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On the detection of elemental Li on AlLi alloy surfaces using Auger electron spectroscopy
~i~iii!ii!ii!ili~ii~i!iiiiiii!i!i!iiiii!iiiiiiiiiiiiiiiiii, !ii!'~!~,iiii!i!ii!ii~ , Applied Surf:,ce Science 62 (1992) 97-99 North-Holland
aop~
surface science
Letter
On the detection of elemental Li on AI-Li alloy surfaces using Auger electron spectroscopy D.J. Bottomley, G. Liipke, J. Bloch i, H.M. van Driel Department of Physics and Ontario Laser and Lightwave Research Centre. Unit'ersity of Toronto. Toronto. Ontario. Canada td5S 1,47
and R.S. Timsit Alean International Limited. Kingston t~,'seareh and Development (-'entre. Kingston, Ontario, (_anada KTL 5L9 Received 6 January 1992; accepted for publication 30 May 1992
We confirm that the KVV transition of elemental Li produces a line at 52 eV in the Auger electron spectrum of AI-Li alloys,. This eliminates confusion present hitherto on the labelling of Auger electron spectroscopy (AES) Li spectral lines from AI-Li alloy surfaces, A simple procedure is presented for interpreting AES data in order to establish a lower limit on the surface Li concentration, The presence of the elemental Li line at 52 eV means that the Li concentration at the surface of these commercially important alloys can be studied using AES.
Attempts at detecting elemental Li at the surface of Al-Li alloys using Auger electron spectroscopy (AES) have led to confusion on the labelling of AES Li spectral lines. Partridge has claimed that AES cannot detect elemental Li [1]. Gruen et al. attempted to use AES to detect Li at the surface of Cu-Li and Al-Li alloys [2] but stated that AES did not yield a quantitatively measurable Li signal at 43 eV from the surface of their Al-Li alloy with a bulk Li concentration of 0.32 at%. In the present paper, we present data which shows that the LiKVV transition on the clean surface of a dilute AI-Li alloy (bulk Li concentration of 6.5 at%) is located at 52 eV. We believe that this data will eliminate the confusion that has existed in the assignment of AES spectral lines for Li in its elemental and oxidized states, particularly in previous studies of Al-Li Permanent address: Nuclear Research Institute - Negev. P.O. Box 9001. Beer-Sheva, Israel.
alloys. Correct spectral assignment is essential to the use of AES for studying the surface properties of commercially important Al-Li alloys. We have prepared a clean surface of an AI-Li alloy in ultra-high vacuum by sputtering with 3.5 keV Ar ions at room temperature followed by annealing at 400°C. AES spectra were generated using a primary electron beam energy of 3 keV and a cylindrical electron energy analyzer (PHI 600). All spectra were obtained at a primary beam current of approximately 50 nA focused over a disk approximately 1 p.m in diameter. The large current density did not generate a significant temperature rise [3] and did not damage the oxide-free surface [3]. Fig. I shows the resulting AES spectrum taken at a temperature of 400"C and a pressure of 2 x 10 -'; Tort. For comparison, fig. 1 also shows the AES spectrum from a clean, high-purity Al specimen prepared similarly. The line at 68 eV corresponds to the well-known A I L W transition. Both specimens yield sec-
0169-4332/92/$05.(X) ~* 1992 - Elsevier Science Publishers B,V, All rights reserved
D.Z Bottomh'y el aL / Detection of elemental Li on AI-Li alloy surfaces using ,4ES
o8 7-
6-
32103O
----
AI.4~.5;II ¢; l.i AI
4'(I
l,
t r . . . . 50 I~(~ 7(I K i n e t i c E n e r g y (cV~
i ~tl
Fig. 1. Auger electron spectra from the clean AI-Li alloy surface and from pure AI. The two curves are not to scale.
