J. Phys. Chem. C 2010, 114, 17693–17702
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Indium Growth on Reconstructed Si(111)3 × 3 and 4 × 1 In Surfaces Dimitrios Vlachos,*,† Mattheos Kamaratos,† Stylianos D. Foulias,† Federica Bondino,‡ Elena Magnano,‡ and Marco Malvestuto§ Department of Physics, UniVersity of Ioannina, P.O. Box 1186, GR-451 10 Ioannina, Epirus, Greece, IOM-CNR, Laboratorio Nazionale TASC, S.S. 14, km 163.5, I-34012 Trieste, Italy, and Sincrotrone Trieste, S.S. 14 km 163.5, Area Science Park-BasoVizza, 34012 Trieste, Italy ReceiVed: June 8, 2010; ReVised Manuscript ReceiVed: August 25, 2010
The morphology and growth mechanism of nanostructured metals on semiconducting substrates determine crucially the electronic and physicochemical properties of these adsorption systems. In some cases, these properties are affected by modification of the interfacial geometry, induced by the metal adsorbate on the semiconducting substrate. Thus, in this work we investigate indium growth on the Si(111)3 × 3 and Si(111)4 × 1 surfaces reconstructed by indium. The basic motivation of this study is to reveal how reconstruction of the silicon surface affects the growth mode and electronic properties of the indium overlayer. Therefore, the In/Si interface was mainly studied by Si 2p and In 4d photoemission spectra as well as by valence band measurements using synchrotron radiation. In addition, low-energy electron diffraction, Auger electron spectroscopy, thermal desorption spectroscopy, and electron energy loss spectroscopy were used to reveal the structure and adsorption states of the indium adsorbate on the reconstructed silicon substrates. The results indicate that the initial In-Si surface symmetry affects the growth mechanism of the indium overlayer. In particular, the Stransky-Krastanov mode holds for indium adsorption on the clean Si(111)7 × 7 and Si(111)3 × 3 In-reconstructed surface. On the other hand, indium develops on the Si(111)4 × 1 In surface according to the Volmer-Weber mechanism. The adsorbate approaches the metallic phase as the coverage approximates the monolayer irrespective of the substrate symmetry. 1. Introduction Metal-semiconductor interfaces are of great theoretical and technological interest due to their unique electronic and physicochemical properties. In some cases, these properties are affected by modification of the interfacial geometry, which the metal adsorbate induces on the semiconducting substrate within the regime of the first monolayer. For example, it has been demonstrated that the Schottky barrier is crucially affected by such structural transformation with a consequent influence in the electrical properties of the metal-semiconductor junction.1 In addition, just this year it was shown that the interfacial structure and bonding between one Pb or In atomic layer metal film and a Si(111) substrate can enhance the observed twodimensional superconductivity.2 Among the metal adsorbates, indium adsorption on the Si(111) surface gives a number of coverage- and temperature-dependent structural phases, well documented in numerous previous works.3-6 It is generally believed that indium is a nonreactive element with the Si surface in that no silicides or surface disruption have been observed.7 However, the In-Si bonding is quite strong and probably of covalent type since it forms various surface reconstructions. Therefore, the In-Si(111) adsorption system looks suitable in order to investigate if and how these surface reconstructions affect the growth mechanism of the metal adsorbate, as well as the electronic properties of the metal-semiconductor interface. Previous results based on Auger electron spectroscopy and X-ray reflectivity7,8 have shown that In on the Si(111)7 × 7 surface * To whom correspondence should be addressed. Phone: +302651008578. Fax: +302651008694. E-mail:
[email protected]. † University of Ioannina. ‡ Laboratorio Nazionale TASC. § Sincrotrone Trieste.
grows according to the Stransky-Krastanov mechanism. However, the growth rate of In appears to be greatly reduced when adsorption on the Si(111)7 × 7 substrate takes place at temperatures > 300 °C.7 This observation is quite important because the indium-induced silicon reconstructions occur above that temperature.6 Thus, we believe that it is worthy to study in detail the growth and electronic properties of the In overlayer on different reconstructed silicon surfaces such as the 3 × 3-R30° and 4 × 1. We choose these two phases to be the substrate of In adsorption, because both of them are among the most widely studied reconstructed silicon surfaces with wellknown atomic and electronic structures. For comparison, we also briefly report on the In adsorption on the clean Si(111)7 × 7 surface. The study is based on synchrotron radiation measurements such as X-ray photoemission spectroscopy (XPS) and valence band (VB) measurements combined with the use of basic surface analytical techniques such as Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), thermal desorption spectroscopy (TDS), and electron loss spectroscopy (EELS). The work is divided into two main parts. In the first part, we spectroscopically characterize the 3 × 3-R30° and 4 × 1 phases with all of the available techniques at our disposal mainly for reference purposes and discuss the results in comparison with the literature. In the second part, we study adsorption of In on the two different reconstructed silicon phases at room temperature (RT) and compare the results with those on the clean 7 × 7 reconstructed silicon surface. The basic motivation of this work is to reveal how the initial reconstruction of the silicon surface affects the growth mode and electronic properties of the indium overlayer.
