Epitaxial growth of Au on Ge(001) surface: Photoelectron spectroscopy measurements and first-principles calculations

Thin Solid Films 552 (2014) 241–249

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Epitaxial growth of Au on Ge(001) surface: Photoelectron spectroscopy measurements and first-principles calculations Dana G. Popescu, Marius A. Husanu ⁎ National Institute for Materials Physics, Bucharest-Magurele, P.O. Box MG-7, 044125, Romania

a r t i c l e

i n f o

Article history: Received 12 April 2013 Received in revised form 18 December 2013 Accepted 19 December 2013 Available online 30 December 2013 Keywords: Epitaxial growth X-ray photoelectron spectroscopy Angle-resolved photoelectron spectroscopy Band-bending First-principles calculations

a b s t r a c t A single atomic Au layer is grown epitaxially on a Ge(001) surface featured by (2 × 1) reconstruction. The low energy electron diffraction pattern of the Au/Ge(001) surface indicates the formation of a square structure with the length of crystalline domains of ~3 nm. Ab-initio calculations show that Au growth stabilizes the Ge surface in symmetric dimers and angle-resolved photoelectron spectroscopy measurements reveal its metallic character. The modifications in the electronic properties of the Ge surface as a result of annealing are discussed and the consequences as reflected in X-ray photoelectron spectroscopy (XPS) measurements are underlined. The deformation density indicates the regions with covalent Au–Ge bonds. These bonds are identified from Au 4f and Ge 3d XPS data. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Knowledge of the growth mechanism and of the surface states at the interface of Au with insulating or semiconducting materials is a key topic in tailoring small scale electronics. Moreover, Ge(001) surface was identified as excellent candidate for the realization of long range order chain-like structures with electronic characteristics resembling one dimensional (1D) systems, making handy the study of exotic states such as Luttinger liquid phase or charge density waves [1,2] when noble metals such as Pt [3–6] or Au [6–23] were deposited in well-defined conditions. So far Au deposition on heated clean Ge(001) surfaces resulted in ordered atomic chains featured by either (8 × 2) [7–15,18–22] or (4 × 2) [16,17] surface reconstructions. In Refs. [8,11] it is claimed that the electronic signature of the resulting chains hosted at the Ge(001) surface is one dimensional, while Nakatsuji and Komori found it to be essentially two dimensional (2D) [19–21]. The debated 1D or 2D character is assessed based on both the interaction with the substrates as well as on the strong intra-chain correlations manifested in the quasi-linear Au structure [10]. Thus, in assessing the dimensionality of electron dynamics, the role of the chain–substrate interaction should be well understood. The theoretical description of these constructions within density functional theory (DFT) framework was discussed in a number of papers [23,24] and references herein, and there is little doubt about how the Au chains may organize at the Ge(001) surface in the low

⁎ Corresponding author. E-mail address: ahusanu@infim.ro (M.A. Husanu). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.12.049

coverage regime (0.2 ML–0.75 ML). Namely, there were reported chain structures due to replacement of Ge dimers by Au or mixed Au–Ge dimers, and bridged dimer-row structures or Au/Ge wires stabilized by Au-covered low-index facets. Nevertheless, as pointed out in Ref. [23], no model elaborated so far fulfills the combination of low formation energy, agreement with experimental Scanning Tunneling Microscopy images and electronic features observed near the Fermi level. For higher coverage regimes, things are even less clear. It is worth mentioning that unlike Pt for example, which is known to form intermetallic compounds with Ge [25], leading to a mixture of electronic states and thus, to a three dimensional-like character of the electronic signature, Au is believed to be more stable against this type of interaction [26]. However, we show that in the (600–675) K deposition temperature range, binding of Au on Ge occurs, leading to covalent bond formation of Au with the Ge substrate and symmetrization of surface dimers. The low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and valence band photoemission results accompanied by first-principles calculations confirm that both temperature and coverage are key parameters in controlling the growth mechanism of Au on reconstructed Ge(001) and the resulting surface morphology. Depending on the temperature range and amount of gold deposited, epitaxial growth of Au may occur, leading to surface reconstructions other than previously reported. This is the case of our study where the morphology of the Au layer as indicated by LEED is not chain-like, featured by either (8 × 2) or (4 × 2) reconstructions but instead, it points towards an epitaxial growth of Au on top of Ge(001) dimers as an unreconstructed Au (001) surface. Up to date, in all previous experiments connected to the topic of 1D and 2D systems resulted during Au deposition on the Ge(001) surface,

