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DLTS study of the oxygen dimer formation kinetics in silicon
ARTICLE IN PRESS Physica B 404 (2009) 4576–4578
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
DLTS study of the oxygen dimer formation kinetics in silicon ¨ Weber b Nikolai Yarykin a,, Jorg a b
Institute of Microelectronics Technology RAS, 142432 Chernogolovka, Moscow Region, Russia Technische Universit¨ at Dresden, 01062 Dresden, Germany
a r t i c l e in f o
PACS: 61.72.Yx 61.80.Az 61.82.Fk 71.55.Cn Keywords: Silicon Radiation defects Oxygen
a b s t r a c t The introduction rates of radiation defects, in particular the X- and M-centers for which the oxygen dimer is a precursor, are investigated as a function of duration of the pre-irradiation heat treatment at 480 3 C in Czochralski-grown silicon both of n- and p-types. The characteristic annealing time to grow the X-center concentration in the n-type crystal is found to be about 1 h in accordance with the model which implies no significant barrier for the dimer formation. The M-center concentration in the p-type crystal is found to be nearly independent of duration of the pre-irradiation annealing after a few minutes transient period. This behavior is ascribed to the stabilization of dimer concentration due to an effective dimer trapping in these samples. & 2009 Elsevier B.V. All rights reserved.
1. Introduction The oxygen dimers are believed to play a decisive role in the processes of oxygen diffusion and agglomeration in silicon below 700 3 C. These species have been experimentally identified by the IR absorption lines and their binding energy was estimated as 0.3 eV [1]. However, the parameters, which determine the dimer contribution to the oxygen transport, such as concentration, diffusivity, and diffusion length, are still a matter of speculations. For instance, the diffusion experiments provide only a combination of the dimer concentration and diffusivity [2,3]. An independent combination of the parameters can be obtained from a study of the dimer formation and dissociation kinetics. Two deep-level centers observed in the electron-irradiated oxygen-rich silicon were ascribed to a complex of the selfinterstitial and oxygen dimer [4–6]. These are the M-center with the Ev þ 0:36 and Ev þ0:12 eV levels in p-type boron-doped Si and the X-center with an acceptor level at Ec 0:11 eV in n-type material. Although the identical structure of the X- and M-centers was later questioned, the oxygen dimers were confirmed as a precursor both of the X- and M-centers [7]. More precisely, the introduction rates of both radiation defects were shown to be directly proportional to the dimer concentration which was varied by a heat treatment [7]. This finding gives a tool to trace the annealing-induced variations of the dimer concentration with the sensitive electrical methods.
In this work the kinetics of dimer formation at 480 3 C are investigated by measuring the X- and M-center concentrations in the samples which received different heat treatments before the irradiation.
2. Experimental The Czochralski-grown Si crystals of n- and p-types (½P ¼ 1015 , ½O ¼ 7 1017 , and ½B ¼ 7 1014 , [O]= 91017 cm3 , respectively) were used in the work. To minimize the oxygen dimer concentration, the as-received wafers were annealed at 750 3 C for 5 min, the heat treatment was terminated by quenching in liquid nitrogen. Then the samples cut from such wafers were annealed at 480 3 C for various durations. The heat-treated samples were mounted on the water-cooled block (20 3 C) and irradiated with the 7 1014 cm2 fluence of 5 MeV electrons. Schottky diodes were fabricated on the chemically etched wafers by the vacuum evaporation of metals (Au or Al on n- and p-type samples, respectively). The deep-level spectra were measured with a standard DLTS technique in the temperature range of 40–300 K.
3. Results 3.1. X-centers in n-type Si
Corresponding author.
