- Email: [email protected]
XPS study of thin films of titanium oxysulfides
Surface Science 254 (1991) 81-89 North-Holland
81
XPS study of thin films of titanium oxysulfides D. Gonbeau, taboratoire 64000 Pm,
C. Guimon,
de Pltysico-Chin& France
A. Levasseur,
G. Pfister-Guillouzo
Mol&uluire
G. Meunier
2 January
1991; accepted
URA 4?4), Uniuersite de Pau et des Pays de I’Adour, 2, Avenue du PrPsident Angot,
and R. Dormoy
Laboratoire de Chimie du Sofide du CNRS, 33405 Talence Cedex, France Received
(CNRS
Ecole Nationale
for publication
Supkieure
18 March
de Chimie et Physique de Bordeaux,
iJniversii6 de Bordeaux
I,
1991
In this study thin films of titanium oxysulfides which could be used as positive electrode materials in microbatteries were analysed by XPS. In light of the binding energies obtained for reference compounds (different titanium oxides and sulfides) the results have shown the existence of a new type of titanium. It presents an environment with gpairs, S2- and O’- ions, this latter in a proportion half that of sulfur ions.
1. Introduction New amorphous titanium oxysulfides obtained in the form of thin films have been recently described [1,2]. They can be used as positive electrode materials in microbatteries [l]. These thin films are obtained by RF sputtering using a TiS_ target which contains in fact a few percent of oxygen They have been characterized by Auger electron spectroscopy (AES), secondary ion mass spectroscopy (SIMS), X-ray microprobe and Rutherford backscattering (RBS) [1,2]. Because of the surface hydrolysis of the target the composition of the thin films obtained changes with the utilization duration of the target which is in fact the sputtering time. The formula of titanium oxysulfide can be written TiO,S,. Whereas the stoichiometry (x + y) is equal to 2 at the beginning of the utilization of the target; it shifts to 1.75 for the high sulfur-content thin films. This anionic deficit can be explained by consideration of the Ti-S and Ti-0 binding energy. Because of the lower of the energy Ti-S bond com-
pared to Ti-0, 0039-6028/91/$03.50
sulphur atoms or molecules are 0 1991 - Elsevier Science Publishers
mainly present in the plasma and preferentially eliminated by the pumping system. This phenomenon does not occur when the sulfur content is low, i.e., at the beginning of the sputtering process when the oxygen content is high. In order to explain the oxydo-reduction reactions occurring when these new materials are used as positive electrode in microbatteries it was necessary first to identify the different ions present in these tita~um oxysulfides. We report in this work the analysis based on XPS, an appropriate technique for this approach. A further study will be devoted to the positive electrode material after use in the batteries (intercalated compounds).
2. Experimental The XPS analysis were performed with a Surface Science Inst~ments spectrometer (model 301) using a focused (diameter of the irradiated area = 300 pm) monochromatized Al KLY radiation (1486.6 eV) and interfaced to a Hewlett
B.V. (North-Holland)
D. Gonbeau et al. / XPS study of thin films of titanium oxysulfides
82
Packard 9000 microcomputer. The residual pressure inside the analysis chamber was in the 5 X lo-’ Pa range. For the titanium oxides powdered samples were pressed as pellets. The targets have been obtained by pressing powder of “Cerac TiS,” inside a glove box filled with dried argon and containing less than 5 ppm of H,O. The thin layers obtained by RF sputtering using these targets have been deposited on silicon substrates. All the samples were fixed on the sample holders in a glove box filled with continuously purified nitrogen (the 0, and H,O levels were below 3 and 5 ppm respectively) and directly attached to the introduction chamber of the spectrometer. For the titanium oxides the charging effects were minimized with a low-energy flood gun. The calibration of the spectra was done for the titanium oxides with the Cls line (284.6 eV) from the carbon contamination layer. The other compounds were sufficiently good conductors to preclude charging effects. For the etching process an Ar+ ion gun was used (current intensity 10 mA, voltage 4 kV). For the mechanical erosions the surface of the samples was carefully scraped with a steel blade in the glove box then directly transferred to the spectrometer. The reproducibility of the measurements was checked by different mechanical scrapings on each sample and further XPS analysis. A carbon contamination of about 25% (atomic percentage) was found. In the peak fitting routine used the lines are defined according to their centroid position, halfwidth, shape (using a combination of Gaussian (80%) and Lorentzian (20%) distributions). These
Table 1 Ti2p binding half maximum
energies in the titanium in parentheses) .% (ev) (FWHM
TiO z
458.1 and 464.4
Ti ,o,
(1.5) (2.2) 456.6 and 461.8
TiO
(1.4) (2) 454.4 and 460.0 (1.6)
(1.8)
oxides
(ev))
(full widths
at
parameters are then varied to obtain a minimum in the error sum of squares between the values for the experimental curve and the combination of bands. However it is possible to constrain certain parameters within set limits; thus for the analysis of thin films we have chosen these limits (for area ratio, spin-orbit splitting and 2P,,,-2P,,z7 full width at half maximum) in relation with the mean results obtained for the reference compounds. A non linear background was used in all the cases.
3. Results and discussion A preliminary step before the analysis of XPS spectra has been the characterization of several reference compounds with well-defined structure (titanium oxides and sulfides). Published values for the Ti2p peak of titanium oxides are relatively dispersed [3], except for TiO, [3b,3d,4]. Studies of titanium sulfides are rare and most often limited to valence zones [5]. Since SIMS and Auger spectroscopy studies have shown that the composition of titanium oxysulfide films changes from the surface to the core (slight hydrolysis occurs when thin films are moved outside the sputtering machine) it was also of interest to analyse the compounds after elimination of the superficial contamination. 3.1. Results of analysis of the reference compounds 3.1.1. Titanium oxides Three standard samples of titanium oxidized at different degrees of oxidation were analyzed TiO,, Ti,O, and TiO. The results obtained for the Ti2p peak are listed in table 1 and agree with published values for TiO, [3b,3d,4]. Concerning the 01s peak of this last compound the main peak is centered at 530 eV and exhibits a high binding energy shoulder at 531.8 eV, which could be assigned to hydroxyl groups [3d]. 3.1.2. Titanium sulfides Three samples were analyzed: a TiS, monocrystal, a “Cerac TiS,” pellet (used to prepare sputtering target) and a TiS, pellet (for the char-
83
D. Gonbeau et al. / XPS study of thin films of titanium oxysulfides
TiS2 monocrystal
TiS2 monocrystal
056
,"
;',,,, i ,, 'l, \ --_A__ 160.8
“Cerac TiS2"
"Cerac TiS2"
4 162
163.2
/
,'\
! 1 /+.A) I,
,64&ti'f _
/
2 ,'',, ., 4.
