Thickness dependence of the surface plasmon dispersion in ultrathin aluminum films on silicon

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1 Surface Science 600 (2006) Thickness dependence of the surface plasmon dispersion in ultrathin aluminum films on silicon Yinghui Yu, Zhe Tang, Ying Jiang, Kehui Wu *, Enge Wang State Key Laboratory for Surface Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box , Beijing , China Received 15 December 2005; accepted for publication 15 August 2006 Available online 8 September 2006 Abstract The collective excitation in Al films deposited on Si(1 1 1)-7 7 surface was investigated by high-resolution electron-energy-loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). At the Al film thickness d < 10 ML, the surface plasmon of Al film has only a small contribution to the observed energy-loss peaks in the long wavelength limit ðq k 0Þ, while its contribution becomes significant for q k > d 1. More interestingly, for thin Al films, the initial slope of the surface plasmon dispersion curve is positive at q k 0, in a sharp contrast to bulk Al surface where the energy dispersion is negative. These observations may be explained based on the screening interaction of the space charge region at the Al Si interface. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Al thin films; Scanning tunneling microscopy; High-resolution-electron-energy-loss spectroscopy; Surface plasmon 1. Introduction Collective electron excitations in metal films have attracted attention for more than 30 years due to the phenomena such as large work function changes, catalytic promotion and metal insulator transitions [1 4]. The plasmon excitation of thin alkali metal films on metal surfaces was calculated by Gaspar et al. (in a slab geometry) [5] and Liebsch (for films on a semi-infinite substrate) [6]. They found that the spectra strongly depend on the coverage and, in the long-wavelength limit, the loss feature is dominated by the substrate surface plasmon mode. As the parallel momentum transfer (q k ) increases, the loss feature transits to the collective modes of the clean alkali metal surface. In the case of metal films on a dielectric substrate, for example Mg/MgO [7], the dielectric medium changes not only the frequency of the loss peak but also the energy dispersion. The initial slope of the dispersion curve can be * Corresponding author. Tel.: ; fax: address: khwu@aphy.iphy.ac.cn (K. Wu). positive since the screening interaction of substrate decreases with the increasing q k, similar like the case of Ag where the s-d mutual screening interaction decreases with the increasing q k [8]. Previous studies of surface plasmons were mainly performed on low electron-density alkali-metals (Na, K) and transition metals (Ag, Pd, Hg) with occupied d-orbitals, while data on high electron-density simple-metal films, such as Al, are still lacking. Only recently has the theoretical prediction (by the jellium model) of the negative initial energy dispersion been experimentally proven in Al(1 1 1) [9]. In the case of Ag, the dispersion curve is dominated by the presence of d electrons [6 8,11], thus the substrate effects is difficult to observe. Our recent HREELS study on the plasmon excitations in ultrathin Ag films on Si(111) shows no effect of the substrate [10]. While in case of Al, the absence of d electrons should manifest the effect of the substrate. Furthermore, the surface plasmon energy of a clean Si(1 11) is 11.0 ev [12], close to that of a clean Al (10.9 ev [9]), thus it may influence the SP of the Al films grown on the Si(1 11) surface due to the overlapping of the electron-density waves /$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi: /j.susc

2 Y. Yu et al. / Surface Science 600 (2006) In this paper, we studied the plasmon excitation and dispersion in ultrathin Al films grown on the Si(1 11)-7 7 surface by scanning tunneling microscopy (STM), high-resolution electron- energy- loss spectroscopy (HREELS), and X-ray photoelectron spectroscopy (XPS). A two-step method (low-temperature deposition followed by annealing to room temperature) were used for the preparation of Al films [13,14]. We found, at small Al film thickness (d < 10 ML), a significant substrate effect which may arise from the screening interaction of the space charge region at the Al Si interface. At higher film thickness (d > 10 ML), the loss peak has significant contribution from the surface plasmon excitation of the Al film. Interestingly, the initial energy dispersion (at q k 0) is positive for Al film thickness less than 10 ML, in a sharp contrast to bulk Al surfaces where the energy dispersion is negative [9]. 