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Cu systems
400
Applications of Surface Science 11/12(1982)400—407 North-Holland Publishing Company
AUGER LINE SHAPE ANALYSES FOR EPITAXIAL GROWTH IN THE Cu/Cu, Ag/Ag AND Ag/Cu SYSTEMS R.W. VOOK and Y. NAMBA * Department of Chemical Engineering and Materials Science, Syracuse University, Syracuse, New York 13210, USA Received 8 June 1981; revised manuscript received 13 August 1981
A new measure of the MVV doublet Auger line shape called the R-factor has been applied to homoepitaxial layer growth on (Ill )Cu and on (Ill )Ag as well as to the heteroepitaxial growth of (ill )Ag on (111)Cu. AES and TEM show that as thick (— 1500 A) monocrystalline Cu and Ag films grow thicker, R oscillates with a period equal to one atomic layer. Polycrystalline films do not show such a periodicity. The changes in doublet line shape are interpreted as arising from the superposition of two sets of doublets slightly displaced in energy. One set is presumed to come from the flat areas of the film and the other from the edge areas of incomplete layers. The origin of the periodicity in R arises then from the atomic relaxations that can be expected at the edges of surface steps, whose density varies periodically during growth. For the initial stages of (Ill )Ag growth on (11 1)Cu, an initially high RA 8 decreased to bulk values as the Ag thickened beyond one or two monolayers for both polycrystalline and monocrystalline films; but the periodicity in RAS was detected only for monocrystalline growth. The high initial values of RAS are interpreted as arising from changes in the energy levels of the corresponding Auger transitions with changing interatomic spacing. Since the Ag overgrowth is compressed towards the smaller lattice parameter of the Cu substrate, energy levels in the valence band of Ag might be expected to become more widely separated. Consequently a more highly resolved MVV doublet for Ag would occur, resulting in a larger RAS.
I. Introduction Most applications of Auger electron spectroscopy (AES) to the study of thin film growth have used the Auger amplitude (AA) to distinguish layer, island, and Stranski—Krastanov growth as well as possible transitions from one growth mode to another [1—4].Layer growth can be distinguished by its sequence of linear segments terminating at the completion of each layer. The ratio of successive slopes is given by m~± 1/m = exp( h1 /X), where h1 is the layer thickness and A is the escape depth of the Auger electrons [3]. In the case of island growth, the exact AA versus thickness plot depends upon the shapes of the islands during growth. In all cases, however, the slope of the linear —
*
On leave from Tokyo Noko University, Department of Electrical Engineering, Tokyo, Japan.
0378-5963/82/0000—0000/$02.75
©
1982 North-Holland
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/
Auger line shape analvses for epitaxial growth
401
increase in AA is much less than for the case of layer growth. For Stranski— Krastanov systems, an initial linear increase corresponding to layer growth is followed by another linear region having a slope corresponding to island growth. All these studies are extremely useful in understanding the initial stages of growth, that is up until several average monolayers have been deposited. They have not been found useful for characterizing growth in thick films because the changes in AA in that case cannot be detected on the large background that arises from an effectively infinitely thick crystal. Until recently the inherent information in the Auger line profile has not been used to characterize thin films. However, studies on the growth of epitaxial Pd films on Cu have shown that changes in line shape can be used to characterize thick as well as thin film growth [5,6]. The results presented in this report extend these studies to thick Cu and Ag films and the growth of Ag on Cu. They show that a measure of the Auger doublet line profile, called the R-factor, can be used to identify epitaxial layer growth in very thick films (— 1200—1500 A) as well as in thin overgrowths. Moreover these results give information, which has still not been interpreted quantitatively, on the electronic energy band structure in the vicinity of surface steps, as well as changes arising from lattice misfit induced strain. Because the interpretation of the line profile data relies on the assumption that the energy levels at surface steps are slightly displaced from those in the flat regions of the surface, the approach gives qualitative information on surface topography. This observation promises to be extremely important for surface reactions which are sensitive to surface roughness phenomena. It is the first evidence that AES can be used to obtain topographic information.
