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Contribution of the disturbed dislocation slip planes to the electrical properties of plastically deformed silicon
ARTICLE IN PRESS
Physica B 340–342 (2003) 1005–1008
Contribution of the disturbed dislocation slip planes to the electrical properties of plastically deformed silicon O.V. Feklisova, E.B. Yakimov*, N. Yarykin Institute of Microelectronics Technology, Russian Academy of Sciences (RAS), Chernogolovka, Moscow Region 142432, Russia
Abstract The deep-level centers introduced by the plastic deformation at 680 C in p-type silicon are quantitatively studied by the electron beam induced current (EBIC) and DLTS techniques. It is shown that the DLTS signal is higher than that could be ascribed to the centers located at (or close to) dislocations. In opposite, the number of electrically active defects in the dislocation trails, which is estimated from the EBIC contrast, easily explains the DLTS signal. The possible nature of defects in the dislocation trails is discussed. r 2003 Elsevier B.V. All rights reserved. Keywords: Silicon; Dislocations; EBIC; DLTS
1. Introduction It is well known that in addition to dislocations a lot of other defects are introduced in silicon during plastic deformation [1]. The defects created by moving dislocations strongly disturb the properties of regions adjacent to the slip planes swept by dislocations [2,3], forming the dislocation trails. As the electron beam induced current (EBIC) investigations have shown [3–5], not only the dislocations but also the dislocation trails demonstrate the pronounced electrical activity. However, the nature of defects formed in the dislocation trails and their relative contribution to the electrical properties of plastically deformed Si is still unknown. One of the approaches for the solution of the latter problem is to correlate the
number of defects located near dislocations and in the dislocation trails with the total concentration of electrically active defects introduced during plastic deformation. In the present paper the EBIC and DLTS studies of plastically deformed p-type Si samples with different dislocation density are carried out. The concentration of defects in the dislocation trails is estimated from the EBIC images while the total concentration of deep-level centers in the sample is obtained by the DLTS measurements on the same Schottky diode. All the results obtained are consistent with the assumption that most of the defects revealed by the DLTS are located in the dislocation trails.
2. Experimental *Corresponding author. Tel.: +7-095-9628074; fax: +7-0959628047. E-mail address: [email protected] (E.B. Yakimov).
The experiments were carried out on the p-type dislocation-free Si grown by Czohralski method
0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.09.196
ARTICLE IN PRESS 1006
O.V. Feklisova et al. / Physica B 340–342 (2003) 1005–1008
and doped with boron to a concentration of about 1015 cm3. After the damage that resulted from the cutting procedure was removed by chemical etching, the dislocation sources were nucleated by scratching the {1 0 0} surface with a diamond tip. The samples were elastically bent at room temperature around a /1 1 0S axis in the cantilever mode using the setup made of clean ceramics. Then the stressed samples were annealed at 680 C for 2 h in argon atmosphere. To reveal the deformation-induced defects, the {1 0 0} surface was selectively etched in Schimmel solution. The Schottky barriers with a diameter of 2 mm for the EBIC and DLTS measurements were formed by thermal evaporation of Al. Under the deformation procedure used, the dislocation density ND changes along the sample from zero to 106 cm2 due to the linear variation of the resolved shear stress. In every diode, ND was obtained by counting the dislocation etch pits in several areas randomly distributed over the diode. Ohmic contacts were produced by rubbing the eutectic Al–Ga alloy in the backside of the samples. The EBIC measurements were carried out in the temperature range from 90 to 300 K in the scanning electron microscope JSM-840A (Jeol) using the Keithley 428 current amplifier. As usual, the contrast C of defects was determined as C ¼ 1 Ic =Ic0 ; where Ic and Ic0 are the collected current values measured with e-beam located at the defect and far from it, respectively. The deeplevel spectra were measured using the standard DLTS setup included a lock-in amplifier as a correlator. As a rule, filling pulse duration of 1 ms and a repetition rate of 20.8 s1 were used.
