In Situ IR Spectroscopic Investigation of Alumina ALD on Porous Silica

Nov 25, 2014 - Aditya Shankar Sandupatla , Konstantinos Alexopoulos , Marie-Françoise Reyniers , and Guy B. Marin. The Journal of Physical Chemistry ...
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In Situ IR Spectroscopic Investigation of Alumina ALD on Porous Silica Films: Thermal versus Plasma-Enhanced ALD Elisabeth Levrau,† Kevin Van de Kerckhove,† Kilian Devloo-Casier,† Sreeprasanth Pulinthanathu Sree,‡ Johan A. Martens,‡ Christophe Detavernier,† and Jolien Dendooven*,† †

Department of Solid State Sciences, COCOON, Ghent University, Krijgslaan 281/S1, B-9000 Ghent, Belgium Centre for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, B-3001 Leuven, Belgium

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ABSTRACT: A novel in situ infrared (IR) approach is demonstrated for investigating and identifying ALD surface reactions during both the steady state and the initial growth regime. The unique combination of reflection−absorption IR spectroscopy in grazing incidence mode with a high surface area reflecting substrate allows for ALD process monitoring with an acceptable acquisition time and a high sensitivity in the entire mid-IR spectral region. Using a mesoporous silica film deposited on a reflecting platinum layer as substrate, the thermal and plasma-enhanced ALD processes of alumina with use of trimethylaluminum (TMA) are compared. Due to the high sensitivity of the method, the relative amount of surface hydroxyl groups added or removed during the process could be determined versus the number of ALD half-cycles. These data reveal substrate-inhibited growth on the silica surface for the thermal process with use of TMA and water, as compared to direct growth for the plasma-based ALD process with use of TMA and O2 plasma. This different behavior could be linked to the formation of Si−CH3 surface groups after the first precursor pulse, as evidenced by the raw IR spectra. It is found that the oxygen radicals in the plasma can remove these surface groups during the next few ALD cycles, while the H2O molecules cannot, thus explaining the initial slower growth for the thermal process.

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INTRODUCTION Atomic layer deposition (ALD) is a thin film deposition technique well-known for its precise thickness control and excellent conformality on high-aspect-ratio structures. The process consists of sequential alternating pulses of precursor gases that react with the substrate surface in a self-limiting manner. Because the start of the film growth depends on the chemical nature of the substrate, knowledge of the initial growth kinetics and surface chemistry of ALD is important, especially when ultrathin films of a few nanometers thickness are needed. Each ALD precursor pulse leaves at most a monolayer of precursor molecules on the surface. As a consequence, very sensitive characterization techniques are needed to perform a detailed investigation of the surface reactions after each pulse. Infrared spectroscopy has proven to be a valuable tool for studying surface chemistry during ALD processes. Chabal et al. published several papers demonstrating a transmission IR technique that allows for monitoring of ALD processes on planar substrates.1−3 George et al. combined transmission geometry with high surface area powders pressed into a grid to improve signal-to-noise ratio and thus reduce measurement times.4−6 The high surface area improves the quality of the spectra because more surface groups can be detected compared with a planar substrate. However, uniform heating of samples in transmission geometry can be a challenge.7 Attenuated total reflection (ATR)-FTIR spectroscopy of ALD processes has been reported, but the need for an ATR crystal and the accurate alignment that this setup requires complicate the experiment.8,9 © 2014 American Chemical Society

Sperling et al. demonstrated a reflection−absorption IR (RAIRS) setup in grazing incidence mode combined with buried metal layer substrates to generate larger signal intensity.7 Unfortunately, in their work the important spectral region between 1100 and 1300 cm−1, of interest to the observation of CH3 bending modes, was unavailable. In this study, a new experimental approach is demonstrated based on the RAIRS technique in grazing incidence mode. Now the substrate is a high-surface-area porous silica film deposited on a reflecting platinum metal layer. This combination of a high-surface-area reflecting substrate with RAIRS in grazing incidence mode enables excellent sensitivity in the entire midIR spectral region. In this paper, first, we validate this novel approach by focusing on the steady state growth regime of the TMA/H 2O and TMA/O 2 plasma ALD processes and comparing the results with those reported in literature for the thermal10−12 and plasma-enhanced13,14 ALD process, respectively. Next, the high sensitivity of our approach is exploited to monitor the relative amount of surface hydroxyl groups added or removed during each ALD half-cycle, including the very first precursor and reactant pulses. These data show that the in situ RAIRS technique allows for investigating the initial growth regime of ALD processes in a very efficient way, while also providing information on the nature of the surface reactions occurring on the starting material. Received: September 1, 2014 Revised: November 12, 2014 Published: November 25, 2014 29854

