The understanding of iodochloride (ICl) gas is excellent framework for the gas-surface reaction of ICl and semiconductor surface. Adsorption of halogen and heterohalogen molecules on semiconductor surfaces has obtained much attention as model systems for chemisorption dynamic and kinetic processes. Kummel et al. chose to study the heterohalogen molecules of iodine monochloride (ICl) and iodine monobromide (IBr) on the Si(111)-7×7 surface and reported very different adsorption processes for the two similar systems[38, 39]. Their study shows that the adsorption of IBr is essential dissociative and that the adsorption of ICl at room temperature proceeds predominantly (I:Cl=3:1) via the least exothermic channel of iodine abstraction [38, 39]. In other words, the iodine atom in an impinging ICl molecule is selectively abstracted by the surface dangling bond to form silicon monoiodide species, Si-I, on the surface. The second atom–chlorine in ICl, is ejected back to the gas phase. The authors attribute this behavior to the fact that the HOMOs of the gas molecule are preferentially concentrated at the iodine atom. The HOMOs of IBr distribute almost evenly on the I and Br atoms and, therefore, the selectivity over I and Br is absence (I:Br = (1:1) in the absorption of IBr on the Si(111)-7 × 7.
In our present research, ICl and IBr gases are exposed on the symmetric dangling bond Si(100)-2×1 to study of the surface-gas reactions and adsorbate-adsorbate interactions.
Another interesting study is about the ordering characteristic of the mixed adsorbates. For the adsorption of a similar molecule HCl on the Si(100) surface, we have found that the Cl and H adsorbates together generates many local 2×2 patterns[40]. The ordering of mixed adsorbates has been shown to due to the interaction between a dissociated fragment atom and its surrounding adsorbates.
Comparison with the adsorption of ICl or IBr on the Si(100) has also been made to clarify the role of the asymmetric molecule (ICl and IBr) bonding with the symmetric substrate (Si(100)-2×1) in adsorption dynamics. The coverage of the saturation for ICl on Si(100) only has large area pattern of c(2×2) but for IBr on Si(100) has c(2×2)-m, c(4×2)-m, c(4×2)-p, p(2×1)-m, and p(4×1)-m small areas of the local patterns.
5. 2 Experimental details
The Si(100) samples were sliced from Boron-doped wafers with a dopant concentration of approximately 1.5×1015cm-3. After outgassing at ~900 K for ~12 h, a dimerized clean Si(100) surface was obtained by dc Joule heating to ~1450 K for a few seconds.
The vapor of ICl(or IBr) was introduced from neat ICl (or IBr) liquid stored in a glass tube, which is attached to the vacuum chamber by a variable leak valve. ICl (or IBr)was purified by degassing in several freeze-pump-thaw cycles prior to use. The apparent ICl exposure, i.e., P×T, is presumably proportional to the actual dosage of ICl molecules on the surfaces. The molecular ICl flux was not measured directly in the present study. Instead, the apparent exposure in Langmuir (1 L=10-6 Torr‧s) is used as the relative measurement of ICl (or IBr) dosage on the bare Si(100)-2×1 surface.
The photoemission spectra were observed at the Taiwan Light Source laboratory in Hsinchu, Taiwan. Synchrotron radiation from a 1.5 GeV storage ring was dispersed by a wide-range spherical grating monochromator. The photocurrent from a gold mesh positioned in the synchrotron beam path was monitored to calibrate the incident photon flux.
Photoelectrons were collected 15° from the surface normal and analyzed by a 125 mm hemispherical analyzer in a μ-metal shielded UHV system. The overall energy resolution was less than 120 meV. The STM measurement was performed in a separated UHV chamber.
5. 3 Results and discussion
5. 3. 1 Photoemission of ICl / Si(100)
High-resolution core level photoemission spectroscopy can be used to distinguish atoms at nonequivalent sites and in different chemical bonding configurations, according to shifts in their binding energy. Figure 5. 1(a), (b), and (c) show the respective surface-sensitive Si 2p, I 4d and, Cl 2p core level spectra (circles), and their decomposition into constituent components from the ICl–Si(100)-2×1 surface before and after ICl exposed at 325 K for various dosages. All fitting was least-squares fitting.Each component that consists of a pair of spin-orbit split doublets is assumed to have the same Voigt line shape.
