Size Dependent Effects in Nucleation of Ru and Ru Oxide Thin Films

Aug 13, 2013 - Fairview Associates, Woodside, California 94062, United States. ABSTRACT: The crystal structure and growth behavior of Ru and RuO2 on...
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Size Dependent Effects in Nucleation of Ru and Ru Oxide Thin Films by Atomic Layer Deposition Measured by Synchrotron Radiation X‑ray Diffraction Rungthiwa Methaapanon,† Scott M. Geyer,† Sean Brennan,‡ and Stacey F. Bent*,† †

Department of Chemical Engineering, Stanford University, Stanford, California 94035, United States Fairview Associates, Woodside, California 94062, United States

‡

ABSTRACT: The crystal structure and growth behavior of Ru and RuO2 on amorphous SiO2 are measured during atomic layer deposition (ALD) by ex situ and in situ high-resolution synchrotron radiation X-ray diffraction (XRD). In situ XRD studies suggest that RuO2 films grown by bis(2,4dimethylpentadienyl)ruthenium, Ru(DMPD)2, and oxygen at low temperature do not initially nucleate as RuO2. Despite large oxygen exposures during the ALD process, the initial nuclei form as hcp Ru. Then, after higher numbers of ALD cycles, crystalline rutile RuO2 begins to appear. The results suggest that a critical Ru nucleus size is required to initiate the growth of RuO2. We speculate that a rate limiting step in the oxidation of Ru, possibly the formation of subsurface oxygen, is dependent upon the size of the Ru nuclei. Although the hcp Ru films are textured with a (002) preference in the growth direction, the rutile RuO2 films once they nucleate have no preferential orientation. KEYWORDS: Ru, RuO2, XRD, nucleation, atomic layer deposition

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INTRODUCTION Ruthenium is a versatile noble metal that is used in many electronic and catalytic applications. Its physical and electrical properties make it suitable for use as an electrode for dynamic random access memory (DRAM) and ferroelectric random access memory (FRAM), and as a gate electrode material in metal-oxide-semiconductor field effect transistors (MOSFETs).1 Ruthenium is also a potential seed layer for the metallization of copper interconnects.2 Moreover, the most stable oxide of ruthenium, RuO2, is conductive and can be used as a capacitor electrode and as an oxygen diffusion barrier.3 In addition to their utilization in optoelectronics applications, Ru and RuO2 are important as catalysts and electrocatalysts in various reactions, such as hydrogenation, ammonia synthesis, and CO oxidation.4 Crystallographic structure and orientation are important in determining the electrical and chemical properties of Ru and RuO2.5 Crystal grain size and film texturing influence charge transfer in the film and hence directly impact the electrical properties of microelectronic devices.6 Previous studies have also shown that the crystallinity and surface morphology of Ru and RuO2 strongly affect the growth of films on Ru/RuO2 substrates,7 as well as the adhesion to the adjacent copper layer in interconnects.8 In addition, the catalytic activities of the surfaces are known to be crystal orientation dependent.4,9 It is therefore useful to study the crystallographic orientation of deposited films to achieve the desired properties of the film for specific applications. Although many studies have reported the crystal structure of Ru thin films by ALD,1,10 there has been no study that focuses on the nucleation process, which is © 2013 American Chemical Society

important in determining the progression of crystallographic structure and properties of the films. Metallic ruthenium has a hexagonal close-packed (hcp) structure, in which the (002) plane is the most densely packed plane with the lowest surface energy. However, in many cases, Ru thin films by ALD show a preference to grow in the (101) orientation rather than the (002) orientation.10b,11 The (002) preference in thin films on amorphous substrates has mostly been observed with higher deposition temperatures (350 to 400 °C)1,12 or with plasma-enhanced atomic layer deposition (PEALD).13 These results show that some activation energy may be required for Ru to nucleate in the (002) direction under typical ALD thin-film growth conditions. Nevertheless, because of the substrate-dependent nature of the ALD technique,11 in some cases Ru (002) preferred growth can be observed in epitaxial ALD growth on Au(111) and Pt(111).10a In these cases, the (111) close-packed surface works as a seed from which the Ru nuclei can orient in the (002) close-packed plane. While Ru and RuO2 can be produced through the same deposition process and occasionally coexist, the crystal structures of the two materials are different. In contrast to metallic Ru, the RuO2 crystal is rutile in structure, the same structure typically observed in TiO2 crystals. It is important to study how the crystal structure transitions from the hexagonal unit cell (hcp) to the tetragonal unit cell (rutile), since this transformation likely has an influence on the grain size and Received: May 15, 2013 Revised: July 24, 2013 Published: August 13, 2013 3458

