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X-ray induced photoelectron and auger spectra of Cu, CuO, Cu2O, and Cu2S thin films
Journal of Electron Spectroscopy and Related Phenamena, 4 (1974) 213-218 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
X-RAY INDUCED PHOTOELECTRON Cu,O, AND Cu,S THIN FILMS
PAUL
AUGER
SPECTRA
OF Cu, CuO,
E. LARSON
GCA/McPherson, (First
AND
Acton, Mass. 01720 (U.S.A.)
received 25 March
1974; in final form
1 June 1974)
ABSTRACT
Improved techniques have been used to prepare thin films of Cu, CuO, Cu,O and Cu,S for x-ray photoelectron spectral analysis. The Cu 2p and Cu LMM Auger spectra have been obtained. Photoelectron and Auger chemical shifts as well as qualitative spectral features are found to be useful diagnostics for valence-state characterization of unknowns. INTRODUCTION
The x-ray photoelectron spectra of transition metal compounds have attracted interest because they exhibit a variety of phenomena not usually observed in other materials. These include line broadening, the appearance of satellites, and chemical shifts which do not correlate simply with oxidation state or electronegativities of ligands and anions. The first attempts to assign the Cu 2p satellites to specific chemical states of copper were unsuccessful because experiments were inconsistent_ The unsuspected variables in these experiments were air oxidation of cuprous compounds, photoreduction of cupric compounds, and thermal reduction of CuO to Cu,O. In an extensive survey of copper compounds’, Frost et al. discovered the importance of air oxidation and photoreduction. They were able to associate the Cu 2p satellites and line broadening with cupric compounds. Rosencwaig and Wertheim’ independently made the same correlation and demonstrated that CuO is quickly reduced to Cu,O under vacuum at 25O”C, thus accounting for a good deal of earIier confusion. Two mechanisms that account for the broadening and satellites of transition metal compounds are now generally accepted. The multiplet splitting mechanism3* 4 explains the large 3s splittings and the 2p line broadening since the effect is smaller for 2p electrons. A shake-up mechanism5’ 6 accounts for the 2p satellites. More generally, these effects have been associated with paramagnetic species.
214 Cu(II) has a paramagnetic 3dg structure while Cu(I) has a filled 3d subshell. Matienzo et al.’ have shown that shake-up satellites occur on the 2p lines of paramagnetic nickel independent of valence or stereochemistry. Frost et al. * found strong 2p sateIlites on paramagnetic Co(H) complexes but not on diamagnetic Co(II1) complexes. Having achieved a reasonable understanding of the effects observed in the Cu, Cu,O, CuO system, this paper reports on the use of Cu vapor deposition, in situ oxidation to CuO, and in situ thermal reduction to Cu,O to obtain clean, welldefined, non-charging thin films of Cu, CuO, and Cu,O. Reference quality, high resolution spectra of the Cu 2p ,and Cu LMM Auger regions have been obtained in order to illustrate the effects discussed above, provide accurate binding energy measurements referenced to the pure metal, and evaluate the utility of the Auger spectrum for valence state analysis of unknowns. In addition, a method of preparing pure thin films of Cu,S has been found, and the spectra of CuZS are included. EXPERIMENTAL
The spectra were obtained on a GCA/McPherson ESCA 36 Photoelectron Spectrometer equipped with magnesium anode, cryopump, evaporator, and variabletemperature sample probe. Copper was evaporated onto the variable temperature probe under high vacuum. The Cu 2p and LMM Auger spectra were recorded. The sample probe was heated to 250°C and approximately 0.5 atmospheres of air admitted. The probe was allowed to cool, and the sample chamber was re-evacuated. The 2p and Auger spectra from the resulting CuO film were then recorded. The CuO was reduced to Cu,O by heating to 250°C in vacua. Again, the 2p and Auger spectra were recorded. Identical conditions of analyzer geometry were maintained for these three sets of measurements. Data was taken at pressures of approximately lo-* torr in the sample chamber. The analyzer transmission window was approximately 0.2 eV for the Cu 2p spectra and 0.5 eV for the LMM Auger spectra. The C 1s spectra were obtained as a check for sample charging, and the positions were constant within 0.1 eV. Films of the order of 1000 A in thickness formed on copper treated with aqueous Na,S. The XPS spectrum showed only &(I) and sulfur. Dissolved 0, was found to promote the reaction_ On the basis of these data, the reaction is presumed to be: 2Cu + S2- + H,O
+ $0,
-+ Cu,S
+ 20H-.
