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AES studies of surface composition of AgCu alloys
Surface Science 47 (1975) 57-63 0 North-Holland Publishing Company
AES STUDIES OF SURFACE COMPOSITION OF Ag-Cu ALLOYS P. BRAUN and W. FARBER II. Institut fiir Experimentalphysik,
Technische
Hochschule
Wien, A-l 040 Vienna, Austria
The influence of pretreatments like sputtering, annealing and cleaving on the surface composition is important in quantitative Auger electron spectroscopy. The present paper deals with this influence for the binary alloy system AgoCu with 100,95, 80, 60,50,40, 20,5, and 0 at% Ag. Sputtering causes an enrichment of Cu on the surface due to the higher sputter coefficient of Ag. In contrast cleaving yields an enrichment of Ag due to the lower tensile strength of Ag. The surface composition obtained by scribing is used as standard for the bulk composition because this technique is independent of parameters like sputter coefficient or tensile strength. The results are compared with previous measurements of homogeneous alloy systems.
1. Introduction In the past years several modern techniques for surface analysis have been developed such as electron spectroscopy, i.e., AES [l-3] and UPS [4,5], and ion mass spectroscopy, i.e., SIMS [6,7]. In these or similar techniques great care is required to reach quantitative results. A large number of methods for quantitative Auger analysis has been suggested [8-l 31. In principle, one can choose between two ways to obtain a quantitative analysis [ 14,151. The first is to compare the Auger results with those of another corresponding technique for analyzing surfaces. The second one is to perform a series of suitable standard measurements. This last method has been investigated in the present paper for the case of silver-copper alloys. A series of Ag-Cu alloys of varying bulk composition of 95,80,60,50,40,20, and 5 (all +O.l) at% Ag as well as the pure components Ag and Cu have been studied. The kinetic energy and consequently the escape depth of the Auger e,lectrons of the two metallic components are very different. Hence it is easy to recognize the influence of the cleanliness and pretreatments [ 16-201 of the samples. For systems such as Ni-Cu [ 17,211 on the other hand this effect will not be very important because of the nearly equal escape depth of the Auger electrons considered. For systems with components of which Auger electron energies and escape depth differ strongly, the component with the lower escape depth will be more affected by small amounts of contamination. The chemical composition of the sample thus appears to be changed. In the present study no contaminations were detected after
scribing or cleaving, therefore this influence may be neglected. Furthermore. chemical effects will influence the intensity and peak shape of Auger transitions involving the valence band [22--241. It is obvious that the pretreatment may alter the results considerably. Therefore, a short discussion of the actual cleaning methods used, is given here. (a) Heating by electron bombardment or indirect heating is mainly used to clean pure element samples. However. with alloys heating can bring about a surface enrichment of the component with the lowest heat of sublimation [18,25,26]. (b) With sputtering of alloys the sputter coefficients of the components determine the composition of the surface [ 131, the element with the lower sputter coefficient being concentrated at the surface. The experimentally observed surface composition depends also on the sputter ion energy 1271. Because the topography of the sample influences the surface composition resulting from sputtering, the sample should be given an optically flat polish. (c) Cleaving in ultrahigh vacuum is a good method to get clean surfaces. With this method one has to know whether the fracture runs intergranularly or along grain boundaries. For the study of metallurgical problems such as grain boundary segregation /28] cleaving is the only satisfying procedure. With Ag-- Cu alloys, however, the fracture runs intergranularly; the surface composition is influenced by the different tensile strength of the components. Therefore. the fractured surface is enriched in Ag as shown later. (d) The last method to obtain a clean surface involves scribing the sample with a stainless steel tip or diaInoIld [29]. When the surface is smooth scribing is a simple way to clean the sample. Though the danger of transfer of material from one sample to the other is small. for sophisticated measurements a set of tips is required so that for each sample a new tip can be used.