I..___I'
\ '\ '\
.,
162.3 455.9
Ti S3
Ti S3
r\
!\ I I
BlndlnQ
energy
(CV) 451 Blndlnp energy (cV) 157.0
Ti%/2-1/2 Fig. 1. Ti2p,,,_,,,
s p ectra in the titanium
5!P3/2-l/2
sulfides.
Fig. 2. S2p,,,_,,,
acterization of Sip pairs). The results obtained for the Ti 2p and S 2p peaks are listed in table 2 and the corresponding spectra, respectively, in figs. 1 and 2.
Table 2 Ti 2p and S2p binding
energies
in the titanium
Et, (eV) (FWHM
Ti2~,,~-,,~
S2P,,,-I,,
sulfides
in the titanium
sulfides.
To our knowledge, only one report of the binding energies associated with the core peak of TiS, exists [5g]. In the case of the monocrystal, very
(full widths
at half maximum
in parentheses)
(eV))
TiS, monocrystal
“Cerac
456 and 462 (1.45) (1.68)
456 and 462 (2.02) (1.53) 458.6 and 464.5 (1.46) (2.04)
160.7 and 161.8 (0.71) (0.87)
spectra
TiS,”
160.8 and 162 (0.87) (0.9) 163.2 and 164.2 (0.8)
(1)
TiS, 455.9 and 462 (1.4) (1.8) 458.8 and 464.6 (1.6)
(1.9)
160.9 and 161.9 (0.7) (0.8) 162.3 and 163.4 (0.75) (0.8)
D. Gonbeau et al. / XPS study of thin films of titanium oxysulfides
84
slight contamination by oxygen has been observed (- 2.5% in atomic percentage). Binding energies associated with the 2~~,~_i,~ doublets of titanium and sulfur could thus be estabhshed with precision (table 2). We may also point out that a spectrum equivalent to that previously reported by Wertheim et al. [5a] was obtained in the valence zone (O-30 eV). A low-intensity band close to the Fermi level was also observed, which could be correlated with the high conductivity of this compound. The origin _
F
/
I
E3
T12p3/2-w
45213
BindIn@ energy (eV)
of this band remains to be defined. For the monocrystal, it is difficult to associate this phenomenon with an excess of tit~ium, as proposed by some authors [6], since the determination of the S/Ti ratio (2.3) based on ESCA peaks arose more from an excess of sulfur. In the case of “Cerac TiS,” pellets, there was contamination by TiO,. The Ti2p peak was present in the form of two well differentiated components. Based on their energy position, they correspond to TiS, and TiO, (fig. 1). In addition to the main component of the S2p peak, there was a second component with lower intensity (fig. 2). It was located at higher energy and corresponds to another type of sulfur atom, which could not be identified. The 01s peak is similar to the one observed for TiOZ (main peak: 530.1 eV, shoulder: 531.8 eV). In the case of TiS,, oxygen contamination led to the observation of two components in the Ti 2p peak (fig. 1). The doublet with the lowest intensity, on the high-energy side, was associated with TiOZ and the major component, on the low-energy side, with TiS,. The S2p peak appeared as two partially overlapping doublets attributed to the Sip pairs (on the high-energy side) and to a sulfur atom with a higher negative charge S2- (on the low energy side). These results are in agreement with the work of Endo et al. [5e].
160.8
A
...j
B
t
%3/2-M
Binding energy (eV)
I!
0
Fig. 3. “Cerac TiS,“. (A) Before mechanical. erosion. (B) After mechanical erosion.
3.1.3. Effect of ion etching on samples of TiO, and TiS, Ion etching leads to the elimination of superficial contamination but it may modify the atomic environment (depletion of the element correlated with a highest sputtering yield), as shown for TiO, [3b,7]. Since this phenomenon could occur under our conditions of bombardment, in the case of the materials studied, measurements were carried out on a TiO, pellet and the TiS, monocrystal at different times of sputtering. The results obtained with TiO, clearly show a reduction in Ti,O, and TiO but under our conditions of bombardment a full reduction to Ti metal had not occured. Similar results were obtained with TiS,, the preferential sputtering of sulfur probably inducing the observed reduction phenomenon. After 30 s of
D. Gonbeau et al. / XPS study of thin films of riranium oxysulfides
85
etching, two peaks appeared (Ti2~s,~_i,~ (eV): 454.6 and 460.5; 453.4 and 459.1), with the low energy peak increasing in intensity after 5 min of etching. The absence of reference compounds and of reliable published data precluded the more precise identification of these peaks. In the case of this compound, it should be noted that the reduction phenomenon was particularly rapid. In order to partially eliminate superficial contaminations, mechanical erosion (by scraping) was carried out, since ion etching modified the environment of the atoms in the samples to be analyzed. 3.1.4. Influence of mechanical erosion The results obtained with the “Cerac TiS,” sample before and after mechanical erosion (fig. 3) confirm the results obtained by Auger spectrometry, which showed that the surface was the zone undergoing the most hydrolysis. It should nevertheless be noted that this mechanical erosion involved the outermost layers and did not lead to bulk stoichiometry determined by Rutherford backscattering (RBS) [1,2]. It is also noteworthy that the decrease in the Ti2p peak associated with TiO, was accompanied by the practically complete disappearance of sulfur located toward high energies. It was not possible to define the origin of this latter peak, corresponding to a more “positive” type of sulfur than those encountered in TiS, and the TiS, monocrystal. 3.2. Analysis of the results of thin films of amorphous titanium oxysu[fides Among the different samples analyzed, the results involve two particularly representative species. Their composition, TiO,.,S,., and TiOS was determined by RBS. 3.2.1.
