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Formation of two dimension Ge cluster superlattice on Si(111)-(7×7) surface
Surface Science 506 (2002) L255–L260 www.elsevier.com/locate/susc
Surface Science Letters
Formation of two dimension Ge cluster superlattice on Si(1 1 1)-(7 7) surface Long Yan, Yongping Zhang, Hongjun Gao, Sishen Xie *, Shijin Pang Beijing Laboratory of Vacuum Physics, Institute of Physics & Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 2724, Beijing 100080, PR China Received 12 June 2001; accepted for publication 30 October 2001
Abstract The adsorption process of sub-monolayer Ge on Si(1 1 1)-(7 7) surface is studied using ultrahigh-vacuum scanning tunneling microscopy. By carefully controlling Ge deposition condition, a unique sixfold symmetry superlattice of Ge clusters is formed. It is found that almost all the Ge clusters observed have similar shapes and uniform sizes, and that they sit on both halves of the (7 7) unit cells. The formation of the ordered structure is attributed to the fact that the characteristic Si(1 1 1)-(7 7) reconstruction controls the nucleation and growth of the Ge clusters. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Germanium; Silicon; Scanning tunneling microscopy
The epitaxial growth of Ge or Ge/Si on Si surfaces has been one focus of many studies in recent decades. It is an important subject to obtain ordered nanoscale structures in the SiGe system because of their potential application in photoelectronic devices and their compatibility with Si processing. It is well known that the self-organized growth of strained islands in heteroepitaxial growth is currently employed to build dense arrays of nanoscale islands [1–10]. However, it is still difficult to control accurately the order, size uniformity, and density of the grown islands. Another way to achieve order is designing templates with periodic structures [11,12]. The
*
Corresponding author. Fax: +86-1062-556598. E-mail address: [email protected] (S. Xie).
entire approach is based on the fact that the adatoms agglomerate in some regularly distributed sites created by the template to form an ordered array or superlattice of well-defined nanoclusters. Si(1 1 1)-(7 7) surface can be served as a perfect template because of its stable periodic surface structure [13]. One (7 7) unit cell consists of faulted and unfaulted halves in which foreign atoms preferentially occupy or congregate due to the attraction of the dangling bonds of the adatoms and rest atoms. The dimer rows, which separate faulted and unfaulted halves, usually repulse foreign atoms on account of no dangling bonds. Corner hole usually does not attract foreign atoms since site of its dangling bond is too deep. The preferential adsorption of adatoms on Si(1 1 1)-(7 7) surface has been reported for a variety of metals [14–18]. Vitali et al. recently
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 1 ) 0 1 8 2 0 - 9
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reported that Tl atoms agglomerated into a 2D superlattice of well-defined nanoclusters on the Si(1 1 1)-(7 7) surface at room temperature [19]. Besides, K€ ohler et al. found that at room temperature irregular Si and Ge clusters could be arranged in the half unit cells toform an ordered array on Si(1 1 1)-(7 7) surfaces [20,21]. In this letter, we found that the shape and size of the clusters could be controlled uniformly. Thus a sixfold symmetry superlattice of Ge cluster formed on Si(1 1 1)-(7 7) surface can be observed. The experiments were performed using an ultrahigh-vacuum scanning tunneling microscopy system (Omicron, Germany) with a base pressure below 1 1010 Torr. N-doped Si substrates (q 1–2 X cm, thickness 0.51–0.54 mm) were degassed for 12 h at 900 K, and then flashed to 1500 K for 20 s by direct current (DC) heating. Finally, perfect Si(1 1 1)-(7 7) surfaces were obtained. The experimental conditions for Ge deposition are as follow. A small piece of Ge was selected as the deposition source, which was sublimed at about 1100 K by DC heating. The deposition rate was about 0.005 ML/min. Note that the Si surface was not deliberately heated. The distance between the sample and source is 5 cm, and the temperature of the Si substrate might be increased to 320 K in a long Ge deposition process. The increase of the substrate temperature might