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Initial stage of Ag growth on Ge(001) surfaces at room temperature
Surface Science 442 (1999) 300–306 www.elsevier.nl/locate/susc
Initial stage of Ag growth on Ge(001) surfaces at room temperature K. Kushida, K. Hattori *, S. Arai, T. Iimori, F. Komori The Institute for Solid State Physics, The University of Tokyo, Roppongi 7-22-1, Minato-ku, Tokyo 106-8666, Japan Received 4 November 1998; accepted for publication 3 September 1999
Abstract Using scanning tunneling microscopy, we studied Ge(001) surfaces covered with an average of approximately 0.8 monolayers of silver. In the deposition at room temperature, we observed that Ag predominantly grew in a threedimensional (3D) mode on bare Ge substrates, corresponding to Volmer–Weber growth. At the same time, we found two-dimensional (2D) Ag islands elongated in the dimer-row direction with one monolayer height, though the density of the 2D islands was smaller than one-fifth of that of the 3D islands. Images with atomic resolution showed stripe patterns on the 2D islands and enhancement of asymmetry of the Ge dimers on both sides of the islands at high positive bias voltages. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Germanium; Nucleation; Scanning tunneling microscopy; Silver; Superconductivity; Surface structure
1. Introduction In the past two decades, there has been a growing interest in the system of silver–germanium interfaces such as Ge(001) surfaces deposited with Ag because of the anomaly of electric conductivity at low temperatures [1–3]. Superconducting transitions have been suggested for the Ag/Ge(001) system from results of electric resistance measurements [1,2] and low-temperature scanning tunneling spectroscopy (STM ) [3] although neither Ge nor Ag bulk solids are superconducting or mix with each other. However, such a unique superconductivity at the interface has not been evidenced because the observed critical temperatures of or * Corresponding author. Present address: Graduate School of Materials Science, Nara Institute of Science and Technology, Takayama 8916-5, Ikoma, Nara 630-0101, Japan. Fax: +81-743-72-6029. E-mail address: [email protected] ( K. Hattori)
Ag thickness dependence of the anomaly are less reproducible. This is probably due to different treatments for sample surfaces. For instance, the substrate temperatures for the depositions were either room temperature (RT ) [1,3] or 150–200 K [2]. Therefore, in order to discuss superconductivity, we should first study growth modes and structures of Ag on Ge(001) surfaces. Silver-deposited Ge(001) surfaces in ultrahigh vacuum ( UHV ) have been investigated using lowenergy electron diffraction (LEED) [2,4], highenergy electron diffraction (HEED) [5], Auger electron spectroscopy (AES) [2,4,5], photoemission spectroscopy (PES) [5], and STM [3]. There is a discrepancy in the initial growth mode at RT among authors: Lince et al. [4] essentially suggested a first intermediate layer formation followed by island growth, known as Stranski–Krastanov (S–K ) growth, while results obtained by Miller et al. [5] seem to imply three-dimensional (3D)
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island growth, which is known as Volmer–Weber ( V–W ) growth. Iraji-zad and Hardiman [2] suggested that an intermediate layer appears on surfaces deposited at 150–200 K followed by annealing to RT. At higher coverages of Ag, both of Lince et al. and Miller et al. demonstrated the formation of Ag(011) islands on Ge(001) surfaces [4,5]. These studies were performed using HEED, LEED, AES, and PES. Compared with these macroscopically averaged methods, studies with STM should be useful to understand the surface growth and structures. Using STM, Hattori et al. [3] recently observed elongated Ag islands grown at RT for a sample with high Ag coverage. However, there has been no STM study for surfaces with low Ag coverage. In this paper, we present STM results for initial Ag growth on Ge(001) surfaces at RT. We observed the growth of islands with a height of 0.18 nm and a width of three dimer-row separations, which cannot be detected by standard electron diffraction measurements. We will report the results at low temperatures and different Ag thicknesses in Ref. [6 ].
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clear 2×1 LEED patterns, no contamination of carbon and oxygen in AES, and, in addition, dimer and dimer-row images by STM. The base pressure of the chamber was lower than 1×10−8 Pa. In the chamber, Ag metal (99.999%) was deposited onto the clean Ge(001)-2×1 surfaces at various substrate temperatures T using a Ta crucible. In this s report, we will focus on the deposition at T =RT except in the case of Fig. 4. Deposition s rates and silver thicknesses were monitored by a quartz crystal microbalance. The deposition rate was about 6 × 10−3 nm s−1. Here, we define a unit of monolayer (ML) as 0.144 nm in thickness and 8.47×1014 atoms cm−2 in surface density of the bulk Ag(011) plane1. After the deposition, we observed the surfaces using STM at RT in a constant current mode (0.1 nA). The lateral and vertical scales of the scanning region were calibrated using well-known constants of Ge(001) surfaces: the distance between Ge dimers a is 0 0.400 nm, the distance between Ge dimer rows a 1 is 0.800 nm, and the single step height is 0.142 nm.