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no clear evidence of the interaction of Au with the substrate by means of XPS was given. We elaborated a theoretical model that reproduces well the observed features in the LEED and XPS spectra of Ge and Au. The signature of Ge-bonded Au is unambiguously assigned to the higher binding energy component in Au 4f core level spectra, which appears due to covalent bond formation between the Au atom and Ge-dimer. This preserves the surface reconstruction of Ge as (2 × 1), but leads to symmetric dimers formation instead of asymmetric ones as in clean Ge(001) surface. Consequently, in Ge 3d core level spectra, an upshift of the component associated with the surface dimers could also be assigned to Au-bonded Ge dimer. Concerning the metallic character of the Au/Ge surface as seen in valence band photoemission measurements, it is caused by a competition between the finite density of electronic states at Fermi level induced by the symmetrization of the Ge(001) surface and 6s states associated to Au quasi-free electrons as deduced from DFT results. The structure of the paper is as follows: after a brief introduction, the experimental and numerical details are outlined in Sections 2 and 3 respectively. The results are presented in Section 4 and their consequences are further discussed in Section 5. 2. Experimental Sample preparation and measurements were performed under high vacuum (base pressure ~1 × 10−8 Pa) in an Ultra High Vacuum (UHV) facility (SPECS). A single atomic layer—“monolayer” (ML) as related to bulk face centered cubic (fcc) Au surface density (1 ML = 2.04 Å) was deposited on a clean Ge(001) surface at 600 K by molecular beam epitaxy from a Createc effusion cell at a deposition rate of 1 ML/min. The Ge(001) substrate was cut from a p-type Ge wafer (In-doped). The thickness of the Au layer and the deposition rate were estimated using a quartz microbalance. The Ge(001) surface was prepared by several successive flash cycles (T ~ 1000 K) for 20 min, followed by a final flash at 1200 K for 20 s. until sharp LEED spots become visible. Surface temperature was estimated from previously performed heating current and applied voltage calibrations. XPS measurements were performed using Al Kα1 monochromatized radiation (E = 1486.74 eV) in an analysis chamber equipped with a 150 mm hemispherical electron energy analyzer (Phoibos). The analyzer operated in fixed analyzer transmission mode with pass energy of 20 eV and step energy of 0.05 eV; the estimated combined (source + analyzer) resolution is of about (0.75 ± 0.025) eV. During the XPS measurements, a flood gun operating at 1 eV acceleration energy and 100 μA electron current was used in order to achieve sample neutralization. Angle resolved ultra-violet photoelectron spectroscopy (ARUPS) measurements were performed at room temperature using as excitation source the un-polarized He II radiation from a high power UVS 300 UV discharge lamp (40.0816 eV). Measurements were performed with an angular step of 1.3° accounting for a distance in reciprocal space of 0.062 Å‐1 covering 1.12 Å‐1 in [010] direction. 3. Calculation procedure The valence electron configuration employed is Ge: 4s24p2 and Au: 5d106s1 without core correction, using a double − ξ plus polarization basis set, automatically constructed by SIESTA [27] based on an energy shift of 50 meV and a split norm of 0.3 for all elements. The grid interval corresponding to the Fourier cutoff energy of 350 Ry is used for the realspace grid. Calculations were performed using the generalized gradient approximation (GGA) of the exchange-correlation term in the Perdew, Burke and Erzernhof parametrization, which in general is accepted to lead to a better description of the electronic ground state than local density approximation (LDA), despite the tendency to overestimate the bond lengths and underestimate the gaps between valence and conduction states. The Ge unit cell was relaxed using a 20 × 20 × 20 k-point sampling of the first Brillouin zone, using 1 × 10−8 eV as convergence threshold for energy and 0.02 eV/Å for forces. The minimum