ondary weak lines at approximately 40 and 52 eV. The secondary lines from high-purity AI arise from collective excitations (plasmons) [4]. Note that the secondary line at 52 eV yielded by the alloy is more intense than that from pure AI and arises in part from the LiKVV transition. In order to evaluate the contribution made by Li to the 52 eV line from the alloy it is necessary to deconvolve this line from the AI plasmon line. The simplest method is to assume that the ratio of the collective excitation at 52 eV to the AI LVV line at 68 eV is the same for both samples, as done by Testoni and Stair [5]. This assumption is reasonable since the plasmon line at 52 eV is unlikely to shift significantly from the spectral location yielded by pure AI, because the free electron densities in pure AI and the AI-Li alloy do not differ appreciably. Using the deconvolution procedure we calculate that 77% + 3% of the secondary line at 52 cV from the alloy surface is attributable to the presence of Li. In contrast to the line at 52 eV, the magnitude of the secondary line at 40 eV yielded by the alloy agrees with the magnitude of the corresponding line from pure AI. In view of previous work [6,7], it is not surprising that Li at the surface of an AI-Li alloy produces an Auger line at 52 eV. Clausing, Easton and Powell detected the KVV transition from clean Li at 52 eV in 1973 [61. This has been
confirmed in more recent work [71, which also showed that Li20 produces an Auger line at 40 eV. Jardin and Robert [8] have shown that Li20 is difficult to observe non-invasively using AES. They discovered that primary current densities > 1 mA/cm 2 result in decomposition of this oxide in vacuum. The attempt by Gruen et al. [2] to observe Li on metal surfaces at 43 eV rather than at 52 eV is not surprising because only few studits of Li by AES have been carried out and the reference spectrum [9] used by these authors identifies the Li line of LiF at 43 eV. In the LiF Auger spectrum, the energy of the elemental Li line is chemically shifted from 52 to 43 eV. The similarity in the energy of the Li line in Li,_O and LiF, which arises from the chemical similarity of oxygen and fluorine, is fortuitous. Quantitative analysis of the alloy spectrum is hindered by the absence of firm data on the elemental sensitivity factor of Li [9]. 111 the light of this absence we have obtained a value for the elemental sensitivity factor of Li by extrapolating the values at a primary electron beam energy of 3 keV for the KVV transitions in C, B and Be. This yields a value of 0.14 _+0.01 for Li. This value should be treated cautiously for two reasons: extrapolation is inherently unreliable and Auger li-es below 100 eV are especially susceptible to distortion by magnetic fields and geometrical effects [9]. Quantitative analysis is further hindered by the well-known fact that the Li is distributed nonuniformly with depth [2,10]. In order to obtain a lower limit on the surface Li concentration we assume that the Li is distributed uniformly throughout the depth to which the AES lines are sensitive (approximately 10 A). A further correction is necessary since electrons emitted at the energies of the two Auger lines in question have different escape depths., According to Prutton [1 ll, these are 10 and 8 A for electron energies of 52 and 68 eV, respectively. Finally we discount the possibility that a significant fraction of the Li at the surface is oxidized because, from fig. 1, the 40 eV line from the alloy is no larger than that obtained from pure AI. The resulting lower limit on the surface Li concentration is 18_ 2 at%. This value differs from the results of Gruen et al. [2] who detected the formation of a monolayer of
D.J. Bottomley et aL / Detection o fdemental Li on AI-Li alloy surfaces nshlg AES
