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On the structure of ion-implanted bicrystalline gold films
Thin Solid Films, 90 (1982) 17-83 PREPARATION
ON THE STRUCTURE FILMS * A. GREENBERG,
77
AND CHARACTERIZATION
OF ION-IMPLANTED
BICRYSTALLINE
GOLD
Y. KOMEM AND C. L. BALJERT
Department of Materials Engineering, Technion, Israel Institute of Technology, Haifa 32000 (Israel) (Received September
2,198l;
accepted
September
22,198l)
Bicrystalline thin films of gold containing 4”, 16” and 18” [OOl] tilt boundaries were implanted at room temperature with cobalt, chromium, gold and lead at an energy of 40 keV with integrated fluxes ranging from 5 x lOi to 100 x 1Ol3 ions cme2 and were subsequently examined by transmission electron microscopy. In all cases, small dislocation loops are produced by the implantation process; in addition, the discrete strain contrast pattern associated with dislocations in low angle boundaries is obliterated by implantation with cobalt or chromium but not by implantation with gold or lead under identical conditions. It is concluded that cobalt and chromium segregate at grain boundaries, thereby distorting the lattice and obliterating the discrete strain contrast pattern associated with individual dislocations, whereas gold cannot distort the lattice and lead, being totally immiscible in gold, segregates at free surfaces. Moreover, under large integrated fluxes, new grains are nucleated at and expand parallel to the original grain boundary during implantation with cobalt. Such nucleation and anisotropic growth is attributed to intrinsic grain boundary migration in the depletion zone surrounding the original boundary.
1. INTRODUCTION
Carefully doped surface layers are routinely produced by ion implantation in semiconductor devices but, because of the small penetration depth’, this technique is not generally applicable to the production of alloys. Even in the few cases where properties of alloys produced by ion implantation have been investigated2, the microstructure near grain boundaries was not examined as a function of the concentration and species of the implanted ions. The purpose of this particular investigation was to implant specially prepared bicrystalline thin films of gold with
* Paper presented at the Fifth International 21-25, 1981. t Department of Metallurgical Engineering Pittsburgh, PA 15213, U.S.A. 0040-6090/82/0000-OOOO/ooo/$o2.75
Thin Films Congress, and
Materials
Herzlia-on-Sea,
Science,
Israel,
Carnegie-Mellon
0 Elsevier Sequoia/Printed
September University,
in The Netherlands
78
A. GREENBERG, Y. KOMEM, C. L. BAUER
cobalt, chromium, gold and lead and to characterize the resultant microstructure near preselected [OOl] tilt boundaries by transmission electron microscopy (TEM). 2.
EXPERIMENTAL PROCEDURE
Bicrystalline thin films of gold, containing grain boundaries characterized by angles of misorientation of 4, 16” and 18” about the common [OOl] axis, were produced by vapour deposition and subsequent epitaxial growth on an intermediate bicrystalline thin film of silver, produced by a similar process of vapour deposition and epitaxial growth on bicrystalline substrates of NaCl. The gold films, measuring about lo-20nm in thickness, were then removed from their substrates by dissolution of the NaCl and silver in distilled water and dilute nitric acid respectively and were mounted on grids for subsequent implantation with ions of cobalt, chromium, lead and gold perpendicular to the (001) surface and in a manner designed to minimize specimen heating. The implantation energy was 40 keV and integrated fluxes ranged from 5 x 1013 to 100 x 1Ol3ions cm-‘, which areequivalent to ion penetrations ranging from 9 to 15 nm and concentrations ranging from 0.06 to 1.2 at.% of the implanted species. The resultant microstructure near the original grain boundary was then examined by TEM. Further details concerning specimen preparation and grain boundary characterization are presented elsewhere3,4. 3.