of the total energy corresponds to a0 = 5.756 Å, which recovers also the semiconducting properties of Ge. For calculating the electronic properties of the Ge slab, the cell parameters resulted from the bulk unit cell relaxation were used. A 6-layer supercell with the dimer rows aligned along the [110] direction was generated, the bottom layers were saturated with H and fixed at the theoretical lattice constant and the rest of the coordinates allowed to relax until the force acting on each atom was below 0.03 eV/Å. The electronic properties resulted are in line with the characteristics deduced previously [28]. The relaxed supercell was spanned with 4 atoms/unit cell accounting for 1 ML coverage of the surface, and from all configurations tested, the one corresponding to the lowest formation energy was selected. In both Ge slab and Au-covered Ge slab cases, the convergence threshold was 1 × 10−8 eV for energy and 0.03 eV/Å for forces in an 8 × 8 × 1 k-point sampling of the reciprocal unit cell. The slabs are separated by 15 Å of vacuum in order to avoid the interactions with their images, and calculations are performed at 0 K. 4. Experimental and numerical results Fig. 1 shows the LEED patterns on a clean Ge(001) surface (a), recorded at room temperature (RT) and on 1 ML Au deposited on a Ge(001) surface at 600 K (b). The clean Ge(001) surface is featured by p(1 × 2) and p(2 × 1) perpendicular domains while the LEED of 1 ML Au-covered Ge(001) surface recorded at RT shows a square pattern, consisting of the main spots of an unreconstructed (1 × 1) Au(001) surface. Considering the theoretical distance pffiffiffi of two neighboring Au atoms in the ideal bulk fcc crystal, of 4:08= 2 ¼ 2:89 A ̊, we can assume that the valleys between two dimer rows would conveniently allow two Au atoms to accommodate in between, at a distance close to their theoretical value, which would explain the observed square LEED pattern. For the Au-covered surface the shape of the LEED pattern recorded at higher energies remains essentially the same, with four spots, but its intensity fades away. Ge 3d and Au 4f XPS core-level spectra recorded on clean Ge(001) surface, 1 ML Au-covered surface and 4 ML Au-covered surface are presented in Fig. 2. Our fitting procedure is based upon using the minimum number of components in order to correctly reproduce the theoretical features of Ge 3d and Au 4f core spectra, like spin-orbit splittings (SOS) and branching ratios (BR) [29]. Trying to fit the core level spectra of both Ge 3d and Au 4f with only two components did not reproduce adequately the experimental features obtained. Instead, three components were needed. For Ge 3d level at least three components are also reported in the literature [29–32] for properly capturing their core-level structure. For Au 4f core level, the components used for resolving their spectra were associated to a metallic component (E = 84.3 eV), with the binding energy close to (83.8–84.25) eV values reported for metallic Au [33]. This was identified through an additional Au deposition equivalent to 4 ML, Ge-bonded Au (E = 84.9 eV), slightly up-shifted with respect to the metallic components. Additionally, a third component, attributed to Au in an oxidized state of AuOx type (E = 85.6 eV) [34,35] at even higher binding energies, yet not far enough from the metallic component in order to allow its association with an Au2O3 state [36,37] appears. One can note also the different asymmetry of the Au 4f line at 1 ML coverage with respect to a previous study where the deposition was performed at a considerably higher temperature [38], here being evident that the amount of Ge-bonded Au exceeds the metallic content. This could indicate that this deposition regime is more favorable to Au bonding than the higher growth regime. Concerning the first-principles calculations, in the first stage the slab was properly relaxed and the geometric parameters of the resulting clean Ge(001) surface are presented in Fig. 3. The dimer length is 2.59 Å, slightly exceeding the bulk bond length, and the buckling angle is 16.36°, in good agreement with previous calculations [23,24,39,40]. Superimposed on a red-blue scale, the deformation density in a plane

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Fig. 1. LEED patterns recorded on clean Ge(001) surface (a) and on 1 ML Au on Ge surface (b). The spot profile analysis performed along the dotted line indicates the dimension of the crystalline domains for Ge(001) (c) and Au/Ge system (d).

Fig. 2. XPS spectra of Ge 3d core level for clean surface (a), and 1 ML Au covered Ge(001) surface. The Au 4f core level spectra of 1 ML Au and 4 ML Au are presented in (b).

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Fig. 3. Relaxed Ge(001) surface-side view (a) and top-view (b). In (a) the structural parameters are presented and superimposed in a red-blue scale in a plane perpendicular to the dimer rows, the deformation density calculated for the relaxed slab. The grey region in figure (b) represents the (1 × 2) surface unit cell.

perpendicular to the dimer rows, calculated according to the following relation, is presented [41]. ΔρðrÞ ¼ ρSCF ðrÞ‐

X

ρ ðrÞ B B

ð1Þ

In Eq. (1) ρSCF(r) represents the charge density calculated self consistently for the relaxed structure and ∑BρB(r) is the sum performed over the charge density of all isolated B species. Its analysis allows finding the

regions which stand for charge depletion or charge accumulation. In Fig. 3(a), the red regions correspond to charge accumulation, which appears, due to the covalent bonding nature of Ge atoms and has its maximum at approximately half the bond length. In Fig. 3(b), the top-view of the relaxed asymmetric dimer surface is presented and the surface unit cell is suggested with grey-shaded. The band structure of the relaxed slab displayed in Fig. 4(a) was calculated along several important crystalline symmetry directions (see the inset). The band structure of bulk Ge projected on (001)

Fig. 4. (a) Band structure of the Ge(001) slab (thick lines) along important directions in surface Brillouin zone (SBZ) superimposed over the projected bulk band structure on (001) surface (thin grey lines). The inset presents the SBZ generated by the two (1 × 2) and (2 × 1) reciprocally rotated dimer domains. In (b) the EDC featured by dimer surface states ( ) and subsurface electronic resonances ( ) are presented along Γ ‐ K direction and in (c) the second derivative of the photoelectron intensity with respect the binding energy is plotted together with the calculated bands.

