Detachment Limited Kinetics of Gold Diffusion through Ultrathin Oxide

Article pubs.acs.org/JPCC

Detachment Limited Kinetics of Gold Diffusion through Ultrathin Oxide Layers Jan Č echal,†,‡,* Josef Polčaḱ ,†,‡ and Tomás ̌ Šikola†,‡ †

CEITECCentral European Institute of Technology, Brno University of Technology, Technická 3058/10, 616 00 Brno, Czech Republic ‡ Institute of Physical Engineering, Brno University of Technology, Technická 2896/2, 616 69 Brno, Czech Republic S Supporting Information *

ABSTRACT: Gold clusters and nanoparticles on ultrathin oxide layers are used as catalysts and represent essential parts of plasmonic and electronic devices. The stability of these nanostructures at surfaces against the diffusion of their constituents into the bulk is therefore of vital importance regarding their long-term applicability. Here, on the basis of in situ X-ray photoelectron spectroscopy measurements of gold diffusion through ultrathin oxide layers (SiO2 and Al2O3) to a Si substrate, we show that the diffusion from gold clusters/islands into the bulk is a detachment-limited process. Hence, the ultrathin oxide acts principally as a layer preventing a direct contact of metal atoms with the silicon substrate rather than a diffusion barrier. These findings contribute to a quantitative understanding of general design rules of metal/oxide structures.

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INTRODUCTION Gold nanoparticles, clusters and even single atoms are constituents of catalytic systems,1−4 plasmon enhanced catalytic and energy conversion systems,5−8 electronic devices,9−15 and also advanced structures featuring individual gold atoms in defined positions.16 The ability to control the processes involved in fabrication of these structures is of a vital importance to prevent the loss of an active material from the surface and ensure the long-term stability of prepared structures. Here, one of the issues is the loss of an Au material via diffusion of its atoms, which mainly concerns the systems comprising thin layers since the diffusion into bulk silica has not been observed4 due to low solubility of gold in SiO2.17 In this work, we focus on a study of diffusion of gold atoms from clusters/islands at surfaces through thin oxides into a Si substrate. Even though this is an important issue, the relevant literature is very scarce. One of the first studies on diffusion of Au through a SiO2 thin film established a value of 2.14 eV for activation energy related to this process.18 However, there are no later works on description of gold diffusion through thin oxide films. Only the temperatures for diffusion onsets related to a native silicon oxide (750 °C),19 and to ultrathin alumina layers on CuAl (400 °C)20 and Mo (930 °C)21 substrates were observed. Moreover, there is discrepancy between annealing and deposition experiments, e.g., an incomplete condensation of Au on SiO2 in early stages of deposition at 350 °C has been reported.22 In addition, the quality of oxides plays a vital role in the diffusion process; the defects such as an oxygen deficient surface23or pores in oxides24 lower the temperature necessary for the onset of diffusion through the oxide layers. Since gold nanoparticles below a certain size are efficient catalysts,25 much effort has been invested in the determination of properties of small metal clusters as a function of their size. It © XXXX American Chemical Society

has been found that the number of under-coordinated atoms in the near surface layers determine the size dependence of cohesive energy of these clusters26,27 and, consequently, mechanical, thermal and electronic properties. For instance, elastic modulus, surface tension, melting point, and shift of binding energy within these clusters are inverse proportional to the characteristic feature size, e.g., cluster radius.27 In this work, we report on our study of activation energies of relevant processes for Au and its clusters/islands on ultrathin SiO2 and Al2O3 surfaces employing X-ray photoelectron spectroscopy. As we have shown recently, this technique is capable of providing morphological information on surface confined islands28 and on diffusion of copper atoms through SiO2 layers.29 Within this work, we have revealed that the temperature at which the diffusion process of Au atoms starts depends on the gold coverage rather than on the thickness and type of the employed oxide as one would suppose. Moreover, if the deposition is carried out directly at elevated temperature, the onset for diffusion is shifted to even lower temperatures. Here, based on our experiment, we propose that the kinetics of gold diffusion is determined mainly by gold atom detachment from clusters/islands and is cluster-size dependent.