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texturing of the films, as well as on other crystallographicdependent film properties. In this work, we studied the crystallography of Ru and RuO2 films by high-resolution synchrotron radiation X-ray diffraction (XRD) to provide information on the formation and the growth mechanism of crystalline nuclei of these materials. In situ analysis is utilized to further study the transition from Ru to RuO2 during initial ALD growth. Ru and RuO2 thin films were selectively grown by atomic layer deposition (ALD) using bis (2,4-dimethylpentadienyl) ruthenium (Ru(DMPD)2) and oxygen. This is a relatively new Ru ALD system that allows deposition at low temperatures. The previous results show that the oxygen dosing conditions, including both partial pressure and exposure time, are the key parameters for controlling the transition from Ru to RuO2.14 Metallic ruthenium films are produced with low oxygen pressures and short dosing times, while higher oxygen pressures or long dosing times result in RuO2 films. However, the mechanism of the process has yet to be explored.

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EXPERIMENTAL SECTION

Oxide-terminated n-type Si (100) wafers (ρ = 1.0−5.0 Ω-cm) were used as substrates for Ru/RuO2 deposition. The substrates were sequentially rinsed in acetone (OC(CH3)2) and ethanol (C2H5OH), followed by piranha cleaning (30:70, 30% H2O2:H2SO4), to achieve chemical oxide surfaces that were free from carbon contamination. Bis(2,4-dimethylpentadienyl)ruthenium(II) (Ru(DMPD)2) (99%, Sigma-Aldrich), together with high purity oxygen gas (99.999%, Praxair), were used as reactants for the Ru ALD processes. Nitrogen was used as a carrier and purging gas. The ex situ experiments were conducted in a warm-wall reactor at a substrate temperature of 185 °C. Oxygen exposure conditions, that is, partial pressure and dosing time, were controlled to produce the desirable film compositions. Specifically, to obtain Ru deposition, 2 s pulses of 500-mtorr oxygen were used, whereas to obtain RuO2 deposition, 30 s pulses of 50-mtorr oxygen were applied in the ex situ studies and 10 s at 1000 mtorr in the in situ studies. The details of the deposition processes have been described elsewhere.14 The in situ experiments were performed in a compact custom-built reactor that attaches to the phi goniometer of the X-ray diffractometer. Gas manifolds were constructed using flexible stainless steel tubing to accommodate chamber movement. X-rays were transmitted through beryllium windows at the front and back of the vacuum chamber. XRD measurements can be conducted under ALD growth conditions, that is, low vacuum and elevated temperature, without the need for sample transfer. For both in situ and ex situ experiments, X-ray measurements were conducted at the Stanford Synchrotron Radiation Lightsource (SSRL) beamline 7-2, using a photon energy of 12.7 keV. The analyses of the ex situ films were performed using θ-2θ scans for both in-plane and out-of-plane configurations. These measurements allow probing of the crystal planes with their normal parallel and perpendicular to the growth direction of the thin films, respectively. The Ru diffraction peaks at low numbers of cycle samples are more apparent in the inplane measurements, because those from the out-of-plane measurement are concealed by a strong background from the Si(100) substrate. For in situ analysis, XRD was measured with the scattering vector nearly perpendicular to the surface normal to probe the progress of the Ru/RuO2 film in the plane of the sample surface. All XRD spectra are reported both in 2θ as collected with 12.7 keV X-ray (bottom axis) and in Q-space (top axis). The grain sizes of crystallites are calculated from the diffraction peak broadening via the Scherrer equation.