Before ICl exposed, the Si 2p spectrum Figure 5. 1(a), first from bottom) has a prominent peak S at the lower (-0.52 eV) binding energy side and a visually indiscernible peak S’ at the higher (+0.26 eV) binding energy side. These two components are attributed to emissions from the up atoms of asymmetric dimers and atoms in the second layer, respectively. After the ICl exposure, the spectra in Figure 5. 1(a) shows the Si 2p core level spectra for the ICl on Si(100) surface. This Si 2p spectrum consists of two components, B and C, separated by about 0.80 eV. The B component is responsible for emission from the bulk and the C component from the contribution of the surface Si-I and Si-Cl species together. As the exposure of ICl increases, the intensities of the peak S component spectra drop off.
The I 4d spectra in Figure 5. 1 (b) each has only one component, consisting of a pair of 1.70 eV spin-orbit-split peaks. The Cl 2p spectra in Figure 5. 1(c) can be analyzed with a component that consists of a pair of split doublets separated by 1.60 eV. The binding energy of these I 4d and Cl 2p spectra relative to that of the corresponding Si 2p remains at 99.5 eV, suggesting that the ICl atoms form similar Si-Cl and Si-I bonds. The 10L (1 L=10-6 Torr‧s) of the exposure of ICl is saturation on the Si(100) surface.
In Figure 5. 2 X-ray photoemission spectroscopy (XPS) was used to determine the absolute ratio of Cl to I for the Si(100)-2×1 surface. The absolute ratios of Cl to I indicated that the ratio of Cl/I is about 0.95 on the Si(100) surface with various amount exposure of ICl. The I-rich Si(111) surface (Cl/I~0.3) at low coverage but a stoichiometric surface (Cl/I~1) at high coverage that is observed in the ICl/Si(111) system is not observed here. So
the XPS results suggested that the adsorption of ICl on Si(100) is no atomic selectivity and ICl is dissociative adsorption on the Si(100).
5. 3. 2 STM images of ICl on Si(100)-2×1
The initial clean Si(100) surface (not shown) forms a (21) reconstruction consisting of parallel rows of dimers. Each surface Si atom has one dangling bond. On the vicinal Si(100) surface, two different types of single-height steps, SA and SB, of height 0.15 nm separates perpendicular domains of (2 1) reconstruction.
Figure 5. 3 is a high-resolution empty-state STM images (20×10 nm2) with sample bias Vs = + 2.1 V and tunneling current IT = 0.2 nA after the ICl exposed on the Si(100)- 2×1 surface. In Figure 5. 3, the saturation of ICl on the Si(100) shows the c(2×2) atomic structure (89.4 %), Si vacancies (labeled defect) (5.4 %), and a few 2×1 atomic structure (5.2 %). Table 5. 2 shows the calculation of the pattern c(2×2), 2×1, and defect.
In Figure 5. 3, the saturation ICl has a well ordered structure c(2×2) on Si(100) surface and the atomic structure is shown in Figure 5. 4. Since the ratio of the Cl to I coverage is roughly 1：1 (Cl/I~0.95 discussed in section 5. 3. 1), in Figure 5. 4(c) on the one dimer raw zigzag structure chain is an ordered array of Cl-Si-Si-I, in which Cl-sites and I-sites in neighboring dimers are anti-phase with each other. The c(2×2) structure can be regarded as a combination of more than two neighboring zigzag Cl-Si-Si-I chains. According the simulated STM images for ICl/Si(100) (discussed in 5. 3. 3), for the empty states of the simulated STM images the protrusions of Si-I is brighter than Si-Cl at Vs > +2.4 V which is corresponded to the empty state STM images. And the XPS (in section 5. 3. 1) results indicated that the ratio of Cl / I is 0.95 which is agreed with the calculation of the ratio of Cl/I~0.9 in STM image (shown in Table 5. 2) at the saturation of the ICl on Si(100).
Therefore, in the c(2×2) structure as shown in Figure 5. 4(a), the brighter protrusions can be regarded as Si-I and the darker sites can be regarded as Si-Cl.
The XPS results (in section 5. 3. 1) indicated that the ICl is no atomic selection and Cl/I is about 1, suggesting that ICl is dissociative adsorption (Cl : I~1 : 1) on the Si(100) surface for the various coverage of ICl. And the ab-initio radius difference between I (1.27 Å) of Si-I and Cl (0.89 Å) of Si-Cl is large. Therefore, in order to have the lowest surface energy for ICl/Si(100), the STM results revealed that Si-I and Si-Cl atoms have to alternately arrange to form a large areas c(2×2), suggesting that I-Si-Si-Cl has the lowest electron overlap.
