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Optical properties of metastable AgCu alloy films
Thin Solid Films, 37 (1976) 345 350 © Elsevier Sequoia S.A., Lausanne----Printed in Switzerland
345
OPTICAL PROPERTIES OF METASTABLE Ag--Cu ALLOY FILMS *
J. RIVORY Laboratoire d'Optique des Solides, Universit~ P. & M. Curie, 4 place Jussieu, 75230-Paris C~dex 05 (France) (Received August 25, 1975; accepted March 2, 1976)
Thin films of metastable Ag--Cu alloys over the whole concentration range have been obtained by vapor quenching on cold amorphous substrates. Their optical properties were deduced from reflectance and transmittance measurements between 0.5 and 6 eV. The influence of preparation conditions on the film structure and optical properties was carefully investigated.
The electronic structure of substitutionally disordered alloys has recently raised considerable interest and important theoretical effort has been made. Optical and photoemission experiments can lead to very useful information on the electronic density of states in alloys. The Ag-Cu system is particularly interesting because of the respective positions and widths of the d bands of the two constituents, but very few studies have been performed on these alloys because of experimental difficulties 1' 2. It must be recalled that Cu and Ag have only a very small solid solubility in each other 3. We have therefore tried to obtain metastable solid solutions as thin films over the whole composition range by co-evaporation of the two metals from the same tungsten crucible onto amorphous silica substrates. The film thickness was obtained by X-ray interferences in reflection at grazing incidence 4 and the alloy concentration by microprobe analysis 5. Structure studies show that true solid solutions are obtained only under very stringent temperature conditions and that phase separation occurs easily, especially at high concentrations. This point has to be emphasized, because insufficient characterization of the alloy structure may lead to important errors in the interpretation of optical data. 1. PREPARATION CONDITIONS AND STRUCTURE STUDIES
1.1. Deposition on room temperature substrates
At first sight, the films deposited at about 100 A s-1 on substrates at room temperature seem to be homogeneous, both by transmission electron microscopy and by electron microprobe analysis (this technique probes 1 ~tm2 areas). It is Paper presented at the Third International Conference on Thin Films, "Basic Problems, Applications and Trends", Budapest, Hungary, August 25-29, 1975, but appearing in the Conference Proceedings only in abstract form.
346
J. RIVORY
very instructive, however, to look at the electron diffraction patterns. For Ag-rich alloys all the observed diffraction rings are identical with those of pure Ag; similarly, for Cu-rich alloys the diffraction rings are those of pure Cu, except for one or two rings which correspond to the oxide Cu20. For alloys in the 50 concentration range, however, the diffraction pattern consists of rings of pure Ag and some rings of pure Cu. It must be inferred that as-deposited films are already partially phase-separated. It may be suggested that, either during deposition or after the film formation, even at room temperature, the atoms are able to migrate and rearrange themselves into assemblies of like atoms; we assume that the impurity atoms then lie along the grain boundaries of the matrix and/or form clusters and even small crystallites at the film surface, as suggested by the observations performed after annealing. 1.2. Deposition on low temperature substrates
The films evaporated onto low temperature substrates (120 K) and heated to room temperature for structure and optical studies are made of very small crystallites. Between 35 and 65 at. ~o, Mader et al. 6 found amorphous alloys. Figure l(a) shows the micrograph of a 55 at. ~ Cu alloy, for which the mean crystallite size is about 13 A. The corresponding diffraction pattern (Fig. l(b)) consists of broadened and blurred rings, but it seems that this alloy, although mostly amorphous, contains some microcrystallites. Thus all the observations tend to prove that the microcrystalline alloy films deposited on low temperature substrates are still true Ag-Cu solid solutions when studied at room temperature. 1.3. Annealed films
After annealing at 250°-300 °C, large dark areas can be seen on the micrographs; microprobe analysis of these areas shows that they consist of Cu-rich clusters. For a 50 at. ~o Cu alloy, Cu monocrystals seem to grow on top of the matrix crystallites and in epitaxy with them; indeed, honeycomb-like moir6 patterns can sometimes be seen on the micrographs (Fig. 2(a)) and a supplementary dotted ring appears in the diffraction pattern (Fig. 2(b)). 2.