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Characterization. ALD film growth was monitored by in situ mid-IR spectroscopy in a reflection mode setup as represented in Figure 1. A mid-IR spectrometer (Vertex V70, Bruker) with evacuated beam path was connected to the chamber with KBr windows and pneumatic valves to prevent the windows from being coated. The mid-IR beam is directed from the interferometer through an optics compartment onto the sample via off-axis parabolic mirrors that allows for measurements to be taken in grazing incidence mode. The angle of the incident mid-infrared light was set at ca. 83° with respect to the normal plane of the sample surface. The beam is refocused onto a medium band mercury cadmium telluride (MCT) detector cooled with liquid nitrogen. Data were collected from 600 to 4000 cm−1 with a resolution of 4 cm−1. Each mid-IR spectrum was generated by averaging 702 scans, which corresponds to a measurement time of ca. 5 min. To monitor the growth process of thermal and plasma-enhanced ALD of alumina in nanoporous silica substrates, mid-IR spectra were recorded after every half-cycle from the very beginning of the deposition processes. Raw spectra are referenced to the original starting substrate while difference spectra are referenced to the situation of the substrate prior to the current half-cycle of the ALD process, i.e., prior to the exposure of the precursor vapor or the reactant.

EXPERIMENTAL SECTION Substrates. Mesoporous silica films with a tridimensional pore network built of nanoslabs were used as high surface area substrates to study the thermal and plasma-enhanced ALD growth of alumina. These films were synthesized as described by Pulinthanathu Sree et al. and spin-coated on platinumcoated silicon wafers.15 The platinum layer is necessary to enhance reflection of the light of the infrared source. Ellipsometric porosimetry measurements showed that the porosity of the films was around 80% with pore diameters of 10−12 nm. Thus, a typical film with thickness of 150 nm presented a surface area of 20−30 cm2/cm2 substrate.16,17 Due to this higher surface area, the signal of a single monolayer of alumina deposited in one ALD cycle is expected to improve by a similar factor of 20−30. Prior to the ALD depositions, the films were characterized with mid-IR after a thermal treatment at 200 °C for 1 h in the ALD reactor to remove water from the pores. This showed that the surface groups of the mesoporous silica films consisted of silanol groups and siloxane bridges. Experimental Setup. ALD depositions have been carried out in a home-built high-vacuum ALD setup that is schematically represented in Figure 1.

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RESULTS AND DISCUSSION The in situ RAIRS approach with high surface area substrates is first validated during the steady state growth regime of Al2O3 ALD processes. The investigated ALD processes consist of alternating exposures of the mesoporous silica film to vapors of TMA and water in the thermal ALD process, and TMA and oxygen plasma in the plasma-enhanced ALD process. All experiments are performed at a sample temperature of 200 °C, with precursor and reactant pulses of 10 s and pumping times of 40 s. Under the selected ALD conditions, it takes about 20 ALD cycles to cover the pore surface with a thin layer (∼1.5−2 nm) of ALD deposited alumina. Mid-IR difference spectra of the representative half-cycles of the thermal and plasmaenhanced ALD processes of alumina are shown in Figure 2. During the thermal and plasma-enhanced ALD processes, the difference spectra undergo detectable changes. Signal gain and

Figure 1. Schematic representation of the home-built ALD chamber, equipped with a remote plasma source and in situ mid-IR spectroscopy in reflection mode. In addition, optical viewports are available for monitoring the film growth with spectroscopic ellipsometry (not performed in this study).

