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(111)Au epitaxial films
Thin Solid Films, 61 (1979) 33-39 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands
33
THE INTERFACE STRUCTURE OF Pd/(111)Au EPITAXIAL FILMS J. G. ERLINGS AND F. W. SCHAPINK
Laboratory of Metallurgy, Delft University of Technology, Rotterdamseweg 137, 2600 GA Delft (The Netherlands) (Received November 16, 1978 : accepted December 14, 1978 )
It is shown that vacuum-deposited palladium can grow in different modes on (111) gold single-crystal platelets obtained by chemical reduction in a gel. The final interfacial structure depends on the vacuum conditions during deposition and on the condition of the substrate surface. It is found that under certain conditions dislocation-free interfaces are formed in the Pd/(111)Au bicrystals; these interfaces remain dislocation-free after annealing. The phenomenon of alloying of Pd/Au thin film couples during aging at room temperature is also reported.
1. INTRODUCTION The epitaxial growth of palladium from the vapour phase onto (111) gold single crystals and the interfacial structure of thin Pd/(lll)Au bicrystals have been investigated by several authors 1-6. It has been shown that vacuum-deposited palladium grows layer by layer on the (111) surface of gold single crystals. A different type of growth which is based on the energies associated with misfit strain and misfit dislocations has been suggested by Matthews et al.7: an initial layer by layer growth which is followed by an island-type growth. Although all the experiments showed a monolayer overgrowth, different mechanisms to relieve misfit strains were observed. Where palladium has been deposited onto small (111) gold single-crystal islands lying epitaxially on a magnesium oxide substrate, the misfit is mainly compensated for by dislocations that glide into the interface from the brims of the islands 2. This mechanism can also be expected for island overgrowth. Cherns and Stowell 3' ~ and Cherns 5 have shown that dislocation trigons at the external palladium surface grow by climb to relieve misfit strains. In these systems the dislocation Burgers vectors are of the Xa (110) type. Another mechanism to relieve misfit strain is the climb of lattice dislocations into the interface, where the Burgers vectors of the climbed dislocations may be inclined to the interface. Cherns and StowelP have also considered the Shockley partial as a misfit dislocation. It has been shown s that this type of dislocation is expected when the interface is near a coherent twin boundary which occurs when the substrate and the overgrowth are twin related. In this work we report on the effects of the substrate surface conditions and the vacuum conditions on the palladium growth and on the interface structure. Modifications in the interface structure occurring during aging will be discussed.
34
J. G. ERLINGS, F. W. SCHAPINK
2. EXPERIMENTAL Thin (70 nm thick) gold single-crystal platelets with a (111) orientation were prepared using the method described in ref. 9. A palladium layer 35 nm thick was deposited onto the dislocation-free gold crystals either in a high vacuum (HV) or in an ultrahigh vacuum (UHV) system. The pumping arrangement of the HV system contained an oil diffusion pump whilst the UHV pumping system consisted of an ion getter pump, a titanium sublimation pump and a liquid nitrogen panel. The substrate temperature was 550 K and the average deposition rate was 0.1 nm s- 1 (controlled by a Deposition Control Master Omni II (Sloan)). During deposition the total pressure in the HV system was 10-s Torr. In the UHV system the pressure increased from 10- lo to 10- 7 Torr during deposition. Quadrupole mass spectrometry measurements (using a Riber QX 100) showed that the residual gases with the highest partial pressures were H2, H20, N 2 and CO 2. In the UHV system there was a very low partial pressure (less than 10-11 Torr) of hydrocarbons. Prior to deposition a number of specimens were surface cleaned in the UHV system by bombardment with low energy (150 eV) argon ions followed by a heat treatment at 570 K for 8 h at pressures in the range 10-1°-10 - 9 Torr. Transmission electron microscopy (TEM) observations of the bicrystals were made using either a Philips EM 300 or a Philips EM 400 electron microscope equipped with a STEM unit* and a secondary electron detector operating at 100 and 120 kV respectively. 3.
