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Ni MBE thin films and Au1−cNic solid solutions with temperature: a HREM study
Thin Solid Films 318 Ž1998. 209–214
Structural evolution of Auw001xrNi MBE thin films and Au 1yc Ni c solid solutions with temperature: a HREM study Pascale Bayle-Guillemaud ) , Cyril Dressler, Gregory Abadias, Jany Thibault ´
1
Departement de Recherche Fondamentale sur la Matiere ´ ` Condensee, ´ CEA-Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France
Abstract The evolution with temperature of Ni MBE Žmolecular beam epitaxy. ultra-thin films embedded between Au layers is described. Strong oscillations in the contrast, with a period of 4 atomic planes in the growth direction, appear on high-resolution electron microscopy ŽHREM. images. These oscillations are related to chemical modulations. For comparison, the thermal stability of solid solutions has been studied, where the same feature is found. The main conclusion is that anisotropic decomposition occurs only when the film or the solid solution is under an in-plane stress. q 1998 Elsevier Science S.A. Keywords: Auw001xrNi MBE thin films; Au 1y c Ni c solid solutions; High-resolution electron microscopy ŽHREM.
1. Introduction The molecular beam epitaxy ŽMBE. technique permits the growth of metastable structures such as strained AurNi multilayers. Theoretically, Ni cannot grow coherently on w001xAu due to a large difference in the lattice parameter Ž15%.. It has been shown experimentally w1x that films can, however, be grown due to a stress-induced interdiffussion during growth. Consequently, the misfit is reduced, and the coherent growth of Ni can occur for film thickness up to 6 monolayers ŽMLs.. Afterwards, the residual stresses are relaxed by a mechanism of phase transformation instead of the expected dislocation mechanism w1,2x. This paper addresses the structural evolution with temperature of these AurNi multilayers. HREM observations have shown that strong oscillations of contrast Žwith a period of about 0.8 nm. in the growth direction appears in specimens heated to between 200 to 2808C. At higher temperature or longer annealing times, Ni diffusion takes place, and the oscillating configuration disappears. We will describe the occurrence and morphology of the oscillations as a function of the thickness of the Ni layers and the heating temperature. We will compare this with results obtained on annealed Au 1y c Ni c solid solutions and discuss this decomposition. The solid solutions structures have also been studied as a function of temperature by X-ray measurements and these
results are presented elsewhere in this conference by Abadias et al. w3x.
2. Specimen growth, treatments and observation The samples have been grown by MBE at room temperature on a 50-nm thick Auw001x buffer deposited on a w001xMgO substrate. Two sets of specimens have been grown: the first one was made of AurmNi multilayers Žm ranging from 1 to 8 ML. and the second was made of Au 1y c Ni c alloys Ž c s 0.28, 0.4 and 0.5. with various thicknesses. The experimental details of the growth process have been described previously w4x. The samples were cut in order to have one as-grown reference sample and pieces subjected to different heat treatments in a good vacuum chamber Ž1 = 10y6 Torr.. A standard procedure was used to prepare the cross-sections for HREM observations. The HREM observations were carried out on a Jeol 4000EX microscope operating at 400 kV with a spherical aberration Cs s 1 mm and equipped with a top entry goniometer stage.
3. Evolution of thin films with temperature 3.1. Structure of as-grown thin films
) Corresponding author. Fax: q 33-04-76-885097; e-mail: [email protected]. 1 Member of the Centre National de la Recherche Scientifique ŽCNRS..
0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 1 1 6 7 - X
Fig. 1a shows the as-grown thin films with 4 Ni ML embedded in 5-nm thick Au layers. The HREM image is a
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P. Bayle-Guillemaud et al.r Thin Solid Films 318 (1998) 209–214
fects appear. Fig. 1b shows a typical profile of the variation of the lattice spacing in the growth direction extracted by processing the HREM picture by a method described in Ref. w1x. The profile extends over about 10 atomic planes, whereas the nominal number of evaporated Ni plane is only 4. The profile has been used to extract the Ni chemical profile that exhibits the same feature. The main result is that the maximum of Ni per plane is 80%. In fact, as pointed out in Section 1, segregation of Au atoms during growth introduces an intermixing of Au and Ni, which reduces the misfit and allows a coherent growth of Ni up to 5 ML w1x. 3.2. Structure of thin films after heat treatment
Fig. 1. Ža. w011x HREM image showing an MBE as-grown multilayer AurNiŽ4ML.rAu. Žb. Typical profile of the variation of the lattice spacing in the growth direction Žsuperimposed the original profile and profile filtered in order to reduced the noise.. Spacing values are given in ˚ A.