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METHODS All the experiments were carried out in a home-built complex ultrahigh vacuum (UHV) system.30 The substrates were cut from a Si(100) wafer (ON Semiconductor, phosphorus doped, resistivity 0.0089−0.0093 Ωcm) with a native silicon oxide layer on top. Atomic layer deposition (ALD, Cambridge Nanotech) was used to grow thin Al2O3 and HfO2 layers directly on the Received: March 31, 2014 Revised: July 4, 2014

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Figure 1. Schematic description of the experiments: (1) in the first step the oxide layer is prepared (ALD or dry oxidation), the sample introduced in the UHV chamber and thermally cleaned. Then the (2a) gold layer was deposited at 100 °C and (3a) gradually annealed at increasing temperature. After each annealing step the sample was cooled down and XPS analysis performed. Alternatively, (2b) gold was deposited directly at elevated temperature and (3b) analyzed afterward at temperature below 100 °C.

adsorbed impurities on oxide surface. For the first part of our study, we have used the Si(100) substrate with 0.80 nm native silicon oxide as determined by angle resolved XPS.33 Even though this layer is often considered as ill-defined, it provides a reasonable substrate since it is sufficiently thin, stable at ambient conditions and, more importantly, possesses reproducible properties within the same batch from a single manufacturer. These properties follow the fact the oxide is formed in the last step of Si wafer fabrication under welldefined conditions. Further, we have used thin Al2O3 and HfO2/Al2O3 films prepared on the native silicon oxide layer by atomic layer deposition (ALD),34 and SiO2 thin film prepared by dry thermal oxidation. First, we introduce the experiment in which gold was deposited at 100 °C on thermally cleaned Si(100) with a native silicon oxide layer (see Methods for details) and gradually annealed the sample in steps within the range of 100−750 °C (see top part in Figure 1). After each annealing step the in situ XPS analysis was carried out. The measured spectra and details on the peak fitting and assignment are given in the Supporting Information (SI). Deposition conditions, i.e., low deposition rate, elevated temperature, and low coverage lead to the formation of separate clusters/islands which is inferred from surface free energies of gold and SiO2 (cf. 1125 and 307−605 mJ/m2 for gold and SiO2, respectively)35 and confirmed by the measured Au 4f peak intensities (see SI for details). Further, the presence of a significant number of gold atoms within the oxide layer is not expected neither during the XPS measurements (below 100 °C) nor during the deposition (below 600 °C) due to a very low solubility of gold in the SiO2 in this temperature range (i.e., below 107 cm−3 at 600 °C).17 This is corroborated by recent microscopy study in which the gold atoms were imaged on top and below the oxide layer but not inside,36 and the fact that no gold diffusion into the bulk silica is observed.4 The dependence of relative intensity of the Au 4f peak on annealing temperature is presented in Figure 2(a). For lower annealing temperatures, the intensity remains constant up to a certain threshold temperature; afterward the Au intensity starts to decrease, but does not reach the zero value. Simultaneously,

untreated native silicon oxide layer. The oxide thickness was checked with X-ray photoelectron spectroscopy. The atomic force microscopy measurements (NTMDT NTegra) confirmed that the grown layers possess the RMS roughness ∼0.2 nm, i.e., the same as measured at the bare substrate. After introduction to UHV each substrate was degassed by thermal annealing at 600 °C for 2 h. This temperature is below the onset of decomposition temperature of the native silicon oxide, Al2O3 and HfO2 as well, but it is sufficient for cleaning the substrate surface form carbonaceous contamination.31,32 In the series of deposition experiments carried out at elevated temperatures, a new substrate was always used for each deposition. The substrates were heated using a PBN heater (Momentive) calibrated by thermocouple. The deposition of gold was carried out from a PBN crucible using the standard Omicron EFM3 effusion cell mounted on a chamber with a base pressure of 3 × 10−8 Pa. During the deposition the pressure was in the range from 9 × 10−8 to 2 × 10−7 Pa. The deposition flux was calibrated by quartz crystal microbalance (Sycon STM100) prior each deposition. For majority of experiments the deposition flux of 0.0004 ML/s was used; only for the “fast” depositions the flux 0.01 ML/s was employed. In this work, the nominal coverage is given in monolayer (ML) units; 1 ML is equivalent to the number of atoms on the clean unreconstructed Si(100) surface, i.e. 6.78 × 1014 atoms/cm2. The X-ray Photoelectron Spectroscopy analysis was carried out in situ in a separate chamber using the Omicron DAR400 X-ray source and EA125 electron energy spectrometer. All measurements were done in the constant analyzer energy (CAE) mode using Al Kα radiation, pass energy of 25 eV and emission angle of 50° (related to the normal direction).

