We illustrate the application of 1H(15N,αγ)12C resonant nuclear reaction analysis (NRA) to quantitatively evaluate the density of hydrogen atoms on the surface, in the volume, and at an interfacial layer of solid materials. The near-surface hydrogen depth profiling of a Pd(110) single crystal and of SiO2/Si(100) stacks is described.
Nuclear reaction analysis (NRA) via the resonant 1H(15N,αγ)12C reaction is a highly effective method of depth profiling that quantitatively and non-destructively reveals the hydrogen density distribution at surfaces, at interfaces, and in the volume of solid materials with high depth resolution. The technique applies a 15N ion beam of 6.385 MeV provided by an electrostatic accelerator and specifically detects the 1H isotope in depths up to about 2 μm from the target surface. Surface H coverages are measured with a sensitivity in the order of ~1013 cm-2 (~1% of a typical atomic monolayer density) and H volume concentrations with a detection limit of ~1018 cm-3 (~100 at. ppm). The near-surface depth resolution is 2-5 nm for surface-normal 15N ion incidence onto the target and can be enhanced to values below 1 nm for very flat targets by adopting a surface-grazing incidence geometry. The method is versatile and readily applied to any high vacuum compatible homogeneous material with a smooth surface (no pores). Electrically conductive targets usually tolerate the ion beam irradiation with negligible degradation. Hydrogen quantitation and correct depth analysis require knowledge of the elementary composition (besides hydrogen) and mass density of the target material. Especially in combination with ultra-high vacuum methods for in-situ target preparation and characterization, 1H(15N,αγ)12C NRA is ideally suited for hydrogen analysis at atomically controlled surfaces and nanostructured interfaces. We exemplarily demonstrate here the application of 15N NRA at the MALT Tandem accelerator facility of the University of Tokyo to (1) quantitatively measure the surface coverage and the bulk concentration of hydrogen in the near-surface region of a H2 exposed Pd(110) single crystal, and (2) to determine the depth location and layer density of hydrogen near the interfaces of thin SiO2 films on Si(100).
The ubiquity of hydrogen as an impurity or as a constituent of a vast variety of materials and the wealth of hydrogen-induced interaction phenomena make revealing the hydrogen distribution in the near-surface region and at buried interfaces of solids an important task in many areas of engineering and fundamental material science. Prominent contexts include studies of hydrogen absorption in storage and purification materials for hydrogen energy applications, fuel cell, photo-, and hydrogenation catalysis, hydrogen retention and embrittlement in nuclear and fusion reactor engineering, hydrogen-induced surfactant effects in epitaxial growth fabrication and hydrogen-related electrical reliability issues in semiconductor device technology.
Despite its omnipresence and simple atomic structure, the quantitative detection of hydrogen poses analytical challenges. As hydrogen contains only a single electron, otherwise versatile elemental analysis by electron spectroscopy is rendered ineffective. Common hydrogen detection methods through mass analytical, optical, or nuclear resonance techniques such as metallurgical fusion, thermal desorption, infrared absorption or NMR spectroscopy are principally insensitive to the depth location of hydrogen. This precludes, e.g., discriminating between surface-adsorbed and bulk-absorbed hydrogen which differ substantially in their physical and chemical material interactions, and their distinction therefore becomes increasingly important for the analysis of nanostructured materials that comprise small volumes and large surface areas. Hydrogen profiling by secondary ion mass spectroscopy, although providing depth-resolved quantitative H concentrations, is equally destructive to the analyzed target as metallurgical fusion, and sputtering effects may render the depth information obtained near the surface unreliable.
Nuclear reaction analysis with the narrow energy resonance (Eres) of the 1H(15N,αγ)12C reaction at 6.385 MeV1-3, on the other hand, combines the advantages of non-destructive hydrogen quantitation with high depth resolution in the order of a few nanometers near the surface. The method determines surface H coverages with a sensitivity in the order of 1013 cm-2 (~1% of a typical atomic monolayer density). Hydrogen concentrations in the interior of materials can be assessed with a detection limit of several 1018 cm-3 (~100 at. ppm) and a probing depth range of about 2 μm. The near-surface depth resolution is routinely 2-5 nm in surface-normal incidence of the 15N ion beam onto the analyzed target. In surface-grazing incidence geometries, the resolution can be enhanced further to values below 1 nm. See Ref. 3 for a detailed account.