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surface is presented with thin grey lines. The slab bands (thick lines) are associated to electronic resonances or surface states. The band associated with up/down Ge dimers may be observed lying in the (0.1–0.4) eV range below and above the Fermi level, and its maximum is located around the Γ point—the center of the surface Brillouin zone (SBZ). Within this picture, the Ge(001) surface is semiconducting, with the electronic states associated to up and down Ge atom dimers separated by a 0.3 eV gap. Additional electron resonances are seen in the range from −0.5 eV to −3 eV and are associated with subsurface states. The band structure and the charge density distribution of the clean surface are consistent with the results reported by Kruger and Pollmann [28] concerning the properties of the C(001), Si(001) and Ge(001) surfaces. The differences between the lattice constants and dimer lengths calculated in [28] and our results are due to the different functional used in calculations (GGA here, LDA in [28]). The band structure is to be compared with angle resolved ultraviolet photoelectron spectroscopy measurements performed along [010] directions (see Fig. 4b) where electronic surface states associated to surface up-dimers are visible immediately below the Fermi edge together with some other electronic resonances, presumably associated to subsurface states. It is well known that the Ge(001) as well as Si(001) surface consist of reciprocally rotated p(2 × 1) and p(1 × 2) domains [42–46] which lead to an overlapping of the electronic states when valence-band photoemission studies are performed along the Γ ‐ J or Γ ‐ J' directions of the surface Brillouin zone. On the other hand, the Γ ‐ K direction is free from this shortcoming [42–44]. In this context, Fig. 4(b) presents the second derivative of the photoemission intensity with respect to the binding energy and the calculated bands along Γ ‐ K direction (see Fig. 4c). The overall agreement between calculations and experiment is good, especially for the bands associated to the dimer-up Ge atoms. The strategy adopted for assessing the optimal configuration of the Au-covered surface was to span the Ge(001) surface with 4 Au atoms per surface unit cell, accounting for 1 ML coverage and subsequently allow them to accommodate on the Ge surface taking into account the calculated distance between two bulk Ge atoms dGe ‐ Ge = 2.53 Å, the distance between two parallel dimer rows drow = 5.6 Å, and the distance between two bulk Au atoms dAu ‐ Au = 2.89 Å. As the deposition

245

temperature was above 600 K when no (4 × 2) or (2 × 2) phase is present [42–44], it is reasonable to assume the simulation cell in the simplest reconstructed case: the p(2 × 1) surface. This scenario is compatible with the p(2 × 1) Ge(001) reconstruction, the amount of Au grown epitaxially and resulted surface morphology. The effect of 1 ML Au deposition on top of the clean p(1 × 2) dimerized Ge(001) surface is evident from Fig. 5(a). This leads to symmetrization of the surface dimers, and the buckling angle changes from 16.36° to 0.2°and an increase of the dimer length from 2.59 Å to 2.62 Å may be noticed. The distances between the two neighboring Au atoms are 2.84 Å and 2.91 Å, close to their theoretical bulk value (dAu ‐ Au = 2.89 Å), and the Au–Ge distance is dAu ‐ Ge = 2.6 Å. The resulting surface is featured by ripples perpendicular to dimer rows with four times the periodicity of the bulk Au atom distance, and a height variation of 0.4 Å between the highest Au atom, situated at half the distance between two dimer rows, and the lowest one situated on top of the dimer row. The electronic properties of Au and Ge near the Fermi level, associated with the resulting morphology are summarized in Fig. 6. It presents the valence band photoemission results on a 1 ML Au-covered Ge surface, and a 4 ML Au-covered surface (a) recorded along the same [010] direction. For more clarity, only the normal incidence spectra are presented, corresponding to the Γ point emission. Additionally, the calculated density of states for the AuGe slab projected on Ge atoms and the DOS projected on Au atoms, respectively, are presented in (b). The dotted line in Fig. 6(a) is the reference spectrum recorded on the Mo holder as reference for the Fermi level position. A first observation concerns the metallic character of the surfaces, which based on the energy distribution curves (EDC) analysis shows metallic behavior due to mixing of Au 6 s states with dimer states resulting from their symmetrization. A second observation is related to the 5d states of Ge-bonded Au atoms and the Au atoms from the top (Au-top) of the dimer rows. Their electronic signature as derived from the projected density of state (see Fig. 6c) is featured by a maximum at ~5.8 eV, while the electronic signature of the Au atom bridging two dimer rows (Au-hollow) is featured by a pronounced maximum at ~4.6 eV. Due to the additional contribution of the Ge-bulk and subsurface states in the (3–4) eV range, an apparent inversion of the Au 5d5/2 and Au 5d3/2 intensities with respect their bulk value may be observed. On the other hand, when the amount of Au

Fig. 5. Deformation density in a red-blue scale of the relaxed configuration consisting in 1 ML Au on top of Ge(001) surface in a plane perpendicular to the dimer orientation (a), and along the [011] direction (b). Red color is assigned to regions where Δρ is positive, while blue to negative values regions. In (c) the top view of the relaxed structure is presented. Contour lines are given in 1 × 10−3 eÅ−3 units between 0.02 eÅ−3 (red) and −0.02 eÅ−3 (blue).