E-mail address: [email protected] (N. Yarykin). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.08.314
The deep-level spectra shown in Fig. 1 were measured from the irradiated n-type crystals after various heat treatments. It is seen that the pre-irradiation heat treatments have practically no effect
ARTICLE IN PRESS N. Yarykin, J¨ o. Weber / Physica B 404 (2009) 4576–4578
Fig. 1. DLTS spectra (UR ¼ 10 V, UP ¼ 4 V, tP ¼ 1 ms) of the n-type Cz-Si irradiated with 5 MeV electrons. Before the irradiation the samples were quenched from 750 3 C (dash-dot curve) and then additionally annealed at 480 3 C for 25 (short dashes), 80 (dots), and 120 min (solid curve). With dashes is also shown the curve taken in the same conditions for the (non-irradiated) similarly doped n-type Cz-Si annealed at 465 3 C for 2 h.
4577
Fig. 3. Dependence of the apparent concentrations (see text) of the radiation defects in p-type Cz-Si on the pre-irradiation annealing time at 480 3 C. The solid curve is the best fit of the M-center data with exponential function.
the best fit being achieved for a characteristic time of 54 min. The saturation level obtained from the fitting coincides very precisely with the value predicted for 480 3 C by the Arrhenius dependence for concentration of the X-centers on the temperature of preirradiation annealing (see Fig. 4 in Ref. [7]). According to this dependence, the X-center concentration is expected to be about three times lower after the quenching from 750 3 C, while its variation is less than in two times in Fig. 2. This fact is ascribed to the unsuccessful quenching process resulted to an undesired annealing of the particular wafer at intermediate temperature. 3.2. M-centers in p-type Si
Fig. 2. Concentrations of the radiation defects in n-type Cz-Si as a function of the pre-irradiation annealing time at 480 3 C. The solid curve is the best fit of the X-center data with exponential function.
on the concentration of the A-centers (V–O complex) and divacancies. The net donor concentrations in the irradiated samples remain also stable within few percent, indicating that the thermal donor (TD) formation is not significant during the pre-irradiation heat treatments. However, the X-center DLTS peak trends to be higher in the samples which received longer treatments at 480 3 C. Another DLTS peak with a wide maximum at about 54 K also appears in these samples. The nature of this center is unknown. For comparison, the DLTS peak of the double thermal donors is also shown in Fig. 1. This peak was taken from the (non-irradiated) crystal which was annealed for the same time as the last sample in the series of irradiated samples (2 h) but at lower temperature (460 3 C). It is seen that the thermal donor concentration is negligible in the irradiated samples. Variations of the deep-level concentrations with the time of pre-irradiation annealing are presented in Fig. 2. The increase of the X-center concentration is fitted with the exponential function,
The apparent concentrations of the radiation defects in p-type samples are shown in Fig. 3 as a function of duration of the preirradiation annealing at 480 3 C. The DLTS peak of the bistable M-center was found as a difference of the curves taken after cooling the diode under zero or reverse bias [5,7], and the concentration shown in Fig. 3 was calculated from its amplitude in the usual way. However, the true M-center concentration is about 2.14 times less because of the ‘‘zero-U’’ nature of the center [8]. In addition, as the measurements were performed on the asirradiated samples with a very limited exposure to room temperature, the stable part of the signal is composed of two overlapped peaks determined by the Ci Oi and Ci Oi centers [9]. This results in the underestimation of the amount of the carbon– oxygen pairs. It is seen in Fig. 3 that the M-center concentration reaches a saturation after a very short pre-irradiation annealing at 480 3 C. The exponential function, which provides the best fit to the data, has a characteristic time of about 2 min. In contrast, the introduction rate of the carbon–oxygen pairs demonstrates the monotonous decrease due to the pre-irradiation heat treatment. The corresponding dashed curve in Fig. 3 is the exponential function with a characteristic time of 42 min. However, the fitting procedure is not very sensitive to this parameter.