10.1021/jp105278r 2010 American Chemical Society Published on Web 09/22/2010
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2. Experimental Section Experiments were performed in two different ultrahighvacuum chambers with both at a base pressure on the order of 10-10 Torr. The first chamber is located at BACH beamline in the ELETTRA synchrotron radiation source in Trieste in Italy.9 All photoelectron spectroscopy measurements were collected by a 16-channel hemispherical energy VSW electron analyzer with the photon beam in the normal incidence and the take off angle equal to 60°. The substrate was a p type Si(111) single crystal of resistivity 100 Ω cm. A clean Si(111)7 × 7 reconstructed surface was obtained by several cycles of resistive flash heating at ∼850 °C. The sample temperature was measured by a portable infrared pyrometer. Indium deposition was achieved by using a Knudsen cell with an average flux of ∼0.07 monolayers (ML)/min estimated by recording the attenuated intensity of the Si 2p signal after In dosing. LEED measurements were performed to monitor the indium-induced silicon surface reconstructions. The indium-induced 3 × 3-R30° and 4 × 1 silicon-reconstructed surfaces were obtained after annealing ∼1 ML of In dosed on silicon substrate for ∼2 min at ∼440 and 350 °C, respectively. All photoelectron spectra were normalized to the incident photon flux, and energy calibration was performed by measuring the Fermi edge on a Ta foil mounted in electrical contact with the sample. The total energy resolution for measuring the Si 2p was 120 meV, while for the In 4d and valence band spectra it was 40 meV. A second ultrahigh-vacuum chamber equipped with AES, LEED, TDS, and EELS techniques was also used in the Department of Physics of the University of Ioannina in Greece. The Si(111) sample was mounted on a X-Y-Z manipulator and could be heated up to 1200 °C by a Ta tape firmly attached to the back side of the crystal and uniformly pressed between the crystal and a Ta metallic case. The temperature of the sample could be measured by a NiCr-CrAl thermocouple spot welded onto the center of the Ta case and calibrated by an infrared pyrometer. Deposition of In was done by using the same Knudsen cell used in ELETTRA. The AES measurements were performed by utilizing a primary electron beam with energy E ) 2 keV. The Auger electrons were collected and analyzed by a VG semicylindrical mirror analyzer. The AES and EELS spectra were recorded by the analyzer in the first-derivative mode, dN(E)/dE, and the intensity was measured from the peak to peak height (AP-PH). The energy of the primary electron beam used for the EELS measurements was Ep ) 120 eV. Finally, the TDS measurements were received by a UTI quadrupole mass spectrometer (QMS) with the sample annealing rate ∼15 °C/s. 3. Results and Discussion 3.1. 3 × 3-R30° and 4 × 1 Silicon-Reconstructed Surfaces. Two of the most widely studied indium-induced phases on Si(111) are the 3 × 3-R30° and 4 × 1. The typical LEED patterns of these phases as well as that of the clean Si(111)7 × 7 reconstructed silicon surface are shown in Figure 1. The 3 × 3-R30° phase (hereafter referred to as 3 × 3) is known as the β phase with coverage 0.3 ML, while the 4 × 1 phase corresponds to a coverage range from 0.7 to 1 ML.5,6,10 Regarding the 3 × 3 phase, two alternative In adsorption sites have been proposed: (1) the 4-fold-coordinated T4 site above the second layer of Si atoms and (2) the H3 3-fold hollow site in the double-surface Si layer. Although initially the distinction between the two models was not easy,11,12 it seems that there is finally a general agreement that In adatoms occupy most likely T4 sites as the adsorption model in Figure
Figure 1. LEED patterns of (a) the clean Si(111)7 × 7 reconstructed surface at E ) 167 eV, (b) Si(111)3 × 3 In surface at E ) 206 eV, and (c) Si(111)4 × 1 In surface at E ) 159 eV.
