Photoemission study of UNxOy and UCxO y in thin films

Article abstract of DOI:10.1016/j.jallcom.2003.11.121

Thin films of UN_xO_y and UC_xO_y have been prepared by reactive sputtering of U in an Ar atmosphere containing O₂ and N₂ admixtures, and of U and C in an Ar/O ₂ atmosphere. The C content of the films was controlled by the carbon target voltage, and N and O contents were adjusted by the respective partial pressures of O₂ and N₂. Surface composition and electronic structure were studied by X-ray and ultra-violet photoemission spectroscopies (XPS and UPS, respectively). In most cases, the films were mixtures of the binary carbides (UC and UC₂) or nitrides (UN and U₂N ₃), UO₂ and U metal. The high reactivity of oxygen, compared to C and N₂ was demonstrated. At low oxygen pressure, indications for formation of ternary uranium oxycarbides (UC_xO _1-x) or oxynitrides (UN_xO_1-x) were obtained. These ternaries are solid solutions of UO and UC or UN (compounds containing a second constituent which fits into and is distributed in the lattice of the host compound). The metallic nature of uranium oxycarbides and oxynitrides (at low oxygen content) and the itinerant character of their 5f electrons were demonstrated.

Full text of DOI:10.1016/j.jallcom.2003.11.121

Journal of Alloys and Compounds 374 (2004) 261–264
Photoemission study of UNxOy and UCxOy in thin films
M. Eckle∗, T. Gouder
European Commission, Joint Research Centre, Institute for Transuranium Elements, Postfach 2340, D-76175 Karlsruhe, Germany
Abstract
Thin films of UNxOy and UCxOy have been prepared by reactive sputtering of U in an Ar atmosphere containing O2 and N2 admixtures,
and of U and C in an Ar/O2 atmosphere. The C content of the films was controlled by the carbon target voltage, and N and O contents were
adjusted by the respective partial pressures of O2 and N2. Surface composition and electronic structure were studied by X-ray and ultra-violet
photoemission spectroscopies (XPS and UPS, respectively). In most cases, the films were mixtures of the binary carbides (UC and UC2)
or nitrides (UN and U2N3), UO2 and U metal. The high reactivity of oxygen, compared to C and N2 was demonstrated. At low oxygen
pressure, indications for formation of ternary uranium oxycarbides (UCxO1−x) or oxynitrides (UNxO1−x) were obtained. These ternaries are
solid solutions of UO and UC or UN (compounds containing a second constituent which fits into and is distributed in the lattice of the
host compound). The metallic nature of uranium oxycarbides and oxynitrides (at low oxygen content) and the itinerant character of their 5f
electrons were demonstrated.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Thin films; Sputter deposition; Photoelectron spectroscopies; Uranium; Oxycarbide; Oxynitride
1. Introduction
amounts of third elements (S, C) can stabilize the uranium
monoxide [4]. The question of this work is: is it possible
to stabilize UO in thin films prepared by reactive sputtering
when adding stabilizing agents such as C or N, and whether
UO exists at all in thin films.
The films were prepared from the following starting mix-
tures: U/UC, UC and UC2 for carbide compounds and U/UN,
UN and U2N3 for nitride compounds. Oxygen was then
gradually introduced into each starting mixture. For mix-
tures containing U metal (U/UX with X = N, C) oxygen
can first react with uranium without affecting the UN or
UC phase. In the stoichiometric UX systems, introduction
of oxygen can lead to the reorganization of X into a higher
nitride or carbide phase. For U2N3 and UC2, an immediate
competition between X and O for U does occur.
Sputter deposition was used to prepare thin films of
U–N–O and U–C–O ternary compounds. The films were
characterized in situ by photoelectron spectroscopy (XPS
and UPS). This work was motivated by the investigation of
the electronic structure of UX compounds (X = C, N, O)
and by the study of the surface corrosion of UN and UC
nuclear fuels, which produce UOxNy and UOxCy species.
These species may be either true ternary compounds or
mixtures of binaries. Special attention is given to the oxyni-
tride (UOxN1−x) and to the oxycarbide (UOxC1−x), which
are solid solutions of UO and UN, and of UO and UC.
Pure uranium monoxide has never been prepared since UO
is not stable as pure oxide [1]. Instead UO2 forms even in
the presence of U metal, and no intermediate pure oxide
phase is formed. Early XRD data of UO are now thought
to reflect UO stabilized by impurities, i.e. in reality oxyni-
trides [2]. The electronic structure of hypothetical UO has
been calculated [3]. A high DOS at Fermi level, mainly due
to itinerant 5f states, and a symmetrical O-2p anion band
have been predicted for this NaCl-type binary compound.
In addition, it has been shown experimentally that small
2. Experimental
Thin layers of UOxCy and UOxNy have been prepared
in situ by reactive DC sputtering in an Ar atmosphere
(5 × 10−3 Torr Ar, U target (at −800 V) and C target (−100
to −700 V)) containing admixture of O2 and N2. The com-
position of the ternaries was controlled by varying the N2
and O2 pressures (0–2 × 10−5 Torr) for UOxNy, and the O2
pressure together with the C target voltage (−0 to −900 V)
∗
Corresponding author.
E-mail address: michel.eckle@itu.fzk.de (M. Eckle).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2003.11.121