2. Experimenta The apparatus used in this study has been described previously [22]. Data were obtained for pure polycrystalline Ag-Cu alloy samples with a commercial Auger system with a cylindrical mirror analyzer 12.31 and coaxial electron gun. The primary electron energy was 1700 eV, the beam current 30.uA and the diameter of the electron beam on the target 0.3 mm. The modulation signal had an amplitude of f .2 Vpp and a frequency of IO4 cps. The measured resolution at the elastic peak was 0.4%. All spectra were recorded with 1 eV/sec sweep rate and 300 msec time constant. The polished samples were cleaned previously in an ultrasonic bath. The residual pressure in the system was lower than 10m7 Pa. A fracturing device was arranged in the bell jar system to cleave the samples under UHV conditions. In order to check the reproducibility and to estimate the standard deviation the measurements were carried out repeatedly.
P. Braun, W. F&ber/Surface
composition
59
of Ag--Cu ailoys
50 GO 30 20 IO 0
I
1
0
10
20
30
40
tCmin7
Fig. I. Sputtering of Ag/Cu 80/20 at% with ion energies of 500, 1000 and 1500 eV. The ratio of the measured Ag and Cu Auger peak heights is plotted versus sputtering time. (0) Ag/Cu 920 eV;
(A) AgjCu 60 eV. 3. Results and discussion At first the poiished samples were sputtered by argon ion bombardment to study the influence of the primary energy on the composition of the surface. Therefore, the primary ion energy was increased from 500 to 1000 and 1500 eV for the 8Of20 at% Ag/Cu alloy. Fig. 1 shows the ratio of the Ag and Cu Auger peak to peak heights (APPH) for different sputter ion energies. The ratio was estimated both for the Cu M2,3Me5Mb5 (60eV) and for the Cu L3Mh5M4> (920eV) Auger transition. The difference in composition obtained after a sufficiently long sputtering time is obvious. The maxima are caused by initial contamination of the untreated sample. Sputtering of binary alloys leads to some difficulties in quantitative analysis. On analyzing an unknown alloy sample one has to know exactly the different sputter coefficients that depend on the primary ion energy employed. Because of the considerable discrepancies in the sputter coefficients given by different authors [27], it is not possible to infer the initial composition from the final state of the sample without substantial errors. Furthermore, it is not certain that the sputter coefficients of the pure components are valid for the alloy. Generally, cleaving produces clean surfaces, but with heterogeneous systems having an eutectic point as the Cu-Ag system, the tensile strength of the components can affect the surface composition. Therefore, an enrichment in the component with the lower tensile strength (Ag) can be observed. This behaviour is not so obvi-
P. Braun, W. F~rber/Sur@ce composition
60
of’Ag- -Cu allqvs
.*80
20..
eutectic point 100
0 100
80
60
LO
20
0 At%Agb
0
20
LO
60
80
100At%Cub
Pig. 2. The normalized treatments. (A) scribed,
ANN and CUN versus bulk concentration Agb resp. Cub for different (o) cleaved, using Cu 920 eV; (0) cleaved, using Cu 60 eV; (A) scribed, Cu 920 cV: Cu 60 eV; (0) sputtered, Cu 920 eV; (m) sputtered, Cu 60 eV.
ous for homogeneous mixtures as shown elsewhere for the Ag-Au alloy system [30] . The above pertains, however, only to intergranular fractures. When the fracture runs along the grain boundaries cleaving is a valuable method to study grain boundary segregation. The scribing technique is used as a standard method for determining the bulk composition because no material parameters such as tensile strength or sputter coefficients will influence this cleaning technique. The dependence of the surface composition on the pretreatment of the different alloys is shown in fig. 2. Agb resp. Cub are the bulk concentrations of the iavestigated samples. AgN represents the normalized Ag peak height [26] as calculated by: AgN = ]Ag,KAg,
+ Cum)] x 100.
(1)
Ag, and Cum are the measured APPH’s of Ag and Cu. The values of AgN were calculated both for the 60 eV and for the 920 eV Cu transition. No scale factor was used The same calculation can be made for the normalized Cu peak height CuN. Fig. 2 shows the considerable differences in the composition resulting from the pretreatments (sputtering, cleaving and scribing ). The difference between the curves with 60 eV and 920 eV Cu arises from the difference in ionization probability of the two Cu levels and in escape depth, which is a factor of about 3 smaller for Cub0 than for Cu9zo [ 19,201. Compared with scribing, sputtering shows an enrichment in Cu and cleaving an enrichment in Ag.
