Atomic and Electronic Processes during the Formation of an Ionic

Article pubs.acs.org/JPCC

Atomic and Electronic Processes during the Formation of an Ionic NaCl Monolayer on a Covalent Si(100) Surface Chan-Yuen Chang,† Hong-Dao Li,† Shiow-Fon Tsay,‡ Shih-Hsin Chang,§ and Deng-Sung Lin*,† †

Department of Physics, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan Department of Physics, National Sun Yat Sen University, No. 70, Lienhai Road, Kaohsiung 80424, Taiwan § Research Center for Applied Sciences, Academia Sinica, 1001 Ta-Hsueh Road, Hsinchu 30010, Taiwan ‡

ABSTRACT: An atomic layer of stoichiometric NaCl was formed on a covalent Si(100) surface after two successive half-reactions at room temperature. The first half-reaction due to Cl2 exposure generates a square array of Cl adatoms with a distance close to that in a NaCl(100) surface plane. By utilizing scanning tunneling microscopy (STM), corelevel photoemission spectroscopy, and ab initio density functional theory (DFT) calculations, it was found that progressive deposition of Na in the second-half reaction results in surface-supported Na3Cl clusters, onedimensional cluster chains, and (2 × 2) patches, and eventually turns the Cl-adlayer into a single-terrace, wavy NaCl layer at one monolayer Na coverage. The grown NaCl monolayer rolls over atomic steps like a carpet and covers the entire surface.The atomic and electronic structure of the topmost Si layer underneath the NaCl layer resembles that of the initial silicon surface layer with buckled dimers. Results of the comprehensive investigation together suggest that an ionic NaCl monolayer is very weakly bonded to the covalent substrate and appears nearly free-standing.

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remains roughly the same for the subsequent layers.16 As observed experimentally, the weak attraction between NaCl films and the Ge(100) substrate results in the double-layer growth mode. If adsorbate−substrate and adsorbate−adsorbate interactions are compatible and interlayer transport is inhibited, multilayer growth can occur before the first layer or doublelayer wet the surface completely. For NaCl on Si(100), a previous study has established an uncommon growth mode:11 An adlayer consisting of about 0.65 ML NaCl is formed, followed by growth of a double layer. The first adlayer consists mainly of a NaCl network and unreacted single dangling bonds that form several types of local ordering, and is not seen during the MBE growth of NaCl on Ge(100).10 This may be due to the higher reactivity of the Si(100) surface than the Ge(100) surface. Using sequential, self-limiting surface reactions, atomic layer deposition (ALD) of binary compound films satisfies the requirements for atomic layer control and conformal deposition. Because there are only a finite number of surface sites available to source gases, each cycle can only deposit a finite number of surface species. A variation of ALD, denoted herein as ALD-MBE, is to combine one ALD reaction with another growth reaction using MBE with precise thickness control.17 For example, MBE of 1/3 ML Na followed by Cl2 passivation results in Na2Cl trilayer 2D islands.18 The reverse sequence, i.e., Cl2 termination reaction followed by MBE of 1

INTRODUCTION Attractive forces that hold together atoms in ionic crystals are very different from those found in covalent crystals. An ionic crystal has at least two atoms that are ionized and bonded together by their electrostatic attraction. In pure covalent solids, atoms of the same element are held together in the lattice by covalent bonds characterized by the sharing of pairs of electrons. The intersection of a pure covalent crystal and an ionic solid raises many interesting questions. Indeed, the atomic structure at the interface between ionic and covalent materials and the nature and strength of the interface bonding has been a topic of great scientific interest.1−6 Si, Ge, and SiGe alloys are covalent crystals; many advanced high-k dielectrics for CMOS device applications (e.g., HfO2 and Pr2O3) exhibit a primarily ionic bonding with variable atomic coordination numbers.7,8 Therefore, the growth mechanism and interface properties are also important issues for the integration of covalent semiconductors and ionic or partial ionic crystals. Much attention has been devoted to the growth and properties of ultrathin alkali halides, particularlyNaCl, on metal and semiconductor surfaces. By means of molecular beam epitaxy (MBE), NaCl molecules are initially assembled into double-layer islands with upright alternating dipoles on Cu(111), Ge(100), and Si(100).9−11 Multilayer growth of NaCl is easily achieved on metal surfaces such as Ag(100), Al(111), Cu(100), and Ag(111).12−15 The morphology of a growing multilayer film depends critically on the relative strength of adsorbate−adsorbate and adsorbate−substrate interactions. For example, the layer adsorption energy of NaCl on Ge(100) is lowest for the first monolayer (ML) and © 2012 American Chemical Society