RESISTIVITY MEASUREMENTS
The electrical resistivity of the alloy films was measured by a four-point method after deposition and during and after annealing. As shown in Fig. 3, for dilute alloys deposited at room temperature the resistivity increases rapidly with impurity concentration, but at higher concentration its variation becomes much smaller, which is again indicative of the phase separation occurring in these films, especially at high concentrations. In contrast, the values corresponding to the films deposited on substrates at low temperature, whose structure is closer to a solid solution, are in general much higher than those corresponding to the films of the first series; however, the resistivity increase is due not only to an impurity effect but also to the poorer crystallographic structure of the films (very small crystallites). After annealing, the room temperature resistivity of almost all films, independently of their composition, is close to 3 ~ cm,
METASTABLE A g ~ u ALLOY FILMS
347
which is the mean value obtained for pure Ag and pure Cu films with similar structure (the ideal bulk values for pure Ag and pure Cu are about the s a m e 1.7 ~ cm).
3.
OPTICAL MEASUREMENTS
The complex dielectric constant g = e x + ie 2 w a s determined between 0.5 and 6 eV from the film reflectance and transmittance in air at room temperature 7, the film thickness being known.
(a)
(b) Fig. 1. (a) Electron micrograph and (b) electron diffraction pattern for an Ag-55 at. ~ Cu alloy deposited on a low temperature substrate.
348
J. RIVORY
(a)
(b) Fig. 2. (a) Electron micrograph showing honeycomb-like moir6 patterns and (b) electron diffraction diagram with a supplementary ring for an Ag-50 at. ~ Cu film after annealing. Figure 4 shows the imaginary part e 2 of the dielectric constant as a function of energy between 2 and 6 eV for a number of A g - C u alloys as well as for pure Ag and pure Cu. For Cu-rich alloys the e2 spectrum is very similar to the Cu spectrum. For Ag-rich alloys it consists of two parts: a high energy part which is very similar to the interband absorption of pure Ag, and a supplementary absorption between 2.5 and 3.5 eV due to Cu impurities 8. This impurity contribution can be analysed in terms of the virtual bound state model 9. The influence of preparation conditions on the optical absorption was studied on an Ag-30 at.
METASTABLE Ag-Cu ALLOY FILMS
349
~(Ix~ cm)
E2
Cu
I.
/
6
: "
!. 2
Xcu: 0.3
f oee TS = f20°K + + + Ts : 3(X)°K ooo annealed 6O0°K at
bulk
bulk
Ag
I
I
0.50
Cu
0
I
2
3
I
/.
5
p
eV
Fig. 3. Resistivity v s . Cu concentration for Ag-Cu films: O deposited on a room temperature substrate; • deposited on a low temperature substrate; x after annealing. Fig. 4. Imaginary part e2 of the dielectric constant v s . energy for Ag-Cu alloys and for pure Ag and pure Cu.
Cu alloy (Fig. 4). For a real solid solution (film deposited at low temperature), where most of the impurity atoms are isolated within the Ag matrix, the Cu partial d density of states is centered at about 3 eV below the Fermi level. After annealing (complete phase separation), the e2 spectrum can be considered as some weighted sum of the spectra relative to the pure metals, and the Cu partial d density of states has the same position as in pure Cu. For a film deposited at room temperature with partial phase separation, the position and the shape of the impurity absorption are intermediate between the two previous cases. 4. CONCLUSION We have shown that the use of thin films is a powerful way to study metastable Ag-Cu solid solutions obtained by vapor quenching. We have emphasized that a thorough control of the film crystallographic structure is necessary for a correct interpretation of the optical data. Clustering effects have also been investigated. Although no quantitative conclusions can be drawn, we have shown that clustering in Ag-rich alloys modifies the shape and the position of the impurity optical absorption. These observations are important since a theory of the effects of the local environment in disordered alloys has recently been developedl°. ACKNOWLEDGMENTS
The author whishes to thank Professor F. Abel6s for constant encouragement and Dr. M. L. Th6ye and Dr. F. Brouers for many helpful discussions.
350
J. RIVORY
REFERENCES 1 2 3 4 5 6 7 8 9 10
P.O. Nilsson and G. Forssell, J. Phys. (Paris), Colloq., 35 (1974) C4-57. N.J. Shevchik and A. Goldmann, Electron Spectroscopy Conference, Namur, 1974, unpublished. M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 1958, p. 18. W. Umrath, Z. Angew. Phys., 22 (1967) 406. J. Philibert, J. Rivory, D. Bryckaert and R. Tixier, J. Phys. D, 3 (1970) L70. S. Mader, A. S. Nowick and H. Widmer, Acta Metall., 15 (1967) 203. F. Abel6s and M. L. Th6ye, Surf Sci., 5 (1966) 325. J. Rivory, Phys. Rev. B, to be published. J. Rivory, J. Phys. (Paris), 36 (1975) L129. F. Brouers, F. Gautier and J. van der Rest, J. Phys. F, 5 (1975) 975. J. van der Rest, F. Gautier and F. Brouers, J. Phys. F, 5 (1975) 995.
