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Atomic-hydrogen-induced self-organization of Si(111)√3×√3-In surface phase studied by CAICISS and STM
Surface Science 447 (2000) 117–125 www.elsevier.nl/locate/susc
Atomic-hydrogen-induced self-organization of Si(111) 앀3×앀3-In surface phase studied by CAICISS and STM J.T. Ryu a, *, O. Kubo a, T. Fujino a, T. Fuse a, T. Harada a, K. Kawamoto a, M. Katayama a, A.A. Saranin a,b,c, A.V. Zotov a,b,d, K. Oura a a Department of Electronic Engineering, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan b Institute of Automation and Control Processes, 5 Radio Street, 690041 Vladivostok, Russia c Faculty of Physics and Engineering, Far Eastern State University, 690000 Vladivostok, Russia d Department of Electronics, Vladivostok State University of Economics and Service, 690600 Vladivostok, Russia Received 30 July 1999; accepted for publication 8 November 1999
Abstract Using coaxial impact collision ion scattering spectroscopy (CAICISS ), scanning tunneling microscopy (STM ), and low-energy electron diffraction (LEED) techniques, we have investigated the interaction of atomic hydrogen with the Si(111)앀 3×앀3-In surface phase at elevated temperatures and structural behavior of In clusters induced by the interaction. Upon atomic hydrogen interaction, SiMIn bonds are broken and replaced by SiMH bonds. As a result, the 앀3×앀3 reconstruction is destroyed and small In clusters are formed on hydrogen-terminated Si(111)1×1 surface. Using STM, we also have found that the size of the In cluster increases with increasing substrate temperature during hydrogen exposure of the 앀3×앀3-In surface phase. From CAICISS experimental results, we have found that atomichydrogen-induced In clusters for Si(111)앀3×앀3-In surface phase have an In(100) crystalline structure, while those for Si(001)4×3-In surface phase are polycrystalline. In conclusion, we have found that structural differences of surface give rise to different atomic-hydrogen-induced self-organization. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Hydrogen atom; Indium; Ion scattering spectroscopy; Low energy ion scattering (LEIS ); Scanning tunneling microscopy; Silicon; Surface chemical reaction; Surface structure, morphology, roughness, and topography
1. Introduction When adsorbate atoms are deposited on Si substrates, various reconstructed phases are formed [1], depending on both adsorbate coverage and substrate temperature. Among them, the 앀3×앀3 periodicity is representative of the reconstruction phases formed on a Si(111) surface. Although these phases have identical periodicity, however, a great difference in atomic arrangements * Corresponding author. Fax: +81-6-879-7780. E-mail address: [email protected] (J.T. Ryu)
exists. For example, in the case of the Si(111) 앀3×앀3-Ag surface phase [2], a definite amount of Si substrate atoms is incorporated into the surface phase, i.e., 1 ML of Si atoms forms trimers on the bulk Si layer and Ag atoms form honeycomb-chained-trimer structures on the reconstructed Si trimers. In the case of the Si(111) 앀3×앀3-B surface phase [3,4], the adsorbate atoms substitute for Si substrate atoms, and in the case of the Si(111)앀 3×앀3-In surface phase [5,6 ], indium atoms adsorb above the Si atoms in the second layer ( T site) on bulk-like Si substrates 4 where the absolute In coverage is 1/3 ML. In the
0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 11 5 3 -X
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case of the Si(111)앀 3×앀3-Sb surface phase [7,8], Sb atoms form a trimer centered at the T sites on 4 the unreconstructed Si substrate. These structural differences could give rise to different atomichydrogen-induced self-organization phenomena: the three-dimensional metal cluster formation [9– 13] or two-dimensional structural change [14–16 ] due to atomic H interaction. Namely, the interaction of atomic H with adsorbate-induced Si surface phases could cause a peculiar structural transformation, strongly depending on the original surface structure. For instance, as the Si(001)4×3-In surface phase is exposed to atomic H, the uniform In monolayer is transformed into very small polycrystalline In clusters accompanied by a baring of the hydrogenated Si surface with the 4×1 periodicity [17]. However, when the Si(111)앀 3×앀3-Sb surface phase is exposed to atomic H, such atomic-hydrogen-induced metal clustering does not occur on the surface, but only two-dimensional structural change occurs on the surface [14,15]. A similar phenomenon was also observed for the Si(111)앀 3×앀3-Bi surface phase [16 ]. On the other hand, the interaction of atomic H with the Si(111)앀 3×앀3-In surface phase also results in the formation of metal clusters on the Si(111)1×1-H surface. The interaction of atomic H with the surface phase was investigated in detail by Owman and Ma˚rtensson [18] using scanning tunneling microscopy (STM ). They reported that: (1) the 앀3×앀3-In surface phase becomes disordered and small In clusters are formed even with very low H exposure (20 L); (2) increasing H exposure results in the formation of two-dimensional islands exhibiting 2×2, 4×1 and 앀7×앀3 reconstructions; and (3) a further increase in H exposure results in the formation of three-dimensional In islands. Further, they observed a loss of In atoms from the surface during H exposure, which increased with increasing H exposure and substrate temperature. However, they did not report on the structural quality and behavior of In clusters formed by atomic H exposure, which is important for understanding the atomichydrogen-induced self-organization phenomena. As mentioned above, metal clusters formed by
atomic H interaction with Si(111)앀 3×앀3-Ag and -Pb surface phases are monocrystallines [9,11,12], but in the case of Si(001)4×3-In surface In clusters formed by this interaction are polycrystalline [17]. Using scanning tunneling microscopy (STM ), coaxial impact-collision ion scattering spectroscopy (CAICISS), and low-energy electron diffraction (LEED) techniques, we have studied the interaction of atomic H with the Si(111) 앀3×앀3-In surface phase at elevated temperatures and the structural quality and behavior of In clusters formed by atomic H exposure. In this study, we have found that atomic H interaction results in In metal clustering and Si substrate baring. CAICISS data showed that In clusters formed by the interaction of atomic H with the 앀3×앀3-In surface phase have an In(100) crystalline structure with the orientation relationship of In(100) 011//Si(111) 011: , which is in contrast with atomic-hydrogen-induced In clusters on the Si(001)4×3-In surface phase where the In clusters are polycrystalline [17].
