Conference-ISSS-5-Observation of Multi-Step Ordering of Bi Adsorbed on the Si ( 111 ) 7 × 7 Structure by RHEED and STM ∗

The initial adsorption process of Bi onto the Si(111)7 × 7 surface, including nanocluster formation, has been investigated by a real time structural analysis using in-situ RHEED intensity monitoring during the Bi adsorption, as a complementary approach to local and statistical structure analysis by STM. At an elevated temperature, the intensity distribution of 7 × 7 diffraction pattern varied gradually with the Bi adsorption and temporal intensity maxima which indicate the change of ad-layer symmetry without any serious disorder were observed. Subsequent STM observations clearly showed the formation of adsorption structure. From these results, multistep ordering process in Bi initial adsorption onto Si(111)7×7 was concluded, i.e. there are several coverage-dependent adsorption structures. Bi magic nanocluster was clearly observed by STM in atomic resolution. [DOI: 10.1380/ejssnt.2008.291]


I. INTRODUCTION
The topmost layer of surfaces provides experimental model systems for studying fundamental physics related to phase transition, electron correlation in the low dimensional systems [1][2][3][4][5].At the same time, much attempt has been made to establish the novel method for fabrication and control of the artificial nano-structures and their ordered array using such surfaces [6,7] in order to achieve the future nanotechnology.Zero-dimensional (0D) structures such as nanodot and nanocluster were investigated intensively due to their promising applications, such as ultrahigh density recording, quantum operation, and nano-catalysis.However, there is some difficulty in control of their electronic properties because it depends strongly on their size and shape.Because of these factors, an attempt has been made to fabricate monodispersed nanodots or nanoclusters using the surface long-period structures as well-ordered nanoscale templates.Typical case is the Si(111)7 × 7 reconstructed surface which has a very large unit cell [8].
As suggested by Cho and Kaxiras, periodic attractive potential wells on the 7 × 7 half unit cells (HUCs) trap the diffusing adsorbate atoms and fix them in the "basin of attraction" [9].Consequently, nanoclusters which have an ideal size and shape (i.e.so-call "magic cluster") can be created under an appropriately controlled adsorption condition.So far, numerous investigations to fabricate this type of nanoclusters on Si(111)7 × 7 surface were carried out for various adsorption species, and excellent results were reported for the alkali metals, group-III metals, transient metals and the others [10][11][12][13][14][15][16].
In this work, we investigated the ordered adsorption structures in the initial stage of Bi adsorption onto the Si(111)7 × 7 surface which include the formation of nanoclusters.In the surface systems, Bi has attracted much attention recently due to the huge Rashba-type spinsplitting originated in the large spin-orbit interaction in Bi [17][18][19].In this point of view, the electronic property of Bi nanodot or nanocluster might show spin-related phenomena due to its low dimensionality.So, there is a considerable significance in the fabrication of well defined 0D Bi structure.Nagao et al. have observed nanoclustering of Bi adsorbed on Si(111)7 × 7 surface at room temperature (RT) using Scanning Tunneling Microscopy (STM) [20].In their experimental condition, the size and shape of the clusters were irregular in contrast to the well ordered magic nanocluster arrays such as the submonolayer Al/Si(111)7×7 system [12].Sadowski et al. has also reported the formation of a triangular-like cluster and a local honeycomb-like arrangement by post annealing of the wetting layer of 1 ML Bi deposited at RT [21].These reports indicate that there is a good chance to create the ordered Bi nanocluster array, and there is still room for improvement in the growth conditions.So far, Scanning Tunneling Microscopy (STM) has been mainly used to analyze the local atomic arrangement of nanocluster phase in metal/Si(111)7 × 7 systems.In this study, on the other hand, we used Reflection High Energy Electron Diffraction (RHHED) to investigate the initial metal adsorption process as a complementary method to STM.As generally known, a control of the adsorbate atom kinetics is one of the important factors to form well-orderd nanoclusters on the Si(111)7 × 7 surface [12].Therefore, growth parameters such as the substrate temperature and the deposition rate must be determined so carefully from numerous experiments.One of the advantages of RHEED is that it enables us to observe the structure modification in in-situ and real-time during the deposition and desorption at high and low temperatures, and this allows us the easier structural survey and the observation on dynamical changes of the surface structures within a short time.Although the 7 × 7 symmetry is preserved through the nanocluster formation process, the change of the adatom arrangement inside the 7 × 7 unit cell is expected.This leads to the modification of 2D crystal structure factor in the diffraction pattern [22] such as the well known δ7 × 7 phase [23].So, in the present paper, we discussed the characteristic structure transformation of the Si(111)7×7 clean surface by Bi adsorption, probed by the change of RHEED spot intensity distribution.RHEED spot intensity modification revealed an anomalous multistep adsorption process.
Local structure observations by STM were also performed subsequently, so that the detail of the multistep adsorption process was investigated.

