Reflectance difference spectroscopy study of Ag growth on W(1 1 0)

Surface Science 600 (2006) L281–L285 www.elsevier.com/locate/susc

Surface Science Letters

Reflectance difference spectroscopy study of Ag growth on W(1 1 0) L.D. Sun a

a,*

, M. Hohage a, P. Zeppenfeld a, C. Deisl b, E. Bertel

b

Institute of Experimental Physics, Johannes Kepler University Linz, Altenbergerstr. 69, A-4040 Linz, Austria b Institute of Physical Chemistry, University of Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria Received 20 April 2006; accepted for publication 28 July 2006 Available online 17 August 2006

Abstract We report a reflectance difference spectroscopy (RDS) investigation of the epitaxial growth of Ag on the W(1 1 0) surface. Monitoring the growth in real time, the RDS signal at 4.6 eV shows an oscillatory behavior corresponding to the layer-by-layer growth of the first three monolayers. The oscillations are attributed to the variation of the optical anisotropy contributed by the W(1 1 0) substrate and the Ag film. By analyzing the spectral evolution during growth, characteristic optical-electronic fingerprints can be deduced for each added atomic layer. In particular, the binding energy of d-like quantum well states has been used as an indicator for the number of Ag atomic layers and, hence, as a sensitive probe of the Ag thin film growth.  2006 Elsevier B.V. All rights reserved. Keywords: In situ characterization; Thin film growth; Reflectance difference spectroscopy

The growth of thin metal overlayers on metallic substrates has attracted considerable interest during the last decades because their specific physical and chemical properties are of fundamental scientific and technological relevance. On the other hand, all the relevant properties of thin metal films such as the morphology, magnetic behavior, electric and thermal conductivity are related to the electronic structure of these films. Therefore, various analytical methods for the characterization of the electronic properties as a function of the film thickness have been developed and have led to a detailed understanding of the evolution of the electronic properties within the thin metal films [1–5]. The most widespread methods to probe the thin film electronic structure are ultraviolet photoemission (UPS), inverse photoemission (IPE), and two-photon photoemission (2PPE) spectroscopy [1]. The details of the electronic structure, e.g., binding energy and its dispersion can be measured for the occupied and unoccupied states. With increasing film thickness, the transition from a quasi 2D *

Corresponding author. Tel.: +43 732 2468 8519; fax: +43 732 2468 8509. E-mail address: [email protected] (L.D. Sun). 0039-6028/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.07.038

electronic band structure of a single ordered monolayer to the 3D bulk bands can be followed. Detailed information on electronic interface states which are located spatially at the thin film/substrate interface as well as the quantum well states which are trapped between the surface barrier on the vacuum side and band gap on the substrate side has been obtained with those methods [4,6,7]. Especially, the electronic properties of the outermost layers, which may change from layer to layer, strongly influence the epitaxial growth. Therefore, studying the evolution of the surface electronic properties is crucial for the understanding of epitaxial growth [1,8]. Reflectance difference spectroscopy (RDS) is an optical technique which measures the difference of the reflectivity at normal incidence for light linearly polarized along two orthogonal directions [9,10]. In the case of cubic crystals, RDS becomes a highly surface sensitive method since the crystal bulk is electronically isotropic. As a result, the RD anisotropy signal is exclusively generated by the symmetry breaking surface and interface. For cubic metal crystals terminated by an anisotropic surface, the RDS signal can be contributed by electronic transitions either between surface states or surface modified bulk states which are

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incidence with the polarization axis oriented at an angle of 45 with respect to the two main crystallographic axes [0 0 1] and ½1 1 0 of the W(1 1 0) surface. The change of the polarization state of the reflected light is measured using a modulation detection technique. By means of a grating monochromator, the reflected light is further analyzed with respect to the photon energy. As a result, the complex normalized reflectance difference defined as r½001  r½110 Dr ¼2 r r½001 þ r½110

ð1Þ

can be recorded as a function of the photon energy in a range between 1.5 eV and 5.5 eV. Fig. 1 shows the RD signal at fixed photon energy of 4.6 eV as a function of Ag coverage. For comparison we also reproduce the specularly reflected He scattering intensity from Ref. [23]. Both signals reveal growth oscillations with monolayer periodicity for the first three monolayers. Specular He-scattering senses the surface roughness and terrace height distribution and thus probes the evolution of the thin film morphology during the Ag growth. The oscillations of the specular He intensity in Fig. 1 arise from the repeated sequence of island nucleation, coalescence, and completion of a new layer indicating a layer-by-layer growth of the first three Ag monolayers. Note that the oscillations of the specular He intensity are strongly damped and disappear after the third period. Furthermore, the second and third maxima corresponding to the completion of the second and third ML occur around 2.2 ML and 3.5 ML, respectively. (Note that 1 ML is here defined as the Ag coverage needed for the completion of the first monatomic Ag layer.) This observation reflects the different atomic density of the 2 and 3 ML thick Ag films as compared to the first Ag monolayer. Interestingly, a clear oscillation of the RD signal at 4.6 eV whose maxima correlate with those of the He reflectivity curve is observed. This means that the maxima in the RD signal correspond to the completion of first, second and third Ag monolayer.