3. Results and discussion The typical EBIC image of the structure under study presented in Fig. 1 shows the dark broken lines parallel to the acting slip planes. It could be seen that the dark segments of these lines are not associated with dislocations, which are displayed as bright points due to the topography contrast of dislocation etch pits. Thus, the dark lines in the EBIC image could be associated with the recombination centers formed behind dislocations in the
Fig. 1. Typical image of studied Schottky diode in the EBIC mode. Image size is 500 500 mm.
quasi-two-dimensional dislocation trails [3,4]. It is seen in Fig. 1 that the recombination contrast of the dislocation trails varies from one segment to another. The maximum contrast reaches B4% at room temperature and slightly increases at 90 K, while the contrast associated with dislocations themselves is lower than 1% in the whole temperature range from 90 to 300 K. In accordance with Ref. [6], the low dislocation contrast could be considered as an indication of a relatively clean deformation conditions. It is well known [7] that the product of trap concentration near the extended defect and minority carrier capture cross-section s can be obtained from the EBIC contrast. For our samples, we find sNs ¼ 2:5 104 for the dislocation trail with the contrast of 1% (Ns is the sheet defect density) and sNl o107 cm for dislocations (Nl is the linear defect density). The DLTS spectra measured on the diodes with different average dislocation density ND are presented in Fig. 2. The spectra consist of one broad peak with a maximum at about 235 K and with the amplitude increasing with ND : The DLTS spectra obtained in earlier works on the plastically deformed p-type Si with higher ND (see, e.g. Ref. [1]) look different. Nevertheless, they contain
ARTICLE IN PRESS O.V. Feklisova et al. / Physica B 340–342 (2003) 1005–1008
ND, cm-2
5
-4
2.0×10
103 -4
104
105
106
100
101
102
4
1.5×10
1013
-4
1.0×10
3 2
-5
5.0×10
Nt, cm-3
∆C/C
1007
1
0,0 50
100
150
200
250
1012
300
T, K
1011
Fig. 2. The DLTS spectra measured on the diodes with the dislocation density ND ¼ 0 (1), 3 104 (2), 6 104 (3), 1.4 105 (4), 3 105 cm2 (5).
10-1
Effective dislocation trail length, cm-1 Fig. 3. Deep-level center concentration plotted as a function of dislocation density (open circles) and of total effective length of dislocation trails (solid squares).
-4
1×10
∆C/C
the peak (H.49) similar to that observed in the present work. Because of broadening, the concentration of deep-level centers is calculated by integration of the peak area and a comparison with the area of the peak simulated for the point defect with close parameters. The concentration obtained in this way is plotted in Fig. 3 as a function of either the dislocation density ND or the effective length of the dislocation trails. The latter is just the total length of dark segments in the diode reduced to the 1% contrast (this operation takes into account that the number of recombination centers is roughly proportional to the EBIC contrast for the contrast values lower than 10% [7,8]). Both dependences are rather close to the linear trends shown in Fig. 3 with straight lines. These trends give the values of Ns ¼ 3 1010 cm2 and Nl ¼ 6 107 cm1 for densities of the majority carriers trapped by the trail with 1% EBIC contrast and dislocation, respectively. The latter value seems to be too high, although it could be agreed with the value extracted from the EBIC contrast assuming the low enough (o1015 cm2) capture cross-section for the minority carriers. However, the electrostatic barrier related with such density of carriers trapped in (or close to) dislocation core, should be essentially higher than the silicon band gap. Thus, the most part of the deep-level centers observed in the deformed samples cannot be associated with dislocations.
-5
5×10
0 -6
10
-5
10
-4
10
-3
10
tpulse, s Fig. 4. DLTS signal dependence on filling pulse duration.
The dependence on the filling pulse duration shows that the DLTS signal increases about 3 times in the range from 1 to 100 ms following the logarithmic law (Fig. 4). Such dependence, which was reported also for the similar (H.49) centers [1], is a fingerprint of the charged extended defects. It indicates that the defects revealed by DLTS are deep donors conglomerated in some extended defects. Assuming that the defects are uniformly
ARTICLE IN PRESS 1008
O.V. Feklisova et al. / Physica B 340–342 (2003) 1005–1008
distributed over the dislocation trail, the electrostatic barrier is estimated to vary from 0.07 to 0.3 eV for the trails with the EBIC contrast from 1% to 4%. On the other hand, the saturation at B100 ms shown in Fig. 4, results in the B0.14 eV barrier assuming the capture cross-section for majority carriers of B1015 cm2. This barrier is in a perfect agreement with that estimated from the EBIC contrast for the trails with average contrast. Besides, the density of captured carriers for the dislocation trails obtained by fitting the data presented in Fig. 3 well correlated with the sheet density estimated from the EBIC contrast value assuming the rather reasonable minority carrier capture cross-section of B1014 cm2. Speculating on the nature of defects formed in the dislocation trails, it could be noted that the DLTS peak position well correlates with that for the B1 defects observed in this crystal after implantation and annealing at 630 C [9]. The B1 defects are reported to be associated with the interstitial clusters [10]. Although the dependence of the DLTS signal on the filling pulse duration is different in the implanted and plastically deformed samples, this dissimilarity could be attributed to the different spatial distribution of the defects [9]. A confirmation of the effective self-interstitial generation during dislocation motion could be found in the experiments on plastic deformation of Si doped with Au [11,12]. It was observed that the concentration of gold atoms in the substitutional positions essentially decreased after deformation at relatively low (650 C) temperature, at which the concentration of equilibrium self-interstitials is too low for effective pushing the substitutional gold in the mobile interstitial position.