II. EXPERIMENTAL
All experiments were performed in the ultrahigh vacuum (UHV) chamber.Atomically cleaned Si(111)7 × 7 surfaces were obtained by the standard flashing procedure up to 1250 • C after outgassing at ∼ 500 • C for several hours.The substrate temperature was calibrated by a pyrometer before the measurements.Bi was deposited onto the substrate held at constant temperatures at a deposition rate of 0.1 ML/min.The coverage (ML) and deposition rate was calibrated in-situ by observing the completion of the Si(111)β-√ 3× √ 3-Bi phase [24] in the RHEED pattern.During the Bi deposition onto the Si(111)7 × 7 clean surface, the RHEED pattern in the incident azimuth was recorded continuously by a CCD camera to monitor the changes of the surface structures.It should be noted that the glancing angle of the specular beam is not in the surface resonance condition but around 2.1 ± 0.1 • which is somewhat shallower than (333) Bragg reflection angle.This angle is chosen because the characteristic behavior of the RHEED intensity was most clearly observed at this angle.All the STM observations were done at RT.

A. RHEED intensity analysis
Figure 1 shows the evolution of the RHEED patterns and intensity profiles of several fractional-order diffraction spots as a function of Bi coverage at RT ((a), (c)) and around 300 • C ((b), (d)).As seen in Figs.1(a) and (c), in RT deposition, every 7 × 7 fractional spots shows almost monotonic intensity decay.We could understand this feature of intensity decay as a result of a random site adsorption without making long range order in this adsorption condition.However, the attenuation curves of intensity of some spots showed the shoulders at ∼ 0.5 ML as indicated by allows illustrated in Fig. 1(c).These features quantitatively agree with the previous report [20].Namely, these intensity shoulders is considered to correspond to the formation of the irregular clusters in the HUCs reported in Ref. [20].In contrast, at the elevated temperature, around 300 • C, we obtained interesting results by the same measurement.As can be seen in Figs.1(b) and (d), intriguingly, the 7 × 7 fractional spots intensities were not monotonically decreased but showed two temporal intensity maxima at about 0.2 ML and 0.45 ML (indicated by "A" and "B" in Fig. 1(d metry remained but its internal structure which affects the structure factor in the diffraction pattern is changed.This means that the Bi adsorption onto Si(111)7 × 7 is site-selective in the early stage at around 300 • C, and the multistep growth preserving the 7 × 7 symmetry is expected.Subsequently, diffuse √ 3 × √ 3 streaks appeared with disappearing of 7 × 7 spots.These √ 3 × √ 3 streaks reached to a constant value at around 1.0 ML.Hence, in our result, there is a possibility of formation of the ordered adsorption structure such as the above mentioned identical nanocluster before the massive surface reconstruction occurs.As is well-known, the intensity distribution of reciprocal lattice of original DAS (Dimer-Adatom-Stacking fault) structure reflects the local 2 × 2 adatom arrangement [25,26].A similar feature is seen in Fig. 1(a), though some difference exists between Fig. 1(a) and the reported one [25] probably due to the difference in the incident angle of the RHEED electron beam.Figure 2 shows the variation of the RHEED spots intensities calculated using ∆I = (I x − I 0 )/I 0 , for the spots between the 0-th and the first Laue zone.The size of circles indicates the degree of intensity variation.I x is the spot intensity in x ML's pattern and I 0 is the original intensity of the clean 7 × 7 surface.Data was sampled by the detectable spots in the RHEED patterns (Fig. 1(b)).In the adsorption process for 0-0.2 ML, the RHEED intensity distribution does not change so drastically in contrast to the process around ∼ 0.45 ML.This result would lead a simple model, that is, ad-layer symmetry is still unchanged from the original DAS structure until 0.2 ML.But for ∼ 0.45 ML, the intensity of number of fractional spots increased drastically.This increase is attributed to the large scattering factor of Bi atom.It would seem that an ordered adsorption structure, keeping the 7 × 7 periodicity with a new inner structure inside the unit cell different from the original DAS structure, is formed.Although we found the structure changes at 0.2 ML and at 0.45 ML, it is not so easy to discuss the atomic arrangement of Bi ad-layer only from these RHEED results.So, we also carried out STM observations for the several Bi deposited samples; 0.2 ML (A: first intensity maximum), 0.45 ML (B: second intensity maximum), and 0.3 ML (transient structure of A to B) prepared at 300 • C. The choice of these coverages was made by the above mentioned RHEED results.

