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Structural changes induced by helium ion irradiation in silicon carbide crystals
709
Journal of Nuclear Materials 133$134 (1985) 709-713
STRUCTURAL CHANGES INDUCED BY HELIUM ION IRRADIATION IN SILICON CARBIDE CRYSTALS K. HOJOU and K. IZUI Department of Chemistry, Japan Atomic Energy Research Institute, Tokai
mum, Ibaraki 319 11, Japan
Electron microscopic observations were performed on defect structures in polycrystalline Sic crystals irradiated with 30 keV He ions to fluences from lOI to 10” ions/cm2. A number of defect clusters were observed which were hexagonal in shape when observed from the e-axis direction but were seen to be of linear form when viewed from a direction parallel to the c-plane. These clusters increased in density with increasing irradiation fluence, and were considered to be formed by the agglomeration of irradiation-produced point defects or the injected helium atoms on the c-planes. Amorphization was observed for a fluence above a critical value of 7 X 1015 ions/cm’ which corresponds to about one dpa for the 30 keV helium ions incident at random orientations on a Sic crystal. Irradiations of this critical fluence were unable to cause amorphization in the ion channeling directions. Heavy irradiations at the high temperature of 1000 K resulted in a number of fine crystallites, that is, recrystallized zones.
1. Introduction Ceramics are important materials for fusion reactor components such as the first wall, insulators in the eiectronics system and blanket materials. Technological data concerned with the irradiation effects on the mechanical, electrical and thermal properties in ceramics have been reported. However, little study of irradiation-induced structural changes, which are important for understanding the fundamental processes of radiation damage, has been made. In the present study electron microscopic observations were made of the defect structures in silicon carbide (Sic) crystals after irradiation with helium ions, for the purpose of simulating 14 MeV neutron damage and the effects of injected helium atoms resulting from (ii, a) reactions.
images were taken using the ultra-high resolution pole piece (UHP) with a spherical aberration coefficient of 0.7 mm.
3. Experimental results and discussion The thin films of Sic crystals made by ion thinning were observed before being subjected to the irradiation. The films were usually wedge shaped and had a small grain size of 2 to 10 pm with various orientations, as shown typically in fig. 1. From the analysis of the electron diffraction patterns with various orientations,
2. Experimental procedures The specimens used in the present work were SIC sintered polycrystals supplied from Hitachi Research Lab., Hitachi Ltd.. Pulverized SiC obtained by grinding the green body is mixed with a small amount (less than 1 wtW) of the sintering aid and beryllia (BeO) powder. The composite grains obtained are cold-pressed into a plate, then hot-pressed and sintered in a graphite die at about 2300 K for 1 h [I]. Thin films suitable for electron microscopy were made by 2 to 3 keV Ar ion bombardment at room temperature, after mechanical polishing. Irradiations were performed with 30 keV helium ions originating from an RF ion source with the fluences ranging from lOI4 to 10” ions/cm. The irradiation temperature was about 300 K for most cases. High temperature irradiations were performed for some cases at 1000 K using a tantalum ribbon heater. Observations were carried out with a JEM 1OOC electron microscope operating at 100 kV. In some cases, many-beam lattice 0022-3115/85/$03.30 0 Elsevier Science Publishers (forth-Holland Physics Publishing Division)
B.V.
Fig. 1. The general aspect of SiC sintered polycrystal before being subjected to irradiation.
710
K. HOJOU. 1% Izui
j
He ion damage m SK
these crystals were found to be of hexagonal type with a lattice spacing do,, = 2.65 A and do,,, = 2.5 A. After the irradiations to a fluence of 3 X 10n’ ions/cm2, a number of white spots of several tens of angstroms in size were observed, as shown in fig. 2 taken from a direction nearly parallel to the c-axis. Some of the spots were of hexagonal shape. as shown later in the high-resolution lattice images. These spots were found to increase in number and size with increasing fluence, suggesting that they were irradiation-induced defect clusters. When observed from a direction parallel to the basal plane, no white spot was observable but a number of linear defects could be seen as shown in fig. 3, taken in another crystal grain with a [2iO] orientation in the same thin film specimen as in fig. 2. These linear defects were found to be parallel to the (001) plane and also to increase both in density and in total length with increasing irradiation fluence, suggesting the planar defects produced by the irradiation. In order to examine the correspondence between these two figures, high resolution many-beam lattice images were taken from these two crystal grains with the ]OOl] and [2iO] orientations after being subjected to the same irradiation condition. Figs. 4(a) and (b) show the lattice images taken from the direction of the c-axis after irradiations to the fluences of 3 X 10n’ and 1 X 10” ions/cm’, respectively and (c) is an enlargement of (b). Some defect clusters with a hexagonal shape coincident with the crystallo-
Fig. 2. Defect clusters observed from the c-axis direction in a Sic crystal irradiated with 30 keV He ions to a fluence of 3 x IO” ions/cm’ at 300 K.