F. &-am, W. ~~rber~~urface co~p~sitio~l of Ag-Cu
61
ailoys
Mm 2o07 Cum92OeV 100.,
30 *-
10 .'
3 **
1 a' 0,Ol
.. 3
0.03 0.1 5195
0.3 1 3 10 ZOlBO ~O/6~o~O/~0 80120 9515
30' r !aL cub
Fig. 3. Logarithmic plot of the measured APPH ratio AgmiCum and scale factor KC, versus the bulk ratio .&gb/CUb. (0) Agm/Cum; (*) I&.
Fig. 3 displays the measured APPH ratio Ag,/Cu, versus the atomic ratio A&,/ Cub for the scribed samples. It can be seen that there is no linear relationship in contrast to the stoichiometric compounds in the Pt-Sn system [ 181. It is well known that the elements Ag and Cu differ in the Auger current yield [29]. It is possible, however, to determine a scale factor using the results of the scribing technique as a standard. This scale factor Kcu is defined by:
CumKcuIAgmKAg = CubJ&b
2
(2)
where Cum and Ag, are measured APPH’s of the scribed samples. Accepting silver as standard (KA, = I), it follows: KC, = (Agm/Cum)(Cub/Agb).
(3)
The scale factor Kcu is also plotted in fig. 3. As the scale factor Kcu depends strong ly on the concentration, application of a constant scale factor is not possible. The large variation of K, arises from the fact that the Auger electron yield of Ag in alloys with a high content of Cu is much higher than that of Cu in alloys with a high content of Ag. This is probably caused by crystallographic effects. Inserting Kcu into eq. (1) one gets the corrected normalized APPH for Ag: Ag; = fAg,/(Ag,
+ Cum
Kc,>1X 100.
(4)
The expression for Cuk is analogous. Fig. 4 shows a plot of Ag& resp. Cuf*; it displays clearly the influence of the eutectic point (60/40) in case of cleaving. Fig. 3 affords a quantitative analysis of unknown Ag-Cu alloys from the meas-
62
P. Braun, W. FGber/Surface
100
0
80
60
20
LO
Fig. 4. Corrected Agh and Cui (only for Cu 920 eV).
ured ratio Ag,/Cu,
composition oJ’Ag-Cu
versus bulk concentration.
alloys
OAt%Ag,, 100At%Cub
(3) cleaved;
(a) scribed;
(0) sputtered
of the scribed samples.
Acknowledgement It is a pleasure to thank Professor F.P. Viehback for his generous support during the course of this work. The authors are grateful to the Gsterreichische Nationalbank (project No. 424) and the Gsterreichischer Fonds zur Fiirderung der wissenschaftlichen Forschung (project No. 1501) which supported this research financially.
References ill E.H.S. Burhop,
The Auger Effect and Other Radiationless Transitions (University Press, England, 1952). [21 C.C. Chang, Surface Sci. 25 (1971) 53. [31 P.W. Palmberg, G.K. Bohn and J.C. Tracy, Appl. Phys. Letters 15 (1969) 254. 141 W.E. Spicer, in: Proc. 8th Intern. Conf. Phenomena in Ionized Gases, Vienna, Austria, 1967. K. Siegbahn et al., ESCA: Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy (Almqvist and Wiksells, Uppsala, 1967). if31 R.F.K. Herzog and F.P. ViehbGck, Phys. Rev. 76 (1949) 855. Ann. Physik 15 (1965) 113. [71 A. Benninghoven, [81 P.W. Palmberg and T.N. Rhodin, J. Appl. Phys. 39 (1968) 2425. [91 S. Thomas and T.W. Haas, J. Vacuum Sci. Technol. 9 (1971) 840.