1.2

0

RDS 1.0 -3

-1

0.8 0.6

-2

0.4 -3

He 0

1

2

3

4

norm. He intensity

both spatially located in the near surface region [10–15]. When adsorbate layers or films are grown on such surfaces, new specific features may arise in the RD spectra. Those RDS features have been used to monitor the adsorbate-induced modifications at substrate surfaces and interfaces [16–18]. In this letter we will show that in case of heteroepitaxial growth, namely, Ag on W(1 1 0), the RD spectra senses not only the changes of the W surface but also the evolution of the electronic structure within the growing Ag films. This establishes RDS as a complementary tool to photoemission techniques that is particularly well suited for the real time monitoring of the epitaxial growth of metal films. W(1 1 0) is a classic substrate for the epitaxial growth of noble metal overlayers because no alloying occurs and the electronic structure of the adlayers can be well identified. The growth of Ag layers on W(1 1 0) has been extensively studied over the last decades [5,19–26]. Layer-by-layer growth occurs for the first two Ag monolayers, at higher coverage 3D growth sets in. Recent studies using STM and polarization-dependent photoemission [25,26] show that the first Ag monolayer is essentially commensurate with the bcc W(1 1 0) surface thus exhibiting a C2v symmetry. Only around domain walls parallel to the W[0 0 1] direction preset discommensuration lines exist where the surface stress induced by the misfit between the strained Ag(1 1 1)-like layer and the W(1 1 0) substrate is released. In the second monolayer, both bcc and fcc domains are formed, suggesting the transition from the W bcc substrate potential to the Ag fcc holding potential. Once the second layer is almost filled, 3D islands with Ag(1 1 1)-like planes on top start to grow. Upon further growth the geometrical and electronic structure approaches that of the structurally isotropic Ag(1 1 1) surface [5,26]. The C2v symmetry of the W(1 1 0) surface however induces a significant structural anisotropy in the growing Ag films which should make these films easily accessible with the RDS technique. The experiments have been carried out in an UHV chamber [26] with a base pressure of p < 8 · 1011 mbar. The system is equipped with an STM, LEED, a mass spectrometer, and an indirectly heated alumina crucible for Ag evaporation. The W(1 1 0) sample was a circular disk of 8 mm diameter, oriented to better than 0.25, which corre˚. sponds to an average terrace width of several hundred A The sample was cleaned by repeated cycles of heating to T = 1400–1750 K in a 5 · 108 mbar oxygen atmosphere, followed by a rapid flash to T = 2300 K in the absence of oxygen. After successful cleaning no indications for the well-known C-induced reconstructions on W(1 1 0) could be detected in atomically resolved STM images and a sharp (1 · 1) LEED pattern with very low background intensity was obtained. All RD spectra were recorded at room temperature. Ag was evaporated at a constant rate of 0.7 ML/ min, with the substrate at room temperature. An RD spectrometer of the Aspnes type [9] was attached to the UHV chamber via a strain-free optical window. The light from a Xe lamp is directed on the sample at normal

Re(Δr/r) (10 )

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0.2

5

0.0

Ag coverage (ML) Fig. 1. Growth oscillations recorded during Ag deposition on the W(1 1 0) surface at room temperature: RDS signal recorded at a photon energy of 4.6 eV (s) and specular He intensity reproduced from Ref. [23].