3. Results 2. Experimental Sample preparations and observation were performed in a UHV system consisting of a first-entry chamber, a gas treatment chamber with an ion gun ( ULVAC-PHI, USG-3), and an analysis chamber. The analysis chamber was equipped with an evaporator, LEED/AES optics (OCI, BDL-600), and a scanning tunneling microscope (OMICRON, MICRO STM head). A mechanically polished n-type Ge(001) wafer (Sb-doped, 0.2–0.4 V cm, 0.4 mm in thickness) was cut into pieces with a size of 4 mm×10 mm. The sample was rinsed in deionized water before being mounted on a sample holder. The sample was introduced into the first-entry chamber and then transferred to the gas treatment chamber, whose base pressure was less than 1×10−8 Pa. Here, Ge(001) surfaces were cleaned by several repetitions of Ar ion bombardment (500 eV, 1.5 mA cm−2, 10 min) and direct current annealing (700–830°C, 10 min). After the cleaning and transference to the analysis chamber, we confirmed
Fig. 1 shows a typical image of Ag/Ge(001) surfaces of 0.12 nm in average thickness. The image size is 70 nm×70 nm, and the sample bias voltage V is +1.0 V. On the surface, we can see s a step (a short arrow) and two domains of bare Ge terraces with the dimer rows of [11: 0] and [110] directions ( long arrows). Moreover, it can be observed that deposited Ag atoms form two types of islands on the substrate, their heights being high and low. For instance, the island labeled A is high, whereas B is low. Fig. 2 indicates a cross-section of islands A and B. This figure is obtained by averaging cross-sections parallel to the solid line ( Fig. 1) using a magnified image at V =+3.0 V. s The height of island A is 0.85 nm, equivalent to 6 ML of Ag(011) plane and that of B was 0.18 nm. In the present paper, A-type islands and B-type islands are identified as 3D and 2D, respectively. 1 Note that Refs. [2,5] present data in terms of the Ag(011)plane ML, while Refs. [1,4] present data in terms of the Ag(001) ML.
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Fig. 1. An STM image of Ag deposited on Ge(001) surface at RT. The average thickness of Ag was 0.12 nm. The scanning size is 70 nm×70 nm (V =+1.0 V ). Labels A and B indicate s 3D and two-dimensional (2D) Ag islands, respectively. We can see Ge substrate dimer rows between islands. The solid line is one of the cross-section lines for islands A and B.
Fig. 3. A wide STM image of 200 nm×200 nm in size including Fig. 1 (V =+1.0 V ). Many 3D Ag islands can be seen. A round s peak such as the island labeled C at the center is sometimes observed even on clean Ge surfaces.
In Fig. 1, the following features can be observed on the islands. (1) The shape of the 2D islands is elongated in the dimer-row direction, and their width corresponds to about three dimer-rows, 3a . (2) Most of the 3D islands are anisotropic in 1 shape, and some of them have rectangular shapes with principal axes in the dimer-row direction. These can be confirmed in wider images as well. Fig. 3 shows a 200 nm×200 nm image including islands A and B. The short arrows indicate steps, and the long arrows show dimer-row directions on each terrace. A round peak C at the center of Fig. 3 should be noted. Since such kinds of peaks surrounded by multi-steps were sometimes observed even on clean Ge surfaces2, it is clear that the peak was not caused by the Ag deposition but existed on the original substrate. Under these deposition conditions, 2D islands are located on terraces, while 3D islands are located both on terraces and at step edges. The Fig. 2. A cross-section of the 3D island A and the 2D island B in Fig. 1. Between these islands, the corrugation of the Ge dimer substrate can be observed.
2 We consider that these peaks are caused by impurities such as carbon, which are not completely removed by usual cleaning procedures and are undetectable by AES.
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Fig. 4. (a) A high-resolution image of a 2D Ag island grown on Ge(001) at #−50°C, and (b) a schematic interpretation of the STM image. The image size is 14 nm×5.5 nm (V =+2.0 V ). The grid in (b) is drawn guided by c(4×2) s zigzag chains of Ge asymmetric dimers in (a). Dark gray, light gray, and open circles in (b) correspond to visible Ge asymmetric dimers, invisible dimers, and protrusions in the 2D Ag island in (a), respectively. Here, dimer rows are labeled by A–F and a =0.400 nm and a =0.800 nm. 0 1
density of the 2D islands is less than 20% of that of the 3D islands. The heights of the 3D islands are distributed over 0.6–0.9 nm, equivalent to 4– 6 ML of Ag(011). These STM images did not change during long-time scanning over half a day at RT. The LEED image of this surface displayed a clear Ge(001)-2×1 pattern. The Auger peak intensity of Ag (351–356 eV ) was less than 15% of that of Ge (52 eV ). Fig. 4a displays a high-resolution image of a typical 2D island on an Ag/Ge(001) surface deposited at T #−50°C. The image size is s 14 nm×5.5 nm, and V =+2.0 V. Although the s growth temperature is different from previous ones, the atomic structure is essentially the same. When T is lower than RT, the density of 2D s islands increases, and that of 3D islands decreases. The details of T dependence are reported elses where [6 ].