4. Conclusion In the plastically deformed p-type silicon, the number of recombination centers near the
dislocations and in the dislocation trails is estimated from the EBIC measurements. The total concentration of the centers with deep levels in the lower half of the gap is measured by DLTS. It is shown that the main part of the DLTS signal cannot be associated with dislocations. In opposite, the total concentration of the deep-level centers is closely correlated with the number of defects in the dislocation trails. Thus, the main part of the measured DLTS signal in plastically deformed p-type Si could be attributed to the defects in the dislocation trails. The similarity of these defects to the self-interstitial clusters observed in the ion-implanted Si allows to speculate about their self-interstitial nature.
Acknowledgements The work was supported by the INTAS program (INTAS-01-0194).
References [1] C. Kisielowski, E.R. Weber, Phys. Rev. B 44 (1991) 1600. [2] I.E. Bondarenko, et al., Phys. Stat. Sol. (a) 68 (1981) 53. [3] E.B. Yakimov, I.E. Bondarenko, N.A. Yarykin, Mater. Sci. Forum 10–12 (1986) 787. [4] I.E. Bondarenko, et al., Phys. Stat. Sol. (a) 95 (1986) 173. [5] I.E. Bondarenko, E.B. Yakimov, Phys. Stat. Sol. (a) 122 (1990) 121. [6] M. Kittler, C. Ulhaq-Bouillet, V. Higgs, Mater. Sci. Eng. B 24 (1994) 52. [7] C. Donolato, Semicond. Sci. Technol. 7 (1992) 37. [8] E.B. Yakimov, J. Phys.: Condens. Matter 14 (2002) 13069. [9] N. Yarykin, E. Steinman, in these Proceedings (ICDS-22), Physica B 340–342 (2004). [10] S. Libertino, S. Coffa, J.L. Benton, Phys. Rev. B 63 (2001) 195206. [11] V.V. Aristov, et al., Phys. Stat. Sol. (a) 102 (1987) 687. [12] B. Pichaud, G. Mariani-Regula, E.B. Yakimov, Mater. Sci. Eng. B 71 (2000) 272.
Physica B 340–342 (2003) 1005–1008
Contribution of the disturbed dislocation slip planes to the electrical properties of plastically deformed silicon O.V. Feklisova, E.B. Yakimov*, N. Yarykin Institute of Microelectronics Technology, Russian Academy of Sciences (RAS), Chernogolovka, Moscow Region 142432, Russia
Abstract The deep-level centers introduced by the plastic deformation at 680 C in p-type silicon are quantitatively studied by the electron beam induced current (EBIC) and DLTS techniques. It is shown that the DLTS signal is higher than that could be ascribed to the centers located at (or close to) dislocations. In opposite, the number of electrically active defects in the dislocation trails, which is estimated from the EBIC contrast, easily explains the DLTS signal. The possible nature of defects in the dislocation trails is discussed. r 2003 Elsevier B.V. All rights reserved. Keywords: Silicon; Dislocations; EBIC; DLTS
1. Introduction It is well known that in addition to dislocations a lot of other defects are introduced in silicon during plastic deformation [1]. The defects created by moving dislocations strongly disturb the properties of regions adjacent to the slip planes swept by dislocations [2,3], forming the dislocation trails. As the electron beam induced current (EBIC) investigations have shown [3–5], not only the dislocations but also the dislocation trails demonstrate the pronounced electrical activity. However, the nature of defects formed in the dislocation trails and their relative contribution to the electrical properties of plastically deformed Si is still unknown. One of the approaches for the solution of the latter problem is to correlate the
number of defects located near dislocations and in the dislocation trails with the total concentration of electrically active defects introduced during plastic deformation. In the present paper the EBIC and DLTS studies of plastically deformed p-type Si samples with different dislocation density are carried out. The concentration of defects in the dislocation trails is estimated from the EBIC images while the total concentration of deep-level centers in the sample is obtained by the DLTS measurements on the same Schottky diode. All the results obtained are consistent with the assumption that most of the defects revealed by the DLTS are located in the dislocation trails.
2. Experimental *Corresponding author. Tel.: +7-095-9628074; fax: +7-0959628047. E-mail address: [email protected] (E.B. Yakimov).
