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Preparation and applications of thin film specimens containing grain boundaries of controlled geometry
Thin Solid Films, 33 (1976) 1-11 © Elsevier Sequoia S.A., Lausanne---Printed in Switzerland
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PREPARATION AND APPLICATIONS OF THIN FILM SPECIMENS CONTAINING GRAIN BOUNDARIES OF CONTROLLED GEOMETRY
T. Y. TAN*, J. C. M. HWANG, P. J..GOODHEW t AND R. W. BALLUFFI Department of Materials Science and Engineering and Materials Science Center, Cornell University, Ithaca, N. Y. 14853 (U.S.A.) (Received May 9, 1975; accepted June 12, 1975)
A novel welding and annealing method for the convenient production of thin film specimens containing different types of controlled grain boundary structures has been developed. Bicrystals containing controlled grain boundaries in their midplane were produced by welding single-crystal thin films together face-to-face. Polyerystals containing columnar grains of only two crystal orientations were produced by annealing the previous bicrystals. Polycrystals with columnar grains of only three orientations were produced by extensions of the method in which films of three orientations were welded together and annealed. The types of grain structures which can be obtained by these techniques are described, and the potential uses of such specimens are discussed.
1. INTRODUCTION Many areas of materials science and thin film technology call for a more extensive knowledge of grain boundary characteristics such as grain boundary structure, grain boundary energy, grain boundary mobility and impurity diffusion and segregation at-grain boundaries. In recent years, powerful techniques such as transmission electron 'microscopy, scanning electron microscopy, Auger spectroscopy and ion scattering spectroscopy have reached new levels of sophistication so that more precise and systematic grain boundary related studies can be undertaken, Such studies demand convenient and reliable techniques by which specimens containing simple grain boundary structures of predetermined geometry can be routinely produced. In this paper we~describe a novel welding and annealing method for the rapid and easy production of thin film specimens containing several useful types of controlled grain boundary structures. Examples of these structures are given, and their uses in grain boundary studies are discussed. * Permanent address: International Business Machines Corp., Essex Junction, Vermont 05452, U.S.A. t Permanent address: Department of Metallurgy and Material Technology, University of Surrey, * Gt. Britain.
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2 2. SPECIMEN PREPARATION METHOD
2.1. Bicrystal with grain boundary in midplane As has been demonstrated elsewhere 1-5, thin film bicrystals in the configuration shown in Fig. l(a) can be prepared by welding two oriented singlecrystal films together face-to-face. The single-crystal films are prepared initially on flat substrates by, for example, vapor deposition, and are then welded together while still on their substrates by applying a moderate pressure at a slightly elevated temperature. One, or both, of the substrates are then removed by preferential dissolution in a suitable solvent. By controlling the orientations of the two films, and the angle at which the films are welded, a grain boundary of any desired geometry can be produced. For example, bicrystals of gold have been prepared t-4 by a relatively simple procedure. Gold epitaxial films are first prepared by vapor deposition on substrates consisting of single crystals of rocksalt already coated with a thin epitaxial film of evaporated silver. The gold films are then welded together in air, and the substrates are preferentially dissolved away in nitric acid and water. A large variety of grain boundaries have been prepared t-4 by varying the substrate orientations and the welding angle. Similar bicrystals of surface-reactive metals such as copper and aluminum have been prepared s by constructing a special apparatus which allows the entire deposition process and welding operation to be carded out under high vacuum. CRYSTAL 1
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,
CRYSTAL 2 ~I
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(b)
'12[~121~121
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2
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1
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(c)
I I=]1[=111=1 CRY/KSTAL 3
3
(d)
(e)
(f)
Fig. 1. (a) Thin film bicrystal specimen prepared by pressure welding crystal 1 to crystal 2 faeeto-face at a slightly elevated temperature. (b) Partially annealed structure obtained by annealing the specimen in (a) at a more elevated temperature. (c) Polycrystal with columnar grains of only two crystal orientations, i.e. I and 2, obtained by further annealing of the specimen in (b). (d) Specimen with grains of three orientations obtained by welding a thin film single crystal, i.e. crystal 3, to the specimen in (c). (e) Thin film tricrystai prepared by simultaneous welding of three single crystals. (f) Polyerystal with columnar grains of three crystal orientations obtained by annealing the specimens in (d) or (e).
THIN FILM SPECIMENS WITH GRAIN BOUNDARIES
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All of the specimens prepared by this technique have contained a distribution of small bubbles lying in the boundary plane. A typical distribution is shown in Fig. 2. These bubbles evidently result from imperfect welding and gas entrap ment at the interface. The bubbles are remarkably stable, and we have not been able to eliminate them by annealing at characteristic temperatures at which it is known6 that voids of similar size anneal out rapidly in thin metal films. This may be taken as an indication that the bubbles contain insoluble gases which exert a back pressure which hinders their shrinkage. We note that bubbles of this type were also obtained’ in specimens which were fabricated in situ under vacuum at pressures of the order of lo-’ torr. Evidently, enough gas atoms were still present under these conditions to stabilize the bubbles.