RESULTS AND DISCUSSION
A bright field micrograph of a specimen prepared in the HV system is shown in Fig. 1. The deposited layer consists of an aggregate of nearly coalescing crystals of average cross section 25 nm which are surrounded by areas of average width 1.5 nm. The surrounding areas appear bright when a single gold reflection is used for imaging, which indicates the absence of palladium. The diffraction pattern shows that the crystals are partly epitaxially and partly randomly oriented. These observations suggest that under HV conditions palladium grows in a partly epitaxial island mode on (111) gold, in contrast to the layer growth mentioned earlier. This difference is in agreement with experiments on silicon and germanium by Kasper and Herzog 1° who found that surface contamination, in particular carbon contamination occurring at higher pressures in oil-diffusion-pumped systems, can drastically alter the growth mode of the deposited material. Specimens obtained in the UHV system without prior surface treatment consisted of continuous layers which contained networks of misfit dislocations in the interface of initial average spacing 25-30 nm. These networks were observed under weak beam diffraction conditions (Fig. 2). Figure 3 shows the structure of the interface after aging at room temperature for 6 months. In the diffraction patterns, although there was no evidence for separate reflections of the palladium and gold crystals, spots were found at intermediate positions. The spacing of the original *A STEM unit can be used for both scanning and transmission electron microscopy measurements.
INTERFACESTRUCTUREOF Pd/(111)Au EPITAXIALFILMS
35
Fig. 1. A bright field micrograph of an HV-depositedpalladium layer on a gold single crystal.
Fig. 2. A weak beam micrograph of a dislocation network in a UHV-prepared specimen. network had increased to 30-35 nm. Analysis showed that the dislocations were of the edge type with ½a ( i l 0 ~ Burgers vectors. N o evidence was found for the formation of dislocation walls which Matthews and Crawford 11 have observed in Pd/(100)Au interfaces. Curved grains which were formed as a result of diffusion were observed locally. If bulk constants are assumed for the gold and palladium crystals and all the misfit is accommodated in the interface by dislocation arrays, the dislocation spacing is calculated to be 6.3 nm. However, the observed spacing is considerably larger and m a y be the result of a lack of dislocation sources during deposition; this can be explained by layer by layer growth of palladium onto the substrate. In this growth mode the first layers are deposited at pressures of 1 0 - 1 ° - 1 0 - 7 Torr and consequently no foreign atoms are built into the overgrowth lattice. Once the first
36
J . G . ERLINGS, F. W. SCHAPINK
Fig. 3. A weak beam micrograph of a UHV-prepared specimen annealed for 15 x 100 s at room temperature.
few layers are formed no dislocations can be generated at the interface, contrary to the results of Yagi e t al. 2 Since no substrate lattice dislocations are present disclocation climb from the substrate cannot account for the relief of misfit strain. Under these conditions misfit dislocations can only be generated at the palladium surface 4 or at irregularities or impurities in the interface. However, because no dislocation sources were observed at the palladium surface the only possible sources are interface irregularities and impurities. Only part of the misfit can be located in the original dislocation network; the remainder has to be compensated for initially by elastic strain. We suggest that the activation energy for diffusion is lowered by this elastic strain and an alloyed area a few monolayers thick results. The remainder of the misfit is located in this diffusion zone.
Using extrapolated diffusion data for lattice diffusion from ref. 12 the diffusion distance ( D t ) 1/2 after 15 x 106 S at 300 K is 10- 14 m. The diffraction patterns show that during aging at room temperature considerable alloying of the bicrystal occurs despite the very small calculated diffusion distance. This suggests an enlarged diffusion coefficient at room temperature which may be explained by a strainenhanced diffusion coefficient. Moreover the phenomenological diffusion theory may give inaccurate results for diffusion over areas of the order of only 100 atomic distances. After diffusion the change in the lattice parameter at the interface will be reduced and the spacing of the dislocation network must increase to accommodate this smaller misfit. This increased spacing can be seen in Fig. 3. On the external palladium surface initially there were a number of small
INTERFACESTRUCTUREOF Pd/(11 I)Au EPITAXIALFILMS
37
randomly oriented palladium crystals which agglomerated during annealing at room temperature. It was shown using secondary electron detection that these crystals were on the palladium surface (Fig. 4) and consequently did not disturb the interfacial dislocation network (Fig. 5).