2. Experimental STM experiments were carried out in an ultrahigh vacuum ( UHV ) chamber with a base pressure of 8×10−11 Torr equipped with STM (‘Omicron’) and LEED systems, and a load-lock for introduction of samples and tunneling tips without breaking the vacuum. Electro-chemically etched tungsten tips, cleaned by in situ heating, were used. The Si(111) sample (2×13 mm2) was cut from n-type Si(111) wafers with 0.05 V cm resistivity. The sample was outgassed at 600°C for 12 h in the UHV chamber. An atomically clean silicon surface was prepared in situ by direct Joule flashing for several times at 1250°C. After this treatment, a sharp (7×7) LEED pattern was observed and the STM images exhibited a clean 7×7 surface. CAICISS experiments were carried out in an UHV chamber with a base pressure of 8.0×10−11 Torr. The UHV chamber is connected with a beam line for ICISS of TOF type and timeof-flight elastic recoil detection analyzer ( TOF-
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ERDA), a sample transfer load-lock system, and sub-chamber with a base pressure of 5×10−11 Torr for sample cleaning and annealing. The UHV chamber is also equipped with low energy electron diffraction (LEED) facilities. A He+ beam of 2.0 keV was produced by a differentially pumped ion source. The beam could be chopped by two pairs of electrostatic deflection plates with a chopping aperture of 1 mm diameter, resulting in a pulse width of about 50 ns. After passing through the aperture, the pulsed ion beam impinged on the sample in the UHV chamber. The He particles (ions and neutrals) scattered from sample are detected at a scattering angle of 180° (perfect back scattering) using a TOF analyzer, i.e. the apparatus comprises a coaxial arrangement of a pulse-beam ion source and a detector [19]. Beam current and diameter were about 20 nA and 2.0 mm at the sample surface, respectively. A p-type Si(111) wafer with 1–2 V cm resistivity and dimensions of 25×25 mm2 was used. The sample was cleaned in situ by annealing for about 12 h at 600°C followed by flash heating at 1200°C under a base pressure of 6×10−10 Torr using electron bombardment from behind the sample. After the cleaning, a sharp (7×7) LEED pattern was observed.
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Indium of 99.999% purity was deposited from a Knudsen cell in the CAICISS experiment and a Ta foil tube in the STM experiment. The deposition rate was about 0.1 ML/min (where 1 ML is defined as 7.8×1014 atoms/cm2, the ideal Si surface atomic density in the (111) plane). A Si(111)앀 3×앀3-In surface phase was prepared by depositing of 0.4 ML In onto the Si(111)7×7 surfaces held at 550°C. For atomic hydrogen exposure, H gas was 2 admitted through a leak valve. An 1800°C tungsten filament, 10 cm from the Si surface in the CAICISS experiment and 7 cm in the STM experiment, was used to dissociate molecular hydrogen. The exposures were conducted with the specimen facing the filament and by backfilling the chamber with H 2 at 1×10−7 Torr. Since the arrival rate of atomic hydrogen is unknown, the dose of molecular hydrogen is specified, and expressed in units of Langmuir (1 L=1×10−6 Torr s).
3. Results and discussion Fig. 1a, b shows filled- and empty-state STM ˚ ×300 A ˚ ) of the Si(111)앀 3×앀3-In images (300 A
˚ ×300 A ˚ ) of Si(111)앀3×앀3-In surface phase. Tunneling conditions: Fig. 1. Filled state (a) and empty state (b) STM images (300 A ±2.0 V sample bias and 0.2 nA tunneling current.
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surface. These images were taken with a sample biased at ±2.0 V, and a tunneling current of 0.2 nA. Most of the surface is covered with a centered hexagonal array of protrusions. The distance between nearest-neighbor protrusions is ˚ along the [112: ] and [2: 11] crystalloabout 6.7 A graphic directions, consistent with a 앀3×앀3 reconstruction rotated by 30° with respect to the (1×1) unit cell of the ideal Si substrate. An interesting feature of these images is the presence of defects. In Fig. 1a (filled-state), these defects are much brighter than their neighbor protrusions, but in Fig. 1b (empty-state) these defects appear darker than the surrounding protrusions. These observations indicate the strong effect of electronic structure on the apparent height of these defects. About 14% of the 앀3×앀3 surface is occupied by these defects. Similar defects (so-called type-S ) were observed by Owman and Ma˚rtensson [18] and Hamers [20]. They assigned the defects to substitutional Si atoms. In practice, the unit cell of the original Si(111)7×7 surface contains an excess of 8% of one monolayer Si atoms against an ideal bulk terminated Si surface, i.e., 24% of the number of atoms in a 앀3×앀3 surface. Thus, a part of the excess Si atoms should be merged into steps, but the rest should remain on the 앀3×앀3 surface area. Type-V defects [18,20] attributed to vacancies are also seen in Fig. 1a, which appear darker than the surrounding protrusions both at filled- and empty-state STM images. These STM images in
Fig. 1 are consistent with images in previous STM studies of the 앀3×앀3-In surface [18,21]. Fig. 2a shows a filled-state STM image of a ˚ ×500 A ˚ region of the Si(111)앀 3×앀3-In 500 A surface taken after atomic H exposure (500 L). The sample was held at about 300°C during H exposure. This image was taken with a sample bias of −2.0 V, and a tunneling current of 0.2 nA. For this sample, we observed a sharp and bright (1×1) LEED pattern with low background. As one can see in Fig. 2a, after atomic H exposure, no ordered 앀3×앀3 areas remain and small spherical clusters appear on the surface. One can see from the result of ion scattering experiments presented below that these clusters are built of In atoms. The size of the In clusters is not uniform. The size of large clusters, ˚, occupying about 10% of the surface, is about 60 A while that of small clusters, occupying about 15% ˚ . The underlying Si of the surface, is about 20 A surface is exposed to vacuum because of the removal of In atoms, which is caused by atomic H exposure. Thus, most of the surface, besides the clusters, exhibits the 1×1 structure due to the underlying Si atoms terminated by H atoms as shown in Fig. 2c (the high-resolution image of Fig. 2a). This result is in agreement with the observed 1×1 LEED pattern. Fig. 2b shows a ˚ ×500 A ˚ region filled-state STM image of a 500 A of the Si(111)앀 3×앀3-In surface taken after atomic H exposure (500 L) at about 400°C. Small ˚ clusters with average diameters of about 20 A
˚ ×500 A ˚ ) taken from Si(111)앀3×앀3-In surface phase exposed to 500 L of atomic hydrogen at 300°C (a) Fig. 2. STM images (500 A and 400°C (b), respectively. Tunneling conditions: −2.0 V sample bias and 0.2 nA tunneling current. (c) The enlarged STM image ˚ ×100 A ˚ ) of hydrogen-terminated 1×1-H area shown in (a). (100 A
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distribute randomly on the surface, and these clusters occupy about 10% of the surface. The surroundings of the clusters exhibit also the Hterminated 1×1 structure. As can be seen in Fig. 2b, a two-dimensional hexagonal In island exists on the surface. The size of the island was ˚ with an apparent height found to be about 150 A ˚ of about 12 A. The topmost surface of the twodimensional island comprises rows composed of some protrusions, which have a 4×4 arrangement. Owman and Ma˚rtensson [18] observed two-dimensional islands, exhibiting 2×2, 4×1, and 앀7×앀3 reconstructions after atomic H exposure of the 앀3×앀3-In surface phase, but no In island with 4×4 reconstruction was observed in their experiment. Saranin et al. [22], using STM, observed In islands with 4×4 reconstruction after atomic H exposure of the Si(111)4×1-In surface phase at 300°C. Ion scattering experiments were carried out to investigate the structural quality and behavior of In clusters formed after atomic H exposure of the Si(111)앀 3×앀3-In surface phase. Fig. 3 shows typical CAICISS spectra taken before and after 1000 L H exposure of the Si(111)앀3×앀3-In surface phase at 300°C. The incident energy and the direction of primary ion beam are 2.0 keV and a= 90° (normal to the surface), respectively. The peak at 4730 ns corresponds to the He signal scattered by In atoms, and a peak and a broad signal
Fig. 3. TOF spectra taken from Si(111)앀3×앀3-In surface before and after hydrogen exposure of 1000 L at 300°C. He+ primary ions of 2.0 keV were used. The incident direction is normal to the surface (a=90°).