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Around the 2D island, we notice that Ge dimer rows form a c(4×2) zigzag structure. This structure is often observed near pinning sites, e.g. defects or impurities on Ge(001) surfaces, even at RT [7,8]. Guided by these zigzag chains, we can make a grid of asymmetric dimer positions. Fig. 4b shows a schematic interpretation of the STM image using the grid. Here, dark gray circles and light gray circles present a visible and an invisible protrusion at the dimer sites, respectively, and open circles point to bright protrusions on the 2D island. Bright protrusions on dimer rows B and F just on both sides of the 2D island can be observed. Fig. 4b indicates that the asymmetry of the dimers is enhanced alternately toward the 2D island. We observe a slight enhancement of the asymmetry on the second neighbor dimer row from the island (dimer row A). A similar enhanced asymmetry appears on dimer row F in the bottom left region of the image, which belongs to another 2D island. We can easily recognize the approximately 3a 1 width of the 2D island corresponding to dimer rows C, D, and E. In the 2D island, stripes perpendicular to the dimer-row direction can be seen. The stripes are indicated by open elliptic circles in Fig. 4b. The period of the stripes is a couple of dimer–dimer distances toward dimer-row direction, 2a . 0 However, this period is partially disordered, as indicated by arrows in Fig. 4a. The length of this 2D island is approximately 20a . 0 We studied the bias voltage dependence of these features of 2D islands. Fig. 5 demonstrates the same 2D island (island B in Fig. 1) obtained with V =+3.0 V and +0.5 V, respectively. The image s size is 25 nm × 10 nm. Fig. 5a, with a high positive bias voltage, also shows (i) an enhancement of the asymmetry of the dimers nearby the 2D island3, (ii) periodic stripes on the island, and (iii) partial disorder of or absence of the stripes (arrows). In contrast, Fig. 5b, with a low positive bias voltage, reveals no enhancement of the asymmetry and 3 The enhancement of the asymmetry of the dimers at the upper side of the island is imaged more strongly than that at the lower side. This is an artifact caused by conditions of scanning directions and the shape of the STM tip apex. Essentially, the enhancement exists at both sides, as shown in Fig. 4.
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Fig. 5. Sample bias dependence of STM images for a 2D Ag island (island B in Fig. 1): (a) V =+3.0 V and (b) s V =+0.5 V, I =0.1 nA. The image size is 25 nm×10 nm. s t Stripes in (a) and atomic modulation in (b) can be seen on the 2D island. The insets in (a) and (b) show schematic views of the stripes and schematic contours of the 2D island, respectively. Arrows indicate a partial disorder of the stripes and constrictions of the Ag island.
normal dimer rows. Moreover, there is slight modulation in the atomic scale on the island instead of stripes. At the low bias voltage, constrictions of the island are also observed at the positions where the stripes are disordered or absent at the high bias voltage. The width observed at the low bias is slightly greater and the length shorter than those at the high bias voltage. The length of this island is approximately 50a . 0 4. Discussion At T =RT, it is clear that most silver grows s three-dimensionally on the Ge-dimer substrate without any uniform intermediated Ag layer, just as in the V–W growth mode. Once the 3D islands are formed at RT, shapes and positions of the islands are not changed over half a day of scanning.
This stability indicates that Ag islands neither dissociate nor diffuse at RT. So far, at least two groups have studied the growth mode of Ag on Ge(001) surfaces at RT [4,5]. Lince et al. suggested an S–K growth at RT from LEED and AES measurements [4]. This finding is inconsistent with our results. We believe that the discrepancy may arise from non-equivalent thickness in spite of the same represented thickness. Their LEED patterns and the Auger peak ratios are different from ours at the same thickness. Our calibration for Ag thickness is also consistent with the STM results; the total volume of Ag islands in the images with 0.12 nm in thickness corresponds to that of 0.8 ML of a flat Ag(011) plane. Moreover, it would seem that their result in Fig. 1 of Ref. [4], namely the Auger peak ratio I(Ge)/I(Ag) as a function of the coverage, does not have an obvious break point. Thus, it would be difficult to suggest S–K growth at the initial stage from this result. Miller et al. studied Ag/Ge(001) surfaces using HEED and PES [5]. Their HEED patterns displayed bright spots from the Ge substrate and, in addition, additional streaks corresponding to Ag(011) 3D nucleation at about 0.05 nm coverage. At about 0.1 nm, half-order streaks of Ge substrates disappeared completely. The former result is consistent with the V–W growth we observed. However, the latter is inconsistent with our results. This could also be explained by the difference in the calibration of the thickness. We observed that the shape of 3D islands was sometimes rectangular elongated toward the dimer-row directions. The anisotropic shape can be induced by the anisotropic arrangement of substrate Ge atoms. Such kinds of 3D rectangular islands at 1 nm in average thickness were also observed using STM [3]. The size and the density of the 3D islands were larger than those of the present result because of high Ag coverage. At such a high coverage, the LEED [4] and HEED [5] results have indicated 3D growth of Ag islands with (011) plane. In particular, the latter has suggested elongated islands in the Ag[01: 1] direction. Therefore, we consider that the 3D islands in Figs. 1 and 3 are Ag(011) islands, and that the long axes of some islands correspond to the Ag[01: 1] direction.