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RESULTS AND DISCUSSION In this work, we have studied submonolayer coverages of gold on clean oxides under well-defined ultrahigh vacuum conditions. In this way, we can determine the dependence of related processes on gold coverage (cluster size) and separate them from the influence of the surrounding atmosphere and B

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Figure 3. Binding energy of Au 4f7/2 electrons plotted as a function of nominal coverage obtained by deposition of Au on native silicon oxide at 100 °C. The horizontal lines mark the size of clusters; this is given as a number of atoms K in the cluster radius as explained in the inset.

similar systems,41−44 the size effect could be identified as the primary contribution for the binding energy shift to higher energies.37,42 However, the effect of the substrate oxide composition on the magnitude of the shift should be taken into account when different substrate oxides are employed due to a different substrate-cluster charge transfer character38,44 and different ability of metal oxide supports to shield the final-state hole via extra-atomic relaxation.37 Using the results of recent experimental and theoretical works,27,44 one can roughly correlate the measured binding energy with the average size of clusters which is marked by horizontal lines in Figure 3 and characterized by the parameter K in Figure 2(b) together with the nominal coverage. The value of parameter K corresponds to the number of atoms in the cluster radius and can be calculated by the relation K = R/db, where db = 0.288 nm is the covalent bond length. For hemispherical islands containing n atoms K = 0.78 3√(n). The Au 4f peak intensity decrease presented in Figure 2(a) can be explained by the diffusion of Au through the native silicon oxide layer and formation of gold silicide (AuSi) underneath. Compared to gold present on the surface the gold buried in the sample displays a lower intensity due to the attenuation of the corresponding photoelectron intensity (attenuation length in SiO2 is ∼2.5 nm).45 The driving force for diffusion to the bulk is a minimization of surface free energy by removing gold from surface and a decrease of enthalpy of the system by formation of AuSi. Another possible reason for the decrease of XPS intensity could be the growth of the mean cluster size via the Ostwald ripening process. However, the increase of Au 4f electron binding energy with annealing temperature contradicts the trend which would be expected for increasing the size of clusters, i.e., the decrease of BE toward the bulk value presented in Figure 2(b). Next, taking into account the activation energies of 2 eV for gold desorption from Al2O346 and 1 eV for its diffusion to the bulk (this value is derived below), it is evident that the dominant process is diffusion into the bulk and, hence, desorption of gold from oxide surface could be neglected. The original explanation stating that the decrease in Au 4f peak intensity is caused by the diffusion of Au through the native silicon oxide layer and

Figure 2. Gradual annealing of gold deposited with a nominal coverage in the range of 0.1−2.0 ML on the native silicon oxide layer at 100 °C: (a) evolution of the Au 4f peak intensity normalized to the initial value for easier comparison and (b) Au 4f7/2 peak position as a function of annealing temperature. The temperature at which the native silicon oxide layer is decomposed is marked by a vertical line (see SI for details on the oxide decomposition).

the binding energy (BE) of the Au 4f7/2 peak shifts to higher values. In addition, there is a shift in the initial BE to higher values as the coverage becomes smaller. The position of the Au 4f7/2 peak as a function of the nominal gold coverage measured on the samples prepared at 100 °C is demonstrated in Figure 3. The shift in binding energy of Au 4f7/2 peak could be explained by (1) a different chemical environment surrounding gold atoms, (2) binding of gold atoms to charged defect sites present on the oxide surface, or (3) the effect originating from the reduced size of gold islands, i.e., a final state effect due to the reduced core hole screening.37,38 Since the experiments were carried out under UHV conditions and the gold was deposited on the clean oxide surface, the influence of adsorbed species on the peak position can be excluded. Similarly, the interaction of metals with surface defects on SiO2 is weak39 and the binding of small gold clusters to the negatively charged defects on oxide surface would lead to much smaller or even opposite shifts (i.e., to lower binding energies) than it has been observed.40,41 Since the measured dependence of the Au 4f7/2 peak position on the nominal coverage follows the trend well documented in literature for C

