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Growth of the Ge overlayer on Si(100)-(2×1)
Surface Science 461 (2000) L565–L569 www.elsevier.nl/locate/susc
Surface Science Letters
Growth of the Ge overlayer on Si(100)-(2×1) T.-W. Pi a, *, R.-T. Wu a, C.-P. Ouyang b, J.-F. Wen c, G.K. Wertheim 1,d a Synchrotron Radiation Research Center, Hsinchu, Taiwan, ROC b Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, ROC c Department of Physics, Tamkang University, Tamsui, Taiwan, ROC d Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974, USA Received 28 March 2000; accepted for publication 22 May 2000
Abstract The initial stages of the development of a Ge adatom layer on a clean Si(001)-(2×1) surface are consistent with random deposition and limited surface mobility. A critical comparison of the rate of attenuation of the Si buckled dimer up-atom signal by Ge deposition with the growth of the two well-resolved features of the Ge adatom spectrum rules out the substitution of Ge into the Si dimers. Instead, Ge is captured by the dangling bonds of the Si dimers and remains on the surface, initially dominantly as isolated Ge atoms, then as dimers, and finally in islands or clusters. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Germanium; Growth; Photoelectron spectroscopy; Silicon; Single crystal epitaxy
It is well documented that germanium does not grow epitaxially on clean Si(001)-(2×1) surfaces. For coverages exceeding a few monolayers (ML) the additional Ge forms islands or clusters, that is, it exhibits Stanski–Krastonov growth [1,2]. Below 1 ML, there exist three models for the growth of Ge on the Si(001)-(2×1) surface. They are the Ge–Ge surface–dimer model [3–9], the mixed Ge–Si surface–dimer model [10,11], and the interface mixing SiGe alloy model [12–21]. In the Ge–Ge surface–dimer model, the Ge atoms replace totally the surface dimer atoms. However, the mixed Ge–Si surface–dimer model proposes that only the up-atoms in the surface dimers are * Corresponding author. Fax: +886-3578-9816. E-mail address: [email protected] ( T.-W. Pi) 1 Retired.
replaced. More complicated than the first two models, the interface SiGe alloy model postulates that the Ge replacement occurs not only on the top surface layer, but also down to the next three subsurface layers. Interest here focuses on the initial stages of the formation of the Ge layer in contact with the underlying Si. Contrary to the existing models, we found that the Ge adatoms do not replace, but are captured by the dangling bonds of the Si dimers and remain on the surface. They initially appear as isolated Ge atoms, then as dimers, and finally in islands or clusters. Photoemission data illustrating the regimes of interest are shown in Figs. 1 and 2, for Si 2p and Ge 3d, respectively. They were measured with an 125 mm hemispherical analyzer (Omicron Vakuumphysik GmbH ) in a ultrahigh vacuum ( UHV ) chamber with base
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Fig. 1. Si 2p normal emission spectra upon successive exposures of Ge on Si(001)-(2×1) in minutes with hn=130 eV.