4. Discussion It was shown in Ref. [7] that the X- and M-center concentrations are directly proportional to the pre-irradiation amount of oxygen dimers. Therefore, let us consider first which behavior is expected for the dimer concentration during the pre-irradiation
ARTICLE IN PRESS 4578
N. Yarykin, J¨ o. Weber / Physica B 404 (2009) 4576–4578
heat treatment at 480 3 C. A model assumes that the dimer dissociation is a dominant process as compared to trapping, although this is uncertain for 480 3 C [10]. In this case, the characteristic time t of reaching the equilibrium dimer concentration (which is, in fact, the reverse dimer dissociation rate) can be found as
t¼
N2eq 8pD1 rN 21
;
ð1Þ
where N2eq is the equilibrium dimer concentration, N1 and D1 the interstitial oxygen concentration and diffusivity, respectively, and r the capture radius for the dimer formation. N2eq can be calculated as N2 E ð2Þ N2eq ¼ 3 1 exp b2 ; NSO kT where NSO is the number of lattice sites for dimers and Eb2 the dimer binding energy. This statistical formula was shown to be valid under the assumption that the oscillator strength for the IR dimer-related lines is close to that for interstitial oxygen [1]. Combining Eqs. (1) and (2) and taking r ¼ 5 108 cm and D1 from Ref. [11], one can obtain t ¼ 26 min for Eb2 ¼ 0:3 eV [1] or t ¼ 120 min for Eb2 ¼ 0:4 eV [7]. Thus, a characteristic time of 54 min observed for the X-center growth in n-type samples (Fig. 2) is in a good agreement with the estimations above. In particular, this means that there is no significant potential barrier for the dimer formation. The data for p-type samples are not so clear. The very short transient time for the M-centers (Fig. 3) indicates that the dimer trapping may play an important role in the kinetics. The trapping rate can be expressed as
t1 T ¼ 4pD2 rT NT ;
ð3Þ
where D2 is the dimer diffusivity, rT and NT the capture radius and concentration of the trapping centers, respectively. Using the data on oxygen effective diffusivity [2,3] and Eq. (2), we obtain D2 ¼ ð229Þ 1015 cm2 s1 depending on the chosen Eb2 . Taking rT ¼ 5 108 cm, one can see that the transient time in the range
of 3–15 min can only be achieved if the trap density is close to the oxygen concentration in the crystal under study (7 1017 cm3 ). This implies that the higher-order oxygen agglomerates are formed in these samples. Indeed, the net boron concentration exhibits the decrease of 20% due to the 480 3 C annealing for 25 min, which could be attributed to the appearance of thermal donors. However, the reasons for such enhanced TD formation in these samples are not clear. On the other hand, the decrease of the carbon–oxygen complex introduction rate (Fig. 3) shows that additional traps for interstitials are formed during the heat treatment. Taking into account that the M-center concentration is sensitive to the presence of such traps [5], this could compensate the effect of increasing dimer concentration, resulting in the weak total dependence of the M-center concentration on duration of the pre-irradiation annealing.
Acknowledgment The work was partially supported by the Erasmus Mundus External Co-operation programme of the European Union (EM ECW-L04 TUD 08-210).
References ¨ [1] L.I. Murin, T. Hallberg, V.P. Markevich, J.L. Lindstrom, Phys. Rev. Lett. 80 (1998) 93. [2] S. Senkader, P.R. Wilshaw, R. Falster, J. Appl. Phys. 89 (2001) 4803. [3] N. Yarykin, E. Hieckmann, V.I. Vdovin, Phys. Stat. Sol. (c) 4 (2007) 3070. ¨ [4] J.L. Lindstrom, et al., Physica B 308–310 (2001) 284. [5] V.P. Markevich, et al., J. Phys. Condens. Matter 17 (2005) S2331. [6] V.P. Markevich, et al., Solid State Phenom. 108–109 (2005) 273. [7] N. Yarykin, J. Weber, Physica B 401–402 (2007) 483. [8] N. Yarykin, Semicond. Sci. Technol. 23 (2008) 125031. [9] L.I. Khirunenko, et al., Solid State Phenom. 108–109 (2005) 261. [10] L.I. Murin, V.P. Markevich, in: C.A.J. Ammerlaan, B. Pajot (Eds.), Shallow-level Centers in Semiconductors, World Scientific, Singapore, 1997, p. 339. [11] R.C. Newman, J. Phys. Condens. Matter 12 (2000) R335.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