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M. Eckle, T. Gouder / Journal of Alloys and Compounds 374 (2004) 261–264
for UOxCy. The plasma in the diode source was maintained
by injection of electrons of 50–100 eV energy. The exact
experimental set-up of the diode source will be presented
elsewhere. Ultrahigh purity argon (99.9999%), nitrogen
(99.999%) and oxygen (99.999%) were used as sputter gas.
Deposition times varied from 1 to 60 s. The U target and the
substrate were kept at room temperature. Shields were in-
stalled to expose only the substrate to the Ar–U plasma and
keep U contamination of the chamber as low as possible.
The U target was a U (99.9%) metal disc (5 mm radius, 1 mm
thickness), which was cleaned before introduction to the vac-
uum chamber by mechanical polishing and nitric acid etch-
ing. The background pressure of the plasma chamber was
4×10−9 Torr. The substrate was a single crystalline Si (1 1 1)
wafer, cleaned in situ by Ar ion sputtering at 400 ◦C. Photoe-
mission data were obtained in situ, without exposing sam-
ples to atmosphere, using a Leyboldt LHS-10 hemispherical
analyser. XPS spectra were taken using Mg K␣ (1253.6 eV)
radiation with an approximate resolution of 1 eV. UPS
measurements were made using HeI and HeII (21.22 and
40.81 eV) excitation radiation produced by a windowless
UV rare gas discharge source. The total resolution in UPS
was 0.1–0.05 eV for the high resolution scans. The back-
ground pressure in the analysis chamber was 2×10−10 Torr.
low oxygen contents, core level and valence band spectra
indicate the formation of an oxide different from uranium
dioxide. In Fig. 1, core level spectra of metallic U (A), and
UOxNy (B) and UO2 (C) are compared.
The UOxNy has been obtained by sputtering U at pN2 =
5 × 10−6 T and pO2 = 2 × 10−6 T. The binding energy of
this system (389.5 eV for U-4f5/2) is intermediate between
metallic uranium (388.0 eV) and UO2 (391.0 eV), pointing
to an oxide with a lower oxidation state than UIV in UO2.
Valence band spectra give further information on the na-
ture of this oxide. In Fig. 2, two ternary systems (UOxCy and
UOxNy) are compared with reference spectra of UO2 and
UN. The UOxCy film was prepared from a U/UC (57:43)
mixture at pO2 = 1 × 10−6 T. The C-2p signal around
2.5 eV binding energy is barely seen, because of its low
cross-section [5]. The O-2p peak looks very different from
that of UO2. While UO2 has a broad 5.7 eV wide, asym-
metrical line, the O-2p signal in UO/UC is symmetrical and
narrower (2.8 eV). Furthermore, there is no trace of local-
ized 5f electrons characteristic for UO2. Instead the 5f states
form a peak close to the Fermi-level, which proves their
itinerant character. This is consistent with formation of UO,
which is metallic and has itinerant 5f states. The larger width
of the anion O-2p band in UO2 is attributed to the strong
bonding interaction and to the complex structure of UO2,
compared to the simple NaCl-type UO. The smaller width
of the O-2p band in UO/UC compared with the N-2p band
width in UN, is perfectly reproduced in the ground state
density of states calculations [3]. The valence band spec-
trum of UOxNy (spectrum B) shows a broad anion band,
3. Results and discussion
When oxygen is gradually introduced while sputtering
of U, two different pressure regimes can be observed. At
U-4f5/2
391.0e V
U-4f7/2
UO2
(C)
389.5 eV
UOxNy
(B)
(A)
N-1s
388.0 eV
Umetal
430
420
410
400
390
380
370
360
Binding energy [eV]
Fig. 1. Core level spectra of: (A) metallic uranium, (B) U/UN system at pO2 = 2 × 10−6 T, (C) UO2.

