4. 1 Introduction
The epitaxial growth of halides on semiconductor surfaces has attracted much attention because of potential applications in microelectronic and optoelectronic devices, as well we scientific interest in the basic principle of epitaxial growth and heterostructure physics[27, 28]. Halide chlorides are a class of ionic solids with high fractions of ionic character. NaCl/Si(100) is a prototypical system for the heteroepitaxy of small-lattice-mismatch ionic crystals/covalent crystal. The second nearest-neighbor separation R1 for an NaCl crystal is 3.98 Å. The surfac lattice constant a, or the period of unreconstructed Si(100)-1×1, is 3.84 Å. The lattice mismatch at the heterostructure of NaCl/Si(100) is close to 4%. Previous studies have established that NaCl can grow epitaxially on Ge(100) with a high degree of quality under suitable conditions[4, 14-16]. An STM measurement suggests that the growth of NaCl begins with a carpet-like double-layer NaCl film. In an electron energy loss scattering (EELS) measurement Zielasek, Hildebrandt and Henzler found electronic states at the NaCl/Ge interface and suggested that the dimerization of the Ge(100) surface is not eliminated at the NaCl/interface - even if the thickness of NaCl rises to 20 ML[13]. Scanning tunneling microscopy (STM) is powerful tools to investigate surface local atomic structure. Up to know, STM studies of alkali halide thin films grown on semiconducting substrates have been carried out for the systems KCl/Si(100), NaCl/Ge(100), KBr/InSb(100), LiF/Si(100), and KI/Si(100)[4-6, 29, 30].
For large lattice mismatch systems, the growth of alkali halide on the Si and Ge surfaces at around one monolayer coverage does not yield an ordered surface structure. For example, sub-monolayer LiBr (R1=3.89 Å) and LiF (R1 = 2.85Å) are adsorbed randomly onto Si(100) (a=3.84 Å) at room temperature[3, 6]. Guo and Souda observed that KI (nearest-neighbor separation R0 =3.53 Å) dissociative adsorbs on the Si(100) surface at a coverage of less than 0.5 ML. Although thick flat films can be obtained, the growth of KI, LiF and LiBr on Si(100) and Si(111) surfaces proceeds by the Volmer-Weber (VW) mechanism of island growth as a result of the interfacial lattice mismatch.
In the present work, we report on XPS spectra and STM images of NaCl films,
which were grown on clean Si(100)- 2×1 surfaces. The lattice misfit of this system is 4%, and we have already achieved epitaxial growth of NaCl films, and study the growth mode of NaCl on Si(100)-2×1 by STM and XPS.
4. 2 Experimental details
The single crystal Si(100) with size of 1×10 or 3×10 mm2 was sliced from an Antimony doped wafer with the resistance of 0.01 Ω‧cm. The clean Si(100) surface was obtained in situ by direct heating to 1450 K for a few seconds after degassing at 900 K for several hours.
NaCl powder of 99.99% purity was evaporated from an alumina crucible by a feedback current flux controlled electron bombardment beam. The deposition rate was measured by a quartz-crystal thickness monitor. The coverage of NCl adsorbated in ML (denoted by θ), was estimated from the exposure time, assuming the sticking coefficient is 1. The ML is referred to the surface density of the unreconstructed Si(100) surface, i.e. 1 ML=6.8×1014 cm-2. The substrate temperature during growth was approximately 330 K.
The photoemission spectra were recorded in a separated -metal-shielded chamber with a based pressure of ~ 310-10 torr 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 (SGM). The photocurrent from a gold mesh placed in the synchrotron beam path was monitored to determine the relative incident photon beam flux.
Photoelectrons were collected from 45° off normal emission and analyzed by a 125-mm hemispherical analyzer. The overall energy resolution was better than 120 meV. The STM measurement was taken in a separated UHV chamber with a base pressure of 8 10-11 torr.
The tunneling current was about 0.1 nA. The topographic height measurement did not strongly depend on the sample bias around -2.4 V typically used.
