The changes of the Auger line shape during formation of Ag film on polycrystalline Au

Surface Science 123 (1982) L667-L673 North-Holland Publishing Company

L667

S U R F A C E S C I E N C E LETTERS T H E C H A N G E S O F T H E AUGER L I N E S H A P E D U R I N G F O R M A T I O N OF Ag F I L M O N P O L Y C R Y S T A L L I N E Au R. S I U D A Instytut Matematyki i Fizyki, Akademia Techniczno-Rolnicza, S. Kaliskiego 7, 85-763 Bydgoszcz, Poland Received 8 July 1982; accepted for publication 27 September 1982

Recently, Chao, N a m b a and Vook have published several papers in which changes of the Auger line shape were used as a parameter which allows us to distinguish whether the evaporated epitaxially grown layers are smooth or rough in the atomic scale [1-3]. They have introduced the so-called R-factor defined as in the insertion shown in fig. l a. This factor can be used if the observed line has a doublet structure. In the referred papers the R-factor has been applied for studying the mechanism of growth of evaporated Ag, Cu and Pd films. For epitaxial layers the R-factor value manifests the periodicity with a period close to the monolayer thickness. The authors attribute the maximal and minimal values of R to atomically smooth and rough surfaces, respectively. They explain the dependence of R on the surface state as being due to the different numbers of edge atoms at different stages of monolayer growth. Chao, N a m b a and Vook also reported some results obtained for polycrystalline films but these results seem to be less conclusive than in the case of monocrystalline layers. The aim of this paper is to show that the R-factor can also be useful for studying the growth of polycrystalline layers. The sample under investigation was a bilayer composed of Au on Ag. Polycrystalline Ag film of 4100 +__80 A thickness was first evaporated on glass substrate and then the 1180 _+ 40 A Au layer was deposited. After deposition the sample was in contact with the air atmosphere before transferring it to the U H V chamber of the AES spectrometer. The AES spectrometer used has been described elsewhere [4]. The electron energy analyser used was a CMA type with a resolution of 0.007. The spectra were recorded in the derivative form. The modulation voltage was 0.9 Vrms, the primary electron beam energy was 1700 eV and the electron beam current was - 20 /zA. The surface contaminants (S, CI, C, O) and segregated Ag were removed by argon ion etching (E~ = 1000 eV) until their lines were undetectable. After several minutes when the Ag 356 eV line became detectable again, the resistive heating of the glass 0039-6028/82/0000-0000/$02.75 © 1982 North-Holland

L668

R. Siuda / Ag filrn on polycrystalline Au

s u b s t r a t e was switched on and the t e m p e r a t u r e of 75°C was m a i n t a i n e d . A u g e r p e a k - t o - p e a k heights ( A P P H ) of the A u 69 eV a n d the A g 356 eV features were r e c o r d e d a n d the values of the R - f a c t o r were d e t e r m i n e d . The results in the f o r m of R - t a n d A P P H - t plots are shown in figs. l a a n d lb, respectively. The raw values of R were s m o o t h e d with the p r o c e d u r e R~Sm = ) [ R i + I ( R i_ 1 + R,+ i)] [91. Since the electron b e a m d i a m e t e r was of the o r d e r of 0.1 m m and the grain d i m e n s i o n s of a b o u t 0 . 1 - 1 gin, the n u m b e r of grains in the investigated area was - 1 0 3. T h e r e f o r e the p r o p e r t i e s of the investigated surface must be a v e r a g e d over the large n u m b e r of real facets of different structure. N e v e r t h e less, for the sake of clarity the following discussion concerns the idealized c l o s e - p a c k e d surface. It will be seen that some conclusions seem to be valid also for the case of the averaged surface. A l m o s t the s a m e lattice p a r a m e t e r s for A g and A u allow us to a d m i t that the A g a d l a y e r reconstructs the structure of the A u substrate. If on the ideal c l o s e - p a c k e d surface of A u a single a t o m of A g appears, it will be b o n d e d with 3 b o n d s a n d we can assume that a roughly a p p r o x i m a t e d value of the energy p e r b o n d is ½({As + [Au)' where ~Ag a n d ~Au are energies per b o n d for A g a n d Au, respectively. F r o m ref. [5] one can o b t a i n EAg ~ 0.249 eV a n d ~Au = 0.298 eV if only i n t e r a c t i o n with the nearest n e i g h b o u r s is taken into account. W e d e n o t e here ~Ag as ~ and /(~Ag + ~Au) as ~S" T h u s the b i n d i n g energy (BE) of a single a t o m is E1 = 3c s. If a cluster consisting of 2 a t o m s is present on the surface, the interaction of a d a t o m s i n t r o d u c e s an a d d i t i o n a l energy 2~, and the BE p e r a d a t o m is E 2 = 3~ s + c . F o r a cluster consisting of 3 a t o m s two a r r a n g e m e n t s are possible: a triangle a n d a linear one. F o r a triangle configu r a t i o n the BE p e r a t o m is E 3 = 3c s + 2~ a n d is the same for each atom. F o r a l i n e a r c o n f i g u r a t i o n the central a t o m has BE equal to E 3 whereas the two external ones have BE equal to E 2. T h u s the difference of the entire BE of the two types of cluster is 2~. It m e a n s that, e.g., for the t e m p e r a t u r e T = 400 K ( k T = 0 . 0 3 4 eV) the n u m b e r of triangle clusters significantly exceeds the n u m b e r of linear clusters when thermal e q u i l i b r i u m is reached. Therefore if the flux of a t o m s to the surface is low enough for establishing a thermal equilibrium, the existence of linear configurations of a t o m s can be neglected. M o r e o v e r , a t o m s a r r a n g e d in linear configurations have only two possible values of BE: E 2 o r E 3 a n d no o t h e r values of BE can be taken into account. F o r this reason we can neglect the presence of linear clusters (except 2 - a t o m clusters). If we consider a further increase of the n u m b e r of a t o m s in a cluster, the increase of BE per a t o m can be stated (see fig. 2) until the cluster consists o f 7 atoms. In the latter case the central a t o m has a BE of E 7 = 3~ s + 6~, i.e. the m a x i m a l value if only t w o - d i m e n s i o n a l g r o w t h of clusters takes place. Thus o n e can see that the BE ranges from 3c s to 3c s + 6~ if the n u m b e r of a t o m s in a cluster changes from 1 to 7. Since the investigated area consists of a large n u m b e r of different crystalline facets, the range of BE changes is not n a r r o w e r