M. Eckle, T. Gouder / Journal of Alloys and Compounds 374 (2004) 261–264
263
UO2
U-5f 2
O-2p
UN
N-2p
UOxNy
(B)
(A)
UOxCy
O-2p
C-2p
12
10
8
6
4
2
0
-2
Binding energy [eV]
Fig. 2. HeII valence band spectra of: (A) UOxCy, (B) UOxNy. UN and UO2 spectra are given as reference.
which is interpreted as superposition of the narrow O-2p and
N-2p bands. The maximum at 6 eV binding energy clearly
comes from the O-2p contribution while the smaller shoul-
der at 3 eV coincides with the N-2p signal in UN. The O-2p
contribution cannot be associated with the presence of UO2,
which has a main signal around 4 eV. Instead it coincides
well with the O-2p signal in the oxycarbide (UOxCy). This
shows that at low oxygen pressures, prior to the formation
of UO2, small amounts of UO, as part of a UOxCy solid so-
lution, are formed. The tendency for UO formation is more
pronounced for UC than for UN. UPS spectra of U–C–O
compounds preserve the symmetrical shape of the O-2p band
at much higher oxygen contents than in U–N–O ternaries.
This is consistent with the fact that oxycarbides have been
prepared by classical chemical methods [6] and their phase
diagram and region of stability are well established. Oxyni-
trides have not been prepared by such methods.
the linear relationship. This shows that UO2 formation is
suppressed and a different oxide formed. This oxide has a
lower oxidation state than UO2 (low BE), which is consis-
tent with UO.
At higher oxygen pressures, oxygen prevails over N and
C in bonding with uranium, and there is formation of UO2.
In the valence band spectra, the U-5f2 emission of UO2 ap-
pears. Also the O-2p band has binding energy and asym-
metrical shape typical for UO2. In presence of metallic U,
substoichiometric UO2−x is formed. But once all reduced
U has reacted, hyperstoichiometric UO2+x is formed. In the
U–N–O system, formation of UO2 is accompanied by the
gradual disappearance of the N-1s signal and finally no ni-
trogen is incorporated in the films. In the U–C–O system,
the increase of the oxygen pressure leads to the formation of
higher carbide phases. Fig. 4 shows the evolution of the C-1s
1
line with pO2. The lower binding energy peak labelled C,
Formation of UO is also supported by the evolution of
the oxygen content in the oxide. The integration of the O-1s
signal (not shown here) allows us to determine the total
amount of oxygen in the film. The U-4f signal allows one
to distinguish the different types of oxides. The U-4f line
of UO2 appears at 391.0 eV (Fig. 1 spectrum C), while the
UO signal appears at lower BE. If UO2 is the only oxide,
then its U-4f intensity should increase linearly with the O-1s
intensity. However, if another oxide is formed, then the U-4f
intensity of UO2 should be smaller than expected. In Fig. 3,
the area of the U-4f lines of UO2 (dashed line) and the
total oxide (UO2 and hypothetical UO, full line) are plotted
versus the area of the corresponding O-1s lines.
[O-1s]
For low oxygen pressures, we found that the area corre-
sponding to UO2 is lower than the area of the oxidized ura-
nium ([UO2] < [oxidized uranium]). Also, it deviates from
Fig. 3. Evolution of U-4f line area of oxidized uranium vs. O-1s peak
area (circle) and evolution of the U-4f line of UO2 vs. O-1s line peak
area (square) represented for the U/UN starting system.

264
M. Eckle, T. Gouder / Journal of Alloys and Compounds 374 (2004) 261–264
because now excess oxygen (all uranium is used up) reacts
with carbon, transforming it into CO or CO2. These gases
do not adsorb on UO2, therefore there is no carbon signal.
C-1s
pO (T)
2
51 0-5
31 0-5
4. Conclusions
1,21 0-5
91 0-6
Sputter deposition of U–C–O and U–N–O thin films yield
in most cases mixtures of binary carbides (UC and UC2) or
nitrides (UN and U2N3), UO2 and U metal. At low oxygen
content uranium oxycarbides (UOxC1−x) and oxynitrides
(UOxN1−x) were obtained. The itinerant 5f character of these
compounds was demonstrated. At higher oxygen pressure,
the higher reactivity of oxygen toward U prevails. N and C
are displaced from the film and UO2 is formed.
3C
51 0-6
2C
0
1C
Umetal
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Fig. 4. C-1s spectra of U/C mixtures with increasing oxygen pressure.
Three carbon species are identified and attributed to carbon in UC (1C),
UC2 (2C) and graphite (3C).
measured at 282.0 eV, has been observed previously for sub-
stoichiometric UC1−x [7] whereas 2C, measured at 282.5 eV
is typical for substoichiometric uranium dicarbide UC2−x
3
[8]. The peak labelled C (284.5 eV) appearing at pO2 =
1.2 × 10−5 T is characteristic of carbon in graphite [9].
At even higher pressures, oxygen scavenges all the ura-
nium and graphite carbon co-deposits, probably in small in-
clusions. Finally, carbon disappears from the film surface,