Fig. 2. Bubbles preselit in an [OOl]twist boundary in a gold bicrystal film. Twist angle 24.5”. Lines in the boundary are extraneous grain boundary dislocations with strong Burgers vector components normal to the grain boundary.
2.2. Polycrystal with columnar grains of only two crystal orientations A polycrystalline specimen with columnar grains in the configuration shown in Fig. l(c) can be readily obtained by annealing the bicrystal shown in Fig. l(a) to a temperature higher than that used initially in the welding operation. (A suitable annealing temperature is usually about half the absolute melting temperature.) The changes in the grain boundary configuration which occur during the annealing are illustrated schematically in Fig. l(a)-(c). The initial bicrystal configuration is metastable, and during annealing crystal 1 grows into crystal 2 and crystal 2 grows into crystal 1 in various places (Fig. l(b)). Eventually grain boundary migration converts the bicrystal into the relatively stable columnar structure shown in Fig. l(c). Each grain in such a specimen has either orientation 1 or orientation 2, and the crystal misorientation across
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every grain boundary is therefore essentially the same*. An example of such a specimen is shown in Fig. 3. An interesting aspect o f the grain structure is that the boundary of each crystal must be a continuous closed surface, Le. no triple grain junctions can exist.
Fig. 3. Polycrystallinespecimenof gold containing columnar grains of two orientations. Boundaries are tilt boundaries. Tilt angle 10°. (a) and (b)are dark field mierographs utilizing the < 110> 1 and < 110> 2 reflections, respectively;subscripts 1 and 2 designate the two crystal orientations. Notice that alternate grains "light up" in (a) and (b). In the case o f gold the entire process of grain boundary migration takes approximately 0.5 h at 200 °C. At 400 °C the process is essentially instantaneous for practical purposes. Once all horizontal boundaries are eliminated, grain growth essentially stops and a relatively stable structure is achieved. The final grain size seems almost independent o f annealing conditions and appears to be determined mainly by the thickness o f the foil. The tendency is that thicker specimens end up with larger grain sizes. F r o m bicrystal specimens 2000 A thick, polycrystalline specimens with grain diameters o f several microns are usually obtained. When the bicrystal is 800 A thick, polycrystalline specimens o f grain diameter about 0.1 lam are typical. We note that in general every planar segment of grain boundary possesses five degrees of freedom s which consist of three variables which fix the crystal misorientation across the boundary and two variables which fix the orientation o f the grain boundary plane. The first three degrees o f freedom of every boundary segment in a polycrystal o f the present type are therefore fixed. We have also observed that surface tension causes most boundary segments in well-annealed polycrystals to lie closely perpendicular to the foil surface, as seen, for example, in Fig. 4. To a good approximation therefore a fourth degree of freedom describing the orientation o f the boundary plane is also fixed for these boundary segments. Therefore these boundary segments possess only one degree of freedom which may be taken as the angle which their normal makes with respect to some reference direction in the plane o f the specimen surface. * It should be noted that different regions in each initially prepared single-crystal film are slightly misoriented with respect to each other. These misorientationstherefore give rise to small variations in the misorientation across the grain boundaries in different parts of the final welded and annealed specimen.
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Fig. 4. High magnification view of the grain boundary configuration in a polycrystalline specimen of gold containing columnar grains of two orientations. The boundary shown is a tilt boundary. Tilt angle 35.1 °. Secondary grain boundary dislocations are visible edge-on at the arrows.
Visible in Fig. 4 are bubbles in the midplane of the individual grains which were left behind during the grain boundary migration. The transverse boundaries in the polycrystals in Figs. 3 and 4 are, of course, free of bubbles except along their lines of intersection with the specimen midplane.
2.3. Polycrystal with columnar grains of only three crystal orientations A polycrystal with columnar grains of only three crystal orientations can be prepared by further extensions of the method, which are illustrated in Fig. l(c)-(f). Two approaches are possible. A third single-crystal film (crystal 3) can be welded to a columnar bicrystal to produce the specimen shown in Fig. l(d), or alternatively three single-crystal thin films may be welded simultaneously to produce a specimen containing two horizontal boundaries (Fig. l(e)). Annealing of either type of tricrystal (Fig. l(d) or (e)) gives rise to boundary migration, and a columnar polycrystal containing grains of only three orientations is produced (Fig. l(f)). It is possible for two distinct types of polycrystalline regions to be formed in these tricrystal specimens. A metastable configuration similar to that found in the columnar bicrystals is formed under certain conditions (of crystal thickness, temperature etc.) in which grain "islands" of either of two orientations are present in a local matrix having the third orientation, and no grain boundary triple junctions are found. A more stable configuration occurs when a network of grain boundaries is formed and triple boundary junctions are common, as illustrated in Fig. 5. 3. APPLICATIONS We now briefly describe a number of applications of the previous specimens in grain boundary related studies. Several of these studies are either completed or else in progress in our laboratory.