Received: January 17, 2012 Revised: March 28, 2012 Published: May 8, 2012 11526

dx.doi.org/10.1021/jp300578h | J. Phys. Chem. C 2012, 116, 11526−11538

The Journal of Physical Chemistry C

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ML Na, has been shown to grow stoichiometric NaCl film on Ge(100) in a layer-by-layer fashion.19 In the present work, we examine the growth processes of NaCl by ALD-MBE on a Si(100) surface. The material system is chosen in part because a two-dimensional (2D) NaCl square lattice and a Si(100) surface have a very small lattice mismatch, as will be discussed in section 3A. The two-step growth method is unlike the growth of most alkali halides or ionic oxides that use beams of ionically balanced molecular units.20 The consecutive deposition of Cl and Na yields additional information not available from the MBE deposition of NaCl. It is known that a Cl− can diffuse from a stoichiometric NaCl to combine with an excess Na deposited on the surface, which loses an electron. The electron then diffuses to the vacant site and forms a color center.21 Herein, a single-terrace NaCl monolayer is shown to cover the entire Si(100) surface after consecutive adsorption of chlorine and sodium. The atomic resolution of the adsorbate structure and the evolution of surface morphology are observed by STM. The high-resolution core-level spectroscopy reveals the chemical characteristics of the topmost Si and the growing adlayer. DFT programs calculate the mobility of an individual Na adsorbate, the charge transfer between atoms during Na adsorption, the stable surface-supported clusters of Na3Cl, and various atomic structures of the grown NaCl monolayer. The experimental and theoretical findings obtained from this model system can improve the understanding of similar and more complex growth processes and interfacial properties of binary compounds on covalent crystals.

positions and the surface terminated by hydrogen. A vacuum region with a thickness of 15 Å was included on top of the Si surface to form a supercell. Plane waves with kinetic energies up to 22.06 Ry (300 eV) were included. The irreducible Brillouin zone was sampled with a (4 × 4 × 1) Monkhorst−Pack mesh.29 Geometry optimization was performed until the total energy converged to within 10−5 eV. Cell sizes were fixed to yield a Si lattice constant (aSi) of 5.47 Å. After relaxing the atomic structure, electronic density of state calculations were performed. Adsorption reactions are often accompanied by charge transfer between the adsorbates and the substrate. To obtain quantitative information about the charge redistributions induced by adsorbed Na, a Bader atoms-in-molecules (AIM) charge analysis was conducted using a grid-based algorithm with core charges included in the calculations.30−32 Here, the term “charge transfer” is a shift in the Bader charges. According to the AIM scheme, molecules or crystals can be partitioned into atomic volumes, called Bader regions, by interatomic ⎯r ). The gradient of the electron density ρ(→ ⎯r ) at any surfaces S(→ S S ⎯r on S has no component normal to the dividing surface. point→ S Thus, an interatomic surface satisfies the “zero-flux” boundary ⎯r ) n(→ ⎯r ) = 0. Calculations were performed for a condition: ∇ρ(→ S S grid with 100 × 100 × 192 points using the UT theoretical chemistry code.