Fig. 2. Ge 3d normal emission spectra upon successive exposures of Ge on Si(001)-(2×1) in minutes with hn=110 eV
pressure >3×10−11 Torr. The chamber is stationed at the end of the low-energy spherical grating monochromator (LSGM ) in the Synchrotron Radiation Research Center (SRRC ) in Hsinchu, Taiwan, Republic of China. A clean Si(100)-(2×1) surface was pre-oxidized according to the Ishizaki and Shiraki method, and annealed in a step-wise fashion to 875°C in the photoemission chamber. The pressure during deposition from the well-degassed Ge source onto the silicon surface held at 400°C always remained below 1.1× 0−10 Torr. In the Si substrate we see a transition from the characteristic spectrum of the (2×1) surface to one approaching the simple spin-orbit spectrum characteristic of the bulk. The well-resolved component of the spectrum due to the Si up-dimer atom is rapidly attenuated by the Ge. Substantially greater deposition is required to reduce the broadening of the main peaks, which is due to the subsurface layer [22]. The subsurface atom has a shift to larger binding energy in bulk Si, which is reduced as it is converted into the Ge interface layer. For the Ge overlayer we initially see a
relatively sharp spin-orbit pattern with a phonon width of 0.35 eV, which must be due to Ge atoms without Ge neighbors. A secondary, broader peak with a phonon width of 0.45 eV starts to form a little more slowly but then overtakes the entire spectrum and grows into what appears like a broadened spectrum of bulk Ge [23,24]. This sequence of events has been interpreted as showing that the Ge initially replaces the Si up-dimer atoms, then the down-dimer atoms [17], eventually forming a 2 or 3 ML surface layer, and then forming islands [1,2]. However, more detailed qualitative considerations reveal a significant problem. According to Fig. 1, 10 min of deposition is sufficient to replace the entire 0.5 ML of Si dimer up-atoms, so that 20 min would be required to deposit 1 ML. According to this interpretation the Ge spectrum should require ca. 20 min of deposition to reach a condition where there are two doublets of equal intensity, representing the upand down-dimer atoms of Ge. The data show, however, that this condition is achieved in only 10 min. After 20 min of deposition the bulk-like spectrum is well developed. This factor-of-two
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Fig. 3. The exponential attenuation plot of single Ge adatoms and Si dimer up-atoms.
discrepancy suggests that something is amiss and invites a more quantitative investigation. The attenuation of the Si up-dimer atom is readily quantified by introducing an attenuation factor into the model function that is used to fit the spectrum of the clean surface. As in all models of surface layer formation we assume that the adatoms are randomly deposited and that they are immediately captured by the nearest dangling bond preventing diffusion and aggregation, which is commonly found on metallic substrates. The capture is promoted by the common bonding properties of Si and Ge. The fitted results are not in accord with a linear substitution of Ge into Si up-dimer sites, but are in much better agreement with a model of random deposition with vanishing surface diffusion. That model predicts an exponential attenuation of the Si up-dimer sites, which is realized as illustrated in Fig. 3. The exponential decay implies that Ge atoms, which are deposited near the Ge atoms already on the surface, do not contribute to the spectrum of isolated Ge atoms, but must form Ge dimers.This can be investigated further by an analysis of the coverage-dependent Ge 3d spectra. Fig. 4 shows the representative fit to the 4 min spectrum. For the first 8 min of deposition, the data are adequately represented by two spin-orbit doublets whose areas are readily determined. According to the above description of Ge substitution into the existing Si dimers and
Fig. 4. A representative fit to the 4 min Ge 3d spectrum.
random deposition, the fraction of Ge atoms without Ge neighbors should follow an exponential function with a slope identical to that of the Si up-atom attenuation. The data, shown in Fig. 3, do exhibit an exponential attenuation, but with a slope about one half of that of the Si up-dimer attenuation coefficient. Quantitative analysis thus confirms the existence of the problem that became evident even in the visual inspection of the data. An important clue to an alternative interpretation is offered by the analysis of the Ge 3d spectra. They show that the doublet A, which grows initially, does not retain its strength as the broader secondary doublet B develops. That argues against identification with Ge atoms in Si up-dimer sites, which would remain occupied. Furthermore, the random deposition model identifies the B doublet at a larger binding energy as being initially due to isolated dimers. The total Gaussian width f 0.45 eV of this feature is sufficiently large to hide and include the buckling-induced 0.10–0.15 eV splitting of the dimer signal in bulk Ge [23,24]. There is no prospect that the signal from the up-dimers is coincident with the sharper doublet A; the dimer splitting would be much too large. Attempts to fit the data with such a model were unsuccessful. The binding energy of the isolated Ge dimer signal,