2a illustrates.13-16 Concerning the 4 × 1 phase, a widely accepted model states that In atoms form double zigzag rows along the [1j10] direction on top of an unreconstructed silicon substrate. According to that model, first proposed by Nakamura et al.17 (N model), indium adatoms occupy T4, H3, and bridge sites (B) as well, as illustrated in Figure 2b for coverage 1 ML. However, Stevens et al.18 performing STM measurements supported a slightly different model corresponding to 0.5 ML coverage, with the In adatoms relaxing only on T4 and H3 sites. This model is supported by STM measurements,6,19 but it is inconsistent with surface X-ray diffraction measurements which alternatively support the N model.20 The apparent disagreement between the N model and the STM images has been attributed to a height difference (∼0.8 Å) between the four indium atoms within the unit mesh,18 a distance which had also been noticed in Nakamura’s et al. model17 and is drawn in Figure 2b (side view). More recently, Bunk et al.21 doing X-ray diffraction measurements proposed a modified model, where two zigzag indium chains are arranged between parallel zigzag silicon chains, on top of an unreconstructed silicon surface, resulting in 1 ML coverage (B model). The B model is illustrated in Figure 2c and is consistent with a lot of previous experimental data, while it is also supported by numerous subsequent theoretical calculations.22-25 Note that the “inner” In atoms along the zigzag In chains (denoted with number 2 in Figure 2c) are surrounded by six In atoms, while the “outer” In atoms (denoted with number 1 in Figure 2c) are surrounded by only four. As we will see later, the different local atomic environment of these two different types of In atoms gives different electronic states
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Figure 2. Top and side view of the proposed adsorption models for (a) the Si(111)3 × 3 In-reconstructed surface, with the In adatoms relaxing on the T4 adsorption sites, (b) the Si(111)4 × 1 In surface, with the In adatoms relaxing on T4, H3, and bridge adsorption sites (B) named as the N model17 (see text), and (c) the Si(111)4 × 1 In surface, with zigzag indium chains arranged between parallel zigzag top silicon atoms chains named as the B model21 (see text). The “outer” and the “inner” In atoms along the parallel zigzag indium chains are denoted with the numbers 1 and 2, respectively. The unit mesh is drawn in each case by dotted lines.
of the adsorbate. Finally, different models than the aforementioned more popular ones have also been proposed to interpret the 4 × 1 reconstruction.26,27 The valence band of the clean Si(111)7 × 7 surface as well as those of the In-reconstructed 3 × 3 and 4 × 1 silicon surfaces are shown in Figure 3a. The clean Si(111)7 × 7 surface presents three well-known surface states S1, S2, and S3 at ∼0.2, 0.9, and 1.8 eV, respectively, which are due to the different coordinations of the top Si atoms.28 The higher energy VB features can be identified as bulk transitions. The surface states of silicon disappear in the VB of the 3 × 3 phase. This can be attributed to the interaction of the In adatoms with the surface Si atoms, reconstructing the surface by coupling the pz silicon atomic orbitals with the px and py metal adatom ones, as is suggested by angle-resolved photoelectron spectroscopy29 and theoretical calculations.30 The disappearance of the surface states near the Fermi edge and development of a new one at ∼1.4 eV indicates the semiconducting character of the 3 × 3 phase. This is in agreement with the scanning tunneling spectroscopy (STS) measurements by Kraft et al.,6 where a surface-state band gap of 0.95 eV was recorded. The new emission feature at ∼1.4 eV is attributed to an In-induced surface state assigned to the bonds between the In and surface Si atoms.11,31 As the 3 × 3 phase turns into the 4 × 1 one, the VB spectrum changes again with the development of three surface states at energies ∼0.4, 1.0, and 1.5 eV, respectively. In addition, it seems that these states make the 4 × 1 phase more metallic than the 3 × 3 one, as previous STS measurements report.6 The metallic character of the 4 × 1 phase is also supported by angle-resolved photoemission spectroscopy32 and inverse photoemission experi-
ments.33 Furthermore, Nakamura et al.,23 performing firstprinciples total energy calculations, confirmed the surface metallic states, which have been attributed not only to the metallic bonds in the In overlayer but also to covalent-like bonding between In and Si atoms with the In 5p and Si 2p orbitals participating. Even more recently, Lopez-Lozano et al.34 doing band structure calculations also revealed the metallic character of the 4 × 1 phase. The Si 2p photoemission spectra are shown in Figure 3b for the clean Si(111)7 × 7 surface and the 3 × 3 and the 4 × 1 phase. The clean sample shows a small contribution on the low binding energy (BE) side which reflects the surface states of silicon.35 These states disappear in the 3 × 3 phase due to the indium adatoms interaction with the silicon surface atoms. In the 4 × 1 phase, the doublet develops at ∼0.4 eV lower BE compared to the energy of the clean and 3 × 3 phase doublets. This energy difference is consistent with previous results36 and might be attributed to the metallic character of the In overlayer and therefore to the significant role of the extraatomic relaxation effects. The In 4d photoemission spectra for (a) the 3 × 3 and (b) the 4 × 1 phase are shown in Figure 4. The spectra were fitted by a Doniach-Sunjic line shape convoluted with a Gaussian function to describe both experimental resolution and phonon broadening. An integrated background was subtracted. The detailed parameters for the curve fitting are listed in Table 1. As a result of the curve analysis, both of the In 4d spectra were deconvoluted into a pair of doublets noticed as C1 and C2. Each doublet comprises two spin-orbit split peaks representing the 4d5/2 and 4d3/2 atomic levels, respectively. Regarding
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Figure 4. In 4d photoemission spectra for the (a) 3 × 3 and (b) 4 × 1 indium-reconstructed Si(111) surfaces. The curve-fitting parameters for the components C1 and C2 are shown in Table 1.