These capabilities have proven 1H(15N,αγ)12C NRA as a powerful technique to elucidate the static and dynamic behavior of hydrogen at surfaces and interfaces in a large variety of processes and materials3. Established by Lanford4 in 1976, 15N NRA was first used predominantly to quantitatively determine volume H concentrations in bulk materials and thin films. Among other purposes, the absolute hydrogen concentrations obtained through 15N NRA have been used to calibrate other, not directly quantitative, hydrogen detection techniques5,6. Also 15N NRA hydrogen profiling in targets with well-defined interfaces in layered thin film structures has been described7-10. More recently, much progress has been achieved in studying hydrogen in the near-surface region of chemically clean and structurally well-defined targets by combining 15N NRA with surface analytical ultra-high vacuum (UHV) instrumentation to prepare atomically controlled surfaces in situ for the H analysis3.
By quantifying the hydrogen coverage on single crystal surfaces, NRA has contributed significantly to the current microscopic understanding of hydrogen adsorption phases on many materials. 1H(15N,αγ)12C NRA is furthermore the only experimental technique to directly measure the zero-point vibrational energy of surface-adsorbed H atoms11, i.e., it can reveal the quantum-mechanical vibrational motion of adsorbed H atoms in the direction of the incident ion beam. Through the capability of nanometer-scale discrimination between surface-adsorbed and bulk-absorbed H, 15N NRA can provide valuable insight into the hydrogen ingress through material surfaces, such as relevant to mineral hydration dating12 or for observing hydride nucleation underneath surfaces of H-absorbing metals13-15. High-resolution 15N NRA applications have demonstrated the potential to detect sub-monolayer thickness variations of adlayers16 and to distinguish surface-adsorbed from volume-absorbed hydrogen in Pd nanocrystals17. The combination with thermal desorption spectroscopy (TDS) allows for unambiguous identifications of H2 thermal desorption features and for the depth-resolved assessment of the thermal stability of adsorbed and absorbed hydrogen states against desorption and diffusion13,15,18. Due to its non-destructive nature and high depth resolution 1H(15N,αγ)12C NRA is also the ideal method to detect hydrogen buried at intact interfaces, which allows for studying hydrogen trapping at metal/metal19-22 and metal/semiconductor interfaces16,23-25 and for tracking hydrogen diffusion in stacked thin film systems9. By directly visualizing hydrogen redistribution phenomena between interfaces of SiO2/Si-based metal-oxide-semiconductor (MOS) structures that relate to electrical device degradation, NRA has made particularly valuable contributions to device reliability research26.
The hydrogen detection principle in NRA is to irradiate the analyzed target with a 15N ion beam of at least Eres=6.385 MeV to induce the resonant 1H(15N,αγ)12C nuclear reaction between 15N and 1H in the material. This reaction releases characteristic γ-rays of 4.43 MeV that are measured with a scintillation detector nearby the sample. The γ-yield is proportional to the H concentration in a certain depth of the target. Normalizing this signal by the number of incident 15N ions converts it into absolute H density after the γ-detection system has been calibrated with a standard target of known H concentration. 15N ions incident at Eres can react with hydrogen on the target surface. The concentration of buried hydrogen is measured with 15N ions incident at energies (Ei) above Eres. Inside the target material, the 15N ions suffer energy loss due to electronic stopping. This effect provides the high depth resolution, because the 1H(15N,αγ)12C nuclear reaction resonance has a very narrow width (Lorentzian width parameter Γ = 1.8 keV) and the stopping power of materials for 6.4 MeV 15N ranges between 1-4 keV/nm, so that the passage of the 15N ion through only a few atomic layers is sufficient to shift its energy outside the resonance window. Thus, the resonant reaction detects buried H at Ei > Eres in a probing depth d=(Ei–Eres)/S, where S is the electronic stopping power of the analyzed material3.