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Fig. 5. Polycrystailine specimen of gold containing columnar grains of three orientations. All boundaries are high angle tilt boundaries. There is a triple grain boundaryjunction at 1, 2, 3. The specimen has been annealed to the point where "unzipping" at the grain boundaries has occurred (see text) producing grain boundary intersectionswith the film surface, e.g. at 2, 3, V. 3.1. Grain boundary structure The thin film bicrystals have been used extensively9 to study the fine structure of both low and high angle grain boundaries by transmission electron microscopy and electron diffraction. An example is shown in Fig. 6 (see caption for details). The ease of specimen preparation has made it possible to prepare large numbers of grain boundaries of controlled geometry, and this has allowed a systematic study of grain boundary structure as a function of crystal misorientation and the orientation of the grain boundary plane. The specimens have the added advantage that large areas of boundary are available which can be observed at normal incidence. Also, the adjoining grains are fiat slabs and therefore do not produce thickness fringes which can often cause confusion if periodic defect line structure in the boundary is under investigation. We also note that in certain cases the bubbles in the boundary are useful, since they enable one to distinguish 1o between moir6 fringes and actual periodic line structure in the boundary. The polycrystal specimens with grains of only two orientations have also been useful. These specimens allow the observation of grain boundaries edgeon, and they have therefore been employed in the study of grain boundary faceting. An example of a faceted boundary observed edge-on is shown in Fig. 7. Under these conditions the angles between facets can be measured readily, and the facet planes can be identified 11. 3.2. Grain boundary segregation Solute atom segregation to grain boundaries in the thin film alloy bicrystal specimens can be studied as a function of boundary structure in certain systems. If the solute atoms segregate at the boundary in a periodic distribution (probably
THIN FILM SPECIMENS WITH GRAIN BOUNDARIES
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Fig. 6- [001] low angle tilt boundary in a gold bicrystal. The fine horizontal fines are edge dislocations lyingparallel to the [001] tilt axis.
Fig. 7. Polycrystalline specimen of gold containing columnar grains of two orientations. Boundaries are < 110> tilt boundaries. Tilt angle 70.5 °. Extensive faeeting has occurred to form relatively low
energy twin boundarysegments. with the same periodicity as the grain boundary structure) and possess electron scattering factors which differ from those of the host atoms, then it should be feasible to detect the periodic solute atom distribution by means of electron diffraction. In such a case the differences in scattering power due to the presence of the solute atoms should produce extra reflections around (000) in a pattern which is reciprocally related to the geometry of the segregation pattern.
3.3. Grain boundary energy Columnar bicrystal and tricrystal specimens offer a number of ways of studying grain boundary energies. Clearly, tricrystal films containing many
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well-equilibrated triple junctions (as in Fig. 5) can be used to measure the relative energies of well-characterized boundaries using the familiar Herring equilibrium relationship 12 for the junction. By judicious selection of the misorientations across the three boundaries it should also be possible to determine the magnitude of the torque terms, since one specimen will contain many junctions between boundaries of identical misorientation but different inclination with respect to an arbitrary direction in one of the crystals. The effect of boundary inclination on the energy of a boundary of known misorientation can also be studied using the columnar bicrystal specimens. Despite local trapping of the ~ n boundary at bubbles it is possible to deduce, from the relative frequency of occurrence of boundary segments of each inclination, a polar plot similar to the Gibbs-Wulff construction for the orientation dependence of surface energy. A third type of measurement can be made if the bicrystal or tricrystal film is annealed in the electron microscope until it begins to "unzip" along the grain boundaries*. If annealing is stopped at this point a specimen is obtained which contains, in addition to islands and/or grain boundary triple junctions, many intersections between grain boundaries and the film surface. Figure 5 in fact shows such a specimen and from suitable boundary--surface intersections where the boundary is perpendicular to the foil it is possible to relate grain boundary energy to the surface energy. Studies of grain boundary energy in gold using these three approaches are currently in progress la. 3.4. Grain boundary diffusion Solute atom diffusion along well-characterized grain boundaries can be studied conveniently using the columnar polycrystal specimens with grains of two orientations. Such a specimen is first placed in an ultrahigh vacuum chamber. The temperature of the specimen is raised to the diffusion temperature. Solute atoms are then quickly deposited on one side of the film. These atoms diffuse through the specimen thickness along the grain boundaries and spread out on the opposite (exit) surface. The number of solute atoms diffusing through in this manner can be detected by Auger spectroscopy on the exit side. The method is exceedingly sensitive, since a specimen thickness of about 800 A and a grain size of 0.1 pm or less are possible. Furthermore, the Auger technique is capable of detecting between 10 -2 and 10 -a of a monolayer of solute atoms on the exit surface. Preliminary calculations indicate that the grain boundary diffusion of, for example, silver through gold should be detectable at room temperature with this technique. At this temperature lattice diffusion is essentially "frozen out", and it should therefore be possible to study almost pure grain boundary diffusion without accompanying complications due to diffusional leakage to the adjoining lattice. * Upon annealing at relativelyelevatedtemperatures, the thin film specimeneventuallyspheroidizes by thinning along the grain boundaries and thickening near the grain centers. Eventuallythe specimen opens up completelyalong many grain boundaries (" unzips") tending to form islands consisting of thickened grains.