TABLE 1: Curve-Fitting Parameters for the Components C1 and C2 Used To Fit the In 4d Photoemission Spectra for the 3 × 3 and 4 × 1 Phases As Well As for the In Adsorption on These Surfacesa surface 3 × 3 4×1 In(1.4 ML)/3 × 3 In(5.6 ML)/3 × 3 In(1.4 ML)/4 × 1 In(7 ML)/4 × 1 Figure 3. (a) Valence band and (b) Si 2p photoemission spectra of the clean Si(111)7 × 7, 3 × 3, and 4 × 1 reconstructed Si(111) surfaces.
the 3 × 3 phase at 0.3 ML coverage, the two doublets (Figure 4a) represent two different adsorption sites of the In adatoms on the silicon surface. The dominant C1 doublet is obviously due to the In atoms relaxing on the T4 adsorption sites, while the weak C2 doublet is probably related to defects on the 3 × 3 surface.37 Although previous studies report only a single In 4d doublet,31,36 here we detect a significant C2 component at higher BE than the C1. The C2 component might be interpreted with In adatoms adsorbing on defect sites with lower coordination such as step edges. Perhaps the Si atoms at these sites are more reactive, presenting a stronger bonding with In adatoms, interpreting the higher BE of the C2 component compared to that of the C1 one. Similarly, the two doublets in the analyzed In 4d spectrum of the 4 × 1 reconstructed surface at 1 ML coverage, shown in Figure 4b, correspond to two different In adsorption sites. These are the “inner” and “outer” indium atoms among the four within the unit cell, in agreement with previous high-resolution photoemission measurements.38 The outer In atoms in the edge row (outer atoms are labeled as 1 in Figure 2c) are fixed to the silicon substrate by In-Si covalent bonds, while the inner In
component C1 C2 C1 C2 C3 C4 C4 C1 C2 C4 C1 C4
SCLS Gauss (eV) width (eV) asymmetry 0 0.36 0 -0.46 0.08 -0.40 -0.40 0 -0.46 -0.28 0 -0.28
0.48 0.41 0.34 0.36 0.48 0.27 0.23 0.34 0.36 0.23 0.34 0.19
0 0 0.09 0.10 0 0.08 0.13 0.09 0.10 0.12 0.09 0.12
a
SCLS denotes the surface core-level shift. The spin-orbit splitting of 0.88 eV, the Lorentzian width of 0.17 eV, and the branch ratio of 0.66 are common to all spectra. Note that the indium coverage in parentheses describing the adsorbed quantity of In on the 3 × 3 and 4 × 1 surfaces does not include the predeposited 0.3 or 1.0 ML of In on the clean Si(111)7 × 7 surface in order to obtain the 3 × 3 or 4 × 1 reconstruction.
atoms in the central rows (inner atoms are labeled as 2 in Figure 2c) are expected to interact with each other by metallic bonds. Thus, the C1 doublet represents the outer indium atoms in the zigzag indium chains reacting covalently with the neighboring silicon atoms. In contrast, because of the more neighboring In adatoms (6 atoms), the inner indium atoms are probably more weakly bound to the surface, thus giving rise to the lower BE doublet C2 in Figure 4b. Remarkably, theoretical calculations for the 4 × 1 phase predict that the 4d core levels of the inner indium atoms are shifted to lower binding energy by 0.46 eV relative to those of the outer indium atoms.24 This is in excellent agreement with our measured energy shift reported in Table 1. Additionally, theoretical calculations verify the covalent bonding between the outer In atoms with the neighboring Si ones and also reveal a metallic character of the bond between the inner In atoms originating from the In 5s orbitals.23
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Figure 5. TDS spectra of the In(115 amu) QMS signal of both the 3 × 3 and 4 × 1 indium-reconstructed Si(111) surfaces.