4. 3 Results and discussion
4. 3. 1 Photoemission results
High-resolution X-ray photoemission spectroscopy can be performed to distinguish between atoms at nonequivalent sites and in different chemical bonding configurations, base on shifts in their binding energy [31]. Figure 4. 1 (a), 1(b), and 1(c) respectively presents a series of surface-sensitive Si 2p, Cl 2p, and Na 2p core level spectra (dot) for the Si(100)-2×1 surface with various amounts of NaCl. All fitting was least-squares fitting. Identical Voigt line shapes that each consists of a pair of spin-orbit split doublets were used to decompose the Si 2p and Cl 2p core level spectra into overlapping components (curves) [17]. The solid curves represent the fitting results that overlap the data points. Spectra of the chlorine terminated Si(100)-2×1 (Cl/Si(100)) are also presented for reference. The Si 2p core level spectrum (Figure 4. 1 (a), bottom) has two components, B and Si+, that are separated by 0.90 eV. The B component was responsible for the emission from the bulk and the Si+ component from the surface Si-Cl species[21, 31]. The corresponding Cl 2p spectrum for Cl/Si(100) (Figure 4. 1 (b), bottom) can be analyzed in terms of only a single component that has a pair of split doublets separated by 1.60 eV, implying that all Cl has the same Si-Cl monochloride bonding configuration[18, 19].
Before NaCl deposition, the Si 2p spectrum (Figure 4. 1(a), second from bottom), which was obtained from the clean Si(100)-2×1 surface, has a bulk component (B) and surface-shifted (-0.52 eV) components (S). The surface related components of S peak has been attributed to emission from the top atoms of symmetric dimers. The B component was responsible for the emission from the bulk. All core level binding energies are referenced to the bulk Si 2p3/2 position (99.5 eV) relative to the valence band maximum.
At submonolayer coverage of NaCl deposition, the intensity of the S component declines with increasing coverage, suggesting a much reduced charge transfer between the up- and down-atoms in a dimer. Tails on the higher binding energy side for Si 2p with θ＞0.4 ML can be located near the position of the Si+ component. These signatures suggest that a portion of deposited NaCl molecules decomposes and Si-Cl bonds are presented on the surface. Noticeably, at θ=0.4 ML the Si 2p spectra become broader on the higher binding energy side. Thus two additional components, Si+ and I, are included in our fitting. The
component of Si+ peak, shifted by +0.9 eV from B, is apparently responsible from the Si-Cl surface species [21, 32, 33]. Accordingly, a fraction of the adsorbed NaCl dissociate, and the peak of I appears after the NaCl on the Si(100) surface. Thus, we can regard the peak of I as the emission from the top Si layer under the NaCl film. The shift of +0.42eV to the higher binding energy side also could be explained that it is positively charged for the top Si layer under the NaCl film.
Figure 4. 2 (a) plots the intensities of S, I, Si+ components in Si 2p associated with the coverage of deposited NaCl. At θ=0.4ML, the coverage of the Si+ (0.23ML) is larger than I (0.11ML), indicating that a more portion of deposited NaCl (0.23ML) molecules decomposes and Si-Cl bonds and ionized Na are present on the surfaces, and a fewer portion of deposited NaCl to adsorption on the Si atom (Si-Cl-Na) by a form of a dipole molecule of Na-Cl (0.11ML). Because the component of I of Si 2p in Figure 4. 1(a) spectrum, the I shifts to the higher binding energy side by +0.42eV which is less than the Si+ components shifts to the higher binding energy side by +0.9eV, indicating that the adsorbed the Na-Cl dipole molecule has fewer charge transfers from the Si to Na-Cl (Si-Cl-Na) than the covalent bonds of Si-Cl.
So Na-Cl does not completely bond with Si to form a covalent bond to be Si-Cl; instead, Na-Cl bond with Si by Si-Cl-Na. In Figure 4. 2 (a), above the θ=0.6ML the coverage of I (~0.38 ML), S+(~0.23 ML), and S(~0.39 ML) do not change with the furthering deposition of the NaCl. However,Figure 4. 2 (b) plots the integrated intensities of the Si 2p, Cl2p, and Na 2p peaks. Above θ=0.6ML the Cl and Na increases with the amount of NaCl deposited.
Therefore, above θ=0.6ML the second NaCl layer begins to grows on the first NaCl layer, and first NaCl layer has residual dangling bonds (S~0.39ML), Si-Cl-Na (~0.38ML), and Si-Cl (~0.23ML).
4. 3. 2 STM results
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 (21) reconstruction [34].