enhance the mobility of the Ge a little bit. In the other hand, the resulting pressure is almost the same as the base pressure of our system. Thus we believe that there is no significant contamination in the deposition process. To verify this, we annealed the Si sample covered by Ge at 700 K. The nice Ge islands with perfectly ordered 5 5 and/or 7 7 structure can be observed on clean Si(1 1 1)-(7 7) substrates. Fig. 1a shows the results of 0.4 ML Ge deposition on a Si(1 1 1)-(7 7) substrate. It can be found that the deposited Ge atoms nucleate into clusters, as reported by K€ ohler et al. [21]. However, most of the clusters have similar shape and uniform size, which was not observed in previous experiments. We are sure that the uniform shape of the Ge clusters is not due to a limited resolution of the tip because individual adatom of the (7 7) surface can be clearly resolved using the tip. Moreover, the Ge clusters distribute quite homo-
Fig. 1. The STM images of 0.4 ML Ge on the Si(1 1 1)-(7 7) surface. (a) The scan area is 40 40 nm2 . The sample bias and tunneling current are þ1.76 V and 0.44 nA, respectively. The inset shows its Fast Fourier transform, (b) The high resolution STM image. The scan area is 12 12 nm2 . The sample bias and tunneling current are þ1.80 V and 0.43 nA. A (7 7) unit cell is indicated. Note that there is one Ge cluster in each half unit cell.
geneously over the (7 7) substrate on a macroscopic scale and form a nearly ordered 2D superlattice. The 2D fast Fourier transformation (2DFFT) from Fig. 1a is shown in the inset. The spectrum exhibits a clear sixfold pattern like a hexagon, confirming the macroscopic 2D ordering of the Ge clusters. Scrutinizing a high-resolved
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scanning tunneling microscopy (STM) image shown in Fig. 1b, the size of the Ge clusters is consistent with that of one half cell. There is a clear borderline between two nearby clusters, and each hole is usually surrounded with six clusters. It is noted that the borderlines and holes could correspond to the dimer rows and corner holes of the (7 7) substrate, respectively. K€ ohler et al. have demonstrated that irregular clusters sat on the (7 7) halves, leaving the dimer rows and corner holes uncovered [21]. Moreover, the great amount of experiments show that the deposition of Ge atoms on Si(l 1 1)-(7 7) substrate at lower temperature does not transform the Si surface until a few monolayer Ge atoms have been deposited [22– 24]. So we believe that all of the Ge clusters located on the (7 7) halves. Here, it should be pointed out that the clusters equally occupied on the faulted and the unfaulted halves, i.e., no preference for clustering on the faulted half is observed. Thus the 2D structure formed by the Ge clusters naturally have sixfold symmetry. To understand the forming mechanism of the ordered structure, we decrease Ge coverage to study growth process of Ge on Si(1 1 1)-(7 7) substrates. Fig. 2a shows the result of 0.3 ML Ge deposition. It was found that the clusters on most areas have random shapes, which is similar to the results of previous observation [21]. But the clusters in the areas as indicated by the arrows in Fig. 2a appear bigger and higher, and have similar shape, which is similar to Fig. 1. On the average, the difference in height between the Ge clusters is . This might be related to the fluctuation l–1.5 A of the cluster size. The Ge atom number of the bigger clusters is more than that of the smaller one. Increasing the amount of Ge deposition to 0.35 ML, the areas containing such ordered Ge cluster correspondingly increase, as shown in Fig. 2b. It should be noted that the size of any Ge cluster do not exceed one half cell. This demonstrates that the formed Ge clusters can grow with the increase of deposition amount until the clusters reach an ultimate size. How do we understand the growth mechanism of Ge on Si(1 1 1)-(7 7) substrate? At initial adsorption stage, due to the attraction of the dangling bonds of adatoms and rest atoms on the
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Fig. 2. The STM images of the Si(1 1 1)-(7 7) surface with (a) 0.3 ML Ge coverage ML Ge coverage. The scan areas of (a) and (b) are all 35 35 nm2 . The sample bias and tunneling current are (a) þ1.70 V and 0.45 nA, (b) þ1.60 V and 0.48 nA, respectively.