Figure 1. High-resolution ex situ XRD spectra of ruthenium films deposited at 185 °C with 2 s of 500-mtorr oxygen in (a) the out-ofplane configuration and (b) the in-plane configuration. The vertical lines show the expected relative intensities of specific planes with random orientations to show the resemblance of the in-plane diffraction to the powder diffraction pattern.

these films show the presence of hcp Ru starting from nucleation as isolated islands to the growth of continuous thin films, which occurs at approximately 500 cycles, based on results obtained under these same deposition conditions in an earlier study.14 This initial crystalline Ru growth correlates well with the Ru0 content and the lack of oxygen as detected by ex situ XPS shown in the previous study.14 In thicker films, the out-of-plane XRD scans show a strong (002) plane orientation in the growth direction. The peak intensities of the (002) planes grow faster with Ru film thickness than those of other planes, which indicates an increasing degree of texture with film thickness. In contrast, the in-plane XRD scans resemble an hcp powder diffraction pattern, as all peaks grow with intensities similar to those of an isotropic powder. Figures 2a and 2b show the texture factor of different planes with their normal in the film growth direction and planes with their normal parallel to the substrate, respectively, as a function of ALD cycle. The texture factor is defined as the relative peak intensity of a specific diffraction plane when compared with the relative intensity of that peak expected from powder diffraction.15 A texture factor of 1 represents powder diffraction, which is shown as a dotted line in the figures. The (002) preference is evident in the growth direction, as a texture factor of noticeably larger than 1 is observed, even for the lowest number of cycles for which Ru is observable. The 500-cycle film also shows a slight preference for the (100) plane. Unfortunately, the peak analyses of the films with a lower number of ALD cycles than

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RESULTS AND DISCUSSION The Ru films deposited by ALD exhibit a textured hexagonal close packed (hcp) crystalline structure with a preference toward (002) planes. In Figure 1, the ex situ XRD scans of 3459

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precursors, and observed stronger (002) characteristics in Ru films deposited by Ru(DMPD)2.16 In contrast to the hcp Ru films, our results show that RuO2 films exhibit a randomly oriented polycrystalline rutile structure. Although RuO2 is grown under the same ALD conditions used for growth of Ru with the exception of the oxygen exposure conditions, no preferential orientation in RuO2 is observed. The diffraction pattern in Figure 3 is that of

Figure 2. (a) Out-of-plane, and (b) in-plane texture factors of hcp Ru films as a function of the number of ALD cycles, as measured by ex situ XRD. Texture factor is defined as the ratio of the relative peak intensity of the sample to the relative peak intensity of a powder diffraction sample.

500 in the out-of-plane configuration are less accurate because of the overwhelming Si (100) background from the substrate and the broadening of the diffraction peaks due to particle size effects. On the other hand, the XRD spectra from the in-plane configuration show no preferred orientation, especially for thick films. The texture factors of the (002) and (101) planes are close to unity. The occurrence of the (100) plane is higher than would be expected from powder diffraction. This occurrence, however, agrees well with the (002) preference in the growth direction, since the (100) plane is perpendicular to the (002) plane. At a lower number of ALD cycles, the deposited Ru shows a modest (002) preference in the in-plane diffraction, as texture factors of slightly larger than 1 were observed. The preference for the (002) plane at a lower number of cycles, for both in-plane and out-of-plane directions, is not fully understood but may be the result of phenomena such as stacking fault defects. This preference for the close-packed (002) orientation is normally observed only at higher temperatures in thin films grown by ALD.1 As discussed earlier, this implies that some energy may be required to permit the close-packed surface to form. Interestingly, the Ru films in our studies show a (002) preference despite low deposition temperatures. The slower growth rates of Ru films at low temperatures by Ru(DMPD)2, compared with the growth rates by other precursors, may allow sufficient time for the Ru atoms to crystallize in the closepacked orientation. However, the crystal alignment may not only be growth-rate dependent but may also be precursordependent. Similar results were observed in MOCVD despite the generally higher growth rates of MOCVD. Kawano et al. compared the metalorganic chemical vapor deposited (MOCVD) Ru films, grown using the same Ru(DMPD)2 precursor studied here, with those grown using other

Figure 3. High-resolution ex situ XRD spectra of RuO2 films deposited at 185 °C with 30 s of 50-mtorr oxygen in (a) the out-of-plane configuration, and (b) the in-plane configuration. The vertical lines show the expected relative intensities of specific planes with random orientations.

