An els study of the oxidation of polycrystalline Ta and Nb at room temperature

Journal of Electron Spectroscopy and Related Phenomena, 34 (1984) 149-160 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

AN ELS STUDY OF THE OXIDATION Nb AT ROOM TEMPERATURE

OF POLYCRYSTALLINE

Ta AND

J.M. SANZ* and S. HOFMANN Max-Planck-Institut fiir Metallforschung, 92, D-7000 Stuttgart 1 (W. Germany)

Znstitut fib Werkstoffwissenschaften,

Seestrasse

(Received 16 November 1983)

ABSTRACT ELS has been used to characterize the initial steps in the oxidation of polycrystalline Ta and Nb in an oxygen atmosphere at low pressures (- 1.3 x low6 Pa) and room temperature. The results are interpreted in terms of transitions to unoccupied states above EF and plasmon losses, and are discussed in the light of previous studies. The results show important changes in the valence band. The 0 2s and 0 2p bands increase in intensity during oxidation.

INTRODUCTION

Electron energy-loss spectroscopy (ELS) gives useful information concerning the electronic properties of the material under study. This information can be used to identify chemical states [ 11. In the reflection mode, ELS features can be observed on the low-energy side near the elastic peak using any conventional AES spectrometer. Despite this easy detectability, ELS features have not been taken into account systematically in the past. Only recently has ELS become a useful tool for surface analysis as a complementary technique to AES, XPS or valence-band spectroscopies [2--51. Whereas the ELS spectra of pure Ta and Nb have been studied by several authors [6-g], no ELS study of the oxidation of these metals is known to the present authors except for a brief discussion given by Rieder [9] for Nb. This is somewhat surprising, considering the large number of studies of the oxidation of Ta and Nb reported in recent years, using various techniques

[lOI* *Permanent address: Departamento Fisica Aplicada, Faculdad de Ciencias C-XII, Universidad Autonoma de Madrid, Madrid-34, Spain. 0368-2048/84/$03.00

0 1984 Elsevier Science Publishers B.V.

150

The main purpose of this work is to present and interpret the characteristic variations of ELS spectra (obtained in the reflection mode) during the initial steps in the oxidation of polycrystalline Ta and Nb up to - 23 L exposure to O2 (1 L = 1 x 10m6Torr s) at room temperature. Accompanying AES measurements and comparisons with UPS data, as well as bandstructure and electron density-of-states (DOS) calculations, have proved useful for a consistent interpretation of ELS results [ 11.

EXPERIMENTAL

The samples used in this work were polycrystalline sheets of Ta and Nb, 200pm thick and 5 x 10 mm2 in area. The original foils were annealed, decarburized (2600 K at PO, 2 10L4 Pa) and deoxidized (2700 K at Ptot < 1(T6 Pa). After mounting the samples in a differential turbo-molecular and ion-getter-pumped UHV system, a clean surface was obtained by Ar+ ion sputtering at 3 keV. The composition of the surface was checked with AES. At an oxygen pressure of 1.3 x 10v6 Pa and at room temperature, analysis of the surface was performed after various oxygen exposures in a cumulative fashion until an exposure of 23 L was reached. AES and ELS data were obtained in the derivative mode using a doublepass cylindrical-mirror analyzer (CMA) (PHI 15-255 GAR, Physical Electronics Division) with its axis at 30” to the normal of the sample surface. For AES a primary energy of 2 keV and a modulation voltage of 3 V peakto-peak were used. ELS was performed using primary electrons at 0.5 keV and modulation voltages of 1 or 0.5 V peak-to-peak. In both cases the beam current was maintained below 5pA. Although the spectrometer was run at 0.5% energy resolution (i.e., 2.5 eV), loss features at AE < 5 eV were not resolved. The energy positions of the loss peaks were determined more precisely from the negative derivative of the measured spectra (--N”(E)), which was obtained numerically from the raw data. The measured signal at the output of the CMA is proportional to EW(E)/dE + N(E), but it has been shown [ll] that for small modulation voltages it is proportional to dN(E)/dE. Accordingly, we refer to the spectra as N’(E) or N”(E).

