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Adsorption and diffusion of Ag atoms on Si(1 1 1)-(7 × 7) surface
Surface Science 482±485 (2001) 386±390
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Adsorption and diusion of Ag atoms on Si(1 1 1)-(7 ´ 7) surface Tom as Jarolõmek, Josef Myslivecek, Pavel Sobotõk, Ivan Ost' adal * Department of Electronics and Vacuum Physics, Faculty of Mathematics and Physics, Charles University of Prague, V Holesovi ck ach 2, 180 00 Praha 8, Czech Republic
Abstract Very initial stages of Ag nucleation on Si(1 1 1)-(7 7) surface were studied using scanning tunneling microscopy at room temperature (RT). At RT silver atoms form small clusters inside half-unit cells (HUCs) of the 7 7 surface reconstruction. Precise measurement of the amount of deposited material allowed to determine the appearance of the HUCs containing one and two silver atoms. Mobility of Ag atoms at RT was studied and growth of the smallest silver objects was investigated during a real time observation of selected surface region. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Epitaxy; Growth; Surface diusion; Silver; Silicon; Surface structure, morphology, roughness, and topography
1. Introduction Growth of metals on semiconductor surfaces is a subject of many studies due to its technological importance. The basic growth process ± the diusion of adsorbed atoms on a substrate is strongly in¯uenced by the presence of substrate reconstructions. The 7 7 reconstruction of Si(1 1 1) surface consists of large triangular half-unit cells (HUCs) of two dierent types (faulted ± F and unfaulted ± U) which are divided by dimer rows [1]. Surface diusion of Ag on such a structure has not been experimentally studied in a detail yet. Ag is very suitable for studying metal diusion on the
* Corresponding author. Tel.: +420-2-21912336; fax: +420-26885095. E-mail address: ivan.ostadal@m.cuni.cz (I. Ost'adal).
Si(1 1 1)-(7 7) reconstructed surface because it does not react with silicon at temperatures below 500 K [2±4]. Epitaxy of Ag on Si(1 1 1)-(7 7) reconstructed surface has been studied by several techniques including scanning tunneling microscopy (STM). The interest, however, has been mainly directed towards analyzing types of Si (1 1 1)-Ag reconstructions formed at dierent temperatures [5]. Very early stages of Ag nucleation at temperatures below 500 K have been studied without discussing kinetic processes occurring during the growth [2,3,6,7]. In our recent work [8] we observed diusion-limited nucleation at temperatures above 370 K in contrast with hit-and-stick nucleation at room temperature (RT). Morphology of grown ®lms indicated presence of limited mobility of Ag atoms even at RT. Here we present results on the very early stages of Ag nucleation on the Si(1 1 1)-(7 7) reconstructed surface. Very small amount of material ± up to 0.005 ML
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 0 ) 0 1 0 3 9 - 6
T. Jarolõmek et al. / Surface Science 482±485 (2001) 386±390
(1 ML 7:54 1014 atoms cm 2 , which is equivalent to the surface atomic density of Si(1 1 1)) ± was deposited in order to identify an appearance of the smallest silver clusters and study their mobility. 2. Experimental Experiments were performed in a UHV chamber equipped with a home-made STM system. The pressure in the system did not exceed 2 10 8 Pa during experiments. Si(1 1 1) substrates (Sb doped, n-type, resistivity 0.01±0.005 X cm) were used. The 7 7 reconstructed surface was obtained using a standard ¯ashing procedure. Ag was deposited from a tungsten wire evaporator. Samples with very low coverage (0.003±0.004 ML) were prepared at rates from 0.0003 to 0.0005 ML/s, measured by a quartz crystal thickness monitor [6]. For calibration of the p thickness p monitor we measured the coverage of 3 3-Ag reconstructed islands on an annealed (650 K for 10 min) sample