345
OPTICAL PROPERTIES OF METASTABLE Ag--Cu ALLOY FILMS *
J. RIVORY Laboratoire d'Optique des Solides, Universit~ P. & M. Curie, 4 place Jussieu, 75230-Paris C~dex 05 (France) (Received August 25, 1975; accepted March 2, 1976)
Thin films of metastable Ag--Cu alloys over the whole concentration range have been obtained by vapor quenching on cold amorphous substrates. Their optical properties were deduced from reflectance and transmittance measurements between 0.5 and 6 eV. The influence of preparation conditions on the film structure and optical properties was carefully investigated.
The electronic structure of substitutionally disordered alloys has recently raised considerable interest and important theoretical effort has been made. Optical and photoemission experiments can lead to very useful information on the electronic density of states in alloys. The Ag-Cu system is particularly interesting because of the respective positions and widths of the d bands of the two constituents, but very few studies have been performed on these alloys because of experimental difficulties 1' 2. It must be recalled that Cu and Ag have only a very small solid solubility in each other 3. We have therefore tried to obtain metastable solid solutions as thin films over the whole composition range by co-evaporation of the two metals from the same tungsten crucible onto amorphous silica substrates. The film thickness was obtained by X-ray interferences in reflection at grazing incidence 4 and the alloy concentration by microprobe analysis 5. Structure studies show that true solid solutions are obtained only under very stringent temperature conditions and that phase separation occurs easily, especially at high concentrations. This point has to be emphasized, because insufficient characterization of the alloy structure may lead to important errors in the interpretation of optical data. 1. PREPARATION CONDITIONS AND STRUCTURE STUDIES
1.1. Deposition on room temperature substrates
At first sight, the films deposited at about 100 A s-1 on substrates at room temperature seem to be homogeneous, both by transmission electron microscopy and by electron microprobe analysis (this technique probes 1 ~tm2 areas). It is Paper presented at the Third International Conference on Thin Films, "Basic Problems, Applications and Trends", Budapest, Hungary, August 25-29, 1975, but appearing in the Conference Proceedings only in abstract form.
346
J. RIVORY
very instructive, however, to look at the electron diffraction patterns. For Ag-rich alloys all the observed diffraction rings are identical with those of pure Ag; similarly, for Cu-rich alloys the diffraction rings are those of pure Cu, except for one or two rings which correspond to the oxide Cu20. For alloys in the 50 concentration range, however, the diffraction pattern consists of rings of pure Ag and some rings of pure Cu. It must be inferred that as-deposited films are already partially phase-separated. It may be suggested that, either during deposition or after the film formation, even at room temperature, the atoms are able to migrate and rearrange themselves into assemblies of like atoms; we assume that the impurity atoms then lie along the grain boundaries of the matrix and/or form clusters and even small crystallites at the film surface, as suggested by the observations performed after annealing. 1.2. Deposition on low temperature substrates
The films evaporated onto low temperature substrates (120 K) and heated to room temperature for structure and optical studies are made of very small crystallites. Between 35 and 65 at. ~o, Mader et al. 6 found amorphous alloys. Figure l(a) shows the micrograph of a 55 at. ~ Cu alloy, for which the mean crystallite size is about 13 A. The corresponding diffraction pattern (Fig. l(b)) consists of broadened and blurred rings, but it seems that this alloy, although mostly amorphous, contains some microcrystallites. Thus all the observations tend to prove that the microcrystalline alloy films deposited on low temperature substrates are still true Ag-Cu solid solutions when studied at room temperature. 1.3. Annealed films
After annealing at 250°-300 °C, large dark areas can be seen on the micrographs; microprobe analysis of these areas shows that they consist of Cu-rich clusters. For a 50 at. ~o Cu alloy, Cu monocrystals seem to grow on top of the matrix crystallites and in epitaxy with them; indeed, honeycomb-like moir6 patterns can sometimes be seen on the micrographs (Fig. 2(a)) and a supplementary dotted ring appears in the diffraction pattern (Fig. 2(b)). 2.
RESISTIVITY MEASUREMENTS
The electrical resistivity of the alloy films was measured by a four-point method after deposition and during and after annealing. As shown in Fig. 3, for dilute alloys deposited at room temperature the resistivity increases rapidly with impurity concentration, but at higher concentration its variation becomes much smaller, which is again indicative of the phase separation occurring in these films, especially at high concentrations. In contrast, the values corresponding to the films deposited on substrates at low temperature, whose structure is closer to a solid solution, are in general much higher than those corresponding to the films of the first series; however, the resistivity increase is due not only to an impurity effect but also to the poorer crystallographic structure of the films (very small crystallites). After annealing, the room temperature resistivity of almost all films, independently of their composition, is close to 3 ~ cm,
METASTABLE A g ~ u ALLOY FILMS
347
which is the mean value obtained for pure Ag and pure Cu films with similar structure (the ideal bulk values for pure Ag and pure Cu are about the s a m e 1.7 ~ cm).