RESULTS

Figures 1 and 2 show ELS spectra near the elastic peak at room temperature and PO, z 1.3 x 10s6 Pa as a function of oxygen exposure for Ta and Nb, respectively. The Auger peaks of Ta, Nb, C and 0 at the various exposures indicate the respective chemical states at the surface of the sample. The small but detectable contamination of carbon in both figures is probably due

151

Fig. I, Electron energy-loss and AES spectra (N’(E)) for Ta as a function of oxygen emosure: (A) 0 L; (B) 0.8 1;; (C) 4 L;(D) 23 L.

I

C e B A A

Fig, 2. Electron energy-loss and AES spectra exposure: (A) OL;(B) O&L;(C) BL;(D) 23L.

(IV’(E)) for Nb as a function

of oxygen

152

to residual gas. It is important to note that the initial surface (curve A) is a highly reactive, sputtered surface. Although for curve B (0.8 L) the carbon coverage is almost one-third that of oxygen, it remains constant whereas the oxygen coverage increases (curves C, D). Therefore, it may be concluded that the observed changes in the ELS spectra are due to oxygen coverage. In Figs. 1 and 2 the signals are given in the derivative mode (N’(E)) and plotted versus the loss energy AE with respect to the elastic peak in the range 5 eV =GAE < 60 eV, so that possible features related to Ta 5p and Nb 4p can be observed. The different spectra labelled A-D correspond to exposures of 0, 0.8, 4 and 23 L, respectively. From XPS and AES measurements [lo] it is known that 10 L is a “saturation” exposure characterized by formation of the pentoxide for both metals. Therefore, only small changes are expected for higher exposures. Figures 3 and 4 show the negative derivatives of the measured signals, i.e., --N”(E). These were obtained numerically from the “as-measured” spectra of Figs. 1 and 2, respectively. Smoothing of the data was carried out as described by Proctor [12] both before and after obtaining the derivative, using the Lagrange formula for equally spaced abscissae and using five points 1131. Whereas N’(E) locates structures in N(E), maxima of N(E) are observed as peaks in --N”(E). As pointed out by Henrich [14], higher derivatives can

1

50

I

40

I

I

30 20 AEleV)

I

I

10

0

I

50

I

40

I

30

I

20

I

10

I

0

AE (eV1

Fig. 3. Electron energy-loss spectra (-N"(E))for Ta as a function of oxygen exposure. Arrows in (A) indicate the values of Weaver et al. [15, 161 for surface (s) and bulk (b) plasmons. Fig. 4. Electron energy-loss spectra (- N”(E)) for Nb as a function of oxygen exposure. Arrows in (A) indicate the values of Weaver et al. [15, 161 for surface (s) and bulk (b) plasmons.

153 TABLE 1 CHARACTERISTIC

ELECTRON ENERGY LOSSES FOR Ta AND Tao,

Ta Energy-loss peaks (eV)

Reference Apholte and Ulmer [ 7 ] Lynch and Swan [ 81 Schubert and Wolf [6] This work (N”(E))

12.4 4

9.4 8 9

12.5 (12.5)

18

22.2 20 20 20

27 28

33 33

39.7 38.7 31.5 38

48.8 46.5 48

Tao, Exposure (L) 0.8

4 23

Energy-loss peaks (eV) 7.5 7.5 8

12.5 12.5 13

27.5 28 28

20 21 21.5

38.5 39 39

48 48-50 48-50

produce artificial features, and so N’(E) must be used as a check for peaks observed in N”(E). Tables 1 and 2 show the energy positions of the loss features measured in this work for the pure metals and for exposures of oxygen up to 23 L for Ta and Nb, respectively; these features correspond to the maxima of the peaks in -N”(E). Values for pure Ta and Nb as reported by other workers are also TABLE 2 CHARACTERISTIC Nb

ELECTRON ENERGY LOSSES FOR Nb AND NbO,

Reference

Energy-loss peaks (eV)

Apholte and Ulmer [ 7 ] Lynch and Swan [ 81 Rieder [ 9 ] Schubert and Wolf [6] This work (NN(E))

11.3 9.5 11 9.6 9.5

21 19.6 20 20.8 20

18.3

43 32.4 45 27

30.3 31.5

38 40

NbO, Exposure (L)

Energy-loss peaks (eV)

0.8 4 23

9

NbzOs 191

11 12.5

21 21.5 22

14.5

25

27 27 27.6

32.5 33

40 42.5 44 46’