DLTS study of the oxygen dimer formation kinetics in silicon ¨ Weber b Nikolai Yarykin a,, Jorg a b
Institute of Microelectronics Technology RAS, 142432 Chernogolovka, Moscow Region, Russia Technische Universit¨ at Dresden, 01062 Dresden, Germany
a r t i c l e in f o
PACS: 61.72.Yx 61.80.Az 61.82.Fk 71.55.Cn Keywords: Silicon Radiation defects Oxygen
a b s t r a c t The introduction rates of radiation defects, in particular the X- and M-centers for which the oxygen dimer is a precursor, are investigated as a function of duration of the pre-irradiation heat treatment at 480 3 C in Czochralski-grown silicon both of n- and p-types. The characteristic annealing time to grow the X-center concentration in the n-type crystal is found to be about 1 h in accordance with the model which implies no significant barrier for the dimer formation. The M-center concentration in the p-type crystal is found to be nearly independent of duration of the pre-irradiation annealing after a few minutes transient period. This behavior is ascribed to the stabilization of dimer concentration due to an effective dimer trapping in these samples. & 2009 Elsevier B.V. All rights reserved.
1. Introduction The oxygen dimers are believed to play a decisive role in the processes of oxygen diffusion and agglomeration in silicon below 700 3 C. These species have been experimentally identified by the IR absorption lines and their binding energy was estimated as 0.3 eV [1]. However, the parameters, which determine the dimer contribution to the oxygen transport, such as concentration, diffusivity, and diffusion length, are still a matter of speculations. For instance, the diffusion experiments provide only a combination of the dimer concentration and diffusivity [2,3]. An independent combination of the parameters can be obtained from a study of the dimer formation and dissociation kinetics. Two deep-level centers observed in the electron-irradiated oxygen-rich silicon were ascribed to a complex of the selfinterstitial and oxygen dimer [4–6]. These are the M-center with the Ev þ 0:36 and Ev þ0:12 eV levels in p-type boron-doped Si and the X-center with an acceptor level at Ec 0:11 eV in n-type material. Although the identical structure of the X- and M-centers was later questioned, the oxygen dimers were confirmed as a precursor both of the X- and M-centers [7]. More precisely, the introduction rates of both radiation defects were shown to be directly proportional to the dimer concentration which was varied by a heat treatment [7]. This finding gives a tool to trace the annealing-induced variations of the dimer concentration with the sensitive electrical methods.
In this work the kinetics of dimer formation at 480 3 C are investigated by measuring the X- and M-center concentrations in the samples which received different heat treatments before the irradiation.
2. Experimental The Czochralski-grown Si crystals of n- and p-types (½P ¼ 1015 , ½O ¼ 7 1017 , and ½B ¼ 7 1014 , [O]= 91017 cm3 , respectively) were used in the work. To minimize the oxygen dimer concentration, the as-received wafers were annealed at 750 3 C for 5 min, the heat treatment was terminated by quenching in liquid nitrogen. Then the samples cut from such wafers were annealed at 480 3 C for various durations. The heat-treated samples were mounted on the water-cooled block (20 3 C) and irradiated with the 7 1014 cm2 fluence of 5 MeV electrons. Schottky diodes were fabricated on the chemically etched wafers by the vacuum evaporation of metals (Au or Al on n- and p-type samples, respectively). The deep-level spectra were measured with a standard DLTS technique in the temperature range of 40–300 K.
3. Results 3.1. X-centers in n-type Si
Corresponding author.