EXPERIMENTAL RESULTS
Electron micrographs before and after implantation were taken along the same tilt boundary but not from the same section. However, many observations were made by TEM and typical microstructures near 4”, 16” and 18” [OOl] tilt boundaries in gold prior and subsequent to implantation with cobalt at 40 keV and 5 x 1013 ions cm-’ 10 x 1Ol3 ions cm-’ and 100 x 1013 ions cm-’ are presented in Figs. I,2 and 3 respectively. It is evident that the discrete strain contrast patterns associated with dislocations in the 4” and 16” grain boundaries (cfi arrows in Figs. l(a) and 2(a)) are obliterated by the implantation process (c$ arrows in Figs. l(b) and 2(b)). Similar effects were observed for implantation with chromium under identical conditions. In addition, small dislocation loops associated with the radiation damage produced by the implantation process can be observed. At integrated fluxes greater than about 1Ol4 ions cm-‘, a third grain is observed to nucleate at and expand parallel to the original grain boundary. A typical example is presented in Fig. 3 for the 18” boundary, wherein the new grain is revealed after an integrated flux of 1 x 1015 Co+ cm-’ by both bright and dark field TEM. According to the associated electron diffraction pattern in Fig. 3(c), the [Ol l] axis of the new grain is parallel to the original [OOl] axis; however, this particular orientation is not unique, since sometimes other axes of the new grain, e.g. (112) and (i14), were also found to be parallel to the original [OOl] axis. Bicrystalline thin films of gold were also implanted with gold and lead, and typical results are presented in Figs. 4(a) and 4(b) respectively for an integrated flux of 5 x 1013 ions cm-‘. In contrast with prior results for implantation with either cobalt or chromium, the discrete strain contrast patterns associated with dislocations at the original grain boundary are not obliterated under identical conditions, although the usual distribution of small dislocation loops is observed.
(b)
(b)
Fig. 1. Tilt boundary in a bicrystalline thin film of gold produced by a relative rotation of 4” about a common [OOl] axis: (a) before implantation;(b) after implantation with cobalt at an energy of 40 keV and an integrated flux of 5 x 1or3 co+ cn-r,
Fig. 2. Tilt boundary in a bicrystalline thin film of gold produced by a relative rotationof 16”about acommon [Ool] axis: (a) before implantation; (b) after implantation with cobalt at an energy of 40 keV and an integrated flux of 1 x 1Or4Co+ cm-‘.
64
(4
64
Fig. 3. Tilt boundary in a bicrystalhne thin film of gold produced by a relative rotation of 18” about a common [OOl] axis: (a) before implantation; (b), (c) bright field and dark field images after implantation with cobalt at an energy of 40 keV and an integrated flux of 1 x 1Or5 Co+ cm-‘. The associated electron diffraction pattern is also included in (c) wherein the (200) reflections from the original grains and the (111) reflection from the new grain are labelled.
STRUCTURE OF ION-IMPLANTED BICRYSTALLINE
Au FILMS
81
Fig. 4. Tilt boundary in a bicrystalline thin film of gold produced by a relative rotation of 4” about a common [OOl] axis after implantation with (a) gold and (b) lead at an energy of 40 keV with an integrated flux of 1 x 1Ol4ions cm-‘.