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Fig. 6. ARUPS spectra of a 1 ML Au covered Ge(001) surface and a 4 ML Au covered Ge(001) surface along Γ − K direction (a). The electronic densities of states projected on Ge and Au contributions are figured in (b). In (a), the dotted line is the reference spectrum recorded on the Mo holder as reference for the Fermi level. Represented with (■,●) are the Au 5d5/2 and Au 5d3/2 levels and with (○,□) the electronic resonances associated to the Ge-surface.

deposited on the Ge substrate increases, the bulk ratio of Au 5d5/2 and Au 5d3/2 intensities recovered. A brief inspection of the projected density of states of Ge-atoms (see Fig. 6b) reveals that the states indexed with ○ and □ situated at ~3 eV and 1 eV could not possibly be associated to Au, but rather to subsurface states of Ge. 5. Discussion In order to estimate the coherence domain of the crystalline surface, spot profile analysis was performed for each of the two cases. Intensity profiles along a line passing through the center of the spots are presented in Fig. 1(c) for the Ge(001) clean surface and in Fig. 1(d) for the Au-covered Ge surface. Full details about the evaluation procedure of the coherence domain are given in Ref. [47]. We only recall here briefly the main steps. Using q, the momentum in reciprocal space, we define the coherence length connected to the average domain size as follows: bDN≈Δx≈2π=Δq

ð2Þ

The average surface coherence domain was estimated according to the expression: bDN=a ¼ q0 =Δq

ð3Þ

where α is the surface lattice constant a = aGe/2 = 4.03 Å and Δq/q0 is the normalized width of a diffraction pattern (centered at q0). Thus, the coherence domain for Ge(001) surface gives D = 81.6 Å, while for the Au covered surface D = 43.2 Å. One can observe that the coherence domain of Au covered Ge is approximately half of the clean (2 × 1) surface. Note that in Ref. [10] Blumenstein and co. reported, in addition to the signature of the one-dimensional, chain-like structure featured by (2 × 8) sharp spots, a similar weak LEED pattern, which they associated to a supra-structure. Unlike them, we did not observe any higher-order reconstruction i.e. (4 × 2) or (2 × 8). This is consistent with the observation formulated in Refs. [8–12,16,17], that both deposition temperature and coverage are key-factors in obtaining different surface reconstructions when the Au/Ge(001) system is considered. Previously, it was suggested that the most appropriate models energetically for Au chain formation on Ge(001) should take into account the formation of Au–Ge heterodimers or the bridging of two dimer rows together with the removal of every second dimer row of clean Ge(001) [23,24].

Thus, we tested the surface stoichiometry of the AuGe system against the assumption of Ge substitution at the surface. A priori, if any Ge atom is removed from the surface layer and substituted with an Au one, a change should appear in the XPS spectrum. The escape depth (ED) of Ge 3d photoelectrons, which is the distance from which 95% of the collected photoelectrons originate, is generally taken as ~3λ, where λ is inelastic mean free path of photoelectrons in a material at a given energy. For Ge 3d photoelectrons λ3d is reported between 23.82 Å [48] and 26.45 Å [49], leading to an escape depth of almost 10 nm (~60 atomic monolayers). Assessing the variation of the Ge atoms concentration in the first surface layer based on Ge 3d peak analysis is thus arguable. Yet, the ED of Ge 2p photoelectrons is more than four times smaller than for 3d ones, and their peak analysis is more feasible for studying the surface concentration. As a first test, instead of directly calculating the Au and Ge concentration at the surface based on integral intensities corrected with the empirically derived atomic sensitivity factors, we evaluated the ratio between the integral intensity of Ge 2p and Ge 3d on the clean surface and compared it with that of the Au covered system. A numerical difference of the two would signal a variation in the surface stoichiometry. Supposing that one of every four Ge surface atoms is replaced by an Au atom, this will result in an integral intensity of the Ge 2p peak 2% lower with respect to the clean surface, and a 0.04% variation of the intensity calculated based on Ge 3d ED would remain completely unnoticed in the Ge 3d peak. Evaluating the Ge 2p/Ge 3d before and after the Au deposition, we obtain an inverse variation, as expected: c¼

  IGe2p IGe3d Au

 covered

  IGe2p ‐1 ¼ 1:18 IGe3d clean

ð4Þ

suggesting a Ge enrichment at the surface, which is probably due to migration of bulk atoms during the pre-deposition thermal flashes, in accordance with the observations from [42,43]. Even if these considerations do not completely exclude the possibility of Ge substitution, it is unlikely that both a uniform coverage of the Ge surface and a square geometry of the adsorbed layer that presumably forms can be reproduced at a 1 ML coverage on the p(2 × 1) surface reconstruction, with one or more Ge surface atoms replaced by Au. Based on these considerations, a model which does not take into account substituted surface Ge atoms was elaborated. In order to recover both the amount of gold covering the surface and the low energy diffraction pattern, 4 Au atoms per surface unit cell subsequently relaxed were added, accounting for 1 ML Au deposited. Keeping the relative distance of the Au atoms the same as in their bulk structure, we span

