Name: quantification of hydrogen concentrations in surface and interface layers and bulk materials through depth profiling with nuclear reaction analysis
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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 &#39;Material Design through Computics: Complex Correlation and Non-Equilibrium Dynamics&#39; from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

1. Planning of Experiments
<ol>
	<li>Identify the MALT accelerator beam line of interest depending on the measurement task (BL-1E for surface hydrogen, BL-2C for bulk or interfacial hydrogen). Contact the assisting scientist (currently M.W. or K.F.) to discuss details of the NRA measurements and their necessary preparations.</li>
	<li>Download a beam time application form and observe the submission deadline on the MALT website31. 
	Note: The MALT facility invites new project proposals each March and Sep...

<ol>
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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 ...

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,&#945;&#947;)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 &#956;m. 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,&#945;&#947;)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,&#945;&#947;)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,&#945;&#947;)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,&#945;&#947;)12C nuclear reaction between 15N and 1H in the material. This reaction releases characteristic &#947;-rays of 4.43 MeV that are measured with a scintillation detector nearby the sample. The &#947;-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 &#947;-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,&#945;&#947;)12C nuclear reaction resonance has a very narrow width (Lorentzian width parameter &#915; = 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 &#62; Eres in a probing depth d=(Ei-Eres)/S, where S is the electronic stopping power of the analyzed material3.
By measuring the &#947;-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 (&#947;-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 &#62; 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 (&#916;Ei &#8805; 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) &#947;-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 &#947;-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 &#947;-rays penetrate the thin chamber walls with negligible attenuation.
<img alt="Figure 1" src="/files/ftp_upload/53452/53452fig1.jpg" /> 
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. <a href="https://www.jove.com/files/ftp_upload/53452/53452fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
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 &#60;10-8 Pa to maintain surface cleanliness. To provide sample access for the surface-analytical tools, the 4&#34; 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, &#920;) 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&#34; BGO detectors arranged at 90&#176; 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.
<img alt="Figure 2" src="/files/ftp_upload/53452/53452fig2.jpg" /> 
Figure 2. NRA setup at BL-2C. (A) Schematic top view into the high vacuum chamber at BL-2C equipped with two BGO &#947;-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. <a href="https://www.jove.com/files/ftp_upload/53452/53452fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

<table>
	<tbody>
		<tr>
			<td>Pd single crystal&#160;</td>
			<td>SPL (Surface Preparation Laboratory), http://www.spl.eu/products.html, or any other suitable supplier</td>
			<td>Order made to specification</td>
			<td>Disk, 9 mm diam., (110) oriented, aligned to &#60; 0.5 degree or less, one side polished to &#60; 0.3 mm roughness, self-prepared specimen&#160;</td>
		</tr>
		<tr>
			<td>H2 gas</td>
			<td>Joutou Gas Corporation, Ltd., Japan, http://www.jyotougas.co.jp/item/gas.html</td>
			<td></td>
			<td>(99.9995%), or any other suitable supplier</td>
		</tr>
		<tr>
			<td>O2 gas</td>
			<td>Joutou Gas Corporation, Ltd., Japan, http://www.jyotougas.co.jp/item/gas.html</td>
			<td></td>
			<td>(99.99%), or any other suitable supplier</td>
		</tr>
		<tr>
			<td>Ar gas</td>
			<td>Joutou Gas Corporation, Ltd., Japan, http://www.jyotougas.co.jp/item/gas.html</td>
			<td></td>
			<td>(99.99995%), or any other suitable supplier</td>
		</tr>
		<tr>
			<td>Tantalum / Wire</td>
			<td>The Nilaco Corporation, http://nilaco.jp/en/order.php</td>
			<td>TA-411325</td>
			<td>(99.95%), 0.3 mm diam., or any other suitable supplier</td>
		</tr>
		<tr>
			<td>Alumel / Wire&#160;</td>
			<td>The Nilaco Corporation, http://nilaco.jp/en/order.php</td>
			<td>851266</td>
			<td>0.2 mm diam., or any other suitable supplier</td>
		</tr>
		<tr>
			<td>Chromel / Wire (Chromel)</td>
			<td>The Nilaco Corporation, http://nilaco.jp/en/order.php</td>
			<td>861266</td>
			<td>0.2 mm diam., or any other suitable supplier</td>
		</tr>
	</tbody>
</table>

quantification of hydrogen concentrations in surface and interface layers and bulk materials through depth profiling with nuclear reaction analysis

Nuclear reaction analysis (NRA) via the resonant 1H(15N,&#945;&#947;)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 &#956;m 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,&#945;&#947;)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).

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&#215;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 abso...

Watch this Scientific Journal Video about Quantification of Hydrogen Concentrations in Surface and Interface Layers and Bulk Materials through Depth Profiling with Nuclear Reaction Analysis at JoVE.co

Quantification of Hydrogen Concentrations in Surface and Interface Layers and Bulk Materials through Depth Profiling with Nuclear Reaction Analysis

We illustrate the application of 1H(15N,&#945;&#947;)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,&#945;&#947;)12C reaction is a highly effective method of depth profiling that quantitatively and ...

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