It is equipped with a remote plasma source directly above the sample stage and a mid-IR spectrometer in reflection mode. The sample was mounted horizontally in the vacuum chamber and placed onto a heating stage. During the Al2O3 depositions the sample was heated to 200 °C and the chamber wall to 95 °C. Trimethylaluminum (TMA, 97%, Sigma-Aldrich) was used as the aluminum precursor. Deionized water and O2 gas (99.9999%, Praxair) were used as oxygen sources for the thermal and the plasma ALD processes, respectively. All the precursors were kept at room temperature and pulsed inside the chamber through delivery lines. For TMA and deionized water, the delivery lines were heated to 45 and 50 °C, respectively, to prevent condensation of the precursors. The plasma process was performed with a radio frequency remote plasma source of 13.56 MHz and a power of 200 W. The vacuum chamber had a base pressure of 10−6 mbar and each precursor was pulsed at a pressure of ca. 10−3 mbar. A typical ALD half-cycle consisted of a 10 s precursor or reactant pulse followed by a 40 s pumping period.

Figure 2. Mid-IR difference spectra of the half-cycles of the 20th ALD cycles of (a) thermal and (b) plasma-enhanced ALD of alumina. 29855

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first TMA pulse indicating the removal of Si−OH surface groups. In the second ALD cycle this peak is split into a weak absorption at 3778 cm−1 and a strong absorption at 3730 cm−1, indicating the removal of a terminal hydroxyl group on a tetrahedral aluminum atom and a bridged hydroxyl group on an octahedral/tetrahedral aluminum atom, respectively.20 This observation indicates that the active sites of the ALD deposition process evolved from Si−OH to Al−OH surface groups. For the plasma-enhanced ALD process, similar slight peak shifts were found in the region of the carbonates (1350−1700 cm−1) suggesting a more alumina-like surface from the second ALD cycle onward. In addition to the changes in peak positions, also changes in peak intensities were observed. The more a chemical group is present, the more light will be absorbed, thus the higher the intensity of the peak at a specific position. Infrared spectroscopy can thus be used to quantify the change in amount of a specific surface group by integrating its peak area in the difference spectra for every ALD cycle. The amount of active Si−OH surface groups present on the starting substrate can thus be represented by the integrated peak area of the negative Si−OH peak at 3740 cm−1, in the very first difference spectrum (gray area in Figure 3a). The evolution of this integrated peak area during the ALD process can be plotted against the number of ALD cycles to monitor the amount of active surface species in every cycle. Because for both ALD processes the starting substrates were the same, the effect of the first TMA pulse, i.e., the initial removal of Si−OH groups, should be the same, hence it is valid to normalize this initial amount of removed Si−OH groups to −1, allowing us to represent the results for both processes on the same figure (Figure 4).

signal loss is observed at certain wavenumbers indicating surface groups being formed and eliminated, respectively. Hydroxyl surface groups are removed after a TMA pulse as indicated by the signal loss around 3750 cm−1, while CH surface groups are formed as indicated by the signal gain around 3000−2800 and 1212 cm−1.18,19 After the reactant pulses, H2O in case of the thermal ALD process and O2 plasma in case of the plasma ALD process, peaks appear at the exact same positions but if they were positive after the TMA pulse, they are now negative and vice versa. This alternating formation−elimination of surface groups repeats itself every ALD cycle and allows for the elucidation of reaction mechanisms of these processes. More specifically, after the reactant pulse, OH groups are regenerated on the surface while the CH surface groups are removed. This indicates that, for both ALD processes, TMA molecules arriving at the surface during the next ALD half-cycle will react with hydroxyl surface groups.12 For the plasma-enhanced ALD process of alumina some additional peaks are observed in the spectral region from 1350 to 1700 cm−1. These peaks are formed after the O2 plasma pulse and removed after the TMA pulse and were assigned to carbonate species by Agarwal and co-workers.8 The results in Figure 2 are in agreement with previous literature on the thermal and plasma-enhanced ALD processes of alumina, indicating that the proposed in situ RAIRS-based approach for studying reaction mechanisms of ALD processes by using large surface area porous thin films provides useful and relevant information. The combination of a highly porous film on a reflecting substrate with a grazing incident beam path generated enough intensity to measure half-cycles of the ALD deposition process from the very first cycle onward. It is thus possible to monitor the ALD deposition starting from the porous silica surface and moving gradually toward a porous alumina surface. The evolutions of the hydroxyl surface groups, the active species, for the thermal and the plasma-enhanced ALD processes are presented in Figure 3, parts a and b, respectively. As can be seen in both processes, a very sharp negative peak at 3740 cm−1 is formed after the porous silica is exposed to the

Figure 4. Evolution of the normalized integrated peak area of the O− H stretch vibration of thermal and plasma-enhanced ALD of alumina monitored with mid-IR spectrometry in reflection mode.

