Auger analysis of thick silver films and of silver layers grown on copper

Thin Solid Films, 82(1981) 165-177 PREPARATION AND CHARACTERIZATION

165

A U G E R ANALYSIS OF T H I C K SILVER FILMS A N D OF SILVER LAYERS G R O W N ON C O P P E R Y. NAMBA* AND R. W. VOOK

Department of Chemical Engineering and Materials Science, Syracuse University, Syracuse, N Y 13210

(u.s.A.) (Received February 6, 1981 ; accepted March 20, 1981)

A sensitive new measure of the line broadening of an Auger doublet, called the R factor, was applied to the growth of(l) thick silver films and (2) thin silver films on a thick copper substrate. It is shown that periodic fluctuations in R with thickness give evidence for monocrystalline layer growth while the absence of such fluctuations is characteristic of polycrystalline growth. By combining measurements of the Auger amplitude and the R factor as functions of the silver film thickness for silver films deposited onto copper, it was shown that a Stranski-Krastanov growth mode is characteristic of the epitaxial growth of Ag(111) on Cu(111) at 210 :C. The initial layer in this case is two (1111 atom spacings thick on which high flat islands form. Continued growth of these islands is shown to take place by two-atom-thick flat islands.

I. INTRODUCTION

Recent work on Pd(111)/Cu(111) bilayers I and thick Cu(111) films 2 has shown that Auger electron spectroscopy (AES) line shape analyses can be used to identify layer growth during the formation of monocrystalline films by vapor deposition methods. The AES technique is based on measuring changes in line profile obtained in the derivative mode (dN/dE) using a sensitive parameter defined as the R factor. The Auger lines in the particular cases of palladium and copper are MVV doublets that originate in the valence band. When monocrystalline layer growth occurred, the R factor was found to oscillate with a periodicity approximately equal to the thickness of the layer. When a polycrystalline film was formed, no such periodicity was observed. These periodic thickness fluctuations in R were ascribed to the superposition of two sets of Auger doublets, one arising from the flat areas of the growing film and the other from edge areas of the flat islands. For thick copper films, energy measurements showed that the observed doublet also oscillated in energy with approximately the same periodicity as the R factor. The results were interpreted by assuming that the two doublets differed slightly in energy and that the higher energy doublet came from the edge areas. The fluctuations in broadening would * On leavefrom Tokyo Noko University,Departmentof Electrical Engineering,Tokyo, Japan. 0040-6090/81/0000-0000/$02.50

(~' ElsevierSequoia/Printedin The Netherlands

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therefore be directly related to the ratio of edge areas to fiat areas, a quantity that varied periodically as the film grew. Thus the R factor is a measure of the surface t o p o g r a p h y of the specimen, high and low values corresponding to smooth and rough surfaces respectively. In the case of palladium deposited onto copper, the periodic fluctuations were superimposed on a mean value thai decreased as the overgrowth thickened ~. An initially high value for fractional m o n o l a y e r palladium films decreased to values in the range of those reported for sputter-cleaned polycrystalline palladium surfaces ~. This effect appeared to be related to the compressive stresses induced in palladium by the thick copper substrate +. In the present work these results were extended to thick Agl 111 ) films and to thin silver films grown epitaxially on Cu(111 } films. Effects similar to those found earlier for P d / C u and copper films were observed. In all cases no attempt was made to separate the observed line profile into the true Auger line and the instrument response function. The results show that, even w i t h o u t m a k i n g such a deconvolution, the observed R factor is a useful tool for determining whether a film is monocrystalline and is growing by a layer growth mechanism and what the thickness period of the layer is. 2. EXPERIMENTAl. TECHNIQUES

