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Step contrast on ultrathin Au films investigated by TEM
Ultramicroscopy 17 (1985) 73-80 North-Holland, Amsterdam
73
STEP C O N T R A S T ON U L T R A T H I N Au F I L M S I N V E S T I G A T E D BY TEM M. K L A U A and H. B E T H G E Institut fur Festkrrperphysik und Elektronenmikroskopie der Akademie der Wissenschaften der DDR, Weinberg, DDR-401 Halle/Saale, German Dem. Rep.
Received 14 January 1985
Conditions of optimum contrast of atomic steps on ultrathin Au films imaged by transmission electron microscopy are given. Au(111) and Au(100) films less than 10 monolayers thick were prepared by vacuum deposition onto compact Ag growth faces, backed with C films and dissolved electrolytically from the substrate. From systematic tilting experiments it follows that optimum step contrast can be obtained in bright-field and dark-field modes for all orientations where low index (111), (200), (220) and (311) reflections are excited either systematically or as cross-grating patterns. Bright-field step contrasts are in good agreement with theoretical n-beam calculations, whereas discrepancies in dark-field contrasts are due to the background scattering of the supporting C films. 1. Introduction
Real surfaces usually contain geometric surface defects, the most important of which are atomic steps. They have been shown to play a fundamental role in processes of growth, evaporation, thin film growth, and epitaxy as well as in adsorption, reconstruction, and 2D phase transitions. The att e m p t to visualize atomic steps dates back to the sixties when Bassett [1] first succeeded in decorating cleavage steps on rock salt. For alkali halides, the decoration technique has been multiply applied to the imaging of step structures which result from processes of growth, evaporation, thin film growth, cleavage, and deformation [2]. For metal surfaces, mainly because of difficulties in the preparation technique, tl~ere are only a few examples of a successful step decoration [3-5]. On the other h a n d , TEM has been applied to direct step imaging on thin films first by Cherns [6] who observed dark and bright contours on Au(111) films in dark-field images taken with kinematically forbidden reflections. Similar observations were made by other authors [7-9] on Au, Pt, and Cu films. K a m b e and Lehmpfuhl [10] used a weak-beam dark-field technique for step imaging on MgO particles, and Lehmpfuhl and U c h i d a [11] made a quantitative analysis of bright-field and dark-field
contrasts from atomic steps on MgO. K l a u a and Bethge [12] applied these techniques to ultrathin Au films, while Takayanagi [13] studied monolayer growth of Ag on Ag by in situ UHV electron microscopy. Iijima [14] investigated systematically the capability of defocus phase contrast for step imaging down to a resolution of monatomic height. These TEM techniques are well established in connection with investigations of thin film growth, but other powerful methods for step imaging on compact crystal surfaces have been developed. Reflection electron microscopy u n d e r UHV conditions was applied by Yagi et al. [15] to investigate step movements, thin film growth and 2D transitions on Si(111). Binnig and Rohrer [16] showed that scanning tunneling microscopy permits threedimensional imaging of surface topographies on atomic scale. Low-energy electron reflection microscopy has been recently developed by Bauer [17]; its application to step imaging and to the investigation of various other surface phenomena [18] promises the full success expected of this technique. 2. Experimental Ultrathin epitaxial Au films of less than 10 atom layer thickness were prepared by evaporating
0304-3991/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
74
M. Klaua, tl. Bethge / Step contrast on ultrathin Au films investigated by TEM
the metal onto bulk Ag(111) and Ag(100) faces in UHV. The deposition rate was 1 to 2 A min -t, and the substrate temperature ranged from 30 to 200°C. Spherical Ag crystals of 5 mm diameter were obtained by fast solidification of droplets on a graphite mould in high vacuum on which, by cathodic electrocrystallization, growth faces of (111), (100), (110) and (211) orientation were produced having a maximum extension of 1 to 2 mm [19,20]. The growth faces were cleaned in UHV by argon ion bombardment (energy: 600 eV, current density: 2.5 /tA cm -2) and subsequent annealing at 350 to 400°C or by a free evaporation at 650 to 800°C. The cleanliness, surface crystallography and step structure were determined by AES, LEED and gold decoration technique, respectively [21]. Before removing the Au films from the Ag substrates they were supported by depositing C films of some hundred Angstrom thickness. The Au films adhering to the C films were dissolved electrolytically from the substrate, rinsed in water baths and then mounted on electron microscope grids. Transmission electron micrographs and diffraction patterns were taken after tilting the crystal
films by means of a goniometer stage to attain optimum contrast conditions. Step contrasts per monolayer thickness change were determined by measuring the optical densities on the plates by microdensitometry which was calibrated by Faraday-cup measurements.