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RESULTS AND DISCUSSION A. Lattice Constant of 2D NaCl. The Madelung constant rises from 1.386 for the 1D ionic crystal of alternative charge to 1.747 for the 3D sodium chloride lattice. As displayed in Figure 1, our VASP calculations show that the lattice constants for

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EXPERIMENTAL AND THEORETICAL METHODS Our photoemission experiments were conducted at beamline 24A1, at Taiwan’s National Synchrotron Radiation Research Center, using a 125-mm hemispherical analyzer housed in a μmetal-shielded ultrahigh vacuum (UHV) chamber with a base pressure of 2 × 10−10 Torr. Light from the 1.5 GeV storage ring was dispersed by a spherical grating monochromator. The photocurrent from a gold mesh positioned in the synchrotron beam path was monitored to measure the relative incident photon beam flux. Photoelectrons were collectedat 45° from the surface normal with an acceptance angle of ±8°. The overall energy resolution was better than 120 meV. The STM experiments were conducted in another UHV chamber. All images were recorded in constant current mode at room temperature. Low tunneling currents (200 pA or below) were used. The STM images show no apparent bias dependency within ±0.3 V near the sample biases reported herein. Antimony-doped Si(100) samples with a resistance of about 0.01 Ω·cm were first outgassed at ∼800 K over 16 h. Clean Si(100) surfaces were prepared at ∼1400 K by passing a DC current directly through the sample for about 10 s. To deposit Cl, the sample was exposed to a beam of Cl2 gas; the surface uptake of Cl was self-limiting.22 Na was deposited from an SAES getter source,23 and the rate of deposition was determined by monitoring the change in the work function as previously reported.19,24 In the following, the coverage of Na (θNa) is specified in terms of the site density of the Si(100)-(1 × 1) surface: 1 ML = 6.78 × 1014 cm−2. The deposition rate ranging from 0.85 to 3.0 ML/min does not affect the results. DFT calculations were performed using VASP (Vienna Abinitio Simulation Package) employing PBE generalized gradient approximation functionals.25−28 The Si substrate was modeled as a 4 × 4 × 8 slab with the bottom two layers fixed at bulk

Figure 1. Calculated lattice constant for various NaCl-structured halogen halides arranged in 1D (with all atoms constrained to a straight line), 2D (with all atoms constrained to a single plane), and a crystal (3D). Twice of the bond lengths for the halide molecules are plotted as the lattice constant for 0D. The horizontal line indicates the lattice constant of Si crystal (5.47 Å).

various alkali halides increase with the dimensionality as expected. For NaCl, the lattice constant (aNaCl) increases from 5.00 Å for a line of Na−Cl ions to 5.61 Å for 3D NaCl crystal. Even though aNaCl(3D) is about 2.6% larger than aSi, aNaCl(2D) (5.40 Å) is 1.3% smaller. From this prospective, if a 11527



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2D NaCl monolayer exists on the Si(100) surface, it is not under compression but slightly under tension. B. The Starting Si(100) Surface. The fairly high energy of the (1 × 1) bulk-terminated (100) surface is lowered when two neighboring surface atoms move toward each other symmetrically to form a dimer. The clean Si(100)-(2 × 1) surface thus formed consists of rows of symmetric dimers (−Si−Si−) in which each atom has one dangling bond. The surface energy is further lowered by additional freedom, namely, the buckling of dimers. Arranging the asymmetric dimers in an antiferromagnetic or ferromagnetic ordering respectively forms the c(4 × 2) and (2 × 2) phases.33−35 The energy associated with buckling is significantly smaller than that of dimerization.36 Therefore, the consideration of three phases (2 × 1), c(4 × 2), and (2 × 2) as the ground state surface reconstruction of Si(100) has long been a source of controversy. As listed in Table 1, our Table 1. Calculated Structure Parameters of Si(100) Surfaces in (2 × 1), (2 × 2), and c(4 × 2) Configurations and That with 1 ML NaCl on top

surface ordering (2 × 1) c(4 × 2) (2 × 2) (2 × 1):NaCl c(4 × 2):NaCl (2 × 2):NaCl c(2 × 2):NaCl -cluster array

dimer distance

buckling angle

2.29 2.36 2.36 2.24 2.43 2.41 2.41

0 19.2 19.0 0 10.4 9.2 9.3

energy difference (eV/(1 × 1))

layer adsorption energy (eV/NaCl pair) or (eV/(1 × 1))