cross-section along w110x. As shown in Ref. w1x, despite the 15% misfit between Ni and Au, coherent growth of Ni on top of w001x Au occurs: neither dislocations or other de-
Fig. 2a,b shows the same multilayers after a 2-min annealing at 2308C and 2808C, respectively. The main feature is the occurrence of contrast oscillation in the growth direction at the position of the initial Ni layers which were not present in Fig. 1a. Three oscillations are visible and in both cases; the period l is of 4 atomic planes. At 2308C, the contrast intensity is smoother than at 2808C, where one atomic plane exhibits a higher intensity than the three others. At 2308C, the contrast is, moreover, made of two bright and two darker atomic planes. No additional defect has appeared, and the coherency is preserved. Furthermore, the interfaces between the modulated layers and Au seem to be more straight than in the as-grown sample.
Fig. 2. w011x HREM images showing the evolution of Fig. 1 sample after a 2-min annealing at T s 2308C Ža. and 2808C Žb.. Žc. and Žd. show the profile of the variation of the lattice spacing.
P. Bayle-Guillemaud et al.r Thin Solid Films 318 (1998) 209–214
Fig. 2c,d shows the lattice spacing profiles extracted from the samples of Fig. 2a,b. Oscillations of the parameters mirror those of the intensity. Due to the nonlinearity in HREM imaging process, a quantitative chemical profile is difficult to extract. Nevertheless, one can say that these oscillations in intensity and in distortion are undoubtedly related to chemical modulations between the Au and Ni concentrations.
3.3. Modulations occurrence as a function of Ni layer thickness The occurrence of the modulations has been checked as a function of the Ni layer thickness t Žranging from 1 to 8 ML.. The multilayer has been annealed at 2208C for 2 min. Oscillations of the contrast appear on the HREM micrographs for t s 2, 3, 4, 5 ML and are no longer present for thicker layers. One has to note that the period l remains the same, i.e., 4 atomic planes. Nevertheless, the number of periods increases with the thickness Ž1 period for 2 ML up to 4 periods for 5 ML.. No oscillation is seen for a single isolated monolayer. Two conclusions are drawn from these experiments. Ži. The oscillations are driven by the in-plane stress. Indeed, as soon as the stresses are relaxed, i.e., for a Ni thickness larger than 5 ML w1,2x, the oscillations no longer take place for the experimental conditions studied. Žii. Since, in fact, the Ni layer is mixed with Au atoms, the observed oscillations take place in a pseudo-solid solution. In order to get a better knowledge of the decomposition of Au 1y c Ni c as a function of the Ni composition c, a study has been carried out on Au–Ni solid solution embedded between Auw001x layers.
211
4. Evolution of solid solutions with temperature Au 1y c Ni c solid solution have been grown with a wellcontrolled composition on a w001xAu buffer. The goal of this study was to investigate with heating temperature the effect of the initial composition of the film, as well as its thickness on the chemical modulations. Thus, three sets of samples have be grown with c s 0.3, 0.4 and 0.5 with two thicknesses of the layers: t s 20 and 30 ML for the c s 0.3 and 0.5 films. 4.1. Structure of as-grown solid solutions The critical thickness for which the layer relaxes the epitaxial strain due to the misfit Ž m. depends on the Ni concentration of the solid solution. For c s 0.5 Ž m s 7%., few defects are present in the 20-ML thick layer whereas in the 30-ML thick layer, a high density of defects partially releases the strain as observed in the HREM images ŽFig. 3.. For c s 0.4 Ž m s 5%. and t s 20 ML, the layer is homogeneous, and only a few defects have been observed showing that the layer is still highly strained by the Au buffer. For c s 0.3 Ž m s 4%. and t s 20 ML, no defects are detected, and the solid solution is homogeneous ŽFig. 4., while as soon as the thickness is increased Ž t s 30 ML., defects appear. 4.2. Structure of solid solutions after heat treatment It appears from the series of heat treatments of these solid solutions that for the partially relaxed as-grown layers Ž c s 0.5; t s 20, 30 ML and c s 0.3; t s 30 ML., a low heating temperature ŽT - 2208C. for short processing
Fig. 3. w011x HREM images of a Au 1y 0.5 Ni 0.5 solid solution Ž t s 30 ML.: a high-density of defects that release the strain are visible.