Li on the surface of an AI-Li alloy with a bulk Li concentration of 0.32 at% after annealing at 500°C by secondary ion mass spectrometry. However, our results do not necessarily disagree since we have only established a lower limit for the Li concentration at the surface. Before attempting any comparison of this value with theoretical predictions of the surface Li concentration, it is important to realise 'that at the temperature of 400°C used in this experiment, the vapour pressure of Li (in equilibrium) is between 10 -s and 10 -4 Torr [12]. Therefore, in this experiment Li is being lost to the vacuum and being replenished at the surface by outward diffusion from the bulk. However, the chamber used in the experiments performed here was dynamically pumped, and the pressure did not in fact exceed 2 x 10 - ' Torr. The model of Miedema (see ref. [2}) predicts a surface Li concentration of 81 at% for the alloy used in the work reported here. The wide discrepancy between this prediction and our lower limit may therefore be attributable in part to Li vaporization. Since the Li is evaporating from the surface, we surmise that the Li concentration is maximal at the surface consistent with the data of ref. [10]. In conclusion, we have confirmed that Li produces a line at 52 eV in the Auger spectrum of clean AI-Li alloys. The intensity of this line may be used for determining the Li concentration at the surface of these commercially important al-
99
~oys by AES, provided a suitable calibration standard is used, and the depth distribution of the Li is established. We gratefully acknowledge the financial support of the Natural Science and Engineering Research Council of Canada, the Ontario Laser and Lightwave Research Centre and Alcan International Limited. References {ll P.G. Partridge, Int. Mater, Rev. 35 (10901 37. [2] D.M, Gruen, A.R. Krauss, S. Susman. M. Ven_,gopalan and M. Ron. J. Vac. Sci. Technol. A I (1983) 924. [3] L.W. Hohbs, in: Introduction to Analytical Electron Microscopy, Eds. J.J. Hren, J.I. Goldstein and I~.C. Joy IPlenum, New ~'ork, 19"/9) p. 437. [4l C. Kittel, Introduction to Solid State Physics, 6th ed. (Wiley, New York, 1986) p. 262. [5] A.L, Testoni and P.C. Stair, Surf. Sci. 171 (1986) !_491. [6] R.E. Clausing, D.S. Easton and G.L. Powell. Surf. Sci, (1973) 377. [7] G. Hanke and K. Mh|ler, Surf. Sci. 152/153 (1985) 902, [8] C. Jardin and D. Robert. Appl. Surf. Sci. 35 (1988) 495, [9] Handbook of Auger Electron Spectroscopy. Eds. L. Davis, N. MacDonald, K. Palmberg. G. Riach and R. Weber (Perkin-Elmer, Eden Prairie, MN, 19"/8). llOl C. Moreau. E.J. Knystautas, R,S. Timsi: and R. Groleau. NucL Instr. Meth. Phys. Res. 218 (1983) Ill. [I 1] M. Prutton, Surface Physics, 2nd ed. (Oxford Univ. Press, Oxford, 1987~ p. 2.3. [12] D.G. Lord and T.E. Gallon. SurL Sci. 36 (1973) 606: Handbook of the Physio-Chemical Properties of the Elements, Ed, G,V. Samsonov (OIdbourne. London. 1968).
aop~
surface science
Letter
On the detection of elemental Li on AI-Li alloy surfaces using Auger electron spectroscopy D.J. Bottomley, G. Liipke, J. Bloch i, H.M. van Driel Department of Physics and Ontario Laser and Lightwave Research Centre. Unit'ersity of Toronto. Toronto. Ontario. Canada td5S 1,47
and R.S. Timsit Alean International Limited. Kingston t~,'seareh and Development (-'entre. Kingston, Ontario, (_anada KTL 5L9 Received 6 January 1992; accepted for publication 30 May 1992
We confirm that the KVV transition of elemental Li produces a line at 52 eV in the Auger electron spectrum of AI-Li alloys,. This eliminates confusion present hitherto on the labelling of Auger electron spectroscopy (AES) Li spectral lines from AI-Li alloy surfaces, A simple procedure is presented for interpreting AES data in order to establish a lower limit on the surface Li concentration, The presence of the elemental Li line at 52 eV means that the Li concentration at the surface of these commercially important alloys can be studied using AES.
Attempts at detecting elemental Li at the surface of Al-Li alloys using Auger electron spectroscopy (AES) have led to confusion on the labelling of AES Li spectral lines. Partridge has claimed that AES cannot detect elemental Li [1]. Gruen et al. attempted to use AES to detect Li at the surface of Cu-Li and Al-Li alloys [2] but stated that AES did not yield a quantitatively measurable Li signal at 43 eV from the surface of their Al-Li alloy with a bulk Li concentration of 0.32 at%. In the present paper, we present data which shows that the LiKVV transition on the clean surface of a dilute AI-Li alloy (bulk Li concentration of 6.5 at%) is located at 52 eV. We believe that this data will eliminate the confusion that has existed in the assignment of AES spectral lines for Li in its elemental and oxidized states, particularly in previous studies of Al-Li Permanent address: Nuclear Research Institute - Negev. P.O. Box 9001. Beer-Sheva, Israel.