2. Experiments The experiments were carried out in an OMICRON ultrahigh vacuum (UHV) system consisting of a preparation chamber (< mbar) and an analyzing chamber (< mbar). The analyzing chamber is equipped with an OMICRON variable temperature STM, angle-resolved HREELS (LK-5000), low energy electron diffraction (LEED) and XPS. The Si(111) sample was cut from a phosphors-doped (n-type) Si wafer with a resistivity of 2 X cm. The sample was flashed to about 1100 C to obtained a perfect Si(1 11)-7 7 surface, as confirmed by STM observations. A Knudsen cell was used to evaporate Al. During the Al film growth, the substrate temperature was kept at about 120 K by liquid nitrogen cooling. The deposition rate was about 0.2 ML/min (1 ML refers to the atomic density of the Al(111) plane). After growth, the sample was gradually warmed up to room temperature and transferred to the analysis chamber for in situ STM, HREELS, LEED and XPS measurements. The HREELS measurements were carried out with the incident electron energy of 50 ev and an incident angle of 55 with respect to the direction normal to the surface. The energies of the observed loss peaks were obtained after background subtraction and Gaussian fitting. The energy dispersion was obtained by rotating the analyzer around the specular direction while keeping the sample and the monochromator in fixed positions. The uncertainties of plasmon energies [15] are given by p Dhq k ¼ ffiffiffiffiffiffiffiffiffiffi p 2mE i ðcos h i þ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 E loss =E i cos h s ÞDh s ; ð1þ where E loss is the loss energy of the incident electron with kinetic energy E i, h i and h s are the incident and scattering angles of the electron beam with respect to the surface normal direction. When the angular aperture of the dipole lobe (11.5 ), b = E loss /2E i, is larger than Dh s, Dh s equals to the angular acceptance a (typically 1 ) of the HREEL spectrometer. As one can see from Eq. (1), Dhq k is lowest at low impact energy and larger incident angle. Most of our measurements were recorded in this limit. 3. Results and discussions The typical STM images of Al films grown on the Si(1 1 1)-7 7 surface at different coverages are shown in Fig. 1. About 1.1 ML Al atoms are initially consumed to form a disordered wetting layer. Thus the coverages given below are with respect to the wetting layer. As Fig. 1(a) reveals, below the critical thickness (3 ML), Al forms discontinuous flat-top islands [16]. Starting from 3 ML, the film becomes atomically smooth, with only a few islands or voids due to a slight excess or shortage of Al, as shown in Fig. 1(b). The following growth above 4 ML proceeds in a layer-by-layer mode as shown in Fig. 1(c d). Note that the steps in the STM images have a height of 3.1 Å corresponding to the initial steps of the Si(1 1 1)-7 7 substrate. XPS spectra were recorded for Al films with different thicknesses, as shown in Fig. 2. We focus on the Al 2p and Si 2p peaks. For comparison, the spectrum from clean Si(1 1 1)-7 7 surface is also shown. Significant peak shifts were found, at the initial Al coverage, along the opposite directions for the Al 2p [Fig. 2(b)] and the Si 2p [Fig. 2(a)] peaks with respect to their relevant bulk peaks, which should be associated with a charge transfer from Al films to the Si substrate. The amounts of peak shifts reach to 0.3 ev and 0.2 ev for the Al 2p and Si 2p peaks, respectively, at the film thickness of about 3 ML. Note that the two peaks at 89.6 and 91.3 ev (pointed by two black arrows) are the Mg Ka 3,4 satellite peaks of Si 2p (Si 2p peak is generated by Mg Ka 1,2 ), because: (1) their energy displacement (8.4 ev and 10.1 ev, respectively) and intensity relative to Si 2p peak are equal to those of Mg Ka 3,4 relative to Mg Ka 1,2, respectively [17]; (2) The binding energies of both satellite peaks show the same red-shift (0.2 ev) as the Si 2p peak with the increase of Al film thickness from 0 to 3 ML, as seen in Fig. 2(b). In addition, the two satellite peaks at 77.9 ev (Fig. 2b) and ev (Fig. 2a) are from some unknown impurity in our Mg target of the XPS spectroscopy. In the above spectra two additional peaks at ev and ev were well resolved. The energy