XI’S
characterization
of
a
thin
of film
Ti%.& In light of the results obtained for the reference compounds (peak widths, spin-orbit splittings and area ratios). The Ti 2p peak was broken down into three components (fig. 4 and table 3). The most intense component, on the high-energy side, could be assigned to titanium atoms with a [TiO,] oc-
Ti2P3/2-W
Binding
enerqy
(CV)
452.0
160.9
w
S%/2-w
Llindlng
enerqy
(eV)
1570
Fig. 4. XPS spectra of Ti2p and S2p levels in the thin film TiO,,,S,,,. (A) Before mechanical erosion. (B) After mechanical erosion.
tahedral environment as in TiO, (rutile); that on the low-energy side may be assigned to titanium atoms with a [TiS,] octahedral environment as in TiS,. The origin of the intermediate peak will be discussed later. The S 2p peak was fitted into three components in order to have suitable values for half widths. They correspond to three different types of sulfur atoms; the most intense component, on the low-energy side (160.9 and 161.8 eV) could be attributed to S2- sulfur, analogous to
D. Gonbeau
86 Table 3 Ti2p and S2p binding
energies
ef al.
/
XPS sfudy of thin film of fjfaniu~ oxysurfides
in the thin film of TiO,,,S,,,
Before mechanical E, (ev) (FWHM 456.1 and 462.1 (1.55) (1.80) 457.3 and 463.4 (1.72) (1.4) 458.1 and 464.6 (1.48) (1.76) 160.9 and 161.8 (0.85) (0.80) 162.3 and 163.3 (0.80) (0.85) 163.4 and 164.3 (0.80) (0.80)
( full widths
erosion (ev))
at half maximum
in parentheses)
After mechanical Relative (W)
E (eV) (FWHM
20
456.1 and 461.9 (1.56) (1.80) 457.3 and 463.2 (1.46) (1.85) 458.6 and 464.6 (1.82) (1.52)
20 60
60 30
160.9 and 161.9 (0.85) (0.99) 162.4 and 161.9 (0.80) (0.89)
erosion (ev))
Relative
(%)
30 40 30
80 20
10
those present in TiS,, the component in intermediate position (162.3 and 163.3 eV) to Sfpairs, as in TiS,. The doublet with lowest intensity, on the high energy side (163.4 and 164.3 eV) is analogous to that of low intensity and not identified, appearing on the spectrum of “Cerac Ti&” (fig. 3). The 0 Is peak appears similar to the ones observed in TiO, and “Cerac TiS,” (main peak: 530.1 eV; shoulder: 531.9 eV). Following mechanical erosion, there was simply a change in the relative proportions of the ‘three types of titanium in the Ti2p peak (table 3). The component assigned to TiO, decreased, while the other two increased (fig. 4). In the S2p peak, the component situated toward high energies, whose origin could not be determined, disappeared and the doublet in intermediate position decreased. For the 01s peak a decrease of the high binding energy shoulder was observed. 3.2.2. XPS characterization of a thin film of TiOS All the species observed above were also detected in this composition (fig. 5, table 4). In comparison to the previous sample, there was merely a lower intensity of the component attributable to TiS, in the Ti 2p peak, which agrees with the fact that this film is richer in oxygen.
The S2p peak contained the three components previously observed with the thin film of TiO,,,S,_,, but in different relative proportions. The presence of an additional component toward high energies was also noted, corresponding to the non-deconvoluted broad band at around 168.5 eV. This component, assigned to sulfur atoms in the sulfate state, was systematically observed in oxygen rich (0 z 33%) compositions. In the different films analyzed, its proportion varied from 30 to 35% of the total quantity of sulfur present in the sample. Similar broad bands have also been observed by XPS in inorganic sulfides [5d,8]. They have been assigned to the existence of complexes situated at the surface of the sulfides. After the mechanical erosion of the thin film of TiOS, it was noted, as before, that the relative proportions of the three types of titanium were modified (fig. 5). The component associated with TiS, remained lower than that observed for the At the level of the S2p thin film of TiO,,,S,.,. peak, the two components situated toward high energies disappeared (the one around 168.5 eV and the doublet 163.5 and 164.5 eV) (fig. 5). It is to be noted that for compositions particularly rich in oxygen, mechanical erosion led to a decrease but not a disappearance of sulfur atoms in the
D. Gonbeauet al. / XPS studyof thinfilms of tiraniumoxysurfides
87
Table 4 Ti 2p and S 2p binding energies in the thin film TiOS (full widths at half maximum in parentheses) After mechanical erosion
Before mechanical erosion -% (W Ti2~,+,,2
S 2P33,12-1,2
(MHM
456.3 and 462.4 (1.68) (1.40) 457.2 and 463.3 (1.68) (1.40) 458.7 and 464.7 (1.45) (1.75) 161 and 162 (0.95) (0.85) 162.4 and 163.3 (0.88) (0.85) 163.5 and 164.5 (0.99) (0.92)
(eV)
Relative ( W)
E (ev) (FWHM (eV))
Relative (W)
11
456.3 and 462.2 (1.51) (1.81) 457.4 and 463.4 (1.48) (1.80) 458.6 and 464.5 (1.55) (1.76)
15
19 70
41 36
161.0 and 162 (0.90) (0.85) 162.5 and 163.4 (0.93) (1.0)
40 45
65 35
23
sulfate state. For the 01s peak the same observations as previously mentioned for the TiO,,,S,,, film have been made. 3.3. Discussion of the results obtained for titanium oxysulfides