The experiments were carried out on the p-type dislocation-free Si grown by Czohralski method
0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.09.196
ARTICLE IN PRESS 1006
O.V. Feklisova et al. / Physica B 340–342 (2003) 1005–1008
and doped with boron to a concentration of about 1015 cm3. After the damage that resulted from the cutting procedure was removed by chemical etching, the dislocation sources were nucleated by scratching the {1 0 0} surface with a diamond tip. The samples were elastically bent at room temperature around a /1 1 0S axis in the cantilever mode using the setup made of clean ceramics. Then the stressed samples were annealed at 680 C for 2 h in argon atmosphere. To reveal the deformation-induced defects, the {1 0 0} surface was selectively etched in Schimmel solution. The Schottky barriers with a diameter of 2 mm for the EBIC and DLTS measurements were formed by thermal evaporation of Al. Under the deformation procedure used, the dislocation density ND changes along the sample from zero to 106 cm2 due to the linear variation of the resolved shear stress. In every diode, ND was obtained by counting the dislocation etch pits in several areas randomly distributed over the diode. Ohmic contacts were produced by rubbing the eutectic Al–Ga alloy in the backside of the samples. The EBIC measurements were carried out in the temperature range from 90 to 300 K in the scanning electron microscope JSM-840A (Jeol) using the Keithley 428 current amplifier. As usual, the contrast C of defects was determined as C ¼ 1 Ic =Ic0 ; where Ic and Ic0 are the collected current values measured with e-beam located at the defect and far from it, respectively. The deeplevel spectra were measured using the standard DLTS setup included a lock-in amplifier as a correlator. As a rule, filling pulse duration of 1 ms and a repetition rate of 20.8 s1 were used.
3. Results and discussion The typical EBIC image of the structure under study presented in Fig. 1 shows the dark broken lines parallel to the acting slip planes. It could be seen that the dark segments of these lines are not associated with dislocations, which are displayed as bright points due to the topography contrast of dislocation etch pits. Thus, the dark lines in the EBIC image could be associated with the recombination centers formed behind dislocations in the
Fig. 1. Typical image of studied Schottky diode in the EBIC mode. Image size is 500 500 mm.
quasi-two-dimensional dislocation trails [3,4]. It is seen in Fig. 1 that the recombination contrast of the dislocation trails varies from one segment to another. The maximum contrast reaches B4% at room temperature and slightly increases at 90 K, while the contrast associated with dislocations themselves is lower than 1% in the whole temperature range from 90 to 300 K. In accordance with Ref. [6], the low dislocation contrast could be considered as an indication of a relatively clean deformation conditions. It is well known [7] that the product of trap concentration near the extended defect and minority carrier capture cross-section s can be obtained from the EBIC contrast. For our samples, we find sNs ¼ 2:5 104 for the dislocation trail with the contrast of 1% (Ns is the sheet defect density) and sNl o107 cm for dislocations (Nl is the linear defect density). The DLTS spectra measured on the diodes with different average dislocation density ND are presented in Fig. 2. The spectra consist of one broad peak with a maximum at about 235 K and with the amplitude increasing with ND : The DLTS spectra obtained in earlier works on the plastically deformed p-type Si with higher ND (see, e.g. Ref. [1]) look different. Nevertheless, they contain
ARTICLE IN PRESS O.V. Feklisova et al. / Physica B 340–342 (2003) 1005–1008
ND, cm-2
5
-4
2.0×10
103 -4
104
105
106
100
101
102
4
1.5×10
1013
-4
1.0×10
3 2
-5
5.0×10
Nt, cm-3
∆C/C
1007
1
0,0 50
100
150
200
250
1012
300
T, K
1011
Fig. 2. The DLTS spectra measured on the diodes with the dislocation density ND ¼ 0 (1), 3 104 (2), 6 104 (3), 1.4 105 (4), 3 105 cm2 (5).
10-1
Effective dislocation trail length, cm-1 Fig. 3. Deep-level center concentration plotted as a function of dislocation density (open circles) and of total effective length of dislocation trails (solid squares).
-4
1×10
∆C/C
the peak (H.49) similar to that observed in the present work. Because of broadening, the concentration of deep-level centers is calculated by integration of the peak area and a comparison with the area of the peak simulated for the point defect with close parameters. The concentration obtained in this way is plotted in Fig. 3 as a function of either the dislocation density ND or the effective length of the dislocation trails. The latter is just the total length of dark segments in the diode reduced to the 1% contrast (this operation takes into account that the number of recombination centers is roughly proportional to the EBIC contrast for the contrast values lower than 10% [7,8]). Both dependences are rather close to the linear trends shown in Fig. 3 with straight lines. These trends give the values of Ns ¼ 3 1010 cm2 and Nl ¼ 6 107 cm1 for densities of the majority carriers trapped by the trail with 1% EBIC contrast and dislocation, respectively. The latter value seems to be too high, although it could be agreed with the value extracted from the EBIC contrast assuming the low enough (o1015 cm2) capture cross-section for the minority carriers. However, the electrostatic barrier related with such density of carriers trapped in (or close to) dislocation core, should be essentially higher than the silicon band gap. Thus, the most part of the deep-level centers observed in the deformed samples cannot be associated with dislocations.