3. Results and discussion
Step (or monolayer) contrast is discussed first for Au(111) films of submonolayer coverage since here its interpretation is very easy. If a fraction of one Au monolayer is deposited onto an almost perfect Ag(111) face having extended atomically flat regions with monatomic steps of low density as, e.g., in fig. la, the initial stage of thin film growth consists in the formation of islands by two-dimensional nucleation and the deposition of Au atoms at the existing steps. At sufficiently high substrate temperature (above 80°C for Au on Ag(111)) a repeated nucleation on top of the initially formed islands can be neglected for this submonolayer coverage leading to strictly two-di-
l-ig. 1. 2D nucleation and growth of Au on A g ( l l l ) : (a) at 100°C, coverage 0.12; (b) at 207°C, coverage 0.6.
M. Klaua, H. Bethge / Step contrast on ultrathin Au films investigated by TEM
mensional island growth and monolayer growth along the steps. Figs. la and lb show Au crystallites of ¢~ae-atom-layer thickness for two different coverages and substrate temperatures. The Au crystallites proved to be two-dimensional by comparison of the deposited amounts of Au measured by a calibrated quartz oscillator and, on the other hand, by measuring the coverage on the micrographs assuming that reevaporation and volume diffusion can be neglected. The two-dimensionality is further supported by the observation that all Au crystallites at steps grow only in the direction of concave curvature which corresponds to the lower side of the steps because the curved steps were formed during the evaporation process. Finally, both islands and crystallites at steps show the same optical density, i.e. contrast. The nonuniform contrast of the Au monolayer ribbons in fig. lb is due to the granular phase contrast structure of the underlying C film or possibly to a slight interdiffusion between the Au monolayer and the Ag substrate. Interdiffusion in thin A u / A g films in the temperature range of 280 to 400°C was
75
studied systematically by Meinel [22] using TEM and AES to characterize the diffusion process quantitatively. Such monolayer Au crystallites yield a contrast of some percent which depends on the direction of incidence of the electron beam. Figures and a quantitative comparison with theoretical calculations are given at the end of this section. If the thickness of the deposited film increases to some monolayers the contrast of atomic steps depending on the orientation of the film relative to the electron beam has to be optimized by tilting the specimen. Fig. 2 shows the effect of a slight tilting for a cylindrically bent crystal film. Strong step contrast appears solely in the middle region where systematical (111) reflections are excited as can be seen in the inserted diffraction pattern. With increasing deviation from this optimum orientation the contrasts gradually weaken and disappear. Without interpreting the observed step structure in detail we only state the fact that in this bright-field image the brightest parts correspond to the thinnest parts of the film and that with step-wise increasing thickness the contrast
Fig. 2. Bright-field image of atomic steps on a cylindrically bent A u ( l l l ) film of 3.5 monolayer mean thickness, direction of incidence for the middle region near [110].
