- Email: [email protected]
Observation of vacancy clustering in FZ-Si crystals during in situ electron irradiation in a high voltage electron microscope
Nuclear Instruments and Methods in Physics Research B 112 (1996) 133-138
Brm Interactions with Materials&Atoms
ELSEVIER
Observation of vacancy clustering in FZ-Si crystals during in situ electron irradiation in a high voltage electron microscope L. Fedina a2c3 * , J. Van Landuyt
a, J. Vanhellemont
b, A.L. Aseev ’
a Uniuersiryof Antwerp (RCJCA). Groenenborgerlaan 171, B-2020 Antwerp. Belgium b IMEC, Kapeldreef 75. B-3001 Leuven. Belgium ’ Institute of Semiconductor Physics, pr. ak. Lavrentjeva 13, 630090. Novosibirsk. Russian Federation
Abstract Vacancy accumulation in pure FZ-Si crystals covered with thin Si,N, films leads upon irradiation with a high intensity electron beam in a high voltage electron microscope (HVEM) at temperatures between 20 and 250°C, to the suppression of the formation of (113)-defects of interstitial type in the central part of the irradiated area. Very small secondary clusters of interstitial type with a density of about IO” cmm2 appear, however, in this area after prolonged irradiation. Small stacking fault tetrahedra and Frank loops of vacancy type connected with interstitial clusters were analysed on an atomic scale in the thinnest part of the irradiated area in a 400 keV high resolution electron microscope. Vacancy clusters might act as nucleation centres for the observed secondary defect formation.
1. Introduction High fluxes of 1 MeV electrons in a high voltage electron microscope (HvEM) create a supersaturation of self-interstitials in Si crystals covered by thin films of SiO, or Si3N, [ 11. These self-interstitials cluster in the so-called {113}-defects. It was found that the state of impurity atoms such as oxygen, carbon and dopant atoms (P,B) plays an important role during the formation of the {I 13}-defects in Si crystals [ 1.21. The formation of irradiation-induced {I 13)-defects is different between pure floating zone (FZ) and Czochralski (Cz) Si [l-4] because the former has a low concentration of impurity atoms. It was concluded that only defects related to the impinging impurity atoms from the irradiated surface are observed at temperatures from room temperature to 400°C in FZ-Si [2]. To exclude the effect of impurity atoms, introduced in the electron microscope to the surface of the specimen, on the defect formation, the present study was carried out on FZ-Si specimens covered with a thin Si,N, coating to prevent indiffusion of impurities after TEM specimen preparation.
2. Experiments The specimens (110) orientation
were prepared from a FZ-Si crystal in containing 1 X lOI cme3 interstitial
* Corresponding author. Tel. + 32 3 2180259, fax + 32 3 2180257, e-mail [email protected].
oxygen atoms, 3 X 10” crnv3 substitutional carbon atoms and 3 X lOI cms3 substitutional phosphorous atoms. The thinned Si specimens were covered with Si,N, films of 5-15 nm thickness. In situ observation using the high-voltage electron microscope JEOL-1250 at Antwerp University (RUCA) was carried out in a (Ill} dynamical contrast mode in the temperature range of 20-450°C at 1 MeV. The intensities of irradiation were between 3 X IO’* and 3 X lOI electrons/cm2 s. The temperature control during in situ experiments was performed by thermocouple measurement on the specimen cup. Some of the specimens were irradiated and investigated by the JEM-4000-EX high resolution electron microscope operated at 400 keV at the Institute of Semiconductor Physics (Novosibirsk, Russia).
3. Results The defect creation in FZ-Si crystals covered with Si,N, films during electron irradiation in the temperature range 20-250°C depends on the electron beam intensity in the central part of the irradiated area. The density of the defects in this area decreases from about 10” cme2 up to complete suppression by increasing the intensity of irradiation from 3 X IO’* to 3 X lOI electrons/cm2 s, as shown in Fig. 1. The incubation time for nucleation of these initially formed defects is slightly increased by increasing the beam intensity for irradiation temperatures up to 300°C as shown in Fig. 2a. After about 10 min of irradiation with maximum beam intensity secondary (very small) clusters
0168-583X/%/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDlOl68-583X(95)01277-X
VI. SILICON-DEFECTS/DAMAGE
L. Fedinu et al./Nucl.
134
Fig. 1. The defects created in FZ-Si covered intensity 1=6X IO’* ecm-2s-’ (a)and3X
Instr. and Meth. in Phys. Res. B II2 (1996) 133-138
with a 50 nm Si,N, lOI9 ecm-2s-‘(b).