We also performed TDS measurements for the 3 × 3 and 4 × 1 In-reconstructed silicon surfaces as Figure 5 illustrates. The 3 × 3 phase gives a single thermal desorption (TD) peak at ∼800 °C, which might be related to indium desorption from the T4 adsorption sites. On the other hand, the 4 × 1 phase gives three different TD peaks. The one at the highest temperature appears more like a shoulder and can be identified as the peak due to the desorbing atoms from the T4 sites. The two other peaks develop at ∼750 and 710 °C and at first sight look quite equivalent in intensity. However, additional TDS measurements (not shown) illustrated that for the 4 × 1 phase the 750 °C peak develops before the 710 °C one. In addition, the latter peak continues to increase further with coverage, overcoming the intensity of the 750 °C peak. In other words the relative intensity ratio between these two recorded TD peaks in the 4 × 1 phase depends on the indium concentration (0.7-1 ML). This lead us to postulate that the 750 °C peak is probably due to the desorbing atoms from the H3 adsorption sites, while the 710 °C one should be related to the In adatoms desorbing from the B sites (N model). The area of the 710 °C TD peak is much bigger than that of the 750 °C one (not shown), which might be interpreted by the fact that the density of the B sites is twice that of the H3 sites at a coverage of 1 ML. Thus, it seems that during formation of the 4 × 1 phase the H3 sites are filled first while the B sites are occupied later. The EELS spectra of the clean Si(111)7 × 7 and indiuminduced reconstructed 3 × 3, 4 × 1, and 1 × 1 silicon surfaces are shown in Figure 6. The curves represent the differentiated energy loss spectra dN(E)/dE received with primary electron beam energy Ep ) 120 eV. The clean silicon surface shows the usual loss spectrum with the surface and bulk plasmon at about 11.1 and 18.6 eV, respectively. The loss at about 37.7 eV can be assigned to a second-order bulk plasmon oscillation. As the 7 × 7 silicon phase changes into the 3 × 3 one, the surface plasmon loss vanishes while the bulk one is reduced. In addition, two new loss peaks appear at about 6.4 and 13.6 eV. The first one can be attributed to the surface plasmon of In, while the second one is probably related to an In-Si interface plasmon.7 The In 4d loss is expected at ∼17 eV, but it probably overlaps with the Si bulk plasmon. For the 4 × 1 reconstruction, a strong loss at ∼9.2 eV appears which might be assigned to the In surface plasmon in the two-
Figure 6. EELS spectra of the clean Si(111)7 × 7 surface, 3 × 3, 4 × 1, and 1 × 1 indium-reconstructed Si(111) surfaces.
dimensional metallic phase. Both of the characteristic losses of the 3 × 3 phase almost vanish. As regards the 1 × 1 reconstructed phase obtained after deposition of 3 ML of In, two new losses are developing, one at ∼11.1 eV and a second at ∼7.4 eV, with the 9.2 eV loss weakening at the same time. As we will mention later, at this adsorption stage indium adatoms form three-dimensional particles on a predeposited indium atomic layer, so the adsorbate obtains bulk properties with the 11.1 eV loss assigned to the In bulk plasmon and the 7.4 eV to the surface one. The remaining weak loss at ∼5 eV might be attributed to a In-Si interface state. 3.2. Indium on the 3 × 3 and 4 × 1 Reconstructed Si(111) In Surfaces. 3.2.1. Synchrotron Radiation Measurements. The next step was In growth on the 3 × 3 reconstructed Si(111) surface at RT. LEED measurements showed that the symmetry of the surface changes as a function of the indium coverage. Figure 7 illustrates the observed LEED patterns, while Table 2 shows the coverage Θ in ML where each pattern appears. The initial 0.3 ML of the 3 × 3 reconstructed surface is not included in the reported coverage ranges in Table 2. As the LEED observations show, the initial 3 × 3 phase progressively transforms into a 2 × 2 one for Θ g 0.5 ML. Next, the 2 × 2 phase changes into a 1 × 1 one for Θ g 0.8 ML. Finally, the 1 × 1 phase is transformed into a 7 × 3 one for Θ g 1.2 ML. These phase transformations have been observed previously37,39 at almost the same indium coverage ranges. In particular, the 7 × 3 phase has also been described previously as 1 × 1-R30°.7,10 However, Pavlovska et al.40 doing low-energy electron microscopy (LEEM) and LEED measurements showed that these two phases are different and can coexist in various proportions depending upon coverage and preparation conditions. In the present work the
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Figure 8. In 4d photoemission spectra for indium adsorption on the Si(111)3 × 3 In surface at different coverages. The curve-fitting parameters for the components are shown in Table 1.