(7 7) substrate, the arriving Ge atoms have very low mobility, which can increase nucleation probability. So the formed Ge clusters homogeneously cover the (7 7) substrate. When the Ge clusters are big enough to saturate all dangling bonds of adatoms and rest atoms in the halves, the adsorption ability of the dangling bonds for additional atoms will become very weak. Moreover,
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it is well known that the attraction ability of such Ge clusters for foreign atoms is also weak. Hence, the mobility of the additional Ge atoms will increase. So once the additional Ge atoms sit on the Ge clusters, they move preferentially to the sites of the smaller Ge clusters. Besides, a little increase of the substrate temperature in the deposition process might enhance the mobility of the additional atoms. Thus, our STM images show that the growth of the Ge clusters is suppressed after they reached certain size. This shows that the dangling bonds of the (7 7) reconstruction play an important role in the formation and growth of the Ge clusters. On the other hand, the experiments strongly demonstrate the initial growth of the amorphous Ge film on Si(1 1 1)-(7 7) surface is not disordered as expected. We next investigate higher coverage of Ge on Si(1 1 1)-(7 7) surface. Fig. 3a and b are the STM images of 0.5 ML Ge deposition, which were scanned at high positive bias voltages. From Fig. 3a, it can be found that some compact Ge islands occur on the 2D Ge cluster superlattice formed on the Si(1 1 1)-(7 7) surface. The Ge islands at step edges are also observed, as shown in Fig. 3b. These in height, which is obviously islands are 5 A higher than one bilayer step of Ge surface. Otherwise, it is difficult to observe the existence of small clusters. These facts indicate that the mobility of the Ge atoms on the 2D Ge cluster superlattice is indeed much higher than that on the (7 7) substrate. Besides, it is noted that the morphology of the 2D Ge cluster superlattice appears not to be modified, i.e., it still keeps the sixfold symmetry structure consisted of uniform Ge cluster. This further demonstrates that the additional Ge atoms do not make the Ge clusters grow continuously. The electronic characteristics of such Ge cluster on Si(1 1 1)-(7 7) surface are also investigated by STM technique. For the Ge cluster superlattice on Si(111 )-(7 7) surface, the STM images observed at either low positive bias or negative bias show smaller corrugation than that at high positive bias. This means that the local density of state of the Ge cluster near and far below the Fermi level is drastically reduced, which can be supported by the scanning tunneling spectroscopy (STS) measuring
Fig. 3. The STM images of 0.5 ML Ge on the Si(1 1 1)-(7 7) surface. The sample bias and tunneling current are þ1.86 V and 0.43 nA, respectively. The scan areas are (a) 55 55 nm2 (b) 35 35 nm2 , respectively.
on the Ge clusters (see Fig. 4). The dI=dV –V curve (i.e. STS) in Fig. 4 was obtained through averaging data taking from a few individual clusters in order to have a better signal-to-noise ration. From the STS we can clearly see the gap of electronic states near Fermi level, which indicates that the population of the electrons near Fermi level is very small. Therefore, it is difficult to obtain clear STM
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Fig. 4. The scanning tunneling spectroscopy of the ordered Ge clusters in the 2D Ge cluster superlattice on the Si(1 1 1)-(7 7) surface.