film after a 10 min in situ irradiation
of point defects also appear in the central part of the irradiated area (Fig. 3). The density of these secondary defects increases upon prolonged irradiation (45-120 min) at a dose of about 10” cm-*. The incubation time for the creation of these defects was found to be strongly dependent on the electron beam intensity as also shown in Fig. 2a. Both the initial and the secondary defects are of the interstitial type and thus reveal a supersaturation of self-interstitials. This two-step nucleation mechanism has also been reported elsewhere but without real understanding of why the secondary defects nucleate (e.g. Refs. [1,2]). Irradiation at temperatures above 300°C leads to the formation of the (113}-defects which do not disappe.. even at higher intensity of irradiation. The incubation time for the nucleation of these defects is also decreased by increasing the irradiation intensity in the available range (see Fig. 2a). The creation rate of the initially formed defects at T= 210-250 (curve 2) and at T- 310-350°C (curve 1) as a function of the electron beam intensity is shown in Fig. 2b. Curve (2) illustrates the decrease of initial defects with increasing electron beam intensity, while curve (1) illustrates that the initial defects formed at higher temperature are stable.
in HVEM at 250°C and with an
The two dark field micrographs in Fig. 4 show the changes in size and defect density for changing thickness of the wedge-shaped TEM specimens irradiated at T= 250°C in the central part. It can be seen in Fig. 4a that in the thinnest part of the irradiated area (first extinction band), only very small defects with black-white contrast are observed. The mixing of these and the {113)-defects is observed in the thicker part of the third extinction contour (Fig. 4b). The small size of the defects (up to about 2 nm) makes it difficult to determine the type of the created defects. Therefore the same type of specimens were irradiated and investigated on an atomic scale in a 400 keV high resolution electron microscope. Fig. 5 shows various types of defects found in the thinnest part of FZ-Si crystals during irradiation in a high resolution electron microscope. From Fig. 5c we can conclude that the image of one plane of the V-shaped defect differs from the image of the other plane. It can be clearly seen that the first one is of the vacancy prismatic type and the second one is a non-prismatic defect. An atomic model of the V-shaped defect based on the HREM-image is shown in Fig. 5d. The model illustrates that the V-shaped defect corresponds to a Frank loop of the vacancy type connected with a Shockley
a bd
E
b
un-3.p
1
10” < 10’
3
-
10”
I IllI lo’s
I IIll
I I III 11)‘~ I,Ecm‘2 .s‘ ld9
;
,,,,,
2
<,, lo’s
1o19
10ZO I,&cm-2 .s-1
Fig. 2. The dependencies of the incubation time (a) and of the creation rate (b) of the defects on the intensity of the electron beam. In (a) 1 and 3 represent the data for the initial defects, while 2 represents the data for the secondary defects. Data 1 and 2 were obtained at 25O”C, while data 3 were obtained at 350°C. In (b) the rate of creation of initial defects is given at 250 (2) and at 350°C (1).
L. Fedina et al./Nucl.
Fig. 3. Formation
of the secondary
135
Insrr. and Merh. in Phys. Res. B 112 (1996) 133-138
defects in the central part of the irradiated
area. Irradiation
conditions
are 250°C.
ecm-* s- ’ for 6 (a) and 16 min (b).
dislocation gliding from the core of Frank dislocation. The defect appears to be the classical stacking fault tetrahedron of vacancy type which has one plane of prismatic vacancy defects perpendicular to the (110) surface of the specimen. Fig. 5b reveals an undissociated vacancy Frank loop which may be interconnected to an interstitial cluster close to the core of the dislocation. Another type of defect on the (11 l} plane, after prolonged irradiation (about 30 min), was found in the thicker irradiated areas (see Fig. 6a). The HREM-images of these defects distinguish them from the vacancy defects in the thinnest irradiated area. Very small distortions of the (111) planes parallel to the defect plane are observed, enabling the determination of the type of defects from the HREM-images. These images reveal regular sequences of complicated configurations of black and white dots. It can be seen from Fig. 6a that these defects can also be connected with {113) defects.
