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Comparison of Cu valence band structures from X-ray spectroscopy and photoemission
Volume 44A, number 1
PHYSICS LETTERS
7 May 1973
C O M P A R I S O N O F Cu V A L E N C E B A N D S T R U C T U R E S F R O M X-RAY SPECTROSCOPY AND PHOTOEMISSION S. HUFNER* and G.K. WERTHEIM Bell Laboratories, Murray Hill, New Jersey 07974, USA Received 12 March 1973 The valence band structure of Cu obtained from M X-ray emission is considerably wider than from X-ray photoemission. It is shown that the width of the Cu 3p hole state is sufficiently large to account for the discrepancy. This width also limits the resolution which can be attained in M X-ray emission spectroscopy.
The density of states of the occupied band structure of copper has in recent years been obtained by three different techniques: ultraviolet photoemission spectroscopy (UPS) [ 1,2], X-ray photoelectron spectroscopy (XPS or ESCA) [3], and soft X-ray spectroscopy (SXS) [4]. The results of UPS and XPS are in satisfactory agreement, while those of SXS are distincly different. We consider here the origin of this disagreement. Potentially a fundamental difference could arise from the fact that the volume of material under observation in photoelectron spectroscopy using ultraviolet or X-radiation lies within 15 A of the surface while SXS samples to a depth an order of magnitude greater. Since the band structure of surface atoms is expected to be narrower than that of the atoms in the bulk one may be tempted to ascribe the smaller width of UPS and XPS spectra to a surface effect. However, the band width has been shown to vary as Z - 1/2 (Z is the coordination number) so that the change in band width is largely confined to the outer layer of atoms. These make up only a small though not entirely negligible fraction of the atoms in a 15 layer, but cannot account for the pronounced difference between the results of the two types of experiments. Such surface effects may, however, become important in experiments using synchrotron radiation for photoemission since the range of electrons drops to circa 5 A at 100 eV, and may dominate the spectrum obtained by ion neutralization spectroscopy [5]. The good agreement between UPS and XPS densi* Permanent address: Fachbereich Physik, Freie Universit~it, Berlin, Germany.
t.O SXS
tU
t.
CU METAL
/
i
l
I 4
.....
A
0.5
10
I 8
t
I 6
I
I 2
I
[ O
I
I
I
-2
BINDING ENERGY ( e V )
Fig. 1. Comparison of the Cu valence band obtained from M X-ray emission spectroscopy (solid line, from ref. [4]) with the results of X-ray photoelectron spectroscopy (dotted). The XPS data were obtained on an evaporated sample using monochromatized A1Kc~radiation in a Hewlett-Packard
spectrometer with a resolution of 0.5 eV. ties of state implicit is the comparison made in ref. [3] is gratifying and gives confidence that the unfolding of the UPS joint optical density of states can be done without ambiguity. It also indicates that the variation of the photoelectric transition element across the d-band does not cause major errors in XPS data. The unfolding of the M2,3 SXS emission spectrum presents a greater challenge, but has apparently been successfully carried out [4]. The resulting spectrum is 47
Volume 44A, number t
3,4
PHYSICS LETTERS
Cu METAL3~
3.0 i2.6
2:.2
1,8 90
I
I
85
I
I
I
I
80 75 BINDINGENERGY(eV)
I
1
TO
i
(;5
Fig. 2. The Cu 3p X-ray photoelectron spectrum obtained with MgKa radiation in a Varian spectrometer. The data have been fitted with two symmetrical lines on a sloping background. compared with the XPS density of states in fig. 1. A resolution of 0.4 to 0.5 eV is claimed for the SXS spectrum, but no correction has been applied for the width of the initial-state core hole. The XPS spectrum was obtained with a resolution of 0.5 eV and has been corrected only for the inelastic background. The difference in width between the two spectra, as well as the difference in the amount of structural detail in the two spectra supposedly obtained at similar resolution require explanation. An immediate source of concern is the width of the 3p hole state which was not known at the time of publication of ref. [4]. We have measured the Cu 3p XPS spectrum fig. 2, on an argon ion sputtered sampie using MgK~ radiation in a Varian lEE-15 spectrometer. Least squares analysis yields a spin-orbit splitting of 2.5 eV and a linewidth of 2.7 eV. The spin-