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the (2 × 1) surface unit cell with all possible relative orientations of Au layer. The most stable configuration obtained after the complete relaxation of the coordinates and subsequent accommodation of Au on top of surface Ge atoms was presented in Fig. 5. The arrangement of the Au atoms resembles a square-quasi planar, seesaw-like reconstruction, characteristic mainly to d8 electron complexes, and to Au(III), Pt(II), Ir(I) systems [50]. According to the relaxed geometry, ½ ML Au (2 atoms per surface unit cell) binds the Ge dimers and lead to their symmetrization, the Au–Ge distance being 2.6 Å, and ¼ ML Au is accommodated on top of the dimer row binding with another Au atom, while 1 atom per surface unit cell, accounting for another ¼ ML, is accommodated in hollow sites in the valleys between dimers rows, bridging the Ge-bonded Au atoms. The formation energy is calculated according to the following relation: −1

Eform ¼ A

 fEðn1 Ge; n2 Au; n3 HÞ−Eðn1 Ge; 0Au; n3 HÞ−n2 EAu g

ð5Þ

with E(n1Ge,n2Au,n3H) the energy of the relaxed (Au + Ge) slab, E(n1Ge,0Au,n3H) the energy of the relaxed clean Ge(001) surface, EAu the energy of an bulk Au atom and A—the surface area of the supercell. In Eq. (4), ni stands for the number of Ge, Au and H atoms in the slab. The value for the formation energy obtained is Eform = 0.036 eV. This value suggests on one hand that the structure should not be stable at 0 K, since according to our calculations its formation energy is positive. On the other hand, one can speculate that the high temperature deposition regime (600 K) would satisfactorily account for the 0.036 eV formation energy (for T = 600 K, kBT = 51.7 meV with kB the Boltzmann constant). This assumption is consistent with the fact that when deposition at room temperature is performed, cluster-like, disordered domains appear instead of an epitaxial growth. Likewise, no LEED was observed, indicating that indeed, the layer-by-layer growth is not energetically favorable. According to the map calculated by Sauer and co-workers [23] for the adsorption energy of a single Au ad-atom, the position between the dimer rows in line with the dimers is indicated as less probable for Au adsorption. On the other hand, in our case this is exactly where an Au atom is located. This might be one of the causes the system presented here exhibits such a high formation energy. Another aspect concerning the stability of the resulted surface may be related with our surface reconstruction, which is (1 × 2) and not (4 × 2) like in Refs. [23,24] or [30], which is known to be more stable than the one considered in our work. The fact that this configuration is the most favorable among all tested, should be related to the geometrical constraints necessary for the accommodation of gold atoms at such high coverage, at the same time keeping the given surface morphology of the resulted Au surface. Concerning the interaction between Au and the Ge surface, the calculations suggest that no charge transfer from Ge to Au or vice versa occurs, and instead we observe regions where charge accumulation occurs, approximately at half the distance between the Ge dimer atom

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and the nearest Au one, suggesting that both species share electrons in order to form a covalent bond. This was identified analyzing the deformation density computed in a plane perpendicular to the dimer rows Fig. 5(a), and in a diagonal plane—Fig. 5(b). Computing the difference of atomic charge distribution calculated self-consistently for the relaxed slab taking the sum over all free constituent charge densities according to Eq. (1), one basically obtains that, for a certain atomic entity, when Δρ(r) is negative that specie stands for charge depletion, and when positive it signal charge accumulation [41]. In our case, the charge migrates from the centers of Au and Ge towards half the Au–Ge distance, leading to covalent Au–Ge bonds. On the other hand, the “hollow-site” Au atom, the one situated at half the distance between two dimer rows, and the Au atom located on top of the dimer rows do not seem to interact with the Ge atoms. Instead, as observed in Fig. 5(b) where Δρ(r) was calculated along a diagonal [011] direction, the 2 Au atoms share charge and form crystalline, covalent bonds with other Au atoms exclusively. We will further use these considerations in order to explain the XPS Au 4f core level structure as it is seen in Fig. 2(b). Concerning the components of the core levels, the spectra were fitted with Voigt lines and associated integrals of the Voigt profile, both extracted from Ref. [51]. The inelastic background is associated to the inelastic scattering of photoelectrons leaving the sample. Consequently, when the photoelectrons are originating from the topmost layer, one might expect that no inelastic background occurs [52]. Actually, this is the case for the “surface” components in the spectrum of the Au 4f component, at higher BE. Therefore, we will use this “inelastic background coefficient” (IBF) as a sign to whether the emitters are located at the sample surface (when this inelastic background is very small). As stated in Section 4, three components were needed in order to correctly reproduce the theoretical values of spin-orbit splitting and branching ratio in the case of both Au 4f and Ge 3d levels. Their values as resulted from the fitting procedure are listed in Table 1 and are in close agreement with the values presented in Ref. [29]. Based on the considerations outlined above, concerning both the XPS fitting procedure and the electronic structure deduced for the relaxed AuGe slab, we assigned the three Au 4f components as follows: the first one at 84.3 eV increasing with Au deposition, up to 4 ML identified as metallic Au, an up-shifted component at E = 84.9 eV associated to Ge-bonded Au atoms, and a weaker one at E = 85.6 eV. The later one presumably represents the signature of Au in an oxidized state, although not of Au2O3 type, which should have been observed at E = 85.9 eV [36]. It seems rather reasonable to assign it to an oxidized AuOx state [34,35]. Note that the sum of the integral intensities of the metallic and oxidized components is equal to the integral intensity of the Ge-bonded component, in accordance with our model, which assumes that only half the atoms deposited on Ge(001) surface bind the dimers, leading to their symmetrization. Also, from the inspection of the three Au 4f components at 4 ML Au coverage, one observe that the integral background of the oxidized components is closest to zero,