76
M. Klaua, 1t. Bethge / Step contrast on ultrathin Au films investigated by TEM
also increases. But this monotonous increase of contrast only occurs if the stacking sequence of subsequent (111) layers is the normal fcc one. On the other hand, in twinning processes, i.e. if stacking faults exist in planes parallel to the film surface, these processes are visible as irregular contrast changes (white arrow). But under our experimental conditions this phenomenon seldom occurs; it seems to be connected with impurities or macroscopic defects in the surface. In addition to the step contrast sometimes bright and dark lines are observed generally running in [110] directions. The diffraction contrast of these lines is caused by stacking faults or dislocations on (111) planes inclined to the surface which are formed during a strong bending of the A u / C sandwich film in the course of electrolytic dissolution. Systematic tilting experiments of A u ( l l l ) and Au(100) films led to the following optimum contrast conditions for step imaging on ultrathin films: generally speaking, all orientations give strong contrasts for which low index reflections (111),
(200), (220), (311) are strongly excited either as systematic rows or as cross-grating patterns. For (111) films these are [211], [321], etc. or [111] and [332] orientations, and for (100) films [100], [210], etc. and [110] directions of incidence. Furthermore, those orientations near the latter ones where one or two low-index reflections are exactly excited (2-beam or 3-beam cases) were found to have relatively strong step contrast. Figs. 3a and 3b show an example of bright-field and dark-field imaging of atomic steps on an A u ( l l l ) film of 3 monolayer mean thickness. In the bright-field micrograph, the optical density increases step-wise with increasing thickness whereas the darkfield image taken with one of the (111) reflections reveals the complementarity but with much higher contrast. The observed film growth mode is the result of a special pretreatment of the A g ( l l l ) substrate. After cleaning by ion bombardment the Ag crystal was heated up to 650°C. During free evaporation, large step-free regions were formed while existing atomic steps bunched to form higher substrate steps. The high
Fig. 3. Bright-field (a) and dark-field (b) image of a A u ( l l l ) film of 3 monolayer mean thickness; substrate temperature 142.5°C; orientation [112].
M. Klaua, H. Bethge / Step contrast on ultrathin Au films investigated by TEM
steps are decorated with thick Au ribbons (black and white, respectively), and in the flat regions the interaction of nucleation and growth of two-dimensional islands and the spreading of monolayers from high substrate steps yield the observed morphology. The actual step structure of the growing Au film depends on the initial substrate morphology, i.e. the mean step density and the substrate temperature [23]. Fig. 4 shows an example of thin film growth of Au on Ag(111) at a higher temperature of 172°C, where the only growth mechanism is the spreading of monolayers from high substrate steps whereas no nucleation on Au terraces occurs. This brightfield micrograph was taken after tilting the Au(111) film by 43 ° to a [130] near direction of incidence where two (200) reflections are strongly excited (see inserted diffraction pattern). Three types of steps can be distinguished. High substrate steps are identified as dark bands in which atomic steps are not resolved. Second, atomic steps appear on the surface of the grown Au film with step-wise increase of optical density. And, finally, due to contrast changes because of increasing or decreas-
77
ing thickness [12], atomic steps are occasionally detected on the substrate side of the Au film (white arrows). For comparing step contrasts measured with those theoretically obtained, the thickness dependence of the transmitted and diffracted beams has been calculated for perfect crystals using the Bloch-wave matrix formulation of the dynamical theory of high-energy electron diffraction [24]. The absorption was treated by perturbation theory. As input parameters, extinction lengths and absorption lengths after Goringe [24] based on Doyle and Turner [25], Humphreys and Hirsch [26] and Radi [27] have been used. Fig. 5 shows the thickness dependence of the intensities of the (000) beam, two (111) beams, two (222) beams and two (220) beams for a 17-beam calculation of a perfect A u ( l l l ) film tilted to an exact [112] direction of incidence. The (000) beam with which the bright-field micrographs are taken does not show any thickness oscillation so that for this orientation strong step contrast can be expected only for thicknesses below 10 monolayers. Accordingly, for much thicker films other special
Fig. 4. Bright-field image of a 3.5 monolayer thick Au(111) film; substrate temperature 172°C; orientation [130].
M. Klaua, H. Bethge / Step contrast on ultrathin Au films investigated by TEM
78
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directions of incidence have to be chosen to o b t a i n sufficiently strong step contrast [11]. For dark-field micrographs t a k e n with one of the diffracted b e a m s the complementary step contrasts are always much higher than bright-field ones for ultrathin films, but also for thicker films conditions of sufficiently s t r o n g step contrast can be f o u n d more easily t h o u g h at intensities orders of m a g n i t u d e lower. Such intensity curves were calculated for many orientations, and the a b o v e m e n t i o n e d rules of o p t i m u m contrast for ultrathin films have been confirmed. The contrast is defined as the absolute v a l u e of the difference of the intensities at the thickness t and t + At divided by the m a x i m u m
b e a
m
yo
2
. . . . . . . . . . . . .i ! , , , , ! , Fig. 5. Thickness dependence of intensities of 100 keV electrons for a A u ( l l l ) film in e x a c t [112] direction of incidence, 17-beam calculation.