The above results obtained with the two types of compounds have shown the existence of a type of titanium (Ti2p 1: 457.3 and 463.4 eV) which is very different from those encountered in all the reference compounds examined. Considering the results obtained with TiO, and TiS,, its closest neighbors are very probably oxygen atoms, since its Ti2p peak is located at a higher energy than that of the sulfides and a fortiori the reduced forms of these sulfur containing compounds. Examination of the 01s peak indicates an environment with oxygen ions O*- similar to those present in TiO,. When the S2p peak is examined, the existence of S2- sulfurs and Sipolysulfide groups is seen, the latter very probably associated with the new species of titanium. Even though XPS does not enable precise quantitative determinations to be carried out, analysis of the results obtained with thin layers (with reference to reference molecules) shows that: _ all the type S*- sulfur atoms cannot be associated only with titanium atoms with a TiS, environment;
- the “oxysulfide” form of titanium presents an environment in which sulfur ions (S*- and S,‘-) are present in a proportion practically twice to that of oxygen. For the first thin film analyzed, the stoichiometry determined by RBS, TiO,,,S,.,, showed a clear anionic deficit. In order to satisfy electric neutrality, the hypothesis of one titanium at oxidation level + 3 was suggested. However ESR study did not show any paramagnetic signal (Ti3’). The entity we have demonstrated by XPS is not a +3 titanium because of the binding energy associated with the Ti2p peak (457.3 and 463.4 eV) compared to that of Ti3+ with only an oxygen environment in Ti,O, (456.6 and 461.8 eV). Nevertheless, these observations do not rule out the presence of a small proportion of a reduced form with a similar environment (Si-, S*- and O*ions), for which the titanium binding energy could be of the same order as that of TiS,. The use of an analysis technique such as EXAFS, enabling the local order of a structure to be characterized (interatomic distances and nature of the closest neighbors) would be necessary in order to test the preceding hypotheses. Another interesting result is the presence of sulfate ions in the oxygen-rich compositions. They are obtained from the most highly hydrolyzed zone, since they are formed at the beginning of sputtering. The partial pressure formed by
D. Gonbeuu et al. / XPS study of thin films of titanium mysulfides
88
7
T12p
Binding
energy
(eV)
452
4. Conclusion For the different thin films of titanium oxysulfides the XPS results have shown; _ the existence of a new type of titanium characterized by a Ti2p binding energy intermediate between those of TiO, and TiS,; the same species is present in oxygen-rich and sulfur-rich compositions, but with different proportion; _ the closest neighbors of this “oxysulfide form” appear to be oxygen and sulfur atoms, pairs of S ions (S,“.-) and isolated S ions (S2-); the sulfur ions are present in a proportion practically twice that of oxygen; - for the oxygen-rich compositions the presence of sulfate ions has always been detected.
312-M 162.4
References [l] G. Meunier, R. Dormoy and A. Levasseur,
1 S2p3/*-M
f5i Binding
energy
feV)
Fig. 5. XPS spectra of Ti2p and S2p levels in the thin film TiOS. (A) Before mechanical erosion. (B) After mechanical erosion.
sputtered oxygen atoms must thus be sufficient to oxidize the ejected atoms or molecules of sulfur. In these conditions, the situation is as if we sputtered a sulfide in a reactive oxygen atmosphere. For longer sputtering times, it should no longer be possible to form sulfates, since the partial pressure of oxygen in the plasma would be too low. Their presence was thus not detected in sulfur-rich compositions.
Mater. Sci. Eng. B 3 (1989) 19. [2] G. Meunier, R. Dormoy and A. Levasseur, Thin Solid Films, submitted. 131 (a) H.F. Franzen, M.X. Umana, J.R. McCreary and R.J. Thorn, J. Solid State Chem. 18 (1976) 363; (b) C.N. Sayers and N.R. Armstrong, Surf. Sci. 77 (1978) 301; (c) C.N.R. Rao, D.D. Sarma, S. Vasudevan and M.S. Hegde, Proc. R. Sot. London A 367 (1979) 239; (d) M.E. Levin, M. Salmeron, A.T. Bell and G.A. Somorjai, Surf. Sci. 195 (1988) 429. [4] (a) M. Murata and K. Wakino, J. Electron Spectrosc. Relat. Phenom. 6 (1975) 459; (b) B. Wallbank, C.E. Johnson and LG. Main, J. Phys. C 6 (1973) L340; (c) I. Ikemoto, K. Ishii, H. Kuroda and J.M. Thomas, Chem. Phys. Lett. 28 (1974) 55; (d) KS. Kim and N. Winograd, Chem. Phys. Lett. 31 (1975) 312; (e) S.K. Sen, J. Riga and J. Verbist, Chem. Phys. Lett. 39 (1976) 560; (f) C. Ocal and S. Ferrer, Surf Sci 191 (1987) 147. [S] (a) G.K. Wertheim, F.J. Di Salvo and D.N.E. Buchanan, Solid State Commun. 13 (1973) 1225; (b) H.F. Franzen, M.X. Umana, J.R. McCreary and R.J. Thorn, J. Solid State Chem. 18 (1976) 363; (c) J. Gopalakrishnan, T. Murugesan, M.S. Hedge and C.N.R. Rao, J. Phys. C (Solid State Phys.) 12 (1979) 5255; (d) D. Lichtman, J.H. Craig, Jr., V. Sailer and M. Drinkwine, Appl. Surf. Sci. 7 (1981) 325; (e) K. Endo, H. Ihara. K. Watanabe and S.I. Gonda. J. Solid State Chem. 44 (1982) 268;
D. Gonbeuu et al. / XPS si&y of thin firms of firanium oxysu~~de~ (f) K. Endo, H. Ihara, K. Watanabe and S.I. Gonda, J. Solid State Chem. 39 (1981) 215; (g) A. Fujimori, S. Suga, H. Negishi and M. Inoue, Phys. Rev. B 38 (1988) 3676. [6] (a) C.R.H. Friend, D. Jerome, W.Y. Liang, J.C. Mikkelsen and A.D. Yoffe, J. Phys. C 10 (1977) L705; (b) R.B. Murray, R.A. Bromley and A.D. Yoffe, J. Phys. C 5 (1972) 746; (c) J.C. Phillips, Phys. Rev. Lett. 28 (1972) 1196.
89
171 S.O. Saied, J.L. Sullivan, T. Choudhury and C.G. Pearce, Vacuum 38 (1988) 917. [S] (a) AS. Manocha and R.L. Park, Appl. Surf. Sci. 1 (1977) 129; (b) D. Brion, Appl. Surf. Sci. 5 (1980) 133.