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Fig. 4. w011x HREM images of a Au 1y 0.3 Ni 0.3 solid solution Ž t s 20 ML.: homogeneous layer without any defect.
times Ž2 min. does not give rise to modulation. As soon as the time and temperature is increased Ž2 h at 2808C., it seems that a few modulations appear on the HREM images with a very low contrast as compared to the ones obtained on the AurNi multilayers. It is therefore difficult to conclude if a decomposition has occurred. Fig. 5 shows a HREM view in cross-section along the w110x direction of the Au 0.6 Ni 0.4 solid solution Ž t s 20
ML. heated 95 h at 2008C. Modulations of the contrast are clearly visible in the growth direction localised in the area of the solid solution layer. They exhibit the same kind of contrast observed in the multilayers described in Section 3.2. Periods of 3 and 4 monolayers have been detected. Both could be observed in the growth direction. In the in-plane direction, the characteristic distance between the domains is of about 10 nm; they are separated by steps or
Fig. 5. w001x HREM images of a Au 1y 0.4 Ni 0.4 solid solution Ž t s 20 ML. after a heat treatment at 2008C for 95 h. In inset: the electron diffraction showing the additional spots due to the modulated structure.
P. Bayle-Guillemaud et al.r Thin Solid Films 318 (1998) 209–214
213
Fig. 6. w011x HREM images of a Au 1y 0.28 Ni 0.28 solid solution Ž t s 20 ML. after a heat treatment at 2208C for 24 h.
antiphase. The inset in Fig. 5 gives the electron diffraction pattern where the spots due to the AuNi solid solution and the modulated structure are present. This last spot corresponds to an average period l of 3.6 Ž002. monolayers, which is in agreement with the period observed on the HREM images. The contrast of the modulations is relatively high as we can see on the Fig. 5. The images often consist of a plane with a high intensity followed by 2 or 3 planes with a less intensity. This has been interpreted as due to the presence of Ni-rich planes followed by Au-rich planes, the difference in composition being more than 20%. Coherency in the growth plane is kept, and no additional defects appear. Fig. 6 shows a HREM view in cross-section along the w110x direction of the Au 0.7 Ni 0.3 solid solution Ž t s 30 ML. heated for 24 h at 2208C. Modulations of the contrast are visible with the same characteristics as before. The periods observed in this sample are slightly larger. Areas with a period l of 4 and 5 monolayers have been observed. The modulations are less homogeneous than the ones of the previously discussed samples, and the characteristic size of the domains is of few nanometers. A X-ray diffraction study of this sample as a function of temperature is presented in Ref. w3x.
5. Discussion and conclusion The main result of this study is the occurrence after heat treatments of strongly anisotropic composition modulations in Au–Ni strained thin films and solid solutions. In this case, the HREM technique has been a powerful tool to
detect the premises of the phenomenon that affects only a small volume in the materials observed. The X-ray technique allow one to follow the growth of these oscillations or their disappearance over a wide range of time and temperature w3x, which would be tedious using TEM. In that sense, the two techniques are complementary. The results obtained by HREM are, at the moment, qualitative. The quantitative interpretation of the images is rather difficult due to the number of parameters to be introduced. In particular, the preparation may introduce some artefacts, and these have to be controlled as explained in Ref. w5x. Furthermore, due to strong nonlinearities in the imaging process in a TEM, even with a precise measurement of the contrast and of the lattice spacing, it is difficult to give an accurate composition profile for such strong chemical variations with short period. The occurrence of decomposition in AuNi bulk system after ageing treatments has been known for a long time. The paper by Hofer and Warbicher w6x gives a good review of the theories on spinodal decomposition, as well as experiments made on bulk AuNi. They made electron diffraction experiments themselves and recalculated the chemical and the coherent spinodals: they found reasonable agreement between calculated and observed spinodal. In particular, they gave a curve relating the decomposition period to the composition lŽ c .. The spinodal decomposition was found to take place along the three elastically soft ²001: axes as expected. In all experiments, the uncertainties are important. In particular, the increase of l with temperature as expected by theory is difficult to detect because simultaneously, the amplitude of the oscillation decreases. Furthermore, since the temperature is low, the
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diffusion is small, and consequently the detection of the variation is difficult. Our results are located on the curve lŽ c . given in Ref. w6x, and it seems that l increases as c decreases. This point has to be confirmed on another composition with c in the range of 0.15 to 0.2, where the variation of l would be more significant. In our case, due to the in-plane stress and the resulting tetragonal structure of the solid solution, the decomposition is favoured in only one 001 axis: the tetragonal axis Žparallel to the growth direction.. The role of the stress seems to be dominant in the anisotropy of the oscillations. Thus, the modulation wave vector is parallel to the tetragonal axis, and, depending on the stress level, the modulations are more or less coherent laterally: if the strain is high as in the case of multilayers, only a few antiphase boundaries are present. When the stress is reduced, the oscillations are not so homogeneous laterally, and if the solid solution completely relaxes, no oscillation appears in our experiments. One may ask, whether there are modulations simultaneously in all three directions, which could not be detected in our experiments. This point has to be explored. The main point discussed has been related to the amplitude of chemical variations. The spinodal decomposition in the bulk gives rise to modulations with amplitude of only a
few percent. In our HREM images, the contrast is certainly due to strong compositional variations over a few planes in each period: more than 20%, one plane being strongly enriched in Ni. The question arises as to whether the modulated structure may be considered as a Au 1y xrNi x ordered compound or corresponds to a continuous anisotropic decomposition.