images at low bias voltage. Moreover, the density of the electronic states near 1.8 eV is much lower than that near þ1.8 eV, which demonstrates that the tunneling probability of electrons from the cluster to the tip is less than that from the tip to the clusters. So the high-resolved STM images can be only obtained at high positive bias. On account of poor electronic conductivity of the Ge cluster near and far below the Fermi level, we speculate that the Ge clusters may be amorphous. Unfortunately, the STM images at a high positive bias do not display clear geometry structure of the higher Ge islands as shown in Fig. 4. However, the STM images at low positive bias can do more for the Ge islands. Fig. 5 is another STM image of 0.5 ML Ge deposition on the Si(1 1 1)(7 7) surface at the sample bias voltage of þ1.10 V. In the STM image, the height of the island is . So the height of the islands at different bias 4 A is different. Besides, the 2D Ge cluster superlattice on the Si(1 1 1)-(7 7) surface is hazy. But the holes and the clusters in the island located above the superlattice can be observed, which is very similar to the morphology of 2D Ge cluster superlattice on the Si(1 1 1)-(7 7) surface at high positive bias. This indicates that the state density of Ge clusters in the island is higher near Fermi level that that at far above Fermi level. The morphology and electronic characteristics of the
Fig. 5. The STM image of 0.5 ML Ge on the Si(1 1 1)-(7 7) surface acquired at a sample bias of þ1.10 V. The tunneling current is 0.43 nA. The scan areas are 37 35 nm2 .
islands are still open in many aspects and need to be further investigated. However, the STM image indeed shows that the Ge islands have ordered structure and there exists preferential adsorption of Ge on the 2D Ge cluster superlattice. In summary, we have demonstrated the ordered structure of submonolayer Ge on Si(1 1 1)-(7 7) surface. The Si(1 1 1)-(7 7) reconstruction can be employed to control the nucleation and growth of the Ge clusters. At 0.4 ML Ge coverage, the clusters with similar shape and uniform size are homogeneously distributed over the (7 7) substrate and form a sixfold symmetry superlattice. When the coverage exceed 0.4 ML some compact Ge islands can be formed on the 2D Ge cluster superlattice.
Acknowledgements One of the authors (Long Yan) wishes to express thanks to Prof. Q.K. Xue for very valuable discussions and Prof. Y.S. Gu for the revision of language. This work has been supported by the National Nature Science Foundation of China.
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References [1] Y.-W. Mo, D.E. Savage, B.S. Swartzentruber, M.G. Lagally, Phys. Rev. Lett. 65 (1990) 1020. [2] B. Voigtl€ ander, A. Zinner, Appl. Phys. Lett. 63 (1993) 3055. [3] M.A. Lutz, R.M. Feenstra, P.M. Mooney, J. Tersoff, J.O. Chu, Surf. Sci. 316 (1994) L1075. [4] J. Tersoff, C. Teichert, M.G. Lagally, Phys. Rev. Lett. 76 (1996) 1675. [5] D.E. Jesson, K.M. Chen, S.J. Pennycook, T. Thundat, R.J. Warmack, Phys. Rev. Lett. 77 (1996) 1330. [6] S.Y. Shiryaev, F. Jensen, J. Lundsgaard Hansen, J. Wulff Petersen, A. Nylandsted Larsen, Phys. Rev. Lett. 78 (1997) 503. [7] E. Mateeva, P. Sutler, J.C. Bean, M.G. Lagally, Appl. Phys. Lett. 71 (1997) 3233. [8] Y.H. Xie, S.B. Samavedam, M. Bulsara, T.A. Langdo, E.A. Fitzgerald, Appl. Phys. Lett. 71 (1997) 3567. [9] J.-H. Zhu, K. Brunner, G. Abstreiter, Appl. Phys. Lett. 73 (1998) 620. [10] F.M. Ross, R.M. Tromp, M.C. Reuter, Science 286 (1999) 1931. [11] D.S.L. Mui, D. Leonard, L.A. Coldren, P.M. Petroff, Appl. Phys. Lett. 66 (1995) 1620.