rutile RuO2, and the relative diffraction peak intensities of different crystal planes are similar to those of the powder diffraction pattern for both in-plane and out-of-plane configurations. The random polycrystalline structure is confirmed in Figure 4, which shows the texture factors of all planes to be close to one, from small to large numbers of ALD cycles. This observation is different from that of strong (110) orientation detected in previous MOCVD studies of RuO2 growth using this precursor,17 which suggests different mechanisms between the two processes. Moreover, while we observe a random polycrystalline structure at low growth temperature by ALD, this randomly oriented structure was only observed at higher temperature (>425 °C) in the MOCVD films.18 MOCVD growth at lower temperatures resulted in textured RuO2 films.18,19 In addition, the formation mechanism for RuO2 in our studies is likely to be different from simple oxidation of Ru films, since it has been shown in previous studies that oxidation of Ru (002) single crystals at elevated temperatures resulted in exclusively RuO2 (110).4 Therefore, if RuO2 films had formed from the oxidation of the textured metallic thin films, they should be textured. 3460

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Figure 5. In situ in-plane diffraction spectra during the growth of RuO2 ALD by Ru(DMPD)2 and oxygen (10 s at 1000 m-torr). The spectra were collected in situ under ALD conditions using a fixed grazing incident angle of 0.40°.

Figure 4. (a) Out-of-plane, and (b) in-plane texture factors of RuO2 films as a function of the number of ALD cycles, measured by ex situ XRD. No preferential growth orientation is observed in both directions.

RuO2 nanocrystallites that cannot be detected by XRD during early stages of growth. However, regardless of the presence of amorphous RuO2, the termination of crystalline Ru growth af ter crystalline RuO2 begins to grow suggests that the growth of crystalline RuO2 is competitive with crystalline Ru growth. The data further suggest that rutile RuO2 growth is more favorable once the hcp Ru crystals have attained a certain size. We propose the following explanation for this initial absence of crystalline oxide formation: a critical Ru island size may be required to either overcome the kinetic barriers associated with formation of the oxide phase or to thermodynamically stabilize the solid oxide phase relative to loss of oxygen. Here we present a possible reason. We hypothesize that the rate limiting step for formation of RuO2 in small particles is the reaction to form subsurface oxygen sites, which comprises the initial stage of Ru oxide formation according to literature reports,14,20 and that this step is influenced by the Ru island size. Density functional theory (DFT) calculations on a Ru(0001) surface by Reuter et al. suggested an aggregation of subsurface oxygen into islands preceding the oxide formation.21 Importantly, it is known that the diffusion of the first oxygen atoms into these subsurface sites is the most difficult22 and results in significant distortion of Ru lattice.23 The activation energy of this process is reduced considerably with the coverage of chemisorbed oxygen on the surface because of relaxation of the metal lattice which mediates strong oxygen−oxygen interaction,24 and the incorporation of oxygen to the subsurface of Ru starts by oxygen transfer from the surface to the subsurface location after a full coverage of chemisorbed oxygen has formed.25 We consider that the required incorporation of oxygen into the subsurface may favor larger particles. A recent DFT study showed that O binds more strongly on transition metal nanoparticles than on the bulk metal surface;26 as a result, the energetics in small particles may favor chemisorbed O over the subsurface state. Moreover, previous studies of Ru oxidation showed that subsurface oxygen formation is promoted by surface defect sites.25 However, the number of defects is small in small particles, similar to the selfpurification process known in semiconductor nanocrystals,

Interestingly, in the present system, the deposited crystalline films seem to initially nucleate as hcp Ru which transforms into RuO2 after a large number of ALD cycles. As shown in Figure 3, the low-ALD-cycle films (300 cycles) show some contribution from hcp Ru, which cannot be detected in thicker films (1000 cycles), together with RuO2. In addition, the hcp Ru nuclei exhibit similar texturing as do samples grown directly as metallic films (i.e., under low oxygen conditions, Figure 1). This presence of hcp Ru under the highly oxidizing environment used during the RuO2 film deposition is surprising. However, because of strong RuO2 peak intensities, the ex situ results do not give conclusive evidence as to whether metallic Ru disappears at a larger number of ALD cycles. To further investigate the growth mechanism, we performed in situ XRD experiments to study the development of the ALD film. In situ measurement avoids the postdeposition oxidation that may alter the structure of the films before they can be measured. From the ex situ results discussed earlier, a random crystal orientation is anticipated in the plane of the substrate. The measurement is hence conducted at a fixed grazing incident angle and takeoff angle, instead of with the scattering angle along the surface normal, to minimize the movement of the chamber. A grazing incidence angle of 0.40°, which is slightly larger than the critical angle of ruthenium, was selected to provide surface sensitivity. ALD conditions with high oxygen partial pressure and long oxygen exposure time (10 s at 1000 mtorr) were selected to ensure RuO2 film production. Despite the highly oxidizing conditions, the in situ XRD spectra in Figure 5 clearly exhibit the initial formation of hcp Ru nuclei during the first 200 cycles, with no evidence for crystalline RuO2 at these early cycle numbers. After 300 cycles, the growth of rutile RuO2 becomes clearly visible, and the growth of hcp Ru slows down and ceases. The in situ observations are consistent with the ex situ studies. We note that there is a possibility that the film contains amorphous RuO2 or very small 3461