48
7 May 1973
nrbit splitting is slightly larger than ihat used in the unfolding of the SXS M2, 3 emission spectrum. A lower limit for the width of the 2p hole state is obtained by subtracting the instrumental resoluiion ~i 1.3 e g from the measured widih. I A lower limit is obtained because the instrumental resolulion inchldes the unknown width ol'the Au 4f line used in ltle determination.} The restllting width of 1.4 eV Iol the initial state of the M emissi~m princess provides a Iowe~ limit t\~r the resolution which can be achieved in the unfolding of Cu M emission spectra. This suggests strongly thal the detailed st ructure obtained in re i [4] does not have physical significance, and that the greater width of the SXS spectruin is due to the width t)t the initial state. It is also worth noting thai the L3 emission specl rum has a width considerably smaller that the M3 spectrum, but comparable to that of the XPS spectrum. This is in accord with the smaller width tound by XPS for the 2p core leveE, and indicates that transition probability effects need not be invoked to account for the difference between L and M emission spectra. We thank D. N. E. Buchanan for technical assistance
References [ 1 ] N.V. Smith, Phys. Rev. B3 ( 1971 ) 1862 ; see also Electronic density of states, ed. L.H. Bennett, NBS Special Publication No. 323 (USGPO Washington, D.C., 1971 ) p. 191. [2] D.E. Eastman, in Electron spectroscopy, ed. D.A. Shirley (North Holland, Amsterdam, 1972) p. 487, a~ld references cited therin [3] S. Htifner, G.K. Wertheim, N.V. Smith and M.M. Traum. Solid State Communications / l (1972) 323. 141 R.C. Dobbyn, M.L. Williams,J.R. Cuthill and A.J. McAlister, Phys. Rev. B2 (1970) 1563. [5] R. Haydock, V. Heine, M.J. Kelly and J.B. Pardry, Phys, Rev. Lett. 29 (1972) 868. [6] H.D. Hagstrum, Science 178 11972) 275.
PHYSICS LETTERS
7 May 1973
C O M P A R I S O N O F Cu V A L E N C E B A N D S T R U C T U R E S F R O M X-RAY SPECTROSCOPY AND PHOTOEMISSION S. HUFNER* and G.K. WERTHEIM Bell Laboratories, Murray Hill, New Jersey 07974, USA Received 12 March 1973 The valence band structure of Cu obtained from M X-ray emission is considerably wider than from X-ray photoemission. It is shown that the width of the Cu 3p hole state is sufficiently large to account for the discrepancy. This width also limits the resolution which can be attained in M X-ray emission spectroscopy.
The density of states of the occupied band structure of copper has in recent years been obtained by three different techniques: ultraviolet photoemission spectroscopy (UPS) [ 1,2], X-ray photoelectron spectroscopy (XPS or ESCA) [3], and soft X-ray spectroscopy (SXS) [4]. The results of UPS and XPS are in satisfactory agreement, while those of SXS are distincly different. We consider here the origin of this disagreement. Potentially a fundamental difference could arise from the fact that the volume of material under observation in photoelectron spectroscopy using ultraviolet or X-radiation lies within 15 A of the surface while SXS samples to a depth an order of magnitude greater. Since the band structure of surface atoms is expected to be narrower than that of the atoms in the bulk one may be tempted to ascribe the smaller width of UPS and XPS spectra to a surface effect. However, the band width has been shown to vary as Z - 1/2 (Z is the coordination number) so that the change in band width is largely confined to the outer layer of atoms. These make up only a small though not entirely negligible fraction of the atoms in a 15 layer, but cannot account for the pronounced difference between the results of the two types of experiments. Such surface effects may, however, become important in experiments using synchrotron radiation for photoemission since the range of electrons drops to circa 5 A at 100 eV, and may dominate the spectrum obtained by ion neutralization spectroscopy [5]. The good agreement between UPS and XPS densi* Permanent address: Fachbereich Physik, Freie Universit~it, Berlin, Germany.
t.O SXS
tU
t.