[12] T.I. Kamins, R. Stanley Williams, Appl. Phys. Lett. 71 (1998) 1201. [13] K. Takayanagi, Y. Tanishiro, M. Takahashi, S. Takahashi, J. Vac. Sci. Technol. A 3 (1985) 1502. [14] U.K. K€ ohler, J.E. Demuth, R.J. Hamers, Phys. Rev. Lett. 60 (1988) 2499. [15] S. Tosch, H. Neddermeyer, Phys. Rev. Lett. 61 (1988) 349. [16] X.F. Lin, I. Chizhov, H.A. Mai, R.F. Willis, Surf. Sci. 366 (1996) 51. [17] D. Tang, H.E. Elsayed-Ali, J. Wendelken, J. Xu, S. Tosch, H. Neddermeyer, Phys. Rev. B 52 (1995) 1481. [18] A. Watanabe, M. Naitoh, S. Nishigaki, Jpn. J. Appl. Phys. 37 (1998) 3778. [19] L. Vitali, M.G. Ramsey, F.P. Netzer, Phys. Rev. Lett. 83 (1999) 316. [20] U. K€ ohler, J.E. Demuth, R.J. Hamers, J. Vac. Sci. Technol. A 7 (1989) 2860. [21] U. K€ ohler, O. Jusko, G. Pietsch, B. M€ uller, M. Henzler, Surf. Sci. 321 (1991) 248. [22] H.-J. Gossmann, L.C. Feldman, W.M. Gibson, Phys. Rev. Lett. 53 (1984) 294. [23] H.-J. Gossmann, L.C. Feldman, W.M. Gibson, Surf. Sci. 155 (1985) 413. [24] H. Hibino, T. Ogino, Appl. Surf. Sci. 82/83 (1994) 374.
Surface Science Letters
Formation of two dimension Ge cluster superlattice on Si(1 1 1)-(7 7) surface Long Yan, Yongping Zhang, Hongjun Gao, Sishen Xie *, Shijin Pang Beijing Laboratory of Vacuum Physics, Institute of Physics & Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 2724, Beijing 100080, PR China Received 12 June 2001; accepted for publication 30 October 2001
Abstract The adsorption process of sub-monolayer Ge on Si(1 1 1)-(7 7) surface is studied using ultrahigh-vacuum scanning tunneling microscopy. By carefully controlling Ge deposition condition, a unique sixfold symmetry superlattice of Ge clusters is formed. It is found that almost all the Ge clusters observed have similar shapes and uniform sizes, and that they sit on both halves of the (7 7) unit cells. The formation of the ordered structure is attributed to the fact that the characteristic Si(1 1 1)-(7 7) reconstruction controls the nucleation and growth of the Ge clusters. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Germanium; Silicon; Scanning tunneling microscopy
The epitaxial growth of Ge or Ge/Si on Si surfaces has been one focus of many studies in recent decades. It is an important subject to obtain ordered nanoscale structures in the SiGe system because of their potential application in photoelectronic devices and their compatibility with Si processing. It is well known that the self-organized growth of strained islands in heteroepitaxial growth is currently employed to build dense arrays of nanoscale islands [1–10]. However, it is still difficult to control accurately the order, size uniformity, and density of the grown islands. Another way to achieve order is designing templates with periodic structures [11,12]. The
*
Corresponding author. Fax: +86-1062-556598. E-mail address: [email protected] (S. Xie).