Fig. 4. Dark field electron micrographs of the secondary Irradiation conditions as in Fig. 3 for 45 min.
defects
4. Discussion The results reported here reveal a vacancy accumulation in FZ-Si crystals covered by Si,N, films during irradiation with a high intensity electron beam at temperatures ranging from 20 to 250°C. This leads to the suppression of the creation of interstitial clusters. Upon continued irradiation, vacancy clusters are formed in the shape of stacking fault tetrahedra and separated undissociated Frank loops. The decrease of the creation rate of interstitial clusters with increasing intensity of irradiation gives some evidence to conclude that free vacancies accumulate at high intensity in pure FZ-Si and may dissolve the nuclei of interstitial clusters before their appearance. After prolonged irradiation the accumulation of free vacancies leads to the formation of vacancy clusters in the thinnest part of the irradiated crystal in the form of stack-
in specimen
areas with thicknesses
of about
120 (a) and 360 nm (b).
VI. SILICON-DEFECTS/DAMAGE
136
L. Fedina et al./Nucl.
Instr. and Meth. in Phys. Res. B 112 (1996) 133-138
ing fault tetrahedra (SIT). It seems that the strain at the interface Si-Si,N, may play an important role in the relaxation of the deformed crystal area around the Frank dislocation core. We assume that the strain promotes the formation of a gliding Shockley dislocation. Indeed the stacking fault energy in Si is high enough for the mechanism of the formation of Sfl, well-known for metals (51, to be realised. There are few reports about SFT in Si, all of which concern the formation of Sfl in the top silicon layer between the surface and the amorphous buried layer
A much more complicated behaviour of vacancy clusters occurs in the thicker irradiated areas where the concentration of interstitials becomes higher and the process of point defect recombination must be strong. As recently found, the interstitial atoms may be located in (11 l} atomic plane in a metastable intermediate configuration if they interact with the core of interstitial Frank dislocation [9]. On the basis of the HREM-images, an atomic model of a transformed dislocation core has been developed including a regular sequence of double five-membered rings and single eight-membered atomic rings. as illustrated in Fig.
Fig. 5. HREM images of different vacancy type defects in FZ-Si formed during in situ electron irradiation: (a) SFT; (b) isolated Frank loops interco~ected by interstitial type defects (arrow); (c) high magnification of SFT (single arrow shows a prismatic vacancy loop while double arrow shows a Shockley dislocation); (d) structural model of SW.
L. Fedina et al./Nucl.
Instr. and Meth. in Phys. Res. B 112 (1996) 133-138
6b-6e. From the comparison of the HREM-images of the defects in the (111) plane shown in Fig. 6a with the transformed core of the Frank dislocation in Fig. 6b, we may assume that the core of the vacancy Frank dislocation can also interact with interstitial atoms. This process will lead to the filling up of the plane of the vacancy loops with interstitials having a similar configuration as in the core of the interstitial Frank dislocation (Fig. 6e). At the same time, the interaction of interstitials with stable vacancy loops may lead to the compensation of the strain around the vacancy loops and to the formation of a new configuration of interstitial atoms in the (111) planes without full recombination of the extended defects. This assumption agrees well with the low probability for insertion of inter-
137
stitial atoms to the lattice points in the dislocation loop planes due to the energy barrier at low temperatures [lo]. For this reason the interstitial atoms do form a metastable intermediate configuration on the different planes as observed. 5. Conclusion In situ high-voltage and high-resolution electron microscopy of FZ-Si specimens gives rise to characteristic secondary defects of both vacancy and interstitial type which are probably related to vacancy clustering during prolonged irradiation at temperatures below 250°C. Vacancy accumulation leads to the formation of stacking fault
Fig. 6. HREM images of interstitial type defects formed on a (111) plane (single arrow) and on a (113) plane (double arrow) in I%-% (a) and transformation of the dislocation conz of a pre-existing Frank partial (b) during in situ electron irradiation in an HREM at room temperature. Calculated (c) and experimentally observed (d) HREM images of a part of the transformed core. In (e) a structural model is superimposed on the experimental image 191.