The XPS spectrum indicates that the films are quite pure, and stable against air oxidation over a period of at least several days. The Cu 2p and Auger spectra were recorded. Figure 1 shows the full copper spectrum. The Cu 2p spectra are given in Figure 2, and Figure 3 shows a portion of the LMM Auger spectra. The effect of the Mg Ka,, 4 satellites has been removed from the 2p spectra by a shift-and-subtract
215 COPPER M.W100 WATTS
I
1 ,000
I
800
Figure
I 970
500
1. Full spectrum
of copper
I 100
BINDING ENERGY
foil cleaned
I zoo
I 0
by argon ion sputtering.
I
I
I
I
I
960
SE4
94a
030
920
BINDING ENERGY W/l
Figure 2. Cu 2p spectra from Cu, CuO, CuaO and CuzS thin films excited by Mg Ka radiation. CuO has a significant chemical shift, multiplet broadening, and shake-up satellites.
Only
216
1
I BR.3
633.3
I 333.3
D16.3
I 338.3
I WV.3
KlNETW2ENERGY WI
Figure 3. The Cu LMM radiation.
f 176
Auger
spectra
from Cu, CuO,
I
I
165
155
CUZO, and CL&
thin films using Mg KQ
BlNDlNG ENERGY
Figure 4. The sulfur 2p spectrum from CuzS. The binding energy is charactetistic sulfate position is indicated for reference.
of a sulfide. The
217 algorithm. Figure 4 shows the S 2p spectrum from the Cu,S film. The positions of the sulfur 2p lines are characteristic of a sulfide, and the sulfate position is indicated for comparison’. Although these thin films are typical of those encountered in many surface chemistry problems, the spectra need not correspond identically with those of the perfect crystalline materials since transition metal oxides and sulfides are well known for their lack of stoichiometry and high degree of defect structure which can vary with the preparative conditions. The use of air oxidation for preparation of CuO raises the question of contamination by atmospheric components other than 0,. No nitrogen 1 s signal could be observed, but an asymmetry of the 0 1s spectrum indicated the presence of a small amount of the hydroxide13. RESULTS
Of the four Cu 2p spectra in Figure 2, only CuO exhibits multiplet line broadening and the shake-up satellites which Frost et al. found to be characteristic of all cupric compounds. The spectra of the cuprous compounds are nearly identical with the copper spectrum in terms of line width, binding energy, and absence of satellites. The Cu 2p chemical shifts, referenced to the metal, are given in Table 1. In addition to its other features, the CuO spectrum has a substantial chemical shift of 1.2 eV. The cuprous compounds have chemical shifts so small as to be nearly insignificant since error limits of + 0. I eV must be attached to these measurements. The Cu 2p spectrum readily identifies Cu(II) species, but fails to distinguish between Cu(0) and Cu(1). TABLE
1
CHEMICAL Species
SHIFTS 2P112 and 2Pw.1
LzM4,5M4,5
and L3M4.5M4,5 -~
cu cue cuno cuzs
0 1.2
0 1.0
0.1 0.1
2.3
~.._
1.8
The chemical shifts for the three compounds measured here are internally consistent with the measurements of Frost et al., within their stated reproducibility of -to.3 eV. Copper metal was omitted from the Frost survey, but the present data allows us to assign a binding energy of 934.7 f0.3 eV on their scale, which centers it among the Cu(1) binding energies. The LMM Auger spectra are shown in Figure 3. Schon” and Kowalczyk et al.” have analyzed the copper Auger spectrum. The chemical effects on the Auger