E-mail address: [email protected] (N. Yarykin). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.08.314
The deep-level spectra shown in Fig. 1 were measured from the irradiated n-type crystals after various heat treatments. It is seen that the pre-irradiation heat treatments have practically no effect
ARTICLE IN PRESS N. Yarykin, J¨ o. Weber / Physica B 404 (2009) 4576–4578
Fig. 1. DLTS spectra (UR ¼ 10 V, UP ¼ 4 V, tP ¼ 1 ms) of the n-type Cz-Si irradiated with 5 MeV electrons. Before the irradiation the samples were quenched from 750 3 C (dash-dot curve) and then additionally annealed at 480 3 C for 25 (short dashes), 80 (dots), and 120 min (solid curve). With dashes is also shown the curve taken in the same conditions for the (non-irradiated) similarly doped n-type Cz-Si annealed at 465 3 C for 2 h.
4577
Fig. 3. Dependence of the apparent concentrations (see text) of the radiation defects in p-type Cz-Si on the pre-irradiation annealing time at 480 3 C. The solid curve is the best fit of the M-center data with exponential function.
the best fit being achieved for a characteristic time of 54 min. The saturation level obtained from the fitting coincides very precisely with the value predicted for 480 3 C by the Arrhenius dependence for concentration of the X-centers on the temperature of preirradiation annealing (see Fig. 4 in Ref. [7]). According to this dependence, the X-center concentration is expected to be about three times lower after the quenching from 750 3 C, while its variation is less than in two times in Fig. 2. This fact is ascribed to the unsuccessful quenching process resulted to an undesired annealing of the particular wafer at intermediate temperature. 3.2. M-centers in p-type Si
Fig. 2. Concentrations of the radiation defects in n-type Cz-Si as a function of the pre-irradiation annealing time at 480 3 C. The solid curve is the best fit of the X-center data with exponential function.
on the concentration of the A-centers (V–O complex) and divacancies. The net donor concentrations in the irradiated samples remain also stable within few percent, indicating that the thermal donor (TD) formation is not significant during the pre-irradiation heat treatments. However, the X-center DLTS peak trends to be higher in the samples which received longer treatments at 480 3 C. Another DLTS peak with a wide maximum at about 54 K also appears in these samples. The nature of this center is unknown. For comparison, the DLTS peak of the double thermal donors is also shown in Fig. 1. This peak was taken from the (non-irradiated) crystal which was annealed for the same time as the last sample in the series of irradiated samples (2 h) but at lower temperature (460 3 C). It is seen that the thermal donor concentration is negligible in the irradiated samples. Variations of the deep-level concentrations with the time of pre-irradiation annealing are presented in Fig. 2. The increase of the X-center concentration is fitted with the exponential function,
The apparent concentrations of the radiation defects in p-type samples are shown in Fig. 3 as a function of duration of the preirradiation annealing at 480 3 C. The DLTS peak of the bistable M-center was found as a difference of the curves taken after cooling the diode under zero or reverse bias [5,7], and the concentration shown in Fig. 3 was calculated from its amplitude in the usual way. However, the true M-center concentration is about 2.14 times less because of the ‘‘zero-U’’ nature of the center [8]. In addition, as the measurements were performed on the asirradiated samples with a very limited exposure to room temperature, the stable part of the signal is composed of two overlapped peaks determined by the Ci Oi and Ci Oi centers [9]. This results in the underestimation of the amount of the carbon– oxygen pairs. It is seen in Fig. 3 that the M-center concentration reaches a saturation after a very short pre-irradiation annealing at 480 3 C. The exponential function, which provides the best fit to the data, has a characteristic time of about 2 min. In contrast, the introduction rate of the carbon–oxygen pairs demonstrates the monotonous decrease due to the pre-irradiation heat treatment. The corresponding dashed curve in Fig. 3 is the exponential function with a characteristic time of 42 min. However, the fitting procedure is not very sensitive to this parameter.