CU METAL
/
i
l
I 4
.....
A
0.5
10
I 8
t
I 6
I
I 2
I
[ O
I
I
I
-2
BINDING ENERGY ( e V )
Fig. 1. Comparison of the Cu valence band obtained from M X-ray emission spectroscopy (solid line, from ref. [4]) with the results of X-ray photoelectron spectroscopy (dotted). The XPS data were obtained on an evaporated sample using monochromatized A1Kc~radiation in a Hewlett-Packard
spectrometer with a resolution of 0.5 eV. ties of state implicit is the comparison made in ref. [3] is gratifying and gives confidence that the unfolding of the UPS joint optical density of states can be done without ambiguity. It also indicates that the variation of the photoelectric transition element across the d-band does not cause major errors in XPS data. The unfolding of the M2,3 SXS emission spectrum presents a greater challenge, but has apparently been successfully carried out [4]. The resulting spectrum is 47
Volume 44A, number t
3,4
PHYSICS LETTERS
Cu METAL3~
3.0 i2.6
2:.2
1,8 90
I
I
85
I
I
I
I
80 75 BINDINGENERGY(eV)
I
1
TO
i
(;5
Fig. 2. The Cu 3p X-ray photoelectron spectrum obtained with MgKa radiation in a Varian spectrometer. The data have been fitted with two symmetrical lines on a sloping background. compared with the XPS density of states in fig. 1. A resolution of 0.4 to 0.5 eV is claimed for the SXS spectrum, but no correction has been applied for the width of the initial-state core hole. The XPS spectrum was obtained with a resolution of 0.5 eV and has been corrected only for the inelastic background. The difference in width between the two spectra, as well as the difference in the amount of structural detail in the two spectra supposedly obtained at similar resolution require explanation. An immediate source of concern is the width of the 3p hole state which was not known at the time of publication of ref. [4]. We have measured the Cu 3p XPS spectrum fig. 2, on an argon ion sputtered sampie using MgK~ radiation in a Varian lEE-15 spectrometer. Least squares analysis yields a spin-orbit splitting of 2.5 eV and a linewidth of 2.7 eV. The spin-
48
7 May 1973
nrbit splitting is slightly larger than ihat used in the unfolding of the SXS M2, 3 emission spectrum. A lower limit for the width of the 2p hole state is obtained by subtracting the instrumental resoluiion ~i 1.3 e g from the measured widih. I A lower limit is obtained because the instrumental resolulion inchldes the unknown width ol'the Au 4f line used in ltle determination.} The restllting width of 1.4 eV Iol the initial state of the M emissi~m princess provides a Iowe~ limit t\~r the resolution which can be achieved in the unfolding of Cu M emission spectra. This suggests strongly thal the detailed st ructure obtained in re i [4] does not have physical significance, and that the greater width of the SXS spectruin is due to the width t)t the initial state. It is also worth noting thai the L3 emission specl rum has a width considerably smaller that the M3 spectrum, but comparable to that of the XPS spectrum. This is in accord with the smaller width tound by XPS for the 2p core leveE, and indicates that transition probability effects need not be invoked to account for the difference between L and M emission spectra. We thank D. N. E. Buchanan for technical assistance
References [ 1 ] N.V. Smith, Phys. Rev. B3 ( 1971 ) 1862 ; see also Electronic density of states, ed. L.H. Bennett, NBS Special Publication No. 323 (USGPO Washington, D.C., 1971 ) p. 191. [2] D.E. Eastman, in Electron spectroscopy, ed. D.A. Shirley (North Holland, Amsterdam, 1972) p. 487, a~ld references cited therin [3] S. Htifner, G.K. Wertheim, N.V. Smith and M.M. Traum. Solid State Communications / l (1972) 323. 141 R.C. Dobbyn, M.L. Williams,J.R. Cuthill and A.J. McAlister, Phys. Rev. B2 (1970) 1563. [5] R. Haydock, V. Heine, M.J. Kelly and J.B. Pardry, Phys, Rev. Lett. 29 (1972) 868. [6] H.D. Hagstrum, Science 178 11972) 275.