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originating from single and multiple scattering by substrate Si atoms were also observed at 5220 ns and above 5370 ns (not shown here), respectively. The positions of the peaks due to In and Si atoms are in agreement with those calculated on the basis of kinematics of elastic single scattering between He+ and those atoms. As one can see in Fig. 3, after atomic H exposure, the peak due to In atoms has a small tail on the low-energy (high flight time) side and is slightly broader than that before atomic H exposure, indicating that small In clusters were formed on the surface. This result is in agreement with our present STM observations ( Fig. 2). One can also see in Fig. 3 that the intensity of the In peak increases after atomic H exposure in spite of the desorption of In atoms from the surface during atomic H exposure, as mentioned below. Therefore, the increase of the In scattering intensities should be caused by the slight focusing effect in the ion scattering process due to the structural change of the In layer. At this time, no shadowing effect occurs because the clusters are smaller, as shown in Fig. 2a. If further large islands were formed on the surface, scattering intensity should reduced due to the shadowing effect, and the peak should have a broader tail because of the multiple scattering of incident ions in three-dimensional large clusters, as observed for the H exposure of Si(001)4×3-In surface [17]. To confirm the loss of In atoms by atomic H interaction indicated by Owman and Ma˚rtensson [18], we estimated the amount of In atoms from the surface annealed at 500°C for 10 min after H adsorption of 1000 L on a 4×3-In surface at 300°C. It is well known that H atoms existing on the Si surface desorb at a temperature of 500°C. Therefore, the 앀3×앀3-In surface phase is recovered because of dissociation of In clusters, and the scattering intensity should be reverted. After the annealing, however, the scattering intensity due to In atoms was reduced, as shown in the inset in Fig. 3, and the tail shown after atomic H exposure disappeared, indicating that In atoms form a two-dimensional layer caused by the dissociation of In clusters. The scattering intensity due to In atoms was reduced to about 80% of the original intensity after the annealing, indicating
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that the desorption of In atoms from the Si(111) surface is caused by atomic H interaction. We also found from STM images in Fig. 2a and b that the amount of In islands formed by atomic H exposure at 400°C is 60% with respect to that formed by atomic H exposure at 300°C. This result suggests that a higher temperature during atomic H exposure results in an increased loss of In atoms. These results are in agreement with previous work by Owman and Ma˚rtensson [18]. Such atomic H induced desorption of adsorbate was also observed for the Si(001)2×1-Sb surface phase [23,24]. Fig. 4 shows the CAICISS intensity (azimuth scan) of He scattered from In atoms on the Si(111)앀 3×앀3-In surface phase as a function of azimuth angle w (the origin of w is [1: 1: 2]) for various ion incidence angles a from 8° to 70°, where a is measured from the surface plane. The incident energy of the primary ion beam was 2.0 keV. The variations of scattering intensity observed in Fig. 4 are due to various shadowing and focusing effects, which strongly depend on the structure of the In layer. The intensity curves measured at a=8° and 10° are sensitive to the topmost surface structure, while those for a=20°, 30°, 50°, and 70° reflect the structure of deeper layers. Therefore, we can obtain structural infor-
Fig. 4. Azimuth angle scan: variation of He scattering intensity due to In atoms on the Si(111)앀3×앀3-In surface phase (a) and the surface exposed to atomic hydrogen (1000 L) at 300°C (b). Parameters are azimuth angle w (measured from [1: 1: 2]) and incidence angle a (measured from the surface) of the ion incidence direction. He+ primary ions of 2.0 keV were used.
mation from the intensity variations. Fig. 4a shows azimuth scans of In scattering intensity for the Si(111)앀 3×앀3-In surface phase before atomic H exposure. The drops and enhancements of the scattering intensities for a=8° and 10° reflect the structure of the 앀3×앀3-In surface phase. The intensity curve has a periodicity of 60° as expected. However, the drops and enhancements as well as periodicity of the intensity are absent in the incidence angle ranges from a=20° to 70°. This result indicates that In atoms in the Si(111)앀 3×앀3-In phase compose the topmost two-dimensional layer. Fig. 4b shows azimuth scans of In scattering intensity taken after atomic H exposure (1000 L) of the Si(111)앀 3×앀3-In phase at 300°C. After atomic H exposure, for incidence angle from a= 8° to 10°, the periodicity seen in Fig. 4a disappeared, but intensity curves having a periodicity of 30° newly appeared. Moreover, this feature appears only for an incidence angle of less than a=20°, but is absent for larger incidence angle ranges from a=30° to 70°. These results indicate that the In monolayer with a 앀3×앀3 periodicity is transformed into very small crystalline In agglomerate. Atomic-hydrogen-induced In metal clustering and the structural quality of the clusters can be also confirmed from a-scans taken before and after atomic H exposure of the 앀3×앀3-In surface phase. We measured the intensity variations (ascan) of the surface peak due to In atoms as a function of the incident angle a, along the [1: 1: 2] and [011: ] azimuths. Fig. 5 shows the results for the [1: 1: 2] azimuth measured for two kinds of specimens; (a) the Si(111)앀 3×앀3-In surface without H exposure and (b) the surface after 1000 L atomic H exposure of the surface shown in Fig. 5a at 300°C. As we can see in Fig. 5a, there are two dominant peaks in the a-scan of the 앀3×앀3-In surface. As already discussed in detail, these peaks can be explained in terms of the focusing effects of the In atoms with the 앀3×앀3 periodicity overlying the Si surface. The presence of only two dominant peaks in the a-scan indicates the formation of a uniform In overlayer with no detectable In agglomerate, which is in agreement with the experimental results of the present STM ( Fig. 1)
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Fig. 5. Incidence angle scan: variation of He scattering intensity due to In atoms as a function of incidence angle, a (measured from the surface plane), along the [1: 1: 2] azimuth, measured for two kinds of the specimens; (a) the Si(111)앀3×앀3-In surface without hydrogen exposure and (b) the surface exposed to atomic hydrogen of 1000 L at 300°C.
and azimuth scan (Fig. 4a). On the other hand, when the 앀3×앀3 surface phase was exposed to atomic H at 300°C, quite a different a-scan was observed as shown in Fig. 5b. Although the intensity is small, several peaks are definitely observed. All these peaks can be well explained in terms of the focusing effect due to In atoms in the In agglomerate, which form very small In(100) crystallite grown on the Si(111) surface. In other words, the a-scan results of Fig. 5a and b reveal that the In monolayer with a 앀3×앀3 periodicity is transformed into an In(100) agglomerate by atomic H exposure. As shown in Fig. 5b, however, the intensities have very slight enhancements and dips. This means that these clusters may have poor quality and be very small, which is consistent with our STM results. In bulk has a face-centered tetragonal structure with lattice constants of a= ˚ and c=4.938 A ˚ . As mentioned above, b=4.588 A however, the azimuth scan in Fig. 4b shows that the He intensity changes with a periodicity of 30°. It seems to be difficult to explain this result in terms of In(100) agglomerates with only one orientation type of alignments with relation to a orientation of Si substrate, because In bulk has a fourfold
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Fig. 6. Incidence angle scan: variation of He scattering intensity due to In atoms as a function of incidence angle, a (measured from the surface plane), along the Si [011: ] azimuth. (a) The simulation result with three orientations of In(100) 011 direction; (b) the film grown In 12 ML on Si(111) at room temperature; and (c) the surface exposed to atomic hydrogen of 1000 L at 300°C.