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~
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Fig. 6. Thickness dependence of bright-field monolayer contrast on Au(111) films in exact [123] orientation ( x ) and [123] near orientation ( O ) w h e r e a (531) and (420) reflection are exactly excited.
value of the intensity at either t or t + At:
K=
II(t)--I(t+At)l I( t +,at)] "
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In fig. 6 the bright-field contrasts of a t o m i c steps up to 10 monolayers were calculated for an exact [123] incidence ( u p p e r curve) and for a [123] near incidence (lower curve) where a (331) and a (420) beam are exactly excited. A slight tilting by only 1 ° out of the [123] orientation results in a m a r k e d decrease in contrast. The two sets of experimental p o i n t s were measured on a A u ( l l l ) film of 3 m o n o l a y e r mean thickness tilted to the corresponding orientations. In spite of some
Table 1 C o m p a r i s o n between experimental and theoretical contrasts of one monolayer Au for different directions of incidence n-Beam calculation
Tilting angle (deg)
13 7 7
0 0 ~1
17 9 15 17
19.5 19.5 22.2 ~ 23
Direction of incidence
C o n t r a s t of 1 monolayer (%)
[111] exact [111] exact = [1111, (2~o) e x a c t excited [112] e x a c t [112] e x a c t [123] e x a c t ~- [123], (531) and (420) exact excited
6.4 5.2 3.4
Theor.
3.7 4.6 4.1 2.8
Expt.
5.7 3.4 4.5 3.8 2.9
M. Klaua, H. Bethge / Step contrast on ultrathin Au films investigated by TEM
scattering of the points a good agreement is observed. In table 1 a further comparison is made between experimental and theoretical contrasts of one monolayer Au for different directions of incidence. The experimental values were measured on micrographs with submonolayer coverage of Au as in fig. 1. The calculations were carried out for different n-beam cases taking into account only the allowed reflections. The question of how many b e a m s have to be treated cannot yet be answered definitely for ultrathin films. The solution of this question requires a systematic study of bright-field contrasts for film thicknesses up to 30 monolayers to be carried out in o r d e r to measure contrast oscillations [22]. Such os.cillations in the bright-field intensity strongly d e p e n d on the number of b e a m s used. With increasing number of beams the intensity oscillations vanish, as can be seen in fig. 5 for a 17-beam case.
4. Conclusion
Experimental conditions of o p t i m u m step contrast of ultrathin Au films backed by C films are correlated with n-beam intensity calculations for perfect crystals. Bright-field contrasts from 1 to 10 monolayer thickness are in good agreement with theoretical values, whereas experimental dark-field contrasts are much smaller than theoretical ones due to influence of the background scattering of the ,supporting C film. But practically, the darkfield mode is most suitable for step imaging on ultrathin films and thicker films.
79
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
[16] [17] [18] [19] [20] [21] [22] [23] [24]
[25] [26] [27]
G.A. Bassett, Phil. Mag. 3 (1958) 1042. H. Bethge, Phys. Status Solidi 2 (1962) 3, 775. J.P. Allpress and J.V. Sanders, Phil. Mag. 9 (1964) 645. S. Assunmaa and N.A. Tiner, Trans. Met. Soc. AIME 230 (1964) 1507. M. Klaua, in: Proc. 2nd Colloq. on Thin Films, Budapest, 1967, p. 86. D. Chems, Phil. Mag. 30 (1974) 549. R.L. Hines, Thin Solid Films 35 (1976) 229. J.P.F. Levitt and A. Howie, J. Microscopy 116 (1979) 89. W. Krakow and D.G. Ast, Surface Sci. 58 (1976) Kl15. K. Kambe and G. Lehmpfuhl, Optik 42 (1975) 187. G. Lehmpfuhl and Y. Uchida, Ultramicroscopy 4 (1979) 275. M. Klaua and H. Bethge, Ultramicroscopy 11 (1983) 125. K. Takayanagi, Ultramicroscopy 8 (1982) 145. S. Iijima, Ultramicroscopy 6 (1981) 41. K. Takayanagi, K. Kobayashi, K. Yagi and G. Honjo, J. Phys. Ell (1978) 441; G. Honjo and K. Yagi, in: Current Topics in Materials Science, Vol. 6, Ed. E. Kaldis (North-Holland, Amsterdam, 1980) p. 196. G. Binnig and H. Rohrer, Surface Sci. 126 (1983) 236. E. Bauer, Ultramicroscopy 17 (1985) 51. W. Telieps and E. Bauer, Ultramicroscopy 17 (1985) 57. R. Kaischew, E. Budewski and J. Malinowski, Compt. Rend. Acad. Bulg. Sci. 2 (1949) 29. H. Bethge and M. Klaua, Ann. Physik 17 (1966) 177. C. Ammer, M. Klaua and H. Bethge, Phys. Status Solidi (a) 71 (1982) 415. K. Meinel, Thesis, University Halle (1984). K. Meinel and M. Klaua, to be published. A.J.F. Methereil, Diffraction of Electrons by Perfect Crystals, in: Electron Microscopy in Materials Science, Part 2, Eds. U. Valdr6 and E. Ruedl (Luxemburg, 1976); G. Thomas and M.J. Goringe, Transmission Electron Microscopy of Materials (Wiley, New York, 1979) p. 241. A.P. Doyle and P.S. Turner, Acta Cryst. A24 (1968) 390. C.J. Humphreys and P.B. Hirsch, Phil. Mag. 18 (1968) 115. G. Radi, Acta Cryst. A26 (1970) 41.
