Influence of gaseous environments on beryllium–tungsten and tungsten surfaces
investigated by XPS
Alexandru MarinCristian P. Lungu and Corneliu Porosnicu
Citation: J. Vac. Sci. Technol. A 35, 021403 (2017); doi: 10.1116/1.4972513
View online: http://dx.doi.org/10.1116/1.4972513
View Table of Contents: http://avs.scitation.org/toc/jva/35/2
Published by the American Vacuum Society
Influence of gaseous environments on beryllium–tungsten and tungsten
surfaces investigated by XPS
Alexandru Marina)
Institute for Nuclear Research, Pitesti, Str. Campului 1, Mioveni, 115400 Arges, Romania and Institute
of Physical Chemistry “Ilie Murgulescu” of the Romanian Academy, Spl. Independentei 202, Bucharest 060021,
Romania
Cristian P. Lungu and Corneliu Porosnicu
National Institute for Lasers, Plasma and Radiation Physics, Str. Atomistilor 409, Magurele-Bucharest
077125, Romania
(Received 26 October 2016; accepted 5 December 2016; published 22 December 2016)
Beryllium–tungsten and tungsten layers were deposited by thermionic vacuum arc technique in
gaseous environments, in order to simulate the plasma–wall phenomena appearing during fusion
device operation. In-depth x-ray photoelectron spectroscopy investigations highlighted the
chemical changes on (Be-W) and W systems subjected to different plasma environments similar to
a fusion device: deuterium, neon, helium, nitrogen, and oxygen. The metallic feature among oxides
of beryllium can be detected only for deuterium and neon plasma atmosphere. Tungsten displays
only the metallic contribution for the mentioned plasmas, deuterium, and neon. Full oxidation
state of tungsten (W6þ) is found in Be-W system for nitrogen and oxygen plasma environment
deposition and for W system in oxygen atmosphere. Finally, the nitrogen atmosphere has a
protective behavior by increasing the metallic feature followed by a decrease in the W oxidation
state, from 19.7% for oxygen plasma to 31.5% for nitrogen plasma and 14.0% to 6.6%,
C 2016 American Vacuum Society. [http://dx.doi.org/10.1116/1.4972513]
respectively. V
I. INTRODUCTION
Previous studies have indicated the crucial importance of
plasma–wall interactions in a fusion device (tokamak) with
the first wall material. The most suitable plasma-facing components of the first wall are beryllium and tungsten which
will be used for the construction of the future test reactor—
international thermonuclear experimental reactor (ITER).1,2
Beryllium is a low-Z metal, which acts as an oxygen getter
and does not exhibit chemical erosion from hydrogen
plasma.3 Tungsten has a high Z, high melting point, and low
sputtering yield.4 Carbon, an initial selected material for
ITER first wall, incorporates hydrogen from the plasma,5,6
and this has an impact on first wall performance.7 Efforts
have been made in order to indicate the importance of relationship between the incident Be, D, and O fluxes.8–10 Fusion
related characteristics of components involve material migration and erosion/redeposition processes during operation,
which led to the formation of mixed compounds.11 The
behavior of tungsten and beryllium at the high flux irradiation is not yet fully understood and is extensively studied
using powerful plasma devices and laser irradiation.12
Therefore, the aim of this work is to highlight the chemical changes on (Be-W) and W systems subjected to different
plasma environments similar to a fusion device: deuterium,
neon, helium, nitrogen, and oxygen. In this work, we investigate surface and “in-depth” chemistry of beryllium and tungsten related to oxygen, nitrogen, neon, helium, and
deuterium deposition environments. Noble gases (Ne) or
nitrogen can be seeded into the divertor plasma in order to
lower the power loads in a heating scenario.13
a)
Electronic mail: [email protected]
021403-1 J. Vac. Sci. Technol. A 35(2), Mar/Apr 2017
Thereby, under these circumstances, x-ray photoelectron
spectroscopy (XPS) investigations will shed light on the
chemistry behind the seeding species and the plasma-facing
materials, beryllium and tungsten.
This comprehensive study provides a chemical state profile from surface to subsurface region of beryllium and tungsten behavior under different environment conditions. In the
approach of low Z impurities such as oxygen which can be
absorbed and react with the materials’ surface, an oxygen
gaseous environment has been used in order to simulate
plasma vessel conditions.
FIG. 1. (Color online) Schematic diagram of the mixed films depositions
with gas inclusions.