K. Kushida et al. / Surface Science 442 (1999) 300–306
The 2D islands have some common features, e.g. the enhancement of the asymmetry on dimer rows B and F in Fig. 4a. The enhancement is also slightly seen even on the second neighbor dimer row, namely dimer row A in Fig. 4a, and is observed only at the high positive bias but not at the low positive bias ( Fig. 5). It is natural to consider that these enhanced dimer sites are Ge substrate atoms, not the substituted Ag atoms. The enhancement is probably caused by the charge transfer from the 2D island to the Ge dimers and/or by further buckling of the Ge dimers. In contrast, the bright protrusions at the 2D island on dimer rows C, D, and E in Fig. 4 are clearly made of deposited Ag atoms. The images at various T show that 2D Ag islands are stable s and approximately 3a in width [6 ]. For length, 1 the order of the stripes implies that Ag atoms on the 2D island form a periodic lattice with 2a 0 toward the dimer-row direction. Thus, the interaction between Ag atoms and Ge substrate dominates the periodicity inside the 2D island in principle. The periodicity is not always perfect and has defects in this direction. Actually, partial disorder of the stripes or constrictions of the width are found in the middle of the 2D islands in Figs. 4 and 5. We consider that the 2D island experiences stress in the dimer-row periodic direction, resulting in the disorder of the periodicity. The bias voltage dependence ( Fig. 5) indicates the energy dependence of the electronic structures of 2D Ag islands: the stripe images at high bias voltages and slight atomic modulation at low bias voltages suggest extended empty states along the dimer direction and localized empty states, respectively. As described in Ref. [6 ], the density of the 2D island increases with decreasing T from RT. s Moreover, once the 2D island is formed at low T , it is stable and does not to change to a 3D s island after annealing to RT. On the basis of all the above results, we consider that the Ag islands do not grow under thermal equilibrium conditions which is assumed as the usual explanation of island growth. Instead, the growth mechanism is essentially non-equilibrium and accompanied with dissipation of the thermal energy of Ag atoms on the surface, as follows. First, in the case of low T , the s evaporated Ag atom with finite kinetic energy
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collides on the substrate and is trapped in the surface potential. At this stage, the kinetic energy of Ag atoms is higher than the local potential barrier on the substrate, and Ag atoms hop or diffuse on the substrate with loss of energy. Finally, when the kinetic energy becomes lower than the barrier along the surface, the Ag atom is trapped at a site neighboring a 2D island not a site on a bare terrace at low T , resulting in an extension of s the 2D island. Since the barrier is higher than RT thermal energy, the 2D island structure is stable after annealing to RT. However, in the case of T =RT, the diffused Ag atom is not only trapped s at the 2D island neighbor but also climbs up the 2D island over the barrier assisted by phonons before dissipating its kinetic energy completely, resulting in the 3D island formation. Superconductivity has been suggested in the Ag/Ge(001) system [1–3]. The average Ag thickness in these studies (≥0.5 nm) is higher than that in our experiment (0.12 nm). Therefore, we can expect that the surface is covered mainly by 3D islands with a small area of 2D islands in the previous studies. Since 3D Ag normal-metal islands or films have a tendency to destroy the superconducting proximately, a candidate for superconductivity results from the interfaces between Ge(001) substrates and thin Ag islands. Actually, the observations by low-temperature scanning tunneling spectroscopy suggest the superconducting behavior occurs at the surface area with thin Ag coverage [4], such as the area covered by 2D islands. In contrast, resistance measurements have indicated non-reproducible critical temperatures of the resistance anomaly [1,2]. The scattered critical temperatures would arise from different electric paths through the high density of 3D Ag normal-metal islands and the low density of superconducting thin Ag islands. This underscores the importance of the study of the detailed electronic structures of 2D Ag islands. Previous LEED and HEED studies cannot provide structural information on 2D islands because the density of 2D islands is low at T =RT. Irajis zad and Hardiman [2] have reported the LEED patterns of Ag/Ge(001) surfaces prepared by Ag deposition at T =150–200 K, and a streaky two s domain Ag(011) LEED pattern was obtained at
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RT. However, the Ag thickness in the experiments was several monolayers, and it is expected that 3D islands will form predominantly after annealing to RT. Thus, this study did not provide information on 2D islands either.
atomically modulated patterns at the low bias voltage. The images indicate the enhancement of asymmetry at Ge dimer sites along both sides of the 2D islands. This also depends on bias voltages. We suggest that the enhancement is caused by a charge transfer to or from 2D islands and/or further relaxation of the asymmetric dimers.
5. Conclusions We have observed the initial stages of Ag deposition on Ge(001) surfaces at T =RT using STM. s At an average of approximately 0.8 ML coverage, 3D Ag islands are predominantly formed on the bare Ge substrate. This corresponds to the V–W growth mode. The shape of most 3D islands is anisotropic and rectangular toward Ge dimer-row directions. From the results of previous electron diffraction measurements, we consider 3D islands to have an Ag(011) plane. The interaction between Ag and Ge weaker than RT would result in diffusion of Ag on Ge substrates and the formation of 3D Ag islands. At the same time, we found 2D islands with a low density. The shape of these 2D islands is also rectangular and elongated toward the Ge dimerrow direction. The width of the 2D islands is approximately 3a . The periodicity toward the long 1 axis is 2a , and the length is typically a few tens 0 or several tens a . The interaction between Ag 0 atoms and Ge substrate dominates the periodicity. The stress in the 2D island could induce the partial disorder of the periodicity. The 2D islands show a stripe pattern at high positive bias voltages but
Acknowledgements The authors thank H. Daimon for his stimulating discussions. This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, and Culture of Japan.
References [1] M.J. Burns, J.R. Lince, R.S. Williams, P.M. Chaikin, Solid State Commun. 51 (1984) 865. [2] A. Iraji-zad, M. Hardiman, Solid State Commun. 83 (1992) 467. [3] K. Hattori, Y. Takahashi, T. Iimori, F. Komori, Surf. Sci. 357–358 (1996) 361. [4] J.R. Lince, J.G. Nelson, R.S. Williams, J. Vac. Sci. Technol. B 1 (1983) 553. [5] T. Miller, E. Rosenwinkel, T.-C. Chiang, Phys. Rev. B 30 (1984) 570. [6 ] F. Komori, K. Kushida, K. Hattori, S. Arai, T. Iimori, Surf. Sci. 438 (1999) 123. [7] J. Kubby, J.E. Griffith, R.S. Becker, J.S. Vickers, Phys. Rev. B 36 (1987) 6079. [8] S.J. Murray, M. Mian, F.M. Leibsle, R. McGrath, Surf. Sci 307–309 (1994) 741.