THIN FILM SPECIMENSWITH GRAIN BOUNDARIES
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3.5. Grain boundary migration Grain boundary migration can be studied during the annealing process which converts the bicrystal specimen to the polycrystal (Fig. l(a)-(c)). In this process the single-crystal regions grow at the expense of the bicrystal regions by the migration of the vertical boundary segments which are normal to the specimen surface (Fig. l(b)). The driving force for this migration is, of course, derived from the energy of the sections of the horizontal boundary which are consumed in the process. The migration of these boundary segments can be easily observed and measured, e.g. by hot-stage electron microscopy. The possibility that the migration occurs by the movement of grain boundary ledges or steps t4 could be tested by tilting the specimen and observing the boundary plane directly. A further advantage of the method is the fact that the driving force for the boundary migration remains constant. An example of a migrating boundary is shown in Fig. 8. It may be seen that the migration of the vertical boundary tends to be restrained by the bubbles lying in the horizontal boundary (at the arrows in Fig. 8). A schematic drawing of the situation directly at a bubble is presented in Fig. 9. It is readily seen that the restraint occurs only when the migrating boundary pulls away from the bubble. However, the increase in grain boundary area required locally at the bubble in this process is small compared with the accompanying decrease in total area of the horizontal boundary. Since the energies per unit area of the
Fig. 8. Appearance of a migratingboundary in gold during the conversionof a bicrystalto a polycrystal with columnar grains of two orientations by the process illustrated in Fig. l(a)-(c). The. horizontal boundary is a low angle < 001 > twist boundary consisting of a grid of orthogonal screw dislocations. The arrows indicate bubbles in the bicrystalmidplane.
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Fig. 9. Schematic drawing of the constraining effect of a bubble on the migration of a vertical boundary in the process illustrated in Fig. l(a)-(c) and Fig. 8.
vertical and horizontal boundaries will generally be quite similar, it may be concluded that the restraining effect of the bubbles will not be of much significance under usual conditions. This conclusion is consistent with the experimental result that the vertical boundaries always migrate readily through the array of bubbles. 3.6. Additional applications Additional applications also exist. For example, plans are underwayi in our laboratory to study flux lattice pinning in superconducting specimens of the present type which contain controlled arrays of high angle grain boundaries and dislocations in low angle boundaries. Also, the influence of grain boundaries on the electrical properties of polycrystalline silicon can be studiedi conveniently using the polycrystal specimens with columnar grains of two orientations. 4. CONCLUDING REMARKS We mention that it should be possible to produce bicrystals (and therefore polycrystals) of any material which can be deposited epitaxially on suitably soluble substrates in the form of uniform thin films. Controlled grain boundary studies in ionic and other non-metallic materials would therefore be feasible. Further developments of the technique also appear possible. In certain applications the presence of the bubbles and possible impurities at the interface may be troublesome. In such cases polycrystal specimens of the present columnar type could be used as substrates for the epitaxial growth of entirely new Urns of either similar or dissimilar material. The substrates could then be removed by chemical dissolution or ion milling. Each newly grown specimen would then possess the same columnar grain boundary configuration as its substrate but would be free of bubbles and any impurities in the midplane.
THIN FILM SPECIMENS WITH GRAIN BOUNDARIES
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ACKNOWLEDGMENT
This work was supported by the Energy Research and Development Administration under Contract AT(11-1)-3158. Additional support was received from the National Science Foundation through the Materials Science Center at Cornell University. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
T. Schober and R. W. Ballufli, Philos. Mag., 20 (1969) 511. T. Schober and R. W. Balluffi, Philos. Mag., 21 (1970) 109. T. Schober and R. W. Ballufli, Phys. Status Solidi (b), 44 (1971) 103. W . R . Wagner, T. Y. Tan and R. W. Ballulli, Philos. Mag., 29 (1974) 895. W . R . Wagner and R. W. Ballufii, Philos. Mag., 30 (1974) 673. For example, H. G. Bowden and R. W. Balluili, Philos. Mag., 19 (1969) 1001. W . R . Wagner and R. W. Balluiii, unpublished research. W.T. Read, Jr., Dislocations in Crystala, McGraw-Hill, New York, 1953, p. 173. R.W. Ballufli, Y. Komen and T. Schober, Surf Sci., 31 (1972) 68. T.Y. Tan, S. L. Sass and R. W. Balluffi, Philos. Mag., 31 (1975) 575. T.Y. Tan and R. W. Ballutii, research in progress. H. Brooks, in Metal Interfaces, American Society for Metals, Cleveland, Ohio, 1952, p. 27. P.J. Goodhew, research in progress. T. Schober and R. W. Balluffi, Philos. Mag., 24 (1971) 469. E.J. Kramer, J. E. Clemans, B. D. Lauterwasser and R. W. Balluffi, research in progress. D . G . Ast, personal communication.