symmetry. This probably means that three direction types of In(100) agglomerates are formed on a direction of Si substrate due to atomic H exposure. Thus, we considered three orientation types of In(100) agglomerates with relation to Si 011 direction: the simulation result is the summation of the three orientation rotated by 0°, 120°, and 240° relative to the In bulk [011] azimuth. The result is showed in Fig. 6a. The computer simulation results based on ‘‘two atoms approximation’’ technique introduced by Williams et al. [25]. Fig. 6c shows the incident angle scan for Si [011: ] azimuth after 1000 L atomic H exposure at 300°C. The simulation result with the three orientations corresponds well to the experimental results. At lower angles, however, the simulation result was in slight disagreement on the In signal intensity, because the formed In agglomerates is poor quality and very small. These results mean that the orientational relationship between the In agglomerate and the substrate is In(100) 011//Si(111) 011: . We have also found that In films with the same orientation grow on H-terminated Si(111) surface as well
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as on clean Si(111) surface. Fig. 6b shows an incident angle scan of the film grown 12 ML In on Si(111) surface at room temperature. On the other hand, as mentioned above, the 2D island formed after atomic H exposure at 400°C (Fig. 2b) has a hexagonal shape and protrusions exhibit a 4×4 periodicity on the surface of the island. It seems to be difficult to explain these results in terms of In(100) agglomerates because of their fourfold symmetry. Therefore, we infer that the 2D island probably has a structure different from those of the small In clusters formed at 300°C in Figs. 2a and 5b. In a previous work for atomic H exposure of the Si(001)4×3-In surface phase [17], we found that the atomic-hydrogen-induced In clusters were not monocrystalline, but rather polycrystalline. However, in the present work for atomic H exposure of the Si(111)앀 3×앀3-In surface phase, In clusters were found to be monocrystalline. The reason why such differences occur in spite of the same element is not clear as yet, but the explanation could be the different effects of H termination on In thin film growth for both surfaces. We investigated the effect of H termination on the crystallinity of the initial In thin film growth on both Si(001) and Si(111) surfaces [26,27]. In the In/Si(111) experiment, we found that H termination did not have an important effect on the crystallinity of In thin film. For the In/Si(001) experiment, however, H termination had a serious effect on the crystallinity of In thin film, i.e., epitaxial growth of In thin film on the Si(001) surface was disrupted by the presence of H atoms residing at the interface between the films and the Si substrate. Thus, we inferred that the disturbance of epitaxial growth is caused by the suppression of surface diffusion and the loss of preferable adsorption sites by H termination [26,27]. These results suggest that H termination on both Si(111) and Si(001) surfaces would cause the different surface diffusion of In atoms on these surfaces. Therefore, we infer that the difference of the crystallinity between In clusters induced by the interaction of atomic H with Si(001)4×3-In and Si(111)앀 3×앀3-In surface phases is caused by the different surface diffusion of In atoms on the
H-terminated Si surfaces. Another reason could be the structural difference of Si substrates. No Si atoms in the 앀3×앀3-In phase are reconstructed, a bulk-like structure of 1×1, while those in the 4×3-In phase are reconstructed into a 4×1-Si structure with missing dimer rows [17,28]. Thus, when these surfaces were exposed by atomic H, the Si substrate in the 앀3×앀3-In phase formed a relatively flat H-terminated 1×1-H structure and that in the 4×3-In phase formed a 4×1-H structure with monohydride Si dimers and dihydride Si atoms [28]. This implies that Si(111)앀 3×앀3-In phase forms more atomic flat H-terminated surfaces (1×1-H ) than Si(001)4×3-In phase (4×1-H ). Therefore, these different substrate structures would cause different In cluster formation. On the other hand, it is interesting that where H atom exist. Fukutani et al. [29] reported that hydrogen atoms in the Pb/H/Si system exist at the interface, while hydrogen atoms in the Ag/H/Si system do not exist at the interface. Unfortunately, it is impossible to confirm a location of H atom from our experiment.
4. Conclusion We have used STM, CAICISS, and LEED to study the interaction of atomic H with a Si(111) 앀3×앀3-In surface phase and the structural quality and behavior of In clusters formed by atomic H interaction. A schematic illustration of the structural transformation of the Si(111)앀 3×앀3-In surface phase induced by the atomic H interaction at 300°C is shown in Fig. 7. Upon atomic H interaction, most of the SiMIn bonds were broken and replaced by SiMH bonds. As a result, the 앀3×앀3 reconstruction was destroyed and In atoms formed small clusters on the H-terminated 1×1 surface. The size of the In clusters increased with increasing substrate temperature during H exposure of the 앀3×앀3-In surface phase. On the basis of CAICISS experimental results, we have found that atomic-hydrogen-induced In clusters have a monocrystalline structure with the orientation relationship of In(100) 011//Si(111) 011: , which is in contrast with that on the
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Fig. 7. Schematic illustration of the morphology and the structural transformation of the Si(111)앀 3×앀3-In phase induced by atomic hydrogen exposure.
Si(001)4×3-In surface phase where the In clusters are polycrystalline.
Acknowledgement This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan.