4. DISCUSSION OF RESULTS The two principal results of this investigation are (1) that the discrete strain contrast pattern associated with dislocations in low angle [OOl] tilt boundaries in gold is obliterated by implantation with cobalt or chromium but not by implantation with gold or lead under identical conditions and (2) that dynamic recrystallization occurs by nucleation of a new grain at and subsequent growth parallel to the original grain boundary during implantation of cobalt at room temperature and under relatively large integrated fluxes. These results, and others, are analysed in the following paragraphs. Obliteration of the strain contrast pattern in low angle [OOl] tilt boundaries in bicrystalline thin films of gold by cobalt implantation is attributed to local distortions produced by the short-range diffusion of cobalt in a supersaturated solid solution to these boundaries, assisted by a large (transient) density of intrinsic
82
A. GREENBERG, Y. KOMBM, C. L. BAUER
defects produced by the implantation process. This effect, however, is not observed for (self-)implantation with gold under identical conditions. In this particular case, the implanted ions may be assimilated into the lattice either by combination with irradiation-produced vacancies or by dislocation climb in the tilt boundary’, thus preserving the discrete strain contrast pattern. Implantation with lead under identical conditions also does not obliterate the discrete strain contrast pattern, even though the lattice distortion produced by lead is much greater than that produced by cobalt. Therefore, it must be concluded that lead does not segregate to dislocations in the grain boundary during the implantation process but remains in the matrix as a metastable solid solution, in the form of small (less than 2.5 nm) precipitates or in the form of a thin layer on the external surfaces of the specimen. Since the room temperature solubility of lead in gold is even less than that for cobalt in gold6, the second possibility seems to be the most likely. Dynamic recrystallization produced by high integrated fluxes of cobalt, which is effected by nucleation of a new grain at and subsequent growth parallel to the original grain boundary, is attributed to nucleation of a new grain at the original boundary, which forms a high angle (mobility) boundary with one of the original grains, and subsequent growth in the cobalt-depleted region. It has recently been demonstrated by direct observation that electron irradiation produces solute segregation and a concomitant depletion zone at grain boundaries in copper’. Moreover, it has been demonstrated that the annealing of ion-implanted bicrystalline thin films of gold results in the formation of new grains at the original boundary rather than in the migration of the original boundarys. Therefore, it may be concluded that annihilation of intrinsic defects produced by the implantation process occurs by migration of an incoherent grain boundary until its movement is arrested by the adsorption of cobalt atoms. Accordingly, a depletion zone near the original grain boundary is necessary to explain the anisotropic growth of the new grain. Since the boundary accumulates one monolayer of cobalt after migrating a distance equivalent to the atomic diameter of gold divided by the atomic fraction of cobalt in gold, or about 20 nm for an implantation energy of 40 keV and an integrated flux of 1 x 1Or5Co + cm-‘, boundary motion would be expected to cease within this distance, especially at temperatures where cobalt cannot easily diffuse. This distance, however, is about ten times less than the observed grain width (cf: Fig. 3(b)). Therefore, grain boundary migration is limited to the depletion zone width perpendicular to the original grain boundary but is almost unlimited within the depletion zone parallel to the original grain boundary. Further details concerning the probable driving forces for dynamic recrystallization are presented elsewheres. ACKNOWLEDGMENTS
The authors express appreciation to S. Sen and R. Troetschel for preparation of bicrystalline thin films of gold at Carnegie-Mellon University and to the Fund for Promotion of Research at the Technion, the National Science Foundation (Grant 76-l 1373) and the U.S.A.-Israel Binational Science Foundation (Grant 040-240) for support of this research.
STRUCTURE OF ION-IMPLANTED BICRYSTALLII’E
Au
FILMS
83
REFERENCES
1 G. Deamaley, J. H. Freeman, R. S. Nelson and J. Stephen, Ion Impluntafion, North-Holland, Amsterdam, 1973, p. 42. 2 I. W. Hall, in H. M. Ortner (ed.), Proc. 10th Plansee Semi& Reutte, 1981, Vol. 1, Metallwerk Plansee, Reutte, 1981, p. 153. 3 F. Cosandey, Y. Komem and C. L. Bauer, Phys. Status Solidi A, 48 (1978) 555. 4 F. Cosandey, Y. Komem and C. L. Bauer, Thin Solid Films, 59 (1979) 165. 5 Y. Komem, P. Petroff and R. W. Balluffi, Philos. Mug., 26 (1972) 239. 6 M. Hansen, Constitufion of Binary Alloys, McGraw-Hill, New York, 1958. 7 T. Takeyama, Ohnaki and H. Takahashi, ScrMetuN., 14 (1980) 1105. 8 Y. Komem, A. Greenberg and C. L. Bauer, Phys. Status Sofidi A, 64 (198 1) 3 17.