218 spectrum are quite remarkable considering the conventional opinion that chemical effects are much more pronounced in XPS spectra than in Auger spectra. This impression probably results from the lower resolution and derivative presentation usually employed in electron-excited Auger spectrometry. There are several known cases where Auger chemical shifts are larger than photoelectron chemical shifts”. The Auger chemical shifts are listed in Table 1 along with the photoelectron chemical shifts. CuO has a chemical shift of 1.0 eV, but the cuprous compounds have even larger shifts averaging 2.0 eV. On the basis of this limited data base, the Augerphotoelectron energy difference is about the same for Cu(0) and Cu(II), but about 2 eV less for Cu(1). This could be the basis for a useful diagnostic technique for unknown samples which is independent of spectrometer calibration or sample charging. REFERENCES 1 2 3 4 5 6 8 9 10 11 12 13
D. C. Frost, A. Ishitani and C. A. McDowell, Mol. Phys., 24 (1972) 861. A. Rosencwaig and G. K. Wertheim, J. Electron Spectrosc., 1 (1973) 493. C. S. Fadley and D. A. Shirley, Phys. Rev. A, 2 (1970) 1109. C. S. Fadley, D. A. Shirley, A. J. Freeman, P. S. Bagus and J. V. Mallow, Phys. Rev. Lett., 23 (1969) 1397. T. Novakov and R. Prins, in D. A. Shirley (editor), Electron Spectroscopy, North HoIland Publ. Co., Amsterdam, 1972, p. 821. A. Rosencwaig, G. K. Wertheim and H. J. Guggenheim, Whys. Rev. Lett., 27 (1971) 479. L. J. Matienzo, L. I. Yin, S. 0. Grim and W. E. Swartz Jr., Inorg. Ckem., 12 (1973) 2762. D. C. Frost, C. A. McDowell and L. S. Woolsey, Chem. Whys. Lett., 17 (1972) 320. K. Siegbahn et al., ESCA-Atomic, Molecular and Solid State Strucfure Studied by Means of Electron Spectroscopy, Almqvist & Wiksells, Uppsala, 1967, p. 134. G. Schon, J. Electron Spectrosc., 1 (1973) 377. S. P. Kowalczyk, R. A. Pollak, F. R. Mcfeely, L. Ley and D. A. Shirley, Whys. Rev. B, 8 (1973) 2387. G. Schon, J. Electron Spectrosc., 2 (1973) 75. N. S. McIntyre and M. G. Cook, “XPS Spectra of the Oxides and Hydroxides of Cobalt, Nickel and Copper” presented at the 25th Pittsburgh Conference.
X-RAY INDUCED PHOTOELECTRON Cu,O, AND Cu,S THIN FILMS
PAUL
AUGER
SPECTRA
OF Cu, CuO,
E. LARSON
GCA/McPherson, (First
AND
Acton, Mass. 01720 (U.S.A.)
received 25 March
1974; in final form
1 June 1974)
ABSTRACT
Improved techniques have been used to prepare thin films of Cu, CuO, Cu,O and Cu,S for x-ray photoelectron spectral analysis. The Cu 2p and Cu LMM Auger spectra have been obtained. Photoelectron and Auger chemical shifts as well as qualitative spectral features are found to be useful diagnostics for valence-state characterization of unknowns. INTRODUCTION
The x-ray photoelectron spectra of transition metal compounds have attracted interest because they exhibit a variety of phenomena not usually observed in other materials. These include line broadening, the appearance of satellites, and chemical shifts which do not correlate simply with oxidation state or electronegativities of ligands and anions. The first attempts to assign the Cu 2p satellites to specific chemical states of copper were unsuccessful because experiments were inconsistent_ The unsuspected variables in these experiments were air oxidation of cuprous compounds, photoreduction of cupric compounds, and thermal reduction of CuO to Cu,O. In an extensive survey of copper compounds’, Frost et al. discovered the importance of air oxidation and photoreduction. They were able to associate the Cu 2p satellites and line broadening with cupric compounds. Rosencwaig and Wertheim’ independently made the same correlation and demonstrated that CuO is quickly reduced to Cu,O under vacuum at 25O”C, thus accounting for a good deal of earIier confusion. Two mechanisms that account for the broadening and satellites of transition metal compounds are now generally accepted. The multiplet splitting mechanism3* 4 explains the large 3s splittings and the 2p line broadening since the effect is smaller for 2p electrons. A shake-up mechanism5’ 6 accounts for the 2p satellites. More generally, these effects have been associated with paramagnetic species.