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doublet B, is somewhat larger than that in bulk Ge, but shifts to smaller binding energy as the dimers begin to interact. The shift to smaller binding energy of the sharper doublet A due to isolated Ge atoms is about twice as large as that of the up-dimer in bulk Ge, providing one argument against substitution into the existing Si dimers. The smaller binding energy is in accord with expectations for a surface atom with two dangling bonds [25]. The 0.45 eV phonon width of the dimer spectrum is in good accord with the 0.50 eV width of the dimer spectrum on the bulk (2×1) surface [22]. The width of the sharper doublet A at smaller binding energy is closer in width to that of the bulk signal. The large phonon width associated with the buckled dimers on both Si(100) and Ge(100) is due to the magnitude of the atomic motion associated with dimer buckling, which is large compared to the thermal motion of bulk atoms. Photoionization of a dimer atom changes the equilibrium dimer configuration. According to the theory of phonon broadening this will produce a large phonon width. The smaller phonon width associated with the Ge atoms initially deposited, provides another argument against their incorporation into the Si dimer structure. The alternative is that they are captured by the dangling bonds of the reconstructed surface and remain on the surface. In accord with expectations for an isolated surface atom, the width is larger than the bulk width because the atom has more freedom of motion relative to the surface than a bulk atom has relative to its neighbors. To make such a small phonon width compatible with dimer substitution would require that the buckling of the mixed dimers is insensitive to photoionization of the Ge member, which seems unlikely. Since there is no energy advantage in ejecting a Si atom from a dimer and replacing it with a Ge atom [26 ], the formation of mixed dimers seems, a priori, improbable. The well-known surface segregation of Ge in equiatomic GeSi alloys offers some additional support for the notion that the Ge adatom will remain on top of the existing Si surface. The immediate question is whether such surface Ge adatoms can account for the attenuation of the Si up-dimer signal. The most likely adatom
binding sites should correspond to the lattice sites of the next surface layer. One such site exists between two in-line dimers, which have dangling bonds with the proper orientation, except for the buckling, to bond to an atom like Si or Ge. The other site would be at the dimer bond itself. The former seems more likely to capture an adatom since it does not require the rupture of an existing bond. The effect of capturing an atom at such a site would have profound consequences on both dimers. The atoms bonded to the adatom are halfway to becoming subsurface atoms and should have much smaller shifts that the dimer up-atoms. The buckling would certainly be greatly modified and the dimer bond would become less favorable energetically and may be disrupted. Note that two dimers are affected by the bonding of one adatom so that only 0.25 ML is required to remove the entire up-atom signal. That serves to explain the factor-of-two discrepancy between the attenuation of the Si up-dimer signal and the growth of the isolated adatom signal noted above. In Fig. 3, we, from the slope of the fraction of the total Ge 3d signal which appears in doublet A, obtain a deposition rate of 23.6 min ML−1. From the slope of the attenuation of the Si 2p up-dimer signal we obtain a slope of 10.9 min ML−1. Since two Si atoms are removed from this signal for each Ge atom, the corresponding deposition rate is 21.8 min ML−1. The resulting average deposition rate is 22.7 min ML−1. Because of the large scatter in the data at large deposition, a reasonable value is 23±3 min ML−1. That value is in good accord with the fact that the signal from the isolated Ge atoms is effectively eliminated by a 24 min exposure. With continued deposition beyond monolayer coverage, the dimer signal transforms into one resembling a greatly broadened bulk Ge(100) spectrum [23]. The broadening is in accord with the formation of small clusters or islands, implied by the Stanski–Krastonov growth. It cannot be attributed to the formation of a mixed Si–Ge layer, because neither the Si 2p nor Ge 3d spectra of the equiatomic alloy are greatly broadened or shifted compared to those of the respective elemental solid. It is known that deposition in excess of a few monolayers will result in the growth of Ge
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islands or clusters islands or massive clusters. The data in Fig. 2 suggest only 1 ML of Ge dimers covers most of the Si(001) surface. The weak and non-exponential attenuation of the Si 2p signal in the present regime of coverage supports this conclusion. The conclusion that the Ge adatoms are not incorporated into the existing dimers but remain on top of the Si surface is in accord with all aspects of their behavior, including the binding energy, the phonon width, and the rate of attenuation of the Si up-dimer signal. The identification of the broader Ge doublet, that forms more slowly but ultimately dominates the behavior, with isolated Ge dimers is in accord with their binding energy, phonon width, rate of production by random deposition, and eventual transformation into a bulk-like spectrum. The initial notion that Ge adatoms substitute into the existing Si dimers cannot be reconciled with the data.