Initial stage of Ag growth on Ge(001) surfaces at room temperature K. Kushida, K. Hattori *, S. Arai, T. Iimori, F. Komori The Institute for Solid State Physics, The University of Tokyo, Roppongi 7-22-1, Minato-ku, Tokyo 106-8666, Japan Received 4 November 1998; accepted for publication 3 September 1999
Abstract Using scanning tunneling microscopy, we studied Ge(001) surfaces covered with an average of approximately 0.8 monolayers of silver. In the deposition at room temperature, we observed that Ag predominantly grew in a threedimensional (3D) mode on bare Ge substrates, corresponding to Volmer–Weber growth. At the same time, we found two-dimensional (2D) Ag islands elongated in the dimer-row direction with one monolayer height, though the density of the 2D islands was smaller than one-fifth of that of the 3D islands. Images with atomic resolution showed stripe patterns on the 2D islands and enhancement of asymmetry of the Ge dimers on both sides of the islands at high positive bias voltages. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Germanium; Nucleation; Scanning tunneling microscopy; Silver; Superconductivity; Surface structure
1. Introduction In the past two decades, there has been a growing interest in the system of silver–germanium interfaces such as Ge(001) surfaces deposited with Ag because of the anomaly of electric conductivity at low temperatures [1–3]. Superconducting transitions have been suggested for the Ag/Ge(001) system from results of electric resistance measurements [1,2] and low-temperature scanning tunneling spectroscopy (STM ) [3] although neither Ge nor Ag bulk solids are superconducting or mix with each other. However, such a unique superconductivity at the interface has not been evidenced because the observed critical temperatures of or * Corresponding author. Present address: Graduate School of Materials Science, Nara Institute of Science and Technology, Takayama 8916-5, Ikoma, Nara 630-0101, Japan. Fax: +81-743-72-6029. E-mail address: [email protected] ( K. Hattori)
Ag thickness dependence of the anomaly are less reproducible. This is probably due to different treatments for sample surfaces. For instance, the substrate temperatures for the depositions were either room temperature (RT ) [1,3] or 150–200 K [2]. Therefore, in order to discuss superconductivity, we should first study growth modes and structures of Ag on Ge(001) surfaces. Silver-deposited Ge(001) surfaces in ultrahigh vacuum ( UHV ) have been investigated using lowenergy electron diffraction (LEED) [2,4], highenergy electron diffraction (HEED) [5], Auger electron spectroscopy (AES) [2,4,5], photoemission spectroscopy (PES) [5], and STM [3]. There is a discrepancy in the initial growth mode at RT among authors: Lince et al. [4] essentially suggested a first intermediate layer formation followed by island growth, known as Stranski–Krastanov (S–K ) growth, while results obtained by Miller et al. [5] seem to imply three-dimensional (3D)
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island growth, which is known as Volmer–Weber ( V–W ) growth. Iraji-zad and Hardiman [2] suggested that an intermediate layer appears on surfaces deposited at 150–200 K followed by annealing to RT. At higher coverages of Ag, both of Lince et al. and Miller et al. demonstrated the formation of Ag(011) islands on Ge(001) surfaces [4,5]. These studies were performed using HEED, LEED, AES, and PES. Compared with these macroscopically averaged methods, studies with STM should be useful to understand the surface growth and structures. Using STM, Hattori et al. [3] recently observed elongated Ag islands grown at RT for a sample with high Ag coverage. However, there has been no STM study for surfaces with low Ag coverage. In this paper, we present STM results for initial Ag growth on Ge(001) surfaces at RT. We observed the growth of islands with a height of 0.18 nm and a width of three dimer-row separations, which cannot be detected by standard electron diffraction measurements. We will report the results at low temperatures and different Ag thicknesses in Ref. [6 ].
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clear 2×1 LEED patterns, no contamination of carbon and oxygen in AES, and, in addition, dimer and dimer-row images by STM. The base pressure of the chamber was lower than 1×10−8 Pa. In the chamber, Ag metal (99.999%) was deposited onto the clean Ge(001)-2×1 surfaces at various substrate temperatures T using a Ta crucible. In this s report, we will focus on the deposition at T =RT except in the case of Fig. 4. Deposition s rates and silver thicknesses were monitored by a quartz crystal microbalance. The deposition rate was about 6 × 10−3 nm s−1. Here, we define a unit of monolayer (ML) as 0.144 nm in thickness and 8.47×1014 atoms cm−2 in surface density of the bulk Ag(011) plane1. After the deposition, we observed the surfaces using STM at RT in a constant current mode (0.1 nA). The lateral and vertical scales of the scanning region were calibrated using well-known constants of Ge(001) surfaces: the distance between Ge dimers a is 0 0.400 nm, the distance between Ge dimer rows a 1 is 0.800 nm, and the single step height is 0.142 nm.