4. Discussion It was shown in Ref. [7] that the X- and M-center concentrations are directly proportional to the pre-irradiation amount of oxygen dimers. Therefore, let us consider first which behavior is expected for the dimer concentration during the pre-irradiation
ARTICLE IN PRESS 4578
N. Yarykin, J¨ o. Weber / Physica B 404 (2009) 4576–4578
heat treatment at 480 3 C. A model assumes that the dimer dissociation is a dominant process as compared to trapping, although this is uncertain for 480 3 C [10]. In this case, the characteristic time t of reaching the equilibrium dimer concentration (which is, in fact, the reverse dimer dissociation rate) can be found as
t¼
N2eq 8pD1 rN 21
;
ð1Þ
where N2eq is the equilibrium dimer concentration, N1 and D1 the interstitial oxygen concentration and diffusivity, respectively, and r the capture radius for the dimer formation. N2eq can be calculated as N2 E ð2Þ N2eq ¼ 3 1 exp b2 ; NSO kT where NSO is the number of lattice sites for dimers and Eb2 the dimer binding energy. This statistical formula was shown to be valid under the assumption that the oscillator strength for the IR dimer-related lines is close to that for interstitial oxygen [1]. Combining Eqs. (1) and (2) and taking r ¼ 5 108 cm and D1 from Ref. [11], one can obtain t ¼ 26 min for Eb2 ¼ 0:3 eV [1] or t ¼ 120 min for Eb2 ¼ 0:4 eV [7]. Thus, a characteristic time of 54 min observed for the X-center growth in n-type samples (Fig. 2) is in a good agreement with the estimations above. In particular, this means that there is no significant potential barrier for the dimer formation. The data for p-type samples are not so clear. The very short transient time for the M-centers (Fig. 3) indicates that the dimer trapping may play an important role in the kinetics. The trapping rate can be expressed as
t1 T ¼ 4pD2 rT NT ;
ð3Þ
where D2 is the dimer diffusivity, rT and NT the capture radius and concentration of the trapping centers, respectively. Using the data on oxygen effective diffusivity [2,3] and Eq. (2), we obtain D2 ¼ ð229Þ 1015 cm2 s1 depending on the chosen Eb2 . Taking rT ¼ 5 108 cm, one can see that the transient time in the range
of 3–15 min can only be achieved if the trap density is close to the oxygen concentration in the crystal under study (7 1017 cm3 ). This implies that the higher-order oxygen agglomerates are formed in these samples. Indeed, the net boron concentration exhibits the decrease of 20% due to the 480 3 C annealing for 25 min, which could be attributed to the appearance of thermal donors. However, the reasons for such enhanced TD formation in these samples are not clear. On the other hand, the decrease of the carbon–oxygen complex introduction rate (Fig. 3) shows that additional traps for interstitials are formed during the heat treatment. Taking into account that the M-center concentration is sensitive to the presence of such traps [5], this could compensate the effect of increasing dimer concentration, resulting in the weak total dependence of the M-center concentration on duration of the pre-irradiation annealing.
Acknowledgment The work was partially supported by the Erasmus Mundus External Co-operation programme of the European Union (EM ECW-L04 TUD 08-210).
References ¨ [1] L.I. Murin, T. Hallberg, V.P. Markevich, J.L. Lindstrom, Phys. Rev. Lett. 80 (1998) 93. [2] S. Senkader, P.R. Wilshaw, R. Falster, J. Appl. Phys. 89 (2001) 4803. [3] N. Yarykin, E. Hieckmann, V.I. Vdovin, Phys. Stat. Sol. (c) 4 (2007) 3070. ¨ [4] J.L. Lindstrom, et al., Physica B 308–310 (2001) 284. [5] V.P. Markevich, et al., J. Phys. Condens. Matter 17 (2005) S2331. [6] V.P. Markevich, et al., Solid State Phenom. 108–109 (2005) 273. [7] N. Yarykin, J. Weber, Physica B 401–402 (2007) 483. [8] N. Yarykin, Semicond. Sci. Technol. 23 (2008) 125031. [9] L.I. Khirunenko, et al., Solid State Phenom. 108–109 (2005) 261. [10] L.I. Murin, V.P. Markevich, in: C.A.J. Ammerlaan, B. Pajot (Eds.), Shallow-level Centers in Semiconductors, World Scientific, Singapore, 1997, p. 339. [11] R.C. Newman, J. Phys. Condens. Matter 12 (2000) R335.