with 0.6 ML of deposited Ag. All samples were prepared and investigated at RT. We used electrolytically polished tungsten tips, cleaned by electron bombardment prior to their ®rst use. 3. Results 3.1. Monomers Ag atoms deposited on the Si(1 1 1)-(7 7) surface form several types of islands [2,3,7] inside HUCs of the reconstruction. A triangular HUC with highlighted corners has been already identi®ed as the HUC containing single Ag atom [6,7]. Fig. 1 shows an example of both faulted and unfaulted HUCs occupied by one Ag atom. Images were recorded at both negative and positive sample bias in order to show dierences between appearance of the occupied faulted and unfaulted HUCs. At RT we observed preference of Ag nucleation in F HUCs [6,7]. The appearance of a HUC containing one Ag atom is probably related with high mobility of an Ag atom inside the HUC. When a single Ag atom
387
occupies a HUC with one missing Si corner adatom the two rest corner adatoms of defect-free part of the HUC (over which the Ag atom can hop only) are highlighted in a STM image [7] ± the Ag atom is more localized. A hop of the silver atom to a neighboring non-defect HUC produces occupied HUC with ``normal'' appearance [7]. Hopping rate of Ag atoms between HUCs is much lower than the rate inside one HUC. To estimate the value of the hopping rate between the HUCs, following experiments were performed: samples were observed about 2 or 3 h after deposition. Images were acquired from a surface region of 36 36 nm2 , where 50 HUCs occupied by single Ag atoms were observed. During a period of more than 30 min we registered only one or two hops between neighboring HUCs which corresponds to the hopping rate of a single atom 16 5 10 6 s 1 . Most of the observed hops were from U to F HUCs in spite of very high number of occupied F HUC (see Fig. 2). A comparable result has been obtained from our recent KMC simulations of growth experiments [9]. G omez-Rodrõguez reported for the hopping rate of single Pb atoms between dierent HUCs of the Si(1 1 1)-(7 7) surface at RT a similar value of 2 10 5 s 1 [10]. 3.2. Dimers Three dierent types of HUCs occupied by Ag atoms can be found on the surface with sucient amount of deposited Ag (0.004 ML) ± Fig. 2. The ®rst one (marked A in Fig. 2) is the HUC containing one Ag atom. A HUC with highlighted center adatoms (height of 0.1 nm above the plane of Si adatoms in the HUC) belongs to the second type (B). The third type (C) appears as a bright spot with a height of 0.17 nm. To determine the number of Ag atoms contained in the B-type HUCs we compared experimental distribution of Ag cluster types with results of a statistical model of cluster size distribution. Calculation of the size distribution was based on a simple model with Poisson distribution [10]. Denoting Cn the average number of Ag clusters containing n Ag atoms, at 1 the end of the deposition we have Cn sn! n sh exp sh, where h is the average number of
388
T. Jarolõmek et al. / Surface Science 482±485 (2001) 386±390
Fig. 1. Appearance of F and U HUCs occupied with single silver atom. The orientation of F HUC and U HUC is marked and the occupied HUCs are indicated by arrows.
Fig. 2. Three dierent types of silver objects: (A) HUC containing one silver atom, (B) HUC occupied with a silver dimer and (C) HUC with a silver cluster.
atoms per HUC and s is the capture area. In case of Ag on Si(1 1 1)-(7 7) a value s 4 gives the best agreement with experimental results [8]. It means that the capture area of an occupied HUC contains three nearest-neighbor HUCs (the same value was reported by G omez-Rodrõguez for the Pb/Si(1 1 1)(7 7) system [10]). Comparing experimental distribution with the model we can determine B-type
Fig. 3. Comparison of the experimental cluster type distribution with calculated cluster size distribution was used for identi®cation of silver dimers (see text for details).