0734-2101/2017/35(2)/021403/6/$30.00
C 2016 American Vacuum Society
V
021403-1
021403-2 Marin, Lungu, and Porosnicu: Influence of gaseous environments on beryllium-tungsten
II. EXPERIMENT
A. Film preparation
Thermionic vacuum arc (TVA) method14 was used to
obtain pure and mixed layers. This type of discharge ignites
in vacuum conditions in the vapors of the anode material,
continuously generated by the electron bombardment of the
anode. The electrons, emitted from a heated tungsten cathode, are accelerated toward the anode, by a dc high voltage
applied across the electrodes. After switching on, the anode
material (Be, W) first locally melts and then starts to evaporate, and a steady-state metal vapor atoms density is established in the interelectrodes space. By further increasing the
applied high voltage, a stable metal vapor discharge is established. In order to include gases (O2, N2, He, Ne, and D2)
during film formation, a supplementary plasma is produced
in any of mentioned gas (by a rf or dc driven plasma torch),
as can be seen in Fig. 1.15–17 The pressure of the gas was
controlled in the reaction chamber and kept was at 1 103 Pa (usually the TVA discharge could run in pure Be or
W vapors at pressures below 1.5 103 Pa). During deposition, the substrate temperature reaches 400 C; although in
the beginning of the deposition process, samples were at
room temperature. By water cooling the substrate holder, the
sample temperature was kept below 100 C. Under these
conditions, the deposition rate was 0.5 nm/s. (Q is the sensor
FIG. 2. (Color online) Be1s and W4f depth profile spectra in different plasma environments.
J. Vac. Sci. Technol. A, Vol. 35, No. 2, Mar/Apr 2017
021403-2
021403-3 Marin, Lungu, and Porosnicu: Influence of gaseous environments on beryllium-tungsten
021403-3
TABLE I. Tungsten chemistry: Binding energies, surface chemical species,
and chemical states relative concentrations.
Sample
Be þ W þ N2
Be þ W þ O2
W þ O2
Tungsten chemical
species
Binding
energy (eV)
Tungsten chemical
states relative
concentrations (%)
Wmetallic
WN
WN2
W5þ
W6þ
Wmetallic
W4þ
WO2X
WO3X
W6þ
Wmetallic
W4þ
WO2X
W5þ
W6þ
31.6
32.5
33.6
34.6
36.2
31.1
32.3
33.5
35.1
36.2
31.3
32.4
33.5
34.5
35.9
31.5
33.7
17.9
10.3
6.6
19.7
18.5
18.3
29.5
14.0
46.5
21.8
14.5
8.9
8.3
FIG. 3. (Color online) XPS deconvoluted W4f spectrum for Be-W system in
nitrogen environment.
that the errors in our quantitative analysis (relative concentrations) were estimated in the range of 610%, while the accuracy for BEs assignments was 60.2 eV.
III. RESULTS AND DISCUSSION
A. Surface chemistry
of a crystal quartz balance which monitors the deposition
rate and total thickness of the prepared film.)
B. Film characterization
Surface analysis performed by XPS was carried out on
Quantera SXM equipment, with a base pressure in the analysis chamber of 107 Pa. The x-ray source was Al Ka radiation
(1486.6 eV, monochromatized), and the overall energy resolution is estimated at 0.65 eV by the full width at half maximum of the Au4f7/2 line. In order to take into account the
charging effect on the measured binding energies (BEs), the
spectra were calibrated using the C1s line [BE ¼ 284.8 eV,
C–C (CH)n bondings] of the adsorbed hydrocarbon on the
sample surface. An electron flood gun has been used to compensate the charging effect in insulating samples. The atomic
composition of surface was evaluated using PHI sensitivity
factors, specific for the instrument. The inelastic mean free
path (IMFP) was calculated using the TPP-2M algorithm.26 It
is appropriate to note here that all the calculations were performed assuming that the samples were homogeneous within
the XPS detected volume (<10 nm). We have to emphasize
XPS method was used to establish the chemical states of
the elements present on the surface and subsurface region
and after quantitative analysis to find the element and chemical state relative concentrations.
Depth-profiling x-ray photoelectron spectroscopy was
involved to reveal the chemical distribution of the elements
across the surface of the films using an Arþ ion gun20 operated with a beam voltage of 1 kV in a (3 3) mm raster area
(gentle etching). After scanning survey XPS spectra, the
high resolution photoelectron spectra of the most prominent
XPS transitions (C1s, O1s, Be1s, and W4f) were recorded
for the samples A:(Be þ W þ O2), B:(Be þ W þ N2), C:(Be
þ W þ Ne), D:(Be þ W þ D2), E:(W þ N2), F:(W þ He), and
G:(W þ O2).
Figure 2 shows the high resolution depth profile spectra
of Be1s and W4f of the mixed Be/W layers prepared using
TVA technique in different gaseous environments (O2, N2,
Ne, and D2). The metallic feature among oxides of beryllium
can be detected only for samples prepared in deuterium and
neon atmosphere [Figs. 2(e) and 2(g)]. Comparing nitrogen
and oxygen codeposited samples, Be1s spectra exhibit
TABLE II. Beryllium chemistry: Binding energies, surface chemical species,
and chemical states relative concentrations.