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PREPARATION AND APPLICATIONS OF THIN FILM SPECIMENS CONTAINING GRAIN BOUNDARIES OF CONTROLLED GEOMETRY
T. Y. TAN*, J. C. M. HWANG, P. J..GOODHEW t AND R. W. BALLUFFI Department of Materials Science and Engineering and Materials Science Center, Cornell University, Ithaca, N. Y. 14853 (U.S.A.) (Received May 9, 1975; accepted June 12, 1975)
A novel welding and annealing method for the convenient production of thin film specimens containing different types of controlled grain boundary structures has been developed. Bicrystals containing controlled grain boundaries in their midplane were produced by welding single-crystal thin films together face-to-face. Polyerystals containing columnar grains of only two crystal orientations were produced by annealing the previous bicrystals. Polycrystals with columnar grains of only three orientations were produced by extensions of the method in which films of three orientations were welded together and annealed. The types of grain structures which can be obtained by these techniques are described, and the potential uses of such specimens are discussed.
1. INTRODUCTION Many areas of materials science and thin film technology call for a more extensive knowledge of grain boundary characteristics such as grain boundary structure, grain boundary energy, grain boundary mobility and impurity diffusion and segregation at-grain boundaries. In recent years, powerful techniques such as transmission electron 'microscopy, scanning electron microscopy, Auger spectroscopy and ion scattering spectroscopy have reached new levels of sophistication so that more precise and systematic grain boundary related studies can be undertaken, Such studies demand convenient and reliable techniques by which specimens containing simple grain boundary structures of predetermined geometry can be routinely produced. In this paper we~describe a novel welding and annealing method for the rapid and easy production of thin film specimens containing several useful types of controlled grain boundary structures. Examples of these structures are given, and their uses in grain boundary studies are discussed. * Permanent address: International Business Machines Corp., Essex Junction, Vermont 05452, U.S.A. t Permanent address: Department of Metallurgy and Material Technology, University of Surrey, * Gt. Britain.
T.Y. TAN et al.
2 2. SPECIMEN PREPARATION METHOD
2.1. Bicrystal with grain boundary in midplane As has been demonstrated elsewhere 1-5, thin film bicrystals in the configuration shown in Fig. l(a) can be prepared by welding two oriented singlecrystal films together face-to-face. The single-crystal films are prepared initially on flat substrates by, for example, vapor deposition, and are then welded together while still on their substrates by applying a moderate pressure at a slightly elevated temperature. One, or both, of the substrates are then removed by preferential dissolution in a suitable solvent. By controlling the orientations of the two films, and the angle at which the films are welded, a grain boundary of any desired geometry can be produced. For example, bicrystals of gold have been prepared t-4 by a relatively simple procedure. Gold epitaxial films are first prepared by vapor deposition on substrates consisting of single crystals of rocksalt already coated with a thin epitaxial film of evaporated silver. The gold films are then welded together in air, and the substrates are preferentially dissolved away in nitric acid and water. A large variety of grain boundaries have been prepared t-4 by varying the substrate orientations and the welding angle. Similar bicrystals of surface-reactive metals such as copper and aluminum have been prepared s by constructing a special apparatus which allows the entire deposition process and welding operation to be carded out under high vacuum. CRYSTAL 1
\
GRAIN BOUNDARY
~
/
,
CRYSTAL 2 ~I
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(o)
121
ill
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121
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2
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2
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3
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Fig. 1. (a) Thin film bicrystal specimen prepared by pressure welding crystal 1 to crystal 2 faeeto-face at a slightly elevated temperature. (b) Partially annealed structure obtained by annealing the specimen in (a) at a more elevated temperature. (c) Polycrystal with columnar grains of only two crystal orientations, i.e. I and 2, obtained by further annealing of the specimen in (b). (d) Specimen with grains of three orientations obtained by welding a thin film single crystal, i.e. crystal 3, to the specimen in (c). (e) Thin film tricrystai prepared by simultaneous welding of three single crystals. (f) Polyerystal with columnar grains of three crystal orientations obtained by annealing the specimens in (d) or (e).
THIN FILM SPECIMENS WITH GRAIN BOUNDARIES
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All of the specimens prepared by this technique have contained a distribution of small bubbles lying in the boundary plane. A typical distribution is shown in Fig. 2. These bubbles evidently result from imperfect welding and gas entrap ment at the interface. The bubbles are remarkably stable, and we have not been able to eliminate them by annealing at characteristic temperatures at which it is known6 that voids of similar size anneal out rapidly in thin metal films. This may be taken as an indication that the bubbles contain insoluble gases which exert a back pressure which hinders their shrinkage. We note that bubbles of this type were also obtained’ in specimens which were fabricated in situ under vacuum at pressures of the order of lo-’ torr. Evidently, enough gas atoms were still present under these conditions to stabilize the bubbles.