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[7] P. Ma˚rtensson, G. Meyer, N.M. Amer, E. Kaxiras, K.C. Pnadey, Phys. Rev. B 42 (1990) 7230. [8] S. Nakatani, Y. Kuwahara, T. Takahashi, M. Aono, Surf. Sci. 357/358 (1996) 65. [9] H. Watanabe, M. Ichikawa, Surf. Sci. 383 (1997) 95. [10] M. Yoshimura, B. An, K. Yokogawa, K. Ueda, Appl. Phys. A 66 (1998) S1051. [11] K. Oura, H. Ohnishi, Y. Yamamoto, I. Katayama, Y. Ohba, J. Vac. Sci. Technol. B 14 (1996) 988. [12] K. Oura, Y. Tanaka, H. Morishita, F. Shoji, I. Katayama, Nucl. Instrum. Methods B 85 (1994) 439. [13] M. Naitoh, H. Ohnishi, Y. Ozaki, F. Shoji, K. Oura, Appl. Surf. Sci. 60/61 (1992) 190. [14] J.T. Ryu, K. Kui, K. Noda, M. Katayama, K. Oura, Nucl. Instrum. Methods B 136–138 (1997) 1102. [15] J.T. Ryu, Ph.D. Thesis, Osaka University, 1999, in English. [16 ] M. Naitoh, H. Shimaya, N. Oishi, F. Shoji, S. Nishigaki, Appl. Surf. Sci. 123/124 (1998) 171. [17] J.T. Ryu, T. Fuse, O. Kubo, T. Fujino, H. Tani, T. Harada, A.A. Saranin, A.V. Zotov, M. Katayama, K. Oura, J. Vac. Sci. Technol. B 17 (1999) 983. [18] F. Owman, P. Ma˚rtensson, Surf. Sci. 359 (1996) 122. [19] M. Katayama, E. Nomura, N. Kanekama, H. Soejima, M. Aono, Nucl. Instrum. Methods B 33 (1988) 857. [20] R.J. Hamers, Phys. Rev. B 40 (1989) 1657. [21] J. Nogami, Sang-il Park, C.F. Quate, Surf. Sci. 203 (1988) L631. [22] A.A. Saranin, E.A. Khramtsova, K.V. Ignatovich, V.G. Lifshits, T. Numata, O. Kubo, M. Katayama, I. Katayama, K. Oura, Phys. Rev. B 55 (1998) 5353. [23] J.T. Ryu, K. Kui, K. Noda, M. Katayama, K. Oura, Jpn. J. Appl. Phys. 36 (1997) 4435. [24] J.T. Ryu, K. Kui, M. Katayama, K. Oura, Appl. Surf. Sci. 121/122 (1997) 223. [25] R.S. Willams, M. Kata, R.S. Daley, M. Aono, Surf. Sci. 225 (1990) 355. [26 ] J.T. Ryu, K. Kui, K. Noda, M. Katayama, K. Oura, Surf. Sci. 401 (1998) L425. [27] J.T. Ryu, O. Kubo, H. Tani, T. Harada, M. Katayama, K. Oura, Surf. Sci. 433–435 (1999) 575. [28] J.T. Ryu, O. Kubo, H. Tani, M. Katayama, A.A. Saranin, A.V. Zotov, K. Oura, Jpn. J. Appl. Phys. 37 (1998) 3774. [29] K. Fukutani, H. Iwai, Y. Murata, H. Yamashita, Phys. Rev. B 59 (1999) 13020.
Atomic-hydrogen-induced self-organization of Si(111) 앀3×앀3-In surface phase studied by CAICISS and STM J.T. Ryu a, *, O. Kubo a, T. Fujino a, T. Fuse a, T. Harada a, K. Kawamoto a, M. Katayama a, A.A. Saranin a,b,c, A.V. Zotov a,b,d, K. Oura a a Department of Electronic Engineering, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan b Institute of Automation and Control Processes, 5 Radio Street, 690041 Vladivostok, Russia c Faculty of Physics and Engineering, Far Eastern State University, 690000 Vladivostok, Russia d Department of Electronics, Vladivostok State University of Economics and Service, 690600 Vladivostok, Russia Received 30 July 1999; accepted for publication 8 November 1999
Abstract Using coaxial impact collision ion scattering spectroscopy (CAICISS ), scanning tunneling microscopy (STM ), and low-energy electron diffraction (LEED) techniques, we have investigated the interaction of atomic hydrogen with the Si(111)앀 3×앀3-In surface phase at elevated temperatures and structural behavior of In clusters induced by the interaction. Upon atomic hydrogen interaction, SiMIn bonds are broken and replaced by SiMH bonds. As a result, the 앀3×앀3 reconstruction is destroyed and small In clusters are formed on hydrogen-terminated Si(111)1×1 surface. Using STM, we also have found that the size of the In cluster increases with increasing substrate temperature during hydrogen exposure of the 앀3×앀3-In surface phase. From CAICISS experimental results, we have found that atomichydrogen-induced In clusters for Si(111)앀3×앀3-In surface phase have an In(100) crystalline structure, while those for Si(001)4×3-In surface phase are polycrystalline. In conclusion, we have found that structural differences of surface give rise to different atomic-hydrogen-induced self-organization. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Hydrogen atom; Indium; Ion scattering spectroscopy; Low energy ion scattering (LEIS ); Scanning tunneling microscopy; Silicon; Surface chemical reaction; Surface structure, morphology, roughness, and topography
1. Introduction When adsorbate atoms are deposited on Si substrates, various reconstructed phases are formed [1], depending on both adsorbate coverage and substrate temperature. Among them, the 앀3×앀3 periodicity is representative of the reconstruction phases formed on a Si(111) surface. Although these phases have identical periodicity, however, a great difference in atomic arrangements * Corresponding author. Fax: +81-6-879-7780. E-mail address: [email protected] (J.T. Ryu)
exists. For example, in the case of the Si(111) 앀3×앀3-Ag surface phase [2], a definite amount of Si substrate atoms is incorporated into the surface phase, i.e., 1 ML of Si atoms forms trimers on the bulk Si layer and Ag atoms form honeycomb-chained-trimer structures on the reconstructed Si trimers. In the case of the Si(111) 앀3×앀3-B surface phase [3,4], the adsorbate atoms substitute for Si substrate atoms, and in the case of the Si(111)앀 3×앀3-In surface phase [5,6 ], indium atoms adsorb above the Si atoms in the second layer ( T site) on bulk-like Si substrates 4 where the absolute In coverage is 1/3 ML. In the
0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 11 5 3 -X
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case of the Si(111)앀 3×앀3-Sb surface phase [7,8], Sb atoms form a trimer centered at the T sites on 4 the unreconstructed Si substrate. These structural differences could give rise to different atomichydrogen-induced self-organization phenomena: the three-dimensional metal cluster formation [9– 13] or two-dimensional structural change [14–16 ] due to atomic H interaction. Namely, the interaction of atomic H with adsorbate-induced Si surface phases could cause a peculiar structural transformation, strongly depending on the original surface structure. For instance, as the Si(001)4×3-In surface phase is exposed to atomic H, the uniform In monolayer is transformed into very small polycrystalline In clusters accompanied by a baring of the hydrogenated Si surface with the 4×1 periodicity [17]. However, when the Si(111)앀 3×앀3-Sb surface phase is exposed to atomic H, such atomic-hydrogen-induced metal clustering does not occur on the surface, but only two-dimensional structural change occurs on the surface [14,15]. A similar phenomenon was also observed for the Si(111)앀 3×앀3-Bi surface phase [16 ]. On the other hand, the interaction of atomic H with the Si(111)앀 3×앀3-In surface phase also results in the formation of metal clusters on the Si(111)1×1-H surface. The interaction of atomic H with the surface phase was investigated in detail by Owman and Ma˚rtensson [18] using scanning tunneling microscopy (STM ). They reported that: (1) the 앀3×앀3-In surface phase becomes disordered and small In clusters are formed even with very low H exposure (20 L); (2) increasing H exposure results in the formation of two-dimensional islands exhibiting 2×2, 4×1 and 앀7×앀3 reconstructions; and (3) a further increase in H exposure results in the formation of three-dimensional In islands. Further, they observed a loss of In atoms from the surface during H exposure, which increased with increasing H exposure and substrate temperature. However, they did not report on the structural quality and behavior of In clusters formed by atomic H exposure, which is important for understanding the atomichydrogen-induced self-organization phenomena. As mentioned above, metal clusters formed by
atomic H interaction with Si(111)앀 3×앀3-Ag and -Pb surface phases are monocrystallines [9,11,12], but in the case of Si(001)4×3-In surface In clusters formed by this interaction are polycrystalline [17]. Using scanning tunneling microscopy (STM ), coaxial impact-collision ion scattering spectroscopy (CAICISS), and low-energy electron diffraction (LEED) techniques, we have studied the interaction of atomic H with the Si(111) 앀3×앀3-In surface phase at elevated temperatures and the structural quality and behavior of In clusters formed by atomic H exposure. In this study, we have found that atomic H interaction results in In metal clustering and Si substrate baring. CAICISS data showed that In clusters formed by the interaction of atomic H with the 앀3×앀3-In surface phase have an In(100) crystalline structure with the orientation relationship of In(100) 011//Si(111) 011: , which is in contrast with atomic-hydrogen-induced In clusters on the Si(001)4×3-In surface phase where the In clusters are polycrystalline [17].