alloys. Correct spectral assignment is essential to the use of AES for studying the surface properties of commercially important Al-Li alloys. We have prepared a clean surface of an AI-Li alloy in ultra-high vacuum by sputtering with 3.5 keV Ar ions at room temperature followed by annealing at 400°C. AES spectra were generated using a primary electron beam energy of 3 keV and a cylindrical electron energy analyzer (PHI 600). All spectra were obtained at a primary beam current of approximately 50 nA focused over a disk approximately 1 p.m in diameter. The large current density did not generate a significant temperature rise [3] and did not damage the oxide-free surface [3]. Fig. I shows the resulting AES spectrum taken at a temperature of 400"C and a pressure of 2 x 10 -'; Tort. For comparison, fig. 1 also shows the AES spectrum from a clean, high-purity Al specimen prepared similarly. The line at 68 eV corresponds to the well-known A I L W transition. Both specimens yield sec-
0169-4332/92/$05.(X) ~* 1992 - Elsevier Science Publishers B,V, All rights reserved
D.Z Bottomh'y el aL / Detection of elemental Li on AI-Li alloy surfaces using ,4ES
o8 7-
6-
32103O
----
AI.4~.5;II ¢; l.i AI
4'(I
l,
t r . . . . 50 I~(~ 7(I K i n e t i c E n e r g y (cV~
i ~tl
Fig. 1. Auger electron spectra from the clean AI-Li alloy surface and from pure AI. The two curves are not to scale.
ondary weak lines at approximately 40 and 52 eV. The secondary lines from high-purity AI arise from collective excitations (plasmons) [4]. Note that the secondary line at 52 eV yielded by the alloy is more intense than that from pure AI and arises in part from the LiKVV transition. In order to evaluate the contribution made by Li to the 52 eV line from the alloy it is necessary to deconvolve this line from the AI plasmon line. The simplest method is to assume that the ratio of the collective excitation at 52 eV to the AI LVV line at 68 eV is the same for both samples, as done by Testoni and Stair [5]. This assumption is reasonable since the plasmon line at 52 eV is unlikely to shift significantly from the spectral location yielded by pure AI, because the free electron densities in pure AI and the AI-Li alloy do not differ appreciably. Using the deconvolution procedure we calculate that 77% + 3% of the secondary line at 52 cV from the alloy surface is attributable to the presence of Li. In contrast to the line at 52 eV, the magnitude of the secondary line at 40 eV yielded by the alloy agrees with the magnitude of the corresponding line from pure AI. In view of previous work [6,7], it is not surprising that Li at the surface of an AI-Li alloy produces an Auger line at 52 eV. Clausing, Easton and Powell detected the KVV transition from clean Li at 52 eV in 1973 [61. This has been
confirmed in more recent work [71, which also showed that Li20 produces an Auger line at 40 eV. Jardin and Robert [8] have shown that Li20 is difficult to observe non-invasively using AES. They discovered that primary current densities > 1 mA/cm 2 result in decomposition of this oxide in vacuum. The attempt by Gruen et al. [2] to observe Li on metal surfaces at 43 eV rather than at 52 eV is not surprising because only few studits of Li by AES have been carried out and the reference spectrum [9] used by these authors identifies the Li line of LiF at 43 eV. In the LiF Auger spectrum, the energy of the elemental Li line is chemically shifted from 52 to 43 eV. The similarity in the energy of the Li line in Li,_O and LiF, which arises from the chemical similarity of oxygen and fluorine, is fortuitous. Quantitative analysis of the alloy spectrum is hindered by the absence of firm data on the elemental sensitivity factor of Li [9]. 