differences of these two peaks with respect to the Al 2p peak are ev and ev, respectively, in agreement with the surface plasmon and bulk plasmon energies of Al [18]. As seen from Fig. 2(b), the Al bulk plasmon peak shifts to lower value by 1.0 ev with the Al film thickness increased from 0 to 8 ML. After subtracting the background peak shifts of Al 2p (0.3 ev) and the Mg Ka 3,4 satellite peaks (0.2 ev), a net peak shift of 0.9 ev is obtained. On the other hand, the surface plasmon peak does not show any detectable peak shift in Fig. 2(b). Due to the complexity [19] of energy loss processes in XPS spectra, we are not able to give a clear model for the above observed shift of the Al film bulk plasmon without detail theoretical analysis. Interestingly, even at the

3 4968 Y. Yu et al. / Surface Science 600 (2006) Fig. 1. STM images of Al films with different thicknesses deposited on Si(111)-7 7 at 120 K followed by annealing to RT. (a) 1 ML; (b) 3 ML; (c) 4 ML; (d) 6 ML. The scale of (a) and (b) is 100 nm 100 nm and the scale of (c) and (d) 200 nm 200 nm. Si 2p Al 2p Intensity (Arb. units) 15 ML 11 ML 9 ML 8 ML 7 ML 6 ML 4 ML 3 ML 1 ML Si Binding Energy (ev) Intensity (Arb. units) bulk plasmon SP Θ (ML) Si Binding Energy (ev) Fig. 2. XPS of Si 2p (a) and Al 2p (b) as a function of Al film thicknesses. The curves labeled Si correspond to the relevant XPS from the clean Si(111) surface and two black arrows point to the Mg Ka 3,4 satellite peaks, respectively. The lines are just for eye-guiding to show the shifts of emission peaks with the increasing Al film thicknesses. All spectra are recorded in nearly specular direction.

4 Y. Yu et al. / Surface Science 600 (2006) Fig. 4. HREEL spectra obtained at different scattering angle h s for two Al film thicknesses with an electron primary energy E 0 = 50 ev and an incident angle h i =55. (a) 10 ML; (b) 80 ML. The loss peaks are fitted by Gauss curves. coverage of only 1 ML, the plasmon-like feature can be observed at ev. It may be attributed to the single particle excitations since at this coverage the deposited Al only forms discontinuous flat-top islands (see Fig. 1(a)) instead of a smooth film. More detail information of the surface plasmon dispersion was measured by HREELS. We first consider the surface plasmon excitation in the long wavelength limit (q k 0). Fig. 3 shows the thickness-dependent HREEL spectra of Al films at q k ¼ 0:04 1. Due to the small-angle dipole-scattering mechanism (the angular aperture of the dipole lobe 11.5 with respect to the specularly reflected beam direction) of the surface plasmon, the loss intensity is very weak at q k 0 and the resolution is reduced to obtain a reasonable signal-to-noise ratio. A loss peak can be observed at ev, close to the peak position of the surface plasmon excitation on clean bulk Al surfaces. The peak position shows an initial red-shift from 10.4 ev (4 ML) to 9.9 ev (10 ML), and eventually a slight blue-shift to 10.2 ev as the film thickness further increases up to 80 ML. To explain the above data, we note that the penetration depth of electrons in an inelastic scattering is 1=q k [20]. In case of film thickness d 1=q k (small q k or small film thickness), the surface plasmon field has a large penetration depth and the surface plasmon of the Si(1 11) substrate may have a large contribution to the energy loss. In contrast, in case of d 1=q k (large q k or large film thickness), the surface plasmon field decays rapidly across the Al film and substrate effect vanishes. In the spectra in Fig. 3, q k ¼ 0:04 Å 1, thus 1=q k is about 25 Å, corresponding to a film thickness of about 10 ML. Therefore, at film thickness <10 ML, the energy loss mode have a large contribution from the surface plasmon of the Si substrate (11.0 ev for a clean Si surface [12]). With increasing Al film thickness, this mode shifts downwards and, near q k d 1, it approaches the Al Si interface mode given by [7]: x i p xp ffiffiffiffiffiffiffiffi, 1þe Si where x p is the bulk plasmon frequency of Al, and e Si the dielectric function of silicon (e Si 15 [21]). This explains the observed peak shift in Fig. 3. On the other hand, at 80 ML, the energy of the Al surface plasmon shows a slight blue-shift as compared with that at 10 ML, which indicates a gradual transforming into the bulk regime and the substrate effect is gradually switched off. However, the above calculated x i (3.9 ev) is much lower than our measured value (10 ev) at 