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and subsurface oxygen to maintain RuO2 growth. As shown in Figure 6, the Ru grain size starts plateauing once RuO2 is detectable in the spectra, suggesting that the particles retain an hcp Ru core and therefore that oxygen does not advance into the inner layers to further oxidize the Ru particles. The continuous growth of RuO2 likely occurs on the exterior surface after nucleation. The data in Figure 6 provide further insight into the transformation between Ru and RuO2. A transition region in both the integrated area (Figure 6a) and grain size (Figure 6b) plots occurs between 150 and 250 ALD cycles. At this stage, the deposited Ru should be in nanoparticle form as the coalescence of nanoparticles under highly oxidizing condition occurs at approximately 300 cycles, based on results from an earlier study.14 The grain size of hcp Ru extracted from the Scherrer equation (Figure 6b) indicates that the average size of Ru particles saturates at ∼8 nm. This clearly shows that the growth of hcp Ru ceases once a certain size is approached and suggests that the deposition on larger diameter particles is likely in the form of RuO2, consistent with the rising signal from RuO2, and that particles smaller than this size will be stable in the hcp Ru form. Because there is a distribution of particle sizes during the ALD process, the growth of RuO2 may occur on some larger particles, while the majority of Ru particles are still too small for RuO2 nucleation and hence continue growing as hcp Ru. Therefore, the increase in calculated hcp Ru particle size from 200 cycles to 250 cycles (Figure 6b) may not strictly imply continuous growth of hcp Ru after the establishment of rutile RuO2 growth on the hcp particles. Once the majority of particles reach a critical size for RuO 2 nucleation, a discontinuation of hcp Ru growth can be detected, as observed in the plateauing of hcp Ru size after 250 cycles. A small reduction of Ru peak intensity at later cycles (Figure 6a) is likely from the loss of hcp Ru because of oxidation to form RuO2. This effect is not noticeable at the earlier cycles because of the continuous growth of small Ru hcp particles. Nevertheless, because of oxygen diffusion limitations, the oxidation is restricted to the outer surface; therefore, we do not observe a continuous reduction of hcp Ru particle size. The stable Ru particle size after 250 cycles reinforces the conclusion that hcp Ru is still present even after RuO2 is observed.

where formation energies of defects increases as the nanocrystal size decreases.27 The calculation by Muller and Albe also predicted lower concentration of vacancy defects in nanoparticles.28 Therefore, the formation of subsurface oxygen may be hindered, leading in turn to a rate of oxide formation in these small particles that is significantly reduced. Further theoretical studies are required to confirm the mechanism. In a related system, Chakraborty et al. reported a strong correlation between the stability of clusters of subsurface oxide with the radius of Ni particles.24 In that work, it was argued that the establishment of a critical size of the oxide nucleus was required to overcome the out-diffusion to the chemisorbed state.24 Similarly, in our studies, RuO2 is detectable when the radius of Ru nuclei is between 3 and 4 nm (grain size of 6−8 nm), as calculated from the Scherrer equation (see Figure 6 and

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Figure 6. (a) Integrated area under the peaks of selected crystallographic planes of Ru and RuO2 during the in situ ALD growth and (b) grain size, as calculated from the Scherrer equation.