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The Ag coverage determined from the growth oscillation of the RD signal has been cross checked with STM measurements [26]. Therefore, in the current study, the RDS signal at 4.6 eV during Ag deposition was used to calibrate the film thickness. The open question here concerns the origin of the observed growth oscillation of the RD signal. Actually, growth oscillations with monolayer periodicity probed by RDS have been reported earlier for the homo-epitaxial growth of GaAs(1 0 0) [27,28]. In this case a surface resonant electronic transition related to the presence of Ga or As dimers is probed, whose concentration is affected by the (oscillating) density of surface steps during growth. More recently, we have shown that RDS is sensitive to the strain distribution, i.e., the intrinsic atomic displacement induced in the substrate during thin film growth. The variation of this strain field with monolayer periodicity also gives rise to an oscillating RDS signal [29]. From the following discussion, however, it will become clear that the oscillations observed here arise from the layer-by-layer evolution of the electronic structure of the growing Ag film due to the strong electronic confinement in the thin Ag slab, i.e., the presence of quantum well states. In Fig. 2, the real part of the RD spectra for different Ag coverages is plotted. The thick solid line at the bottom of the graph corresponds to the bare W(1 1 0) surface. The overall line shape is similar to that reported earlier by Martin et al. [30]. The main features of this RD spectrum located around 3 eV and 4.5 eV are attributed to interband transitions in the topmost surface layer and surface modified bulk transitions, respectively [30,31]. After deposition of the first 1/3 ML of Ag, the substrate related features are strongly quenched. Increasing the Ag coverage further,

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several new features appear over the entire photon energy range. When the Ag nominal thickness increases above 3 ML, the spectrum shows a dip-like feature located around 3.9 eV which is very close to the onset of the d-band transition of bulk Ag. This observation strongly suggests the formation of a bulk-like electronic structure when the Ag films become several layers thick. This conclusion agrees with photoemission and STM results [5,26]. The observed anisotropy at 3.9 eV is thus due to the residual strain in the Ag film induced by the anisotropic W(1 1 0) substrate. The details of the evolution of the RD spectra become more apparent in Fig. 3 where we have plotted the difference (DRDS) between subsequent RD spectra in Fig. 2. These difference spectra thus reflect the incremental changes of the optical anisotropy after sequential deposition of 1/3 ML of Ag. The most important conclusion that can be drawn from Fig. 3 is that the changes strongly depend on the number of the added Ag atomic layers. This reveals the layer-by-layer evolution of the electronic structure in the Ag film: For the first ML (bottom panel of Fig. 3), the DRDS curve for the first 1/3 ML Ag is very close to the negative spectrum of clean W(1 1 0). This means that initially, the surface optical anisotropy of the W(1 1 0) substrate is quenched by the deposited Ag. This observation suggests that the Ag forms small, randomly distributed islands which disturb the electronic structure of the W(1 1 0) surface without allowing for new Ag related optical transitions in the energy range of interest. As a result, the overall surface related optical anisotropy is quenched.

0.5 5 ML

2

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3 ML

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3rd ML

Δ RDS (10 )

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-3

Re(Δr/r) (10 )

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0 ML

0.0

×0.4

-0.5 1.5

2.0

2.5

3.0

3.5

4.0

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Photon energy (eV) Fig. 2. Real part of the RD spectra recorded from the bare W(1 1 0) surface (thick solid line) and after successive deposition of Ag at room temperature. The Ag coverage was increased in steps of 1/3 ML up to a thickness of 3 ML. The topmost RD spectrum corresponds to a 5 ML thick Ag film.

2

3

4

5

Photon energy (eV) Fig. 3. Difference between two subsequent RD spectra in Fig. 1 up to a total coverage of 3 ML Ag, showing the incremental changes of the optical anisotropy for Ag grown on the W(1 1 0) surface. The solid lines, dashed lines and the dotted lines are the DRDS spectra corresponding to the first, second and third deposited dose 1/3 ML of each layer.