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one equal to ES−B = 2.14 eV derived from MOS structure electrical measurements.18 However, it also lies within the reported interval of cohesive energy of gold clusters, i.e., the energy required for detachment of a gold atom from the cluster. On the basis of the generalized bond energy model, this ranges from 1.9 to 2.9 eV/atom for the clusters containing 50 (K = 2.8) to 500 (K = 6.2) atoms.49 This could be altered for clusters containing few atoms where the values 1.22−2.79 eV/atom for clusters with the size 2 to 20 atoms were obtained using DFT.50 As shown later, the deposition experiment gives the value of the activation energy ES−B = 1.0 eV which is much lower than that one obtained from the annealing experiment. To further elucidate the observed behavior, we have carried out experiments in which 0.8 ML-thick gold layers have been deposited directly at elevated temperatures (see the bottom part in Figure 1). For the future discussion, it will be helpful to briefly introduce elementary processes taking place during the deposition and thin film (island) growth. The impinging atoms (monomers) diffuse on the surface until two (or more) atoms meet and form a stable nucleus or attach to existing islands and thus contribute to their growth. Once a saturation density of nuclei is reached, the islands only grow by monomer attachment. Hence, during deposition, there is a significant population of single gold atoms (monomers) on the surface. These atoms can directly diffuse into the bulk without the need to be detached from existing islands. This results in a shift of the onset of Au diffusion to the bulk to even lower temperatures compared to the 0.1 ML-thick layer deposited at room temperature and annealed subsequently. Figure 5(a) presents the relative intensity of the Au 4f peak and Figure 5(b) the position of the Au 4f7/2 peak measured after deposition of 0.8 ML of gold on native silicon oxide at different deposition temperatures. At the first sight, there is a monotonic decrease of the intensity with increasing deposition temperature. Contrary to that the binding energy starts to shift only when deposition temperature rises over a certain limit (∼200 °C) and the shift saturates by reaching the value corresponding to the AuSi. Additionally, there is also a decrease in Au 5d spin−orbit splitting toward the silicide value42 for layers prepared at temperatures higher than 300 °C (not shown). Contrary to this, a clear change of the slope is present in the Arrhenius-like plot given in Figure 6. This plot displays two mutually different slopes which suggest that two distinct processes are involved in the behavior of gold layers on oxide surfaces. Even though the theoretical modeling is explained later in the text, we will already present one of the results here. The fit to the theory is given in Figure 5 together with the hypothetical curve depicting the case when no diffusion takes place. Hence, the monotonic decrease contains two contributions: the first represents the growth of larger islands with increasing temperature and the second diffusion of gold into the bulk at higher temperatures. Next, similar deposition experiments have been carried out using the 1 nm-thick Al2O3 substrates. Compared to the native oxide sample, the change of the slope in the Arrhenius-like plot given in Figure 6 takes place at higher temperatures which implies an increase in activation energy for surface-to-bulk diffusion. The results presented above suggest that only the single gold atoms, i.e., monomers, can diffuse to the bulk. This explains the observed difference between annealing and deposition experiments. During annealing of predeposited islands, the atoms have to be first detached from these islands to form the

formation of gold silicide there is further corroborated by the fact that the measured Au 4f intensity increases again when the oxide layer is removed by annealing at temperatures beyond 650 °C (see SI for details on the oxide film decomposition).47,48 In Figure 2(a) one can clearly observe that the lower gold coverage, the lower temperature at which the Au 4f peak intensity starts to decrease, i.e., ∼300 °C for 0.1 ML compared with ∼500 °C for the 2 ML nominal coverage. This interesting observation indicates a cluster size-dependent-detachment kinetics of gold atoms from clusters. Next, we have extended the experiment to other oxide substrates, but only to a single gold coverage of 0.3 ML which is sufficiently high to be easily detected. The dependence of the relative intensity of the Au 4f peak on the annealing temperature is presented in Figure 4. Surprisingly, it is very

Figure 4. Evolution of normalized Au 4f peak intensity during gradual annealing of 0.3 ML of gold deposited on various substrates: (1) native silicon oxide layer, (2) Al2O3 with thicknesses in the range of 0.5−15 nm prepared by ALD on native silicon oxide; data for all thicknesses are presented in the inset together with average values while in the main plot only the average values are presented, (3) 10 nm-thick SiO2 prepared by thermal oxidation, and (4) multilayer structure consisting of 8 nm-thick HfO2 and 2 nm-thick Al2O3 on native silicon oxide layer prepared by ALD. All these plots show almost identical behavior.

similar for all studied oxide layers: (1) native silicon oxide layer with a thickness of 0.8 nm, (2) Al2O3 with thicknesses in range of 0.5 to 15 nm, (3) 10 nm-thick thermal SiO2, and (4) 8 nm HfO2/2 nm Al2O3 bilayer. Assumption that the transport through the oxide layer is fast allows us to formally describe the removal of gold atoms from the surface by second order desorption kinetics. This can be justified via comparison of the diffusion length and thickness of oxide layers. Calculating the values of bulk diffusivity D using the activation energy 1 eV (see below) and the pre-exponential factor 1 × 10−13 s−1 (attempt frequency), the diffusion length 2√(D t) is evaluated to 1 nm for time t = 1 ns. Hence, we assume that the actual process of diffusion though the ultrathin oxide layer (200 °C) the diffusion of the material into the bulk is the main reason for the observed intensity decrease.