Acknowledgements Special thanks are due to B. Gilles, A. Marty and I. Schuster for stimulating and fruitful discussions.
References w1x P. Bayle, T. Deutsch, B. Gilles, F. Lanc¸on, A. Marty, J. Thibault, Ultramicroscopy 94 Ž1994. 94. w2x P. Bayle, PhD Thesis, Universite´ Joseph Fourier, Grenoble, 1994. w3x G. Abadias, I. Schuster, B. Gilles, A. Marty, this symposium. w4x B. Gilles, J. Eymery, A. Marty, J.C. Joud, A. Chamberod, Mater. Res. Soc. Symp. Proc. 237 Ž1992. 511. w5x P. Bayle-Guillemaud, J. Thibault, Microsc. Microanal. Microstru. 8 Ž1997. under press. w6x F. Hofer, P. Warbicher, Z. Metalld. 76 Ž1985. 11.
Structural evolution of Auw001xrNi MBE thin films and Au 1yc Ni c solid solutions with temperature: a HREM study Pascale Bayle-Guillemaud ) , Cyril Dressler, Gregory Abadias, Jany Thibault ´
1
Departement de Recherche Fondamentale sur la Matiere ´ ` Condensee, ´ CEA-Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France
Abstract The evolution with temperature of Ni MBE Žmolecular beam epitaxy. ultra-thin films embedded between Au layers is described. Strong oscillations in the contrast, with a period of 4 atomic planes in the growth direction, appear on high-resolution electron microscopy ŽHREM. images. These oscillations are related to chemical modulations. For comparison, the thermal stability of solid solutions has been studied, where the same feature is found. The main conclusion is that anisotropic decomposition occurs only when the film or the solid solution is under an in-plane stress. q 1998 Elsevier Science S.A. Keywords: Auw001xrNi MBE thin films; Au 1y c Ni c solid solutions; High-resolution electron microscopy ŽHREM.
1. Introduction The molecular beam epitaxy ŽMBE. technique permits the growth of metastable structures such as strained AurNi multilayers. Theoretically, Ni cannot grow coherently on w001xAu due to a large difference in the lattice parameter Ž15%.. It has been shown experimentally w1x that films can, however, be grown due to a stress-induced interdiffussion during growth. Consequently, the misfit is reduced, and the coherent growth of Ni can occur for film thickness up to 6 monolayers ŽMLs.. Afterwards, the residual stresses are relaxed by a mechanism of phase transformation instead of the expected dislocation mechanism w1,2x. This paper addresses the structural evolution with temperature of these AurNi multilayers. HREM observations have shown that strong oscillations of contrast Žwith a period of about 0.8 nm. in the growth direction appears in specimens heated to between 200 to 2808C. At higher temperature or longer annealing times, Ni diffusion takes place, and the oscillating configuration disappears. We will describe the occurrence and morphology of the oscillations as a function of the thickness of the Ni layers and the heating temperature. We will compare this with results obtained on annealed Au 1y c Ni c solid solutions and discuss this decomposition. The solid solutions structures have also been studied as a function of temperature by X-ray measurements and these
results are presented elsewhere in this conference by Abadias et al. w3x.