214 Cu(II) has a paramagnetic 3dg structure while Cu(I) has a filled 3d subshell. Matienzo et al.’ have shown that shake-up satellites occur on the 2p lines of paramagnetic nickel independent of valence or stereochemistry. Frost et al. * found strong 2p sateIlites on paramagnetic Co(H) complexes but not on diamagnetic Co(II1) complexes. Having achieved a reasonable understanding of the effects observed in the Cu, Cu,O, CuO system, this paper reports on the use of Cu vapor deposition, in situ oxidation to CuO, and in situ thermal reduction to Cu,O to obtain clean, welldefined, non-charging thin films of Cu, CuO, and Cu,O. Reference quality, high resolution spectra of the Cu 2p ,and Cu LMM Auger regions have been obtained in order to illustrate the effects discussed above, provide accurate binding energy measurements referenced to the pure metal, and evaluate the utility of the Auger spectrum for valence state analysis of unknowns. In addition, a method of preparing pure thin films of Cu,S has been found, and the spectra of CuZS are included. EXPERIMENTAL
The spectra were obtained on a GCA/McPherson ESCA 36 Photoelectron Spectrometer equipped with magnesium anode, cryopump, evaporator, and variabletemperature sample probe. Copper was evaporated onto the variable temperature probe under high vacuum. The Cu 2p and LMM Auger spectra were recorded. The sample probe was heated to 250°C and approximately 0.5 atmospheres of air admitted. The probe was allowed to cool, and the sample chamber was re-evacuated. The 2p and Auger spectra from the resulting CuO film were then recorded. The CuO was reduced to Cu,O by heating to 250°C in vacua. Again, the 2p and Auger spectra were recorded. Identical conditions of analyzer geometry were maintained for these three sets of measurements. Data was taken at pressures of approximately lo-* torr in the sample chamber. The analyzer transmission window was approximately 0.2 eV for the Cu 2p spectra and 0.5 eV for the LMM Auger spectra. The C 1s spectra were obtained as a check for sample charging, and the positions were constant within 0.1 eV. Films of the order of 1000 A in thickness formed on copper treated with aqueous Na,S. The XPS spectrum showed only &(I) and sulfur. Dissolved 0, was found to promote the reaction_ On the basis of these data, the reaction is presumed to be: 2Cu + S2- + H,O
+ $0,
-+ Cu,S
+ 20H-.
The XPS spectrum indicates that the films are quite pure, and stable against air oxidation over a period of at least several days. The Cu 2p and Auger spectra were recorded. Figure 1 shows the full copper spectrum. The Cu 2p spectra are given in Figure 2, and Figure 3 shows a portion of the LMM Auger spectra. The effect of the Mg Ka,, 4 satellites has been removed from the 2p spectra by a shift-and-subtract
215 COPPER M.W100 WATTS
I
1 ,000
I
800
Figure
I 970
500
1. Full spectrum
of copper
I 100
BINDING ENERGY
foil cleaned
I zoo
I 0
by argon ion sputtering.
I
I
I
I
I
960
SE4
94a
030
920
BINDING ENERGY W/l
Figure 2. Cu 2p spectra from Cu, CuO, CuaO and CuzS thin films excited by Mg Ka radiation. CuO has a significant chemical shift, multiplet broadening, and shake-up satellites.
Only
216
1
I BR.3
633.3
I 333.3
D16.3
I 338.3
I WV.3
KlNETW2ENERGY WI
Figure 3. The Cu LMM radiation.
f 176
Auger
spectra
from Cu, CuO,
I
I
165
155
CUZO, and CL&
thin films using Mg KQ
BlNDlNG ENERGY
Figure 4. The sulfur 2p spectrum from CuzS. The binding energy is charactetistic sulfate position is indicated for reference.