73
STEP C O N T R A S T ON U L T R A T H I N Au F I L M S I N V E S T I G A T E D BY TEM M. K L A U A and H. B E T H G E Institut fur Festkrrperphysik und Elektronenmikroskopie der Akademie der Wissenschaften der DDR, Weinberg, DDR-401 Halle/Saale, German Dem. Rep.
Received 14 January 1985
Conditions of optimum contrast of atomic steps on ultrathin Au films imaged by transmission electron microscopy are given. Au(111) and Au(100) films less than 10 monolayers thick were prepared by vacuum deposition onto compact Ag growth faces, backed with C films and dissolved electrolytically from the substrate. From systematic tilting experiments it follows that optimum step contrast can be obtained in bright-field and dark-field modes for all orientations where low index (111), (200), (220) and (311) reflections are excited either systematically or as cross-grating patterns. Bright-field step contrasts are in good agreement with theoretical n-beam calculations, whereas discrepancies in dark-field contrasts are due to the background scattering of the supporting C films. 1. Introduction
Real surfaces usually contain geometric surface defects, the most important of which are atomic steps. They have been shown to play a fundamental role in processes of growth, evaporation, thin film growth, and epitaxy as well as in adsorption, reconstruction, and 2D phase transitions. The att e m p t to visualize atomic steps dates back to the sixties when Bassett [1] first succeeded in decorating cleavage steps on rock salt. For alkali halides, the decoration technique has been multiply applied to the imaging of step structures which result from processes of growth, evaporation, thin film growth, cleavage, and deformation [2]. For metal surfaces, mainly because of difficulties in the preparation technique, tl~ere are only a few examples of a successful step decoration [3-5]. On the other h a n d , TEM has been applied to direct step imaging on thin films first by Cherns [6] who observed dark and bright contours on Au(111) films in dark-field images taken with kinematically forbidden reflections. Similar observations were made by other authors [7-9] on Au, Pt, and Cu films. K a m b e and Lehmpfuhl [10] used a weak-beam dark-field technique for step imaging on MgO particles, and Lehmpfuhl and U c h i d a [11] made a quantitative analysis of bright-field and dark-field
contrasts from atomic steps on MgO. K l a u a and Bethge [12] applied these techniques to ultrathin Au films, while Takayanagi [13] studied monolayer growth of Ag on Ag by in situ UHV electron microscopy. Iijima [14] investigated systematically the capability of defocus phase contrast for step imaging down to a resolution of monatomic height. These TEM techniques are well established in connection with investigations of thin film growth, but other powerful methods for step imaging on compact crystal surfaces have been developed. Reflection electron microscopy u n d e r UHV conditions was applied by Yagi et al. [15] to investigate step movements, thin film growth and 2D transitions on Si(111). Binnig and Rohrer [16] showed that scanning tunneling microscopy permits threedimensional imaging of surface topographies on atomic scale. Low-energy electron reflection microscopy has been recently developed by Bauer [17]; its application to step imaging and to the investigation of various other surface phenomena [18] promises the full success expected of this technique. 2. Experimental Ultrathin epitaxial Au films of less than 10 atom layer thickness were prepared by evaporating
0304-3991/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
74
M. Klaua, tl. Bethge / Step contrast on ultrathin Au films investigated by TEM