crystal
Fig. 3. Defect clusters observed parallel to the c-plane m a SiC crystal irradiated with 30 keV He ions to a fluence of 3 x IO" ions/cm* at 300 K.
graphic orientation can be seen. An interesting moduiation of the lattice images due to the defect cluster is seen in fig. 4(c), where the (100) lattice fringes are shifted by about a half period of the lattice spacing within the hexagonal area of the defect cluster. This may be caused by a stacking disorder associated with the cluster, although a detailed analysis of the structure of the defect clusters is not possible at the present stage without an image simulation investigation. Figs. 5 (a). (b) and (c) show the lattice images taken from the [210] direction parallel to the basal plane after the irradiations under the same conditions as for figs. 4(a) and (b) respectively. A number of linear defect fringes running parallel to the c-plane can be seen. As seen in the enlarged image of fig. 5(c), a number of complex modulations of the lattice fringes occur both in the directions parallel and normal to the linear defects. Although the direct correspondence between the two kinds of images of figs. 4 and 5 is difficult to establish, it may be concluded from these images that a large number of stacking disorders with various modes along the basal planes are introduced by the ion irradiation. Amorphous zones were sometimes observed for ion fluences higher than 7 X IO” ions/cm2, as shown in fig. 6. Electron diffraction patterns from the relatively thin areas of the specimen show only halo patterns. as shown in the upper part of fig. 6, while those from thick areas show both halo and normai net patterns. This indicates that the amorphous zones are confined to regions of some thickness corresponding to the projected ion range
K. Hojou, K. lout / He ion damage in SiC cpstal
711
Fig. 4. Defect clusters observed in the lattice image of an SiC crystal in the (001) orientation irradiated with 30 keV He ions at 300 K. (a) 3 x lOI ions/cm’, (b) 1 x lOI ions/cm*, (c) enlarged photograph of (ht.
within which a high density of damage is produced. Amorphization phenomena due to ion irradiations have been reported in some ceramics materials [2], in which critical doses for amo~hization were shown to be of the order of 10’” ions/cm2 [3], consistent with the present result. By using the TRIM code [4] the damage density in the Sic amorphous structure is estimated to be about
1 dpa for the 30 keV helium ions of a fluence of 7 x lOI ions/cm*. This dpa value is considered to be critical for amorphization. On the other hand, no amorphous zone was FFroduced even by the irradiation above the critical flue nce (lo’“-10” ions/cm2) in the crystal grains oriented in the (OOl] and (2101 directions. respectively, as has al-
Fig. 5. Defect clusters observed in the lattice image of an Sic crystal in the (270) orientation, irradiated with 30 keV He ions at 300 K. (a) 3 x 1016 ions/cm2, (b) 1 X 10”’ ions/cm*, (c) enlarged photograph of (b).
712
K. Hqou, K. 1x1 / He ion damage in SIC crystul
Fig. 6. The amorphous zone produced in an Sic crystal irradiated with 30 keV He ions to a fluence of 3 X lO’(’ ions/cm2 at 300 K.
Fig. 7. The recrystallized zone formed in an Sic crystal irradiated with 30 keV He ions to a fluence of I x10” ion\/cm’ at 1000 K.