Fig. 2. Bubbles preselit in an [OOl]twist boundary in a gold bicrystal film. Twist angle 24.5”. Lines in the boundary are extraneous grain boundary dislocations with strong Burgers vector components normal to the grain boundary.
2.2. Polycrystal with columnar grains of only two crystal orientations A polycrystalline specimen with columnar grains in the configuration shown in Fig. l(c) can be readily obtained by annealing the bicrystal shown in Fig. l(a) to a temperature higher than that used initially in the welding operation. (A suitable annealing temperature is usually about half the absolute melting temperature.) The changes in the grain boundary configuration which occur during the annealing are illustrated schematically in Fig. l(a)-(c). The initial bicrystal configuration is metastable, and during annealing crystal 1 grows into crystal 2 and crystal 2 grows into crystal 1 in various places (Fig. l(b)). Eventually grain boundary migration converts the bicrystal into the relatively stable columnar structure shown in Fig. l(c). Each grain in such a specimen has either orientation 1 or orientation 2, and the crystal misorientation across
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T.Y. TAN et al.
every grain boundary is therefore essentially the same*. An example of such a specimen is shown in Fig. 3. An interesting aspect o f the grain structure is that the boundary of each crystal must be a continuous closed surface, Le. no triple grain junctions can exist.
Fig. 3. Polycrystallinespecimenof gold containing columnar grains of two orientations. Boundaries are tilt boundaries. Tilt angle 10°. (a) and (b)are dark field mierographs utilizing the < 110> 1 and < 110> 2 reflections, respectively;subscripts 1 and 2 designate the two crystal orientations. Notice that alternate grains "light up" in (a) and (b). In the case o f gold the entire process of grain boundary migration takes approximately 0.5 h at 200 °C. At 400 °C the process is essentially instantaneous for practical purposes. Once all horizontal boundaries are eliminated, grain growth essentially stops and a relatively stable structure is achieved. The final grain size seems almost independent o f annealing conditions and appears to be determined mainly by the thickness o f the foil. The tendency is that thicker specimens end up with larger grain sizes. F r o m bicrystal specimens 2000 A thick, polycrystalline specimens with grain diameters o f several microns are usually obtained. When the bicrystal is 800 A thick, polycrystalline specimens o f grain diameter about 0.1 lam are typical. We note that in general every planar segment of grain boundary possesses five degrees of freedom s which consist of three variables which fix the crystal misorientation across the boundary and two variables which fix the orientation o f the grain boundary plane. The first three degrees o f freedom of every boundary segment in a polycrystal o f the present type are therefore fixed. We have also observed that surface tension causes most boundary segments in well-annealed polycrystals to lie closely perpendicular to the foil surface, as seen, for example, in Fig. 4. To a good approximation therefore a fourth degree of freedom describing the orientation o f the boundary plane is also fixed for these boundary segments. Therefore these boundary segments possess only one degree of freedom which may be taken as the angle which their normal makes with respect to some reference direction in the plane o f the specimen surface. * It should be noted that different regions in each initially prepared single-crystal film are slightly misoriented with respect to each other. These misorientationstherefore give rise to small variations in the misorientation across the grain boundaries in different parts of the final welded and annealed specimen.
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Fig. 4. High magnification view of the grain boundary configuration in a polycrystalline specimen of gold containing columnar grains of two orientations. The boundary shown is a tilt boundary. Tilt angle 35.1 °. Secondary grain boundary dislocations are visible edge-on at the arrows.
Visible in Fig. 4 are bubbles in the midplane of the individual grains which were left behind during the grain boundary migration. The transverse boundaries in the polycrystals in Figs. 3 and 4 are, of course, free of bubbles except along their lines of intersection with the specimen midplane.
2.3. Polycrystal with columnar grains of only three crystal orientations A polycrystal with columnar grains of only three crystal orientations can be prepared by further extensions of the method, which are illustrated in Fig. l(c)-(f). Two approaches are possible. A third single-crystal film (crystal 3) can be welded to a columnar bicrystal to produce the specimen shown in Fig. l(d), or alternatively three single-crystal thin films may be welded simultaneously to produce a specimen containing two horizontal boundaries (Fig. l(e)). Annealing of either type of tricrystal (Fig. l(d) or (e)) gives rise to boundary migration, and a columnar polycrystal containing grains of only three orientations is produced (Fig. l(f)). It is possible for two distinct types of polycrystalline regions to be formed in these tricrystal specimens. A metastable configuration similar to that found in the columnar bicrystals is formed under certain conditions (of crystal thickness, temperature etc.) in which grain "islands" of either of two orientations are present in a local matrix having the third orientation, and no grain boundary triple junctions are found. A more stable configuration occurs when a network of grain boundaries is formed and triple boundary junctions are common, as illustrated in Fig. 5. 3. APPLICATIONS We now briefly describe a number of applications of the previous specimens in grain boundary related studies. Several of these studies are either completed or else in progress in our laboratory.