0.08 −0.00 0.00 −0.02 −0.12 −0.12 −0.15

calculations show that the two buckled phases have about the same energy and are favored by 0.08 eV per (1 × 1) supercell over the symmetric (2 × 1) phase, all consistent with previous findings. C. The Si(100)-(2 × 1):Cl Surface with a Large Surface Dipole. Saturation adsorption of monovalent chlorine on the Si(100) surface terminates all surface dangling bonds while preserving the backbone dimer structure. The resulting monochloride surface, Si(100)-(2 × 1):Cl, is displayed in Figure 2a.37−39 In good agreement with earlier calculations and experimental results,40−42 we determine a symmetric dimer bond length of 2.42 Å for a Cl−Si−Si−Cl dimer, slightly increased from that of the clean Si(100)-c(4 × 2) buckled dimers (2.36 Å). The Si−Cl bond length is 2.07 Å. The distances between two adjacent Cl adatoms on the same dimer and across a dimer row are 3.84 and 3.86 Å, respectively. Since one unit cell of unreconstructed Si(100) has a length of 3.86 Å, the Cl adlayer can be viewed as a square lattice. Bader charge analysis showed that the charge transfer mostly occurred between the topmost Si layer and the Cl adlayer, and that the fourth-layer fixed Si atoms are nearly unaffected by the adsorption, indicating that the slab is reasonably thick. The calculated net atomic charges, Qnet = Qvalence − QBader, for Si(100)-(2 × 1):Cl are illustrated in Figure 2a. A positive value indicates a charge gain, while a negative value indicates a charge loss. As shown in Figure 2a, Qnet is −0.70 and 0.70 e, respectively, for each Cl and Si atom on the monochloride surface. This amount of charge transferred to Cl is smaller than that transferred to NaCl crystal (−0.84 e) but larger than the Mulliken atomic charge found in a Si2Cl2 molecule (−0.38 e).43

Figure 2. Bader net charges for Si (in black, upper left in each picture), Cl (in green, lower right), and Na atoms (in purple, middle) on the (a) Si(100)-(2 × 1):Cl, (d) Si(100)-c(4 × 2):NaCl, (e) Si(100)-(2 × 1):NaCl, and (f) Si(100)-(2 × 2):NaCl-cluster surface. (b) One and (c) three Na are each placed on an H site on a (4 × 4) cell of Si(100)(2 × 1):Cl. The charge unit is in the elementary charge e.

In the simple scenario of two point charges, the dipole moment of the Si−Cl bond equals Qpd = 6.96 D (1 D = 2.998 × 1029 C·m) if the Si−Cl bond length (2.07 Å) is used. This value is substantially larger than that of 2−3 D previously estimated.44,45 This discrepancy arises because the polarization of Si+−Cl− is not considered in the point charge calculation. Even though the calculated atomic net charges from the Bader analysis scheme might be overestimated here,46 the trend for the amount of charge transfer during Na adsorption should remain valid. D. Adsorption Energy for a Na Atom on Si(100)-(2 × 1):Cl. Upon adsorption on the well-ordered Si(100)-(2 × 1):Cl at a low substrate temperature, a Na atom is likely to rest near high-symmetry sites, such as those indicated in Figure 3a. Six different adsorption sites are considered: hollow (H), edge (E), cross (C), bridge (B), top (T), and dimer (D) sites. The adsorption geometries of the Na/Si(100)-(2 × 1):Cl systems at these sites (termed configurations H, E, C, B, T, and D, respectively) have been optimized, and the calculated mean adsorption energies Ead are presented in Table 2. As listed in Table 2, the distance between adsorbed Na and Cl in an on-top 11528