Acknowledgements This project is sponsored by the National Science Council under the Contract Nos. NSC-87-2613-M-213-005 and NSC-89-2112-M213-006
References [1] Y.-W. Mo, D.E. Savage, B.S. Swartzentruber, M.G. Lagally, Phys. Rev. Lett. 65 (1990) 1020.
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[2] D.J. Eaglesham, M. Cerullo, Phys. Rev. Lett. 64 (1990) 1943. [3] D.-S. Lin, T. Miller, T.-C. Chiang, Phys. Rev. Lett. 67 (1991) 2187. [4] F. Iwawaki, M. Tomitori, O. Nishikawa, Surf. Sci. 266 (1992) 285. [5] R.M. Tromp, Phys. Rev. B 47 (1993) 7125. [6 ] E. Fontes, J.R. Patel, F. Comin, Phys. Rev. Lett. 70 (1993) 2790. [7] J.-M. Jin, L.J. Lewis, Phys. Rev. B 49 (1994) 2201. [8] J.-H. Cho, S. Jeong, M.-H. Kang, Phys. Rev. B 50 (1994) 17139. [9] F. Liu, M.G. Legally, Phys. Rev. Lett. 76 (1996) 3156. [10] X. Chen, D.K. Saldin, E.L. Bullock, L. Patthey, L.S.O. Johansson, J. Tani, T. Abukawa, S. Kono, Phys. Rev. B 55 (1997) R7319. [11] R.H. Miwa, Surf. Sci. 418 (1998) 55. [12] P.C. Kelires, J. Tersoff, Phys. Rev. Lett. 63 (1989) 1164. [13] F.K. LeGroues, V.P. Kesan, S.S. Iyer, Phys. Rev. Lett. 64 (1990) 2038. [14] R. Schorer, G. Abstreiter, S. de Gironcoli, E. Molinari, H. Kibbel, H. Presting, Phys. Rev. B 49 (1994) 5406. [15] M. Sasaki, T. Abukawa, H.W. Yeom, M. Yamada, S. Suzuki, S. Sato, S. Kono, Appl. Surf. Sci. 82/83 (1994) 387. [16 ] G. Theodorou, C. Tserbak, Phys. Rev. B 51 (1995) 4723. [17] L. Patthey, E.L. Bullock, T. Abukawa, S. Kono, L.S.O. Johansson, Phys. Rev. Lett. 75 (1995) 2538. [18] H. Oyanagi, K. Sakamoto, R. Shioda, Y. Kuwahara, K. Haga, Phys. Rev. B 52 (1995) 5824. [19] R. Gunnella, P. Castrucci, N. Pinto, I. Davoli, D. Se´billeau, M. De Crescenzi, Phys. Rev. B 54 (1996) 8882. [20] H.W. Yeom, M. Sasaki, S. Suzuki, S. Sato, S. Hosoi, M. Iwabuchi, K. Higashiyama, H. Fukutani, M. Nakamura, T. Abukawa, S. Kono, Surf. Sci. 381 (1997) L533. [21] B.P. Uberuaga, M. Leskovar, A.P. Smith, H. Jo´nsson, M. Olmstead, Phys. Rev. Lett. 84 (2000) 2441. [22] T.-W. Pi, I.-H. Hong, C.-P. Cheng, G.K. Werheim, J. Electron. Spectrosc. Relat. Phenom. 107 (2000) 163. [23] T.-W. Pi, C.-P. Ouyang, J.-F. Wne, R.-T. Wu, G.K. Wertheim, unpublished data. [24] A. Goldoni, S. Modesti, V.R. Dhanak, M. Sancrotti, A. Santoni, Phys. Rev. B 54 (1996) 11340. [25] E. Pehlke, M. Scheffler, Phys. Rev. Lett. 71 (1993) 2338. [26 ] Y. Yoshimoto, M. Tsukada, Surf. Sci. 423 (1999) 32.