2. Experimental STM experiments were carried out in an ultrahigh vacuum ( UHV ) chamber with a base pressure of 8×10−11 Torr equipped with STM (‘Omicron’) and LEED systems, and a load-lock for introduction of samples and tunneling tips without breaking the vacuum. Electro-chemically etched tungsten tips, cleaned by in situ heating, were used. The Si(111) sample (2×13 mm2) was cut from n-type Si(111) wafers with 0.05 V cm resistivity. The sample was outgassed at 600°C for 12 h in the UHV chamber. An atomically clean silicon surface was prepared in situ by direct Joule flashing for several times at 1250°C. After this treatment, a sharp (7×7) LEED pattern was observed and the STM images exhibited a clean 7×7 surface. CAICISS experiments were carried out in an UHV chamber with a base pressure of 8.0×10−11 Torr. The UHV chamber is connected with a beam line for ICISS of TOF type and timeof-flight elastic recoil detection analyzer ( TOF-
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ERDA), a sample transfer load-lock system, and sub-chamber with a base pressure of 5×10−11 Torr for sample cleaning and annealing. The UHV chamber is also equipped with low energy electron diffraction (LEED) facilities. A He+ beam of 2.0 keV was produced by a differentially pumped ion source. The beam could be chopped by two pairs of electrostatic deflection plates with a chopping aperture of 1 mm diameter, resulting in a pulse width of about 50 ns. After passing through the aperture, the pulsed ion beam impinged on the sample in the UHV chamber. The He particles (ions and neutrals) scattered from sample are detected at a scattering angle of 180° (perfect back scattering) using a TOF analyzer, i.e. the apparatus comprises a coaxial arrangement of a pulse-beam ion source and a detector [19]. Beam current and diameter were about 20 nA and 2.0 mm at the sample surface, respectively. A p-type Si(111) wafer with 1–2 V cm resistivity and dimensions of 25×25 mm2 was used. The sample was cleaned in situ by annealing for about 12 h at 600°C followed by flash heating at 1200°C under a base pressure of 6×10−10 Torr using electron bombardment from behind the sample. After the cleaning, a sharp (7×7) LEED pattern was observed.
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Indium of 99.999% purity was deposited from a Knudsen cell in the CAICISS experiment and a Ta foil tube in the STM experiment. The deposition rate was about 0.1 ML/min (where 1 ML is defined as 7.8×1014 atoms/cm2, the ideal Si surface atomic density in the (111) plane). A Si(111)앀 3×앀3-In surface phase was prepared by depositing of 0.4 ML In onto the Si(111)7×7 surfaces held at 550°C. For atomic hydrogen exposure, H gas was 2 admitted through a leak valve. An 1800°C tungsten filament, 10 cm from the Si surface in the CAICISS experiment and 7 cm in the STM experiment, was used to dissociate molecular hydrogen. The exposures were conducted with the specimen facing the filament and by backfilling the chamber with H 2 at 1×10−7 Torr. Since the arrival rate of atomic hydrogen is unknown, the dose of molecular hydrogen is specified, and expressed in units of Langmuir (1 L=1×10−6 Torr s).
3. Results and discussion Fig. 1a, b shows filled- and empty-state STM ˚ ×300 A ˚ ) of the Si(111)앀 3×앀3-In images (300 A
˚ ×300 A ˚ ) of Si(111)앀3×앀3-In surface phase. Tunneling conditions: Fig. 1. Filled state (a) and empty state (b) STM images (300 A ±2.0 V sample bias and 0.2 nA tunneling current.
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surface. These images were taken with a sample biased at ±2.0 V, and a tunneling current of 0.2 nA. Most of the surface is covered with a centered hexagonal array of protrusions. The distance between nearest-neighbor protrusions is ˚ along the [112: ] and [2: 11] crystalloabout 6.7 A graphic directions, consistent with a 앀3×앀3 reconstruction rotated by 30° with respect to the (1×1) unit cell of the ideal Si substrate. An interesting feature of these images is the presence of defects. In Fig. 1a (filled-state), these defects are much brighter than their neighbor protrusions, but in Fig. 1b (empty-state) these defects appear darker than the surrounding protrusions. These observations indicate the strong effect of electronic structure on the apparent height of these defects. About 14% of the 앀3×앀3 surface is occupied by these defects. Similar defects (so-called type-S ) were observed by Owman and Ma˚rtensson [18] and Hamers [20]. They assigned the defects to substitutional Si atoms. In practice, the unit cell of the original Si(111)7×7 surface contains an excess of 8% of one monolayer Si atoms against an ideal bulk terminated Si surface, i.e., 24% of the number of atoms in a 앀3×앀3 surface. Thus, a part of the excess Si atoms should be merged into steps, but the rest should remain on the 앀3×앀3 surface area. Type-V defects [18,20] attributed to vacancies are also seen in Fig. 1a, which appear darker than the surrounding protrusions both at filled- and empty-state STM images. These STM images in
Fig. 1 are consistent with images in previous STM studies of the 앀3×앀3-In surface [18,21]. Fig. 2a shows a filled-state STM image of a ˚ ×500 A ˚ region of the Si(111)앀 3×앀3-In 500 A surface taken after atomic H exposure (500 L). The sample was held at about 300°C during H exposure. This image was taken with a sample bias of −2.0 V, and a tunneling current of 0.2 nA. For this sample, we observed a sharp and bright (1×1) LEED pattern with low background. As one can see in Fig. 2a, after atomic H exposure, no ordered 앀3×앀3 areas remain and small spherical clusters appear on the surface. One can see from the result of ion scattering experiments presented below that these clusters are built of In atoms. The size of the In clusters is not uniform. The size of large clusters, ˚, occupying about 10% of the surface, is about 60 A while that of small clusters, occupying about 15% ˚ . The underlying Si of the surface, is about 20 A surface is exposed to vacuum because of the removal of In atoms, which is caused by atomic H exposure. Thus, most of the surface, besides the clusters, exhibits the 1×1 structure due to the underlying Si atoms terminated by H atoms as shown in Fig. 2c (the high-resolution image of Fig. 2a). This result is in agreement with the observed 1×1 LEED pattern. Fig. 2b shows a ˚ ×500 A ˚ region filled-state STM image of a 500 A of the Si(111)앀 3×앀3-In surface taken after atomic H exposure (500 L) at about 400°C. Small ˚ clusters with average diameters of about 20 A
˚ ×500 A ˚ ) taken from Si(111)앀3×앀3-In surface phase exposed to 500 L of atomic hydrogen at 300°C (a) Fig. 2. STM images (500 A and 400°C (b), respectively. Tunneling conditions: −2.0 V sample bias and 0.2 nA tunneling current. (c) The enlarged STM image ˚ ×100 A ˚ ) of hydrogen-terminated 1×1-H area shown in (a). (100 A
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distribute randomly on the surface, and these clusters occupy about 10% of the surface. The surroundings of the clusters exhibit also the Hterminated 1×1 structure. As can be seen in Fig. 2b, a two-dimensional hexagonal In island exists on the surface. The size of the island was ˚ with an apparent height found to be about 150 A ˚ of about 12 A. The topmost surface of the twodimensional island comprises rows composed of some protrusions, which have a 4×4 arrangement. Owman and Ma˚rtensson [18] observed two-dimensional islands, exhibiting 2×2, 4×1, and 앀7×앀3 reconstructions after atomic H exposure of the 앀3×앀3-In surface phase, but no In island with 4×4 reconstruction was observed in their experiment. Saranin et al. [22], using STM, observed In islands with 4×4 reconstruction after atomic H exposure of the Si(111)4×1-In surface phase at 300°C. Ion scattering experiments were carried out to investigate the structural quality and behavior of In clusters formed after atomic H exposure of the Si(111)앀 3×앀3-In surface phase. Fig. 3 shows typical CAICISS spectra taken before and after 1000 L H exposure of the Si(111)앀3×앀3-In surface phase at 300°C. The incident energy and the direction of primary ion beam are 2.0 keV and a= 90° (normal to the surface), respectively. The peak at 4730 ns corresponds to the He signal scattered by In atoms, and a peak and a broad signal
Fig. 3. TOF spectra taken from Si(111)앀3×앀3-In surface before and after hydrogen exposure of 1000 L at 300°C. He+ primary ions of 2.0 keV were used. The incident direction is normal to the surface (a=90°).