Figure 7. LEED patterns for In adsorption on the Si(111)3 × 3 In surface at RT: (a) 3 × 3 at E ) 51.5 eV, (b) 3 × 3 + 2 × 2 at E ) 51 eV, (c) 2 × 2 at E ) 49.2 eV, (d) 1 × 1 at E ) 77.2 eV, and (e) 7 × 3 at E ) 88.1 eV. The coverage range of the appearance of each LEED pattern is reported in Table 2.
TABLE 2: Observed Symmetries for Successive In Adsorption on the Si(111)3×3 In-Reconstructed Surface at RTa coverage, Θ (ML) LEED pattern
1 ML the increase of the In MNN signal becomes much slower, approaching saturation. At the same time the Si LMM substrate signal does not decrease substantially. This manner of adsorbate and substrate signal variation with the coverage indicates formation of 3D nanoparticles on the predeposited indium wetting layer, described by the StranskyKrastanov growth mechanism.7,45 LEED observations showed that the first indium layer lifts the 7 × 7 reconstruction restoring the 1 × 1 symmetry. This is in contrast with Pavloska’s et al. work,40 where the 7 × 7 pattern remains visible up to high coverages (10 ML) but is fully consistent with others’ work.7,45 ¨ fner et al.7 only approximately specify the coverage However, O where the 3D growth is established (Θ ≈ 1-2 ML). Moreover, Finney et al.,8 based on AES and X-ray reflectivity measurements, claim that there are two consecutive pseudomorphic indium layers before 3D growth. Nevertheless, our results in Figure 12 do not show a second “break” in the In MNN and Si LMM AP-PH curves, which would be evidence for a second uniform layer formation. In contrast, for Θ > 1 ML, both the Auger signals are fairly insensitive to coverage, declaring growth
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Figure 11. (a) Si 2p and (b) valence band photoemission spectra for indium adsorption on the Si(111) 4 × 1 In surface at different coverages.
of 3D particles above the wetting layer. This result is consistent with previous STM measurements,45 where the wetting indium
Figure 12. AP-PH of the In (402 eV) MNN and Si (91 eV) LMM Auger transition lines as a function of In doses on the clean Si(111)7 × 7, Si(111)3 × 3 In, and Si(111)4 × 1 In-reconstructed surfaces.
Vlachos et al. layer is not exactly a continuous monatomic layer but rather an ensemble of small In agglomerates uniformly spread over the Si surface. For In adsorption on the Si(111)3 × 3 In substrate, the In MNN signal also increases linearly in the submonolayer regime, indicating formation of a uniform two-dimensional indium layer. At ∼7.5 min, however, a premonolayer (pML) “break” appears which is probably related to the 2 × 2 phase formed for extra coverage 0.5-0.8 ML on the 3 × 3 phase (Table 2). Considering that the initial sticking coefficient of In on both the clean and the Si(111)3 × 3 In surface is the same (TDS measurements support that consideration), the total coverage corresponding to the pML “break” (including the initial 0.3 ML of the 3 × 3 phase) is ∼0.8 ML. At this coverage the 2 × 2 phase is fully developed. For doses higher than ∼7.5 min, the In MNN signal continues to increase linearly up to ∼17 min, where a second “break” is formed. This dosing corresponds to a total coverage of 1.4 ML, where the 1 × 1 phase is almost fully formed. This coverage can be justified if we take into consideration the negative lattice mismatch (∼15%) between the In(111) and Si(111) lattice constant (dIn/dSi ) 3.24/ 3.83). Thus, an In double layer seems to form at RT with 1 × 1 symmetry. Further dosing of In results in a slower rate of the In MNN AP-PH increase, approaching a maximum intensity. This implies formation of 3D particles with the accompanying phase transformation from 1 × 1 into the 7 × 3 phase. There are experimental indications based on STM measurements46 that this phase contains a single layer of In adatoms, although earlier RHEED observations support the opposite.47 The combination of our LEED and AES results suggest that the 7 × 3 phase is formed after double-layer completion (17 min, 1 × 1 phase), when 3D particles start growing. In this way, the 7 × 3 phase appears at a total coverage higher than 1.5 ML (including the initial ∼0.3 ML of the 3 × 3 phase). This coverage is slightly larger than that estimated by Kraft et al.46 However, as mentioned before (section 3.2.1), the 7 × 3 phase can also be described as 1 × 1-R30° since, although LEEM measurements