3.
OPTICAL MEASUREMENTS
The complex dielectric constant g = e x + ie 2 w a s determined between 0.5 and 6 eV from the film reflectance and transmittance in air at room temperature 7, the film thickness being known.
(a)
(b) Fig. 1. (a) Electron micrograph and (b) electron diffraction pattern for an Ag-55 at. ~ Cu alloy deposited on a low temperature substrate.
348
J. RIVORY
(a)
(b) Fig. 2. (a) Electron micrograph showing honeycomb-like moir6 patterns and (b) electron diffraction diagram with a supplementary ring for an Ag-50 at. ~ Cu film after annealing. Figure 4 shows the imaginary part e 2 of the dielectric constant as a function of energy between 2 and 6 eV for a number of A g - C u alloys as well as for pure Ag and pure Cu. For Cu-rich alloys the e2 spectrum is very similar to the Cu spectrum. For Ag-rich alloys it consists of two parts: a high energy part which is very similar to the interband absorption of pure Ag, and a supplementary absorption between 2.5 and 3.5 eV due to Cu impurities 8. This impurity contribution can be analysed in terms of the virtual bound state model 9. The influence of preparation conditions on the optical absorption was studied on an Ag-30 at.
METASTABLE Ag-Cu ALLOY FILMS
349
~(Ix~ cm)
E2
Cu
I.
/
6
: "
!. 2
Xcu: 0.3
f oee TS = f20°K + + + Ts : 3(X)°K ooo annealed 6O0°K at
bulk
bulk
Ag
I
I
0.50
Cu
0
I
2
3
I
/.
5
p
eV
Fig. 3. Resistivity v s . Cu concentration for Ag-Cu films: O deposited on a room temperature substrate; • deposited on a low temperature substrate; x after annealing. Fig. 4. Imaginary part e2 of the dielectric constant v s . energy for Ag-Cu alloys and for pure Ag and pure Cu.
Cu alloy (Fig. 4). For a real solid solution (film deposited at low temperature), where most of the impurity atoms are isolated within the Ag matrix, the Cu partial d density of states is centered at about 3 eV below the Fermi level. After annealing (complete phase separation), the e2 spectrum can be considered as some weighted sum of the spectra relative to the pure metals, and the Cu partial d density of states has the same position as in pure Cu. For a film deposited at room temperature with partial phase separation, the position and the shape of the impurity absorption are intermediate between the two previous cases. 4. CONCLUSION We have shown that the use of thin films is a powerful way to study metastable Ag-Cu solid solutions obtained by vapor quenching. We have emphasized that a thorough control of the film crystallographic structure is necessary for a correct interpretation of the optical data. Clustering effects have also been investigated. Although no quantitative conclusions can be drawn, we have shown that clustering in Ag-rich alloys modifies the shape and the position of the impurity optical absorption. These observations are important since a theory of the effects of the local environment in disordered alloys has recently been developedl°. ACKNOWLEDGMENTS
The author whishes to thank Professor F. Abel6s for constant encouragement and Dr. M. L. Th6ye and Dr. F. Brouers for many helpful discussions.
350
J. RIVORY
REFERENCES 1 2 3 4 5 6 7 8 9 10
P.O. Nilsson and G. Forssell, J. Phys. (Paris), Colloq., 35 (1974) C4-57. N.J. Shevchik and A. Goldmann, Electron Spectroscopy Conference, Namur, 1974, unpublished. M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 1958, p. 18. W. Umrath, Z. Angew. Phys., 22 (1967) 406. J. Philibert, J. Rivory, D. Bryckaert and R. Tixier, J. Phys. D, 3 (1970) L70. S. Mader, A. S. Nowick and H. Widmer, Acta Metall., 15 (1967) 203. F. Abel6s and M. L. Th6ye, Surf Sci., 5 (1966) 325. J. Rivory, Phys. Rev. B, to be published. J. Rivory, J. Phys. (Paris), 36 (1975) L129. F. Brouers, F. Gautier and J. van der Rest, J. Phys. F, 5 (1975) 975. J. van der Rest, F. Gautier and F. Brouers, J. Phys. F, 5 (1975) 995.