ON THE STRUCTURE FILMS * A. GREENBERG,
77
AND CHARACTERIZATION
OF ION-IMPLANTED
BICRYSTALLINE
GOLD
Y. KOMEM AND C. L. BALJERT
Department of Materials Engineering, Technion, Israel Institute of Technology, Haifa 32000 (Israel) (Received September
2,198l;
accepted
September
22,198l)
Bicrystalline thin films of gold containing 4”, 16” and 18” [OOl] tilt boundaries were implanted at room temperature with cobalt, chromium, gold and lead at an energy of 40 keV with integrated fluxes ranging from 5 x lOi to 100 x 1Ol3 ions cme2 and were subsequently examined by transmission electron microscopy. In all cases, small dislocation loops are produced by the implantation process; in addition, the discrete strain contrast pattern associated with dislocations in low angle boundaries is obliterated by implantation with cobalt or chromium but not by implantation with gold or lead under identical conditions. It is concluded that cobalt and chromium segregate at grain boundaries, thereby distorting the lattice and obliterating the discrete strain contrast pattern associated with individual dislocations, whereas gold cannot distort the lattice and lead, being totally immiscible in gold, segregates at free surfaces. Moreover, under large integrated fluxes, new grains are nucleated at and expand parallel to the original grain boundary during implantation with cobalt. Such nucleation and anisotropic growth is attributed to intrinsic grain boundary migration in the depletion zone surrounding the original boundary.
1. INTRODUCTION
Carefully doped surface layers are routinely produced by ion implantation in semiconductor devices but, because of the small penetration depth’, this technique is not generally applicable to the production of alloys. Even in the few cases where properties of alloys produced by ion implantation have been investigated2, the microstructure near grain boundaries was not examined as a function of the concentration and species of the implanted ions. The purpose of this particular investigation was to implant specially prepared bicrystalline thin films of gold with
* Paper presented at the Fifth International 21-25, 1981. t Department of Metallurgical Engineering Pittsburgh, PA 15213, U.S.A. 0040-6090/82/0000-OOOO/ooo/$o2.75
Thin Films Congress, and
Materials
Herzlia-on-Sea,
Science,
Israel,
Carnegie-Mellon
0 Elsevier Sequoia/Printed
September University,
in The Netherlands
78
A. GREENBERG, Y. KOMEM, C. L. BAUER
cobalt, chromium, gold and lead and to characterize the resultant microstructure near preselected [OOl] tilt boundaries by transmission electron microscopy (TEM). 2.
EXPERIMENTAL PROCEDURE
Bicrystalline thin films of gold, containing grain boundaries characterized by angles of misorientation of 4, 16” and 18” about the common [OOl] axis, were produced by vapour deposition and subsequent epitaxial growth on an intermediate bicrystalline thin film of silver, produced by a similar process of vapour deposition and epitaxial growth on bicrystalline substrates of NaCl. The gold films, measuring about lo-20nm in thickness, were then removed from their substrates by dissolution of the NaCl and silver in distilled water and dilute nitric acid respectively and were mounted on grids for subsequent implantation with ions of cobalt, chromium, lead and gold perpendicular to the (001) surface and in a manner designed to minimize specimen heating. The implantation energy was 40 keV and integrated fluxes ranged from 5 x 1013 to 100 x 1Ol3ions cm-‘, which areequivalent to ion penetrations ranging from 9 to 15 nm and concentrations ranging from 0.06 to 1.2 at.% of the implanted species. The resultant microstructure near the original grain boundary was then examined by TEM. Further details concerning specimen preparation and grain boundary characterization are presented elsewhere3,4. 3.