L669

R. Siuda / Ag film on polycrystalline Au

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Fig. 1. (a) Smoothed R-factor plot versus time for Au (I 180_+40 ,~) on Ag (4100+ 80 A) polycrystalline bilayer evaporated on glass substrate. Substrate temperature was 75°C. The R-value characterizes the shape of the Ag 356 eV feature. (b) For the same sample A P P H - t plots: Ag 356 eV ( + ) and Au 69 eV (O) Auger transitions.

than for an ideal close-packed facet. Therefore the existence of atoms with different values of BE (or different numbers of bonds) causes broadening of the observed Auger lines. For the case in which the Auger feature exhibits doublet structure (like the Ag 356 eV line), the decrease of the R-factor can be observed if the spreading of BE of emitting atoms occurs. Now one can qualitatively explain the R - t plot as follows. Just after ion etching of the Au surface the Ag atom coverage (0) can be assumed as being nearly zero. The grain boundary diffusion supplies the Au surface with Ag

R. Siuda / Ag film on polycrystalline Au

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atoms which in Spite of surface migration do not create clusters if 0 is maintained low enough. It means that the spread of the Ag valence electron energy is narrow and that the R-factor has a large value. If 0 increases the surface density of Ag atoms and the probability of their impingement increases as well. The existence of clusters consisting of a few atoms is more probable and a rapid decrease of the R-value is observed. For a certain time, mainly small clusters are present and hence the maximal spread of BE takes place ~nd a minimal value of R is observed. With the increase of O the growth of existing clusters occurs and the R-value becomes larger. From fig. la one can see ~hat up to t I = 26 min linear dependence for both Au and Ag APPHs is observed. It is well known that if A P P H changes linearly two-dimensional growth of layer takes place ( F r a n k - v a n der Merwe or, for only first layer, Stranski-Krastanov mechanism) (see for example refs. [6,7]). For t > t~ two features of presented A P P H - t dependences should be explained, namely the change of slope in the

R. Siuda / A g film on polycrystalline A u

L671

Ag signal and the distinct deviation from linearity in the Au signal. The decrease of the slope of the A P P H - t curve for Ag can be ascribed to the change in growth mechanism from two- to three-dimensional. If one assumes that on flat islands the second layer begins to rise, then the signal depends on coverages o f the first (81) and the second (0 2) layer according to the relation: IA = I ° ( 1 -- a A ) ( P , O ,

+ 02[ P2 -- Pl( 1 -- aA)] ),

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where I ° is the signal from the adlayer of infinite thickness, a A is the so-called attenuation factor of monoadlayer [6], and Pl and P2 take into account the fact that both for the first and for the second adlayer the backscattering factors cannot a priori be taken as equal to that of bulk material. If the latter is denoted as rA then Pl rl/rA and P2 r2//rA. It can be seen from eq. (1) that the slope of the A P P H - t plot should decrease when the second layer origins on islands of the first one. Taking this into consideration also for the substrate signal, one can obtain: =