Figure 4. 3 (a) shows a STM topographic image scanned at a sample bias of -1.8 V and a tunneling current of 0.23 nA for a Si(100)-21 surface predosed with 0.1 ML NaCl molecules at room temperature. The unreacted dangling bonds of Si atoms appear as bright dimer rows and the reacted dangling bonds of Si atoms appear as dim color. The XPS revealed that below θ< 0.4ML the adsorbed NaCl to form a Si-Cl and Si-Cl-Na, so in Figure 4. 3 (a) the Si atoms dangling bonds have been eliminated by the newly formed Si-Cl and Si-Cl-Na to be a dim color [19]. In Figure 4. 3 (b) the small area pattern in the solid boxes enclose can be found on the surface, and the bright spots in the small area pattern have the same bright as the unreacted dimer rows of Si. Therefore we suggest that the bright spots in the small area pattern are the dangling bonds, and the dim color is Si-Cl or Si-Cl-Na.
Figure 4. 4 shows the evolution of the Si(100) surface after the various amounts of NaCl deposition at room temperature. At the SA step edges, the direction of the dimer bonds on the upper terrace is oriented perpendicular to the step edge, whereas it is oriented parallel to the step edge at SB steps. In Figure 4. 4 (a), at θ= 0.65ML the dimer rows of Si(100)-21 cannot be observed, and the bright spots (0.35ML) are scattered on the Si surface. Because Figure 4.
4 presented that dimer colors of the Si(100)-21 surface is due to the adsorption of the molecule NaCl, the bright spots in Figure 4. 4 (a) are the dangling bonds of the Si and these dangling bonds have the c(2×4), c(2×2), and p(2×2) structure due to the arrangements of Si-Cl and Si-Cl-Na as shown in Figure 4. 4. The growth first layer of NaCl/Si(100) and KCl/Si(100) (c(4×4) structure) can be observed the ordered structure, but the an STM measurement suggests that the growth of NaCl/Ge(100) begins with a carpet like double-layer NaCl films [4, 30]. It is well known that the growth mode in heteroepitaxy is dependent upon the surface free energy and on the lattice mismatch. The lattice mismatch of NaCl/Si(100) (~4%), KCl/Si(100) (~13%) are larger than NaCl/Ge(100) (~0.5%). So at submonolayer of NaCl molecules adsorbed on Si(100)-2×1 surface, the NaCl molecules cannot be combined with other NaCl molecules to be a double layer islands on Si(100)-2×1.
The larger lattice mismatch of NaCl/Si(100) (4%) results in partial of the dissociated NaCl (Si-Cl) and partial of the Si-Cl-Na.
The STM image such as that in Figure 4. 4 (b) reveals that two dimensions islands emerge and grow as NaCl grow above 0.65ML. The carpet-like growth that is observed in the NaCl/Ge(100) system is not observed here. The apparent height of these islands Figure 4.
5(b)) extracted from line scans is above 3.8 Å above the first NaCl layer, which cover much of the surface. The first layer NaCl on Si(100) reduced the dangling bonds of Si at θ=0.65ML and also reduce the reactive interface between the dangling bonds and NaCl molecules;
moreover, the XPS indicated that θ > 0.6 ML the dangling bonds are still under the second NaCl films. Therefore, the second layer of NaCl can grow to double layer islands without the effect from the dangling bonds of Si. Another notably study in Figure 4. 4 (b) is that these small islands much prefer to grow close the step A edge on the lower terraces. Figure 4. 4 (c) presents the surface morphology after 1.55 ML of NaCl deposition. The main features in the image are large area islands on flat terraces or cross two step (step A and step B). Figure 4. 5 shows the apparent topographic height profiles and corresponding schematic of NaCl films (large yellow rectangles) of one or two layers thick on Si(100) along the arrows in (a) Figure 4. 4 (a), (b) Figure 4. 4(b), and (c) Figure 4. 4 (c) respectively. The apparent layer thickness of double- layer NaCl films are about 0.38 nm. The NaCl clusters and dissociated species dispersed on the Si(100) surface are represented by concave boxes of the top Si surafecs.
Figure 4. 7 shows the sphere model of the first layer at 0.65ML NaCl on Si(100). Figure 4. 7 (a) p(2×4), (b) c(2×2), and (c) p(2×2) correspond to Figure 4. 6 (b), (c), and (d), respectively. NaCl crystal which evaporated from the EFM3 is a diatom molecules of NaCl (Na-Cl), so the bond length 2.3 Å of the NaCl diatom molecule is considered and the NaCl diatom molecule is a dipole molecule. According the previous study for the Na/Si(100)-2×1, the Na atoms sit on the hollow sites or the valley sides of the Si(100)-2×1 surface[35, 36].