81
XPS study of thin films of titanium oxysulfides D. Gonbeau, taboratoire 64000 Pm,
C. Guimon,
de Pltysico-Chin& France
A. Levasseur,
G. Pfister-Guillouzo
Mol&uluire
G. Meunier
2 January
1991; accepted
URA 4?4), Uniuersite de Pau et des Pays de I’Adour, 2, Avenue du PrPsident Angot,
and R. Dormoy
Laboratoire de Chimie du Sofide du CNRS, 33405 Talence Cedex, France Received
(CNRS
Ecole Nationale
for publication
Supkieure
18 March
de Chimie et Physique de Bordeaux,
iJniversii6 de Bordeaux
I,
1991
In this study thin films of titanium oxysulfides which could be used as positive electrode materials in microbatteries were analysed by XPS. In light of the binding energies obtained for reference compounds (different titanium oxides and sulfides) the results have shown the existence of a new type of titanium. It presents an environment with gpairs, S2- and O’- ions, this latter in a proportion half that of sulfur ions.
1. Introduction New amorphous titanium oxysulfides obtained in the form of thin films have been recently described [1,2]. They can be used as positive electrode materials in microbatteries [l]. These thin films are obtained by RF sputtering using a TiS_ target which contains in fact a few percent of oxygen They have been characterized by Auger electron spectroscopy (AES), secondary ion mass spectroscopy (SIMS), X-ray microprobe and Rutherford backscattering (RBS) [1,2]. Because of the surface hydrolysis of the target the composition of the thin films obtained changes with the utilization duration of the target which is in fact the sputtering time. The formula of titanium oxysulfide can be written TiO,S,. Whereas the stoichiometry (x + y) is equal to 2 at the beginning of the utilization of the target; it shifts to 1.75 for the high sulfur-content thin films. This anionic deficit can be explained by consideration of the Ti-S and Ti-0 binding energy. Because of the lower of the energy Ti-S bond com-
pared to Ti-0, 0039-6028/91/$03.50
sulphur atoms or molecules are 0 1991 - Elsevier Science Publishers
mainly present in the plasma and preferentially eliminated by the pumping system. This phenomenon does not occur when the sulfur content is low, i.e., at the beginning of the sputtering process when the oxygen content is high. In order to explain the oxydo-reduction reactions occurring when these new materials are used as positive electrode in microbatteries it was necessary first to identify the different ions present in these tita~um oxysulfides. We report in this work the analysis based on XPS, an appropriate technique for this approach. A further study will be devoted to the positive electrode material after use in the batteries (intercalated compounds).
2. Experimental The XPS analysis were performed with a Surface Science Inst~ments spectrometer (model 301) using a focused (diameter of the irradiated area = 300 pm) monochromatized Al KLY radiation (1486.6 eV) and interfaced to a Hewlett
B.V. (North-Holland)
D. Gonbeau et al. / XPS study of thin films of titanium oxysulfides
82
Packard 9000 microcomputer. The residual pressure inside the analysis chamber was in the 5 X lo-’ Pa range. For the titanium oxides powdered samples were pressed as pellets. The targets have been obtained by pressing powder of “Cerac TiS,” inside a glove box filled with dried argon and containing less than 5 ppm of H,O. The thin layers obtained by RF sputtering using these targets have been deposited on silicon substrates. All the samples were fixed on the sample holders in a glove box filled with continuously purified nitrogen (the 0, and H,O levels were below 3 and 5 ppm respectively) and directly attached to the introduction chamber of the spectrometer. For the titanium oxides the charging effects were minimized with a low-energy flood gun. The calibration of the spectra was done for the titanium oxides with the Cls line (284.6 eV) from the carbon contamination layer. The other compounds were sufficiently good conductors to preclude charging effects. For the etching process an Ar+ ion gun was used (current intensity 10 mA, voltage 4 kV). For the mechanical erosions the surface of the samples was carefully scraped with a steel blade in the glove box then directly transferred to the spectrometer. The reproducibility of the measurements was checked by different mechanical scrapings on each sample and further XPS analysis. A carbon contamination of about 25% (atomic percentage) was found. In the peak fitting routine used the lines are defined according to their centroid position, halfwidth, shape (using a combination of Gaussian (80%) and Lorentzian (20%) distributions). These
Table 1 Ti2p binding half maximum
energies in the titanium in parentheses) .% (ev) (FWHM
TiO z
458.1 and 464.4
Ti ,o,
(1.5) (2.2) 456.6 and 461.8
TiO
(1.4) (2) 454.4 and 460.0 (1.6)
(1.8)
oxides
(ev))
(full widths
at
parameters are then varied to obtain a minimum in the error sum of squares between the values for the experimental curve and the combination of bands. However it is possible to constrain certain parameters within set limits; thus for the analysis of thin films we have chosen these limits (for area ratio, spin-orbit splitting and 2P,,,-2P,,z7 full width at half maximum) in relation with the mean results obtained for the reference compounds. A non linear background was used in all the cases.
3. Results and discussion A preliminary step before the analysis of XPS spectra has been the characterization of several reference compounds with well-defined structure (titanium oxides and sulfides). Published values for the Ti2p peak of titanium oxides are relatively dispersed [3], except for TiO, [3b,3d,4]. Studies of titanium sulfides are rare and most often limited to valence zones [5]. Since SIMS and Auger spectroscopy studies have shown that the composition of titanium oxysulfide films changes from the surface to the core (slight hydrolysis occurs when thin films are moved outside the sputtering machine) it was also of interest to analyse the compounds after elimination of the superficial contamination. 3.1. Results of analysis of the reference compounds 3.1.1. Titanium oxides Three standard samples of titanium oxidized at different degrees of oxidation were analyzed TiO,, Ti,O, and TiO. The results obtained for the Ti2p peak are listed in table 1 and agree with published values for TiO, [3b,3d,4]. Concerning the 01s peak of this last compound the main peak is centered at 530 eV and exhibits a high binding energy shoulder at 531.8 eV, which could be assigned to hydroxyl groups [3d]. 3.1.2. Titanium sulfides Three samples were analyzed: a TiS, monocrystal, a “Cerac TiS,” pellet (used to prepare sputtering target) and a TiS, pellet (for the char-
83
D. Gonbeau et al. / XPS study of thin films of titanium oxysulfides
TiS2 monocrystal
TiS2 monocrystal
056
,"
;',,,, i ,, 'l, \ --_A__ 160.8
“Cerac TiS2"
"Cerac TiS2"
4 162
163.2
/
,'\
! 1 /+.A) I,
,64&ti'f _
/
2 ,'',, ., 4.