revealed that these two phases are different, both of them coexistonthesiliconsurface.40 Moreover,duetotheindium-silicon lattice mismatch pointed out earlier, it is possible that a small fraction of the 3D indium particles incorporates into the 1 × 1 double layer, resulting in the 7 × 3 phase. Overall, out of the AES measurements In growth on the 3 × 3 phase seems to follow the Stransky-Krastanov mechanism as it happens on the clean Si(111)7 × 7 surface too. In the case of indium adsorption on the Si(111)4 × 1 surface, the In MNN signal increases slowly and not linearly, suggesting clearly that In grows in 3D nanoparticles. Thus, the previously developed wetting layer of 4 × 1 symmetry dictates the Volmer-Weber growth mode. Thus, comparing the In growth on the three different reconstructed silicon surfaces 7 × 7, 3 × 3, and4 × 1, we conclude that the symmetry of the reconstructed silicon surface plays a decisive role to the ensuing growth mechanism. We also performed TDS measurements for In adsorption on the reconstructed silicon surfaces. Figure 13 shows the TDS spectra of the In(115 amu) QMS signal for indium desorption from the Si(111)7 × 7 surface. Within the submonolayer coverage, the a TD peak at ∼820 °C grows with coverage and is postulated to be due to the T4 adsorption sites in the 3 × 3 phase. In addition, a second peak b develops at lower temperature and drifts to higher desorption temperature as the coverage increases. Near the completion of 1 ML (curve 4), a new desorption state c appears at ∼750 °C while the a peak
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In order to investigate further the role of the surface symmetry in the indium overlayer growth, we also performed TDS measurements (not shown) for indium adsorption on both the 3 × 3 and 4 × 1 reconstructed silicon surface. The results showed the existence of all four different adsorption states at similar temperature ranges as in Figure 13. 4. Conclusions
Figure 13. In(115 amu) QMS signal for indium desorption from the Si(111)7 × 7 surface. The dashed lines denote roughly the thermal evolution of each desorption peak, while the surface symmetry is also noed (see text).
seems to saturate. For Θ > 1 ML (curves 5 and 6), the b peak continues to grow and drift toward higher temperatures while the c one grows and moves to lower temperatures. Moreover, another desorption state d develops at the temperature range between the a and the c ones, which appears more as a shoulder than as a well-defined peak. In order to investigate if all four desorption states are related to the surface symmetry, we performed in situ LEED observations during the desorption process. In particular, when each desorption peak was near completion, we interrupted the desorption process and performed LEED at RT. In this manner, by carrying out TDS measurement for Θ > 1 ML and just before recording the b state, the 7 × 3 phase appeared, which means that the b state mainly involves desorption of In atoms from the second layer. In the same manner, just after recording the b peak and just before recording of the c and d ones, the 4 × 1 phase was observed. This indicates that the two latter states are related to two different adsorption sites of indium in the 4 × 1 phase as we discussed in the description of the 4 × 1 TD curve in Figure 5. The state c at ∼720 °C might be attributed to the inner zigzag indium atoms chains, while the d state at ∼765 °C can be assigned to the outer zigzag indium atoms chains. The c state appears at a lower temperature than the d one because the inner zigzag indium atoms interact more with each other, approaching the metallic phase, while the outer zigzag indium atoms interact also with the neighboring silicon atoms through a covalent type of bonding. The drift of the finally dominant c state to lower temperatures might be attributed to the increasing metallicity of the In overlayer as the coverage increases. Perhaps this drift also affects the weaker b state, which appears moving toward higher temperatures due to peak overlapping. When the c and d TD peaks are complete, the 3 × 3 symmetry appears, supporting the idea that the following a desorption state is due to the remaining In adatoms relaxing on the T4 sites. Finally, when the a peak is fully recorded the initial 7 × 7 silicon surface symmetry is restored.