2. Specimen growth, treatments and observation The samples have been grown by MBE at room temperature on a 50-nm thick Auw001x buffer deposited on a w001xMgO substrate. Two sets of specimens have been grown: the first one was made of AurmNi multilayers Žm ranging from 1 to 8 ML. and the second was made of Au 1y c Ni c alloys Ž c s 0.28, 0.4 and 0.5. with various thicknesses. The experimental details of the growth process have been described previously w4x. The samples were cut in order to have one as-grown reference sample and pieces subjected to different heat treatments in a good vacuum chamber Ž1 = 10y6 Torr.. A standard procedure was used to prepare the cross-sections for HREM observations. The HREM observations were carried out on a Jeol 4000EX microscope operating at 400 kV with a spherical aberration Cs s 1 mm and equipped with a top entry goniometer stage.
3. Evolution of thin films with temperature 3.1. Structure of as-grown thin films
) Corresponding author. Fax: q 33-04-76-885097; e-mail: [email protected]. 1 Member of the Centre National de la Recherche Scientifique ŽCNRS..
0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 1 1 6 7 - X
Fig. 1a shows the as-grown thin films with 4 Ni ML embedded in 5-nm thick Au layers. The HREM image is a
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P. Bayle-Guillemaud et al.r Thin Solid Films 318 (1998) 209–214
fects appear. Fig. 1b shows a typical profile of the variation of the lattice spacing in the growth direction extracted by processing the HREM picture by a method described in Ref. w1x. The profile extends over about 10 atomic planes, whereas the nominal number of evaporated Ni plane is only 4. The profile has been used to extract the Ni chemical profile that exhibits the same feature. The main result is that the maximum of Ni per plane is 80%. In fact, as pointed out in Section 1, segregation of Au atoms during growth introduces an intermixing of Au and Ni, which reduces the misfit and allows a coherent growth of Ni up to 5 ML w1x. 3.2. Structure of thin films after heat treatment
Fig. 1. Ža. w011x HREM image showing an MBE as-grown multilayer AurNiŽ4ML.rAu. Žb. Typical profile of the variation of the lattice spacing in the growth direction Žsuperimposed the original profile and profile filtered in order to reduced the noise.. Spacing values are given in ˚ A.
cross-section along w110x. As shown in Ref. w1x, despite the 15% misfit between Ni and Au, coherent growth of Ni on top of w001x Au occurs: neither dislocations or other de-
Fig. 2a,b shows the same multilayers after a 2-min annealing at 2308C and 2808C, respectively. The main feature is the occurrence of contrast oscillation in the growth direction at the position of the initial Ni layers which were not present in Fig. 1a. Three oscillations are visible and in both cases; the period l is of 4 atomic planes. At 2308C, the contrast intensity is smoother than at 2808C, where one atomic plane exhibits a higher intensity than the three others. At 2308C, the contrast is, moreover, made of two bright and two darker atomic planes. No additional defect has appeared, and the coherency is preserved. Furthermore, the interfaces between the modulated layers and Au seem to be more straight than in the as-grown sample.
Fig. 2. w011x HREM images showing the evolution of Fig. 1 sample after a 2-min annealing at T s 2308C Ža. and 2808C Žb.. Žc. and Žd. show the profile of the variation of the lattice spacing.
P. Bayle-Guillemaud et al.r Thin Solid Films 318 (1998) 209–214
Fig. 2c,d shows the lattice spacing profiles extracted from the samples of Fig. 2a,b. Oscillations of the parameters mirror those of the intensity. Due to the nonlinearity in HREM imaging process, a quantitative chemical profile is difficult to extract. Nevertheless, one can say that these oscillations in intensity and in distortion are undoubtedly related to chemical modulations between the Au and Ni concentrations.
3.3. Modulations occurrence as a function of Ni layer thickness The occurrence of the modulations has been checked as a function of the Ni layer thickness t Žranging from 1 to 8 ML.. The multilayer has been annealed at 2208C for 2 min. Oscillations of the contrast appear on the HREM micrographs for t s 2, 3, 4, 5 ML and are no longer present for thicker layers. One has to note that the period l remains the same, i.e., 4 atomic planes. Nevertheless, the number of periods increases with the thickness Ž1 period for 2 ML up to 4 periods for 5 ML.. No oscillation is seen for a single isolated monolayer. Two conclusions are drawn from these experiments. Ži. The oscillations are driven by the in-plane stress. Indeed, as soon as the stresses are relaxed, i.e., for a Ni thickness larger than 5 ML w1,2x, the oscillations no longer take place for the experimental conditions studied. Žii. Since, in fact, the Ni layer is mixed with Au atoms, the observed oscillations take place in a pseudo-solid solution. In order to get a better knowledge of the decomposition of Au 1y c Ni c as a function of the Ni composition c, a study has been carried out on Au–Ni solid solution embedded between Auw001x layers.