3. Results 2. Experimental Sample preparations and observation were performed in a UHV system consisting of a first-entry chamber, a gas treatment chamber with an ion gun ( ULVAC-PHI, USG-3), and an analysis chamber. The analysis chamber was equipped with an evaporator, LEED/AES optics (OCI, BDL-600), and a scanning tunneling microscope (OMICRON, MICRO STM head). A mechanically polished n-type Ge(001) wafer (Sb-doped, 0.2–0.4 V cm, 0.4 mm in thickness) was cut into pieces with a size of 4 mm×10 mm. The sample was rinsed in deionized water before being mounted on a sample holder. The sample was introduced into the first-entry chamber and then transferred to the gas treatment chamber, whose base pressure was less than 1×10−8 Pa. Here, Ge(001) surfaces were cleaned by several repetitions of Ar ion bombardment (500 eV, 1.5 mA cm−2, 10 min) and direct current annealing (700–830°C, 10 min). After the cleaning and transference to the analysis chamber, we confirmed
Fig. 1 shows a typical image of Ag/Ge(001) surfaces of 0.12 nm in average thickness. The image size is 70 nm×70 nm, and the sample bias voltage V is +1.0 V. On the surface, we can see s a step (a short arrow) and two domains of bare Ge terraces with the dimer rows of [11: 0] and [110] directions ( long arrows). Moreover, it can be observed that deposited Ag atoms form two types of islands on the substrate, their heights being high and low. For instance, the island labeled A is high, whereas B is low. Fig. 2 indicates a cross-section of islands A and B. This figure is obtained by averaging cross-sections parallel to the solid line ( Fig. 1) using a magnified image at V =+3.0 V. s The height of island A is 0.85 nm, equivalent to 6 ML of Ag(011) plane and that of B was 0.18 nm. In the present paper, A-type islands and B-type islands are identified as 3D and 2D, respectively. 1 Note that Refs. [2,5] present data in terms of the Ag(011)plane ML, while Refs. [1,4] present data in terms of the Ag(001) ML.
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Fig. 1. An STM image of Ag deposited on Ge(001) surface at RT. The average thickness of Ag was 0.12 nm. The scanning size is 70 nm×70 nm (V =+1.0 V ). Labels A and B indicate s 3D and two-dimensional (2D) Ag islands, respectively. We can see Ge substrate dimer rows between islands. The solid line is one of the cross-section lines for islands A and B.
Fig. 3. A wide STM image of 200 nm×200 nm in size including Fig. 1 (V =+1.0 V ). Many 3D Ag islands can be seen. A round s peak such as the island labeled C at the center is sometimes observed even on clean Ge surfaces.
In Fig. 1, the following features can be observed on the islands. (1) The shape of the 2D islands is elongated in the dimer-row direction, and their width corresponds to about three dimer-rows, 3a . (2) Most of the 3D islands are anisotropic in 1 shape, and some of them have rectangular shapes with principal axes in the dimer-row direction. These can be confirmed in wider images as well. Fig. 3 shows a 200 nm×200 nm image including islands A and B. The short arrows indicate steps, and the long arrows show dimer-row directions on each terrace. A round peak C at the center of Fig. 3 should be noted. Since such kinds of peaks surrounded by multi-steps were sometimes observed even on clean Ge surfaces2, it is clear that the peak was not caused by the Ag deposition but existed on the original substrate. Under these deposition conditions, 2D islands are located on terraces, while 3D islands are located both on terraces and at step edges. The Fig. 2. A cross-section of the 3D island A and the 2D island B in Fig. 1. Between these islands, the corrugation of the Ge dimer substrate can be observed.
2 We consider that these peaks are caused by impurities such as carbon, which are not completely removed by usual cleaning procedures and are undetectable by AES.
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Fig. 4. (a) A high-resolution image of a 2D Ag island grown on Ge(001) at #−50°C, and (b) a schematic interpretation of the STM image. The image size is 14 nm×5.5 nm (V =+2.0 V ). The grid in (b) is drawn guided by c(4×2) s zigzag chains of Ge asymmetric dimers in (a). Dark gray, light gray, and open circles in (b) correspond to visible Ge asymmetric dimers, invisible dimers, and protrusions in the 2D Ag island in (a), respectively. Here, dimer rows are labeled by A–F and a =0.400 nm and a =0.800 nm. 0 1
density of the 2D islands is less than 20% of that of the 3D islands. The heights of the 3D islands are distributed over 0.6–0.9 nm, equivalent to 4– 6 ML of Ag(011). These STM images did not change during long-time scanning over half a day at RT. The LEED image of this surface displayed a clear Ge(001)-2×1 pattern. The Auger peak intensity of Ag (351–356 eV ) was less than 15% of that of Ge (52 eV ). Fig. 4a displays a high-resolution image of a typical 2D island on an Ag/Ge(001) surface deposited at T #−50°C. The image size is s 14 nm×5.5 nm, and V =+2.0 V. Although the s growth temperature is different from previous ones, the atomic structure is essentially the same. When T is lower than RT, the density of 2D s islands increases, and that of 3D islands decreases. The details of T dependence are reported elses where [6 ].