the metal onto bulk Ag(111) and Ag(100) faces in UHV. The deposition rate was 1 to 2 A min -t, and the substrate temperature ranged from 30 to 200°C. Spherical Ag crystals of 5 mm diameter were obtained by fast solidification of droplets on a graphite mould in high vacuum on which, by cathodic electrocrystallization, growth faces of (111), (100), (110) and (211) orientation were produced having a maximum extension of 1 to 2 mm [19,20]. The growth faces were cleaned in UHV by argon ion bombardment (energy: 600 eV, current density: 2.5 /tA cm -2) and subsequent annealing at 350 to 400°C or by a free evaporation at 650 to 800°C. The cleanliness, surface crystallography and step structure were determined by AES, LEED and gold decoration technique, respectively [21]. Before removing the Au films from the Ag substrates they were supported by depositing C films of some hundred Angstrom thickness. The Au films adhering to the C films were dissolved electrolytically from the substrate, rinsed in water baths and then mounted on electron microscope grids. Transmission electron micrographs and diffraction patterns were taken after tilting the crystal
films by means of a goniometer stage to attain optimum contrast conditions. Step contrasts per monolayer thickness change were determined by measuring the optical densities on the plates by microdensitometry which was calibrated by Faraday-cup measurements.
3. Results and discussion
Step (or monolayer) contrast is discussed first for Au(111) films of submonolayer coverage since here its interpretation is very easy. If a fraction of one Au monolayer is deposited onto an almost perfect Ag(111) face having extended atomically flat regions with monatomic steps of low density as, e.g., in fig. la, the initial stage of thin film growth consists in the formation of islands by two-dimensional nucleation and the deposition of Au atoms at the existing steps. At sufficiently high substrate temperature (above 80°C for Au on Ag(111)) a repeated nucleation on top of the initially formed islands can be neglected for this submonolayer coverage leading to strictly two-di-
l-ig. 1. 2D nucleation and growth of Au on A g ( l l l ) : (a) at 100°C, coverage 0.12; (b) at 207°C, coverage 0.6.
M. Klaua, H. Bethge / Step contrast on ultrathin Au films investigated by TEM
mensional island growth and monolayer growth along the steps. Figs. la and lb show Au crystallites of ¢~ae-atom-layer thickness for two different coverages and substrate temperatures. The Au crystallites proved to be two-dimensional by comparison of the deposited amounts of Au measured by a calibrated quartz oscillator and, on the other hand, by measuring the coverage on the micrographs assuming that reevaporation and volume diffusion can be neglected. The two-dimensionality is further supported by the observation that all Au crystallites at steps grow only in the direction of concave curvature which corresponds to the lower side of the steps because the curved steps were formed during the evaporation process. Finally, both islands and crystallites at steps show the same optical density, i.e. contrast. The nonuniform contrast of the Au monolayer ribbons in fig. lb is due to the granular phase contrast structure of the underlying C film or possibly to a slight interdiffusion between the Au monolayer and the Ag substrate. Interdiffusion in thin A u / A g films in the temperature range of 280 to 400°C was
75
studied systematically by Meinel [22] using TEM and AES to characterize the diffusion process quantitatively. Such monolayer Au crystallites yield a contrast of some percent which depends on the direction of incidence of the electron beam. Figures and a quantitative comparison with theoretical calculations are given at the end of this section. If the thickness of the deposited film increases to some monolayers the contrast of atomic steps depending on the orientation of the film relative to the electron beam has to be optimized by tilting the specimen. Fig. 2 shows the effect of a slight tilting for a cylindrically bent crystal film. Strong step contrast appears solely in the middle region where systematical (111) reflections are excited as can be seen in the inserted diffraction pattern. With increasing deviation from this optimum orientation the contrasts gradually weaken and disappear. Without interpreting the observed step structure in detail we only state the fact that in this bright-field image the brightest parts correspond to the thinnest parts of the film and that with step-wise increasing thickness the contrast
Fig. 2. Bright-field image of atomic steps on a cylindrically bent A u ( l l l ) film of 3.5 monolayer mean thickness, direction of incidence for the middle region near [110].