VI. SILICON-DEFECTS
/DAMAGE
138
L. Fedina et al./Nucl.
Instr. and Meth. in Phys. Res. B I12 (1996) 133-138
tetrahedra and separated Frank loops in the thinnest part of the irradiated area. In the thicker part of the irradiated area the interaction of interstitials with stable vacancy loops may lead to the formation of new configurations of interstitials in the plane of the vacancy loops. The vacancy clusters might thus act as nucleation centres for the secondary interstitial type defects.
Acknowledgements
The authors would like to thank Dr. A. Gutakovski for valuable discussions and assistance with experiments in the high resolution electron microscopy in Novosibirsk. One of us (L.F.) is grateful for her research fellowship from the Belgian Federal Office for Scientific, Technical and Cultural Affairs. J.V.L and J.V gratefully acknowledge the financial support of the Belgian National Science Foundation (NFWO).
References [l] A. Aseev, L. Fedina, D. Hoehl and H. Barsch, Clusters of Interstitial Atoms in Si and Ge (Academy, Berlin, 1994). [21 R. Oshima and G.C. Hua, Ultramicroscopy 39 (1991) 160. [3] M. Kuwabara, H. Endoh, Y. Tsubokawa, H. Hashimoto, Y. Yokota and R. Shimuzi, In Situ Experiments with HVEM, ed. H. Fujita (Osaka University, Osaka, 1985) p. 433. [4] M. Hirata and M. Kiritani, Physica B 116 (1983) 616. [5] J. Silcox and P.B. Hirsch, Philos. Mag. 4 (1958) 72. [6] D.K. Sadana and 1. Washburn, Philos. Mag. B 46 (19821 611. [7] W. Coene, H. Bender and S. Amelinkx, Philos. Mag. 52 (1985) 369. [8] G. Nabert and H.-U. Habermeier, Appl. Phys. Lett. 58 (1991) 1074. [9] L. Fedina, A. Gutakovski and A. Chuvilin. ICEM-13, 2A (1994) 99. [lo] L. Fedina and A. Aseev, Phys. Status Solidi (al 95 (1986) 517.
Brm Interactions with Materials&Atoms
ELSEVIER
Observation of vacancy clustering in FZ-Si crystals during in situ electron irradiation in a high voltage electron microscope L. Fedina a2c3 * , J. Van Landuyt
a, J. Vanhellemont
b, A.L. Aseev ’
a Uniuersiryof Antwerp (RCJCA). Groenenborgerlaan 171, B-2020 Antwerp. Belgium b IMEC, Kapeldreef 75. B-3001 Leuven. Belgium ’ Institute of Semiconductor Physics, pr. ak. Lavrentjeva 13, 630090. Novosibirsk. Russian Federation
Abstract Vacancy accumulation in pure FZ-Si crystals covered with thin Si,N, films leads upon irradiation with a high intensity electron beam in a high voltage electron microscope (HVEM) at temperatures between 20 and 250°C, to the suppression of the formation of (113)-defects of interstitial type in the central part of the irradiated area. Very small secondary clusters of interstitial type with a density of about IO” cmm2 appear, however, in this area after prolonged irradiation. Small stacking fault tetrahedra and Frank loops of vacancy type connected with interstitial clusters were analysed on an atomic scale in the thinnest part of the irradiated area in a 400 keV high resolution electron microscope. Vacancy clusters might act as nucleation centres for the observed secondary defect formation.
1. Introduction High fluxes of 1 MeV electrons in a high voltage electron microscope (HvEM) create a supersaturation of self-interstitials in Si crystals covered by thin films of SiO, or Si3N, [ 11. These self-interstitials cluster in the so-called {113}-defects. It was found that the state of impurity atoms such as oxygen, carbon and dopant atoms (P,B) plays an important role during the formation of the {I 13}-defects in Si crystals [ 1.21. The formation of irradiation-induced {I 13)-defects is different between pure floating zone (FZ) and Czochralski (Cz) Si [l-4] because the former has a low concentration of impurity atoms. It was concluded that only defects related to the impinging impurity atoms from the irradiated surface are observed at temperatures from room temperature to 400°C in FZ-Si [2]. To exclude the effect of impurity atoms, introduced in the electron microscope to the surface of the specimen, on the defect formation, the present study was carried out on FZ-Si specimens covered with a thin Si,N, coating to prevent indiffusion of impurities after TEM specimen preparation.