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configuration at a T site in Figure 3c is 2.49 Å, which is about 5.5% larger than the NaCl molecular bond length (2.36 Å). Assuming the adsorbed Na forms a NaCl molecule with the Cl adatom, energies of relevant reactions involving configuration T can be estimated using common bond energy values as follows:47 Na + 5.14 eV → Na + + e−

Si+ + e− → Si + 5.17 eV Na + + Cl− → Na +Cl− (molecule) + 6.10 eV

Si+Cl− + 3.90 eV → Si+ + Cl−

Combining the above equations yields Si+Cl− + Na → Si + Na +Cl− + 2.23 eV

While close to that obtained in VASP calculations for the C and H sites, the estimated magnitude of the adsorption energy (2.23 eV) is markedly larger than that calculated for configuration T (1.28 eV). In our calculations, both the longer Na−Cl bond distance (2.49 Å vs 2.36 Å in a NaCl molecule) and the less exothermic adsorption for configuration T indicate that the Si− Cl bond is not completely broken upon Na adsorption. Table 2 also indicates that configurations H and C are, respectively, the most and second most energetically favorable with a small difference of 60 meV. The other configurations are less favorable for adsorption by over 0.50 eV. In the potential surface, configurations B, D, and E are located each at a saddle point and, therefore, not stable. The adsorbed Na atoms for the stable configurations H and C reside slightly above the Cl plane, as displayed in Figure 3b. The nearest Na−Cl distance (2.77 Å) is slightly shorter than that of bulk NaCl crystal (2.82 Å). Upon adsorption at an H site, Figure 2b shows that the Na atom becomes fully ionized (0.84 e, identical to that in NaCl crystal). However, the electrons from the Na atom transfer little to the surrounding four Cl− anions, since their partial charges increase only slightly, i.e., from −0.70 to −0.72 e. Instead, about two-thirds of an electron from the adsorbed Na atom flow to the nearest four Si atoms, leading to a change in their partial charge from 0.70 to 0.50 e. The other one-third mainly flows to the next nearest four Si atoms along the trench and causes a decrease of their partial charge from 0.70 to 0.60 e. The decrease of the Si net charge suggests a reduction of the Si−Cl bond strength, in response to the formation of local Na−Cl bond formation. As Figure 2c shows, the Si net charge and the corresponding Si−Cl bond strength are further reduced as the Na coverage increases. E. Na Diffusion on Si(100)-(2 × 1):Cl and Formation of Clusters. As will be discussed in section F, the STM images for Si(100)-(2 × 1):Cl with submonolayer Na coverage show adsorbates nucleate into clusters and 2D islands. The coalescence of adsorbates indicates that Na at initial adsorption sites (e.g., H and C sites) has low diffusion barriers and, therefore, is mobile at RT. Results of NEB calculations for the diffusion barriers summarized in Figure 4confirm this presumption: the diffusion barriers between the H and C sites are all below 0.70 eV. With these small barriers, the Na adsorbates can hop freely before being incorporated into stable adsorption configurations. Since configurations H and C have relatively low adsorption energies, it is natural to expect that mobile Na adsorbates on Si(100)-(2 × 1):Cl can merge to form rows or patches of

Figure 3. (a) A schematic top view of Si(100)-(2 × 1):Cl for various adsorption sites for Na. Side view of (b) configuration H and (c) configuration T along [0, −1, 1]. The bond lengths are indicated in angstroms.