Fig. 4. A secondary electron micrograph of palladium islands on the palladium surface of the bicrystal.
Fig. 5. A bright field micrograph ofa palladium agglomerate at thepalladiumsurfaceandofadislocation network in the interface. The lower left corner shows a static beam microdiffraction pattern of the bicrystal and the upper right corner a microdiffraction pattern of the agglomerate.
During argon ion sputtering a few (111) layers of the gold crystal and any surface impurities were removed. The damage produced during ion bombardment disappeared during the subsequent heat treatment. No evidence was found (under weak beam conditions or in the diffraction patterns 5) for the presence of misfit
38
J.G. ERLINGS, F. W. SCHAPINK
dislocation networks in specimens prepared in this manner (Fig. 6). This dislocationfree interface is attributed to a combination of the dislocation-free substrate, the impurity-free surface and the UHV evaporation conditions.
Fig. 6. A dark field micrograph of a UHV-prepared specimen, with the surface cleaned by argon ion sputtering prior to deposition, showing (~11) moir6 patterns.
No misfit dislocations were generated on annealing at temperatures in the range 450-600 K for about 12 h ((Dr) 1/2 < 25 nm). In local regions alloying and recrystallization of the bicrystal were observed and the new grains were in different orientations. The complete absence of misfit dislocations suggests that all the misfit is located in a thin alloyed layer. However, the diffraction patterns showed no indication of such layers which suggests that the diffused layer is only a few atomic layers thick. 4. CONCLUSIONS The nature of the substrate surface and the vacuum conditions determine the growth mode of the vacuum-deposited material and the final interface structure. Under certain conditions P d / ( l l l ) A u bicrystals in which the misfit is not compensated for by dislocation networks in the boundary plane can be prepared. It is suggested that the elastic strain present near the interface promotes the formation of a thin alloyed intermediate layer which accommodates the misfit. ACKNOWLEDGMENT
The authors would like to thank Mr. H. Weerheijm for his assistance in constructing the UHV apparatus and in preparing the specimens.
INTERFACE STRUCTURE OF P d / ( 1 1 1 ) A u EPITAXIAL FILMS
39
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
G.S. Bassett, J. W. Menter and D. W. Pashley, Proc. R. Soc. London, Ser. A, 246 (1958) 345. K. Yagi, K. Takayanagi, K. Kobayashi and G. Honjo, J. Cryst. Growth, 9 ( 1971 ) 84. D. Cherns and M. J. Stowell, Scr. Metall., 7 (1973) 489. D. Cherns and M. J. Stowell, Thin Solid Films, 29 (1975) 127. D. Cherns, ThinSolidFilms, 48(1978) 385. Y. Matsushita, K. Yagi, T. Narusawa and G. Honjo, Jpn. J. Appl. Phys., Suppl. 2, Part 1 (1974) 567. J.W. Matthews, D. C. Jackson and A. Chambers, Thin Solid Films, 26 (1975) 129. D.H. WarringtonandH. Grimmer, Philos. Mag.,30(1974)461. J.G. Erlings and F. W. Schapink, Scr. Metall., 11 (1977) 427. E. Kasper and H. J. Herzog, Wiss. Ber. A EG Allg. Elektricitaets Ges. Telefunken, 49 (1976) 6. J.W. Matthews and J. L. Crawford, Philos. Mag., 11 (1965) 977. O. Neukam, Galvanotechnik, 61 (1970) 626.