Every negative data point indicates the amount of Si/Al−OH surface groups being removed after a TMA pulse whereas every positive data point is the amount of Si/Al−OH surface groups being formed after the oxidant pulse, H2O or O2 plasma. This allows for monitoring the filling process of a porous material with ALD deposited material. Let us first consider the O2 plasma-enhanced ALD process. In the first TMA half-cycle, a high amount of Si−OH surface groups is being removed (normalized to −1). The next O2 plasma half-cycle will regenerate OH surface groups but this amount is much less

Figure 3. Evolution of the O−H stretches after the TMA pulses of thermal (a) and plasma-enhanced (b) ALD of alumina monitored with mid-IR spectrometry in reflection mode. All spectra presented are difference spectra of the TMA half-cycle. 29856

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TMA or the oxidant pulse, respectively. Very close to this peak, however, is another very strong absorption peak observed at 1275 cm−1 that can be assigned to Si−CH3 surface groups.24 Obviously this peak is positive after the TMA pulse, hence being formed on the surface during both ALD processes. The formation of Si−CH3 surface groups after exposure of silica to TMA has previously been reported in the literature, typically based on very detailed analysis of the 2900−3000 cm−1 region of the spectrum where many peaks overlap making quantification very difficult.25−27 Since the starting substrates are exactly the same and both processes begin with the same TMA pulse, the first difference spectra are identical. The real distinction between the two ALD processes is what happens in the next difference spectra, after the oxidant pulse. In case of the thermal ALD process, the OH groups are regenerated and the Al−CH3 groups are removed while no negative signal is observed in the difference spectrum around 1275 cm−1, indicating that the Si−CH3 groups are not removed and remain on the surface. In the case of the plasma-enhanced ALD process, OH and carbonate surface species are formed and both the Al−CH3 as well as the Si−CH3 groups are removed during the O2 plasma exposure. This reveals that O2 plasma is strong enough to remove the Si−CH3 surface groups and restore them to active surface groups but H2O cannot. In the next ALD halfcycle, when the surface reacts again with TMA molecules, the TMA will be able to react with OH and carbonate species, but not with Si−CH3 groups. These Si−CH3 groups hinder the reaction of TMA molecules with the surface; consequently less alumina will be deposited in the second cycle of the thermal H2O-based ALD process. During the following ALD cycles, the amount of active OH surface groups is slowly increasing meaning that the alumina is gradually overgrowing the Si−CH3 groups until the silica is fully covered after 15 ALD cycles and there is no longer any influence of the substrate. In other words, the ALD process of thermal alumina is substrate inhibited because of the Si−CH3 surface groups formed after the first TMA pulse. This delay in growth has earlier been observed in literature, e.g., using NMR,28 TXRF,29 or QCM,30 and is often referred to as surface poisoning. Once this inhibition is overcome, the amount of surface groups decreases again because of the surface area that is decreasing when more alumina is deposited in the porous film. If O2 plasma is strong enough to remove these Si−CH3 groups but H2O is not, then Si−CH3 groups are expected to remain present in the interface layer throughout the thermal ALD process, while they are expected to be removed during each plasma exposure in the case of plasma-enhanced ALD. This can be clearly seen in Figure 6 were the raw mid-IR spectra after TMA pulses of both ALD processes are presented. After the fifth ALD cycle, the amount of Si−CH3 groups present on the substrate during the thermal ALD process has not changed, while it decreased for the plasma ALD process. This decrease in Si−CH3 surface groups as a function of the number of ALD cycles for the plasma process can be explained by the gradual covering of the silica surface with alumina as the process advances. After 17 ALD cycles, the Si−CH3 peak is still present during thermal ALD whereas plasma-enhanced ALD was able to remove this peak completely.