ready been shown in figs. 2-5. The reason for this lack of amorphization can be attributed to the effect of the ion channeling. It should be noted that the observed direction, that is the direction of incident electrons for illumination, is almost equal to the direction of the incident ions used for the irradiation to the specimen. Therefore, the results shown in figs. 2-5 correspond to the cases when the [OOl] and [2iO] channeling of ions occurs. Since the range of the channeled ions is one order of magnitude longer than that in a random orientation [3], the damage production rate along the channeled direction is remarkably reduced and unable to reach the dpa value required for the amorphization, even if the incident ion fluence is over the critical value of 7 X lOI ions/cm’. When irradiated at the high temperature of 1000 K to a fluence of 1 x 10” ions/cm*, recrystallized zones were sometimes observed, as shown in fig. 7. These recrystallized zones were found to consist of fine grains of several hundred angstrom in size by using selected area electron diffraction patterns, as shown in the upper part of fig. 7, where the selected area is taken as a circle of about O.lpm in diameter. For the high fluence of 1 x 10” ions/cm2, both knock-ons and sec-
ondary electrons are produced in extremely high densities and interact with the lattice atoms, leading to amorphization through bond destruction and atomic displacements. On the other hand the effect of a high temperature during the irradiation would tend to anneal out the amorphous structure once produced to allow atoms to rearrange by thermal agitation. Both the process towards amorphization and that to recrystallization compete with each other by dynamical interactions among knock-on, secondary electrons and host atoms in thermal motion, and finally the latter process overcomes the former around the recrystallization temperature; i.e.. about 1000 K for SIC [5], resulting in the formation of the regions of fine crystallites, as is actually observed.
4. Conclusions (1) Planar defect clusters lying on the basal plane were observed in SIC after irradiation with 30 keV ions. The density of these clusters increased with inceasing the ion fluence. (2) For fluences exceeding 7 X lOI ions/cm’, for which the damage is about one dpa, amorphization was
K. HOJOU,K. Izui / He ion damage in SIC crystal
found to occur in random orientations of the crystal, but not in the ion channeling orientations. (3) After heavy irradiation to a fluence of 1 x 10” ions/cm* at lOOOK, recrystallization took place, resulting in the production of a number of fine crystallites of several hundred angstrom in size.
Acknowledgment
The authors are indebted to Mr. S. Usami of New Materials, Castings and Forgings Division, Hitachi Ltd. for supplying us the SIC samples.
713
References
PI M. Ura and 0. Asai, F.C. Report 1 (1983) 5 (in Japanese). PI Hj. Matzke, Radiation Effects 64 (1982) 3. [31J.L. Whitton, in: Channeling, Ed. D.V. Morgan (John Wiley and Sons, 1973) p. 225. 141J.P. Biersack and L.G. Haggmark. Nucl. Instrum. Methods 174 (1980) 257. [51 H.N. Naguib and R. Kelly, Radiation Effects 25 (1975) 1.
Journal of Nuclear Materials 133$134 (1985) 709-713
STRUCTURAL CHANGES INDUCED BY HELIUM ION IRRADIATION IN SILICON CARBIDE CRYSTALS K. HOJOU and K. IZUI Department of Chemistry, Japan Atomic Energy Research Institute, Tokai
mum, Ibaraki 319 11, Japan
Electron microscopic observations were performed on defect structures in polycrystalline Sic crystals irradiated with 30 keV He ions to fluences from lOI to 10” ions/cm2. A number of defect clusters were observed which were hexagonal in shape when observed from the e-axis direction but were seen to be of linear form when viewed from a direction parallel to the c-plane. These clusters increased in density with increasing irradiation fluence, and were considered to be formed by the agglomeration of irradiation-produced point defects or the injected helium atoms on the c-planes. Amorphization was observed for a fluence above a critical value of 7 X 1015 ions/cm’ which corresponds to about one dpa for the 30 keV helium ions incident at random orientations on a Sic crystal. Irradiations of this critical fluence were unable to cause amorphization in the ion channeling directions. Heavy irradiations at the high temperature of 1000 K resulted in a number of fine crystallites, that is, recrystallized zones.
1. Introduction Ceramics are important materials for fusion reactor components such as the first wall, insulators in the eiectronics system and blanket materials. Technological data concerned with the irradiation effects on the mechanical, electrical and thermal properties in ceramics have been reported. However, little study of irradiation-induced structural changes, which are important for understanding the fundamental processes of radiation damage, has been made. In the present study electron microscopic observations were made of the defect structures in silicon carbide (Sic) crystals after irradiation with helium ions, for the purpose of simulating 14 MeV neutron damage and the effects of injected helium atoms resulting from (ii, a) reactions.
images were taken using the ultra-high resolution pole piece (UHP) with a spherical aberration coefficient of 0.7 mm.