33
THE INTERFACE STRUCTURE OF Pd/(111)Au EPITAXIAL FILMS J. G. ERLINGS AND F. W. SCHAPINK
Laboratory of Metallurgy, Delft University of Technology, Rotterdamseweg 137, 2600 GA Delft (The Netherlands) (Received November 16, 1978 : accepted December 14, 1978 )
It is shown that vacuum-deposited palladium can grow in different modes on (111) gold single-crystal platelets obtained by chemical reduction in a gel. The final interfacial structure depends on the vacuum conditions during deposition and on the condition of the substrate surface. It is found that under certain conditions dislocation-free interfaces are formed in the Pd/(111)Au bicrystals; these interfaces remain dislocation-free after annealing. The phenomenon of alloying of Pd/Au thin film couples during aging at room temperature is also reported.
1. INTRODUCTION The epitaxial growth of palladium from the vapour phase onto (111) gold single crystals and the interfacial structure of thin Pd/(lll)Au bicrystals have been investigated by several authors 1-6. It has been shown that vacuum-deposited palladium grows layer by layer on the (111) surface of gold single crystals. A different type of growth which is based on the energies associated with misfit strain and misfit dislocations has been suggested by Matthews et al.7: an initial layer by layer growth which is followed by an island-type growth. Although all the experiments showed a monolayer overgrowth, different mechanisms to relieve misfit strains were observed. Where palladium has been deposited onto small (111) gold single-crystal islands lying epitaxially on a magnesium oxide substrate, the misfit is mainly compensated for by dislocations that glide into the interface from the brims of the islands 2. This mechanism can also be expected for island overgrowth. Cherns and Stowell 3' ~ and Cherns 5 have shown that dislocation trigons at the external palladium surface grow by climb to relieve misfit strains. In these systems the dislocation Burgers vectors are of the Xa (110) type. Another mechanism to relieve misfit strain is the climb of lattice dislocations into the interface, where the Burgers vectors of the climbed dislocations may be inclined to the interface. Cherns and StowelP have also considered the Shockley partial as a misfit dislocation. It has been shown s that this type of dislocation is expected when the interface is near a coherent twin boundary which occurs when the substrate and the overgrowth are twin related. In this work we report on the effects of the substrate surface conditions and the vacuum conditions on the palladium growth and on the interface structure. Modifications in the interface structure occurring during aging will be discussed.
34
J. G. ERLINGS, F. W. SCHAPINK
2. EXPERIMENTAL Thin (70 nm thick) gold single-crystal platelets with a (111) orientation were prepared using the method described in ref. 9. A palladium layer 35 nm thick was deposited onto the dislocation-free gold crystals either in a high vacuum (HV) or in an ultrahigh vacuum (UHV) system. The pumping arrangement of the HV system contained an oil diffusion pump whilst the UHV pumping system consisted of an ion getter pump, a titanium sublimation pump and a liquid nitrogen panel. The substrate temperature was 550 K and the average deposition rate was 0.1 nm s- 1 (controlled by a Deposition Control Master Omni II (Sloan)). During deposition the total pressure in the HV system was 10-s Torr. In the UHV system the pressure increased from 10- lo to 10- 7 Torr during deposition. Quadrupole mass spectrometry measurements (using a Riber QX 100) showed that the residual gases with the highest partial pressures were H2, H20, N 2 and CO 2. In the UHV system there was a very low partial pressure (less than 10-11 Torr) of hydrocarbons. Prior to deposition a number of specimens were surface cleaned in the UHV system by bombardment with low energy (150 eV) argon ions followed by a heat treatment at 570 K for 8 h at pressures in the range 10-1°-10 - 9 Torr. Transmission electron microscopy (TEM) observations of the bicrystals were made using either a Philips EM 300 or a Philips EM 400 electron microscope equipped with a STEM unit* and a secondary electron detector operating at 100 and 120 kV respectively. 3.