In Figure 5. 3, the 2×1 structure (5.2 %) can be observed. The atoms in the 2×1 structure are the same as bright as the Si-I in the area of c(2×2) and the XPS results(in section 5. 3. 1) indicated that I atoms is more than Cl atoms by 5%., so the atoms in the area of 2×1 is suggested consisting of I-Si-Si-I. In Figure 5. 3, some darkest area also can be observed. Dimer vacancies, where both Si dimer atoms are missing, appear as dark features spanning the full width of the dimer row, and the single Si-atom vacancies are imaged as small dark features half the width of the dimer row. So in Figure 5. 3 the darkest area can be regarded as the dimer vacancies or single Si-atom vacancies.
5. 3. 3 STM simulation of ICl/Si(100)
According to the Tersoff-Hamann approximation, the tunneling current in STM is proportional to the local density of states (LDOS) near the Fermi level at the position of the tip. To account for the STM tunneling current, we integrate the LDOS between the sample bias and the Fermi energy level. The partial density:
r
EEFFeVSdE
n n E n k k(r)2 ( k)STM
should reflect the STM tunneling currents. Figure 5. 5 shows the simulated empty-state STM images above the top ICl layer by 1.5 Å at various sample voltages for the saturation ICl on Si(100). Figure 5. 5 (a), (b), and (c) display c(2×2) images for Vs > +2.0 V. The c(2×
2) structure is consistent with the experimental STM images as shown in Figure 5. 4 (a). It also reveals that the bright protrusions of the empty-state STM image at the sample bias of Vs = +2.1 V should be assigned to the I atoms.
5. 3. 4 Photoemission of IBr / Si (100)
High-resolution core level photoemission spectroscopy can be used to distinguish atoms at nonequivalent sites and in different chemical bonding configurations, according to shifts in their binding energy. Figure 5. 6 (a), (b), and (c) show the respective surface-sensitive Si 2p, I 4d and, Br 3d core level spectra (circles), and their decomposition into constituent components from the IBr–Si(100)-2×1 surface before and after IBr exposed at 325 K for various dosages. All fitting was least-squares fitting.Each component that consists of a pair of spin-orbit split doublets is assumed to have the same Voigt line shape.
Before IBr exposed, the Si 2p spectrum Figure 5. 6 (a), first from bottom) has a prominent peak S at the lower (-0.52 eV) binding energy side and a visually indiscernible peak S’ at the higher (+0.26 eV) binding energy side. These two components are attributed to emissions from the up atoms of asymmetric dimers and atoms in the second layer, respectively. After the IBr exposure, the spectra in Figure 5. 6 (a) shows the Si 2p core level spectra for the IBr on Si(100) surface. Upon IBr adsorption the peak S decrease with the various amounts of IBr and a chemically shifted shoulder develops on the high binding energy side. For the fitted Si spectrum from the saturated surface (10 L), this Si 2p spectrum consists of two components, B and C, separated by about 0.72 eV. The B component is responsible for emission from the bulk and the C component from the contribution of the surface Si-I and Si-Br species together. As the exposure of IBr increases, the intensities of the peak S component spectra decreased with the amount of the exposure IBr.
The I 4d spectra in Figure 5. 6 (b) each has only one component, consisting of a pair of 1.70 eV spin-orbit-split peaks. The Br 3d spectra in Figure 5. 6 (c) can be analyzed with a component that consists of a pair of split doublets separated by 1.0 eV. The binding energy of these I 4d and Br 3d spectra relative to that of the corresponding Si 2p remains at 99.5 eV, suggesting that the IBr atoms form similar Si-I and Si-Br bonds. The 10L of the exposure of IBr is saturation on the Si(100) surface.
In Figure 5. 2 the absolute ratios of Br to I indicated that the ratio of Br/I is about 0.85 on the Si(100) surface with various amount exposure of IBr. In contrast to ICl/Si(111)-7×7 and IBr/Si(111)-7×7[38, 39], their study shows that the adsorption of IBr is dissociative and
that the adsorption of ICl at room temperature proceeds predominantly (I:Cl=3:1) at low coverages via the least exothermic channel of iodine abstraction [38, 39]. The I-rich Si(111) surface (Cl/I~0.3) at low coverage but a stoichiometric surface (Cl/I~1) at high coverage that is observed in the ICl/Si(111) system is not observed here. The adsorption of IBr on Si(100) is no apparently atomic selectivity is presented and the IBr dissociative adsorption on the Si(100) surface because the Br/I is about 0.85 which is similar the results of IBr/Si(111) with the various exposure of IBr.
