difference in the Au s concentration between the samples cut from the head and tail parts of ingots. The difference in the Au s concentration between NCz and Cz samples was found to increase with decreasing diffusion temperature. It is seen that the Au s concentration in Cz samples after 800 8C diffusion is close to the extrapolated value but after 700 8C diffusion it is about two to three times larger. This could be explained under an assumption that some grown-in sinks for self-interstitials are present in the Cz samples without nitrogen. An increase in diffusion temperature leads to a corresponding increase of self-interstitial generation rate that can lead to the disappearance or reconstruction of these grown-in defects. The Au s concentration in NCz samples at all temperatures used is lower than that predicted for defect-free Si. This decrease can be explained as the result of an enhanced oxygen precipitation in such crystals [6–8]. Self-interstitials generated under oxygen precipitation can reverse the kick-out reaction and decrease the Au s concentration. No effect of grown-in defects on the Au s concentration was observed in NCz Si at all diffusion temperatures used. This result contradicts those obtained on FZ Si [17] where the pronounced increase in Pt concentration was observed after diffusion at similar temperatures. The increased in Pt concentration was explained [17] under the assumption that the simple nitrogen–vacancy complexes were formed during the ingot cooling after growth that decreased vacancy concentration and suppressed the formation of large vacancy-type clusters. In NCz Si such defects could present in a configuration which is not such an efficient sink for the self-interstitials. Besides, self-interstitials created during oxygen precipitation could annihilate or reconstruct such vacancy-type defects.
3.2. Hydrogen diffusion experiments The distribution of hydrogen-donor complexes in Si after WCE can be reconstructed from the measurements of charge carrier distribution. As shown in Refs. [27,28], the decay of depth distribution of these complexes can be described by the exponential law with the characteristic depth L 5 (4p rNd )21 / 2 , where Nd is the dopant concentration and r is the hydrogen capture radius. In most Si crystals hydrogen capture is mainly determined by the dopant atoms but if some additional traps are present in the sample under study this should lead to a decrease of L. The hydrogen penetration in Si during WCE is characterized also by the second independent parameter z 0 , which was measured as the depth, at which 10% of shallow donors were passivated with hydrogen, and is approximately proportional to (Nd )21 / 2 . The examples of charge carrier profiles measured in Cz and NCz Si with very close dopant concentration are presented in Fig. 1. It is seen that near the surface the charge carrier concentration decreases due to the hydrogen passivation of donors. Analysis of such profiles allows to obtain z 0 and L values for the NCz and Cz samples studied. The L values measured in this study were found to correlate rather well with the L(Nd ) dependence obtained in [29] and is practically the same for the both types of crystals. That means that no effective traps for hydrogen are associated with nitrogen. As shown in Ref. [29], nitrogen–oxygen donors are passivated by hydrogen and their capture radius is close to that of phosphorus. However, the concentration of such donors in our samples did not exceed 20% of phosphorus concentration therefore their effect on L is rather small. On the contrary, the z 0 value, was found to be larger in NCz crystals than in Cz ones (Fig. 2). Just this difference determines the difference in charge carrier profiles shown in Fig. 1. It should be noted
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Fig. 1. Charge carrier depth profiles in Cz and NCz silicon after WCE.
that the difference in the hydrogen penetration in Cz and FZ samples was observed in Refs. [23,28], where it was explained under the assumption that some oxygen-related traps for hydrogen are formed in Cz Si. The capture radius and / or concentration of these traps are too low to effect hydrogen trapping in the region where shallow donors are not passivated, therefore they do not effect on L. But close to the surface, where the hydrogen concentration increases and the concentration of nonpassivated shallow donors decreases, their relative effect on hydrogen trapping increases and for this reason these defects contribute to z 0 . In n-type Cz Si, hydrogen is trapped by both the shallow donors and these oxygen-related traps; therefore hydrogen penetrates deeper into FZ Si than into Cz one. In NCz Si hydrogen penetration is larger than that in Cz Si and smaller or comparable with hydrogen penetration in FZ Si. Thus, it could be assumed that nitrogen effects the formation of oxygen-related traps for hydrogen decreasing their effect on hydrogen penetration in Cz Si.
Fig. 2. The dependence of the penetration depth z 0 on the shallow donor concentration for Cz (j) and NCz (s) n-Si. The corresponding (Nd )21 / 2 dependencies are presented by solid lines. For the comparison the dependence obtained for FZ n-Si (3) is presented.
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