of a sulfide. The
217 algorithm. Figure 4 shows the S 2p spectrum from the Cu,S film. The positions of the sulfur 2p lines are characteristic of a sulfide, and the sulfate position is indicated for comparison’. Although these thin films are typical of those encountered in many surface chemistry problems, the spectra need not correspond identically with those of the perfect crystalline materials since transition metal oxides and sulfides are well known for their lack of stoichiometry and high degree of defect structure which can vary with the preparative conditions. The use of air oxidation for preparation of CuO raises the question of contamination by atmospheric components other than 0,. No nitrogen 1 s signal could be observed, but an asymmetry of the 0 1s spectrum indicated the presence of a small amount of the hydroxide13. RESULTS
Of the four Cu 2p spectra in Figure 2, only CuO exhibits multiplet line broadening and the shake-up satellites which Frost et al. found to be characteristic of all cupric compounds. The spectra of the cuprous compounds are nearly identical with the copper spectrum in terms of line width, binding energy, and absence of satellites. The Cu 2p chemical shifts, referenced to the metal, are given in Table 1. In addition to its other features, the CuO spectrum has a substantial chemical shift of 1.2 eV. The cuprous compounds have chemical shifts so small as to be nearly insignificant since error limits of + 0. I eV must be attached to these measurements. The Cu 2p spectrum readily identifies Cu(II) species, but fails to distinguish between Cu(0) and Cu(1). TABLE
1
CHEMICAL Species
SHIFTS 2P112 and 2Pw.1
LzM4,5M4,5
and L3M4.5M4,5 -~
cu cue cuno cuzs
0 1.2
0 1.0
0.1 0.1
2.3
~.._
1.8
The chemical shifts for the three compounds measured here are internally consistent with the measurements of Frost et al., within their stated reproducibility of -to.3 eV. Copper metal was omitted from the Frost survey, but the present data allows us to assign a binding energy of 934.7 f0.3 eV on their scale, which centers it among the Cu(1) binding energies. The LMM Auger spectra are shown in Figure 3. Schon” and Kowalczyk et al.” have analyzed the copper Auger spectrum. The chemical effects on the Auger
218 spectrum are quite remarkable considering the conventional opinion that chemical effects are much more pronounced in XPS spectra than in Auger spectra. This impression probably results from the lower resolution and derivative presentation usually employed in electron-excited Auger spectrometry. There are several known cases where Auger chemical shifts are larger than photoelectron chemical shifts”. The Auger chemical shifts are listed in Table 1 along with the photoelectron chemical shifts. CuO has a chemical shift of 1.0 eV, but the cuprous compounds have even larger shifts averaging 2.0 eV. On the basis of this limited data base, the Augerphotoelectron energy difference is about the same for Cu(0) and Cu(II), but about 2 eV less for Cu(1). This could be the basis for a useful diagnostic technique for unknown samples which is independent of spectrometer calibration or sample charging. REFERENCES 1 2 3 4 5 6 8 9 10 11 12 13
D. C. Frost, A. Ishitani and C. A. McDowell, Mol. Phys., 24 (1972) 861. A. Rosencwaig and G. K. Wertheim, J. Electron Spectrosc., 1 (1973) 493. C. S. Fadley and D. A. Shirley, Phys. Rev. A, 2 (1970) 1109. C. S. Fadley, D. A. Shirley, A. J. Freeman, P. S. Bagus and J. V. Mallow, Phys. Rev. Lett., 23 (1969) 1397. T. Novakov and R. Prins, in D. A. Shirley (editor), Electron Spectroscopy, North HoIland Publ. Co., Amsterdam, 1972, p. 821. A. Rosencwaig, G. K. Wertheim and H. J. Guggenheim, Whys. Rev. Lett., 27 (1971) 479. L. J. Matienzo, L. I. Yin, S. 0. Grim and W. E. Swartz Jr., Inorg. Ckem., 12 (1973) 2762. D. C. Frost, C. A. McDowell and L. S. Woolsey, Chem. Whys. Lett., 17 (1972) 320. K. Siegbahn et al., ESCA-Atomic, Molecular and Solid State Strucfure Studied by Means of Electron Spectroscopy, Almqvist & Wiksells, Uppsala, 1967, p. 134. G. Schon, J. Electron Spectrosc., 1 (1973) 377. S. P. Kowalczyk, R. A. Pollak, F. R. Mcfeely, L. Ley and D. A. Shirley, Whys. Rev. B, 8 (1973) 2387. G. Schon, J. Electron Spectrosc., 2 (1973) 75. N. S. McIntyre and M. G. Cook, “XPS Spectra of the Oxides and Hydroxides of Cobalt, Nickel and Copper” presented at the 25th Pittsburgh Conference.