HUCs as HUCs occupied by two Ag atoms ± Ag dimer (see Fig. 3). Images of the HUC with the dimer taken at both polarities of tip bias are in Fig. 4. In contrast to Pb/Si(1 1 1)-(7 7) we cannot specify the orientation of the Ag dimer inside the HUC. It can be explained either by the higher mobility of the Ag dimer inside HUC, where the dimer switches between three possible orientations (over center adatoms) in a way similar to the Pb
T. Jarolõmek et al. / Surface Science 482±485 (2001) 386±390
389
Fig. 4. A detail of the silver dimer (B-type of Ag objects ± indicated by an arrow) occupying an F HUC taken at both positive and negative sample bias. Highlighted center adatoms are visible at the positive bias in contrast with highlighted corner adatoms in a neighboring HUC containing single Ag atom.
Fig. 5. Real time observation of the formation of the silver dimer. The images were taken at negative sample bias with a delay of about 5 min.
dimer [11] or (in case of low interaction between Ag atoms in a HUC) by higher probability of visiting sites above center adatoms by almost independent Ag atoms. Real-time observations showed formation of the Ag cluster of the type B from two single Ag atoms in neighboring HUCs (see Fig. 5) and provided a direct evidence of the appearance of the dimer. We observed that a single Ag atom can rest in a HUC adjacent to another occupied HUC for minutes. This supported an idea that trapping of Ag adatoms into neighboring occupied HUCs [8] takes
place only immediately after arrival of the Ag adatoms at the surface. 3.3. Clusters Images of HUCs occupied by more than two Ag atoms do not allow to distinguish accurately number of silver atoms contained. Appearance of clusters is various, but observed dierences are very small and it is dicult to distinguish them from the dierences caused by resolution limits of the STM. In contrast to Ag dimers occupying
390
T. Jarolõmek et al. / Surface Science 482±485 (2001) 386±390
centers of HUCs, clusters with more silver atoms are often shifted towards a dimer row of the 7 7 reconstruction (see Fig. 2).
202/97/1109, by the Grant Agency of Charles University, project no.: GAUK 147/99.
4. Conclusions
References
Experimental data show, that only three types of silver objects (monomers, dimers and clusters with more atoms) grown on Si(1 1 1)-(7 7) surface can be resolved using STM technique at RT. The appearance of a HUC containing single silver atom indicates high mobility of Ag atom inside the HUC in comparison with a value estimated for the hopping rate of single Ag atom between HUCs (16 10 6 s 1 ). In case of a Ag object identi®ed as the dimer, the movement of Ag atoms inside the HUC is obvious, but various interpretations of resulting image are still possible. Acknowledgements The presented work was supported by the Grant Agency of the Czech Republic, project no.: GACR
[1] K. Takayanagi, Y. Tanishiro, M. Takahashi, S. Takahashi, J. Vac. Sci. Tech. A 3 (1985) 1502. [2] St. Tosch, H. Neddermeyer, Phys. Rev. Lett. 61 (1988) 349. [3] St. Tosch, H. Neddermeyer, J. Microsc. 152 (1988) 415. [4] E. Ganz, I.-S. Hwang, F. Xiong, S.K. Theiss, J. Golovchenko, Surf. Sci. 275 (1991) 259. [5] K.J. Wan, X.F. Lin, J. Nogami, Phys. Rev. B 47 (1992) 13700. [6] I. Ost'adal, P. Sobotõk, J. Myslivecek, T. Jarolõmek, Czech. J. Phys. 49 (1999) 1613. [7] H. Hirayama, H. Okamoto, K. Takayanagi, Phys. Rev. B 60 (1999) 14260. [8] P. Sobotõk, I. Ost'adal, J. Myslivecek, T. Jarolõmek, Surf. Sci. 454±456 (2000) 847. [9] J. Myslivecek, P. Sobotõk, I. Ost'adal, T. Jarolõmek, P. Smilauer, Phys. Rev. B 63 (2001) 45403. [10] J.M. G omez-Rodrõguez, J.J. Saenz, A.M. Bar o, J.-Y. Veuillen, R.C. Cinti, Phys. Rev. Lett. 76 (1996) 799. [11] J.M. G omez-Rodrõguez, J.-Y. Veuillen, R.C. Cinti, J. Vac. Sci. Tech. B 14 (2) (1996) 1005.