The role of the symmetry of the indium-induced Si(111)reconstructed surfaces has been investigated on how it affects the structural and electronic properties of the developed indium overlayer at room temperature. For this purpose, a variety of techniques was used including synchrotron radiation. The results show that in the case of the clean Si(111)7 × 7 and Si(111)3 × 3 In surfaces the indium adsorbate develops in the Stransky-Krastanov growth mode. On the contrary, when In adsorbs on the Si(111)4 × 1 In surface, the Volmer-Weber growth mechanism takes place. The adsorbate approaches the metallic phase as the coverage exceeds one monolayer in all the studied reconstructed silicon surfaces. Therefore, we conclude that although the symmetry of the indium-induced silicon surface reconstruction does not significantly change the electronic properties of the developing indium overlayer, it determines crucially the growth mechanism of it. Acknowledgment. This work was financially supported by the EU under the contract RII3-CT-2004-506008 (IA-SFS). References and Notes (1) Heslinga, D. R.; Weitering, H. H.; Van der Werf, D. P.; Klapwijk, T. M.; Hibma, T. Phys. ReV. Lett. 1990, 64, 1589–1592. (2) Zhang, T.; Cheng, P.; Li, W. J.; Sun, Y. J.; Wang, G.; Zhu, X. G.; He, K.; Wang, L.; Ma, X.; Chen, X.; Wang, Y.; Liu, Y.; Lin, H. Q.; Jia, J. F.; Xue, Q. K. Nat. Phys. 2010, 6, 104–108. (3) Lander, J. J.; Morrison, J. J. Appl. Phys. 1965, 36, 1706–1713. (4) Kawaji, M.; Baba, S.; Kinbara, A. Appl. Phys. Lett. 1979, 34, 748– 749. (5) Baba, S.; Hirayama, H.; Zhou, J. M.; Kinbara, A. Thin Solid Films 1982, 90, 57–61. (6) Kraft, J.; Ramsey, M. G.; Netzer, F. P. Phys. ReV. B 1997, 55, 5384–5393. ¨ fner, H.; Surnev, S. L.; Shapira, Y.; Netzer, F. P. Phys. ReV. B (7) O 1993, 48, 10940–10949. (8) Finney, M. S.; Norris, C.; Howes, P. B.; Vlieg, E. Surf. Sci. 1992, 277, 330–336. (9) Zangrando, M.; Finazzi, M.; Zacchigna, M.; Cocco, D.; Rochow, R.; Parmigiani, F. ReV. Sci. Instrum. 2004, 75, 31–36. (10) Nogami, J.; Park, S.-I.; Quate, C. F. Phys. ReV. B 1987, 36, 6221– 6224. (11) Nicholls, J. M.; Mårtensson, P.; Hansson, G. V.; Northrup, J. E. Phys. ReV. B 1985, 32, 1333–1335. (12) Kinoshita, T.; Kono, S.; Sagawa, T. Phys. ReV. B 1985, 32, 2714– 2716. (13) Nogami, J.; Park, S.-I.; Quate, C. F. J. Vac. Sci. Technol. B 1988, 6, 1479–1482. (14) Finney, M. S.; Norris, C.; Howes, P. B.; Van Silfhout, R. G.; Clark, G. F.; Thornton, J. M. C. Surf. Sci. 1993, 291, 99–109. (15) Hanada, T.; Daimon, H.; Ino, S. Phys. ReV. B 1995, 51, 13320– 13325. (16) Mizuno, S.; Mizuno, Y. O.; Tochihara, H. Phys. ReV. B 2003, 67, 195410. (17) Nakamura, N.; Anno, K.; Kono, S. Surf. Sci. 1991, 256, 129–134. (18) Stevens, J. L.; Worthington, M. S.; Tsong, I. S. T. Phys. ReV. B 1993, 47, 1453–1459. (19) Park, S.-I.; Nogami, J.; Quate, C. F. J. Microsc. 1988, 152, 727– 734. (20) Finney, M. S.; Norris, C.; Howes, P. B.; James, M. A.; Macdonald, J. E.; Johnson, A. D.; Vlieg, E. Physica B 1994, 198, 246–248. (21) Bunk, O.; Falkenberg, G.; Zeysing, J. H.; Lottermoser, L.; Johnson, R. L.; Nielsen, M.; Berg-Rasmussen, F.; Baker, J.; Feidenhans’l, R. Phys. ReV.B 1999, 59, 12228–12231. (22) Miwa, R. H.; Srivastava, G. P. Surf. Sci. 2001, 473, 123–132. (23) Nakamura, J.; Watanabe, S.; Aono, M. Phys. ReV. B 2001, 63, 193307.
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