Sample
Be þ W þ Ne
Be þ W þ D2
Be þ W þ N2
Be þ W þ O2
Beryllium chemical
species
Binding
energy (eV)
Beryllium chemical
states relative
concentrations (%)
Bemetallic
Be (BeO)
BeO
Bemetallic
Be (BeO)
BeO
BeOXNY
BeO
111.4
112.2
114.0
111.1
112.2
114.4
113.4
113.9
15.9
6.6
77.5
17.7
2.4
79.9
100
100
JVST A - Vacuum, Surfaces, and Films
FIG. 4. (Color online) XPS deconvoluted W4f spectrum for Be-W system in
oxygen environment.
021403-4 Marin, Lungu, and Porosnicu: Influence of gaseous environments on beryllium-tungsten
021403-4
FIG. 6. (Color online) O1s XPS superimposed spectra for Be-W system in
neon plasma.
FIG. 5. (Color online) W4f depth profile spectra in different plasma
environments.
beryllium oxynitrides and oxides, respectively [Figs. 2(a)
and 2(c)]. The binding energies ascribed to these chemical
species can be found in the Table II together with their relative concentrations.18,19,22,24
The W4f core level spectra display also the metallic
contribution for the above mentioned codeposited gases,
deuterium and neon. A broadening of W4f photoelectron
peak is primarily due to a mixture of different tungsten
J. Vac. Sci. Technol. A, Vol. 35, No. 2, Mar/Apr 2017
chemical species that can be noticed for oxygen and nitrogen
environment and, therefore, enabled us to deconvolute them
(Figs. 3 and 4).
The most significant difference between (Be-W) and W
system consist in the larger amount of the metallic tungsten
illustrated for vacuum, nitrogen, helium, and oxygen plasma
environments (Fig. 5).
Thus, the original O1s spectra (not normalized) reveal
the increasing amount of oxygen with the sampling depth
(Fig. 6) in neon plasma environment, leading to a higher
content of beryllium oxide available into the “bulk” of the
beryllium–tungsten system compared to deuterium atmosphere [Figs. 2(e) and 2(g)]. Moreover, the metallic tungsten
is protected in both neon and deuterium gaseous environments while the metallic feature of beryllium is enhanced in
deuterium atmosphere, as compared to neon plasma environment [Figs. 2(e) and 2(g)]. Additionally, a close inspection
of the O1s intensities (Fig. 7) displays a decreasing tendency
of the oxygen content in the depth range, more pronounced
after 2.0 min argon etching. A clear trend of decreasing
beryllium oxide followed by the increase in metallic beryllium can be identified in Fig. 2(g).
The experimental observations of tungsten in different
plasma environments are plotted in Fig. 5. As described
above, the XPS investigations suggest a constant behavior of
the metallic feature in vacuum, nitrogen, and helium. It is
worth noting that there is a similar chemical behavior of W
and (W-Be) systems in oxygen atmosphere [Figs. 2(b), 5(d)
and Table I), meaning that combining the oxides as in our system does not influence the W oxidation properties. However,
the amount of metallic contribution is significantly enhanced
in (W þ O2) system by a factor of >2 (Table I). A slight difference can be observed in an interchange between 5þ oxidation state (W þ O2) and WO3X appearance in (W þ Be þ O2)
system although their percentages are quite the same (35%).
The W4f XPS bandlike spectrum (Fig. 3) recorded on the
nitrogen plasma environment exhibits a complex structure
with five contributions which are labeled on the figure. The
first doublet peak at 31.8 eV is associated with metallic tungsten. The second features located at 32.5 and 33.6 eV can be
assigned to WN and WN2, respectively.25 The other two
doublets are ascribed to W5þ and W6þ oxidation states.23
021403-5 Marin, Lungu, and Porosnicu: Influence of gaseous environments on beryllium-tungsten
FIG. 7. (Color online) O1s XPS superimposed spectra for Be-W system in
deuterium plasma.
Interestingly, the W5þ oxidation state contribution can be
distinguished for the Be-W system in both the nitrogen deposition plasma and the W system in the oxygen environment
(Fig. 9).
Full oxidation state of tungsten (W6þ) is found in Be-W
system for nitrogen and oxygen plasma environment deposition and for W system in oxygen atmosphere.
To perform data processing, we followed the requirements for an accurate deconvolution of the W4f bandlike
transitions by imposing all the constraints recommended by
ISO-TC201 (“Surface Chemical Analysis”) Committee.