than the original amount due to bridging of the adsorbed alumina or the reaction of two neighboring hydroxyl groups to form a siloxane or alumina bridge. In the following ALD halfcycle, these created OH groups can again react with TMA molecules but the amount is decreasing with each ALD cycle. This almost linear decrease, with the exception of the very first data point, can be explained by the gradual decrease in surface area of the porous substrate as the empty pores are being filled with the ALD deposited material.21−23 As the ALD process continues, more material is deposited on and in the pores of the high surface area film, thus decreasing the pore size, porosity, and surface area. As a result, in any next ALD cycle, less material will be deposited because there is less surface area. This decrease in surface area will continue until the pores are filled and the ALD material is deposited on top of the substrate, having the surface area equivalent of a planar substrate.21 In this study, after 35 ALD cycles, the pores (10−12 nm diameter) of the substrates are not completely filled yet with alumina. However, it is already clear that the signal is much smaller than the signal measured on the original porous film, illustrating the importance of using a high surface area substrate to obtain good quality data. For the thermal ALD process of alumina there is a different evolution in the first 15 ALD cycles. The amount of OH surface groups that can be regenerated after the first TMA pulse is much lower than in the case of plasma-enhanced ALD but increases in the next few cycles until it reaches 15 ALD cycles. At that time the amount of OH groups changing is in agreement with the amounts for the plasma-enhanced ALD deposition and both processes continue with the same progress. To explain this deviating behavior of the thermal process, the difference spectra of the very first cycles need to be considered for both processes (Figure 5).

Figure 5. Difference spectra of the 1st ALD cycles of (a) thermal and (b) plasma-enhanced ALD of alumina monitored with mid-IR spectrometry in reflection mode.

Evidently, the peaks in this graph are more intense than the difference spectra after 19 ALD cycles (Figure 2) because of the initial higher surface area of the porous film. The spectral region below 1200 cm−1, called the fingerprint area, is more difficult to interpret because of the many peaks overlapping. On the edge of this complex area is the Al−CH3 stretch vibration at 1214 cm−1 alternatively being positive and negative after the

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CONCLUSIONS Using an in situ reflection adsorption infrared spectroscopic approach, thermal and O2 plasma-enhanced ALD of alumina were investigated. The unique combination of a grazing 29857