Two sets of experiments were carried out, one involving thick monocrystalline Ag(111 } films and one in which silver was deposited epitaxially onto thick Cul 11 I ) films. In all cases the basic substrate was air-cleaved mica onto which NaC1 (200 25() /~) was deposited at 2 5 C in situ in an ultrahigh vacuum chamber. This NaCI acted as a release layer after the Auger measurements had been carried out. At that time the films were removed from the vacuum system, immersed in water and m o u n t e d onto copper grids for transmission electron microscopy (TEMt investigation. Both the silver and copper films grew epitaxially on NaCIt I 1 Ii at a calibrated temperature of 210 C . When silver was deposited at r o o m temperature a textured polycrystalline film was formed. While the base pressure in the vacuum system was in the low 10 10 Torr (1.3 x 10 -8 Pa) range, silver and copper were deposited in pressures ranging up to the low 10 s T o r t (1.3 x 10 ~ Pat range. A single-pass cylindrical mirror analyzer operating in the derivative mode was used to obtain the Auger electron spectrum. The intensity data were sent directly to a Hewlett-Packard 9825A desktop c o m p u t e r with m u l t i p r o g r a m m e r for storage on tape, analysis and plotting. The Cu Me.3M,,.sM4. s (61 eV} doublet was scanned from 30 to 9 0 e V a i a r a t e o f 0 . 2 e V s 1 using a m o d u l a t i o n of l V peak to peak. a 40 x sensitivity on the lock-in amplifier, a time constant of 0.3 s, a beam energy of 2 keV and a target current of 2 gA. For" the Ag M4,sN4.sN,,.5 (356 eV) doublet, the AES operating conditions were the same except that the spectrurn was scanned from 320 to 380 eV. 3. RESUI,TS

3.1. Silverfilms Silver films were deposited to thicknesses ranging t'rom 1200 to 1500 A onto

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thin NaCI(lll) films formed epitaxialty on mica at room temperature. Figure 1 shows a typical Auger spectrum illustrating the clean surface conditions which prevailed. After an initial thick pinhole-free film had been formed, additional silver was deposited in steps of 0.5 A. After each layer had been added, the 356 eV Ag Auger doublet was recorded. Thus the film thicknesses given in Fig. 2 represent silver added I

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to an initially thick layer. The w m a t i o n in the R factor l\~r silver with added thickness is essentially constant for polycrystalline silver films {Fig. 2ta)l while it oscillates for monocrystalline films (Fig. 2tb)). The average period of oscillation for the monocrystalline films is 2.7 A. This result should be c o m p a r e d with the I I 11) interplanar spacing for bulk silver, namely 2.36 A. It is concluded thai the period is a p p r o x i m a t e l y one (1 1 1) a t o m layer of silver. Clearly, since thc R factor is a ratio of two p e a k - t o - p e a k heights in the deriwltive spectrum, wtriations in this quantit? represent changes in line broadening and peak shifts m the NtEt spectrum. It is in fact a very sensitive measurc of such changes. Figure 3 shows transmission electron micrographs and ditTraction patterns for typical polycrystalline and monocrystallinc silver films prepared under essentially identical conditions with those for Fig. 2. The results of Figs. 2 and 3 wcrc obtained three times for polycrystalline films and live times for monocrvstalline films.

Fig. 3. Typical transmission clecmm micrographs ofta)a polycrystalline silver lilm 1200 A thick t\~rmcd a/. 25 ('and Ib)an cpilaxial Ag(ll l) film 15(X)A thick formed al 210 C.

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The estimated precision in the R factor measurements was obtained in three different ways. As for copper z, doublets were recorded at different points on the surface of one particular silver sample. The second method involved determining the spread in R values from a polycrystalline film (Fig. 2(a)). A third method, shown in Fig. 4, gives the results for RAg at one point on the silver surface as a function of time. All three methods measure somewhat different Rs but, since the values obtained fluctuate randomly about a more or less common mean value, the fluctuations may be used to determine a reasonable value for the precision in R. The results show that Rag can be measured reproducibly to a precision of -+ 0.01. This value is significantly less than the amplitude of the oscillations given in Fig. 2(b) for a monocrystalline film. The precision in RAg for silver films is therefore much greater than the precision in Rcu for copper films z, where the amplitude of the oscillation for monocrystalline copper films (about 0.05) was somewhat less than twice the total spread (0.03) in precision (-+0.015). The reason is most probably due to the higher resolution of the overlap oscillation in the case of silver (see Fig. 1). I