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given for comparison. The results of Rieder [ 91 for Nbz05 have also been included. The small discrepancies observed in Tables 1 and 2 between the various authors are due at least partly to the use of differing measurement modes: N(E), N’(E) or --N”(E). Schubert and Wolf [6] observed a loss at 18 eV for Ta which is not resolved in our broad main loss peak at -20 eV. The peak observed by Schubert and Wolf [6] at 12.5 eV is probably due to oxygen contamination, as discussed below. The 4eV peak measured by Schubert and Wolf [6] could not be resolved in our experiments. During the oxidation of both metals, pronounced changes in the shape of the spectra (B, C, D), as well as small energy shifts, can be observed clearly (Figs. l-4) on comparison with respect to the pure-metal spectra (A). Changes in the shape of the Ta and Nb Auger lines are also observed during oxidation, as shown in Figs. 1 and 2. These changes and the formation of suboxides have been discussed in an earlier paper [lo] . For Ta (Fig. 3), the feature at - 9 eV splits into two peaks at 12.5 and 7.5 eV after an exposure of 0.8 L (curve B). At higher exposures the intensity of the 12.5 eV peak increases. With increasing oxygen exposure the broad peak at 20 eV related to the main bulk plasmon loss shows a small energy shift of - 1.5eV (curve D) and becomes sharper for the oxidized surface compared to the pure metal. The structures observed in the range 60 eV > AE > 25 eV show pronounced variations which are not easy to follow because of the broadness of the different peaks. It is interesting to note the increase in the intensity of the peak at 27 eV, the disappearance of the 33.5 eV peak, and the broadening of the 39 and 48 eV loss peaks as the oxygen exposure increases (Fig. 3). For Nb (Fig. 4), the observed changes in the -N”(E) spectra are very similar to those for Ta. Pure Nb (curve A) shows peaks at 9.5, 20, 27, 31.5 and 4OeV, which undergo various changes as the exposure to oxygen increases. After an oxygen exposure of only 0.8 L, the most pronounced effect is the change in the peak at 9 eV, whose intensity and sharpness are affected considerably. The main bulk plasmon loss at 20 eV becomes smaller and sharper. At higher exposures a new feature appears at 12.5 eV. After 20 L exposure the main plasmon-loss peak (26 eV) is shifted by - 2 eV with respect to that observed for the pure metal, similar to the case for Ta. Note how the loss structure observed for the clean Nb surface at energies higher than 30 eV disappears gradually as the surface oxidizes. After 23 L (curve D), only a very broad peak with a maximum at 44 eV can be observed. The intensity of the loss peak at 27 eV increases with oxygen exposure, analogous to the 12.5 eV loss peak. DISCUSSION

Clean surfaces

of Ta and Nb

A detailed discussion of the ELS spectra of Ta and Nb can be found in the work of Schubert and Wolf [6]. The results may be interpreted in the light

155

of previous optical measurements due to Weaver et al. [15, 161 and other studies [ 7, 81, which can be summarized as follows. Apparently, most of the low-energy structure (AE < 25 V) can be identified as collective plasmons, as reported by Weaver et al. [ 15,161 in their UV optical studies. These authors reported surface and bulk loss functions with two maxima each. Four peaks appear for Ta at 20.7, 8.9 and 17.2,8.6 eV for the bulk and surface loss functions, respectively, and at 20.8, 9.7 and 17.7, 9 eV for Nb. With the present method of measurement, i.e., using the reflection mode, we observe a superposition of the two functions. A direct measurement of the bulk loss function in Nb was achieved recently by Meixner et al. [17] in transmission experiments, which gave peaks at 21.7 and 10.3 eV. The two peaks observed in the present work at - 20 and - 9 eV for Ta and Nb correspond to an overlap of the bulk and surface losses. The energy positions of the two sets of plasmons reported by Weaver et al. [15,16] are indicated in Figs. 3 and 4 by arrows, and explain adequately the structures observed for clean Ta and Nb. In the case of Nb (Fig. 4), the peak at - 20 eV decreases in intensity and becomes sharper, whereas the region of the spectrum at - 9 eV suffers a pronounced transformation. This is a consequence of the effect of oxidation on the surface plasmon loss of the pure metal. This plasmon loss vanishes during oxidation of the surface, so that the corresponding contribution to the main peak at - 20 eV disappears. Therefore only the bulk plasmon loss from the metallic substrate remains, giving a peak at - 21-22 eV, in agreement with the value of 21.7 eV given by Meixner et al. [17]. During the exposure to oxygen the appearance of 0 2p states at - 6 eV below EF was observed recently by Miller et al. [22] using UPS. These same authors [22] also observed broadening and the appearance of a shoulder at - 8 eV below EF in the 0 2p band after 7 L oxygen exposure. This could explain the loss peak at 12.5 eV in -N”(E) (Fig. 4) as due to the excitation of an electron from this new band to the empty states above EF (these are expected not to change greatly). The oxygen 2s level appears at 22.5 eV below EF [22]. It is the origin of the loss peak observed at - 27.5 eV (Fig. 4), so that the peak of empty states at - 5 eV above EF is probably the final state for these excited electrons. It is believed [22] that the 0 2p orbitals hybridize with the 4d band from the metal, thus producklg a monotonic decrease of the DOS in the band edge at the Fermi level and the growth of a new 0 2p band. This process indicates the transition from metal to insulator during oxidation, and is the cause of the intensity decrease of the loss peak at - 33 eV corresponding to the excitation 4p + EF . The structure observed in the high-energy region has usually been interpreted as originating from core-level excitations. The loss peaks observed for Nb at 32.5eV (Fig. 2) or 31.5 eV (Fig. 4) are ascribed to excitations of electrons from the 4p level (BE ~32.7 eV [18,19,26]) to the Fermi level.