Table 1 Parameters used for the fit of Au 4f and Ge 3d core-level components.

Ge3d

Clean surface

1 ML Au-covered surface

Au4f

1 ML Au 4 ML Au

Bulk (c1) Dimer (c2) Sub-surface (c3) Bulk (c1) Dimer (c2) Sub-surface (c3) Au-metal Ge-bonded Au Au-metal Ge-bonded Au

BE (eV)

Lorentzian width (eV)

Gaussian width (eV)

SOS (eV)

BR

29.5 29.15 29.8 29.65 29.35 29.95 84.31 84.9 83.9 84.29

0.201

0.542

0.582

1.544

0.210

0.542

0.604

1.676

0.915

0.078

3.681

1.308

0.876

0.074

3.679

1.308

248

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indicating that the region the photoelectrons are coming from is located at the surface, where no attenuation occurs. The contamination with oxygen, given the low reactivity of Au and the UHV working conditions, appears rather surprising. However, it was previously shown that an oxide term resulting from the reaction of CO with atomic oxygen pre-adsorbed on metallic Au even in samples cryogenically cooled may appear, even if such a reaction would seem highly improbable [53–59]. It was also found that even in a low pressure environment (10−7 Pa), some oxidation effects may be associated with the presence of CO solely [53,54]. It was also indicated that this oxidation effects saturates for an adsorbate coverage of at most 1 ML. On the other hand, one can observe that the presence of AuOx term is related rather to metallic gold presence than to Ge-bonded Au as the third component, associated to AuOx slightly increases when Au is subsequently grown up to 4 ML while the second, central component—the Ge-reacted gold, keeps the same intensity. In this context, we speculate that a CO reaction with the first Au layer or an oxidation between the adsorbed oxygen resulted from the cracking of CO2 molecules and the gold ad-layers seems plausible. Unfortunately, the oxygen contamination is difficult to estimate, owing to the superposition of the O 1s peak with the Auger manifold of Ge (LMM, LMV, LVV). On the other hand it is worth mentioning that the influence of the natural asymmetry in the intrinsic XPS lines featuring metallic samples cannot be completely excluded for the third component in the Au 4f spectrum, associated to AuOx. Such asymmetry appears due to nonzero density of states at Fermi level, as how hypothesized by Mahan [60] and calculated by Nozieres and De Dominicis [61], Doniach and Sunjic [62]. Thus, estimating the amount of oxygen contamination from the intensity of the AuOx may lead to erroneous values. For resolving the Ge 3d structure, three components were needed as well: a central component associated to bulk Ge and two additional ones associated to dimer and subsurface components [31,32,42]. It is widely accepted that the “up” and “down” dimer components are shifted towards lower binding energies with 0.5 eV and 0.1 eV. However, our resolution allowed the assignation of a single component shifted towards lower binding energies with respect to the bulk one was considered as coming from either “up” or “down” Ge atoms of the surface dimers. The component of higher binding energy was taken associated as a signature of the subsurface layers. Considering the standard Au 4f core-level binding energy to be that of the 4ML Au covered surface (with the metallic, most intense component at E = 83.9 eV) and the standard Ge 3d binding energy to be that of the clean surface (E = 29.5 eV), one can observe that when 1 ML Au is deposited, both Au 4f and Ge 3d spectra shift towards higher binding energies. This behavior indicates a band bending effect which interplays at the Au–Ge interface, following a similar mechanism described in detail by Apostol and the co-workers [63]. Theoretically, the work function for Au is ΦAu = 5.1 eV [64] and for bulk Ge, ΦGe = 5.0 eV [65]. On the other hand, in our case we deal with a p-doped Ge substrate, which during thermal flashes becomes Ge-enriched at surface, presumably leading to even higher p-doping which pushes the Fermi level upwards. In that case, the ΦGe N ΦAu assumption makes sense and explains the shifts of the Ge and Au core level. Following the mechanism suggested in Fig. 7, the apparent binding energies increase for both Au and Ge, causing a general shift of the Au covered surface by 0.4 eV towards higher binding energies while the dimer component shifts with 0.2 eV. The difference between the shift of the overall spectrum and that of the dimer component is ~0.2 eV, which corresponds to for the chemical shift of the Ge dimer atoms covalently bonded with ½ ML Au from the surface. It is also interesting to note that the intensities of the Ge components are not in accordance with the estimation based on the escape depth of the Ge 3d photoelectrons. The intensity of the bulk component is approximately equal to the total intensity of the surface components. Such an intensity ratio may be realistic assuming that a large fraction