Table 2. Adsorption Energy per Na, Ead, and Structure Parameters for Na Atoms on High-Symmetry Sites and Various Adsorption Configurationsa NNa

configuration

dSi−Cl

dNa−Cl

Ead (eV)

1 1 1 1 1 1 2 2 3 8 16 16 16

T B E D C H HH CC Na3Cl 1D cluster chain Si(100)-(2 × 2):NaCl-cluster array Si(100)-(2 × 1):NaCl Si(100)-c(4 × 2):NaCl

2.19 2.18 2.17 2.17 2.14 2.14

2.49 2.54 2.56 2.59 2.76 2.77

2.36 2.70 2.46

2.62 2.62 2.62 2.81 2.83

−1.28 −1.75 −1.84 −1.84 −2.31 −2.37 −2.06 −2.10 −2.55 −2.55 −2.41 −2.28 −2.38

The numbers of Na adsorbates on a (4 × 4) cell, NNa, is indicated. The adsorption energy is averaged for NNa > 1. The lengths for the relevant, shortest Si−Cl bond (dSi−Cl) and Na−Cl bond (dNa−Cl) are labeled in angstroms. a

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Figure 4. Calculated diffusion barriers for a Na adsorbate on Si(100)(2 × 1):Cl. The reaction coordinate refers to the diffusion paths that begin and end at high-symmetry points as labeled.

configurations H and/or C. However, calculated results show that two Na atoms occupying neighboring H sites (configuration HH in Table 2) have a smaller mean adsorption energy Ead (2.06 eV) than that (2.37 eV) of configuraion H. The mean adsorption energy is defined as Ead = (ESi(100) ‐ (2 × 1):Cl + Na − ESi(100) ‐ (2 × 1):Cl − NNaE iso ‐ Na) /NNa

Figure 5. Top view, side view, and space fill model for (a) the Na3Cl cluster and (b) the NaCl zigzag chain. The bond lengths are indicated in angstroms.

where ESi(100)‑(2×1):Cl+Na and ESi(100)‑(2×1):Cl are the respective calculated total energies of the Si(100)-(2 × 1):Cl system with or without adsorbed Na atoms. Eiso‑Na is the total energy of an isolated Na atom, and NNa is the number of the Na atoms adsorbed on a 4 × 4 cell. Accordingly, the coverage of Na is NNa/16. In other words, the combination of two isolated configurations H into one configuration HH is energetically unfavorable. The decrease in the magnitude of adsorption energy is understandable. The first Na adsorbate configuration HH (or similarly a configuration CC) establishes four Na−Cl ionic bonds, while the second Na introduces only two new ones. As just discussed, individual Na adsorbates are mobile at RT and do not group at neighboring H and/or C sites as one might intuitively expect. It is thus questionable what species are present on Si(100)-(2 × 1):Cl with submonolayer Na coverage. Using an ab initio molecular dynamics simulation at 300 K, we found a new type of stable nucleation configurations for three or more Na adsorbates. As shown in Figure 5a, three nearby Na adsorbates nearly form a close-packed triangle and the associated Cl atom floats over the center of the triangle. This arrangement can be viewed as a microscopic NaCl(111)oriented face. This cluster is denoted herein as a Na3Cl cluster, for it locally involves three Na and one Cl atoms. The distances between the three Na atoms are all about 3.74 Å; the nearestneighboring Na−Cl distances are 2.63 Å. Both distances are ∼5% shorter than those in the NaCl(111) plane (3.97 Å, 2.80 Å). The mean adsorption energy is −2.55 eV, whose magnitude is 0.18 eV larger than that found for configuration H. As shown in Figure 5b, this triangle structure can extend, upon more Na adsorption, along the dimer-row direction with the Na triangles alternatively pointing to opposite sides of the dimer row with the same mean adsorption energy (−2.55 eV).