3. Experimental results and discussion The thin films of Sic crystals made by ion thinning were observed before being subjected to the irradiation. The films were usually wedge shaped and had a small grain size of 2 to 10 pm with various orientations, as shown typically in fig. 1. From the analysis of the electron diffraction patterns with various orientations,
2. Experimental procedures The specimens used in the present work were SIC sintered polycrystals supplied from Hitachi Research Lab., Hitachi Ltd.. Pulverized SiC obtained by grinding the green body is mixed with a small amount (less than 1 wtW) of the sintering aid and beryllia (BeO) powder. The composite grains obtained are cold-pressed into a plate, then hot-pressed and sintered in a graphite die at about 2300 K for 1 h [I]. Thin films suitable for electron microscopy were made by 2 to 3 keV Ar ion bombardment at room temperature, after mechanical polishing. Irradiations were performed with 30 keV helium ions originating from an RF ion source with the fluences ranging from lOI4 to 10” ions/cm. The irradiation temperature was about 300 K for most cases. High temperature irradiations were performed for some cases at 1000 K using a tantalum ribbon heater. Observations were carried out with a JEM 1OOC electron microscope operating at 100 kV. In some cases, many-beam lattice 0022-3115/85/$03.30 0 Elsevier Science Publishers (forth-Holland Physics Publishing Division)
B.V.
Fig. 1. The general aspect of SiC sintered polycrystal before being subjected to irradiation.
710
K. HOJOU. 1% Izui
j
He ion damage m SK
these crystals were found to be of hexagonal type with a lattice spacing do,, = 2.65 A and do,,, = 2.5 A. After the irradiations to a fluence of 3 X 10n’ ions/cm2, a number of white spots of several tens of angstroms in size were observed, as shown in fig. 2 taken from a direction nearly parallel to the c-axis. Some of the spots were of hexagonal shape. as shown later in the high-resolution lattice images. These spots were found to increase in number and size with increasing fluence, suggesting that they were irradiation-induced defect clusters. When observed from a direction parallel to the basal plane, no white spot was observable but a number of linear defects could be seen as shown in fig. 3, taken in another crystal grain with a [2iO] orientation in the same thin film specimen as in fig. 2. These linear defects were found to be parallel to the (001) plane and also to increase both in density and in total length with increasing irradiation fluence, suggesting the planar defects produced by the irradiation. In order to examine the correspondence between these two figures, high resolution many-beam lattice images were taken from these two crystal grains with the ]OOl] and [2iO] orientations after being subjected to the same irradiation condition. Figs. 4(a) and (b) show the lattice images taken from the direction of the c-axis after irradiations to the fluences of 3 X 10n’ and 1 X 10” ions/cm’, respectively and (c) is an enlargement of (b). Some defect clusters with a hexagonal shape coincident with the crystallo-
Fig. 2. Defect clusters observed from the c-axis direction in a Sic crystal irradiated with 30 keV He ions to a fluence of 3 x IO” ions/cm’ at 300 K.
crystal
Fig. 3. Defect clusters observed parallel to the c-plane m a SiC crystal irradiated with 30 keV He ions to a fluence of 3 x IO" ions/cm* at 300 K.
graphic orientation can be seen. An interesting moduiation of the lattice images due to the defect cluster is seen in fig. 4(c), where the (100) lattice fringes are shifted by about a half period of the lattice spacing within the hexagonal area of the defect cluster. This may be caused by a stacking disorder associated with the cluster, although a detailed analysis of the structure of the defect clusters is not possible at the present stage without an image simulation investigation. Figs. 5 (a). (b) and (c) show the lattice images taken from the [210] direction parallel to the basal plane after the irradiations under the same conditions as for figs. 4(a) and (b) respectively. A number of linear defect fringes running parallel to the c-plane can be seen. As seen in the enlarged image of fig. 5(c), a number of complex modulations of the lattice fringes occur both in the directions parallel and normal to the linear defects. Although the direct correspondence between the two kinds of images of figs. 4 and 5 is difficult to establish, it may be concluded from these images that a large number of stacking disorders with various modes along the basal planes are introduced by the ion irradiation. Amorphous zones were sometimes observed for ion fluences higher than 7 X IO” ions/cm2, as shown in fig. 6. Electron diffraction patterns from the relatively thin areas of the specimen show only halo patterns. as shown in the upper part of fig. 6, while those from thick areas show both halo and normai net patterns. This indicates that the amorphous zones are confined to regions of some thickness corresponding to the projected ion range
K. Hojou, K. lout / He ion damage in SiC cpstal
711
Fig. 4. Defect clusters observed in the lattice image of an SiC crystal in the (001) orientation irradiated with 30 keV He ions at 300 K. (a) 3 x lOI ions/cm’, (b) 1 x lOI ions/cm*, (c) enlarged photograph of (ht.