By measuring the γ-yield while scanning the incident 15N ion energy in small increments, one obtains a nuclear reaction excitation curve that contains the density-depth distribution of hydrogen in the target. In this excitation curve (γ-yield vs. 15N energy), the actual H depth distribution is convolved with the NRA instrumental function that adds a predominantly Gaussian broadening and is the main limitation for the depth resolution3. At the surface (i.e., at Ei = Eres) the Gaussian width is dominated by a Doppler Effect due to zero-point vibration of the H atoms against the target surface.11,27,28 The yield curve of buried hydrogen detected at Ei > Eres is affected by an additional Gaussian broadening component due to random 15N ion energy straggling inside the target. The straggling width increases in proportion to the square root of the ion trajectory length in the material 29,30 and becomes the dominant resolution limiting factor above probing depths of 10-20 nm.
To demonstrate a few very typical hydrogen profiling applications with 15N NRA, we here exemplarily describe (1) the quantitative evaluation of the surface H coverage and of the bulk-absorbed hydrogen concentration in a H2 exposed palladium (Pd) single crystal, and (2) the evaluation of the depth location and hydrogen layer densities at buried interfaces of SiO2/Si(100) stacks. The NRA measurements are performed at the MALT 5 MV van-de-Graaf tandem accelerator31 of the University of Tokyo, which delivers a highly stable and well-monochromatized (ΔEi ≥ 2 keV) 15N ion beam of 6-13 MeV. The authors have developed a computer control system for the accelerator to enable automated energy scanning and data acquisition for hydrogen profiling. Reflecting the two different NRA measurement tasks presented by the above H profiling applications, the MALT facility provides two ion beam lines with specialized experimental stations: (1) a UHV surface analytical system with a single bismuth germanate (BGO, Bi4Ge3O12) γ-scintillation detector dedicated to the NRA quantitation of hydrogen surface coverages, to zero-point vibration spectroscopy, and to H depth profiling at atomically controlled single crystal targets in a unique combination with TDS; and (2) a high vacuum chamber equipped with two BGO detectors positioned very close to the target for increased γ-detection efficiency, providing for a lower H detection limit and faster data acquisition. This setup has no sample preparation facilities but allows for rapid sample exchange (~30 min) and thus for a higher throughput of targets for which a well-controlled surface layer is not an essential part of the analytical task, such as H profiling at buried interfaces or the quantitation of bulk H concentrations. At both beam lines, the BGO detectors are placed conveniently outside of the vacuum systems because the γ-rays penetrate the thin chamber walls with negligible attenuation.
Figure 1. NRA setup in the BL-1E UHV system. (A) Schematic top view into the BL-1E UHV system equipped with sputter ion gun, low energy electron diffraction (LEED), and Auger electron spectroscopy (AES) for the in-situ preparation of atomically ordered and chemically clean single crystal surface targets and combined NRA and TDS measurements with a quadrupole mass spectrometer (QMS) mounted on a linear translation stage. (B) Pd single crystal specimen attached on the sample holder of the cryogenic manipulator. Please click here to view a larger version of this figure.
Figure 1 (A) illustrates the UHV system at beam line (BL)-1E, which is fully equipped for the in-situ preparation of atomically ordered single crystal surfaces and has a base pressure <10-8 Pa to maintain surface cleanliness. To provide sample access for the surface-analytical tools, the 4" BGO scintillator is placed on the 15N ion beam axis ~30 mm behind the target. The sample is mounted on a 4-axis manipulation stage for precise (x, y, z, Θ) positioning and can be cooled with liquid nitrogen to ~80 K or with compressed He to ~20 K. Figure 1 (B) shows a Pd single crystal target mounted by spot-welded Ta support wires to a He compression cryostat. Quartz sheet spacers insulate the sample holder plate electrically from the cryostat body. This enables the incident 15N ion beam current measurement necessary for quantitative NRA and allows for electron bombardment heating from the tungsten filament on the backside of the sample holder. A type K thermocouple is spot-welded to the edge of the Pd specimen. A quartz plate attached on the manipulator axis above the sample is used to monitor the ion beam profile and for sample-beam alignment. Figure 2 (A) shows the setup at BL-2C with two 4" BGO detectors arranged at 90° with respect to the 15N beam with their front face no further than 19.5 mm apart from the beam axis. The sample holder (Figure 2 (B)) provides a simple clamping mechanism for quick sample exchange and allows for rotation of the sample around the vertical axis to adjust the 15N incidence angle.