211
4. Evolution of solid solutions with temperature Au 1y c Ni c solid solution have been grown with a wellcontrolled composition on a w001xAu buffer. The goal of this study was to investigate with heating temperature the effect of the initial composition of the film, as well as its thickness on the chemical modulations. Thus, three sets of samples have be grown with c s 0.3, 0.4 and 0.5 with two thicknesses of the layers: t s 20 and 30 ML for the c s 0.3 and 0.5 films. 4.1. Structure of as-grown solid solutions The critical thickness for which the layer relaxes the epitaxial strain due to the misfit Ž m. depends on the Ni concentration of the solid solution. For c s 0.5 Ž m s 7%., few defects are present in the 20-ML thick layer whereas in the 30-ML thick layer, a high density of defects partially releases the strain as observed in the HREM images ŽFig. 3.. For c s 0.4 Ž m s 5%. and t s 20 ML, the layer is homogeneous, and only a few defects have been observed showing that the layer is still highly strained by the Au buffer. For c s 0.3 Ž m s 4%. and t s 20 ML, no defects are detected, and the solid solution is homogeneous ŽFig. 4., while as soon as the thickness is increased Ž t s 30 ML., defects appear. 4.2. Structure of solid solutions after heat treatment It appears from the series of heat treatments of these solid solutions that for the partially relaxed as-grown layers Ž c s 0.5; t s 20, 30 ML and c s 0.3; t s 30 ML., a low heating temperature ŽT - 2208C. for short processing
Fig. 3. w011x HREM images of a Au 1y 0.5 Ni 0.5 solid solution Ž t s 30 ML.: a high-density of defects that release the strain are visible.
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P. Bayle-Guillemaud et al.r Thin Solid Films 318 (1998) 209–214
Fig. 4. w011x HREM images of a Au 1y 0.3 Ni 0.3 solid solution Ž t s 20 ML.: homogeneous layer without any defect.
times Ž2 min. does not give rise to modulation. As soon as the time and temperature is increased Ž2 h at 2808C., it seems that a few modulations appear on the HREM images with a very low contrast as compared to the ones obtained on the AurNi multilayers. It is therefore difficult to conclude if a decomposition has occurred. Fig. 5 shows a HREM view in cross-section along the w110x direction of the Au 0.6 Ni 0.4 solid solution Ž t s 20
ML. heated 95 h at 2008C. Modulations of the contrast are clearly visible in the growth direction localised in the area of the solid solution layer. They exhibit the same kind of contrast observed in the multilayers described in Section 3.2. Periods of 3 and 4 monolayers have been detected. Both could be observed in the growth direction. In the in-plane direction, the characteristic distance between the domains is of about 10 nm; they are separated by steps or
Fig. 5. w001x HREM images of a Au 1y 0.4 Ni 0.4 solid solution Ž t s 20 ML. after a heat treatment at 2008C for 95 h. In inset: the electron diffraction showing the additional spots due to the modulated structure.
P. Bayle-Guillemaud et al.r Thin Solid Films 318 (1998) 209–214
213
Fig. 6. w011x HREM images of a Au 1y 0.28 Ni 0.28 solid solution Ž t s 20 ML. after a heat treatment at 2208C for 24 h.
antiphase. The inset in Fig. 5 gives the electron diffraction pattern where the spots due to the AuNi solid solution and the modulated structure are present. This last spot corresponds to an average period l of 3.6 Ž002. monolayers, which is in agreement with the period observed on the HREM images. The contrast of the modulations is relatively high as we can see on the Fig. 5. The images often consist of a plane with a high intensity followed by 2 or 3 planes with a less intensity. This has been interpreted as due to the presence of Ni-rich planes followed by Au-rich planes, the difference in composition being more than 20%. Coherency in the growth plane is kept, and no additional defects appear. Fig. 6 shows a HREM view in cross-section along the w110x direction of the Au 0.7 Ni 0.3 solid solution Ž t s 30 ML. heated for 24 h at 2208C. Modulations of the contrast are visible with the same characteristics as before. The periods observed in this sample are slightly larger. Areas with a period l of 4 and 5 monolayers have been observed. The modulations are less homogeneous than the ones of the previously discussed samples, and the characteristic size of the domains is of few nanometers. A X-ray diffraction study of this sample as a function of temperature is presented in Ref. w3x.