P. Braun, [lo] [ll] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] (231 [24] [25] [26] [27] [28] [29] [30]
W. Firber/Surface
composition
of Ag-Cu
alloys
63
L.L. Levenson, L.E. Davis, C.C. Bryson, J.J. Melles and W.H. Kov, J. Vacuum Sci. Technol. 9 (1972) 608. M.P. Seah, Surface Sci. 32 (1972) 703. F. Meyer and J.J. Vrakking, Surface Sci. 33 (1972) 271. H. Shimiza, M. Ono and K. Nakayama, Surface Sci. 36 (1973) 817. M.P. Seah, Surface Sci. 40 (1973) 595. R. Bouwman and P. Biloen, Surface Sci. 41 (1974) 348. C.A. Shell and J.C. Riviere, Surface Sci. 40 (1973) 149. K. Nakayama, M. Ono and H. Shimizu, J. Vacuum Sci. Technol. 9 (1971) 749. R. Bouwman, L.H. Toneman and A.A. Holscher, Surface Sci. 35 (1973) 8. M.L. Tarng and G.K. Wehner, 3. Appl. Phys. 44 (1973) 1534. G. Ertl and J. Ktppers, Low Energy Electrons and Surface Chemistry (Verlag Chemie, Germany, 1973). D.T. Quinto, V.S. Sundaram and W.D. Robertson, Surface Sci. 28 (1971) 504. P. Braun, G. Betz and W. Farber, Mikrochim. Acta 5 (1974) 365. W. Farber and P. Braun, Surface Sci. 41 (1974) 195. P. Braun and W. Farber, Vakuum-Technik 1 (1974) 7. W.M.H. Sachtler and G.J.H. Dorgelo, J. Catalysis 4 (1965) 654. W. Farber and P. Braun, in: Proc. 6th Jugoslovanski Vakuumski Kongres;Postojna, Jugoslavia, 1973. G. Carter and J.S. Colligon, Ion Bombardment of Solids (Heinemann, London, 1968). E. Lassner, private communication. P.W. Palmberg et al., Handbook of Auger Electron Spectroscopy (Physical Electronics, Minnesota, 1972). W. Farber and P. Braun, Vakuum-Technik, to be published.
AES STUDIES OF SURFACE COMPOSITION OF Ag-Cu ALLOYS P. BRAUN and W. FARBER II. Institut fiir Experimentalphysik,
Technische
Hochschule
Wien, A-l 040 Vienna, Austria
The influence of pretreatments like sputtering, annealing and cleaving on the surface composition is important in quantitative Auger electron spectroscopy. The present paper deals with this influence for the binary alloy system AgoCu with 100,95, 80, 60,50,40, 20,5, and 0 at% Ag. Sputtering causes an enrichment of Cu on the surface due to the higher sputter coefficient of Ag. In contrast cleaving yields an enrichment of Ag due to the lower tensile strength of Ag. The surface composition obtained by scribing is used as standard for the bulk composition because this technique is independent of parameters like sputter coefficient or tensile strength. The results are compared with previous measurements of homogeneous alloy systems.