EXPERIMENTAL RESULTS
Electron micrographs before and after implantation were taken along the same tilt boundary but not from the same section. However, many observations were made by TEM and typical microstructures near 4”, 16” and 18” [OOl] tilt boundaries in gold prior and subsequent to implantation with cobalt at 40 keV and 5 x 1013 ions cm-’ 10 x 1Ol3 ions cm-’ and 100 x 1013 ions cm-’ are presented in Figs. I,2 and 3 respectively. It is evident that the discrete strain contrast patterns associated with dislocations in the 4” and 16” grain boundaries (cfi arrows in Figs. l(a) and 2(a)) are obliterated by the implantation process (c$ arrows in Figs. l(b) and 2(b)). Similar effects were observed for implantation with chromium under identical conditions. In addition, small dislocation loops associated with the radiation damage produced by the implantation process can be observed. At integrated fluxes greater than about 1Ol4 ions cm-‘, a third grain is observed to nucleate at and expand parallel to the original grain boundary. A typical example is presented in Fig. 3 for the 18” boundary, wherein the new grain is revealed after an integrated flux of 1 x 1015 Co+ cm-’ by both bright and dark field TEM. According to the associated electron diffraction pattern in Fig. 3(c), the [Ol l] axis of the new grain is parallel to the original [OOl] axis; however, this particular orientation is not unique, since sometimes other axes of the new grain, e.g. (112) and (i14), were also found to be parallel to the original [OOl] axis. Bicrystalline thin films of gold were also implanted with gold and lead, and typical results are presented in Figs. 4(a) and 4(b) respectively for an integrated flux of 5 x 1013 ions cm-‘. In contrast with prior results for implantation with either cobalt or chromium, the discrete strain contrast patterns associated with dislocations at the original grain boundary are not obliterated under identical conditions, although the usual distribution of small dislocation loops is observed.
(b)
(b)
Fig. 1. Tilt boundary in a bicrystalline thin film of gold produced by a relative rotation of 4” about a common [OOl] axis: (a) before implantation;(b) after implantation with cobalt at an energy of 40 keV and an integrated flux of 5 x 1or3 co+ cn-r,
Fig. 2. Tilt boundary in a bicrystalline thin film of gold produced by a relative rotationof 16”about acommon [Ool] axis: (a) before implantation; (b) after implantation with cobalt at an energy of 40 keV and an integrated flux of 1 x 1Or4Co+ cm-‘.
64
(4
64
Fig. 3. Tilt boundary in a bicrystalhne thin film of gold produced by a relative rotation of 18” about a common [OOl] axis: (a) before implantation; (b), (c) bright field and dark field images after implantation with cobalt at an energy of 40 keV and an integrated flux of 1 x 1Or5 Co+ cm-‘. The associated electron diffraction pattern is also included in (c) wherein the (200) reflections from the original grains and the (111) reflection from the new grain are labelled.
STRUCTURE OF ION-IMPLANTED BICRYSTALLINE
Au FILMS
81
Fig. 4. Tilt boundary in a bicrystalline thin film of gold produced by a relative rotation of 4” about a common [OOl] axis after implantation with (a) gold and (b) lead at an energy of 40 keV with an integrated flux of 1 x 1Ol4ions cm-‘.