RESULTS AND DISCUSSION
A bright field micrograph of a specimen prepared in the HV system is shown in Fig. 1. The deposited layer consists of an aggregate of nearly coalescing crystals of average cross section 25 nm which are surrounded by areas of average width 1.5 nm. The surrounding areas appear bright when a single gold reflection is used for imaging, which indicates the absence of palladium. The diffraction pattern shows that the crystals are partly epitaxially and partly randomly oriented. These observations suggest that under HV conditions palladium grows in a partly epitaxial island mode on (111) gold, in contrast to the layer growth mentioned earlier. This difference is in agreement with experiments on silicon and germanium by Kasper and Herzog 1° who found that surface contamination, in particular carbon contamination occurring at higher pressures in oil-diffusion-pumped systems, can drastically alter the growth mode of the deposited material. Specimens obtained in the UHV system without prior surface treatment consisted of continuous layers which contained networks of misfit dislocations in the interface of initial average spacing 25-30 nm. These networks were observed under weak beam diffraction conditions (Fig. 2). Figure 3 shows the structure of the interface after aging at room temperature for 6 months. In the diffraction patterns, although there was no evidence for separate reflections of the palladium and gold crystals, spots were found at intermediate positions. The spacing of the original *A STEM unit can be used for both scanning and transmission electron microscopy measurements.
INTERFACESTRUCTUREOF Pd/(111)Au EPITAXIALFILMS
35
Fig. 1. A bright field micrograph of an HV-depositedpalladium layer on a gold single crystal.
Fig. 2. A weak beam micrograph of a dislocation network in a UHV-prepared specimen. network had increased to 30-35 nm. Analysis showed that the dislocations were of the edge type with ½a ( i l 0 ~ Burgers vectors. N o evidence was found for the formation of dislocation walls which Matthews and Crawford 11 have observed in Pd/(100)Au interfaces. Curved grains which were formed as a result of diffusion were observed locally. If bulk constants are assumed for the gold and palladium crystals and all the misfit is accommodated in the interface by dislocation arrays, the dislocation spacing is calculated to be 6.3 nm. However, the observed spacing is considerably larger and m a y be the result of a lack of dislocation sources during deposition; this can be explained by layer by layer growth of palladium onto the substrate. In this growth mode the first layers are deposited at pressures of 1 0 - 1 ° - 1 0 - 7 Torr and consequently no foreign atoms are built into the overgrowth lattice. Once the first
36
J . G . ERLINGS, F. W. SCHAPINK
Fig. 3. A weak beam micrograph of a UHV-prepared specimen annealed for 15 x 100 s at room temperature.
few layers are formed no dislocations can be generated at the interface, contrary to the results of Yagi e t al. 2 Since no substrate lattice dislocations are present disclocation climb from the substrate cannot account for the relief of misfit strain. Under these conditions misfit dislocations can only be generated at the palladium surface 4 or at irregularities or impurities in the interface. However, because no dislocation sources were observed at the palladium surface the only possible sources are interface irregularities and impurities. Only part of the misfit can be located in the original dislocation network; the remainder has to be compensated for initially by elastic strain. We suggest that the activation energy for diffusion is lowered by this elastic strain and an alloyed area a few monolayers thick results. The remainder of the misfit is located in this diffusion zone.
Using extrapolated diffusion data for lattice diffusion from ref. 12 the diffusion distance ( D t ) 1/2 after 15 x 106 S at 300 K is 10- 14 m. The diffraction patterns show that during aging at room temperature considerable alloying of the bicrystal occurs despite the very small calculated diffusion distance. This suggests an enlarged diffusion coefficient at room temperature which may be explained by a strainenhanced diffusion coefficient. Moreover the phenomenological diffusion theory may give inaccurate results for diffusion over areas of the order of only 100 atomic distances. After diffusion the change in the lattice parameter at the interface will be reduced and the spacing of the dislocation network must increase to accommodate this smaller misfit. This increased spacing can be seen in Fig. 3. On the external palladium surface initially there were a number of small
INTERFACESTRUCTUREOF Pd/(11 I)Au EPITAXIALFILMS
37
randomly oriented palladium crystals which agglomerated during annealing at room temperature. It was shown using secondary electron detection that these crystals were on the palladium surface (Fig. 4) and consequently did not disturb the interfacial dislocation network (Fig. 5).