2. Experiments The specimens (110) orientation
were prepared from a FZ-Si crystal in containing 1 X lOI cme3 interstitial
* Corresponding author. Tel. + 32 3 2180259, fax + 32 3 2180257, e-mail [email protected].
oxygen atoms, 3 X 10” crnv3 substitutional carbon atoms and 3 X lOI cms3 substitutional phosphorous atoms. The thinned Si specimens were covered with Si,N, films of 5-15 nm thickness. In situ observation using the high-voltage electron microscope JEOL-1250 at Antwerp University (RUCA) was carried out in a (Ill} dynamical contrast mode in the temperature range of 20-450°C at 1 MeV. The intensities of irradiation were between 3 X IO’* and 3 X lOI electrons/cm2 s. The temperature control during in situ experiments was performed by thermocouple measurement on the specimen cup. Some of the specimens were irradiated and investigated by the JEM-4000-EX high resolution electron microscope operated at 400 keV at the Institute of Semiconductor Physics (Novosibirsk, Russia).
3. Results The defect creation in FZ-Si crystals covered with Si,N, films during electron irradiation in the temperature range 20-250°C depends on the electron beam intensity in the central part of the irradiated area. The density of the defects in this area decreases from about 10” cme2 up to complete suppression by increasing the intensity of irradiation from 3 X IO’* to 3 X lOI electrons/cm2 s, as shown in Fig. 1. The incubation time for nucleation of these initially formed defects is slightly increased by increasing the beam intensity for irradiation temperatures up to 300°C as shown in Fig. 2a. After about 10 min of irradiation with maximum beam intensity secondary (very small) clusters
0168-583X/%/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDlOl68-583X(95)01277-X
VI. SILICON-DEFECTS/DAMAGE
L. Fedinu et al./Nucl.
134
Fig. 1. The defects created in FZ-Si covered intensity 1=6X IO’* ecm-2s-’ (a)and3X
Instr. and Meth. in Phys. Res. B II2 (1996) 133-138
with a 50 nm Si,N, lOI9 ecm-2s-‘(b).
film after a 10 min in situ irradiation
of point defects also appear in the central part of the irradiated area (Fig. 3). The density of these secondary defects increases upon prolonged irradiation (45-120 min) at a dose of about 10” cm-*. The incubation time for the creation of these defects was found to be strongly dependent on the electron beam intensity as also shown in Fig. 2a. Both the initial and the secondary defects are of the interstitial type and thus reveal a supersaturation of self-interstitials. This two-step nucleation mechanism has also been reported elsewhere but without real understanding of why the secondary defects nucleate (e.g. Refs. [1,2]). Irradiation at temperatures above 300°C leads to the formation of the (113}-defects which do not disappe.. even at higher intensity of irradiation. The incubation time for the nucleation of these defects is also decreased by increasing the irradiation intensity in the available range (see Fig. 2a). The creation rate of the initially formed defects at T= 210-250 (curve 2) and at T- 310-350°C (curve 1) as a function of the electron beam intensity is shown in Fig. 2b. Curve (2) illustrates the decrease of initial defects with increasing electron beam intensity, while curve (1) illustrates that the initial defects formed at higher temperature are stable.