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Figure 4 also shows that, for two different emission currents, RAg remains constant with time at a particular point on the surface of the film. The current of 20 mA gives rise to a target current of 2 pA, the value used in all the other experiments. The target current was measured with a + 4 0 V potential on the substrate. The Auger peak-to-peak heights, however, initially decreased slightly and then leveled off, suggesting either that some slight surface contamination occurs which essentially terminates after approximately 4 min (20 mA case) or that a structural rearrangement of the monocrystalline Ag(lll) surface occurs in this time. The complete Auger spectrum taken at the end of these experiments did not indicate that any contamination had taken place: However, a less than detectable contamination may still have occurred. The changes in peak-to-peak heights with time were found

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to be beam induced since they did not occur in unirradiated regions of the silver surface. If beam-induced contamination is.responsible for the dependence of the higher amplitude on time (Fig. 4), some explanation for the approximately 4 min termination (20 mA case) must be found. One possibility is that there may be a limited n u m b e r of"active sites" on the surface and that it takes about 4 min for them to be filled with contaminants in our experimental situation. However, a beaminduced surface atomic rearrangement also has a high probability. It is clear also from the peak-to-peak heights versus time results that experiments carried out continuously may give results that differ from those from experiments in which the deposition is intermittent.

3.2. Ag. Cu bilayers Again two sets of experiments were carried out. In the first set, thick polycrystalline copper films were deposited onto NaCl/mica at r o o m temperature. O n t o this substrate silver was deposited, again at r o o m temperature, in steps of 0.5 A to a total thickness of about 10/t,. Measurements on Rc, (61 eV) for the copper substrate were carried out to silver overlayer thicknesses at which Re, could no longer be measured with sufficient precision. For polycrystalline films the results for Rcu tended to be non-reproducible. RAg (356 eV), however, always started high and decreased to a steady state value as shown in Fig. 5. As shown in Figs. 2(a) and 5, RAg for polycrystalline films tended to have values significantly above the sputter-etched h a n d b o o k value shown as an open square 3. F u r t h e r m o r e the rapid decrease in the Auger amplitude of the copper signal as silver is added indicated that the silver film I

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grew fairly uniformly on the copper substrate. These experiments on polycrystalline films were repeated three times with essentially the same results. Monocrystalline films were also studied. Figure 6 gives typical R factor results for single-crystal silver films formed epitaxially on Cu(lll) films at 210 °C. The fluctuations observed for both Rcu (61 eV) and RAg (356 eV) are typical for this system and were reproduced five times. The fluctuations in both Rcu and RAg are approximately equal to their respective (111) interplanar spacings (2.09 A for copper and 2.36 A for silver). After approximately two monolayers of silver had been deposited, the fluctuations in RAg became essentially the same in periodicity and amplitude as those recorded for the thick silver film case shown in Fig. 2(b). The general overall decrease in RAg as the first three monolayers are deposited is greater than that observed for the polycrystalline case shown in Fig. 5. In the monocrystalline case of Fig. 6, RAg starts out at a somewhat higher value and continues decreasing until minima in the oscillations reach values comparable with those for the sputter-etched case 3. RAg in the polycrystalline case starts out at a somewhat lower value and decreases to the range of the maxima in the oscillations of the thick film monocrystalline silver case. Finally the reason RAg in Fig. 4 is relatively high for a monocrystalline film (RAg ,~ 0.64 in Fig. 4) is that in this case only approximately 4 A of silver had been deposited. Comparison with Fig. 6 shows that, for 4 A of silver on copper, a value of about 0.64 is indeed obtained for RAg. I

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Typical transmission electron micrographs and diffraction patterns are shown in Fig. 7 for the polycrystalline and monocrystalline cases. Figure 7(b) shows doublepositioning twin boundaries and some localized areas of misfit dislocations. The results are similar to earlier results obtained using somewhat different experimental

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conditions ~. In both cases misfit dislocations were not observed everywhere across the bilayer film.