2. Experimental procedure Vapor deposited films of Cu (99.9999%) were deposited in vacua of 10 ‘° to low l0 8 Torr (—~~o8 10 6 Pa) on substrates consisting of air cleaved mica on which approximately 200 A of NaCI had been deposited in situ at 25°C. Epitaxial Cu and Ag films were formed when deposition was carried out on this NaC1/mica substrate at a calibrated 210°C. Room temperature depositions, however, resulted in polycrystalline overgrowths. All film microstructures were examined by transmission electron microscopy (TEM) and diffraction (TED) after the in situ, AES experiments were completed. When the films were removed from the vacuum system, they were floated off their substrates in water and then mounted on copper grids for TEM examination. The AES measurements were carried out with a single pass cylindrical mirror analyzer (CMA) operating in the derivative mode. Data taking was controlled by a Hewlett-Packard 9825A desktop computer with multiprogrammer, which also analyzed, plotted, and stored the results on tape. In order to enhance system resolution (at the expense of sensitivity) a 1 Vp_p modulation
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signal was used. Higher modulations could not resolve the Auger doublets that were of interest: Cu(M23 M45 M45—59, 61 eV) and Ag(M45 N45 N45 351, —
356 eV). Both of these doublets are of the MVV type. The Cu and Ag doublets were scanned from 30 to 90 eV and 320 to 380 eV respectively at a rate of 0.2 eV/s. A beam energy of 2keV and a current of 21.tA were used. The Auger spectrum from 0—1000 eV was also run periodically on the surfaces of interest in order to monitor the presence of adsorbed contaminants. Provided the experiments were carried out immediately after deposition, only trace contaminants, if any at all, were detected. Occasionally chlorine was observed, either as a result of a few pinholes in the films (thereby allowing detection of the rock salt substrate) or as a result of redeposition onto the surface from other, heated areas of the vacuum chamber (such as from the W evaporation baskets and shields). In any case contamination effects were neglibible in most cases.
3. Results A typical derivative Auger trace of the 58—60 MVV Auger doublet is shown in fig. I for the purpose of defining the R-factor. In the case of Ag, R is larger because the corresponding B1 is larger. It is clear from this figure that R is a measure of Auger line broadening in doublets as well as a measure of relative doublet peak shifts. If in the former case, the doublets become sharper the magnitude of B1 will increase. Similarly if the doublets become farther apart, each line will be better resolved and again B1 will increase. In both cases a
Cu
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ELECTRON EAERGY(eV)
Fig. I. Definition of the Auger doublet R-factor in the case of Cu.
R. W. Vook, V. Namba / Auger line shape analyses for epitaxial growth
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Fig. 2. RAS (a) and R Cu (b) versus added film thickness in units of (Ill) interplanar spacings for monocrystalline films formed at 210°C.Initial film thicknesses: Ag (1500 A), Cu (1580 A). Open squares were calculated from handbook doublets for sputter-etched surfaces [7].
larger R-factor will result. Clearly the results depend on the resolution of the CMA used in a particular experiment. In the present case, the elastic peak was continually monitored in order to confirm the constancy of resolution (~E/E) at a value of 0.005. It should also be noted that changes in target current from experiment to experiment, which will severely affect AA data, will have no effect on R if the current remains constant while the peak to peak trace is being taken, a matter of several seconds in our case. Clearly the absolute value of the current is not significant when R-factors are determined so long as it remains constant over this interval. For thick monocrystalline Ag and Cu films, the R-factors vary as shown in fig. 2 with added film thickness, measured in (111) monolayers. In these cases a thick film was formed in a single deposition. On this substrate successive depositions were made as indicated in the figure. Clearly there is a thickness periodicity of the order of a single (111) monolayer. The (Ill) interplanar spacings for Ag and Cu are 2.36 and 2.09 A respectively. The R-factors calculated from the handbook values for sputter-etched samples are also shown in fig. 2, in good agreement with our results.
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/ Auger line shape analyses for epitaxial growth
According to the model to be presented below for interpreting these results, the periodicity should be exactly one monoatomic layer in its simplest case. Apparent deviations from this exact periodicity may have arisen from the thickness measurements in which a quartz crystal oscillator was used. In the range of thicluiesses shown in fig. 2, the depositions were carried out by stabilizing the evaporation rate and then opening and closing a shutter for a finite time in order to deposit thickness steps of 0.50 and 0.56 A in the cases of Ag and Cu respectively. While the digital reading quartz oscillator can read in 0.1 A intervals, it is expected that the actual error is somewhat larger. Also for the 3—4 monolayers deposited, such errors would be cumulative, even though positive as well as negative deviations could be expected. With such arguments the precision in each thickness measurement would be 20% at best, in reasonable agreement with the data given in fig. 2. The results for thick polycrystalline Ag and Cu films are given in fig. 3. These films were made in essentially the same way as those in fig. 2 except that depositions were performed on NaCl/mica substrates at room temperature. Clearly thickness periodicities are not detectable in the polycrystalline cases. When Ag was deposited on a Cu substrate film, the results shown in fig. 4
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Fig. 3. RAg (a) and RCu (b) versus added film thickness in units of (Ill) interplanar spacings for polycrystalline films formed at 25°C.Initial film thicknesses: Ag (1200 A), Cu (1500 A).