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Fig. 5. Polycrystailine specimen of gold containing columnar grains of three orientations. All boundaries are high angle tilt boundaries. There is a triple grain boundaryjunction at 1, 2, 3. The specimen has been annealed to the point where "unzipping" at the grain boundaries has occurred (see text) producing grain boundary intersectionswith the film surface, e.g. at 2, 3, V. 3.1. Grain boundary structure The thin film bicrystals have been used extensively9 to study the fine structure of both low and high angle grain boundaries by transmission electron microscopy and electron diffraction. An example is shown in Fig. 6 (see caption for details). The ease of specimen preparation has made it possible to prepare large numbers of grain boundaries of controlled geometry, and this has allowed a systematic study of grain boundary structure as a function of crystal misorientation and the orientation of the grain boundary plane. The specimens have the added advantage that large areas of boundary are available which can be observed at normal incidence. Also, the adjoining grains are fiat slabs and therefore do not produce thickness fringes which can often cause confusion if periodic defect line structure in the boundary is under investigation. We also note that in certain cases the bubbles in the boundary are useful, since they enable one to distinguish 1o between moir6 fringes and actual periodic line structure in the boundary. The polycrystal specimens with grains of only two orientations have also been useful. These specimens allow the observation of grain boundaries edgeon, and they have therefore been employed in the study of grain boundary faceting. An example of a faceted boundary observed edge-on is shown in Fig. 7. Under these conditions the angles between facets can be measured readily, and the facet planes can be identified 11. 3.2. Grain boundary segregation Solute atom segregation to grain boundaries in the thin film alloy bicrystal specimens can be studied as a function of boundary structure in certain systems. If the solute atoms segregate at the boundary in a periodic distribution (probably
THIN FILM SPECIMENS WITH GRAIN BOUNDARIES
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Fig. 6- [001] low angle tilt boundary in a gold bicrystal. The fine horizontal fines are edge dislocations lyingparallel to the [001] tilt axis.
Fig. 7. Polycrystalline specimen of gold containing columnar grains of two orientations. Boundaries are < 110> tilt boundaries. Tilt angle 70.5 °. Extensive faeeting has occurred to form relatively low
energy twin boundarysegments. with the same periodicity as the grain boundary structure) and possess electron scattering factors which differ from those of the host atoms, then it should be feasible to detect the periodic solute atom distribution by means of electron diffraction. In such a case the differences in scattering power due to the presence of the solute atoms should produce extra reflections around (000) in a pattern which is reciprocally related to the geometry of the segregation pattern.
3.3. Grain boundary energy Columnar bicrystal and tricrystal specimens offer a number of ways of studying grain boundary energies. Clearly, tricrystal films containing many
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well-equilibrated triple junctions (as in Fig. 5) can be used to measure the relative energies of well-characterized boundaries using the familiar Herring equilibrium relationship 12 for the junction. By judicious selection of the misorientations across the three boundaries it should also be possible to determine the magnitude of the torque terms, since one specimen will contain many junctions between boundaries of identical misorientation but different inclination with respect to an arbitrary direction in one of the crystals. The effect of boundary inclination on the energy of a boundary of known misorientation can also be studied using the columnar bicrystal specimens. Despite local trapping of the ~ n boundary at bubbles it is possible to deduce, from the relative frequency of occurrence of boundary segments of each inclination, a polar plot similar to the Gibbs-Wulff construction for the orientation dependence of surface energy. A third type of measurement can be made if the bicrystal or tricrystal film is annealed in the electron microscope until it begins to "unzip" along the grain boundaries*. If annealing is stopped at this point a specimen is obtained which contains, in addition to islands and/or grain boundary triple junctions, many intersections between grain boundaries and the film surface. Figure 5 in fact shows such a specimen and from suitable boundary--surface intersections where the boundary is perpendicular to the foil it is possible to relate grain boundary energy to the surface energy. Studies of grain boundary energy in gold using these three approaches are currently in progress la. 3.4. Grain boundary diffusion Solute atom diffusion along well-characterized grain boundaries can be studied conveniently using the columnar polycrystal specimens with grains of two orientations. Such a specimen is first placed in an ultrahigh vacuum chamber. The temperature of the specimen is raised to the diffusion temperature. Solute atoms are then quickly deposited on one side of the film. These atoms diffuse through the specimen thickness along the grain boundaries and spread out on the opposite (exit) surface. The number of solute atoms diffusing through in this manner can be detected by Auger spectroscopy on the exit side. The method is exceedingly sensitive, since a specimen thickness of about 800 A and a grain size of 0.1 pm or less are possible. Furthermore, the Auger technique is capable of detecting between 10 -2 and 10 -a of a monolayer of solute atoms on the exit surface. Preliminary calculations indicate that the grain boundary diffusion of, for example, silver through gold should be detectable at room temperature with this technique. At this temperature lattice diffusion is essentially "frozen out", and it should therefore be possible to study almost pure grain boundary diffusion without accompanying complications due to diffusional leakage to the adjoining lattice. * Upon annealing at relativelyelevatedtemperatures, the thin film specimeneventuallyspheroidizes by thinning along the grain boundaries and thickening near the grain centers. Eventuallythe specimen opens up completelyalong many grain boundaries (" unzips") tending to form islands consisting of thickened grains.