76
M. Klaua, 1t. Bethge / Step contrast on ultrathin Au films investigated by TEM
also increases. But this monotonous increase of contrast only occurs if the stacking sequence of subsequent (111) layers is the normal fcc one. On the other hand, in twinning processes, i.e. if stacking faults exist in planes parallel to the film surface, these processes are visible as irregular contrast changes (white arrow). But under our experimental conditions this phenomenon seldom occurs; it seems to be connected with impurities or macroscopic defects in the surface. In addition to the step contrast sometimes bright and dark lines are observed generally running in [110] directions. The diffraction contrast of these lines is caused by stacking faults or dislocations on (111) planes inclined to the surface which are formed during a strong bending of the A u / C sandwich film in the course of electrolytic dissolution. Systematic tilting experiments of A u ( l l l ) and Au(100) films led to the following optimum contrast conditions for step imaging on ultrathin films: generally speaking, all orientations give strong contrasts for which low index reflections (111),
(200), (220), (311) are strongly excited either as systematic rows or as cross-grating patterns. For (111) films these are [211], [321], etc. or [111] and [332] orientations, and for (100) films [100], [210], etc. and [110] directions of incidence. Furthermore, those orientations near the latter ones where one or two low-index reflections are exactly excited (2-beam or 3-beam cases) were found to have relatively strong step contrast. Figs. 3a and 3b show an example of bright-field and dark-field imaging of atomic steps on an A u ( l l l ) film of 3 monolayer mean thickness. In the bright-field micrograph, the optical density increases step-wise with increasing thickness whereas the darkfield image taken with one of the (111) reflections reveals the complementarity but with much higher contrast. The observed film growth mode is the result of a special pretreatment of the A g ( l l l ) substrate. After cleaning by ion bombardment the Ag crystal was heated up to 650°C. During free evaporation, large step-free regions were formed while existing atomic steps bunched to form higher substrate steps. The high
Fig. 3. Bright-field (a) and dark-field (b) image of a A u ( l l l ) film of 3 monolayer mean thickness; substrate temperature 142.5°C; orientation [112].
M. Klaua, H. Bethge / Step contrast on ultrathin Au films investigated by TEM
steps are decorated with thick Au ribbons (black and white, respectively), and in the flat regions the interaction of nucleation and growth of two-dimensional islands and the spreading of monolayers from high substrate steps yield the observed morphology. The actual step structure of the growing Au film depends on the initial substrate morphology, i.e. the mean step density and the substrate temperature [23]. Fig. 4 shows an example of thin film growth of Au on Ag(111) at a higher temperature of 172°C, where the only growth mechanism is the spreading of monolayers from high substrate steps whereas no nucleation on Au terraces occurs. This brightfield micrograph was taken after tilting the Au(111) film by 43 ° to a [130] near direction of incidence where two (200) reflections are strongly excited (see inserted diffraction pattern). Three types of steps can be distinguished. High substrate steps are identified as dark bands in which atomic steps are not resolved. Second, atomic steps appear on the surface of the grown Au film with step-wise increase of optical density. And, finally, due to contrast changes because of increasing or decreas-
77
ing thickness [12], atomic steps are occasionally detected on the substrate side of the Au film (white arrows). For comparing step contrasts measured with those theoretically obtained, the thickness dependence of the transmitted and diffracted beams has been calculated for perfect crystals using the Bloch-wave matrix formulation of the dynamical theory of high-energy electron diffraction [24]. The absorption was treated by perturbation theory. As input parameters, extinction lengths and absorption lengths after Goringe [24] based on Doyle and Turner [25], Humphreys and Hirsch [26] and Radi [27] have been used. Fig. 5 shows the thickness dependence of the intensities of the (000) beam, two (111) beams, two (222) beams and two (220) beams for a 17-beam calculation of a perfect A u ( l l l ) film tilted to an exact [112] direction of incidence. The (000) beam with which the bright-field micrographs are taken does not show any thickness oscillation so that for this orientation strong step contrast can be expected only for thicknesses below 10 monolayers. Accordingly, for much thicker films other special
Fig. 4. Bright-field image of a 3.5 monolayer thick Au(111) film; substrate temperature 172°C; orientation [130].