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~
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-
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Ag MNN(351.356 cv) doublet
0.58-
-
I 0
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I 2 3 Ag FILM rHICICNESS/ (III) MQIVLLAYERS
I 4
Fig. 4. RA
8 for Ag growth on monocrystalline (Ill )Cu (solid line) at 2 10°Cand polycrystalline (dashed line) Cu at 25°Cas a function of coverage. Open square is calculated handbook value [7].
were obtained for polycrystalline and monocrystalline films. In both cases the initial RA8 starts out at a high value and then decreases rapidly to values that approach the sputter-etched handbook value [7]. Beyond the first monolayer, fairly erratic RAS values were obtained for polycrystalline bilayers. In the monocrystalline case, except for the region arround 2 monolayers, fairly smooth fluctuations are superimposed on the general trend. From about 2.5 monolayers to beyond 4 monolayers, the oscillation is very similar to what is shown in fig. 2 for thick Ag films. The curve drawn in fig. 4 shows fluctuations which appear to have periods of close to one (111) Ag interplanar spacing.
4. Discussion It has been shown that when Ag is deposited on Cu, an initially high value of RAS decreases and levels off to a more or less fluctuating steady state value as the thickness of the Ag overgrowth increases. In an earlier TEM, TED, and RHEED (reflection high energy diffraction) study of the epitaxial (11 l)Ag/(I I l)Cu system [8],it was shown that the lattice parameter of Ag was
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compressed towards that of the Cu substrate. Such a compression would be expected to cause an increase in the energy spread between the doublet Auger Ag lines used in this work [9,10]. As a result these two lines would become better resolved, giving rise to a larger R-factor, in agreement with experiment. The monolayer periodicity in thick Ag and Cu films may be interpreted in terms of the layer film growth mode. Wynblatt [11] has shown theoretically that there are atomic relaxations at the edges of steps on a crystalline surface. If these relaxations result in energy level shifts in more or less the same way as occurs when the interatomic spacing is changed [9,10], then the Auger electrons emitted by atoms at surface ledges would be expected to be slightly displaced in energy from those emitted from atoms in the flat areas of the surface. The Auger electrons detected by a CMA would then be a summation of all such electrons emitted by the irradiated surface; and the resulting Auger line or doublet would consist of two sets of electrons, slightly displaced in energy. In this case, line broadening should occur. With these assumptions, the occurrence of a thickness periodicity in R during layer growth is apparent. When a perfect, complete monolayer surface occurs only one set of Auger electrons is emitted, the Auger doublet is sharp, and a high value of R is observed. As the film thickens, monolayer high flat islands are formed and at some fractional monolayer increase in thickness a maximum density of ledges will occur. At this point two sets of Auger electrons will be emitted giving rise to a maximum amount of doublet line broadening and a minimum in R. As more material is deposited, the flat monolayer high islands will coalesce, reducing the ledge density until another perfect, complete monolayer surface forms. While this model is highly idealized, it does show how periodic fluctuations in R can occur. Finally it should be mentioned that the thickness periodicity in R depends upon both the layer thickness and the coverage. If flat islands form to a coverage of 50%, for example, and growth occurs on top of those islands by a bi-atomic layer growth model (the islands are two monolayers high during growth), then the periodicity in R will still be a single monolayer. This work on the R-factor also shows that the Auger line shape gives information on surface topography, since the amount of line broadening observed depends directly on the ledge density on the surface. Chemical reactions which may or may not depend on surface steps, as for example in catalysis and corrosion phenomena, may now be related to the step density using AES. In past work the step densities on single crystals, cut along vicinal surfaces and annealed, were determined by LEED [12]. In this technique the characteristics of only the periodic steps can be determined. The new AES method, however, gives information from all the steps that are actually present on the surface. Another possible application of the new R-factor technique could be in the analysis of fluctuating growth mode phenomena such as have been observed by Gillet and Gruzza for Au deposited on (110) Mo [13]. In this case plots of the
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/ A uger line shape analyses for epitaxial growth
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Auger electron intensities versus coverage (or added film thickness) show linear segments followed by regions in which the Auger amplitude oscillates. The actual plot depends upon the temperature of deposition. The data were interpreted as indicating changes in growth mode from layer to island and even the agglomeration of layers into islands when the Auger intensity decreases with added overgrowth material. Auger line shape analyses in terms of R-factor plots should also be useful here in confirming or amplifying the Auger intensity results. It is clear also that the R-factor technique should apply to bulk crystals. That is, bulk monocrystals formed in the standard way by cutting, polishing, sputter-etching and annealing could be used as substrates for homoepitaxial growth. When the R-factor starts fluctuating periodically as deposition continues, one can identify layer growth on the bulk crystal. Moreover by terminating the deposition appropriately one can obtain “smooth” or “rough” surfaces at will. The R-factor thus provides an additional important tool for characterizing the structure and topography of a surface in addition to the more traditional role of AES in determining elemental surface composition.