10 ML. Thus we should consider other effects. One possibility is that the surface plasmon energy of a clean Si surface and a clean Al surface is very close to each other (about 11 ev at q k 0 [9]), thus at the interface the charge density waves of both surface plasmons can overlap at q k 0, 1 and localized to the same electron hole pairs in their decay processes. This effect may shift the interface plasmon energy to a large value, in agreement with our experimental observations. Fig. 4 shows the HREEL spectra recorded in 10 ML and 80 ML Al films at different q k. The parallel momentum transfer, q k, is along the C K direction of the Al(111) surface Brillouin zone (SBZ), as measured by LEED. Interestingly, at the coverage below 10 ML (Fig. 4(a)), we observe a positive initial dispersion of the plasmon peak energy with increasing q k. A positive dispersion had been reported in an early study on polycrystalline films of Al and In [22]. Fig. 3. HREEL spectra obtained at the same q k (0.04 Å 1 ) for different thicknesses of Al films with an electron primary energy E 0 = 50 ev and an incident angle h i =55. The loss peaks are fitted by Gauss curves and the short lines are just for eye-guiding to show the position of the loss peaks. 1 Although the Si plasmon energy may be modified due to the existence of Al overlayer, it is very weak in comparison with the SP energy of Si and the modified Si plasmon energy should still be close to that of Al overlayer.

5 4970 Y. Yu et al. / Surface Science 600 (2006) Fig. 5. The surface plasmon dispersion versus the parallel momentum q k for four Al film thicknesses. The best-fitting curves for the dispersion are also shown. The inset is the LEED pattern, where the measuring direction in the surface Brillouin zone of Al (111) is indicated. However, theoretical studies suggest negative dispersions for simple metals including Al [15,23], which has been proven by a recent HREELS measurement on single crystal Al(111) surface [9]. In order to validate the dispersion of the bulk Al, we measured the surface plasmon of Al film with a thickness (80 ML) approaching to the bulk regime (Fig. 4(b)). We indeed found a negative initial dispersion, as shown in Fig. 5, in agreement with the result from single crystal Al(111). The reversed initial energy dispersion in thin Al films should be induced by the substrate effect. In the cases of Mg/MgO [7], the polarizable Mg MgO interface (the induced space-charge region) has a screening interaction with the free electrons in Mg, which lowers the energy of the SP. For our system, as the chemical shifts in the XPS spectra (Fig. 2) indicate a charge transfer from the Al films to the Si substrate, there should be a space charge region formed at the Al Si interface, with the positive charges at the Al side. The space-charge regions can have a screening interaction with the Al surface plasmon, especially for small film thickness or large electric field penetration depth, similar like the screening effect of d electrons in Ag surface plasmon excitation [6,8]. As a result, the energy of the surface plasmon shifts to lower value. As q k increases, the plasmon fields of the Al films have a smaller penetration depth and thus the screening interaction of the induced space-charge region becomes weaker, resulting in a positive dispersion. On the other hand, if the film becomes thick enough, the screening action near the Al Si interface has almost no influence on the SP energy even in the long wavelength limit. Therefore, the energy dispersion of the SP approaches to that of a clean bulk Al surface. At the large q k region, the plasmon shows a parallel dispersion for different Al film thicknesses due to the small penetration depth of plasmon field. 4. Conclusion In this work, the plasmon excitations of Al films have been investigated using HREELS and XPS. We found that, with the Al film thickness increasing to 10 ML, the observed energy-loss peaks show red-shifts at q k 0, where the interface plasmon dominates. Furthermore, for thin Al films, the initial slope of the dispersion is positive, which can be explained by the screening effect at the interface. However, for the bulklike Al films, the surface plasmon presents an initially negative dispersion as the case of single crystal Al. References [1] A.U. Macrae, K. Müller, J.J. Lander, J. Morrison, J.C. Phillips, Phys. Rev. Lett. 22 (1969) 1048.

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