B. STM observation
Figure 3 shows STM images of Si(111)7 × 7 covered with ∼ 0.2 ML of Bi.In the empty state STM image (Fig. 3(a)), adatom arrays in the original DAS structure could still be observed, although a slight disordering was recognized.However, there is a clear difference from the clean surface in the filled state image (Fig. 3(b)), that is, bright adatoms appeared.They should be different from the Si adatoms that have a different brightness in the image.The area surrounded by the white lines in Fig. 3(b) is enlarged in Fig. 3(c).Approximately 40% of the adatoms are converted to be bright.A similar behavior is reported in the initial stage of the adsorption of Sb and group-III metals on the Si(111)7 × 7 surface and it is understood to be the behavior originated in extrinsic adatoms substituted to Si adatoms [27,28].In the case of Bi adatom [29], Shioda et al. have observed the same behavior and explained the STM appearance of Bi atom substituted to Si adatom as the difference of the number of electron in Bi and Si.Since an extra electron would reside in a lone-pair state for a group-V adatom whereas a Si adatom would have only a partially occupied dangling bond extending above the adatom, the former might be brighter in a filled state image.Segregation behavior at the 7 × 7 anti-phase domain boundary as seen in Fig. 3(a) might be a piece of evidence for the above speculation.That means, in short, it can be presumed that islands seen in the domain boundary were formed by Si atoms released from adatom sites.For these reasons, we attributed brighter adatoms in the filled state STM image to Bi adatoms substituted to original Si adatoms.Therefore, this ad-layer is supposed to be a "2D random binary alloy" formed by Bi and Si adatom which is randomly distributed on the adatom site.These STM results qualitatively confirmed our interpretation of RHEED results, which is, the topmost layer still keeps the 2 × 2 periodicity at 0.2 ML.
Figures 4(a) and (b) shows the empty state STM images of the Si(111) 7 × 7 surface covered by ∼ 0.45 ML Bi.A number of protrusions are seen, and they obviously form a honeycomb-like arrangement, reflecting the nucleation in each 7 × 7 HUCs.These protrusions are assumed to be Bi nanocluster grown in 3D.The size of the nanocluster is rather irregular.RHEED intensity behavior shown in the previous section may be ascribable to the diffraction from irregularly-sized Bi nanocluster array.
We also performed STM observation for Si(111)7 × 7 covered with ∼ 0.3 ML of Bi, to reveal the structure transformation from 0.2 ML to 0.45 ML, motivated by the search for the nanocluster which consisted of a specific (magic) number of Bi atoms.As a result, a structure unexpected from the result of RHEED experiments was obtained.In an empty state STM image shown in Fig. 5, triangular nanoclusters, similar to the group-III (Al, Ga, In) type magic cluster, were observed with atomic resolution (enlarged view was shown in Fig. 5(b)), despite a lack of any characteristic feature in the RHEED intensity behavior at this coverage.The reason of this would be originated in their disorder in contrast to the well-ordered magic cluster array such as Al/Si(111)7×7.It is also seen in the same image in Fig. 5 that some clusters already began to grow as 3D (enlarged view was shown in Fig. 5(c)).It is considered that this growing cluster would be evolved to the 3D cluster seen at 0.45 ML in Fig. 4. Considering the Bi substitution with adatom at earlier adsorption step, protrusions in corner adatom site seen in Fig. 4(b) are assumed to be Bi adatom contrary to the structure model of group-III type magic clusters [22].While further structure analysis such as dynamical diffraction calculations is required to estimate the structure model, magic clustering of Bi on the Si(111)7 × 7 was shown clearly for the first time.More careful control of the deposition parameter such as temperature and deposition rate, which relate to the adsorbate kinetics, may lead to the formation of the highly ordered Bi magic cluster array.