in HVEM at 250°C and with an
The two dark field micrographs in Fig. 4 show the changes in size and defect density for changing thickness of the wedge-shaped TEM specimens irradiated at T= 250°C in the central part. It can be seen in Fig. 4a that in the thinnest part of the irradiated area (first extinction band), only very small defects with black-white contrast are observed. The mixing of these and the {113)-defects is observed in the thicker part of the third extinction contour (Fig. 4b). The small size of the defects (up to about 2 nm) makes it difficult to determine the type of the created defects. Therefore the same type of specimens were irradiated and investigated on an atomic scale in a 400 keV high resolution electron microscope. Fig. 5 shows various types of defects found in the thinnest part of FZ-Si crystals during irradiation in a high resolution electron microscope. From Fig. 5c we can conclude that the image of one plane of the V-shaped defect differs from the image of the other plane. It can be clearly seen that the first one is of the vacancy prismatic type and the second one is a non-prismatic defect. An atomic model of the V-shaped defect based on the HREM-image is shown in Fig. 5d. The model illustrates that the V-shaped defect corresponds to a Frank loop of the vacancy type connected with a Shockley
a bd
E
b
un-3.p
1
10” < 10’
3
-
10”
I IllI lo’s
I IIll
I I III 11)‘~ I,Ecm‘2 .s‘ ld9
;
,,,,,
2
<,, lo’s
1o19
10ZO I,&cm-2 .s-1
Fig. 2. The dependencies of the incubation time (a) and of the creation rate (b) of the defects on the intensity of the electron beam. In (a) 1 and 3 represent the data for the initial defects, while 2 represents the data for the secondary defects. Data 1 and 2 were obtained at 25O”C, while data 3 were obtained at 350°C. In (b) the rate of creation of initial defects is given at 250 (2) and at 350°C (1).
L. Fedina et al./Nucl.
Fig. 3. Formation
of the secondary
135
Insrr. and Merh. in Phys. Res. B 112 (1996) 133-138
defects in the central part of the irradiated
area. Irradiation
conditions
are 250°C.
ecm-* s- ’ for 6 (a) and 16 min (b).
dislocation gliding from the core of Frank dislocation. The defect appears to be the classical stacking fault tetrahedron of vacancy type which has one plane of prismatic vacancy defects perpendicular to the (110) surface of the specimen. Fig. 5b reveals an undissociated vacancy Frank loop which may be interconnected to an interstitial cluster close to the core of the dislocation. Another type of defect on the (11 l} plane, after prolonged irradiation (about 30 min), was found in the thicker irradiated areas (see Fig. 6a). The HREM-images of these defects distinguish them from the vacancy defects in the thinnest irradiated area. Very small distortions of the (111) planes parallel to the defect plane are observed, enabling the determination of the type of defects from the HREM-images. These images reveal regular sequences of complicated configurations of black and white dots. It can be seen from Fig. 6a that these defects can also be connected with {113) defects.
Fig. 4. Dark field electron micrographs of the secondary Irradiation conditions as in Fig. 3 for 45 min.
defects
4. Discussion The results reported here reveal a vacancy accumulation in FZ-Si crystals covered by Si,N, films during irradiation with a high intensity electron beam at temperatures ranging from 20 to 250°C. This leads to the suppression of the creation of interstitial clusters. Upon continued irradiation, vacancy clusters are formed in the shape of stacking fault tetrahedra and separated undissociated Frank loops. The decrease of the creation rate of interstitial clusters with increasing intensity of irradiation gives some evidence to conclude that free vacancies accumulate at high intensity in pure FZ-Si and may dissolve the nuclei of interstitial clusters before their appearance. After prolonged irradiation the accumulation of free vacancies leads to the formation of vacancy clusters in the thinnest part of the irradiated crystal in the form of stack-
in specimen
areas with thicknesses
of about
120 (a) and 360 nm (b).
VI. SILICON-DEFECTS/DAMAGE
136
L. Fedina et al./Nucl.
Instr. and Meth. in Phys. Res. B 112 (1996) 133-138
ing fault tetrahedra (SIT). It seems that the strain at the interface Si-Si,N, may play an important role in the relaxation of the deformed crystal area around the Frank dislocation core. We assume that the strain promotes the formation of a gliding Shockley dislocation. Indeed the stacking fault energy in Si is high enough for the mechanism of the formation of Sfl, well-known for metals (51, to be realised. There are few reports about SFT in Si, all of which concern the formation of Sfl in the top silicon layer between the surface and the amorphous buried layer
A much more complicated behaviour of vacancy clusters occurs in the thicker irradiated areas where the concentration of interstitials becomes higher and the process of point defect recombination must be strong. As recently found, the interstitial atoms may be located in (11 l} atomic plane in a metastable intermediate configuration if they interact with the core of interstitial Frank dislocation [9]. On the basis of the HREM-images, an atomic model of a transformed dislocation core has been developed including a regular sequence of double five-membered rings and single eight-membered atomic rings. as illustrated in Fig.