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This interpretation is supported by the STM study [26] where the formation of small and stable Ag islands on terraces has been observed at room temperature. The DRDS curves for the subsequent two 1/3 ML doses of Ag are clearly different than the first one but are almost identical to one another. Their main common feature, a well defined peak at 4.6 eV, must be related to the electronic structure within the first Ag layer. In fact, for 1 ML Ag on W(1 1 0), d-like quantum well states with binding energies of 4.2 eV, 4.6 eV and 5 eV have been observed in photoemission [5,32]. A recent polarization-dependent photoemission study shows that the quantum well states at 4.6 eV and 5 eV, which are derived from Ag xz, yz d-orbitals, exhibit a C2v symmetry induced by the anisotropic W(1 1 0) substrate [26]. Based on its energetic position and its characteristic evolution with coverage, the DRDS peak at 4.6 eV can be attributed to an electronic transition from the 4.6 eV quantum well state to the Fermi-level. The absence of well defined DRDS peak at 5 eV could be due to the increasing noise level at higher photon energies. The observation that the DRDS spectra do not change their shape and the fact that their amplitude is directly proportional to the incremental Ag coverage, strongly suggest the growth of monolayer islands with increasing size. This conclusion is fully consistent with the initial layer-by-layer growth deduced from Fig. 1 as well as from the photoemission and STM results in Refs. [5,26]. After the completion of the first monolayer, the DRDS curves reveal the formation of new electronic features characteristic for the second ML of Ag. The DRDS curve for the first 1/3 ML fraction of the Ag bilayer shows almost a flat line around zero, with no obvious change of the optical anisotropy. This can be interpreted by the fact that the size of Ag islands nucleating on top of the first monolayer is still too small to allow new optical transitions and/or by the lack of anisotropy at this coverage. Further deposition of Ag induces three positive peaks in the DRDS curves located at 3 eV, 4.2 eV and 4.7 eV, respectively. The 4.2 eV and 4.7 eV peaks are still contributed by those d-like quantum well states which were originally located at 4.6 eV and 5 eV for the 1 ML thick film, respectively, but are now shifted to lower energies as a result of the increasing film thickness [32]. In addition, the peak at 3 eV can be attributed to the transition from another s-like quantum well state with a reported binding energy of 3 eV in the 2 ML thick Ag film [5,32] to the Fermi level. The line shapes of the DRDS curves are again identical and scale with the incremental Ag coverage. As for the first monolayer, this is consistent with the 2D growth mode of the Ag bilayer. During the growth of the third Ag layer, the DRDS curves change again. The curves corresponding to the first two 1/3 ML deposition of Ag are still dominated by the peaks at 4.2 eV and 4.7 eV but now with a negative sign which suggests the quenching of the optical anisotropy originally formed in the 2 ML thick Ag film. This observation can be explained in two different ways, namely, the

vanishing of the quantum well states or the reduction of the anisotropy of the atomic and electronic structure. According to the photoemission results, the binding energy of the 4.7 eV quantum well state in the Ag bilayer should shift to 4.1 eV when the film becomes 3 ML thick. The absence of a 4.1 eV peak in the 3 ML DRDS curves thus suggests the vanishing of the anisotropy of the d-like quantum well states, i.e., the formation of a more isotropic fcc(1 1 1)like Ag film, for instance, due to a more efficient strain/ stress release within the thicker film and the onset of 3D Ag growth. The DRDS curve corresponding to the last 1/ 3 ML deposition of Ag is different again and is characterized by the formation of a negative peak at 4 eV. This energy is close to the known onset of the d-band transition of bulk Ag (3.9 eV) and thus suggests the development of a bulk-like Ag electronic structure, again consistent with the onset of 3D Ag growth. In summary, we have studied the epitaxial growth of Ag on W(1 1 0) at room temperature by monitoring the optical anisotropy during Ag deposition. The fast quenching of the substrate optical anisotropy at the initial stages of growth confirms the 2D island growth on terraces rather than Ag growth from the step edges. The RD spectra and their differences clearly reveal a layer-by-layer change of the electronic structure of the Ag film. We are able to establish a straight correspondence between quantum well states in the growing film and RDS features, which apparently arise from electronic transitions to the Fermi level. As quantum well states in general exhibit a characteristic evolution with film thickness, this transfers to the RDS spectra in a unique way. The present observation therefore qualifies RDS spectroscopy as a versatile tool for in situ monitoring of metal thin film growth in all anisotropic systems where quantum well states are formed. In the present case the Ag grows in a layer-by-layer mode for the first two monolayers. The film on the anisotropic W(1 1 0) surface is uniaxially strained and thus exhibits a two fold symmetry (anisotropy) which gradually relaxes with increasing film thickness. Finally, a bulk-like Ag electronic structure is formed when the nominal film thickness reaches about 3 ML. Acknowledgement We are grateful for financial support by the Austrian Science Fund through the Joint Research Program (JRP) NSOS. References [1] N. Memmel, Surf. Sci. Rep. 32 (1998) 91. [2] A.P. Shapiro, Tc. Hsieh, A.L. Wachs, T. Miller, T.-C. Chiang, Phys. Rev. B 38 (1988) 7394. [3] B. Schmiedeskamp, B. Kessler, B. Vogt, U. Heinzmann, Surf. Sci. 223 (1989) 465. [4] T. Valla, P. Pervan, M. Milun, F.B. Hayden, D.P. Woodruff, Phys. Rev. B 54 (1996) 11786. [5] J. Feydt, A. Elbe, H. Engelhard, G. Meister, A. Goldmann, Surf. Sci. 452 (2000) 33.

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