1. Introduction In the past years several modern techniques for surface analysis have been developed such as electron spectroscopy, i.e., AES [l-3] and UPS [4,5], and ion mass spectroscopy, i.e., SIMS [6,7]. In these or similar techniques great care is required to reach quantitative results. A large number of methods for quantitative Auger analysis has been suggested [8-l 31. In principle, one can choose between two ways to obtain a quantitative analysis [ 14,151. The first is to compare the Auger results with those of another corresponding technique for analyzing surfaces. The second one is to perform a series of suitable standard measurements. This last method has been investigated in the present paper for the case of silver-copper alloys. A series of Ag-Cu alloys of varying bulk composition of 95,80,60,50,40,20, and 5 (all +O.l) at% Ag as well as the pure components Ag and Cu have been studied. The kinetic energy and consequently the escape depth of the Auger e,lectrons of the two metallic components are very different. Hence it is easy to recognize the influence of the cleanliness and pretreatments [ 16-201 of the samples. For systems such as Ni-Cu [ 17,211 on the other hand this effect will not be very important because of the nearly equal escape depth of the Auger electrons considered. For systems with components of which Auger electron energies and escape depth differ strongly, the component with the lower escape depth will be more affected by small amounts of contamination. The chemical composition of the sample thus appears to be changed. In the present study no contaminations were detected after
scribing or cleaving, therefore this influence may be neglected. Furthermore. chemical effects will influence the intensity and peak shape of Auger transitions involving the valence band [22--241. It is obvious that the pretreatment may alter the results considerably. Therefore, a short discussion of the actual cleaning methods used, is given here. (a) Heating by electron bombardment or indirect heating is mainly used to clean pure element samples. However. with alloys heating can bring about a surface enrichment of the component with the lowest heat of sublimation [18,25,26]. (b) With sputtering of alloys the sputter coefficients of the components determine the composition of the surface [ 131, the element with the lower sputter coefficient being concentrated at the surface. The experimentally observed surface composition depends also on the sputter ion energy 1271. Because the topography of the sample influences the surface composition resulting from sputtering, the sample should be given an optically flat polish. (c) Cleaving in ultrahigh vacuum is a good method to get clean surfaces. With this method one has to know whether the fracture runs intergranularly or along grain boundaries. For the study of metallurgical problems such as grain boundary segregation /28] cleaving is the only satisfying procedure. With Ag-- Cu alloys, however, the fracture runs intergranularly; the surface composition is influenced by the different tensile strength of the components. Therefore. the fractured surface is enriched in Ag as shown later. (d) The last method to obtain a clean surface involves scribing the sample with a stainless steel tip or diaInoIld [29]. When the surface is smooth scribing is a simple way to clean the sample. Though the danger of transfer of material from one sample to the other is small. for sophisticated measurements a set of tips is required so that for each sample a new tip can be used.
2. Experimenta The apparatus used in this study has been described previously [22]. Data were obtained for pure polycrystalline Ag-Cu alloy samples with a commercial Auger system with a cylindrical mirror analyzer 12.31 and coaxial electron gun. The primary electron energy was 1700 eV, the beam current 30.uA and the diameter of the electron beam on the target 0.3 mm. The modulation signal had an amplitude of f .2 Vpp and a frequency of IO4 cps. The measured resolution at the elastic peak was 0.4%. All spectra were recorded with 1 eV/sec sweep rate and 300 msec time constant. The polished samples were cleaned previously in an ultrasonic bath. The residual pressure in the system was lower than 10m7 Pa. A fracturing device was arranged in the bell jar system to cleave the samples under UHV conditions. In order to check the reproducibility and to estimate the standard deviation the measurements were carried out repeatedly.
P. Braun, W. F&ber/Surface
composition
59
of Ag--Cu ailoys
50 GO 30 20 IO 0
I
1
0
10
20
30
40
tCmin7
Fig. I. Sputtering of Ag/Cu 80/20 at% with ion energies of 500, 1000 and 1500 eV. The ratio of the measured Ag and Cu Auger peak heights is plotted versus sputtering time. (0) Ag/Cu 920 eV;
(A) AgjCu 60 eV. 3. Results and discussion At first the poiished samples were sputtered by argon ion bombardment to study the influence of the primary energy on the composition of the surface. Therefore, the primary ion energy was increased from 500 to 1000 and 1500 eV for the 8Of20 at% Ag/Cu alloy. Fig. 1 shows the ratio of the Ag and Cu Auger peak to peak heights (APPH) for different sputter ion energies. The ratio was estimated both for the Cu M2,3Me5Mb5 (60eV) and for the Cu L3Mh5M4> (920eV) Auger transition. The difference in composition obtained after a sufficiently long sputtering time is obvious. The maxima are caused by initial contamination of the untreated sample. Sputtering of binary alloys leads to some difficulties in quantitative analysis. On analyzing an unknown alloy sample one has to know exactly the different sputter coefficients that depend on the primary ion energy employed. Because of the considerable discrepancies in the sputter coefficients given by different authors [27], it is not possible to infer the initial composition from the final state of the sample without substantial errors. Furthermore, it is not certain that the sputter coefficients of the pure components are valid for the alloy. Generally, cleaving produces clean surfaces, but with heterogeneous systems having an eutectic point as the Cu-Ag system, the tensile strength of the components can affect the surface composition. Therefore, an enrichment in the component with the lower tensile strength (Ag) can be observed. This behaviour is not so obvi-
P. Braun, W. F~rber/Sur@ce composition
60
of’Ag- -Cu allqvs
.*80
20..