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The fluctuations in energy E 1 lor both the copper substrate and the silver overgrowth for the monocrystalline films of Fig. 6 are shown in Fig. 8. While thc precision is not particularly good, periodic variations appear to be present in both cases. Comparison of the variations in Re. and RAg with film thickness (Fig. 6) with the corresponding variations in E 1 shown in Fig. 8 indicates that maxima in R tend to correspond to minima in El. Similar results were obtained for thick monocrystalline copper films 2. They were interpreted as signifying that the Auger doublet from the edge areas arises from valence energy levels that are slightly displaced to higher energies.

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The dependence of the peak-to-peak Auger dN:dE amplitude on the silver overgrowth thickness is shown in Fig. 9 for the monocrystalline case of Fig. 6. Except for a dip in the silver thickness range 2.5-3.0 A, a more or less linear increase occurs until approximately two monolayers have been deposited. Beyond the break at approximately 4.7 A, a linear increase is again observed but with a much reduced slope S 2. The results for the zero-to-two monolayer region are interpreted as being due basically to the growth of two-atom-high (111) islands of silver on copper that become completely continuous at about 4.7 A. The average slope of the plot in this region is m21 ( = SI). 70

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If monolayer-high islands had grown in this region, a break should have occurred at 2.36 A. The expected curves would then have followed the slopes indicated in the figures as mll and rn~2. The slight leveling off of the curve from about 2.5 to 3/~ may be interpreted in a way similar to that proposed by Gillet and Gruzza ~' for gold on Mo(1101, namely that in this region fiat three-dimensional islands form. The slope of the experimental curve in the second monolayer region is approximately equal to I1111for monolayer growth. Careful inspection of the curve in the zero-to-two monolayer region suggests that the initial silver atoms are deposited in a monolayer fashion which changes to biatomic layer growth up to the deposition of the first monolayer. Then three-dimensional fiat island growth occurs h o m 2.5 to 3/~. This mode is then followed by m o n o a t o m i c layer growth occurring next to the fiat islands up to a thickness of around 4.7 A. The net effect is an average biatomic growth mode in the zero-to-two monolayer region. The low slope in the thickness range beyond 4.7 A is characteristic of Stranski Krastanov growth v. Thus three-dimensional islands appear to iorm on top of the initial two monolayers of silver. Taking a simple model for the expected Auger intensities 8 we can calculate the ratio $2/S ~ of slopes in the case of an initial twoatom layer of silver followed by high fiat island growth. We obtain "~ 1 $2 = p [ e x p ( - 2 ¢ ~ l + e x p l - ' ~ e , l + . . . . . e x p ' ~ - I P + I)¢~ 1] f + e ~ p i z ~ l St

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In this case we have taken 42' as the average entrance angle of the cylindrical mirror analyzer s. The number of layers of interplanar spacing d 11 ~ in the flat islands is P while 2 is the extinction distance. If we take )4Ag) as equal to about 7.7 A, we obtain ~i = 0.42. Solving eqn. (1) numerically gives P = 28. This model is undoubtedly too simple but it supports the principle of Stranski Krastanov growth. Indeed from earlier T E M work on this same system it was concluded that high fiat island growth occurred along with neighboring thin regions 5. 4. DISCUSSION A sensitive measure of changes in the shape of an Auger line doublet has been defined as the R factor. It was found that, for silver films formed epitaxially on NaCI(111 },/mica substrates, R varied periodically with a period equal approximately to the (11 lj interplanar distance. No periodicity was observed for polycrystalline films. Previous reflection high energy electron diffraction studies of this system have shown that monocrystalline films are very fiat 9. The present R factor results were interpreted as arising from the superposition of two silver doublets (351 and 356 eV) differing slightly in energy. One doublet was assumed to come from flat areas of the surface while the other would come from edge areas of flat monolayer-high islands. It is expected that the atomic relaxations in the edge regions would give rise to slightly altered valence energy levels. As the silver film thickened by a layer growth mechanism, the relative amount of edge areas to flat