entire approach is based on the fact that the adatoms agglomerate in some regularly distributed sites created by the template to form an ordered array or superlattice of well-defined nanoclusters. Si(1 1 1)-(7 7) surface can be served as a perfect template because of its stable periodic surface structure [13]. One (7 7) unit cell consists of faulted and unfaulted halves in which foreign atoms preferentially occupy or congregate due to the attraction of the dangling bonds of the adatoms and rest atoms. The dimer rows, which separate faulted and unfaulted halves, usually repulse foreign atoms on account of no dangling bonds. Corner hole usually does not attract foreign atoms since site of its dangling bond is too deep. The preferential adsorption of adatoms on Si(1 1 1)-(7 7) surface has been reported for a variety of metals [14–18]. Vitali et al. recently
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 1 ) 0 1 8 2 0 - 9
CE RF A LE C E TT S E R C IE S N
SU
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L. Yan et al. / Surface Science 506 (2002) L255–L260
reported that Tl atoms agglomerated into a 2D superlattice of well-defined nanoclusters on the Si(1 1 1)-(7 7) surface at room temperature [19]. Besides, K€ ohler et al. found that at room temperature irregular Si and Ge clusters could be arranged in the half unit cells toform an ordered array on Si(1 1 1)-(7 7) surfaces [20,21]. In this letter, we found that the shape and size of the clusters could be controlled uniformly. Thus a sixfold symmetry superlattice of Ge cluster formed on Si(1 1 1)-(7 7) surface can be observed. The experiments were performed using an ultrahigh-vacuum scanning tunneling microscopy system (Omicron, Germany) with a base pressure below 1 1010 Torr. N-doped Si substrates (q 1–2 X cm, thickness 0.51–0.54 mm) were degassed for 12 h at 900 K, and then flashed to 1500 K for 20 s by direct current (DC) heating. Finally, perfect Si(1 1 1)-(7 7) surfaces were obtained. The experimental conditions for Ge deposition are as follow. A small piece of Ge was selected as the deposition source, which was sublimed at about 1100 K by DC heating. The deposition rate was about 0.005 ML/min. Note that the Si surface was not deliberately heated. The distance between the sample and source is 5 cm, and the temperature of the Si substrate might be increased to 320 K in a long Ge deposition process. The increase of the substrate temperature might enhance the mobility of the Ge a little bit. In the other hand, the resulting pressure is almost the same as the base pressure of our system. Thus we believe that there is no significant contamination in the deposition process. To verify this, we annealed the Si sample covered by Ge at 700 K. The nice Ge islands with perfectly ordered 5 5 and/or 7 7 structure can be observed on clean Si(1 1 1)-(7 7) substrates. Fig. 1a shows the results of 0.4 ML Ge deposition on a Si(1 1 1)-(7 7) substrate. It can be found that the deposited Ge atoms nucleate into clusters, as reported by K€ ohler et al. [21]. However, most of the clusters have similar shape and uniform size, which was not observed in previous experiments. We are sure that the uniform shape of the Ge clusters is not due to a limited resolution of the tip because individual adatom of the (7 7) surface can be clearly resolved using the tip. Moreover, the Ge clusters distribute quite homo-
Fig. 1. The STM images of 0.4 ML Ge on the Si(1 1 1)-(7 7) surface. (a) The scan area is 40 40 nm2 . The sample bias and tunneling current are þ1.76 V and 0.44 nA, respectively. The inset shows its Fast Fourier transform, (b) The high resolution STM image. The scan area is 12 12 nm2 . The sample bias and tunneling current are þ1.80 V and 0.43 nA. A (7 7) unit cell is indicated. Note that there is one Ge cluster in each half unit cell.