Figure 2. NRA setup at BL-2C. (A) Schematic top view into the high vacuum chamber at BL-2C equipped with two BGO γ-detectors close to the target position. (B) Sample holder with a large chip target of SiO2/Si(100) clamped on. Fogging up this sample type with water vapor after the NRA analysis visualizes the spots that were irradiated by the 15N ion beam. Please click here to view a larger version of this figure.
1. Planning of Experiments
2. Preparation for NRA Measurements at BL-1E (UHV)
Note: Always wear gloves when handling instruments and materials intended for use in vacuum, including the clean tools.
Figure 3. LEED pattern (223 eV) of the cleaned Pd(110) surface in the BL-1E UHV system. The clear (1×1) pattern with bright diffraction spots on a low background signifies an atomically well-ordered surface structure. Please click here to view a larger version of this figure.
3. Preparation for NRA Measurement at BL-2C
4. NRA Measurement at BL-1E
5. NRA Measurement at BL-2C
6. Data Analysis
Figure 4 shows near-surface NRA H profiles of H2-exposed Pd(110) measured in the BL-1E UHV system at a sample temperature of 90 K under a H2 background pressure of 1.33×10-6 Pa. The 15N ion incidence energy has been converted into probing depth using the stopping power of Pd (S = 3.90 keV/nm). The open symbol profile was obtained after pre-exposing the Pd(110) sample to 2,000 L H2 at 145 K to induce absorption of hydrogen into the Pd bulk15. This profile can be decomposed into a peak at Eres = 6.385 MeV and a broad γ-yield plateau stretching over the entire profiled depth region. The peak at Eres corresponding to H on the target surface, whereas the plateau region proves that hydrogen absorption into the Pd bulk has taken place. The surface peak has a nearly Gaussian shape. To demonstrate this better, Figure 4 includes two additional NRA excitation curves (gray and black solid symbols) that were recorded at 170 K under the same H2 background pressure from a freshly prepared Pd(110) surface without H2 pre-dosage. At this temperature only surface chemisorbed hydrogen is stable at Pd(110); any absorbed H would diffuse into the deeper bulk and escape from NRA detection in the near-surface region. Fitting the surface peak to a Gaussian profile yields a peak height of Imax = 86.9 ± 4.5 cts/μC and an: where σ is the Gaussian width parameter. From the Gaussian peak area: and the γ-ray detection efficiency of the BL-1E setup (calibrated with a Kapton (C22H10O5N2)n foil standard of precisely known H concentration (cbulk = 2.28×1022 cm-3) and stopping power S = 1.2879 keV/nm) α = 7.56×10-13 [(cts/μC) keV cm2], one determines the absolute H density (Ns) on the H2-exposed Pd(110) singe crystal as Ns = IS/α = 1.42×1015 cm-2. Relative to the Pd atom density on the Pd(110) surface of 9.35×1014 cm-2 this corresponds to a H coverage of 1.52 ± 0.13 ML (monolayers), in very good agreement with the literature value (1.5 ML)15,33,34.