Fig. 4. A secondary electron micrograph of palladium islands on the palladium surface of the bicrystal.
Fig. 5. A bright field micrograph ofa palladium agglomerate at thepalladiumsurfaceandofadislocation network in the interface. The lower left corner shows a static beam microdiffraction pattern of the bicrystal and the upper right corner a microdiffraction pattern of the agglomerate.
During argon ion sputtering a few (111) layers of the gold crystal and any surface impurities were removed. The damage produced during ion bombardment disappeared during the subsequent heat treatment. No evidence was found (under weak beam conditions or in the diffraction patterns 5) for the presence of misfit
38
J.G. ERLINGS, F. W. SCHAPINK
dislocation networks in specimens prepared in this manner (Fig. 6). This dislocationfree interface is attributed to a combination of the dislocation-free substrate, the impurity-free surface and the UHV evaporation conditions.
Fig. 6. A dark field micrograph of a UHV-prepared specimen, with the surface cleaned by argon ion sputtering prior to deposition, showing (~11) moir6 patterns.
No misfit dislocations were generated on annealing at temperatures in the range 450-600 K for about 12 h ((Dr) 1/2 < 25 nm). In local regions alloying and recrystallization of the bicrystal were observed and the new grains were in different orientations. The complete absence of misfit dislocations suggests that all the misfit is located in a thin alloyed layer. However, the diffraction patterns showed no indication of such layers which suggests that the diffused layer is only a few atomic layers thick. 4. CONCLUSIONS The nature of the substrate surface and the vacuum conditions determine the growth mode of the vacuum-deposited material and the final interface structure. Under certain conditions P d / ( l l l ) A u bicrystals in which the misfit is not compensated for by dislocation networks in the boundary plane can be prepared. It is suggested that the elastic strain present near the interface promotes the formation of a thin alloyed intermediate layer which accommodates the misfit. ACKNOWLEDGMENT
The authors would like to thank Mr. H. Weerheijm for his assistance in constructing the UHV apparatus and in preparing the specimens.
INTERFACE STRUCTURE OF P d / ( 1 1 1 ) A u EPITAXIAL FILMS
39
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
G.S. Bassett, J. W. Menter and D. W. Pashley, Proc. R. Soc. London, Ser. A, 246 (1958) 345. K. Yagi, K. Takayanagi, K. Kobayashi and G. Honjo, J. Cryst. Growth, 9 ( 1971 ) 84. D. Cherns and M. J. Stowell, Scr. Metall., 7 (1973) 489. D. Cherns and M. J. Stowell, Thin Solid Films, 29 (1975) 127. D. Cherns, ThinSolidFilms, 48(1978) 385. Y. Matsushita, K. Yagi, T. Narusawa and G. Honjo, Jpn. J. Appl. Phys., Suppl. 2, Part 1 (1974) 567. J.W. Matthews, D. C. Jackson and A. Chambers, Thin Solid Films, 26 (1975) 129. D.H. WarringtonandH. Grimmer, Philos. Mag.,30(1974)461. J.G. Erlings and F. W. Schapink, Scr. Metall., 11 (1977) 427. E. Kasper and H. J. Herzog, Wiss. Ber. A EG Allg. Elektricitaets Ges. Telefunken, 49 (1976) 6. J.W. Matthews and J. L. Crawford, Philos. Mag., 11 (1965) 977. O. Neukam, Galvanotechnik, 61 (1970) 626.