I..___I'
\ '\ '\
.,
162.3 455.9
Ti S3
Ti S3
r\
!\ I I
BlndlnQ
energy
(CV) 451 Blndlnp energy (cV) 157.0
Ti%/2-1/2 Fig. 1. Ti2p,,,_,,,
s p ectra in the titanium
5!P3/2-l/2
sulfides.
Fig. 2. S2p,,,_,,,
acterization of Sip pairs). The results obtained for the Ti 2p and S 2p peaks are listed in table 2 and the corresponding spectra, respectively, in figs. 1 and 2.
Table 2 Ti 2p and S2p binding
energies
in the titanium
Et, (eV) (FWHM
Ti2~,,~-,,~
S2P,,,-I,,
sulfides
in the titanium
sulfides.
To our knowledge, only one report of the binding energies associated with the core peak of TiS, exists [5g]. In the case of the monocrystal, very
(full widths
at half maximum
in parentheses)
(eV))
TiS, monocrystal
“Cerac
456 and 462 (1.45) (1.68)
456 and 462 (2.02) (1.53) 458.6 and 464.5 (1.46) (2.04)
160.7 and 161.8 (0.71) (0.87)
spectra
TiS,”
160.8 and 162 (0.87) (0.9) 163.2 and 164.2 (0.8)
(1)
TiS, 455.9 and 462 (1.4) (1.8) 458.8 and 464.6 (1.6)
(1.9)
160.9 and 161.9 (0.7) (0.8) 162.3 and 163.4 (0.75) (0.8)
D. Gonbeau et al. / XPS study of thin films of titanium oxysulfides
84
slight contamination by oxygen has been observed (- 2.5% in atomic percentage). Binding energies associated with the 2~~,~_i,~ doublets of titanium and sulfur could thus be estabhshed with precision (table 2). We may also point out that a spectrum equivalent to that previously reported by Wertheim et al. [5a] was obtained in the valence zone (O-30 eV). A low-intensity band close to the Fermi level was also observed, which could be correlated with the high conductivity of this compound. The origin _
F
/
I
E3
T12p3/2-w
45213
BindIn@ energy (eV)
of this band remains to be defined. For the monocrystal, it is difficult to associate this phenomenon with an excess of tit~ium, as proposed by some authors [6], since the determination of the S/Ti ratio (2.3) based on ESCA peaks arose more from an excess of sulfur. In the case of “Cerac TiS,” pellets, there was contamination by TiO,. The Ti2p peak was present in the form of two well differentiated components. Based on their energy position, they correspond to TiS, and TiO, (fig. 1). In addition to the main component of the S2p peak, there was a second component with lower intensity (fig. 2). It was located at higher energy and corresponds to another type of sulfur atom, which could not be identified. The 01s peak is similar to the one observed for TiOZ (main peak: 530.1 eV, shoulder: 531.8 eV). In the case of TiS,, oxygen contamination led to the observation of two components in the Ti 2p peak (fig. 1). The doublet with the lowest intensity, on the high-energy side, was associated with TiOZ and the major component, on the low-energy side, with TiS,. The S2p peak appeared as two partially overlapping doublets attributed to the Sip pairs (on the high-energy side) and to a sulfur atom with a higher negative charge S2- (on the low energy side). These results are in agreement with the work of Endo et al. [5e].
160.8
A
...j
B
t
%3/2-M
Binding energy (eV)
I!
0
Fig. 3. “Cerac TiS,“. (A) Before mechanical. erosion. (B) After mechanical erosion.
3.1.3. Effect of ion etching on samples of TiO, and TiS, Ion etching leads to the elimination of superficial contamination but it may modify the atomic environment (depletion of the element correlated with a highest sputtering yield), as shown for TiO, [3b,7]. Since this phenomenon could occur under our conditions of bombardment, in the case of the materials studied, measurements were carried out on a TiO, pellet and the TiS, monocrystal at different times of sputtering. The results obtained with TiO, clearly show a reduction in Ti,O, and TiO but under our conditions of bombardment a full reduction to Ti metal had not occured. Similar results were obtained with TiS,, the preferential sputtering of sulfur probably inducing the observed reduction phenomenon. After 30 s of
D. Gonbeau et al. / XPS study of thin films of riranium oxysulfides
85
etching, two peaks appeared (Ti2~s,~_i,~ (eV): 454.6 and 460.5; 453.4 and 459.1), with the low energy peak increasing in intensity after 5 min of etching. The absence of reference compounds and of reliable published data precluded the more precise identification of these peaks. In the case of this compound, it should be noted that the reduction phenomenon was particularly rapid. In order to partially eliminate superficial contaminations, mechanical erosion (by scraping) was carried out, since ion etching modified the environment of the atoms in the samples to be analyzed. 3.1.4. Influence of mechanical erosion The results obtained with the “Cerac TiS,” sample before and after mechanical erosion (fig. 3) confirm the results obtained by Auger spectrometry, which showed that the surface was the zone undergoing the most hydrolysis. It should nevertheless be noted that this mechanical erosion involved the outermost layers and did not lead to bulk stoichiometry determined by Rutherford backscattering (RBS) [1,2]. It is also noteworthy that the decrease in the Ti2p peak associated with TiO, was accompanied by the practically complete disappearance of sulfur located toward high energies. It was not possible to define the origin of this latter peak, corresponding to a more “positive” type of sulfur than those encountered in TiS, and the TiS, monocrystal. 3.2. Analysis of the results of thin films of amorphous titanium oxysu[fides Among the different samples analyzed, the results involve two particularly representative species. Their composition, TiO,.,S,., and TiOS was determined by RBS. 3.2.1.