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originating from single and multiple scattering by substrate Si atoms were also observed at 5220 ns and above 5370 ns (not shown here), respectively. The positions of the peaks due to In and Si atoms are in agreement with those calculated on the basis of kinematics of elastic single scattering between He+ and those atoms. As one can see in Fig. 3, after atomic H exposure, the peak due to In atoms has a small tail on the low-energy (high flight time) side and is slightly broader than that before atomic H exposure, indicating that small In clusters were formed on the surface. This result is in agreement with our present STM observations ( Fig. 2). One can also see in Fig. 3 that the intensity of the In peak increases after atomic H exposure in spite of the desorption of In atoms from the surface during atomic H exposure, as mentioned below. Therefore, the increase of the In scattering intensities should be caused by the slight focusing effect in the ion scattering process due to the structural change of the In layer. At this time, no shadowing effect occurs because the clusters are smaller, as shown in Fig. 2a. If further large islands were formed on the surface, scattering intensity should reduced due to the shadowing effect, and the peak should have a broader tail because of the multiple scattering of incident ions in three-dimensional large clusters, as observed for the H exposure of Si(001)4×3-In surface [17]. To confirm the loss of In atoms by atomic H interaction indicated by Owman and Ma˚rtensson [18], we estimated the amount of In atoms from the surface annealed at 500°C for 10 min after H adsorption of 1000 L on a 4×3-In surface at 300°C. It is well known that H atoms existing on the Si surface desorb at a temperature of 500°C. Therefore, the 앀3×앀3-In surface phase is recovered because of dissociation of In clusters, and the scattering intensity should be reverted. After the annealing, however, the scattering intensity due to In atoms was reduced, as shown in the inset in Fig. 3, and the tail shown after atomic H exposure disappeared, indicating that In atoms form a two-dimensional layer caused by the dissociation of In clusters. The scattering intensity due to In atoms was reduced to about 80% of the original intensity after the annealing, indicating
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that the desorption of In atoms from the Si(111) surface is caused by atomic H interaction. We also found from STM images in Fig. 2a and b that the amount of In islands formed by atomic H exposure at 400°C is 60% with respect to that formed by atomic H exposure at 300°C. This result suggests that a higher temperature during atomic H exposure results in an increased loss of In atoms. These results are in agreement with previous work by Owman and Ma˚rtensson [18]. Such atomic H induced desorption of adsorbate was also observed for the Si(001)2×1-Sb surface phase [23,24]. Fig. 4 shows the CAICISS intensity (azimuth scan) of He scattered from In atoms on the Si(111)앀 3×앀3-In surface phase as a function of azimuth angle w (the origin of w is [1: 1: 2]) for various ion incidence angles a from 8° to 70°, where a is measured from the surface plane. The incident energy of the primary ion beam was 2.0 keV. The variations of scattering intensity observed in Fig. 4 are due to various shadowing and focusing effects, which strongly depend on the structure of the In layer. The intensity curves measured at a=8° and 10° are sensitive to the topmost surface structure, while those for a=20°, 30°, 50°, and 70° reflect the structure of deeper layers. Therefore, we can obtain structural infor-
Fig. 4. Azimuth angle scan: variation of He scattering intensity due to In atoms on the Si(111)앀3×앀3-In surface phase (a) and the surface exposed to atomic hydrogen (1000 L) at 300°C (b). Parameters are azimuth angle w (measured from [1: 1: 2]) and incidence angle a (measured from the surface) of the ion incidence direction. He+ primary ions of 2.0 keV were used.
mation from the intensity variations. Fig. 4a shows azimuth scans of In scattering intensity for the Si(111)앀 3×앀3-In surface phase before atomic H exposure. The drops and enhancements of the scattering intensities for a=8° and 10° reflect the structure of the 앀3×앀3-In surface phase. The intensity curve has a periodicity of 60° as expected. However, the drops and enhancements as well as periodicity of the intensity are absent in the incidence angle ranges from a=20° to 70°. This result indicates that In atoms in the Si(111)앀 3×앀3-In phase compose the topmost two-dimensional layer. Fig. 4b shows azimuth scans of In scattering intensity taken after atomic H exposure (1000 L) of the Si(111)앀 3×앀3-In phase at 300°C. After atomic H exposure, for incidence angle from a= 8° to 10°, the periodicity seen in Fig. 4a disappeared, but intensity curves having a periodicity of 30° newly appeared. Moreover, this feature appears only for an incidence angle of less than a=20°, but is absent for larger incidence angle ranges from a=30° to 70°. These results indicate that the In monolayer with a 앀3×앀3 periodicity is transformed into very small crystalline In agglomerate. Atomic-hydrogen-induced In metal clustering and the structural quality of the clusters can be also confirmed from a-scans taken before and after atomic H exposure of the 앀3×앀3-In surface phase. We measured the intensity variations (ascan) of the surface peak due to In atoms as a function of the incident angle a, along the [1: 1: 2] and [011: ] azimuths. Fig. 5 shows the results for the [1: 1: 2] azimuth measured for two kinds of specimens; (a) the Si(111)앀 3×앀3-In surface without H exposure and (b) the surface after 1000 L atomic H exposure of the surface shown in Fig. 5a at 300°C. As we can see in Fig. 5a, there are two dominant peaks in the a-scan of the 앀3×앀3-In surface. As already discussed in detail, these peaks can be explained in terms of the focusing effects of the In atoms with the 앀3×앀3 periodicity overlying the Si surface. The presence of only two dominant peaks in the a-scan indicates the formation of a uniform In overlayer with no detectable In agglomerate, which is in agreement with the experimental results of the present STM ( Fig. 1)
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Fig. 5. Incidence angle scan: variation of He scattering intensity due to In atoms as a function of incidence angle, a (measured from the surface plane), along the [1: 1: 2] azimuth, measured for two kinds of the specimens; (a) the Si(111)앀3×앀3-In surface without hydrogen exposure and (b) the surface exposed to atomic hydrogen of 1000 L at 300°C.