Acknowledgments The authors gratefully acknowledge the financial support of the US Department of Energy under Contract Number DE-ASO2-77ER0-4496. The technical assistance of S.S. Chao, A.J. Isabelle, and B. Singh was also appreciated.
References [II E.
Bauer, H. Poppa and G. Todd, Thin Solid Films 28 (1975) 19. [2] G.E. Rhead, J. Vacuum Sci. Technol. 13 (1976) 603. [31M.P. Seah, Surface Sci. 32 (1972) 703. [4] E. Gillet and B. Gruzza, in: Proc. 4th Intern. Conf. on Solid Surfaces Cannes, 1980, Vol. 1 [Suppl. Le Vide, Les Couches Minces 201 (1980) 6691. [5] S.S. Chao, E.-A. Knabbe and R.W. Vook, Surface Sci. 100 (1980) 581. [61 S.S. Chao, R.W. Vook and Y. Namba, J. Vacuum Sci. Technol. 18 (1981) 695. [7] L.E. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach and R.E. Weber, Handbook of Auger Electron Spectroscopy (Physical Electronics Industries, Eden Prairie, MN, 1976). [81 R.W. Vook and C.T. Horng, Phil. Mag. 33 (1976) 843. [9] H.M. Knitter, Phys. Rev. 48 (1935) 664. [101 C. Kittel, Introduction to Solid State Physics (Wiley, New York, 1953) p. 250. [111 P. Wynblatt, in: Interatomic Potentials and Simulation of Lattice Defects, Eds. P.C. Gehlen, J.R. Beeler, Jr. and R.I. Jaffee (Plenum, New York, 1972) p. 633. [12] H. Wagner, in: Solid Surface Physics, Springer Tracts in Modern Physics, Vol. 85 (Springer, Berlin, 1979) p. 151. [13] E. Gillet and B. Gruzza, Surface Sci. 97 (1980) 553.
Applications of Surface Science 11/12(1982)400—407 North-Holland Publishing Company
AUGER LINE SHAPE ANALYSES FOR EPITAXIAL GROWTH IN THE Cu/Cu, Ag/Ag AND Ag/Cu SYSTEMS R.W. VOOK and Y. NAMBA * Department of Chemical Engineering and Materials Science, Syracuse University, Syracuse, New York 13210, USA Received 8 June 1981; revised manuscript received 13 August 1981
A new measure of the MVV doublet Auger line shape called the R-factor has been applied to homoepitaxial layer growth on (Ill )Cu and on (Ill )Ag as well as to the heteroepitaxial growth of (ill )Ag on (111)Cu. AES and TEM show that as thick (— 1500 A) monocrystalline Cu and Ag films grow thicker, R oscillates with a period equal to one atomic layer. Polycrystalline films do not show such a periodicity. The changes in doublet line shape are interpreted as arising from the superposition of two sets of doublets slightly displaced in energy. One set is presumed to come from the flat areas of the film and the other from the edge areas of incomplete layers. The origin of the periodicity in R arises then from the atomic relaxations that can be expected at the edges of surface steps, whose density varies periodically during growth. For the initial stages of (Ill )Ag growth on (11 1)Cu, an initially high RA 8 decreased to bulk values as the Ag thickened beyond one or two monolayers for both polycrystalline and monocrystalline films; but the periodicity in RAS was detected only for monocrystalline growth. The high initial values of RAS are interpreted as arising from changes in the energy levels of the corresponding Auger transitions with changing interatomic spacing. Since the Ag overgrowth is compressed towards the smaller lattice parameter of the Cu substrate, energy levels in the valence band of Ag might be expected to become more widely separated. Consequently a more highly resolved MVV doublet for Ag would occur, resulting in a larger RAS.