Fig. 5. HREM images of different vacancy type defects in FZ-Si formed during in situ electron irradiation: (a) SFT; (b) isolated Frank loops interco~ected by interstitial type defects (arrow); (c) high magnification of SFT (single arrow shows a prismatic vacancy loop while double arrow shows a Shockley dislocation); (d) structural model of SW.
L. Fedina et al./Nucl.
Instr. and Meth. in Phys. Res. B 112 (1996) 133-138
6b-6e. From the comparison of the HREM-images of the defects in the (111) plane shown in Fig. 6a with the transformed core of the Frank dislocation in Fig. 6b, we may assume that the core of the vacancy Frank dislocation can also interact with interstitial atoms. This process will lead to the filling up of the plane of the vacancy loops with interstitials having a similar configuration as in the core of the interstitial Frank dislocation (Fig. 6e). At the same time, the interaction of interstitials with stable vacancy loops may lead to the compensation of the strain around the vacancy loops and to the formation of a new configuration of interstitial atoms in the (111) planes without full recombination of the extended defects. This assumption agrees well with the low probability for insertion of inter-
137
stitial atoms to the lattice points in the dislocation loop planes due to the energy barrier at low temperatures [lo]. For this reason the interstitial atoms do form a metastable intermediate configuration on the different planes as observed. 5. Conclusion In situ high-voltage and high-resolution electron microscopy of FZ-Si specimens gives rise to characteristic secondary defects of both vacancy and interstitial type which are probably related to vacancy clustering during prolonged irradiation at temperatures below 250°C. Vacancy accumulation leads to the formation of stacking fault
Fig. 6. HREM images of interstitial type defects formed on a (111) plane (single arrow) and on a (113) plane (double arrow) in I%-% (a) and transformation of the dislocation conz of a pre-existing Frank partial (b) during in situ electron irradiation in an HREM at room temperature. Calculated (c) and experimentally observed (d) HREM images of a part of the transformed core. In (e) a structural model is superimposed on the experimental image 191.
VI. SILICON-DEFECTS
/DAMAGE
138
L. Fedina et al./Nucl.
Instr. and Meth. in Phys. Res. B I12 (1996) 133-138
tetrahedra and separated Frank loops in the thinnest part of the irradiated area. In the thicker part of the irradiated area the interaction of interstitials with stable vacancy loops may lead to the formation of new configurations of interstitials in the plane of the vacancy loops. The vacancy clusters might thus act as nucleation centres for the secondary interstitial type defects.
Acknowledgements
The authors would like to thank Dr. A. Gutakovski for valuable discussions and assistance with experiments in the high resolution electron microscopy in Novosibirsk. One of us (L.F.) is grateful for her research fellowship from the Belgian Federal Office for Scientific, Technical and Cultural Affairs. J.V.L and J.V gratefully acknowledge the financial support of the Belgian National Science Foundation (NFWO).
References [l] A. Aseev, L. Fedina, D. Hoehl and H. Barsch, Clusters of Interstitial Atoms in Si and Ge (Academy, Berlin, 1994). [21 R. Oshima and G.C. Hua, Ultramicroscopy 39 (1991) 160. [3] M. Kuwabara, H. Endoh, Y. Tsubokawa, H. Hashimoto, Y. Yokota and R. Shimuzi, In Situ Experiments with HVEM, ed. H. Fujita (Osaka University, Osaka, 1985) p. 433. [4] M. Hirata and M. Kiritani, Physica B 116 (1983) 616. [5] J. Silcox and P.B. Hirsch, Philos. Mag. 4 (1958) 72. [6] D.K. Sadana and 1. Washburn, Philos. Mag. B 46 (19821 611. [7] W. Coene, H. Bender and S. Amelinkx, Philos. Mag. 52 (1985) 369. [8] G. Nabert and H.-U. Habermeier, Appl. Phys. Lett. 58 (1991) 1074. [9] L. Fedina, A. Gutakovski and A. Chuvilin. ICEM-13, 2A (1994) 99. [lo] L. Fedina and A. Aseev, Phys. Status Solidi (al 95 (1986) 517.