XI’S
characterization
of
a
thin
of film
Ti%.& In light of the results obtained for the reference compounds (peak widths, spin-orbit splittings and area ratios). The Ti 2p peak was broken down into three components (fig. 4 and table 3). The most intense component, on the high-energy side, could be assigned to titanium atoms with a [TiO,] oc-
Ti2P3/2-W
Binding
enerqy
(CV)
452.0
160.9
w
S%/2-w
Llindlng
enerqy
(eV)
1570
Fig. 4. XPS spectra of Ti2p and S2p levels in the thin film TiO,,,S,,,. (A) Before mechanical erosion. (B) After mechanical erosion.
tahedral environment as in TiO, (rutile); that on the low-energy side may be assigned to titanium atoms with a [TiS,] octahedral environment as in TiS,. The origin of the intermediate peak will be discussed later. The S 2p peak was fitted into three components in order to have suitable values for half widths. They correspond to three different types of sulfur atoms; the most intense component, on the low-energy side (160.9 and 161.8 eV) could be attributed to S2- sulfur, analogous to
D. Gonbeau
86 Table 3 Ti2p and S2p binding
energies
ef al.
/
XPS sfudy of thin film of fjfaniu~ oxysurfides
in the thin film of TiO,,,S,,,
Before mechanical E, (ev) (FWHM 456.1 and 462.1 (1.55) (1.80) 457.3 and 463.4 (1.72) (1.4) 458.1 and 464.6 (1.48) (1.76) 160.9 and 161.8 (0.85) (0.80) 162.3 and 163.3 (0.80) (0.85) 163.4 and 164.3 (0.80) (0.80)
( full widths
erosion (ev))
at half maximum
in parentheses)
After mechanical Relative (W)
E (eV) (FWHM
20
456.1 and 461.9 (1.56) (1.80) 457.3 and 463.2 (1.46) (1.85) 458.6 and 464.6 (1.82) (1.52)
20 60
60 30
160.9 and 161.9 (0.85) (0.99) 162.4 and 161.9 (0.80) (0.89)
erosion (ev))
Relative
(%)
30 40 30
80 20
10
those present in TiS,, the component in intermediate position (162.3 and 163.3 eV) to Sfpairs, as in TiS,. The doublet with lowest intensity, on the high energy side (163.4 and 164.3 eV) is analogous to that of low intensity and not identified, appearing on the spectrum of “Cerac Ti&” (fig. 3). The 0 Is peak appears similar to the ones observed in TiO, and “Cerac TiS,” (main peak: 530.1 eV; shoulder: 531.9 eV). Following mechanical erosion, there was simply a change in the relative proportions of the ‘three types of titanium in the Ti2p peak (table 3). The component assigned to TiO, decreased, while the other two increased (fig. 4). In the S2p peak, the component situated toward high energies, whose origin could not be determined, disappeared and the doublet in intermediate position decreased. For the 01s peak a decrease of the high binding energy shoulder was observed. 3.2.2. XPS characterization of a thin film of TiOS All the species observed above were also detected in this composition (fig. 5, table 4). In comparison to the previous sample, there was merely a lower intensity of the component attributable to TiS, in the Ti 2p peak, which agrees with the fact that this film is richer in oxygen.
The S2p peak contained the three components previously observed with the thin film of TiO,,,S,_,, but in different relative proportions. The presence of an additional component toward high energies was also noted, corresponding to the non-deconvoluted broad band at around 168.5 eV. This component, assigned to sulfur atoms in the sulfate state, was systematically observed in oxygen rich (0 z 33%) compositions. In the different films analyzed, its proportion varied from 30 to 35% of the total quantity of sulfur present in the sample. Similar broad bands have also been observed by XPS in inorganic sulfides [5d,8]. They have been assigned to the existence of complexes situated at the surface of the sulfides. After the mechanical erosion of the thin film of TiOS, it was noted, as before, that the relative proportions of the three types of titanium were modified (fig. 5). The component associated with TiS, remained lower than that observed for the At the level of the S2p thin film of TiO,,,S,.,. peak, the two components situated toward high energies disappeared (the one around 168.5 eV and the doublet 163.5 and 164.5 eV) (fig. 5). It is to be noted that for compositions particularly rich in oxygen, mechanical erosion led to a decrease but not a disappearance of sulfur atoms in the
D. Gonbeauet al. / XPS studyof thinfilms of tiraniumoxysurfides
87
Table 4 Ti 2p and S 2p binding energies in the thin film TiOS (full widths at half maximum in parentheses) After mechanical erosion
Before mechanical erosion -% (W Ti2~,+,,2
S 2P33,12-1,2
(MHM
456.3 and 462.4 (1.68) (1.40) 457.2 and 463.3 (1.68) (1.40) 458.7 and 464.7 (1.45) (1.75) 161 and 162 (0.95) (0.85) 162.4 and 163.3 (0.88) (0.85) 163.5 and 164.5 (0.99) (0.92)
(eV)
Relative ( W)
E (ev) (FWHM (eV))
Relative (W)
11
456.3 and 462.2 (1.51) (1.81) 457.4 and 463.4 (1.48) (1.80) 458.6 and 464.5 (1.55) (1.76)
15
19 70
41 36
161.0 and 162 (0.90) (0.85) 162.5 and 163.4 (0.93) (1.0)
40 45
65 35
23
sulfate state. For the 01s peak the same observations as previously mentioned for the TiO,,,S,,, film have been made. 3.3. Discussion of the results obtained for titanium oxysulfides