Fig. 7. The mechanism of the band-bending at the interface between a semiconductor with higher work function (Ge) and a metal with lower work function (Au). Due to the alignment of the Fermi levels, the holes are injected into the valence band of the semiconductor and electrons in the conduction band. Thus, over the distance l, the bands curve “downwards”.

of the surface area is heavily destroyed and the disordered regions penetrate deep into the bulk. This observation also explains on one hand i) the rather poor LEED pattern, characteristic for a surface with a rather high defect ratio (i.e. low coherence length as is the case here), ii) the discrepancy between the theoretical band structure calculated for the Ge(001) surface and the features observed experimentally in Fig. 4 [66], and on the other hand support the conclusion drawn based on Eq. (4), which asserts the modification in Ge surface stoichiometry during flash annealing. Finally, the metallic character of the Au/Ge(001) system was assessed by correlating the ARUPS spectra with the electronic density of states calculated for the relaxed geometry of the 6 layer Ge slab covered with 1 ML Au. We observe in Fig. 6 that the system is not semiconducting anymore, with the filled, bonding π(Dup) states, and the empty, antibonding π*(Ddown) one, separated by a gap as in the case of the clean Ge surface. One would expect that due to symmetrization of the dimers, the electronic states localized at the “up” and “down” atom overlap, leading to a finite density of states at Fermi level, thus at a metallic character [67]. In Fig. 6(a) the ARUPS spectra recorded along the [010] direction are presented. Comparing their structure for the case of 1 ML and 4 ML coverage with the calculated density of states, Fig. 6 (b) one can formulate the following observations: i) The metallic character is due to both the contribution of symmetric dimers and the Au 5d Ge 4s4p hybridized states, ii) With respect to the bulk Au 5d states, both calculated (Fig. 6b) and obtained in other ARPES experiments [68], the single layer deposition results in a narrowing of the Au states, partly because of their contribution to the Au–Ge covalent bond. Upon an additional deposition of Au up to 4 ML, the 5d level broadens. This fact must be related to the bulk signature of Au 5d states, which range up to ~2 eV below Fermi level [68]. In addition to the Au 5d5/2 and Au 5d3/2 (● and ■) features associated to Au layer, two sub-surface states were identified: (○) with maximum intensity at ~0.4 eV and (□) peaking at ~2.8 eV, but assessing their dispersion was below the resolution of our experimental setup.

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6. Conclusions We studied both numerically, using ab-initio electronic structure calculations, and experimentally, by means of X-ray photoelectron spectroscopy, and valence-band photoemission spectroscopy the system resulted after the epitaxial growth of 1 ML Au on clean Ge(001) featured by (2 × 1) reconstructions. The Ge-enriched surface stays semiconducting, no additional electronic states due to self-doping being visible. The Au layer grown on the Ge surface is not decoupled from the substrate, and as a result of the epitaxial growth the surface stabilizes in symmetric dimers. A model was given which explains the XPS Au 4f and Ge 3d core-level structure as due to covalent bond formation of Ge atoms dimers with ½ ML of the Au atoms. We identified the signature of the Ge-bonded Au in the XPS spectrum. The correlation with a similar signature in the Ge 3d spectrum was performed taking into account the band-bending effect which manifest at Au/Ge interface for a p-doped Ge sample. As resulted of numerical calculations and ARUPS measurements, the system is metallic due to Au 5d–Ge 4s4p states resulted from surface dimers bonding with Au layer. Acknowledgment This work was funded by UEFISCDI PCCE ID_3/2012 project and by the Kernel Programme 45N/2009 of the National Institute for Materials Physics. The authors gratefully acknowledge the useful discussions and suggestions from Dr. Cristian M. Teodorescu and Dr. Dan Macovei. References [1] [2] [3] [4] [5] [6]

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