Surface Science Letters
Growth of the Ge overlayer on Si(100)-(2×1) T.-W. Pi a, *, R.-T. Wu a, C.-P. Ouyang b, J.-F. Wen c, G.K. Wertheim 1,d a Synchrotron Radiation Research Center, Hsinchu, Taiwan, ROC b Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, ROC c Department of Physics, Tamkang University, Tamsui, Taiwan, ROC d Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974, USA Received 28 March 2000; accepted for publication 22 May 2000
Abstract The initial stages of the development of a Ge adatom layer on a clean Si(001)-(2×1) surface are consistent with random deposition and limited surface mobility. A critical comparison of the rate of attenuation of the Si buckled dimer up-atom signal by Ge deposition with the growth of the two well-resolved features of the Ge adatom spectrum rules out the substitution of Ge into the Si dimers. Instead, Ge is captured by the dangling bonds of the Si dimers and remains on the surface, initially dominantly as isolated Ge atoms, then as dimers, and finally in islands or clusters. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Germanium; Growth; Photoelectron spectroscopy; Silicon; Single crystal epitaxy
It is well documented that germanium does not grow epitaxially on clean Si(001)-(2×1) surfaces. For coverages exceeding a few monolayers (ML) the additional Ge forms islands or clusters, that is, it exhibits Stanski–Krastonov growth [1,2]. Below 1 ML, there exist three models for the growth of Ge on the Si(001)-(2×1) surface. They are the Ge–Ge surface–dimer model [3–9], the mixed Ge–Si surface–dimer model [10,11], and the interface mixing SiGe alloy model [12–21]. In the Ge–Ge surface–dimer model, the Ge atoms replace totally the surface dimer atoms. However, the mixed Ge–Si surface–dimer model proposes that only the up-atoms in the surface dimers are * Corresponding author. Fax: +886-3578-9816. E-mail address: [email protected] ( T.-W. Pi) 1 Retired.
replaced. More complicated than the first two models, the interface SiGe alloy model postulates that the Ge replacement occurs not only on the top surface layer, but also down to the next three subsurface layers. Interest here focuses on the initial stages of the formation of the Ge layer in contact with the underlying Si. Contrary to the existing models, we found that the Ge adatoms do not replace, but are captured by the dangling bonds of the Si dimers and remain on the surface. They initially appear as isolated Ge atoms, then as dimers, and finally in islands or clusters. Photoemission data illustrating the regimes of interest are shown in Figs. 1 and 2, for Si 2p and Ge 3d, respectively. They were measured with an 125 mm hemispherical analyzer (Omicron Vakuumphysik GmbH ) in a ultrahigh vacuum ( UHV ) chamber with base
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Fig. 1. Si 2p normal emission spectra upon successive exposures of Ge on Si(001)-(2×1) in minutes with hn=130 eV.
Fig. 2. Ge 3d normal emission spectra upon successive exposures of Ge on Si(001)-(2×1) in minutes with hn=110 eV
pressure >3×10−11 Torr. The chamber is stationed at the end of the low-energy spherical grating monochromator (LSGM ) in the Synchrotron Radiation Research Center (SRRC ) in Hsinchu, Taiwan, Republic of China. A clean Si(100)-(2×1) surface was pre-oxidized according to the Ishizaki and Shiraki method, and annealed in a step-wise fashion to 875°C in the photoemission chamber. The pressure during deposition from the well-degassed Ge source onto the silicon surface held at 400°C always remained below 1.1× 0−10 Torr. In the Si substrate we see a transition from the characteristic spectrum of the (2×1) surface to one approaching the simple spin-orbit spectrum characteristic of the bulk. The well-resolved component of the spectrum due to the Si up-dimer atom is rapidly attenuated by the Ge. Substantially greater deposition is required to reduce the broadening of the main peaks, which is due to the subsurface layer [22]. The subsurface atom has a shift to larger binding energy in bulk Si, which is reduced as it is converted into the Ge interface layer. For the Ge overlayer we initially see a
relatively sharp spin-orbit pattern with a phonon width of 0.35 eV, which must be due to Ge atoms without Ge neighbors. A secondary, broader peak with a phonon width of 0.45 eV starts to form a little more slowly but then overtakes the entire spectrum and grows into what appears like a broadened spectrum of bulk Ge [23,24]. This sequence of events has been interpreted as showing that the Ge initially replaces the Si up-dimer atoms, then the down-dimer atoms [17], eventually forming a 2 or 3 ML surface layer, and then forming islands [1,2]. However, more detailed qualitative considerations reveal a significant problem. According to Fig. 1, 10 min of deposition is sufficient to replace the entire 0.5 ML of Si dimer up-atoms, so that 20 min would be required to deposit 1 ML. According to this interpretation the Ge spectrum should require ca. 20 min of deposition to reach a condition where there are two doublets of equal intensity, representing the upand down-dimer atoms of Ge. The data show, however, that this condition is achieved in only 10 min. After 20 min of deposition the bulk-like spectrum is well developed. This factor-of-two
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Fig. 3. The exponential attenuation plot of single Ge adatoms and Si dimer up-atoms.