I. Introduction Most applications of Auger electron spectroscopy (AES) to the study of thin film growth have used the Auger amplitude (AA) to distinguish layer, island, and Stranski—Krastanov growth as well as possible transitions from one growth mode to another [1—4].Layer growth can be distinguished by its sequence of linear segments terminating at the completion of each layer. The ratio of successive slopes is given by m~± 1/m = exp( h1 /X), where h1 is the layer thickness and A is the escape depth of the Auger electrons [3]. In the case of island growth, the exact AA versus thickness plot depends upon the shapes of the islands during growth. In all cases, however, the slope of the linear —
*
On leave from Tokyo Noko University, Department of Electrical Engineering, Tokyo, Japan.
0378-5963/82/0000—0000/$02.75
©
1982 North-Holland
R. W. Vook, V. Namba
/
Auger line shape analvses for epitaxial growth
401
increase in AA is much less than for the case of layer growth. For Stranski— Krastanov systems, an initial linear increase corresponding to layer growth is followed by another linear region having a slope corresponding to island growth. All these studies are extremely useful in understanding the initial stages of growth, that is up until several average monolayers have been deposited. They have not been found useful for characterizing growth in thick films because the changes in AA in that case cannot be detected on the large background that arises from an effectively infinitely thick crystal. Until recently the inherent information in the Auger line profile has not been used to characterize thin films. However, studies on the growth of epitaxial Pd films on Cu have shown that changes in line shape can be used to characterize thick as well as thin film growth [5,6]. The results presented in this report extend these studies to thick Cu and Ag films and the growth of Ag on Cu. They show that a measure of the Auger doublet line profile, called the R-factor, can be used to identify epitaxial layer growth in very thick films (— 1200—1500 A) as well as in thin overgrowths. Moreover these results give information, which has still not been interpreted quantitatively, on the electronic energy band structure in the vicinity of surface steps, as well as changes arising from lattice misfit induced strain. Because the interpretation of the line profile data relies on the assumption that the energy levels at surface steps are slightly displaced from those in the flat regions of the surface, the approach gives qualitative information on surface topography. This observation promises to be extremely important for surface reactions which are sensitive to surface roughness phenomena. It is the first evidence that AES can be used to obtain topographic information.
2. Experimental procedure Vapor deposited films of Cu (99.9999%) were deposited in vacua of 10 ‘° to low l0 8 Torr (—~~o8 10 6 Pa) on substrates consisting of air cleaved mica on which approximately 200 A of NaCI had been deposited in situ at 25°C. Epitaxial Cu and Ag films were formed when deposition was carried out on this NaC1/mica substrate at a calibrated 210°C. Room temperature depositions, however, resulted in polycrystalline overgrowths. All film microstructures were examined by transmission electron microscopy (TEM) and diffraction (TED) after the in situ, AES experiments were completed. When the films were removed from the vacuum system, they were floated off their substrates in water and then mounted on copper grids for TEM examination. The AES measurements were carried out with a single pass cylindrical mirror analyzer (CMA) operating in the derivative mode. Data taking was controlled by a Hewlett-Packard 9825A desktop computer with multiprogrammer, which also analyzed, plotted, and stored the results on tape. In order to enhance system resolution (at the expense of sensitivity) a 1 Vp_p modulation
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signal was used. Higher modulations could not resolve the Auger doublets that were of interest: Cu(M23 M45 M45—59, 61 eV) and Ag(M45 N45 N45 351, —
356 eV). Both of these doublets are of the MVV type. The Cu and Ag doublets were scanned from 30 to 90 eV and 320 to 380 eV respectively at a rate of 0.2 eV/s. A beam energy of 2keV and a current of 21.tA were used. The Auger spectrum from 0—1000 eV was also run periodically on the surfaces of interest in order to monitor the presence of adsorbed contaminants. Provided the experiments were carried out immediately after deposition, only trace contaminants, if any at all, were detected. Occasionally chlorine was observed, either as a result of a few pinholes in the films (thereby allowing detection of the rock salt substrate) or as a result of redeposition onto the surface from other, heated areas of the vacuum chamber (such as from the W evaporation baskets and shields). In any case contamination effects were neglibible in most cases.