geneously over the (7 7) substrate on a macroscopic scale and form a nearly ordered 2D superlattice. The 2D fast Fourier transformation (2DFFT) from Fig. 1a is shown in the inset. The spectrum exhibits a clear sixfold pattern like a hexagon, confirming the macroscopic 2D ordering of the Ge clusters. Scrutinizing a high-resolved
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L. Yan et al. / Surface Science 506 (2002) L255–L260
scanning tunneling microscopy (STM) image shown in Fig. 1b, the size of the Ge clusters is consistent with that of one half cell. There is a clear borderline between two nearby clusters, and each hole is usually surrounded with six clusters. It is noted that the borderlines and holes could correspond to the dimer rows and corner holes of the (7 7) substrate, respectively. K€ ohler et al. have demonstrated that irregular clusters sat on the (7 7) halves, leaving the dimer rows and corner holes uncovered [21]. Moreover, the great amount of experiments show that the deposition of Ge atoms on Si(l 1 1)-(7 7) substrate at lower temperature does not transform the Si surface until a few monolayer Ge atoms have been deposited [22– 24]. So we believe that all of the Ge clusters located on the (7 7) halves. Here, it should be pointed out that the clusters equally occupied on the faulted and the unfaulted halves, i.e., no preference for clustering on the faulted half is observed. Thus the 2D structure formed by the Ge clusters naturally have sixfold symmetry. To understand the forming mechanism of the ordered structure, we decrease Ge coverage to study growth process of Ge on Si(1 1 1)-(7 7) substrates. Fig. 2a shows the result of 0.3 ML Ge deposition. It was found that the clusters on most areas have random shapes, which is similar to the results of previous observation [21]. But the clusters in the areas as indicated by the arrows in Fig. 2a appear bigger and higher, and have similar shape, which is similar to Fig. 1. On the average, the difference in height between the Ge clusters is . This might be related to the fluctuation l–1.5 A of the cluster size. The Ge atom number of the bigger clusters is more than that of the smaller one. Increasing the amount of Ge deposition to 0.35 ML, the areas containing such ordered Ge cluster correspondingly increase, as shown in Fig. 2b. It should be noted that the size of any Ge cluster do not exceed one half cell. This demonstrates that the formed Ge clusters can grow with the increase of deposition amount until the clusters reach an ultimate size. How do we understand the growth mechanism of Ge on Si(1 1 1)-(7 7) substrate? At initial adsorption stage, due to the attraction of the dangling bonds of adatoms and rest atoms on the
L257
Fig. 2. The STM images of the Si(1 1 1)-(7 7) surface with (a) 0.3 ML Ge coverage ML Ge coverage. The scan areas of (a) and (b) are all 35 35 nm2 . The sample bias and tunneling current are (a) þ1.70 V and 0.45 nA, (b) þ1.60 V and 0.48 nA, respectively.
(7 7) substrate, the arriving Ge atoms have very low mobility, which can increase nucleation probability. So the formed Ge clusters homogeneously cover the (7 7) substrate. When the Ge clusters are big enough to saturate all dangling bonds of adatoms and rest atoms in the halves, the adsorption ability of the dangling bonds for additional atoms will become very weak. Moreover,
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it is well known that the attraction ability of such Ge clusters for foreign atoms is also weak. Hence, the mobility of the additional Ge atoms will increase. So once the additional Ge atoms sit on the Ge clusters, they move preferentially to the sites of the smaller Ge clusters. Besides, a little increase of the substrate temperature in the deposition process might enhance the mobility of the additional atoms. Thus, our STM images show that the growth of the Ge clusters is suppressed after they reached certain size. This shows that the dangling bonds of the (7 7) reconstruction play an important role in the formation and growth of the Ge clusters. On the other hand, the experiments strongly demonstrate the initial growth of the amorphous Ge film on Si(1 1 1)-(7 7) surface is not disordered as expected. We next investigate higher coverage of Ge on Si(1 1 1)-(7 7) surface. Fig. 3a and b are the STM images of 0.5 ML Ge deposition, which were scanned at high positive bias voltages. From Fig. 3a, it can be found that some compact Ge islands occur on the 2D Ge cluster superlattice formed on the Si(1 1 1)-(7 7) surface. The Ge islands at step edges are also observed, as shown in Fig. 3b. These in height, which is obviously islands are 5 A higher than one bilayer step of Ge surface. Otherwise, it is difficult to observe the existence of small clusters. These facts indicate that the mobility of the Ge atoms on the 2D Ge cluster superlattice is indeed much higher than that on the (7 7) substrate. Besides, it is noted that the morphology of the 2D Ge cluster superlattice appears not to be modified, i.e., it still keeps the sixfold symmetry structure consisted of uniform Ge cluster. This further demonstrates that the additional Ge atoms do not make the Ge clusters grow continuously. The electronic characteristics of such Ge cluster on Si(1 1 1)-(7 7) surface are also investigated by STM technique. For the Ge cluster superlattice on Si(111 )-(7 7) surface, the STM images observed at either low positive bias or negative bias show smaller corrugation than that at high positive bias. This means that the local density of state of the Ge cluster near and far below the Fermi level is drastically reduced, which can be supported by the scanning tunneling spectroscopy (STS) measuring
Fig. 3. The STM images of 0.5 ML Ge on the Si(1 1 1)-(7 7) surface. The sample bias and tunneling current are þ1.86 V and 0.43 nA, respectively. The scan areas are (a) 55 55 nm2 (b) 35 35 nm2 , respectively.