IV. CONCLUSIONS
Using the in-situ RHEED intensity monitoring combined with STM, we found the multistep ordering process in the initial Bi adsorption onto Si(111)7 × 7 clean surface.Intensities of several 7 × 7 spots in the RHEED patterns showed two temporal maxima during the Bi adsorption.This indicates that there are several coverage dependent adsorption structures with certain degree of orders.Subsequent STM observation clearly showed the formation process of the adsorption structure with the multistep mechanism, as shown below; • ∼ 0.2 ML; formation of "substitutional Bi ad-layer" phase • ∼ 0.3 ML; formation of "Bi magic cluster" phase • ∼ 0.45 ML; formation of "3D cluster" phase However, the degree of order of each adsorption structure estimated from the STM images was not so high.It is necessary to optimize the deposition parameter to fabricate highly ordered nanocluster arrays.

FIG. 1 :
FIG. 1: Evolution of RHEED pattern during the adsorption of Bi onto Si(111)7 × 7 surface at (a) RT and (b) 300 • C. Integrated intensities of several fractional order diffraction spots are plotted as a function of Bi coverage at (a) RT and (b) 300 • C. The selected spots are indicated by several color triangles in (a) and (b).
Figure1shows the evolution of the RHEED patterns and intensity profiles of several fractional-order diffraction spots as a function of Bi coverage at RT ((a), (c)) and around 300 • C ((b), (d)).As seen in Figs.1(a) and (c), in RT deposition, every 7 × 7 fractional spots shows almost monotonic intensity decay.We could understand this feature of intensity decay as a result of a random site adsorption without making long range order in this adsorption condition.However, the attenuation curves of intensity of some spots showed the shoulders at ∼ 0.5 ML as indicated by allows illustrated in Fig.1(c).These features quantitatively agree with the previous report[20].Namely, these intensity shoulders is considered to correspond to the formation of the irregular clusters in the HUCs reported in Ref.[20].In contrast, at the elevated temperature, around 300 • C, we obtained interesting results by the same measurement.As can be seen in Figs.1(b) and (d), intriguingly, the 7 × 7 fractional spots intensities were not monotonically decreased but showed two temporal intensity maxima at about 0.2 ML and 0.45 ML (indicated by "A" and "B" in Fig.1(d)).This indicates that the 7 × 7 sym-

FIG. 3 :
FIG. 3: Empty and filled state STM images of a 0.2 ML Bi covered Si(111)7×7 surface measured at RT. Sample bias voltages were (a) 1.0 V and (b) −1.0 V.The scanning area was (a) 400 × 400 Å2 and (b) 200 × 200 Å2 .Anti-phase domain boundary of 7 × 7 surface is indicated by white dotted line in (a).(c) Enlarged view of the area surrounded by white lines in (b).Faulted and unfaulted half unit cells are indicated as FH and UFH in the right hand side of the STM image.

FIG. 4 :
FIG. 4: Empty state STM images of a 0.45 ML Bi covered Si(111)7 × 7 surface measured at RT.The sample bias voltages were (a) 1.0 V and (b) 3.0 V.The scanning area was 400 × 400 Å2 for both (a) and (b).White corral drawn in (b) indicates 7 × 7 half unit cells.