F. STM Results. Upon Cl saturation, each Cl−Si−Si−Cl monochloride dimer appears in rows as two well-resolved bright protrusions (Cl protrusions) in the empty state images in Figure 6a and as an elongated bright protrusion in the filled state images in Figure 7a. Aside from a few missing dimer defects, some randomly located dark sites each reside on one side of a monochloride dimer. Each of these sites has been shown as a hydrogen atom substituted for a Cl atom, that is, a Si−H surface species.48,49 They are referred to as HSi sites. Figures 6b−d and 7b,c respectively display the empty- and filled-state STM images for the evolution of the Si(100)-(2 × 1):Cl surface morphology during various stages of Na deposition at room temperature. Upon deposition of submonolayer Na, the adsorption is evidently heterogeneous. Negative islands (labeled NI) are present in both empty- and filled-state images. Close examination of Figures 6b and 7b also reveals the emergence of many dim sites (labeled Dm), each of which apparently replaces one Cl protrusion. An isolated Dm site has a similar appearance to a HSi site but slightly enlarges the two neighboring Cl-sites (enclosed in the ellipse in Figure 6b) on the same row in the unoccupied state images. The density of Dm sites is typically higher than that of HSi sites prior to Na adsorption ( 0.2 ML, NaCl 2D islands form and grow in size along with some dispersed clusters. The relative binding energy positions of an atom’s core level emission typically reflect its

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CONCLUSIONS The heteroepitaxy of ionic dielectric films on covalent semiconductors has important applications for current and future nanomaterials and devices. The growth of NaCl on Si(100) is an excellent model system to test current understandings of ionic/covalent interfaces because of its small lattice mismatch. To achieve better control and measurements of the growth process and film morphology, we have used a novel growth technique with two successive half-reactions at room temperature. Complementary synchrotron core level photoemission spectroscopy, atomic resolved microscopy, and first principle calculations were used to study the atomistic growth process, the electronic structure of the NaCl adlayer, and the chemistry at the interface. 11536