AUGER ANALYSIS OF THICK

Ag FILMS AND Ag LAYERS ON C u

175

areas would vary in a periodic manner, the period being again one monoatomic layer. As in previous cases1' z, Rmaxis associated with the expected high resolution in a doublet coming from a flat surface, while Rmin is associated with a surface having a maximum of island edge atoms, i.e. maximum atomic roughness. When silver is deposited onto copper, significant changes again occur in both RAgand Rcu. In both cases RAgdecreases from an initially high value to lower values characteristic of thick films. The effect is somewhat smaller for polycrystalline than for monocrystalline films, suggesting that here too some form of layer growth is occurring. The general decrease in RAg is interpreted as being due to the misfitinduced strain in the silver overgrowth 5. The effect is explained as follows. It is well known that, if individual atoms are compressed into a solid, the energy levels spread into bands consisting of many levels. In a similar manner, silver growing epitaxially on copper is compressed initially towards the lattice parameter of copper, the natural lattice misfit being - 1 3 ~o5. This compression would tend to increase the energy separation of the valence energy levels. Thus we would expect the 356 eV Ag doublet to be better resolved in thinner films of silver on copper than in thicker films. Such better doublet resolution in thinner films would lead to larger RAg values, as was observed for both polycrystalline and monocrystalline bilayers. Energy measurements were carried out on the 356 Ag Auger line from a monocrystalline bilayer. The data obtained were not very precise but they clearly indicated a decrease in doublet peak separation (on an N(E) scale) of approximately 0.14 eV as the silver overgrowth increased in thickness from 0 to 10 A. The actual separation decreased from 5.42 to 5.28 eV with a precision of about -+0.04 eV. These results are in agreement with the model presented in the previous paragraph. In the case of monocrystalline Ag/Cu bilayers, an oscillatory pattern ks superimposed on RAg as the overgrowth becomes thicker. No such oscillation was observed for polycrystalline layers. After about two monolayers have been deposited, the oscillation in RAg resembles very closely that observed for thick silver films. It too has a period approximately equal to the (111) interplanar spacing of silver. The Auger signal from the 61 eV Cu substrate doublet, while lacking the precision obtained from the silver overgrowth, also appears to have a periodic oscillation characteristic of the Cu(lll) interplanar spacing. Energy shift measurements were also made but they were not very precise. Nevertheless they tended to confirm the periodicities observed in RAgand Rcu. In order to interpret the R factor results for monocrystalline Ag/Cu bilayers, we must take into account the peak height data of Fig. 9. Here it was shown that a Stranski-Krastanov growth mode occurred with the formation of high islands after two complete monolayers had been deposited. In this early region of zero-to-two monolayers, the various slope changes suggest that an initial monolayer growth mode is joined by thicker islands which in the 2.5-3 ,& range give rise to high island growth. From 3 to 5 A, the high islands are supplemented by monolayer growth in neighboring regions. In the actual case many of these processes may be occurring simultaneously with varying intensity. The average effect in the zero-to-two monolayer region is that of a biatomic flat island growth mode. Beyond two monolayers (4.7/~) high island growth occurs. A simple calculation indicated that these islands were 28 layers thick. This result is completely unreasonable and indicates a breakdown of the model. Thus it is not likely that, after

176

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two Sliver monolayers have formed, islands which are 28 layers thick form one b~ one as more silver is deposited. The Auger amplitude results of Fig. 9 do show that beyond two monolayers tile surface coverage does not wiry rapidly. Previously it has been shown that thick flat islands of silver formed and thai they were bounded by very thin regions 5. At an average silver thickness of 20 A, the thick islands covered the surface by only about 70'),. At 10 A average thickness, the thick island silver coverage was only 45~,, a value that decreases only slightly as the silver overgrowths become thinner 5. Thus the model which appears to fit the experimental data best consists of a thin silver film two atom layers thick on top of which large flat high islands of silver form. The lateral dimensions of these islands would not increase rapidly with added silver as the TEM ~ and Auger amplitude data of Fig. 9 suggest. These islands would then grow thicker by a layer mechanism giving rise to the fluctuations in RAg observed in Fig. 6. The observed pcriod in RA~ must also bc explained. This can bc done by again going back to the observed high island coverage data obtained by TEM~. Clearly. if this coverage remains approximately 4 0 45?, in the 0 10 A range as reported here. then a biatomic layer growth model will explain a monolayer periodicity. Thus, il growth occurs one atom layer at a time, RA~ is expected to have a periodicity of one monolayer. However, if the coverage is only 50?, (q/i our 40 452,,) and islands two atom layers high form and grow on top of the thick silver islands, then a monolayer periodicity in RAg will again appear. This result is schematically illustrated in Fig. 10, in which it is assumed in Fig. 10(c) that incident silver atoms, after surfacc difl'usion. cud up almost entirely on top of the high islands. Thus without the TEM coverage data it would be impossible to distinguish the models of Figs. 10(a) and 10it) by R factor measurements. As the epitaxial silver films grow thicker on top of the copper substrate, the high island coverage gradually increases 5. Eventually it must reach 100?i,. At that point monolayer growth would be expected to dominate over a biatomic layer growth mode, as is the case in the thick cpitaxial silver film of Fig. 2(b). Thus at some intermediate high island coverage the R factor periodicity may become somewhat diffuse as the new monolayer growth mode is established. This range of thicknesses was not investigated in the present experiments.