Figure 4 also shows that the depth resolution of the NRA H profile in the near-surface region is limited by the width of the surface resonance peak to about 2-3 nm (≈FWHM/S). Therefore, any abrupt features in the H profile such as the point-to-point γ-yield variations seen around 16 nm depth cannot correspond to actually existing steep H concentration gradients, because such would be smeared out by the surface peak width and additional energy broadening due to 15N ion straggling3. Hence, the γ-yield corrugations in the plateau region of the H profile (5 to 22 nm depth) reflect fluctuations of the BGO background count rate (separate background measurements confirm that such random fluctuations occur) and do not contain physical information on the depth distribution of the Pd-absorbed hydrogen. The latter distribution is expected to be rather smooth in the homogenous single crystal, where H diffusion is rapid (several 100 nm/sec even at 145 K)3,13,15. Thus, the approximately constant concentration of the bulk-absorbed hydrogen in the near-surface region of the Pd crystal after exposure to 2,000 L H2 at 145 K can be evaluated by fitting the plateau data in Figure 4 to a sigmoid function that rises to its half-height at Eres with the same width as the Gaussian surface peak3. This analysis determines the plateau height Ibulk to 15.4 ± 1.3 cts/μC. With the Pd stopping power S = 3.9 keV/nm, one thus obtains the H bulk concentration as cbulk = IbulkS / α = 7.94×1020 cm-3, or 1.2% of the atomic density in bulk Pd (the latter equation is also used to calibrate α after having measured Ibulk in the Kapton standard3). This bulk H concentration signifies that finely distributed grains of Pd hydride (which has a H/Pd ratio of about 0.65 at this temperature) has nucleated in the near-surface region of the Pd crystal during its exposure to 2,000 L (2.66×10-3 Pa × 100 sec) H2 at 145 K, because the H solubility in the dilute solid solution phase of Pd amounts to less than H/Pd = 5×10-4 at this temperature and H2 pressure.
Figure 5 displays 15N NRA depth profiles from a series of SiO2/Si(100) stacks as they can be measured at BL-2C. The SiO2 films had thicknesses of 19.0, 30.0, and 41.5 nm35. In addition to a peak of surface hydrogen at Eres, all profiles show a second peak in larger depth, indicating that the H distribution within the oxide film is apparently not uniform. The position of this second peak shifts to larger depth with increasing SiO2 film thickness. The optically determined SiO2 film thickness is indicated by a vertical dashed line in each panel (i)-(iii). Close inspection reveals that the center positions of the NRA profile peaks are located with a small offset of ~4 nm above the respective SiO2/Si interfaces. This evidences that hydrogen tends to accumulate not exactly at but in a region extending over a few nanometers of the oxide in front of the interface. This peculiar H localization behavior has been attributed to the near-interfacial SiO2 structure, where a large number of strain and sub-oxide36 related defects provide preferential binding sites for hydrogen species37. The strained interfacial region results from the reduction of the Si atom density by a factor of ~2 in the abrupt transition from crystalline Si to amorphous SiO2. For quantitative evaluation the respective surface and near-interfacial peaks were fitted to Gaussian functions that are shown as solid lines in Figure 5. An analysis equivalent to the one described above for the surface H peak on Pd(100) reveals that the near-interfacial oxide regions of the SiO2/Si(100) film stacks contained a H layer density of (1.0-1.3)×1014 cm-2.
Figure 4. Near-surface NRA H profiles of Pd(110) obtained in the BL-1E UHV system under a H2 background of 1.33×10-6 Pa. Open symbols: Pre-exposed to 2,000 L H2 at 145 K, NRA measured at 90 K. Filled grey and black symbols: Two profiles measured at 170 K without H2 pre-dosage. Refer to the Note in Procedure Step 6.8 for the error bar calculation. Please click here to view a larger version of this figure.
Figure 5. NRA H profile of three SiO2 film on Si(100) measured in the BL-2C system. NRA H-profiles for three SiO2 films on Si(100) with thicknesses of (i) 19.0 nm, (ii) 30.0 nm, and (iii) 41.5 nm. SiO2/Si(100) interface positions are indicated by dashed vertical lines. This figure has been adapted from Ref. 35 with permission from AIP. Refer to the Note in Procedure Step 6.8 for the error bar calculation. Please click here to view a larger version of this figure.