and azimuth scan (Fig. 4a). On the other hand, when the 앀3×앀3 surface phase was exposed to atomic H at 300°C, quite a different a-scan was observed as shown in Fig. 5b. Although the intensity is small, several peaks are definitely observed. All these peaks can be well explained in terms of the focusing effect due to In atoms in the In agglomerate, which form very small In(100) crystallite grown on the Si(111) surface. In other words, the a-scan results of Fig. 5a and b reveal that the In monolayer with a 앀3×앀3 periodicity is transformed into an In(100) agglomerate by atomic H exposure. As shown in Fig. 5b, however, the intensities have very slight enhancements and dips. This means that these clusters may have poor quality and be very small, which is consistent with our STM results. In bulk has a face-centered tetragonal structure with lattice constants of a= ˚ and c=4.938 A ˚ . As mentioned above, b=4.588 A however, the azimuth scan in Fig. 4b shows that the He intensity changes with a periodicity of 30°. It seems to be difficult to explain this result in terms of In(100) agglomerates with only one orientation type of alignments with relation to a orientation of Si substrate, because In bulk has a fourfold
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Fig. 6. Incidence angle scan: variation of He scattering intensity due to In atoms as a function of incidence angle, a (measured from the surface plane), along the Si [011: ] azimuth. (a) The simulation result with three orientations of In(100) 011 direction; (b) the film grown In 12 ML on Si(111) at room temperature; and (c) the surface exposed to atomic hydrogen of 1000 L at 300°C.
symmetry. This probably means that three direction types of In(100) agglomerates are formed on a direction of Si substrate due to atomic H exposure. Thus, we considered three orientation types of In(100) agglomerates with relation to Si 011 direction: the simulation result is the summation of the three orientation rotated by 0°, 120°, and 240° relative to the In bulk [011] azimuth. The result is showed in Fig. 6a. The computer simulation results based on ‘‘two atoms approximation’’ technique introduced by Williams et al. [25]. Fig. 6c shows the incident angle scan for Si [011: ] azimuth after 1000 L atomic H exposure at 300°C. The simulation result with the three orientations corresponds well to the experimental results. At lower angles, however, the simulation result was in slight disagreement on the In signal intensity, because the formed In agglomerates is poor quality and very small. These results mean that the orientational relationship between the In agglomerate and the substrate is In(100) 011//Si(111) 011: . We have also found that In films with the same orientation grow on H-terminated Si(111) surface as well
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as on clean Si(111) surface. Fig. 6b shows an incident angle scan of the film grown 12 ML In on Si(111) surface at room temperature. On the other hand, as mentioned above, the 2D island formed after atomic H exposure at 400°C (Fig. 2b) has a hexagonal shape and protrusions exhibit a 4×4 periodicity on the surface of the island. It seems to be difficult to explain these results in terms of In(100) agglomerates because of their fourfold symmetry. Therefore, we infer that the 2D island probably has a structure different from those of the small In clusters formed at 300°C in Figs. 2a and 5b. In a previous work for atomic H exposure of the Si(001)4×3-In surface phase [17], we found that the atomic-hydrogen-induced In clusters were not monocrystalline, but rather polycrystalline. However, in the present work for atomic H exposure of the Si(111)앀 3×앀3-In surface phase, In clusters were found to be monocrystalline. The reason why such differences occur in spite of the same element is not clear as yet, but the explanation could be the different effects of H termination on In thin film growth for both surfaces. We investigated the effect of H termination on the crystallinity of the initial In thin film growth on both Si(001) and Si(111) surfaces [26,27]. In the In/Si(111) experiment, we found that H termination did not have an important effect on the crystallinity of In thin film. For the In/Si(001) experiment, however, H termination had a serious effect on the crystallinity of In thin film, i.e., epitaxial growth of In thin film on the Si(001) surface was disrupted by the presence of H atoms residing at the interface between the films and the Si substrate. Thus, we inferred that the disturbance of epitaxial growth is caused by the suppression of surface diffusion and the loss of preferable adsorption sites by H termination [26,27]. These results suggest that H termination on both Si(111) and Si(001) surfaces would cause the different surface diffusion of In atoms on these surfaces. Therefore, we infer that the difference of the crystallinity between In clusters induced by the interaction of atomic H with Si(001)4×3-In and Si(111)앀 3×앀3-In surface phases is caused by the different surface diffusion of In atoms on the
H-terminated Si surfaces. Another reason could be the structural difference of Si substrates. No Si atoms in the 앀3×앀3-In phase are reconstructed, a bulk-like structure of 1×1, while those in the 4×3-In phase are reconstructed into a 4×1-Si structure with missing dimer rows [17,28]. Thus, when these surfaces were exposed by atomic H, the Si substrate in the 앀3×앀3-In phase formed a relatively flat H-terminated 1×1-H structure and that in the 4×3-In phase formed a 4×1-H structure with monohydride Si dimers and dihydride Si atoms [28]. This implies that Si(111)앀 3×앀3-In phase forms more atomic flat H-terminated surfaces (1×1-H ) than Si(001)4×3-In phase (4×1-H ). Therefore, these different substrate structures would cause different In cluster formation. On the other hand, it is interesting that where H atom exist. Fukutani et al. [29] reported that hydrogen atoms in the Pb/H/Si system exist at the interface, while hydrogen atoms in the Ag/H/Si system do not exist at the interface. Unfortunately, it is impossible to confirm a location of H atom from our experiment.
4. Conclusion We have used STM, CAICISS, and LEED to study the interaction of atomic H with a Si(111) 앀3×앀3-In surface phase and the structural quality and behavior of In clusters formed by atomic H interaction. A schematic illustration of the structural transformation of the Si(111)앀 3×앀3-In surface phase induced by the atomic H interaction at 300°C is shown in Fig. 7. Upon atomic H interaction, most of the SiMIn bonds were broken and replaced by SiMH bonds. As a result, the 앀3×앀3 reconstruction was destroyed and In atoms formed small clusters on the H-terminated 1×1 surface. The size of the In clusters increased with increasing substrate temperature during H exposure of the 앀3×앀3-In surface phase. On the basis of CAICISS experimental results, we have found that atomic-hydrogen-induced In clusters have a monocrystalline structure with the orientation relationship of In(100) 011//Si(111) 011: , which is in contrast with that on the
J.T. Ryu et al. / Surface Science 447 (2000) 117–125
Fig. 7. Schematic illustration of the morphology and the structural transformation of the Si(111)앀 3×앀3-In phase induced by atomic hydrogen exposure.
Si(001)4×3-In surface phase where the In clusters are polycrystalline.
Acknowledgement This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan.
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