on the Ge clusters (see Fig. 4). The dI=dV –V curve (i.e. STS) in Fig. 4 was obtained through averaging data taking from a few individual clusters in order to have a better signal-to-noise ration. From the STS we can clearly see the gap of electronic states near Fermi level, which indicates that the population of the electrons near Fermi level is very small. Therefore, it is difficult to obtain clear STM
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L. Yan et al. / Surface Science 506 (2002) L255–L260
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Fig. 4. The scanning tunneling spectroscopy of the ordered Ge clusters in the 2D Ge cluster superlattice on the Si(1 1 1)-(7 7) surface.
images at low bias voltage. Moreover, the density of the electronic states near 1.8 eV is much lower than that near þ1.8 eV, which demonstrates that the tunneling probability of electrons from the cluster to the tip is less than that from the tip to the clusters. So the high-resolved STM images can be only obtained at high positive bias. On account of poor electronic conductivity of the Ge cluster near and far below the Fermi level, we speculate that the Ge clusters may be amorphous. Unfortunately, the STM images at a high positive bias do not display clear geometry structure of the higher Ge islands as shown in Fig. 4. However, the STM images at low positive bias can do more for the Ge islands. Fig. 5 is another STM image of 0.5 ML Ge deposition on the Si(1 1 1)(7 7) surface at the sample bias voltage of þ1.10 V. In the STM image, the height of the island is . So the height of the islands at different bias 4 A is different. Besides, the 2D Ge cluster superlattice on the Si(1 1 1)-(7 7) surface is hazy. But the holes and the clusters in the island located above the superlattice can be observed, which is very similar to the morphology of 2D Ge cluster superlattice on the Si(1 1 1)-(7 7) surface at high positive bias. This indicates that the state density of Ge clusters in the island is higher near Fermi level that that at far above Fermi level. The morphology and electronic characteristics of the
Fig. 5. The STM image of 0.5 ML Ge on the Si(1 1 1)-(7 7) surface acquired at a sample bias of þ1.10 V. The tunneling current is 0.43 nA. The scan areas are 37 35 nm2 .
islands are still open in many aspects and need to be further investigated. However, the STM image indeed shows that the Ge islands have ordered structure and there exists preferential adsorption of Ge on the 2D Ge cluster superlattice. In summary, we have demonstrated the ordered structure of submonolayer Ge on Si(1 1 1)-(7 7) surface. The Si(1 1 1)-(7 7) reconstruction can be employed to control the nucleation and growth of the Ge clusters. At 0.4 ML Ge coverage, the clusters with similar shape and uniform size are homogeneously distributed over the (7 7) substrate and form a sixfold symmetry superlattice. When the coverage exceed 0.4 ML some compact Ge islands can be formed on the 2D Ge cluster superlattice.
Acknowledgements One of the authors (Long Yan) wishes to express thanks to Prof. Q.K. Xue for very valuable discussions and Prof. Y.S. Gu for the revision of language. This work has been supported by the National Nature Science Foundation of China.
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