So in our model for first NaCl layer on Si(100), Na atoms is only on the hollow and valley sites of the Si(100)- 2×1 surface to be considered. As shown in Figure 4. 7 (a) p(2×4) and (c) p(2×2), if the Na-Cl molecules are adsorbed at the same dimer (Si-Cl-Na) and the orientations of the two dipole Na-Cl are anti-parallel and the Na is at the hollow sites of the Si(100), above or below the Si-Cl-Na dimer only has a pair of Si-Cl and dangling bond at the same dimer and the Na atome sites at the nearest valley sites position between the two dimer
rows. The distance between the top Si atom and the nearest hollow side is about 2.3 Å which close the bond length of Na-Cl (2.3 Å), so the Na-Cl can bond with Si atom by the Si-Cl-Na. Because the distance between the Si-Cl and the valley sites of Si(100) (~ 3Å) is larger than the bond length of Na-Cl (2.3Å), the ion bond of the Na-Cl molecule breaks.
Therefore Cl forms chemical bond with Si (Si-Cl) and left Na on the surface. The left Na has no nearest hollow sites of Si(100) surface to occupy, so the Na is onto the nearest valley sites between the two dimer rows. For the Figure 4. 7 (b), the dangling bond and Na-Cl form a zigzag-structure, and a sigzag-structured chain is an ordered array of DB-Si-Si-(Cl-Na) dimers, in which DB sites and (Cl-Na) 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 DB-Si-Si-(Cl-Na) chain.
Table 4. 1 shows the calculated results of the STM images. In calculation of the NaCl coverage, more than 3600 Si sites were counted from a 50×50 nm2 image. The calculation for boundary (Si-Cl-Na) indicated that the Na-Cl must to be in the boundary, ex. the boundary between c(2×4) and p(2×2). The XPS results revealed that I component is ~40% and “Si+”is
~23% for the NaCl coverage over 0.6ML. For the STM results, the I (~40%) can be regarded as the contribution of pattern (Si-Cl-Na = 33%) and boundary (Si-Cl-Na = 5%) and the Si+ (~23%) can be regarded as the contribution of pattern (Si-Cl--Na=14%) and Si-Cl =10%. The Si-Cl is the calculation from the darkest color in the STM images. The darkest color in the STM image cannot be distinguished, but from XPS results the darkest color can be suggested as Cl-Si-Si-Cl without Na atoms around the Cl atoms.
Figure 4. 8 (a) shows the atomic resolution for the second layer of NaCl on Si (100) surface and the well-defined protrusion which has a lattice constant of the square lattice of 3.82 Å is observed. As the previous study of the NaCl layers, only one and two ions (Na+ or Cl-) is observed in STM images[1, 37]. The First principles calculations performed for NaCl/Al [1] and STM images of NaCl/Cu(311)[9] indicated that the species is the Cl ion of the NaCl is imaged as white protrusion. So the well-defined protrusion is the Cl ion of NaCl(100) plane on Si(100) surface as shown in Figure 4. 8 (a).
Figure 4. 9 shows an filled state STM images of the 0.95ML NaCl with different voltage (a) -2.3 V (b) -2.5 V. In Figure 4. 9 (a) and (b), the section height profiles in Figure 4. 9 (c) indicates that the height of the islands are both about 3.8 Å, suggesting that the second NaCl
layer grew on Si(100) in a double-layer fashion, and proving that the height profile measurement of the second NaCl was voltage independent for STM image between Vs = -2.3V to -2.5V. The thickness value of 3.8 nm is larger than 2.8 Å which expected for a single NaCl monolayer. However, it is clearly below the idea double layer height of 5.6 Å. This is certainly due to the fact that the tunneling barrier is not only determined by the vacuum between tip and NaCl layer, but also by the NaCl itself, which does not provide the density of states between the Fermi energies of the tip and Si substrate.
Figure 4. 10 (a) shows the 200×100 nm2 morphology of the Si(100) surface with 0.95ML of NaCl. Figure 4. 10 (b) is the roomed-in STM image from the solid box enclosed selected area in Figure 4. 10 (a). In Figure 4. 10 (b) beside the second of double layer NaCl, the first NaCl layer which had the local pattern of the residual dangling bond was observed as discussed in Figure 4. 10. So Figure 4. 10 (b) indicated the second layer of the double layer NaCl really grew on the fist NaCl. The second layer of NaCl which grows on the first layer of NaCl islands is appear at the coverage over 0.65 ML.