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Around the 2D island, we notice that Ge dimer rows form a c(4×2) zigzag structure. This structure is often observed near pinning sites, e.g. defects or impurities on Ge(001) surfaces, even at RT [7,8]. Guided by these zigzag chains, we can make a grid of asymmetric dimer positions. Fig. 4b shows a schematic interpretation of the STM image using the grid. Here, dark gray circles and light gray circles present a visible and an invisible protrusion at the dimer sites, respectively, and open circles point to bright protrusions on the 2D island. Bright protrusions on dimer rows B and F just on both sides of the 2D island can be observed. Fig. 4b indicates that the asymmetry of the dimers is enhanced alternately toward the 2D island. We observe a slight enhancement of the asymmetry on the second neighbor dimer row from the island (dimer row A). A similar enhanced asymmetry appears on dimer row F in the bottom left region of the image, which belongs to another 2D island. We can easily recognize the approximately 3a 1 width of the 2D island corresponding to dimer rows C, D, and E. In the 2D island, stripes perpendicular to the dimer-row direction can be seen. The stripes are indicated by open elliptic circles in Fig. 4b. The period of the stripes is a couple of dimer–dimer distances toward dimer-row direction, 2a . 0 However, this period is partially disordered, as indicated by arrows in Fig. 4a. The length of this 2D island is approximately 20a . 0 We studied the bias voltage dependence of these features of 2D islands. Fig. 5 demonstrates the same 2D island (island B in Fig. 1) obtained with V =+3.0 V and +0.5 V, respectively. The image s size is 25 nm × 10 nm. Fig. 5a, with a high positive bias voltage, also shows (i) an enhancement of the asymmetry of the dimers nearby the 2D island3, (ii) periodic stripes on the island, and (iii) partial disorder of or absence of the stripes (arrows). In contrast, Fig. 5b, with a low positive bias voltage, reveals no enhancement of the asymmetry and 3 The enhancement of the asymmetry of the dimers at the upper side of the island is imaged more strongly than that at the lower side. This is an artifact caused by conditions of scanning directions and the shape of the STM tip apex. Essentially, the enhancement exists at both sides, as shown in Fig. 4.
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Fig. 5. Sample bias dependence of STM images for a 2D Ag island (island B in Fig. 1): (a) V =+3.0 V and (b) s V =+0.5 V, I =0.1 nA. The image size is 25 nm×10 nm. s t Stripes in (a) and atomic modulation in (b) can be seen on the 2D island. The insets in (a) and (b) show schematic views of the stripes and schematic contours of the 2D island, respectively. Arrows indicate a partial disorder of the stripes and constrictions of the Ag island.
normal dimer rows. Moreover, there is slight modulation in the atomic scale on the island instead of stripes. At the low bias voltage, constrictions of the island are also observed at the positions where the stripes are disordered or absent at the high bias voltage. The width observed at the low bias is slightly greater and the length shorter than those at the high bias voltage. The length of this island is approximately 50a . 0 4. Discussion At T =RT, it is clear that most silver grows s three-dimensionally on the Ge-dimer substrate without any uniform intermediated Ag layer, just as in the V–W growth mode. Once the 3D islands are formed at RT, shapes and positions of the islands are not changed over half a day of scanning.
This stability indicates that Ag islands neither dissociate nor diffuse at RT. So far, at least two groups have studied the growth mode of Ag on Ge(001) surfaces at RT [4,5]. Lince et al. suggested an S–K growth at RT from LEED and AES measurements [4]. This finding is inconsistent with our results. We believe that the discrepancy may arise from non-equivalent thickness in spite of the same represented thickness. Their LEED patterns and the Auger peak ratios are different from ours at the same thickness. Our calibration for Ag thickness is also consistent with the STM results; the total volume of Ag islands in the images with 0.12 nm in thickness corresponds to that of 0.8 ML of a flat Ag(011) plane. Moreover, it would seem that their result in Fig. 1 of Ref. [4], namely the Auger peak ratio I(Ge)/I(Ag) as a function of the coverage, does not have an obvious break point. Thus, it would be difficult to suggest S–K growth at the initial stage from this result. Miller et al. studied Ag/Ge(001) surfaces using HEED and PES [5]. Their HEED patterns displayed bright spots from the Ge substrate and, in addition, additional streaks corresponding to Ag(011) 3D nucleation at about 0.05 nm coverage. At about 0.1 nm, half-order streaks of Ge substrates disappeared completely. The former result is consistent with the V–W growth we observed. However, the latter is inconsistent with our results. This could also be explained by the difference in the calibration of the thickness. We observed that the shape of 3D islands was sometimes rectangular elongated toward the dimer-row directions. The anisotropic shape can be induced by the anisotropic arrangement of substrate Ge atoms. Such kinds of 3D rectangular islands at 1 nm in average thickness were also observed using STM [3]. The size and the density of the 3D islands were larger than those of the present result because of high Ag coverage. At such a high coverage, the LEED [4] and HEED [5] results have indicated 3D growth of Ag islands with (011) plane. In particular, the latter has suggested elongated islands in the Ag[01: 1] direction. Therefore, we consider that the 3D islands in Figs. 1 and 3 are Ag(011) islands, and that the long axes of some islands correspond to the Ag[01: 1] direction.