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3.5. Grain boundary migration Grain boundary migration can be studied during the annealing process which converts the bicrystal specimen to the polycrystal (Fig. l(a)-(c)). In this process the single-crystal regions grow at the expense of the bicrystal regions by the migration of the vertical boundary segments which are normal to the specimen surface (Fig. l(b)). The driving force for this migration is, of course, derived from the energy of the sections of the horizontal boundary which are consumed in the process. The migration of these boundary segments can be easily observed and measured, e.g. by hot-stage electron microscopy. The possibility that the migration occurs by the movement of grain boundary ledges or steps t4 could be tested by tilting the specimen and observing the boundary plane directly. A further advantage of the method is the fact that the driving force for the boundary migration remains constant. An example of a migrating boundary is shown in Fig. 8. It may be seen that the migration of the vertical boundary tends to be restrained by the bubbles lying in the horizontal boundary (at the arrows in Fig. 8). A schematic drawing of the situation directly at a bubble is presented in Fig. 9. It is readily seen that the restraint occurs only when the migrating boundary pulls away from the bubble. However, the increase in grain boundary area required locally at the bubble in this process is small compared with the accompanying decrease in total area of the horizontal boundary. Since the energies per unit area of the
Fig. 8. Appearance of a migratingboundary in gold during the conversionof a bicrystalto a polycrystal with columnar grains of two orientations by the process illustrated in Fig. l(a)-(c). The. horizontal boundary is a low angle < 001 > twist boundary consisting of a grid of orthogonal screw dislocations. The arrows indicate bubbles in the bicrystalmidplane.
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Fig. 9. Schematic drawing of the constraining effect of a bubble on the migration of a vertical boundary in the process illustrated in Fig. l(a)-(c) and Fig. 8.
vertical and horizontal boundaries will generally be quite similar, it may be concluded that the restraining effect of the bubbles will not be of much significance under usual conditions. This conclusion is consistent with the experimental result that the vertical boundaries always migrate readily through the array of bubbles. 3.6. Additional applications Additional applications also exist. For example, plans are underwayi in our laboratory to study flux lattice pinning in superconducting specimens of the present type which contain controlled arrays of high angle grain boundaries and dislocations in low angle boundaries. Also, the influence of grain boundaries on the electrical properties of polycrystalline silicon can be studiedi conveniently using the polycrystal specimens with columnar grains of two orientations. 4. CONCLUDING REMARKS We mention that it should be possible to produce bicrystals (and therefore polycrystals) of any material which can be deposited epitaxially on suitably soluble substrates in the form of uniform thin films. Controlled grain boundary studies in ionic and other non-metallic materials would therefore be feasible. Further developments of the technique also appear possible. In certain applications the presence of the bubbles and possible impurities at the interface may be troublesome. In such cases polycrystal specimens of the present columnar type could be used as substrates for the epitaxial growth of entirely new Urns of either similar or dissimilar material. The substrates could then be removed by chemical dissolution or ion milling. Each newly grown specimen would then possess the same columnar grain boundary configuration as its substrate but would be free of bubbles and any impurities in the midplane.
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ACKNOWLEDGMENT
This work was supported by the Energy Research and Development Administration under Contract AT(11-1)-3158. Additional support was received from the National Science Foundation through the Materials Science Center at Cornell University. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
T. Schober and R. W. Ballufli, Philos. Mag., 20 (1969) 511. T. Schober and R. W. Balluffi, Philos. Mag., 21 (1970) 109. T. Schober and R. W. Ballufli, Phys. Status Solidi (b), 44 (1971) 103. W . R . Wagner, T. Y. Tan and R. W. Ballulli, Philos. Mag., 29 (1974) 895. W . R . Wagner and R. W. Ballufii, Philos. Mag., 30 (1974) 673. For example, H. G. Bowden and R. W. Balluili, Philos. Mag., 19 (1969) 1001. W . R . Wagner and R. W. Balluiii, unpublished research. W.T. Read, Jr., Dislocations in Crystala, McGraw-Hill, New York, 1953, p. 173. R.W. Ballufli, Y. Komen and T. Schober, Surf Sci., 31 (1972) 68. T.Y. Tan, S. L. Sass and R. W. Balluffi, Philos. Mag., 31 (1975) 575. T.Y. Tan and R. W. Ballutii, research in progress. H. Brooks, in Metal Interfaces, American Society for Metals, Cleveland, Ohio, 1952, p. 27. P.J. Goodhew, research in progress. T. Schober and R. W. Balluffi, Philos. Mag., 24 (1971) 469. E.J. Kramer, J. E. Clemans, B. D. Lauterwasser and R. W. Balluffi, research in progress. D . G . Ast, personal communication.