discrepancy suggests that something is amiss and invites a more quantitative investigation. The attenuation of the Si up-dimer atom is readily quantified by introducing an attenuation factor into the model function that is used to fit the spectrum of the clean surface. As in all models of surface layer formation we assume that the adatoms are randomly deposited and that they are immediately captured by the nearest dangling bond preventing diffusion and aggregation, which is commonly found on metallic substrates. The capture is promoted by the common bonding properties of Si and Ge. The fitted results are not in accord with a linear substitution of Ge into Si up-dimer sites, but are in much better agreement with a model of random deposition with vanishing surface diffusion. That model predicts an exponential attenuation of the Si up-dimer sites, which is realized as illustrated in Fig. 3. The exponential decay implies that Ge atoms, which are deposited near the Ge atoms already on the surface, do not contribute to the spectrum of isolated Ge atoms, but must form Ge dimers.This can be investigated further by an analysis of the coverage-dependent Ge 3d spectra. Fig. 4 shows the representative fit to the 4 min spectrum. For the first 8 min of deposition, the data are adequately represented by two spin-orbit doublets whose areas are readily determined. According to the above description of Ge substitution into the existing Si dimers and
Fig. 4. A representative fit to the 4 min Ge 3d spectrum.
random deposition, the fraction of Ge atoms without Ge neighbors should follow an exponential function with a slope identical to that of the Si up-atom attenuation. The data, shown in Fig. 3, do exhibit an exponential attenuation, but with a slope about one half of that of the Si up-dimer attenuation coefficient. Quantitative analysis thus confirms the existence of the problem that became evident even in the visual inspection of the data. An important clue to an alternative interpretation is offered by the analysis of the Ge 3d spectra. They show that the doublet A, which grows initially, does not retain its strength as the broader secondary doublet B develops. That argues against identification with Ge atoms in Si up-dimer sites, which would remain occupied. Furthermore, the random deposition model identifies the B doublet at a larger binding energy as being initially due to isolated dimers. The total Gaussian width f 0.45 eV of this feature is sufficiently large to hide and include the buckling-induced 0.10–0.15 eV splitting of the dimer signal in bulk Ge [23,24]. There is no prospect that the signal from the up-dimers is coincident with the sharper doublet A; the dimer splitting would be much too large. Attempts to fit the data with such a model were unsuccessful. The binding energy of the isolated Ge dimer signal,
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doublet B, is somewhat larger than that in bulk Ge, but shifts to smaller binding energy as the dimers begin to interact. The shift to smaller binding energy of the sharper doublet A due to isolated Ge atoms is about twice as large as that of the up-dimer in bulk Ge, providing one argument against substitution into the existing Si dimers. The smaller binding energy is in accord with expectations for a surface atom with two dangling bonds [25]. The 0.45 eV phonon width of the dimer spectrum is in good accord with the 0.50 eV width of the dimer spectrum on the bulk (2×1) surface [22]. The width of the sharper doublet A at smaller binding energy is closer in width to that of the bulk signal. The large phonon width associated with the buckled dimers on both Si(100) and Ge(100) is due to the magnitude of the atomic motion associated with dimer buckling, which is large compared to the thermal motion of bulk atoms. Photoionization of a dimer atom changes the equilibrium dimer configuration. According to the theory of phonon broadening this will produce a large phonon width. The smaller phonon width associated with the Ge atoms initially deposited, provides another argument against their incorporation into the Si dimer structure. The alternative is that they are captured by the dangling bonds of the reconstructed surface and remain on the surface. In accord with expectations for an isolated surface atom, the width is larger than the bulk