The above results obtained with the two types of compounds have shown the existence of a type of titanium (Ti2p 1: 457.3 and 463.4 eV) which is very different from those encountered in all the reference compounds examined. Considering the results obtained with TiO, and TiS,, its closest neighbors are very probably oxygen atoms, since its Ti2p peak is located at a higher energy than that of the sulfides and a fortiori the reduced forms of these sulfur containing compounds. Examination of the 01s peak indicates an environment with oxygen ions O*- similar to those present in TiO,. When the S2p peak is examined, the existence of S2- sulfurs and Sipolysulfide groups is seen, the latter very probably associated with the new species of titanium. Even though XPS does not enable precise quantitative determinations to be carried out, analysis of the results obtained with thin layers (with reference to reference molecules) shows that: _ all the type S*- sulfur atoms cannot be associated only with titanium atoms with a TiS, environment;
- the “oxysulfide” form of titanium presents an environment in which sulfur ions (S*- and S,‘-) are present in a proportion practically twice to that of oxygen. For the first thin film analyzed, the stoichiometry determined by RBS, TiO,,,S,.,, showed a clear anionic deficit. In order to satisfy electric neutrality, the hypothesis of one titanium at oxidation level + 3 was suggested. However ESR study did not show any paramagnetic signal (Ti3’). The entity we have demonstrated by XPS is not a +3 titanium because of the binding energy associated with the Ti2p peak (457.3 and 463.4 eV) compared to that of Ti3+ with only an oxygen environment in Ti,O, (456.6 and 461.8 eV). Nevertheless, these observations do not rule out the presence of a small proportion of a reduced form with a similar environment (Si-, S*- and O*ions), for which the titanium binding energy could be of the same order as that of TiS,. The use of an analysis technique such as EXAFS, enabling the local order of a structure to be characterized (interatomic distances and nature of the closest neighbors) would be necessary in order to test the preceding hypotheses. Another interesting result is the presence of sulfate ions in the oxygen-rich compositions. They are obtained from the most highly hydrolyzed zone, since they are formed at the beginning of sputtering. The partial pressure formed by
D. Gonbeuu et al. / XPS study of thin films of titanium mysulfides
88
7
T12p
Binding
energy
(eV)
452
4. Conclusion For the different thin films of titanium oxysulfides the XPS results have shown; _ the existence of a new type of titanium characterized by a Ti2p binding energy intermediate between those of TiO, and TiS,; the same species is present in oxygen-rich and sulfur-rich compositions, but with different proportion; _ the closest neighbors of this “oxysulfide form” appear to be oxygen and sulfur atoms, pairs of S ions (S,“.-) and isolated S ions (S2-); the sulfur ions are present in a proportion practically twice that of oxygen; - for the oxygen-rich compositions the presence of sulfate ions has always been detected.
312-M 162.4
References [l] G. Meunier, R. Dormoy and A. Levasseur,
1 S2p3/*-M
f5i Binding
energy
feV)
Fig. 5. XPS spectra of Ti2p and S2p levels in the thin film TiOS. (A) Before mechanical erosion. (B) After mechanical erosion.
sputtered oxygen atoms must thus be sufficient to oxidize the ejected atoms or molecules of sulfur. In these conditions, the situation is as if we sputtered a sulfide in a reactive oxygen atmosphere. For longer sputtering times, it should no longer be possible to form sulfates, since the partial pressure of oxygen in the plasma would be too low. Their presence was thus not detected in sulfur-rich compositions.
Mater. Sci. Eng. B 3 (1989) 19. [2] G. Meunier, R. Dormoy and A. Levasseur, Thin Solid Films, submitted. 131 (a) H.F. Franzen, M.X. Umana, J.R. McCreary and R.J. Thorn, J. Solid State Chem. 18 (1976) 363; (b) C.N. Sayers and N.R. Armstrong, Surf. Sci. 77 (1978) 301; (c) C.N.R. Rao, D.D. Sarma, S. Vasudevan and M.S. Hegde, Proc. R. Sot. London A 367 (1979) 239; (d) M.E. Levin, M. Salmeron, A.T. Bell and G.A. Somorjai, Surf. Sci. 195 (1988) 429. [4] (a) M. Murata and K. Wakino, J. Electron Spectrosc. Relat. Phenom. 6 (1975) 459; (b) B. Wallbank, C.E. Johnson and LG. Main, J. Phys. C 6 (1973) L340; (c) I. Ikemoto, K. Ishii, H. Kuroda and J.M. Thomas, Chem. Phys. Lett. 28 (1974) 55; (d) KS. Kim and N. Winograd, Chem. Phys. Lett. 31 (1975) 312; (e) S.K. Sen, J. Riga and J. Verbist, Chem. Phys. Lett. 39 (1976) 560; (f) C. Ocal and S. Ferrer, Surf Sci 191 (1987) 147. [S] (a) G.K. Wertheim, F.J. Di Salvo and D.N.E. Buchanan, Solid State Commun. 13 (1973) 1225; (b) H.F. Franzen, M.X. Umana, J.R. McCreary and R.J. Thorn, J. Solid State Chem. 18 (1976) 363; (c) J. Gopalakrishnan, T. Murugesan, M.S. Hedge and C.N.R. Rao, J. Phys. C (Solid State Phys.) 12 (1979) 5255; (d) D. Lichtman, J.H. Craig, Jr., V. Sailer and M. Drinkwine, Appl. Surf. Sci. 7 (1981) 325; (e) K. Endo, H. Ihara. K. Watanabe and S.I. Gonda. J. Solid State Chem. 44 (1982) 268;
D. Gonbeuu et al. / XPS si&y of thin firms of firanium oxysu~~de~ (f) K. Endo, H. Ihara, K. Watanabe and S.I. Gonda, J. Solid State Chem. 39 (1981) 215; (g) A. Fujimori, S. Suga, H. Negishi and M. Inoue, Phys. Rev. B 38 (1988) 3676. [6] (a) C.R.H. Friend, D. Jerome, W.Y. Liang, J.C. Mikkelsen and A.D. Yoffe, J. Phys. C 10 (1977) L705; (b) R.B. Murray, R.A. Bromley and A.D. Yoffe, J. Phys. C 5 (1972) 746; (c) J.C. Phillips, Phys. Rev. Lett. 28 (1972) 1196.
89
171 S.O. Saied, J.L. Sullivan, T. Choudhury and C.G. Pearce, Vacuum 38 (1988) 917. [S] (a) AS. Manocha and R.L. Park, Appl. Surf. Sci. 1 (1977) 129; (b) D. Brion, Appl. Surf. Sci. 5 (1980) 133.