5. 3. 5 STM images of IBr on Si(100)-2×1
The initial clean Si(100) surface (not shown) forms a (21) reconstruction consisting of parallel rows of dimers. Each surface Si atom has one dangling bond. On the vicinal Si(100) surface, two different types of single-height steps, SA and SB, of height 0.15 nm separates perpendicular domains of (2 1) reconstruction.
Figure 5. 7 is a high-resolution empty-state STM images (20×10 nm2) with sample bias Vs = + 2.4 V and tunneling current IT = 0.2 nA after the IBr exposed on the Si(100)- 2×1 surface. In Figure 5. 7 the protrusion spots are presented on the Si(100) surface after the exposure of IBr. The protrusions have local pattern as shown in solid boxes enclose selected areas of Figure 5. 7 (b) p(2×1)-m and p(4×1)-m, Figure 5. 7(c) c(4×2)-p and c(4×2)-m, and Figure 5. 7(d) c(2×2)-m. According to the empty-state of the simulated STM image as shown in Figure 5. 10, the brighter protrusions of Si-I is brighter than Si-Br at Vs > +2.0 eV which is corresponded to the bias voltage Vs= + 2.4 V (empty state) of the experimental STM image. Therefore, the brighter protrusions indicated the Si-I and the dimmer sites indicate Si-Br in the empty state of the saturation of IBr/Si(100) STM images. The sphere models of c(2×2)-m and c(4×2)-m are shown in Figure 5. 8 and Figure 5. 9, respectively.
5. 3. 6 STM simulation of IBr/Si(100)
According to the Tersoff-Hamann approximation, the tunneling current in STM is proportional to the local density of states (LDOS) near the Fermi level at the position of the tip. To account for the STM tunneling current, we integrate the LDOS between the sample bias and the Fermi energy level. The partial density:
r
EEFFeVSdE
n n E n k k(r)2 ( k)STM
should reflect the STM tunneling currents. Figure 5. 10 shows the simulated empty-state STM images above the top IBr layer by 1.5 Å at various sample voltages for the saturation IBr on Si(100). Figure 5. 10 displays c(2×2) images for Vs > +2.0 V. The c(2×2) structure is consistent with the experimental STM images as shown in Figure 5. 7 (d). It also reveals that the bright protrusions of the empty-state STM image at the sample bias of Vs = +2.4 V should be assigned to the I atoms.
5. 4 Conclusions
In contract to ICl and IBr on Si(100)-2×1, the XPS results indicated that the majority adsorption of ICl (Cl/I~0.95) and IBr (Br/I~0.85) are both dissociated adsorption on the Si(100) surface and no apparent atomic selection adsorption with the various exposure amount of ICl or IBr. Because for the Si(100)-2×1 the two Si atoms on the same dimer has the nearest dangling bond being 2.40 Å and the bond length of the two dangling bond for Si(100) is close to the bond length of the ICl (2.32 Å) and IBr (2.49 Å), the ICl (or IBr) can adsorbate together easily to form Si-I and Si-Cl (or Si-I and Si-Br) on the Si(100) surface.
For ICl/Si(100) STM images allow to distinguish between I and Cl adsorbates and reveal that the co-adsorbed I and Cl atoms form a large areas of c(2×2) structure on Si(100) surface at room temperature. For IBr/Si(100), STM images reveal that the co-adsorbed I and Br atoms just form a small local areas of c(2×2 )-m, c(4×2)-m, c(4×2)-p, p(2×1)-m, and p(4×1)-m structure on Si(100) surface at room temperature.
Figure 5. 1 (a) Si 2p and (b) I 4d (c) Cl 2p core level photoemission spectra (circles) of ICl–Si(100)-2×1 surface and Si(100) surface with various amounts of exposed ICl, as labeled. The solid curves are fits to the spectra. The curves labeled B, S, S’, and C are the results of decomposition of the Si 2p spectra into contributions from the bulk, the clean surface, and the Si–ICl species, respectively.
The apparent exposure in Langmuir (1 L=10-6 Torr‧s) is used as the relative measurement of ICl dosage on the bare Si(100)-2×1 surface.
△SO WG (eV) WL (eV) Component △Ei (eV)
Si 2p 0.60 0.38
0.20 S -0.52 0.20 C +0.8
0.20 S’ +0.26
I 4d 1.70 0.46 0.20
Cl 2p 1.60 0.68 0.20
Table 5. 1 Fitting parameters from the analysis of the Si 2p, I 4d, and Cl 2p. In all spectra we obtained the best fit using a spin-orbit split (△SO). WL is Lorenzian full width at half maximum. △Ei is the shift of the peak with respect to the bulk binding energy, and WG is the Gaussian width and width.