4. DISCUSSION OF RESULTS The two principal results of this investigation are (1) that the discrete strain contrast pattern associated with dislocations in low angle [OOl] tilt boundaries in gold is obliterated by implantation with cobalt or chromium but not by implantation with gold or lead under identical conditions and (2) that dynamic recrystallization occurs by nucleation of a new grain at and subsequent growth parallel to the original grain boundary during implantation of cobalt at room temperature and under relatively large integrated fluxes. These results, and others, are analysed in the following paragraphs. Obliteration of the strain contrast pattern in low angle [OOl] tilt boundaries in bicrystalline thin films of gold by cobalt implantation is attributed to local distortions produced by the short-range diffusion of cobalt in a supersaturated solid solution to these boundaries, assisted by a large (transient) density of intrinsic
82
A. GREENBERG, Y. KOMBM, C. L. BAUER
defects produced by the implantation process. This effect, however, is not observed for (self-)implantation with gold under identical conditions. In this particular case, the implanted ions may be assimilated into the lattice either by combination with irradiation-produced vacancies or by dislocation climb in the tilt boundary’, thus preserving the discrete strain contrast pattern. Implantation with lead under identical conditions also does not obliterate the discrete strain contrast pattern, even though the lattice distortion produced by lead is much greater than that produced by cobalt. Therefore, it must be concluded that lead does not segregate to dislocations in the grain boundary during the implantation process but remains in the matrix as a metastable solid solution, in the form of small (less than 2.5 nm) precipitates or in the form of a thin layer on the external surfaces of the specimen. Since the room temperature solubility of lead in gold is even less than that for cobalt in gold6, the second possibility seems to be the most likely. Dynamic recrystallization produced by high integrated fluxes of cobalt, which is effected by nucleation of a new grain at and subsequent growth parallel to the original grain boundary, is attributed to nucleation of a new grain at the original boundary, which forms a high angle (mobility) boundary with one of the original grains, and subsequent growth in the cobalt-depleted region. It has recently been demonstrated by direct observation that electron irradiation produces solute segregation and a concomitant depletion zone at grain boundaries in copper’. Moreover, it has been demonstrated that the annealing of ion-implanted bicrystalline thin films of gold results in the formation of new grains at the original boundary rather than in the migration of the original boundarys. Therefore, it may be concluded that annihilation of intrinsic defects produced by the implantation process occurs by migration of an incoherent grain boundary until its movement is arrested by the adsorption of cobalt atoms. Accordingly, a depletion zone near the original grain boundary is necessary to explain the anisotropic growth of the new grain. Since the boundary accumulates one monolayer of cobalt after migrating a distance equivalent to the atomic diameter of gold divided by the atomic fraction of cobalt in gold, or about 20 nm for an implantation energy of 40 keV and an integrated flux of 1 x 1Or5Co + cm-‘, boundary motion would be expected to cease within this distance, especially at temperatures where cobalt cannot easily diffuse. This distance, however, is about ten times less than the observed grain width (cf: Fig. 3(b)). Therefore, grain boundary migration is limited to the depletion zone width perpendicular to the original grain boundary but is almost unlimited within the depletion zone parallel to the original grain boundary. Further details concerning the probable driving forces for dynamic recrystallization are presented elsewheres. ACKNOWLEDGMENTS
The authors express appreciation to S. Sen and R. Troetschel for preparation of bicrystalline thin films of gold at Carnegie-Mellon University and to the Fund for Promotion of Research at the Technion, the National Science Foundation (Grant 76-l 1373) and the U.S.A.-Israel Binational Science Foundation (Grant 040-240) for support of this research.
STRUCTURE OF ION-IMPLANTED BICRYSTALLII’E
Au
FILMS
83
REFERENCES
1 G. Deamaley, J. H. Freeman, R. S. Nelson and J. Stephen, Ion Impluntafion, North-Holland, Amsterdam, 1973, p. 42. 2 I. W. Hall, in H. M. Ortner (ed.), Proc. 10th Plansee Semi& Reutte, 1981, Vol. 1, Metallwerk Plansee, Reutte, 1981, p. 153. 3 F. Cosandey, Y. Komem and C. L. Bauer, Phys. Status Solidi A, 48 (1978) 555. 4 F. Cosandey, Y. Komem and C. L. Bauer, Thin Solid Films, 59 (1979) 165. 5 Y. Komem, P. Petroff and R. W. Balluffi, Philos. Mug., 26 (1972) 239. 6 M. Hansen, Constitufion of Binary Alloys, McGraw-Hill, New York, 1958. 7 T. Takeyama, Ohnaki and H. Takahashi, ScrMetuN., 14 (1980) 1105. 8 Y. Komem, A. Greenberg and C. L. Bauer, Phys. Status Sofidi A, 64 (198 1) 3 17.