4. 4 Conclusions
We have studied the adsorption of NaCl on the Si(100)-2×1 surface at room temperature from low coverage (0.1 ML) up to high coverage (2.25ML) with synchrotron x-ray core level photoelectron spectroscopy and scanning tunneling microscopy. As θ < 0.6 ML, the XPS and STM results together indicated that the partial NaCl dissociated to form Si-Cl and left Na on the surface, and partial NaCl bonded with Si atoms to form Si-Cl-Na. At θ = 0.65 ML (the first NaCl layer on the Si (100)-2×1), the residual dangling bonds of the Si have c(2×4), c(2×2), p(2×2) ordered structure due to the arrangement of Si-Cl-Na and Si-Cl.
Between 0.65 and 2.25 ML, the double layer islands of NaCl grew on the first NaCl layer and the XPS results indicated that the residual dangling bonds of the first layer is still under the second NaCl layer and the amount of the residual dangling bonds are not decreased above θ
> 0.6ML. The atomic resolution of STM images revealed that the doubled layer islands of NaCl on Si (100) surface have the well-defined protrusion which has a lattice constant of the square lattice of 3.82 Å.
Figure 4. 1 (a) Si 2p, (b) Cl 2p, and (c) Na 2p core level photoemission spectra (circles) of Si (100) surface with various amounts of NaCl deposition, as specified. The solid curves are fits to the spectra. The curves labeled B, S, I and Si+ are the results of the decomposition of the Si 2p spectra into contributions from the bulk, the clean surface, the interface layer and the Si-Cl species, respectively. The energy zero in (a) refers to the 2p3/2 bulk position. To eliminate the band bending effect, the relative binding energy of the Cl 2p and Na 2p corresponds to the Si the 2p3/2 bulk position in (a). Dashed lines through the B, S, and Si+ components are guides for the eye.
Figure 4. 2 (a) Evolution of the components S, S+, and I intensity of the Si 2p photoemission spectra of Figure 4. 1 (a). The intensity of S+ is normalized to the component of S+ of the Cl2 terminated Si(100)-2×1. The intensity of S is normalized to the component of S of the clean Si(100)-2×1 surface. (b) Integrated photoemission intensities of Si 2p, Cl 2p, and Na 2p as functions of NaCl coverage. Data for Cl 2p and Na 2p are normalized to the intensity measured at NaCl coverage of 1 ML. Si 2p is normalized to the clean surface. The dashed and the solid lines are simple guides.
Figure 4. 3 (a) STM images of 0.1 ML NaCl on Si (100) with Vs = -1.8 V. (b) the STM image roomed in from Figure 4.3 (a).
Figure 4. 4 Filled-state STM images showing coverage evolution with deposition of (a) 0.65, (b) 0.95, (c) 1.55, and (d) 2.25 ML NaCl on Si(100) surface as labeled. All images are obtained at room temperature with IT = 0.23 nA and Vs = (a) -2.05, (b) -2.3, (c), and (d) -2.8. The images cover an area of about (a) 80×40 nm2, and (b)-(d) 300×150 nm2. Apparent topographic height profiles along the color line are shown in Figure 4. 5.
Figure 4. 5 shows the apparent topographic height profiles and corresponding schematic of NaCl films (large yellow rectangles) of one or two layers thick on Si(100) along the arrows in (a) Figure 4. 4 (a), (b) Figure 4. 4 (b), and (c) Figure 4. 4 (c) respectively. The apparent layer thickness of double- layer NaCl films are about 0.38 nm. The NaCl clusters and dissociated species dispersed on the Si(100) surface are represented by concave boxes of the top Si
Figure 4. 5 shows the apparent topographic height profiles and corresponding schematic of NaCl films (large yellow rectangles) of one or two layers thick on Si(100) along the arrows in (a) Figure 4. 4 (a), (b) Figure 4. 4 (b), and (c) Figure 4. 4 (c) respectively. The apparent layer thickness of double- layer NaCl films are about 0.38 nm. The NaCl clusters and dissociated species dispersed on the Si(100) surface are represented by concave boxes of the top Si