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Ultra-Thin Hafnium Oxide Films Grown on Silicon by ALD using TEMAHf and H2O Precursors. Chem. Mater. 2007, 19, 3127−3138. (4) Goldstein, D. N.; McCormick, J. A.; George, S. M. Alumina ALD with TMA and Ozone Studied by in Situ Transmission FTIR Spectroscopy and QMS. J. Phys. Chem. C 2008, 112, 19530−19539. (5) Burton, B. B.; Lavoie, A. R.; George, S. M. TaN ALD Using TBTDET and Hydrazine. J. Electrochem. Soc. 2008, 155, D508−D516. (6) Klaus, J. W.; Ott, A. W.; Dillon, A. C.; George, S. M. Atomic Layer Controlled Growth of Si3N4 Films Using Sequential Surface Reactions. Surf. Sci. Lett. 1998, 418, L14−L19. (7) Sperling, B. A.; Kimes, W. A.; Maslar, J. E. Reflection Absorption IR Spectroscopy During ALD of HfO2 Films from TEMAHf and H2O. Appl. Surf. Sci. 2010, 256, 5035−5041. (8) Rai, V. R.; Vandalon, V.; Agarwal, S. Influence of Surface Temperature on the Mechanism of ALD of Alumina Using Oxygen Plasma and Ozone. Langmuir 2012, 28, 350−357. (9) Li, K.; Li, S.; Li, N.; Dixon, D. A.; Klein, T. M. TDMAHf Adsorption and Reaction on Hydrogen Terminated Si(100) Surfaces. J. Phys. Chem. C 2010, 114, 14061−14075. (10) Dillon, A. C.; Ott, A. W.; Way, J. D.; George, S. M. Surface Chemistry of Alumina Deposition Using TMA and H2O in a Binary Reaction Sequence. Surf. Sci. 1995, 322, 230−242. (11) George, S. M.; Sneh, O.; Dillon, A. C.; Wise, M. L.; Ott, A. W.; Okada, L. A. Atomic Layer Controlled Deposition of SiO2 and Alumina Using ABAB··· Binary Reaction Sequence Chemistry. Appl. Surf. Sci. 1994, 82, 460−467. (12) Puurunen, R. Surface Chemistry of ALD: A Case Study for the TMA/H2O Process. Appl. Phys. Rev. 2005, 97, 121301. (13) Langereis, E.; Keijmel, J.; van de Sanden, M. C. M.; Kessels, W. M. M. Surface Chemistry of Plasma-Assisted ALD of Alumina Studied by IR Spectroscopy. Appl. Phys. Lett. 2008, 92, 231904. (14) Rai, V. R.; Vandalon, V.; Argarwal, S. Surface Reaction Mechanisms During O3 and O2 Plasma Assisted ALD of Alumina. Langmuir 2010, 26, 13732−13735. (15) Pulinthanathu Sree, S.; Dendooven, J.; Smeets, D.; Deduytsche, D.; Aerts, A.; Vanstreels, K.; Baklanov, M. R.; Seo, J. W.; Temst, K.; Vantomme, A.; et al. Spacious and Mechanically Flexible Mesoporous Silica Thin Film Composed of an Open Network of Interlinked Nanoslabs. J. Mater. Chem. 2011, 21, 7692−7699. (16) Baklanov, M. R.; Mogilnikov, K. P.; Polovinkin, V. G.; Dultsev, F. N. Determination of Pore Size Distribution in Thin Films by Ellipsometric Porosimetry. J. Vac. Sci. Technol., B 2000, 18, 1385− 1391. (17) Dendooven, J.; Devloo-Casier, K.; Levrau, E.; Van Hove, R.; Pulinthanathu Sree, S.; Baklanov, M. R.; Martens, J. A.; Detavernier, C. In Situ Monitoring of ALD in Nanoporous Thin Films Using Ellipsometric Porosimetry. Langmuir 2012, 28, 3852−3859. (18) Tsyganenko, A. A.; Filimonov, V. N. IR Spectra of Surface Hydroxyl Groups and Crystalline Structure of Oxides. J. Mol. Struct. 1973, 19, 579−589. (19) Soto, C.; Tysoe, W. T. The Reaction Pathway for the Growth of Alumina on High Surface Area Alumina and in Ultrahigh Vacuum by a Reaction between TMA and H2O. J. Vac. Sci. Technol., A 1991, 9, 2686−2695. (20) Decanio, E. C.; Edwards, J. C.; Bruno, J. W. Solid-State 1H NMR Characterization of Gamma-Alumina and Modified GammaAluminas. J. Catal. 1994, 148, 76−83. (21) Dendooven, J.; Pulinthanathu Sree, S.; De Keyser, K.; Deduytsche, D.; Martens, J. A.; Ludwig, K. F.; Detavernier, C. In Situ X-ray Fluorescence Measurements During Atomic Layer Deposition: Nucleation and Growth of TiO2 on Planar Substrates and in Nanoporous Films. J. Phys. Chem. C 2011, 115, 6605−6610. (22) Dendooven, J.; Goris, B.; Devloo-Casier, K.; Levrau, E.; Biermans, E.; Baklanov, M. R.; Ludwig, K. F.; Van Der Voort, P.; Bals, S.; Detavernier, C. Tuning the Pore Size of Ink-Bottle Mesopores by ALD. Chem. Mater. 2012, 24, 1992−1994. (23) Detavernier, C.; Dendooven, J.; Pulinthanathu Sree, S.; Ludwig, K. L.; Martens, J. A. Tailoring Nanoporous Materials by ALD. Chem. Soc. Rev. 2011, 40, 5242−5253.

Figure 6. Evolution of the Si−CH3 stretch vibration (1275 cm−1) in the raw mid-IR spectra recorded after the TMA pulses of the thermal ALD process of alumina (left) and of the plasma assisted ALD process of alumina (right).

incidence infrared beam and a highly porous substrate on a reflecting metal layer allowed for monitoring each half-cycle of the deposition processes from the very first pulse onward. The infrared spectra measured after the first TMA pulse of both processes indicated the formation of Si−CH3 groups on the porous silica surface. In the next few ALD cycles it was observed that H2O is not reactive enough to remove the Si− CH3 surface groups, while O2 plasma is. This explained the substrate inhibited growth seen during the thermal ALD process of alumina. It is demonstrated that this novel characterization approach can provide detailed information on the ALD surface reaction mechanisms during the initial and steady state growth regime, making this a promising approach to efficiently study the effect of the substrate on ALD growth.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +3292648572. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.D. is grateful for financial support to the European Research Council through an ERC Starting Grant (Grant No. 239865), the Flemish FWO, and the Special Research Fund BOF of Ghent University (GOA project, Grant No. O1GO1513). J. D. acknowledges the Flemish FWO for a postdoctoral fellowship.

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