3. Results A typical derivative Auger trace of the 58—60 MVV Auger doublet is shown in fig. I for the purpose of defining the R-factor. In the case of Ag, R is larger because the corresponding B1 is larger. It is clear from this figure that R is a measure of Auger line broadening in doublets as well as a measure of relative doublet peak shifts. If in the former case, the doublets become sharper the magnitude of B1 will increase. Similarly if the doublets become farther apart, each line will be better resolved and again B1 will increase. In both cases a
Cu
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ELECTRON EAERGY(eV)
Fig. I. Definition of the Auger doublet R-factor in the case of Cu.
R. W. Vook, V. Namba / Auger line shape analyses for epitaxial growth
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Fig. 2. RAS (a) and R Cu (b) versus added film thickness in units of (Ill) interplanar spacings for monocrystalline films formed at 210°C.Initial film thicknesses: Ag (1500 A), Cu (1580 A). Open squares were calculated from handbook doublets for sputter-etched surfaces [7].
larger R-factor will result. Clearly the results depend on the resolution of the CMA used in a particular experiment. In the present case, the elastic peak was continually monitored in order to confirm the constancy of resolution (~E/E) at a value of 0.005. It should also be noted that changes in target current from experiment to experiment, which will severely affect AA data, will have no effect on R if the current remains constant while the peak to peak trace is being taken, a matter of several seconds in our case. Clearly the absolute value of the current is not significant when R-factors are determined so long as it remains constant over this interval. For thick monocrystalline Ag and Cu films, the R-factors vary as shown in fig. 2 with added film thickness, measured in (111) monolayers. In these cases a thick film was formed in a single deposition. On this substrate successive depositions were made as indicated in the figure. Clearly there is a thickness periodicity of the order of a single (111) monolayer. The (Ill) interplanar spacings for Ag and Cu are 2.36 and 2.09 A respectively. The R-factors calculated from the handbook values for sputter-etched samples are also shown in fig. 2, in good agreement with our results.
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/ Auger line shape analyses for epitaxial growth
According to the model to be presented below for interpreting these results, the periodicity should be exactly one monoatomic layer in its simplest case. Apparent deviations from this exact periodicity may have arisen from the thickness measurements in which a quartz crystal oscillator was used. In the range of thicluiesses shown in fig. 2, the depositions were carried out by stabilizing the evaporation rate and then opening and closing a shutter for a finite time in order to deposit thickness steps of 0.50 and 0.56 A in the cases of Ag and Cu respectively. While the digital reading quartz oscillator can read in 0.1 A intervals, it is expected that the actual error is somewhat larger. Also for the 3—4 monolayers deposited, such errors would be cumulative, even though positive as well as negative deviations could be expected. With such arguments the precision in each thickness measurement would be 20% at best, in reasonable agreement with the data given in fig. 2. The results for thick polycrystalline Ag and Cu films are given in fig. 3. These films were made in essentially the same way as those in fig. 2 except that depositions were performed on NaCl/mica substrates at room temperature. Clearly thickness periodicities are not detectable in the polycrystalline cases. When Ag was deposited on a Cu substrate film, the results shown in fig. 4
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Fig. 3. RAg (a) and RCu (b) versus added film thickness in units of (Ill) interplanar spacings for polycrystalline films formed at 25°C.Initial film thicknesses: Ag (1200 A), Cu (1500 A).
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~
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0.58-
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Fig. 4. RA
8 for Ag growth on monocrystalline (Ill )Cu (solid line) at 2 10°Cand polycrystalline (dashed line) Cu at 25°Cas a function of coverage. Open square is calculated handbook value [7].
were obtained for polycrystalline and monocrystalline films. In both cases the initial RA8 starts out at a high value and then decreases rapidly to values that approach the sputter-etched handbook value [7]. Beyond the first monolayer, fairly erratic RAS values were obtained for polycrystalline bilayers. In the monocrystalline case, except for the region arround 2 monolayers, fairly smooth fluctuations are superimposed on the general trend. From about 2.5 monolayers to beyond 4 monolayers, the oscillation is very similar to what is shown in fig. 2 for thick Ag films. The curve drawn in fig. 4 shows fluctuations which appear to have periods of close to one (111) Ag interplanar spacing.