111 the light of this absence we have obtained a value for the elemental sensitivity factor of Li by extrapolating the values at a primary electron beam energy of 3 keV for the KVV transitions in C, B and Be. This yields a value of 0.14 _+0.01 for Li. This value should be treated cautiously for two reasons: extrapolation is inherently unreliable and Auger li-es below 100 eV are especially susceptible to distortion by magnetic fields and geometrical effects [9]. Quantitative analysis is further hindered by the well-known fact that the Li is distributed nonuniformly with depth [2,10]. In order to obtain a lower limit on the surface Li concentration we assume that the Li is distributed uniformly throughout the depth to which the AES lines are sensitive (approximately 10 A). A further correction is necessary since electrons emitted at the energies of the two Auger lines in question have different escape depths., According to Prutton [1 ll, these are 10 and 8 A for electron energies of 52 and 68 eV, respectively. Finally we discount the possibility that a significant fraction of the Li at the surface is oxidized because, from fig. 1, the 40 eV line from the alloy is no larger than that obtained from pure AI. The resulting lower limit on the surface Li concentration is 18_ 2 at%. This value differs from the results of Gruen et al. [2] who detected the formation of a monolayer of
D.J. Bottomley et aL / Detection o fdemental Li on AI-Li alloy surfaces nshlg AES
Li on the surface of an AI-Li alloy with a bulk Li concentration of 0.32 at% after annealing at 500°C by secondary ion mass spectrometry. However, our results do not necessarily disagree since we have only established a lower limit for the Li concentration at the surface. Before attempting any comparison of this value with theoretical predictions of the surface Li concentration, it is important to realise 'that at the temperature of 400°C used in this experiment, the vapour pressure of Li (in equilibrium) is between 10 -s and 10 -4 Torr [12]. Therefore, in this experiment Li is being lost to the vacuum and being replenished at the surface by outward diffusion from the bulk. However, the chamber used in the experiments performed here was dynamically pumped, and the pressure did not in fact exceed 2 x 10 - ' Torr. The model of Miedema (see ref. [2}) predicts a surface Li concentration of 81 at% for the alloy used in the work reported here. The wide discrepancy between this prediction and our lower limit may therefore be attributable in part to Li vaporization. Since the Li is evaporating from the surface, we surmise that the Li concentration is maximal at the surface consistent with the data of ref. [10]. In conclusion, we have confirmed that Li produces a line at 52 eV in the Auger spectrum of clean AI-Li alloys. The intensity of this line may be used for determining the Li concentration at the surface of these commercially important al-
99
~oys by AES, provided a suitable calibration standard is used, and the depth distribution of the Li is established. We gratefully acknowledge the financial support of the Natural Science and Engineering Research Council of Canada, the Ontario Laser and Lightwave Research Centre and Alcan International Limited. References {ll P.G. Partridge, Int. Mater, Rev. 35 (10901 37. [2] D.M, Gruen, A.R. Krauss, S. Susman. M. Ven_,gopalan and M. Ron. J. Vac. Sci. Technol. A I (1983) 924. [3] L.W. Hohbs, in: Introduction to Analytical Electron Microscopy, Eds. J.J. Hren, J.I. Goldstein and I~.C. Joy IPlenum, New ~'ork, 19"/9) p. 437. [4l C. Kittel, Introduction to Solid State Physics, 6th ed. (Wiley, New York, 1986) p. 262. [5] A.L, Testoni and P.C. Stair, Surf. Sci. 171 (1986) !_491. [6] R.E. Clausing, D.S. Easton and G.L. Powell. Surf. Sci, (1973) 377. [7] G. Hanke and K. Mh|ler, Surf. Sci. 152/153 (1985) 902, [8] C. Jardin and D. Robert. Appl. Surf. Sci. 35 (1988) 495, [9] Handbook of Auger Electron Spectroscopy. Eds. L. Davis, N. MacDonald, K. Palmberg. G. Riach and R. Weber (Perkin-Elmer, Eden Prairie, MN, 19"/8). llOl C. Moreau. E.J. Knystautas, R,S. Timsi: and R. Groleau. NucL Instr. Meth. Phys. Res. 218 (1983) Ill. [I 1] M. Prutton, Surface Physics, 2nd ed. (Oxford Univ. Press, Oxford, 1987~ p. 2.3. [12] D.G. Lord and T.E. Gallon. SurL Sci. 36 (1973) 606: Handbook of the Physio-Chemical Properties of the Elements, Ed, G,V. Samsonov (OIdbourne. London. 1968).