5. Discussion and conclusion The main result of this study is the occurrence after heat treatments of strongly anisotropic composition modulations in Au–Ni strained thin films and solid solutions. In this case, the HREM technique has been a powerful tool to
detect the premises of the phenomenon that affects only a small volume in the materials observed. The X-ray technique allow one to follow the growth of these oscillations or their disappearance over a wide range of time and temperature w3x, which would be tedious using TEM. In that sense, the two techniques are complementary. The results obtained by HREM are, at the moment, qualitative. The quantitative interpretation of the images is rather difficult due to the number of parameters to be introduced. In particular, the preparation may introduce some artefacts, and these have to be controlled as explained in Ref. w5x. Furthermore, due to strong nonlinearities in the imaging process in a TEM, even with a precise measurement of the contrast and of the lattice spacing, it is difficult to give an accurate composition profile for such strong chemical variations with short period. The occurrence of decomposition in AuNi bulk system after ageing treatments has been known for a long time. The paper by Hofer and Warbicher w6x gives a good review of the theories on spinodal decomposition, as well as experiments made on bulk AuNi. They made electron diffraction experiments themselves and recalculated the chemical and the coherent spinodals: they found reasonable agreement between calculated and observed spinodal. In particular, they gave a curve relating the decomposition period to the composition lŽ c .. The spinodal decomposition was found to take place along the three elastically soft ²001: axes as expected. In all experiments, the uncertainties are important. In particular, the increase of l with temperature as expected by theory is difficult to detect because simultaneously, the amplitude of the oscillation decreases. Furthermore, since the temperature is low, the
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P. Bayle-Guillemaud et al.r Thin Solid Films 318 (1998) 209–214
diffusion is small, and consequently the detection of the variation is difficult. Our results are located on the curve lŽ c . given in Ref. w6x, and it seems that l increases as c decreases. This point has to be confirmed on another composition with c in the range of 0.15 to 0.2, where the variation of l would be more significant. In our case, due to the in-plane stress and the resulting tetragonal structure of the solid solution, the decomposition is favoured in only one 001 axis: the tetragonal axis Žparallel to the growth direction.. The role of the stress seems to be dominant in the anisotropy of the oscillations. Thus, the modulation wave vector is parallel to the tetragonal axis, and, depending on the stress level, the modulations are more or less coherent laterally: if the strain is high as in the case of multilayers, only a few antiphase boundaries are present. When the stress is reduced, the oscillations are not so homogeneous laterally, and if the solid solution completely relaxes, no oscillation appears in our experiments. One may ask, whether there are modulations simultaneously in all three directions, which could not be detected in our experiments. This point has to be explored. The main point discussed has been related to the amplitude of chemical variations. The spinodal decomposition in the bulk gives rise to modulations with amplitude of only a
few percent. In our HREM images, the contrast is certainly due to strong compositional variations over a few planes in each period: more than 20%, one plane being strongly enriched in Ni. The question arises as to whether the modulated structure may be considered as a Au 1y xrNi x ordered compound or corresponds to a continuous anisotropic decomposition.
Acknowledgements Special thanks are due to B. Gilles, A. Marty and I. Schuster for stimulating and fruitful discussions.
References w1x P. Bayle, T. Deutsch, B. Gilles, F. Lanc¸on, A. Marty, J. Thibault, Ultramicroscopy 94 Ž1994. 94. w2x P. Bayle, PhD Thesis, Universite´ Joseph Fourier, Grenoble, 1994. w3x G. Abadias, I. Schuster, B. Gilles, A. Marty, this symposium. w4x B. Gilles, J. Eymery, A. Marty, J.C. Joud, A. Chamberod, Mater. Res. Soc. Symp. Proc. 237 Ž1992. 511. w5x P. Bayle-Guillemaud, J. Thibault, Microsc. Microanal. Microstru. 8 Ž1997. under press. w6x F. Hofer, P. Warbicher, Z. Metalld. 76 Ž1985. 11.