Moreover, it should be noted that for Be-W system, the
nitrogen atmosphere has a protective behavior. One can notice
an increase in the metallic feature followed by a decrease in
the W6þ oxidation state, from 19.7% for oxygen plasma to
31.5% for nitrogen plasma and 14.0%–6.6%, respectively.18,21
Be1s deconvoluted spectrum (Fig. 8) reveals a mixture of
metallic and oxidized beryllium and a small feature at
112.2 eV attributed to Be if BeO and the metallic species are
present.22 From the chemical table (Table II) of beryllium, it
is observed that a lower concentration of BeO is present for
the deuterium plasma environment. It can be concluded that
deuterium atmosphere plays an important role on the occurrence of the oxide and metallic states of beryllium. The calculated IMFP values from TPP-2M equation26 are as
follows: kBe 3.3 nm and kW 2.3 nm. Therefore, we can
conclude that the corresponding XPS probing depth (3k) is
in the range of 7–10 nm.
FIG. 8. (Color online) XPS deconvoluted Be1s spectrum for Be-W system in
deuterium environment.
JVST A - Vacuum, Surfaces, and Films
021403-5
FIG. 9. (Color online) XPS deconvoluted W4f spectrum for W system in
oxygen environment.
A detailed literature search revealed that beryllium–
tungsten alloy was reported only for temperature exposed
samples.22,27–29 Indeed, in the work of the present authors,30
experimental results of beryllium–tungsten surface alloying
were observed to occur during high temperature exposure.
The literature found data on beryllium–tungsten systems22,27–29 assigns the Be-W alloying at 111.1 eV for Be1s
photoelectron line and the corresponding W4f transition
located at 31.0 eV, respectively.
In our samples, the XPS binding energy assessments on
beryllium–tungsten system in oxygen environment were as
follows: the metallic W4f7/2 peak was observed at 31.1 eV,
unaccompanied by the corresponding Be1s alloy binding
energy. A single BeO chemical state located at 113.9 eV was
assigned after spectral deconvolution of Be1s core level line
(Table II). A similar chemical behavior was also found for
beryllium–tungsten system in deuterium atmosphere. Based
on this peculiar behavior of both tungsten and beryllium in
oxygen and deuterium gaseous environments, we can conclude that a contribution from alloyed beryllium–tungsten
cannot be completely ruled out.
The results of the quantitative analysis presented in Figs.
10 and 11 indicate that a beryllium segregation process from
bulk to the surface accompanied by the diffusion of tungsten
FIG. 10. Beryllium in-depth concentration for different gaseous environments. The nominal values (at. %) are labeled on the graph.
021403-6 Marin, Lungu, and Porosnicu: Influence of gaseous environments on beryllium-tungsten
021403-6
However, a completely different behavior of beryllium
and tungsten can be drawn for neon and deuterium deposition environments. Thereby, a mixture of metallic and oxidized beryllium accompanied by a metallic tungsten feature
can be observed for the above mentioned plasmas, neon, and
deuterium. Additionally, a similar protective behavior was
noticed for W system in helium deposition plasma.
ACKNOWLEDGMENTS
This work was supported by a grant of the Romanian
National Authority for Scientific Research, CNCS–UEFISCDI,
Project No. PN-II-IDPCE-2011-3-0522.
1
FIG. 11. Tungsten in-depth concentration for different gaseous environments. The nominal values (at. %) are labeled on the graph.
from surface to subsurface region can be noticed for beryllium–tungsten system in deuterium, nitrogen, and neon gaseous environments. Furthermore, an opposite behavior was
observed in oxygen atmosphere.
IV. SUMMARY AND CONCLUSIONS
Be-W and W films were obtained by thermionic vacuum
arc method on silicon substrates for nuclear fusion applications. Film deposition was performed in a plasma characteristic environment similar to a fusion device: deuterium,
neon, helium, nitrogen, and oxygen.
X-ray photoelectron spectroscopy technique was able to
access both the chemical states of the surface and elemental
composition of the films.
The Be-W system was found to exhibit significant difference in the chemical behavior of beryllium and tungsten for
oxygen, nitrogen, neon, and deuterium plasmas. Therefore,
beryllium oxide (BeO) and tungsten oxidation states (W4þ
and W6þ) and suboxides accompanied by metallic tungsten
can be seen for oxygen deposition environment. Interestingly,
the latter remarks are also clearly visible for W system with
the metallic tungsten being enhanced. The evidence suggests
that beryllium is highly reactive toward oxygen, and the
reduction of the metal to the oxide is illustrated in beryllium–
tungsten system.
The nitrogen gaseous environment highlights the formation of beryllium oxynitrides (BeOxNy). One can notice the
tungsten nitrides and the metallic feature besides W5þ and
W6þ oxidation states as a consequence of enhanced beryllium reactivity to oxygen impurities in nitrogen plasma.
Nevertheless, the experimental observation of W system in
nitrogen gaseous environment demonstrates the protective
behavior of nitrogen enriching the metallic contribution of
tungsten. This finding provides convincing evidence that
very fast beryllium oxidation occurs even in oxygen depleted
environments.
J. Vac. Sci. Technol. A, Vol. 35, No. 2, Mar/Apr 2017
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