156

As concerns the broad‘ feature above 40 eV in Fig. 2, many different interpretations have been given by different authors [23-261, but all of them have been questioned. A possible interpretation is the excitation of 4p electrons to empty states above EF [26]. Although DOS calculations [20, 211 show possible final states at - 5 eV above the Fermi level, the threshold energy for this transition (- 38 eV) would be lower than that observed. Another possible interpretation pointed out by Meixner et al. [17] in discussing their transmission experiments is that of multiple scattering. This interpretation was based on broad peaks observed at - 43 and - 65 eV, i.e., at energies which are multiples of the main bulk plasmon peak (21.7 eV). The feature observed at - 27.5 eV in the present work was not reported by Schubert and Wolf [6] or by other authors [7-Q]. Although the AES spectrum of pure Nb (Fig. 2) does not show the presence of oxygen, we believe that the feature at 27.5 eV is related to the 0 2s band (cf. Fig. 6). Dissolved oxygen in the sample could be the origin of such a feature for the “pure” metal. Meixner et al. [17] showed how small quantities of oxygen dissolved in Nb cause a broadening of the main plasmon peak at - 21 eV, in a similar manner as shown in curve (A) of Fig. 2. In the case of Ta (Figs. 1 and 3), in addition to the surface and plasmon loss peaks at energies lower than 25 eV, a loss peak is observed at - 28 eV. This can be interpreted as an excitation of 4f,,2,5,2 electrons with binding energies at - 23.5 and 21.6 eV [18, 19, 271 to empty states - 5 eV above ,

’

I

l

l

23eV Wiw.)

k I I k

40eV

43eV (forbidden?1 ’ 38eV 33eV

l

b 20eV

Too,

, I I

*

20eV 39eV

l

48eV

l

Fig. 5. Tentative assignments of observed ELS peaks for metallic Ta (upper half) and Ta oxide (lower half) corresponding to curves (A) and (D), respectively, in Figs. 1 and 3. The DOS curve at EF is from Ref. 21. The 0 2s and 4 2p binding energies have been adjusted to the ELS results, and the other core-level binding energies are from Refs. 6, 18, 19 and 26.

157

the Fermi level. The prediction of a peak of empty states at - 5 eV above EF has been also obtained by several DOS and band-structure calculations [20, 211. The doublet cannot be resolved. A similar excitation of 4f electrons to the Fermi level would produce a feature at -23 eV which is interpreted as contributing to the main peak at -20 eV, the main bulk plasmon. The losses at 33.5, 38 and 48eV (Fig. 3) can be ascribed to transitions from the 5~s,~ and 5p,,, levels to unoccupied states at the Fermi level or immediately above. Considering the 5ps,, and 5p,,, binding energies (- 34 and 43 eV, respectively [ 18, 191) and the above-mentioned DOS calculations for Ta, various peaks at - 34,39,43 and 48 eV can be expected. These peaks would correspond to excitations of 5p electrons to the Fermi level and to the empty states 5eV above EF. Of these four possible peaks only that at 43 eV has not been observed. This can be explained by the fact that the electronic states at the Fermi edge are mostly of the 5d,,, type, i.e., the . . transition 5p 1,2 + EF (5d,,, ) at AE 3 45 eV would be forbidden because AJ> 1. Correlation diagrams showing the observed losses related to the possible transitions are given in the upper parts of Figs. 5 and 6 for Ta and Nb, respectively. The calculated DOS [20, 211 and the energy positions of the core levels [18, 191 are also included schematically in the figures. For Nb, existing UPS measurements [21] of the valence band have also been included. Effects of oxygen exposure The observed effects due to oxygen exposure can also be interpreted in terms of core-level excitations and collective plasmon oscillations. l