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The 2D islands have some common features, e.g. the enhancement of the asymmetry on dimer rows B and F in Fig. 4a. The enhancement is also slightly seen even on the second neighbor dimer row, namely dimer row A in Fig. 4a, and is observed only at the high positive bias but not at the low positive bias ( Fig. 5). It is natural to consider that these enhanced dimer sites are Ge substrate atoms, not the substituted Ag atoms. The enhancement is probably caused by the charge transfer from the 2D island to the Ge dimers and/or by further buckling of the Ge dimers. In contrast, the bright protrusions at the 2D island on dimer rows C, D, and E in Fig. 4 are clearly made of deposited Ag atoms. The images at various T show that 2D Ag islands are stable s and approximately 3a in width [6 ]. For length, 1 the order of the stripes implies that Ag atoms on the 2D island form a periodic lattice with 2a 0 toward the dimer-row direction. Thus, the interaction between Ag atoms and Ge substrate dominates the periodicity inside the 2D island in principle. The periodicity is not always perfect and has defects in this direction. Actually, partial disorder of the stripes or constrictions of the width are found in the middle of the 2D islands in Figs. 4 and 5. We consider that the 2D island experiences stress in the dimer-row periodic direction, resulting in the disorder of the periodicity. The bias voltage dependence ( Fig. 5) indicates the energy dependence of the electronic structures of 2D Ag islands: the stripe images at high bias voltages and slight atomic modulation at low bias voltages suggest extended empty states along the dimer direction and localized empty states, respectively. As described in Ref. [6 ], the density of the 2D island increases with decreasing T from RT. s Moreover, once the 2D island is formed at low T , it is stable and does not to change to a 3D s island after annealing to RT. On the basis of all the above results, we consider that the Ag islands do not grow under thermal equilibrium conditions which is assumed as the usual explanation of island growth. Instead, the growth mechanism is essentially non-equilibrium and accompanied with dissipation of the thermal energy of Ag atoms on the surface, as follows. First, in the case of low T , the s evaporated Ag atom with finite kinetic energy
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collides on the substrate and is trapped in the surface potential. At this stage, the kinetic energy of Ag atoms is higher than the local potential barrier on the substrate, and Ag atoms hop or diffuse on the substrate with loss of energy. Finally, when the kinetic energy becomes lower than the barrier along the surface, the Ag atom is trapped at a site neighboring a 2D island not a site on a bare terrace at low T , resulting in an extension of s the 2D island. Since the barrier is higher than RT thermal energy, the 2D island structure is stable after annealing to RT. However, in the case of T =RT, the diffused Ag atom is not only trapped s at the 2D island neighbor but also climbs up the 2D island over the barrier assisted by phonons before dissipating its kinetic energy completely, resulting in the 3D island formation. Superconductivity has been suggested in the Ag/Ge(001) system [1–3]. The average Ag thickness in these studies (≥0.5 nm) is higher than that in our experiment (0.12 nm). Therefore, we can expect that the surface is covered mainly by 3D islands with a small area of 2D islands in the previous studies. Since 3D Ag normal-metal islands or films have a tendency to destroy the superconducting proximately, a candidate for superconductivity results from the interfaces between Ge(001) substrates and thin Ag islands. Actually, the observations by low-temperature scanning tunneling spectroscopy suggest the superconducting behavior occurs at the surface area with thin Ag coverage [4], such as the area covered by 2D islands. In contrast, resistance measurements have indicated non-reproducible critical temperatures of the resistance anomaly [1,2]. The scattered critical temperatures would arise from different electric paths through the high density of 3D Ag normal-metal islands and the low density of superconducting thin Ag islands. This underscores the importance of the study of the detailed electronic structures of 2D Ag islands. Previous LEED and HEED studies cannot provide structural information on 2D islands because the density of 2D islands is low at T =RT. Irajis zad and Hardiman [2] have reported the LEED patterns of Ag/Ge(001) surfaces prepared by Ag deposition at T =150–200 K, and a streaky two s domain Ag(011) LEED pattern was obtained at
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RT. However, the Ag thickness in the experiments was several monolayers, and it is expected that 3D islands will form predominantly after annealing to RT. Thus, this study did not provide information on 2D islands either.
atomically modulated patterns at the low bias voltage. The images indicate the enhancement of asymmetry at Ge dimer sites along both sides of the 2D islands. This also depends on bias voltages. We suggest that the enhancement is caused by a charge transfer to or from 2D islands and/or further relaxation of the asymmetric dimers.
5. Conclusions We have observed the initial stages of Ag deposition on Ge(001) surfaces at T =RT using STM. s At an average of approximately 0.8 ML coverage, 3D Ag islands are predominantly formed on the bare Ge substrate. This corresponds to the V–W growth mode. The shape of most 3D islands is anisotropic and rectangular toward Ge dimer-row directions. From the results of previous electron diffraction measurements, we consider 3D islands to have an Ag(011) plane. The interaction between Ag and Ge weaker than RT would result in diffusion of Ag on Ge substrates and the formation of 3D Ag islands. At the same time, we found 2D islands with a low density. The shape of these 2D islands is also rectangular and elongated toward the Ge dimerrow direction. The width of the 2D islands is approximately 3a . The periodicity toward the long 1 axis is 2a , and the length is typically a few tens 0 or several tens a . The interaction between Ag 0 atoms and Ge substrate dominates the periodicity. The stress in the 2D island could induce the partial disorder of the periodicity. The 2D islands show a stripe pattern at high positive bias voltages but
Acknowledgements The authors thank H. Daimon for his stimulating discussions. This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, and Culture of Japan.
References [1] M.J. Burns, J.R. Lince, R.S. Williams, P.M. Chaikin, Solid State Commun. 51 (1984) 865. [2] A. Iraji-zad, M. Hardiman, Solid State Commun. 83 (1992) 467. [3] K. Hattori, Y. Takahashi, T. Iimori, F. Komori, Surf. Sci. 357–358 (1996) 361. [4] J.R. Lince, J.G. Nelson, R.S. Williams, J. Vac. Sci. Technol. B 1 (1983) 553. [5] T. Miller, E. Rosenwinkel, T.-C. Chiang, Phys. Rev. B 30 (1984) 570. [6 ] F. Komori, K. Kushida, K. Hattori, S. Arai, T. Iimori, Surf. Sci. 438 (1999) 123. [7] J. Kubby, J.E. Griffith, R.S. Becker, J.S. Vickers, Phys. Rev. B 36 (1987) 6079. [8] S.J. Murray, M. Mian, F.M. Leibsle, R. McGrath, Surf. Sci 307–309 (1994) 741.