CONCLUSIONS High resolution XRD studies reveal the crystallographic structures of ruthenium and ruthenium dioxide thin films deposited by atomic layer deposition of Ru(DMPD)2 and oxygen at low temperature, and provide insight into the growth mechanism. As-deposited hcp ruthenium films are textured in the growth direction, with a preference for growth along the (002) direction. This result is different from other Ru ALD films deposited at low temperature, which typically show a preference for the (101) direction. This (002) preference may result from the slow deposition rate for this process, as well as the influence of the Ru(DMPD)2 precursor. The rutile RuO2 films, on the other hand, exhibit a randomly oriented polycrystalline structure. In situ studies suggest that, despite the high oxygen exposure conditions, RuO2 is initially absent while hcp Ru nanocrystals nucleate and that rutile RuO2 growth is more favorable once the hcp Ru crystals have attained a certain size. We speculate that a critical Ru island size is required to overcome the kinetic barriers associated with formation of the oxide phase, and suggest that this may be caused by size-dependent effects in the incorporation of oxygen

discussion below), consistent with Chakraborty’s studies of Ni that show that subsurface oxygen starts to be stabilized when the Ni particles are larger than 5 unit cells (∼ 2 nm).24 Therefore, no oxide forms toward the beginning of the ALD process, despite large oxygen exposure. During later ALD cycles, the size of the Ru crystals is sufficiently large (6−8 nm) to accommodate subsurface oxygen islands, and nucleation of RuO2 can be initiated. Although no changes in the hcp Ru XRD pattern are observed that would unambiguously identify the presence of subsurface oxygen, we note that the subsurface oxygen likely resides in interstitial sites without disturbing the Ru lattice. Afterward, RuO2 can grow autocatalytically in two dimensions, when the presence of RuO2 helps to accelerate the Ru oxidation process because oxygen dissociates on RuO2.20b The growth of RuO2 is evident in the XRD patterns indicating rutile structure. The same large oxygen dose is now sufficient for reacting with Ru precursor on the surface and for supplying a full coverage of chemisorbed oxygen 3462

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(17) (a) Kawano, K.; Kosuge, H.; Oshima, N.; Funakubo, H. Electrochem. Solid State 2007, 10 (6), D60−D62. (b) Meda, L.; Breitkopf, R. C.; Haas, T. E.; Kirss, R. U. MRS Proc. 1997, 495, 75−80. (18) Bai, G. R.; Wang, A.; Foster, C. M.; Vetrone, J. Thin Solid Films 1997, 310 (1−2), 75−80. (19) Frohlich, K.; Cambel, V.; Machajdik, D.; Baumann, P. K.; Lindner, J.; Schumacher, M.; Juergensen, H. Mater. Sci. Semicond. Proc. 2002, 5, 173−177. (20) (a) Hrbek, J.; van Campen, D. G.; Malik, I. J. J. Vac. Sci. Technol., A 1995, 13 (3), 1409−1412. (b) Over, H.; Seitsonen, A. P. Science 2002, 297 (5589), 2003−2005. (21) Reuter, K.; Stampfl, C.; Veronica Ganduglia-Pirovano, M.; Scheffler, M. Chem. Phys. Lett. 2002, 352 (5−6), 311−317. (22) Malik, I. J.; Hrbek, J. J. Phys. Chem. 1991, 95 (24), 10188− 10190. (23) Todorova, M.; Li, W. X.; Ganduglia-Pirovano, M. V.; Stampfl, C.; Reuter, K.; Scheffler, M. Phys. Rev. Lett. 2002, 89 (9), 096103. (24) Chakraborty, B.; Holloway, S.; Norskov, J. K. Surf. Sci. 1985, 152/153 (Part 2 (0)), 660−683. (25) Bottcher, A.; Niehus, H. J. Chem. Phys. 1999, 110 (6), 3186− 3195. (26) Peterson, A.; Grabow, L.; Brennan, T.; Shong, B.; Ooi, C.; Wu, D.; Li, C.; Kushwaha, A.; Medford, A.; Mbuga, F.; Li, L.; Norskov, J. Top. Catal. 2012, 55 (19−20), 1276−1282. (27) Dalpian, G. M.; Chelikowsky, J. R. Phys. Rev. Lett. 2006, 96 (22), 226802. (28) Muller, M.; Albe, K. Acta Mater. 2007, 55 (9), 3237−3244.

into the subsurface region which is needed to initiate the oxide formation. Once the hcp Ru particles reach a critical size, rutile RuO2 forms.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the Department of Energy under Award Number DE-SC0004782. R.M. gratefully acknowledges the Anandamahidol Foundation Graduate Fellowship. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University. The authors gratefully express appreciation to Douglas Van Campen, Ron Marks, and Chad Miller for assistance on the SSRL setup, and to Prof. Paul McIntyre and Prof. Bruce Clemens for scientific discussions.

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REFERENCES

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