width because the atom has more freedom of motion relative to the surface than a bulk atom has relative to its neighbors. To make such a small phonon width compatible with dimer substitution would require that the buckling of the mixed dimers is insensitive to photoionization of the Ge member, which seems unlikely. Since there is no energy advantage in ejecting a Si atom from a dimer and replacing it with a Ge atom [26 ], the formation of mixed dimers seems, a priori, improbable. The well-known surface segregation of Ge in equiatomic GeSi alloys offers some additional support for the notion that the Ge adatom will remain on top of the existing Si surface. The immediate question is whether such surface Ge adatoms can account for the attenuation of the Si up-dimer signal. The most likely adatom
binding sites should correspond to the lattice sites of the next surface layer. One such site exists between two in-line dimers, which have dangling bonds with the proper orientation, except for the buckling, to bond to an atom like Si or Ge. The other site would be at the dimer bond itself. The former seems more likely to capture an adatom since it does not require the rupture of an existing bond. The effect of capturing an atom at such a site would have profound consequences on both dimers. The atoms bonded to the adatom are halfway to becoming subsurface atoms and should have much smaller shifts that the dimer up-atoms. The buckling would certainly be greatly modified and the dimer bond would become less favorable energetically and may be disrupted. Note that two dimers are affected by the bonding of one adatom so that only 0.25 ML is required to remove the entire up-atom signal. That serves to explain the factor-of-two discrepancy between the attenuation of the Si up-dimer signal and the growth of the isolated adatom signal noted above. In Fig. 3, we, from the slope of the fraction of the total Ge 3d signal which appears in doublet A, obtain a deposition rate of 23.6 min ML−1. From the slope of the attenuation of the Si 2p up-dimer signal we obtain a slope of 10.9 min ML−1. Since two Si atoms are removed from this signal for each Ge atom, the corresponding deposition rate is 21.8 min ML−1. The resulting average deposition rate is 22.7 min ML−1. Because of the large scatter in the data at large deposition, a reasonable value is 23±3 min ML−1. That value is in good accord with the fact that the signal from the isolated Ge atoms is effectively eliminated by a 24 min exposure. With continued deposition beyond monolayer coverage, the dimer signal transforms into one resembling a greatly broadened bulk Ge(100) spectrum [23]. The broadening is in accord with the formation of small clusters or islands, implied by the Stanski–Krastonov growth. It cannot be attributed to the formation of a mixed Si–Ge layer, because neither the Si 2p nor Ge 3d spectra of the equiatomic alloy are greatly broadened or shifted compared to those of the respective elemental solid. It is known that deposition in excess of a few monolayers will result in the growth of Ge
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islands or clusters islands or massive clusters. The data in Fig. 2 suggest only 1 ML of Ge dimers covers most of the Si(001) surface. The weak and non-exponential attenuation of the Si 2p signal in the present regime of coverage supports this conclusion. The conclusion that the Ge adatoms are not incorporated into the existing dimers but remain on top of the Si surface is in accord with all aspects of their behavior, including the binding energy, the phonon width, and the rate of attenuation of the Si up-dimer signal. The identification of the broader Ge doublet, that forms more slowly but ultimately dominates the behavior, with isolated Ge dimers is in accord with their binding energy, phonon width, rate of production by random deposition, and eventual transformation into a bulk-like spectrum. The initial notion that Ge adatoms substitute into the existing Si dimers cannot be reconciled with the data.
Acknowledgements This project is sponsored by the National Science Council under the Contract Nos. NSC-87-2613-M-213-005 and NSC-89-2112-M213-006
References [1] Y.-W. Mo, D.E. Savage, B.S. Swartzentruber, M.G. Lagally, Phys. Rev. Lett. 65 (1990) 1020.
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