Figure 5. 2 Plot of the ratio between Cl 2p, Br 3d, and I 4d core-level peak intensities as a function of ICl and IBr dose. The dada of ICl/Si(111) is from [39], IBr/Si(111) is from [38], and HCl/Si(100) is from [40].
Figure 5. 3 The (20×10 nm2) STM images of Si(100) after saturation dosage of ICl at room temperature. The image is obtained at room temperature with IT = 0.2 nA and VS = +2.1 V. The bright spots are I atoms, dim spots are Cl atoms, and darkest spots are defect. The white arrows indicate the dimer-row directions in the top Si layer.
structure (%)
C(2×2) 89.4
Si-I (2×1) 5.2
Defect and impurity 5.4
Cl / I 0.9
Table 5. 2 The calculation of Si(100) after saturation dosage of ICl at room temperature from the Figure 5. 3.
Figure 5. 4 (a) Close-up image of c(2×2) structure extracted from Figure 5. 3 dash square denotes unit cell. (b) (-110) projection of atomic model of c(2×2) structure. (c) Top view. Orange, green, and purple circles represent Si, Cl, and I atoms, respectively. According the ab-initio calculation, the bonding lengths of Si-I, Si-Cl are 2.47 Å and 2.07 Å, respectively. The ab-initio radius of I and Cl are 1.27 Å and 0.89 Å.
Figure 5. 5 For c(2×2) the empty states of the simulation STM images for the saturation ICl on Si(100) surface above the top ICl overlayer by 1.5 Å with different sample bias at (a) +2.0 V, (b) +2.5 V, and (c) +3.0 V.
Figure 5. 6 IBr on Si(100) (a) Si 2p and (b) I 4d (c) Br 3d core level photoemission spectra (circles) of IBr–Si(100)-2×1 surface and Si(100) surface with various amounts of exposed IBr, as labeled. The solid curves are fits to the spectra. The curves labeled B, S, S’, and C are the results of decomposition of the Si 2p spectra into contributions from the bulk, the clean surface, and the Si–IBr species, respectively.
△SO WG (eV) WL (eV) Component △Ei (eV)
Si 2p 0.60 0.36 0.20
S -0.52 C 0.72 S’ +0.26
I 4d 1.70 0.43 0.20
Br 3d 1.00 0.53 0.20
Table 5. 3 Fitting parameters from the analysis of the Si 2p, I 4d, and Br 3d. In all spectra we obtained the best fit using a spin-orbit split (ᇞSO). WL is Lorenzian full width at half maximum. ᇞEi is the shift of the peak with respect to the bulk binding energy, and WG is the Gaussian width and width.
Figure 5. 7 (a) 20.0 × 10.0 nm2 STM images of Si(100) after saturation dosage of IBr at sample temperatures of 300 K. Solid boxes enclose selected area with (b) p(4
×1)-m, p(2×1)-m, (c) c(4×2)-p, c(4×2)-m, and (d) c(2×2)-m. (e)-(g) Schematic diagram of the area: Large yellow circles indicated adsorbed Br, small red circles indicated adsorbed I, and white circles indicated dangling bonds. Sample bias voltages used were + 2.4 V (a,b,c,d). The distance between two dimer row is 7.68 Å.
Figure 5. 8 Sphere model for the Figure 5. 7 (d). (a) Side view. (b)Depiction of mixed Iodide- and Bromide-bonded Si(100)-c(2×2) where pairs of IBr alternate along and across the dimer rows. The rhombus shows the periodic boundary of the primitive cell. According the ab-initio calculation, the bonding lengths of Si-I, Si-Br are 2.47 Å and 2.24 Å, respectively. The ab-initio radius of I and Br are 1.27 Å and 1.06 Å.
Figure 5. 9 Sphere model for the Figure 5. 7 (c). (a) Side view. (b)Depiction of mixed Iodide- and Bromide-bonded Si(100)-c(4×2) where pairs of IBr alternate along and across the dimer rows. The rhombus shows the periodic boundary of the primitive cell.
Figure 5. 10 For c(2×2) of the empty states of the simulation STM images for the saturation IBr on Si(100) surface above the top IBr overlayer by 1.5 Å with different sample bias at (a) +2.0 V, (b) +2.5 V, and (c) +3.0 V.