*

32.7eV -5 eV (not resolved1

h

eVl

NbO,

l

I

I

l l

12.SeV 27eV 41eV

Fig. 6. Tentative assignments of observed ELS peaks for metallic Nb (upper half) and Nb oxide (lower half) corresponding to curves (A) and (D), respectively, in Figs. 2 and 4. The valence-band edge and the 0 2p curve are from Miller et aL [22]. The empty DOS curve above EF is from Refs. 20 and 21. Core-level binding energies are from Refs. 18, 19 and 22.

158

Although the shift of the broad peak at - 40 eV (analogous to that of the main bulk plasmon peak at - 20eV) corroborates the idea of multiple scattering (cf. Table 2), transitions from shifted 4p levels to unoccupied electron states above EF could also contribute to this structure. In the lower part of Fig. 6 we have reproduced schematically the 0 2p curve and the energy position of the 0 2s level observed by Miller et al. [22] for Nb after 20 L of oxygen exposure. The position of the 4p level at 36 eV corresponds to that in Nbz O5 [22]. The empty DOS curve above EF has been assumed to be similar to that of pure Nb [20, 211. Arrows indicate tentative assignments of the ELS peaks. The discussion for Ta is very similar to that for Nb. Only some minor points must be considered separately. Figure 3 shows how the contribution of the surface plasmon-loss peak at - 9 eV vanishes, whereas the loss peak associated with the 0 2p band increases in intensity with oxygen exposure. In this case a peak at - 27.5 eV, related to the 0 2s band, also appears for the pure metal. This is because the 4f,12,512 level of Ta has a binding energy similar to that of 0 2s, and the final states for both types of electrons are the unoccupied states above EF (cf. Fig. 5). The peak at -33 eV (Fig. 3) disappears as a consequence of oxidation. This is so because the final states at the Fermi level disappear, probably in a similar way as for Nb, i.e., due to hybridization with 0 2p levels. The energy shifts of the 5p levels [6] during gradual oxidation are probably the explanation for the observed broadening of the peaks at - 39 and 48 eV related to these levels (Fig. 5). A tentative assignment of the observed ELS peaks is shown schematically by the arrows in the lower part of Fig. 5. The curve representing the empty states above EF is the same as for pure Ta [21] (cf. upper part of Fig. 5), and the positions of the 0 2p and 0 2s bands have been adjusted to the observed ELS peaks in accordance with the values for Nb. Interpretation of the observed shifts and the broadening of the ELS peaks during the oxidation of both metals in terms of chemical shifts of core levels and of oxide formation [ 61 is very difficult. As pointed out in the above discussion, what we observe is the overlapping of different peaks, indicating a mixture of various oxides.

CONCLUSIONS

In general, the present work shows that ELS can be used for studying changes in the electronic structure of surfaces exposed to oxygen, resulting in the formation of a surface oxide layer. ELS of Ta and Nb and their initial stages of oxidation at room temperature provides numerous characteristic electron energy losses associated with plasmon excitations, interband transitions and core transitions which have been identified on the basis of band and DOS calculations for the pure metals.

159

The results show that the oxidation of both metals produces important changes in their valence bands. The growth of a loss peak at 12.5 eV and the decrease of the peak characteristic of the metal at - 33 eV have been interpreted, in very good agreement with the UPS data of Miller et al. [22] for Nb, as the growth of a new 0 2p band and progressive suppression of the DOS distribution at the Fermi level. This determines the metal-insulator transition at the surface. Although a direct interpretation of the data in terms of the formation of different oxides has not been possible, the observed shifts and broadening of the ELS peaks related to core levels have been ascribed to chemical shifts of the core levels, thus indicating oxide formation [ 61. The existence of unoccupied states at - 5 eV above EF predicted by bandstructure and DOS calculations [ 20, 211 is confirmed by the present results.

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21

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