Figure 5. (a) Relative Au 4f peak intensity and (b) Au 4f7/2 binding energy for 0.3 ML of gold deposited on the native silicon oxide at various temperatures. The red solid curve depicts theoretical values for the complete process and the dashed line theoretical values without considering surface-to-bulk diffusion of Au atoms. The vertical lines mark the temperature interval in which partial surface-to-bulk diffusion occurs.

monomers which can diffuse to the bulk afterward. Whereas, in the case of deposition, there is already a significant monomer population present on the surface. However, the monomer stays on the surface only for a limited time until it is attached to existing nuclei. This implies that if one would increase the deposition flux (faster deposition), the monomer residence time should decrease accordingly and lower diffusion of gold into the bulk would be observed. Indeed, the increase of the flux from 0.0004 ML/s (slow deposition) to 0.01 ML/s (fast deposition) leads to the shift of the corresponding intensity curve to higher temperatures which can be clearly seen in Figure 6. However, it is necessary to prove that the decrease of the Au 4f peak intensity for lower deposition rates is not due to longer deposition times compared to the fast deposition (and thus longer times for which the samples were at the enhanced temperature). Therefore, we have conducted a control experiment in which the fast deposition (∼65 s) at 400 °C was followed by annealing of the sample at the same temperature for another 30 min to keep the total time of the sample at elevated temperature the same as for the slow deposition (31 min). E

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CONCLUSIONS

From the presented results, it is evident that only the diffusion of individual gold atoms through the oxide layers takes place. Hence, the slower the deposition, the more material diffuses into the bulk. In case of the annealing of substrates with gold clusters/islands upon it, the rate limiting step is the detachment of gold atoms from these structures. These results are not surprising if one compares the activation energies of relevant processes. The cohesive energy of the bulk gold is 3.1 eV56 (this value decreases below 2 eV with decreasing size of cluster) and the activation energy of diffusion through the oxide layer 1 eV. Hence, it is evident that much higher energy is required for the atom detachment from the bulk material than for overcoming the oxide diffusion barrier.57 Consequently, the outcomes of our study can be formulated as follows: (1) Even though various oxides have possessed different activation energies ES−B, the same kinetics has been observed at the equivalent experimental conditions. (2) While using the same oxide barrier layer, variations in relevant kinetics have been observed for different nominal coverages or, more precisely, for different cluster sizes. (3) Diffusion through the oxide barriers proceeds differently when the sample is prepared at low temperatures and annealed afterward, or if the material is deposited directly at elevated temperature, since different atomic processes take place in both cases. (4) The fact that a metal structure is stable on top of a surface layer at a given temperature does not necessarily mean that the present oxide acts as an efficient diffusion barrier against metal atoms. Even in the case that the oxide layer only plays a minor role in suppressing gold from diffusion to a bulk material, its vital role is in preventing gold from a direct contact with silicon and, consequently reaction of gold and silicon.

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ASSOCIATED CONTENT

A more detailed discussion on (1) X-ray photoelectron spectroscopy analysis, (2) gold assisted SiO2 decomposition, (3) calculation of XPS intensities from separate Au islands, and (4) modeling of gold island concentrations using rate equations. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ABBREVIATIONS

XPS, X-ray Photoelectron Spectroscopy; ALD, Atomic Layer Deposition; UHV, Ultra-High Vacuum

S Supporting Information *

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ACKNOWLEDGMENTS

This work was supported by the project “CEITECCentral European Institute of Technology” (CZ.1.05/1.1.00/02.0068) from European Regional Development Fund, the Grant Agency of the Czech Republic (Project No. P102/12/1881), the Technology Agency of the Czech Republic (Project No. TE01020233), and the EU seventh Framework Programme (Project UnivSEM, No. 280566). We thank Miroslav Kolı ́bal for critical reading of the manuscript and insightful comments. F

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dx.doi.org/10.1021/jp5031703 | J. Phys. Chem. C XXXX, XXX, XXX−XXX