eutectic point 100
0 100
80
60
LO
20
0 At%Agb
0
20
LO
60
80
100At%Cub
Pig. 2. The normalized treatments. (A) scribed,
ANN and CUN versus bulk concentration Agb resp. Cub for different (o) cleaved, using Cu 920 eV; (0) cleaved, using Cu 60 eV; (A) scribed, Cu 920 cV: Cu 60 eV; (0) sputtered, Cu 920 eV; (m) sputtered, Cu 60 eV.
ous for homogeneous mixtures as shown elsewhere for the Ag-Au alloy system [30] . The above pertains, however, only to intergranular fractures. When the fracture runs along the grain boundaries cleaving is a valuable method to study grain boundary segregation. The scribing technique is used as a standard method for determining the bulk composition because no material parameters such as tensile strength or sputter coefficients will influence this cleaning technique. The dependence of the surface composition on the pretreatment of the different alloys is shown in fig. 2. Agb resp. Cub are the bulk concentrations of the iavestigated samples. AgN represents the normalized Ag peak height [26] as calculated by: AgN = ]Ag,KAg,
+ Cum)] x 100.
(1)
Ag, and Cum are the measured APPH’s of Ag and Cu. The values of AgN were calculated both for the 60 eV and for the 920 eV Cu transition. No scale factor was used The same calculation can be made for the normalized Cu peak height CuN. Fig. 2 shows the considerable differences in the composition resulting from the pretreatments (sputtering, cleaving and scribing ). The difference between the curves with 60 eV and 920 eV Cu arises from the difference in ionization probability of the two Cu levels and in escape depth, which is a factor of about 3 smaller for Cub0 than for Cu9zo [ 19,201. Compared with scribing, sputtering shows an enrichment in Cu and cleaving an enrichment in Ag.
F. &-am, W. ~~rber~~urface co~p~sitio~l of Ag-Cu
61
ailoys
Mm 2o07 Cum92OeV 100.,
30 *-
10 .'
3 **
1 a' 0,Ol
.. 3
0.03 0.1 5195
0.3 1 3 10 ZOlBO ~O/6~o~O/~0 80120 9515
30' r !aL cub
Fig. 3. Logarithmic plot of the measured APPH ratio AgmiCum and scale factor KC, versus the bulk ratio .&gb/CUb. (0) Agm/Cum; (*) I&.
Fig. 3 displays the measured APPH ratio Ag,/Cu, versus the atomic ratio A&,/ Cub for the scribed samples. It can be seen that there is no linear relationship in contrast to the stoichiometric compounds in the Pt-Sn system [ 181. It is well known that the elements Ag and Cu differ in the Auger current yield [29]. It is possible, however, to determine a scale factor using the results of the scribing technique as a standard. This scale factor Kcu is defined by:
CumKcuIAgmKAg = CubJ&b
2
(2)
where Cum and Ag, are measured APPH’s of the scribed samples. Accepting silver as standard (KA, = I), it follows: KC, = (Agm/Cum)(Cub/Agb).
(3)
The scale factor Kcu is also plotted in fig. 3. As the scale factor Kcu depends strong ly on the concentration, application of a constant scale factor is not possible. The large variation of K, arises from the fact that the Auger electron yield of Ag in alloys with a high content of Cu is much higher than that of Cu in alloys with a high content of Ag. This is probably caused by crystallographic effects. Inserting Kcu into eq. (1) one gets the corrected normalized APPH for Ag: Ag; = fAg,/(Ag,
+ Cum
Kc,>1X 100.