www.elsevier.nl/locate/susc
Adsorption and diusion of Ag atoms on Si(1 1 1)-(7 ´ 7) surface Tom as Jarolõmek, Josef Myslivecek, Pavel Sobotõk, Ivan Ost' adal * Department of Electronics and Vacuum Physics, Faculty of Mathematics and Physics, Charles University of Prague, V Holesovi ck ach 2, 180 00 Praha 8, Czech Republic
Abstract Very initial stages of Ag nucleation on Si(1 1 1)-(7 7) surface were studied using scanning tunneling microscopy at room temperature (RT). At RT silver atoms form small clusters inside half-unit cells (HUCs) of the 7 7 surface reconstruction. Precise measurement of the amount of deposited material allowed to determine the appearance of the HUCs containing one and two silver atoms. Mobility of Ag atoms at RT was studied and growth of the smallest silver objects was investigated during a real time observation of selected surface region. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Epitaxy; Growth; Surface diusion; Silver; Silicon; Surface structure, morphology, roughness, and topography
1. Introduction Growth of metals on semiconductor surfaces is a subject of many studies due to its technological importance. The basic growth process ± the diusion of adsorbed atoms on a substrate is strongly in¯uenced by the presence of substrate reconstructions. The 7 7 reconstruction of Si(1 1 1) surface consists of large triangular half-unit cells (HUCs) of two dierent types (faulted ± F and unfaulted ± U) which are divided by dimer rows [1]. Surface diusion of Ag on such a structure has not been experimentally studied in a detail yet. Ag is very suitable for studying metal diusion on the
* Corresponding author. Tel.: +420-2-21912336; fax: +420-26885095. E-mail address: ivan.ostadal@m.cuni.cz (I. Ost'adal).
Si(1 1 1)-(7 7) reconstructed surface because it does not react with silicon at temperatures below 500 K [2±4]. Epitaxy of Ag on Si(1 1 1)-(7 7) reconstructed surface has been studied by several techniques including scanning tunneling microscopy (STM). The interest, however, has been mainly directed towards analyzing types of Si (1 1 1)-Ag reconstructions formed at dierent temperatures [5]. Very early stages of Ag nucleation at temperatures below 500 K have been studied without discussing kinetic processes occurring during the growth [2,3,6,7]. In our recent work [8] we observed diusion-limited nucleation at temperatures above 370 K in contrast with hit-and-stick nucleation at room temperature (RT). Morphology of grown ®lms indicated presence of limited mobility of Ag atoms even at RT. Here we present results on the very early stages of Ag nucleation on the Si(1 1 1)-(7 7) reconstructed surface. Very small amount of material ± up to 0.005 ML
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 0 ) 0 1 0 3 9 - 6
T. Jarolõmek et al. / Surface Science 482±485 (2001) 386±390
(1 ML 7:54 1014 atoms cm 2 , which is equivalent to the surface atomic density of Si(1 1 1)) ± was deposited in order to identify an appearance of the smallest silver clusters and study their mobility. 2. Experimental Experiments were performed in a UHV chamber equipped with a home-made STM system. The pressure in the system did not exceed 2 10 8 Pa during experiments. Si(1 1 1) substrates (Sb doped, n-type, resistivity 0.01±0.005 X cm) were used. The 7 7 reconstructed surface was obtained using a standard ¯ashing procedure. Ag was deposited from a tungsten wire evaporator. Samples with very low coverage (0.003±0.004 ML) were prepared at rates from 0.0003 to 0.0005 ML/s, measured by a quartz crystal thickness monitor [6]. For calibration of the p thickness p monitor we measured the coverage of 3 3-Ag reconstructed islands on an annealed (650 K for 10 min) sample with 0.6 ML of deposited Ag. All samples were prepared and investigated at RT. We used