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where I ° is the signal from the clean substrate and a s is the attenuation factor of monoadlayer related to the energy of the substrate line. So the slope of the A P P H - t curve for the substrate should change at t = t 1 but with conservation of linearity. This is not the case since a distinct deviation from linearity is seen in fig. lb. In addition this deviation seems to appear slightly before t I. It is believed that the deviation is caused by the increase of the number of bonds of the top surface substrate atoms when adatoms appear. The argument for the validity of such an explanation is the effect of changes in the bond structure on the Au 4f7/2 level of polycrystalline gold reported by Citrin, Wertheim and Baer [8]. Their results prove that the Au 4f7/2 level of polycrystalline gold is composed of two components related to top surface and inner atoms, respectively. The 4f7/2 level components differ in energy by about 0.40 eV. It seems to be most likely that if the top surface of Au atoms is covered with Ag atoms, as a consequence of the increase of bond number, the top surface atoms component decreases. Thus the resulting Auger feature involving both 4f7/2 and valence electrons should be narrower. This means, that the APPH should decrease slower than it follows from eq. (2). If this reasoning is correct, the completion of the first monolayer takes place not for t = t I but later when the first distinct maximum of the R-value occurs. Relying on the linearity of the A P P H - t curve for the substrate, ohe can expect that during the time of a rapid R-value decrease, islands of the second layer exist only. The second distinct maximum of the R-value can be explained in a similar way. The third rapid increase of the R-value seems to be connected with the completion of the third and further monolayers since the presence of a few breaks in the slope of the substrate A P P H - t curve probes that at the same time growth of a few

L672

R. Siuda / Ag film on polycrystalline A u

monolayers occurs• It is worth mentioning that no distinct maxima of the R-value have been observed if the A P P H - t plots were in accordance with the Volmer-Weber mechanism. This can be seen from figs. 3a and 3b where results for Au (1130 _+ 20 A) on Ag (6400 _+ 60 ,~) substrate are presented. All experimental conditions were the same as for the former sample, except the temperature which was 145°C at the beginning, a n d after a few minutes decreased to 85°C and was kept constant. Although the above considerations are rather qualitative, it is possible to draw the following conclusions.

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t/rnjn 0 Fig. 3. (a) Smoothed R-factor plot versus time for Au (1130-t-20/k) on Ag (6400 + 60 A) polycrystalline bilayer evaporated on glass substrat¢. The substrate temperature was 145°C at the beginning, and after a few minutes decreased to 85°C and was kept constant. The R-value characterizes the shape of the Ag 356 eV feature. (b) For the same sample A P P H - t plots: Ag 356 eV ( + ) and Au 69 eV (O) Auger transitions

R. Siuda / Ag film on polycrystalline Au

L673

T h e R - t p l o t o f f e r s a d d i t i o n a l s i g n i f i c a n t i n f o r m a t i o n g i v i n g insight o n d e t a i l s o f l a y e r g r o w t h also w h e n the s u b s t r a t e is p o l y c r y s t a l l i n e . T h e results s h o u l d b e of special i n t e r e s t w h e n s u b s t r a t e crystallites h a v e o n l y o n e t y p e of facet. S i n c e the a n a l y s i s o f the e a r l y stage o f the i n v e s t i g a t e d p r o c e s s i n d i c a t e s that d O / d t is a b o u t 5 × 10 - 4 s - 1 , it is o b v i o u s t h a t such a slow g r o w t h e n a b l e s us to s t u d y t h e d i f f u s i o n a n d s e g r e g a t i o n m e c h a n i s m s in a c a r e f u l m a n n e r . T h e a u t h o r e x p r e s s e s his t h a n k s to Dr. W. Bala for p r e p a r i n g the s a m p l e s a n d to D o c . J. S k o n i e c z n y , D o c . M. R o z w a d o w s k i a n d M . S c . A . B u k a l u k for v a l u a b l e c o m m e n t s o n the m a n u s c r i p t .

References [1] [2] [3] [4] [5]

[6] [7] [8] [9]

S.S. Chao, R.W. Vook and Y. Namba, J. Vacuum Sci. Technol. 18 (1981) 695. Y. Namba, R.W. Vook and S.S. Chao, Surface Sci. 109 (1981) 320. Y. Namba and R.W. Vook, Thin Solid Films 82 (1981) 165. A. Bukaluk, W. Gackowski, M. Rozwadowski and R. Siuda, Prace Inst. Tele-Radiotechn. 89 (1982). The values of CA~and CAu were calculated based on the values of sublimation energy reported by F.R. Bichowski and F.D. Rossini, in: The Thermochemistry of the Chemical Substances (Reinhold, New York, 1936) cited after L. Kalinowski, Fizyka Metali, Vol. 2 (PWN, Warsaw, 1973). J.P. Bibbrian and G.A. Somorjai, Appl. Surface Sci. 2 (1979) 352. C. Argile and G.E. Rhead, Surface Sci. 53 (1975) 659. P.H. Citrin and G.K. Wertheim and Y. Baer, Phys. Rev. Letters 41 (1978) 1425. A relatively small gain of our electron multiplier required a large modulation voltage (2.6 Vp_p) in order to obtain a sufficient signal-to-noise ratio. As a consequence the observed values of the R-factor are lower than those referred to in ref. [3], since the use of a large modulation amplitude causes "smoothing" of the observed feature.