Figure 4 demonstrates the efficient distinction and quantitation of surface-adsorbed from bulk-absorbed hydrogen through 15N NRA at the example of a Pd(110) single crystal in the BL-1E UHV system. The high reproducibility of the surface H peak in the three profiles attests to the reliability of the in-situ UHV sample preparation and to the non-destructive nature of the NRA measurement. The quantitative agreement of the determined H coverage with the expected atomic saturation density further shows the accuracy of the NRA measurement. Close comparison between the NRA profiles of H2 pre-exposed Pd(110) (open symbols) and the two excitation curves of only surface hydrogen on Pd(110) (grey and black symbols) in Figure 4 shows that the surface peak in the former H profile tails asymmetrically towards the plateau of bulk-absorbed hydrogen in the topmost ~5 nm. Such subtle detail of potentially non-uniform hydrogen distributions closely below the surface can only be revealed through NRA. A similar H accumulation in a shallow subsurface region has also been observed in other metals (Pt, Ti)11,14. The origin of this particular H behavior in the subsurface region is presently not clarified but scientifically interesting with regard to properly understanding the peculiar hydrogen absorption properties of nanomaterials17 as opposed to extended bulk metals.
Several critical parameters should be observed in the Protocol in order to obtain high quality data such as those in Figure 4. The H2 background pressure introduced in Step 4.2 (1×10-6 Pa) is a deliberate choice such as to stabilize the H-saturation coverage on the Pd surface by balancing NRA ion beam-induced H-desorption through H-readsorption from the H2 gas on one hand, while at the same time avoiding H uptake into the Pd bulk by H2 absorption, on the other. If the H2 pressure was too high, H absorption during the NRA analysis would cause a gradual increase of the NRA signal at Eres, because the surface resonance peak overlaps with about 2 nm of the topmost Pd bulk region, where H may accumulate, especially at even lower temperatures than those in the experiments of Figure 4.13-15,18 Thus, for H-absorbing materials such as Pd or Ti, the appropriateness of the H2 background setting has to be verified by confirming that the γ-yield at Eres remains constant on the time scale required to measure the surface resonance profile. This complication does not arise for most materials that do not absorb hydrogen at low H2 pressures. Here, quantitation of the surface H saturation coverage is easily performed after observing saturation of the γ-yield at Eres when the H2 background pressure is stepwise increased (up to a maximum of 10-2 Pa tolerable by the vacuum pump system).
Observe further that the data in Figure 4 were taken with a 15N2+ beam current of 15±5 nA (Step 4.5). This beam current has proven to be sufficiently high, on one hand, as to develop reasonably intense γ-signals from the saturated layer density of surface H atoms from the viewpoint of acceptable data statistics and overall measurement time, and also to be still sufficiently gentle, on the other hand, as to avoid excessive H desorption (which would again require higher H2 background pressure for compensation) and sample heating (which may cause modification of the H depth distribution by thermal diffusion).
Although the NRA technique is versatile and easily applied to determine H surface layer densities on most vacuum compatible solid materials, limitations arise with respect to the detection of especially weakly adsorbed H-species that may not be stable against desorption under the incident 15N ion beam irradiation even under loss-compensating H2 gas background (< 10-2 Pa). For example, the authors have not yet succeeded in observing surface H-species with desorption temperatures (TDS) below ~70 K with NRA. The tendency of H to desorb from the sample or to redistribute inside the target by diffusion under the ion beam varies strongly between different target materials and should be evaluated as part of any given analysis by monitoring the γ-yield at the probing depth of interest as a function of the ion beam dose. Without a compensating H2 background as applied here to Pd(110) where H readsorbs readily, in many cases a more or less pronounced exponential decay of the H-signal can be observed. Measuring and extrapolating such H-loss functions to zero 15N exposure allows reproducing the original H density on or inside the target prior to perturbation by the ion beam (for details, see Ref. 3). If the target size permits, reducing the current density (nA/cm2) in the beam-irradiated surface spot by defocusing the ion beam with aid of the MQ04 magnetic lenses (Protocol Steps 2.3.5 and 3.10) may alleviate H losses during the analysis. In general, excessive 15N ion doses should be avoided as they may cause physical damage to the target surface in the form of defects, which can change the H adsorption (and absorption) properties. In case of single crystals, the LEED pattern of the surface and the shape of the H2 TDS spectrum15 should therefore be checked regularly. If any anomaly is observed, re-prepare the surface freshly as instructed (for Pd(110)) in Protocol Steps 2.2.1 through 2.2.6.