electrolytically polished tungsten tips, cleaned by electron bombardment prior to their ®rst use. 3. Results 3.1. Monomers Ag atoms deposited on the Si(1 1 1)-(7 7) surface form several types of islands [2,3,7] inside HUCs of the reconstruction. A triangular HUC with highlighted corners has been already identi®ed as the HUC containing single Ag atom [6,7]. Fig. 1 shows an example of both faulted and unfaulted HUCs occupied by one Ag atom. Images were recorded at both negative and positive sample bias in order to show dierences between appearance of the occupied faulted and unfaulted HUCs. At RT we observed preference of Ag nucleation in F HUCs [6,7]. The appearance of a HUC containing one Ag atom is probably related with high mobility of an Ag atom inside the HUC. When a single Ag atom
387
occupies a HUC with one missing Si corner adatom the two rest corner adatoms of defect-free part of the HUC (over which the Ag atom can hop only) are highlighted in a STM image [7] ± the Ag atom is more localized. A hop of the silver atom to a neighboring non-defect HUC produces occupied HUC with ``normal'' appearance [7]. Hopping rate of Ag atoms between HUCs is much lower than the rate inside one HUC. To estimate the value of the hopping rate between the HUCs, following experiments were performed: samples were observed about 2 or 3 h after deposition. Images were acquired from a surface region of 36 36 nm2 , where 50 HUCs occupied by single Ag atoms were observed. During a period of more than 30 min we registered only one or two hops between neighboring HUCs which corresponds to the hopping rate of a single atom 16 5 10 6 s 1 . Most of the observed hops were from U to F HUCs in spite of very high number of occupied F HUC (see Fig. 2). A comparable result has been obtained from our recent KMC simulations of growth experiments [9]. G omez-Rodrõguez reported for the hopping rate of single Pb atoms between dierent HUCs of the Si(1 1 1)-(7 7) surface at RT a similar value of 2 10 5 s 1 [10]. 3.2. Dimers Three dierent types of HUCs occupied by Ag atoms can be found on the surface with sucient amount of deposited Ag (0.004 ML) ± Fig. 2. The ®rst one (marked A in Fig. 2) is the HUC containing one Ag atom. A HUC with highlighted center adatoms (height of 0.1 nm above the plane of Si adatoms in the HUC) belongs to the second type (B). The third type (C) appears as a bright spot with a height of 0.17 nm. To determine the number of Ag atoms contained in the B-type HUCs we compared experimental distribution of Ag cluster types with results of a statistical model of cluster size distribution. Calculation of the size distribution was based on a simple model with Poisson distribution [10]. Denoting Cn the average number of Ag clusters containing n Ag atoms, at 1 the end of the deposition we have Cn sn! n sh exp sh, where h is the average number of
388
T. Jarolõmek et al. / Surface Science 482±485 (2001) 386±390
Fig. 1. Appearance of F and U HUCs occupied with single silver atom. The orientation of F HUC and U HUC is marked and the occupied HUCs are indicated by arrows.
Fig. 2. Three dierent types of silver objects: (A) HUC containing one silver atom, (B) HUC occupied with a silver dimer and (C) HUC with a silver cluster.
atoms per HUC and s is the capture area. In case of Ag on Si(1 1 1)-(7 7) a value s 4 gives the best agreement with experimental results [8]. It means that the capture area of an occupied HUC contains three nearest-neighbor HUCs (the same value was reported by G omez-Rodrõguez for the Pb/Si(1 1 1)(7 7) system [10]). Comparing experimental distribution with the model we can determine B-type
Fig. 3. Comparison of the experimental cluster type distribution with calculated cluster size distribution was used for identi®cation of silver dimers (see text for details).