(4)
The expression for Cuk is analogous. Fig. 4 shows a plot of Ag& resp. Cuf*; it displays clearly the influence of the eutectic point (60/40) in case of cleaving. Fig. 3 affords a quantitative analysis of unknown Ag-Cu alloys from the meas-
62
P. Braun, W. FGber/Surface
100
0
80
60
20
LO
Fig. 4. Corrected Agh and Cui (only for Cu 920 eV).
ured ratio Ag,/Cu,
composition oJ’Ag-Cu
versus bulk concentration.
alloys
OAt%Ag,, 100At%Cub
(3) cleaved;
(a) scribed;
(0) sputtered
of the scribed samples.
Acknowledgement It is a pleasure to thank Professor F.P. Viehback for his generous support during the course of this work. The authors are grateful to the Gsterreichische Nationalbank (project No. 424) and the Gsterreichischer Fonds zur Fiirderung der wissenschaftlichen Forschung (project No. 1501) which supported this research financially.
References ill E.H.S. Burhop,
The Auger Effect and Other Radiationless Transitions (University Press, England, 1952). [21 C.C. Chang, Surface Sci. 25 (1971) 53. [31 P.W. Palmberg, G.K. Bohn and J.C. Tracy, Appl. Phys. Letters 15 (1969) 254. 141 W.E. Spicer, in: Proc. 8th Intern. Conf. Phenomena in Ionized Gases, Vienna, Austria, 1967. K. Siegbahn et al., ESCA: Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy (Almqvist and Wiksells, Uppsala, 1967). if31 R.F.K. Herzog and F.P. ViehbGck, Phys. Rev. 76 (1949) 855. Ann. Physik 15 (1965) 113. [71 A. Benninghoven, [81 P.W. Palmberg and T.N. Rhodin, J. Appl. Phys. 39 (1968) 2425. [91 S. Thomas and T.W. Haas, J. Vacuum Sci. Technol. 9 (1971) 840.
P. Braun, [lo] [ll] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] (231 [24] [25] [26] [27] [28] [29] [30]
W. Firber/Surface
composition
of Ag-Cu
alloys
63
L.L. Levenson, L.E. Davis, C.C. Bryson, J.J. Melles and W.H. Kov, J. Vacuum Sci. Technol. 9 (1972) 608. M.P. Seah, Surface Sci. 32 (1972) 703. F. Meyer and J.J. Vrakking, Surface Sci. 33 (1972) 271. H. Shimiza, M. Ono and K. Nakayama, Surface Sci. 36 (1973) 817. M.P. Seah, Surface Sci. 40 (1973) 595. R. Bouwman and P. Biloen, Surface Sci. 41 (1974) 348. C.A. Shell and J.C. Riviere, Surface Sci. 40 (1973) 149. K. Nakayama, M. Ono and H. Shimizu, J. Vacuum Sci. Technol. 9 (1971) 749. R. Bouwman, L.H. Toneman and A.A. Holscher, Surface Sci. 35 (1973) 8. M.L. Tarng and G.K. Wehner, 3. Appl. Phys. 44 (1973) 1534. G. Ertl and J. Ktppers, Low Energy Electrons and Surface Chemistry (Verlag Chemie, Germany, 1973). D.T. Quinto, V.S. Sundaram and W.D. Robertson, Surface Sci. 28 (1971) 504. P. Braun, G. Betz and W. Farber, Mikrochim. Acta 5 (1974) 365. W. Farber and P. Braun, Surface Sci. 41 (1974) 195. P. Braun and W. Farber, Vakuum-Technik 1 (1974) 7. W.M.H. Sachtler and G.J.H. Dorgelo, J. Catalysis 4 (1965) 654. W. Farber and P. Braun, in: Proc. 6th Jugoslovanski Vakuumski Kongres;Postojna, Jugoslavia, 1973. G. Carter and J.S. Colligon, Ion Bombardment of Solids (Heinemann, London, 1968). E. Lassner, private communication. P.W. Palmberg et al., Handbook of Auger Electron Spectroscopy (Physical Electronics, Minnesota, 1972). W. Farber and P. Braun, Vakuum-Technik, to be published.