M. Klaua, H. Bethge / Step contrast on ultrathin Au films investigated by TEM
78
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.,
2"6
~4
10
50
100
monoloyer
directions of incidence have to be chosen to o b t a i n sufficiently strong step contrast [11]. For dark-field micrographs t a k e n with one of the diffracted b e a m s the complementary step contrasts are always much higher than bright-field ones for ultrathin films, but also for thicker films conditions of sufficiently s t r o n g step contrast can be f o u n d more easily t h o u g h at intensities orders of m a g n i t u d e lower. Such intensity curves were calculated for many orientations, and the a b o v e m e n t i o n e d rules of o p t i m u m contrast for ultrathin films have been confirmed. The contrast is defined as the absolute v a l u e of the difference of the intensities at the thickness t and t + At divided by the m a x i m u m
b e a
m
yo
2
. . . . . . . . . . . . .i ! , , , , ! , Fig. 5. Thickness dependence of intensities of 100 keV electrons for a A u ( l l l ) film in e x a c t [112] direction of incidence, 17-beam calculation.
exact [12:~]
~
10
i
|
t
i
1
i
1
i
i
i
5
I
i
,
10 monolayer
Fig. 6. Thickness dependence of bright-field monolayer contrast on Au(111) films in exact [123] orientation ( x ) and [123] near orientation ( O ) w h e r e a (531) and (420) reflection are exactly excited.
value of the intensity at either t or t + At:
K=
II(t)--I(t+At)l I( t +,at)] "
max[I(t),
In fig. 6 the bright-field contrasts of a t o m i c steps up to 10 monolayers were calculated for an exact [123] incidence ( u p p e r curve) and for a [123] near incidence (lower curve) where a (331) and a (420) beam are exactly excited. A slight tilting by only 1 ° out of the [123] orientation results in a m a r k e d decrease in contrast. The two sets of experimental p o i n t s were measured on a A u ( l l l ) film of 3 m o n o l a y e r mean thickness tilted to the corresponding orientations. In spite of some
Table 1 C o m p a r i s o n between experimental and theoretical contrasts of one monolayer Au for different directions of incidence n-Beam calculation
Tilting angle (deg)
13 7 7
0 0 ~1
17 9 15 17
19.5 19.5 22.2 ~ 23
Direction of incidence
C o n t r a s t of 1 monolayer (%)
[111] exact [111] exact = [1111, (2~o) e x a c t excited [112] e x a c t [112] e x a c t [123] e x a c t ~- [123], (531) and (420) exact excited
6.4 5.2 3.4
Theor.
3.7 4.6 4.1 2.8
Expt.
5.7 3.4 4.5 3.8 2.9
M. Klaua, H. Bethge / Step contrast on ultrathin Au films investigated by TEM
scattering of the points a good agreement is observed. In table 1 a further comparison is made between experimental and theoretical contrasts of one monolayer Au for different directions of incidence. The experimental values were measured on micrographs with submonolayer coverage of Au as in fig. 1. The calculations were carried out for different n-beam cases taking into account only the allowed reflections. The question of how many b e a m s have to be treated cannot yet be answered definitely for ultrathin films. The solution of this question requires a systematic study of bright-field contrasts for film thicknesses up to 30 monolayers to be carried out in o r d e r to measure contrast oscillations [22]. Such os.cillations in the bright-field intensity strongly d e p e n d on the number of b e a m s used. With increasing number of beams the intensity oscillations vanish, as can be seen in fig. 5 for a 17-beam case.
4. Conclusion
Experimental conditions of o p t i m u m step contrast of ultrathin Au films backed by C films are correlated with n-beam intensity calculations for perfect crystals. Bright-field contrasts from 1 to 10 monolayer thickness are in good agreement with theoretical values, whereas experimental dark-field contrasts are much smaller than theoretical ones due to influence of the background scattering of the ,supporting C film. But practically, the darkfield mode is most suitable for step imaging on ultrathin films and thicker films.
79
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[16] [17] [18] [19] [20] [21] [22] [23] [24]
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