4. Discussion It has been shown that when Ag is deposited on Cu, an initially high value of RAS decreases and levels off to a more or less fluctuating steady state value as the thickness of the Ag overgrowth increases. In an earlier TEM, TED, and RHEED (reflection high energy diffraction) study of the epitaxial (11 l)Ag/(I I l)Cu system [8],it was shown that the lattice parameter of Ag was
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compressed towards that of the Cu substrate. Such a compression would be expected to cause an increase in the energy spread between the doublet Auger Ag lines used in this work [9,10]. As a result these two lines would become better resolved, giving rise to a larger R-factor, in agreement with experiment. The monolayer periodicity in thick Ag and Cu films may be interpreted in terms of the layer film growth mode. Wynblatt [11] has shown theoretically that there are atomic relaxations at the edges of steps on a crystalline surface. If these relaxations result in energy level shifts in more or less the same way as occurs when the interatomic spacing is changed [9,10], then the Auger electrons emitted by atoms at surface ledges would be expected to be slightly displaced in energy from those emitted from atoms in the flat areas of the surface. The Auger electrons detected by a CMA would then be a summation of all such electrons emitted by the irradiated surface; and the resulting Auger line or doublet would consist of two sets of electrons, slightly displaced in energy. In this case, line broadening should occur. With these assumptions, the occurrence of a thickness periodicity in R during layer growth is apparent. When a perfect, complete monolayer surface occurs only one set of Auger electrons is emitted, the Auger doublet is sharp, and a high value of R is observed. As the film thickens, monolayer high flat islands are formed and at some fractional monolayer increase in thickness a maximum density of ledges will occur. At this point two sets of Auger electrons will be emitted giving rise to a maximum amount of doublet line broadening and a minimum in R. As more material is deposited, the flat monolayer high islands will coalesce, reducing the ledge density until another perfect, complete monolayer surface forms. While this model is highly idealized, it does show how periodic fluctuations in R can occur. Finally it should be mentioned that the thickness periodicity in R depends upon both the layer thickness and the coverage. If flat islands form to a coverage of 50%, for example, and growth occurs on top of those islands by a bi-atomic layer growth model (the islands are two monolayers high during growth), then the periodicity in R will still be a single monolayer. This work on the R-factor also shows that the Auger line shape gives information on surface topography, since the amount of line broadening observed depends directly on the ledge density on the surface. Chemical reactions which may or may not depend on surface steps, as for example in catalysis and corrosion phenomena, may now be related to the step density using AES. In past work the step densities on single crystals, cut along vicinal surfaces and annealed, were determined by LEED [12]. In this technique the characteristics of only the periodic steps can be determined. The new AES method, however, gives information from all the steps that are actually present on the surface. Another possible application of the new R-factor technique could be in the analysis of fluctuating growth mode phenomena such as have been observed by Gillet and Gruzza for Au deposited on (110) Mo [13]. In this case plots of the
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Auger electron intensities versus coverage (or added film thickness) show linear segments followed by regions in which the Auger amplitude oscillates. The actual plot depends upon the temperature of deposition. The data were interpreted as indicating changes in growth mode from layer to island and even the agglomeration of layers into islands when the Auger intensity decreases with added overgrowth material. Auger line shape analyses in terms of R-factor plots should also be useful here in confirming or amplifying the Auger intensity results. It is clear also that the R-factor technique should apply to bulk crystals. That is, bulk monocrystals formed in the standard way by cutting, polishing, sputter-etching and annealing could be used as substrates for homoepitaxial growth. When the R-factor starts fluctuating periodically as deposition continues, one can identify layer growth on the bulk crystal. Moreover by terminating the deposition appropriately one can obtain “smooth” or “rough” surfaces at will. The R-factor thus provides an additional important tool for characterizing the structure and topography of a surface in addition to the more traditional role of AES in determining elemental surface composition.
Acknowledgments The authors gratefully acknowledge the financial support of the US Department of Energy under Contract Number DE-ASO2-77ER0-4496. The technical assistance of S.S. Chao, A.J. Isabelle, and B. Singh was also appreciated.
References [II E.
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