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Finally the periodic fluctuations in Rcu for the substrate must be explained. These fluctuations are detectable with reasonable but decreasing precision up to about 4 A of silver, i.e. up to about 12 monolayers of silver. Again a period of about one (111) m o n o a t o m i c layer in Rcu was observed. The Auger amplitude data for silver (Fig. 9) suggest that mixed growth modes occur in the zero-to-two monolayer region. Up to one monolayer of silver deposit, monolayer growth occurs but

AUGER ANALYSIS OF THICK

Ag FILMS AND Ag LAYERS ON C o

177

gradually converts to bilayer growth. Thereafter some thick silver islands form after which monolayer growth again occurs. The fluctuations in Rcu may therefore arise in more or less the same manner as described earlier. We assume that the copper substrate atoms form a more or less rigid lattice because the substrate is very thick. However, in the vicinity of the edges of silver islands, a small perturbation of the copper substrate atom spacings would be expected 1°. Thus two sets of copper Auger 61 eV doublets would appear, one from copper regions near the silver island edge areas and one from the flat areas away from the edges. If these two doublets were slightly displaced in energy, a superposition would result in line broadening and a smaller value for Rc~. In this manner periodic fluctuations in Rcu would arise in the same way as occurs in thick copper or silver films. Finally this behavior of Rcu under the influence of a growing silver film is similar to that observed for Rc~ under a growing palladium film 1. ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support of the U.S. Department of Energy under Contract number DE-AS02-77ER0 4496. The technical assistance of A. J. Isabelle and B. Singh was also appreciated. REFERENCES 1 S.S. Chao, E.-A. Knabbe and R. W. Vook, Surf. Sci., 100 (1980) 581. 2 Y. Namba, R. W. Vook and S. S. Chao, Surf. Sci., in the press. 3 L.E. Davis, N. C. MacDonald, P. W. Palmberg, G. E. Riach and R. E. Weber, Handbook ofA uger Electron Spectroscopy, Physical Electronics Industries, Eden Prairie, MN, 1976. 4 S.S. Chao, R. W. Vook and E.-A. Knabbe, in F. Abeles and M. Croset (eds.), Proc. 8th Int. Vacuum Congr., Cannes, 1980, Vol. 1, Thin Films, in Vide, les Couches Minces, Suppl., 201 (1980) 161. 5 R.W. Vook and C. T. Horng, Philos. Mag., 33 (1976) 843. 6 E. Gillet and B. Gruzza, in D. A. Degras and M. Costa (eds.), Proc. 4th Int. Conf. on Solid Surfi~ces, Cannes, September 22-26, 1980, in Vide, les Couches Minces, Suppl., 201 (1980) 669. 7 G.E. Rhead, J. Vac. Sci. Technol., 13 (1976) 603. 8 E. Bauer, H. Poppa and G. Todd, Thin Solid Films, 28 (1975) 19. 9 F.A. K o c h a n d R . W. Vook, J. Appl. Phys.,42(1971)4510. 10 P. Wynblatt, in P. C. Gehlen, J. R. Beeler, Jr., and R. 1. Jaffee (eds.), lnteratomic Potentials and Simulation o['Lattice Dejects, Plenum, New York, 1972, p. 633.