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(5) Tsay, S. F.; Chung, J. Y.; Hsieh, M. F.; Ferng, S. S.; Lou, C. T.; Lin, D. S. Growth mode and novel structure of ultra-thin KCl layers on the Si(100)-2 × 1 surface. Surf. Sci. 2009, 603 (2), 419−424. (6) Zielasek, V.; Hildebrandt, T.; Henzler, M. Measurement of NaCl/ Ge(001) interface states by inelastic low-energy electron scattering with high momentum resolution. Phys. Rev. B 2004, 69 (20), 205313. (7) Choi, E.; Chang, K. J. Charge-transition levels of oxygen vacancy as the origin of device instability in HfO2 gate stacks through quasiparticle energy calculations. Appl. Phys. Lett. 2009, 94 (12), 122901. (8) Fissel, A.; Da̧browski, J.; Osten, H. J. Photoemission and ab initio theoretical study of interface and film formation during epitaxial growth and annealing of praseodymium oxide on Si(001). J. Appl. Phys. 2002, 91 (11), 8986−8991. (9) Repp, J.; Meyer, G.; Rieder, K.-H. Snell’s Law for Surface Electrons: Refraction of an Electron Gas Imaged in Real Space. Phys. Rev. Lett. 2004, 92 (3), 036803. (10) Glöckler, K.; Sokolowski, M.; Soukopp, A.; Umbach, E. Initial growth of insulating overlayers of NaCl on Ge(100) observed by scanning tunneling microscopy with atomic resolution. Phys. Rev. B 1996, 54 (11), 7705. (11) Chung, J.-Y.; Li, H.-D.; Chang, W.-H.; Leung, T. C.; Lin, D.-S. Sodium chloride on Si(100) grown by molecular beam epitaxy. Phys. Rev. B 2011, 83 (8), 085305. (12) Pivetta, M.; Patthey, F.; Stengel, M.; Baldereschi, A.; Schneider, W.-D. Local work function Moiré pattern on ultrathin ionic films: NaCl on Ag(100). Phys. Rev. B 2005, 72 (11), 115404. (13) Hebenstreit, W.; Redinger, J.; Horozova, Z.; Schmid, M.; Podloucky, R.; Varga, P. Atomic resolution by STM on ultra-thin films of alkali halides: experiment and local density calculations. Surf. Sci. 1999, 424 (2−3), L321−L328. (14) Mauch, I.; Kaindl, G.; Bauer, A. Formation of NaCl stripes on Cu(1 0 0). Surf. Sci. 2003, 522 (1−3), 27−33. (15) Ramoino, L.; von Arx, M.; Schintke, S.; Baratoff, A.; Güntherodt, H. J.; Jung, T. A. Layer-selective epitaxial self-assembly of porphyrins on ultrathin insulators. Chem. Phys. Lett. 2006, 417 (1− 3), 22−27. (16) Tsay, S. F.; Lin, D. S. Atomic and electronic structures of thin NaCl films grown on a Ge(001) surface. Surf. Sci. 2009, 603 (13), 2102−2107. (17) Goh, K. Effect of encapsulation temperature on Si:P ?-doped layers. Appl. Phys. Lett. 2004, 85 (21), 4953. (18) Hebenstreit, W.; Schmid, M.; Redinger, J.; Podloucky, R.; Varga, P. Bulk Terminated NaCl(111) on Aluminum: A Polar Surface of an Ionic Crystal? Phys. Rev. Lett. 2000, 85 (25), 5376. (19) Lou, C. T.; Li, H. D.; Chung, J. Y.; Lin, D. S.; Chiang, T. C. Electronic reconstruction at a buried ionic-covalent interface driven by surface reactions. Phys. Rev. B 2009, 80 (19), 195311−5. (20) Wertheim, G. K.; Rowe, J. E.; Buchanan, D. N. E.; Citrin, P. H. Valence-band structure of alkali halides determined from photoemission data. Phys. Rev. B 1995, 51 (19), 13675−13680. (21) Ueta, M.; Känzig, W. Generation of Electron Traps by Plastic Flow in Alkali Halides. Phys. Rev. 1955, 97 (6), 1591−1595. (22) Lin, D. S.; Wu, J. L.; Pan, S. Y.; Chiang, T. C. Atomistics of Ge Deposition on Si(100) by Atomic Layer Epitaxy. Phys. Rev. Lett. 2003, 90 (4), 046102. (23) SAES Getters S.p.A. Milan, Italy. (24) Chao, Y. C.; Johansson, L. S. O.; Uhrberg, R. I. G. Adsorption of Na on Si(100)-2 × 1 at room temperature studied with photoelectron spectroscopy. Phys. Rev. B 1997, 55 (11), 7198. (25) Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B 1994, 50, 17953. (26) Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 1999, 59 (3), 1758. (27) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. (28) Kresse, G. Dissociation and sticking of H2 on the Ni(111), (100), and (110) substrate. Phys. Rev. B 2000, 62 (12), 8295.

Since Na is highly reducing, many interesting questions arise when a Na atom is adsorbed on the Cl-terminated Si(100) surface. Employing VASP calculations and a thorough analysis of the relaxed structure, nudged elastic band calculation, adsorption energies, Bader charge, and local density of states, the following was found: (1) an adsorbed Na atom has its lowest energy on the hollow site, not the on-top position; (2) charges transferred from an adsorbed Na atom are mostly distributed to not one but many nearby topmost Si atoms; (3) an isolated Na adsorbate is highly mobile at room temperature with a diffusion barrier of 0.64 eV; (4) mobile Na atoms can form Na3Cl clusters; (5) the Na3Cl structure can then grow along the dimer-row direction to become NaCl cluster chains and twodimensionally to form (2 × 2) domains. STM also reveals that, at 1.0 ML coverage, a NaCl monolayer overgrows the atomic steps and covers the entire surface area. The NaCl monolayer appears as a wavy carpet with an apparent height variation of

Atomic and Electronic Processes during the Formation of an Ionic

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