-5
5×10
0 -6
10
-5
10
-4
10
-3
10
tpulse, s Fig. 4. DLTS signal dependence on filling pulse duration.
The dependence on the filling pulse duration shows that the DLTS signal increases about 3 times in the range from 1 to 100 ms following the logarithmic law (Fig. 4). Such dependence, which was reported also for the similar (H.49) centers [1], is a fingerprint of the charged extended defects. It indicates that the defects revealed by DLTS are deep donors conglomerated in some extended defects. Assuming that the defects are uniformly
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O.V. Feklisova et al. / Physica B 340–342 (2003) 1005–1008
distributed over the dislocation trail, the electrostatic barrier is estimated to vary from 0.07 to 0.3 eV for the trails with the EBIC contrast from 1% to 4%. On the other hand, the saturation at B100 ms shown in Fig. 4, results in the B0.14 eV barrier assuming the capture cross-section for majority carriers of B1015 cm2. This barrier is in a perfect agreement with that estimated from the EBIC contrast for the trails with average contrast. Besides, the density of captured carriers for the dislocation trails obtained by fitting the data presented in Fig. 3 well correlated with the sheet density estimated from the EBIC contrast value assuming the rather reasonable minority carrier capture cross-section of B1014 cm2. Speculating on the nature of defects formed in the dislocation trails, it could be noted that the DLTS peak position well correlates with that for the B1 defects observed in this crystal after implantation and annealing at 630 C [9]. The B1 defects are reported to be associated with the interstitial clusters [10]. Although the dependence of the DLTS signal on the filling pulse duration is different in the implanted and plastically deformed samples, this dissimilarity could be attributed to the different spatial distribution of the defects [9]. A confirmation of the effective self-interstitial generation during dislocation motion could be found in the experiments on plastic deformation of Si doped with Au [11,12]. It was observed that the concentration of gold atoms in the substitutional positions essentially decreased after deformation at relatively low (650 C) temperature, at which the concentration of equilibrium self-interstitials is too low for effective pushing the substitutional gold in the mobile interstitial position.
4. Conclusion In the plastically deformed p-type silicon, the number of recombination centers near the
dislocations and in the dislocation trails is estimated from the EBIC measurements. The total concentration of the centers with deep levels in the lower half of the gap is measured by DLTS. It is shown that the main part of the DLTS signal cannot be associated with dislocations. In opposite, the total concentration of the deep-level centers is closely correlated with the number of defects in the dislocation trails. Thus, the main part of the measured DLTS signal in plastically deformed p-type Si could be attributed to the defects in the dislocation trails. The similarity of these defects to the self-interstitial clusters observed in the ion-implanted Si allows to speculate about their self-interstitial nature.
Acknowledgements The work was supported by the INTAS program (INTAS-01-0194).
References [1] C. Kisielowski, E.R. Weber, Phys. Rev. B 44 (1991) 1600. [2] I.E. Bondarenko, et al., Phys. Stat. Sol. (a) 68 (1981) 53. [3] E.B. Yakimov, I.E. Bondarenko, N.A. Yarykin, Mater. Sci. Forum 10–12 (1986) 787. [4] I.E. Bondarenko, et al., Phys. Stat. Sol. (a) 95 (1986) 173. [5] I.E. Bondarenko, E.B. Yakimov, Phys. Stat. Sol. (a) 122 (1990) 121. [6] M. Kittler, C. Ulhaq-Bouillet, V. Higgs, Mater. Sci. Eng. B 24 (1994) 52. [7] C. Donolato, Semicond. Sci. Technol. 7 (1992) 37. [8] E.B. Yakimov, J. Phys.: Condens. Matter 14 (2002) 13069. [9] N. Yarykin, E. Steinman, in these Proceedings (ICDS-22), Physica B 340–342 (2004). [10] S. Libertino, S. Coffa, J.L. Benton, Phys. Rev. B 63 (2001) 195206. [11] V.V. Aristov, et al., Phys. Stat. Sol. (a) 102 (1987) 687. [12] B. Pichaud, G. Mariani-Regula, E.B. Yakimov, Mater. Sci. Eng. B 71 (2000) 272.