within which a high density of damage is produced. Amorphization phenomena due to ion irradiations have been reported in some ceramics materials [2], in which critical doses for amo~hization were shown to be of the order of 10’” ions/cm2 [3], consistent with the present result. By using the TRIM code [4] the damage density in the Sic amorphous structure is estimated to be about
1 dpa for the 30 keV helium ions of a fluence of 7 x lOI ions/cm*. This dpa value is considered to be critical for amorphization. On the other hand, no amorphous zone was FFroduced even by the irradiation above the critical flue nce (lo’“-10” ions/cm2) in the crystal grains oriented in the (OOl] and (2101 directions. respectively, as has al-
Fig. 5. Defect clusters observed in the lattice image of an Sic crystal in the (270) orientation, irradiated with 30 keV He ions at 300 K. (a) 3 x 1016 ions/cm2, (b) 1 X 10”’ ions/cm*, (c) enlarged photograph of (b).
712
K. Hqou, K. 1x1 / He ion damage in SIC crystul
Fig. 6. The amorphous zone produced in an Sic crystal irradiated with 30 keV He ions to a fluence of 3 X lO’(’ ions/cm2 at 300 K.
Fig. 7. The recrystallized zone formed in an Sic crystal irradiated with 30 keV He ions to a fluence of I x10” ion\/cm’ at 1000 K.
ready been shown in figs. 2-5. The reason for this lack of amorphization can be attributed to the effect of the ion channeling. It should be noted that the observed direction, that is the direction of incident electrons for illumination, is almost equal to the direction of the incident ions used for the irradiation to the specimen. Therefore, the results shown in figs. 2-5 correspond to the cases when the [OOl] and [2iO] channeling of ions occurs. Since the range of the channeled ions is one order of magnitude longer than that in a random orientation [3], the damage production rate along the channeled direction is remarkably reduced and unable to reach the dpa value required for the amorphization, even if the incident ion fluence is over the critical value of 7 X lOI ions/cm’. When irradiated at the high temperature of 1000 K to a fluence of 1 x 10” ions/cm*, recrystallized zones were sometimes observed, as shown in fig. 7. These recrystallized zones were found to consist of fine grains of several hundred angstrom in size by using selected area electron diffraction patterns, as shown in the upper part of fig. 7, where the selected area is taken as a circle of about O.lpm in diameter. For the high fluence of 1 x 10” ions/cm2, both knock-ons and sec-
ondary electrons are produced in extremely high densities and interact with the lattice atoms, leading to amorphization through bond destruction and atomic displacements. On the other hand the effect of a high temperature during the irradiation would tend to anneal out the amorphous structure once produced to allow atoms to rearrange by thermal agitation. Both the process towards amorphization and that to recrystallization compete with each other by dynamical interactions among knock-on, secondary electrons and host atoms in thermal motion, and finally the latter process overcomes the former around the recrystallization temperature; i.e.. about 1000 K for SIC [5], resulting in the formation of the regions of fine crystallites, as is actually observed.
4. Conclusions (1) Planar defect clusters lying on the basal plane were observed in SIC after irradiation with 30 keV ions. The density of these clusters increased with inceasing the ion fluence. (2) For fluences exceeding 7 X lOI ions/cm’, for which the damage is about one dpa, amorphization was
K. HOJOU,K. Izui / He ion damage in SIC crystal
found to occur in random orientations of the crystal, but not in the ion channeling orientations. (3) After heavy irradiation to a fluence of 1 x 10” ions/cm* at lOOOK, recrystallization took place, resulting in the production of a number of fine crystallites of several hundred angstrom in size.
Acknowledgment
The authors are indebted to Mr. S. Usami of New Materials, Castings and Forgings Division, Hitachi Ltd. for supplying us the SIC samples.
713
References
PI M. Ura and 0. Asai, F.C. Report 1 (1983) 5 (in Japanese). PI Hj. Matzke, Radiation Effects 64 (1982) 3. [31J.L. Whitton, in: Channeling, Ed. D.V. Morgan (John Wiley and Sons, 1973) p. 225. 141J.P. Biersack and L.G. Haggmark. Nucl. Instrum. Methods 174 (1980) 257. [51 H.N. Naguib and R. Kelly, Radiation Effects 25 (1975) 1.