Figure 5 demonstrated at the example of thin film SiO2/Si(100) stacks that hydrogen depth profiling analysis by 15N NRA can straightforwardly determine the depth location of near-interfacial H layers and the H density therein without destroying the sample material. Note, however, that the H accumulation in the interfacial regions seen in the depth profiles may partially result from the NRA analysis itself, because the 15N ion irradiation can cause redistribution of hydrogen in the material. This is a well-known effect35,38-40 and any possible H relocation during the NRA analysis should be verified by measuring the H concentration evolution at the accumulation peak depth on a non-irradiated sample spot in the course of continued 15N ion irradiation. Although this beam-induced H relocation effect can make determining the original H distribution in a specimen somewhat more difficult, it can be exploited for analytical purposes in dielectric reliability research to evaluate H redistribution trends between intact interfaces of (model) MOS device structures, providing information on relative material-specific H mobilities.3,26
Regarding the H detection limit of the NRA measurement, we note that the setup in BL-2C with its larger BGO detection solid angle (Figure 2 (A) has a more than twice as large calibrated γ-detection efficiency factor (α2C = 1.79×10-19 (cts/μC) (keV/nm) cm3) as in BL-1E (α1E = 7.56×10-20 (cts/μC) (keV/nm) cm3)) and hence provides higher sensitivity, favorable for the measurement of low H concentrations in materials that do not require in-situ surface preparation. The background count rate in our γ-detection system is currently ~0.1 cps, setting a detection limit for volume H concentrations in solids in the order of 100 ppm (several 1018 cm-3) with a 100-nA beam of 15N2+ at BL-2C. In the present examples, this increased sensitivity (in combination with a stronger ion beam), permitted measuring the surface and interfacial layer H densities in the SiO2/Si stacks with the same acquisition time (50 sec) as the about one order of magnitude larger surface H saturation layer on Pd(110) (compare vertical axis scales in Figures 4 and 5). The required acquisition time is determined by the desired counting statistics, the given H density in, and the 15N ion beam current tolerated by the target. The acquisition time also defines the time resolution for the observation of transiently evolving H densities such as upon adsorption, desorption, absorption, or diffusion of hydrogen on or in materials.
The higher vacuum base pressure at BL-2C (~1×10-5 Pa) may cause deposition of H-containing contaminants from the residual gas onto the target surface during the NRA measurement41. This may result in large surface H peaks in the depth profiles that can overwhelm features of interest directly below the surface, similar (but worse than) as seen, e.g., in the overlap with the shallower SiO2/Si interfacial peak in Figure 5 (i). A large H content in the surface layer also negatively affects the evaluation of small bulk H concentrations in large probing depths (Ei > ~9 MeV) by inducing γ-ray background due to non-resonant nuclear reaction yield42. Although the BL-2C system presently already features 10-cm thick lead (Pb) shielding blocks for the BGO detectors (not shown for clarity in Figure 2) that reduce the count rate of environmental γ-radiation background, further improvement of the H detection limit may be achieved by implementing anti-coincidence shielding for the detectors, which can reduce background signals due to highly penetrating cosmic muon radiation43.
The authors have nothing to disclose.
We greatly appreciate M. Matsumoto for implementing the software that enables the automated measurement of NRA H depth profiles by remotely controlling the MALT accelerator parameters from the data acquisition PC. We thank K. Namba for skillfully performing Pd(110) sample preparations and NRA and TDS measurements at the BL-1E UHV system, and C. Nakano for technical assistance in the accelerator operation. The SiO2/Si(100) specimen is gratefully received as a courtesy of Z. Liu of NEC Corporation, Japan. This work is partially supported by Grants-in-Aid for Scientific Research (Grant numbers 24246013 and 26108705) of the Japan Society for the Promotion of Science (JSPS), as well as through a Grant-in-Aid for Scientific Research in Innovative Areas 'Material Design through Computics: Complex Correlation and Non-Equilibrium Dynamics' from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.
Pd single crystal | SPL (Surface Preparation Laboratory), http://www.spl.eu/products.html, or any other suitable supplier | Order made to specification | Disk, 9 mm diam., (110) oriented, aligned to < 0.5 degree or less, one side polished to < 0.3 mm roughness, self-prepared specimen |
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