HUCs as HUCs occupied by two Ag atoms ± Ag dimer (see Fig. 3). Images of the HUC with the dimer taken at both polarities of tip bias are in Fig. 4. In contrast to Pb/Si(1 1 1)-(7 7) we cannot specify the orientation of the Ag dimer inside the HUC. It can be explained either by the higher mobility of the Ag dimer inside HUC, where the dimer switches between three possible orientations (over center adatoms) in a way similar to the Pb
T. Jarolõmek et al. / Surface Science 482±485 (2001) 386±390
389
Fig. 4. A detail of the silver dimer (B-type of Ag objects ± indicated by an arrow) occupying an F HUC taken at both positive and negative sample bias. Highlighted center adatoms are visible at the positive bias in contrast with highlighted corner adatoms in a neighboring HUC containing single Ag atom.
Fig. 5. Real time observation of the formation of the silver dimer. The images were taken at negative sample bias with a delay of about 5 min.
dimer [11] or (in case of low interaction between Ag atoms in a HUC) by higher probability of visiting sites above center adatoms by almost independent Ag atoms. Real-time observations showed formation of the Ag cluster of the type B from two single Ag atoms in neighboring HUCs (see Fig. 5) and provided a direct evidence of the appearance of the dimer. We observed that a single Ag atom can rest in a HUC adjacent to another occupied HUC for minutes. This supported an idea that trapping of Ag adatoms into neighboring occupied HUCs [8] takes
place only immediately after arrival of the Ag adatoms at the surface. 3.3. Clusters Images of HUCs occupied by more than two Ag atoms do not allow to distinguish accurately number of silver atoms contained. Appearance of clusters is various, but observed dierences are very small and it is dicult to distinguish them from the dierences caused by resolution limits of the STM. In contrast to Ag dimers occupying
390
T. Jarolõmek et al. / Surface Science 482±485 (2001) 386±390
centers of HUCs, clusters with more silver atoms are often shifted towards a dimer row of the 7 7 reconstruction (see Fig. 2).
202/97/1109, by the Grant Agency of Charles University, project no.: GAUK 147/99.
4. Conclusions
References
Experimental data show, that only three types of silver objects (monomers, dimers and clusters with more atoms) grown on Si(1 1 1)-(7 7) surface can be resolved using STM technique at RT. The appearance of a HUC containing single silver atom indicates high mobility of Ag atom inside the HUC in comparison with a value estimated for the hopping rate of single Ag atom between HUCs (16 10 6 s 1 ). In case of a Ag object identi®ed as the dimer, the movement of Ag atoms inside the HUC is obvious, but various interpretations of resulting image are still possible. Acknowledgements The presented work was supported by the Grant Agency of the Czech Republic, project no.: GACR
[1] K. Takayanagi, Y. Tanishiro, M. Takahashi, S. Takahashi, J. Vac. Sci. Tech. A 3 (1985) 1502. [2] St. Tosch, H. Neddermeyer, Phys. Rev. Lett. 61 (1988) 349. [3] St. Tosch, H. Neddermeyer, J. Microsc. 152 (1988) 415. [4] E. Ganz, I.-S. Hwang, F. Xiong, S.K. Theiss, J. Golovchenko, Surf. Sci. 275 (1991) 259. [5] K.J. Wan, X.F. Lin, J. Nogami, Phys. Rev. B 47 (1992) 13700. [6] I. Ost'adal, P. Sobotõk, J. Myslivecek, T. Jarolõmek, Czech. J. Phys. 49 (1999) 1613. [7] H. Hirayama, H. Okamoto, K. Takayanagi, Phys. Rev. B 60 (1999) 14260. [8] P. Sobotõk, I. Ost'adal, J. Myslivecek, T. Jarolõmek, Surf. Sci. 454±456 (2000) 847. [9] J. Myslivecek, P. Sobotõk, I. Ost'adal, T. Jarolõmek, P. Smilauer, Phys. Rev. B 63 (2001) 45403. [10] J.M. G omez-Rodrõguez, J.J. Saenz, A.M. Bar o, J.-Y. Veuillen, R.C. Cinti, Phys. Rev. Lett. 76 (1996) 799. [11] J.M. G omez-Rodrõguez, J.-Y. Veuillen, R.C. Cinti, J. Vac. Sci. Tech. B 14 (2) (1996) 1005.