Ar+ and e− excited argon Auger electron emission from Mg, Al, Si, GaP and Ti surfaces

Surface Science 80 (1979) 656-662 0 North-Holland Publishing Company

Ar’ AND e- EXCITED ARGON AUGER ELECTRON EMISSION FROM Mg, Al, Si, GaP AND Ti SURFACES P. VIARIS DE LESEGNO and J.-F. HENNEQUIN, CNRS, Laboratoire PMTM, UniversitiParis-Nord,Avenue Jean-Baptiste Clkment, F-93430 Villetaneuse,France

Argon Auger emission excited by 3-16 keV Ar+ ion bombardment of Mg, Al, Si, GaP and Ti targets is studied as a function of the ion energy and compared to 2 keV electron-excited Auger emission. Contrary to fist series transition metals where collisions such as Ar + Ti are effective for argon excitation, only the rare collisions Ar * Ar (precedently implanted Ar atoms) remain in Mg and Al targets. To this process, another must be added to explain the relatively high argon Auger emission from Si and GaP targets at lower ion energies: collisions of excited energetic recoil Si’or P atoms against pre-implanted Ar atoms seem to become effective too. Application of argon Auger emission to quantitative analysis of implanted argon in any target is considered.

1. Introduction Auger electrons are emitted during the bombardment of solid targets by Ar’ ions having an energy in the keV range. Some of them are characteristic of the target itself [ 1,2], other of the projectiles used [3-S]. As in gaseous collisions, an argon atom could keep one or two vacancies in the 2p level after a sufficiently violent collision against another atom with an equal or slightly higher mass number [6]. In that case, electron transitions occur via radial coupling at the crossings of the 4 fu molecular orbital, correlated to the argon 2p level, with other empty or partially filled molecular orbitals [4]. Other excitation processes appear to be negligible in gases within the primary ion energy range used (below 16 keV). As the argon fluorescence yield is about 10m3[7], de-excitation by Auger effect is much more probable than X-ray emission. If the de-excitation of the excited argon atom occurs near the surface or outside the target, it gives rise to those Auger electrons that appear as peaks around 210 eV in the energy spectrum of secondary electrons. As a result of the excitation process, argon Auger electron emission is particularly high during bombardment of potassium, calcium and first series transition metals targets [4], because asymmetric collisions such as Ar + Ti are numerous and, in that case, effective. On the contrary, with light metals targets, only rare symmetric collisions Ar + Ar against precedently implanted argon atoms can lead 656

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to argon excitation: the argon Auger emission is then much lower and had even escaped the observation in first ‘experiments [4]. In this paper, our purpose is a comparative study of ion- and electron-excited argon Auger emission from different solid targets with a new, more sensitive apparatus.

2. Experimental apparatus The experimental data were taken in a conventional ultrahigh-vacuum system with ion pumping (fig. 1). The base pressure is 5 X lo-” Torr without baking, increasing to 2 X lo-’ Torr during ion bombardment. A commercial cylindrical mirror analyzer (CMA) allows energy analysis of secondary electrons. The electron gun for Auger electron spectroscopy (AES) is coaxial with the CMA whereas the ion beam axis is inclined at 80” to the CMA axis. The electron beam (-5 PA, 2 keV) is focused on a small area with 0.2 mm diameter at the position of optimum energy resolution. The Ar’ ions are emitted from a high-frequency ion source with differential pumping. A first electrostatic lens focuses the ion beam into a small aperture (0.6 mm diameter) which can be closed by a gate valve and which separates the poor vacuum near the source from the high vacuum inside the apparatus. A second lens focuses the ion beam onto the sample, the normal of which has an angle of 30” with the CMA axis and 50” with the ion beam axis. The ion intensity, measured with a Faraday cup, varies between 1 PA at 4 keV and 14 PA at 16 keV; the sputtered area is about 0.3 X 0.5 mm.

HF ion source

sample

1holder

deflection

plates

Fig. 1. Schematic of the experimental apparatus (exact centering of the ion beam into the intermediate aperture is achieved by mechanical displacement, centering on the target by the four pairs of deflection plates. A movable shield allows to protect the electron gun from contamination by sputtered products).

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The different targets studied (polycrystalline Mg, Al, Si and Ti samples, and a GaP monocrystal) are mounted on a carrousel, inside which a Faraday cup with several apertures of different diameters allows us to check the focusing of the ion beam at the same point as the electron beam. Another method allows an exact alignment of the ion beam: if the AES analyzed area does not exactly coincide with the sputtered area (or is not inside it), a carbon peak appears at 272 eV in the AES Auger spectrum. A modulation technique is used to obtain Auger spectra in the usual derivative form dnQ/ti. But Auger spectra can also be obtained as n(E) by measuring the current at the output of the electron multiplier (with dc insulation by optocouplers [S]), and by electronically dividing it by the energy E, as the energy resolution AE/E is a CMA constant (the yield of the electron multiplier is assumed to be nearly constant for small variations of E).

3. Experimental results 3.1. Argon Auger spectra At first sight, the argon Auger spectrum is characteristic for atomic argon de-excitations and should be nearly independent of the nature of the target. In fact, some differences are observed between the Auger spectra shown in derivative mode in fig. 2. With Mg and Al targets, the electron- and ion-excited Auger spectra are quite

A dn(E)

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2lf Jf *r+&

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200

250

150

200

250

150

200

E/eV

Fig. 2. Comparison between AI+ and e- excited argon Auger spectra from Al, Si and Ti targets. The Ar’ ion energy is 7 keV with Si and 10 keV with Al and Ti. Auger spectra from Mg are quite similar to those from Al, Auger spectra from GaP to those from Si.

P. Viatisde Lesegno,J.-F. Hennequin/Argon Auger electronemission

Fig. 3. AES argon peak intensities, indicating the argon concentration versus primary ion energy (saturation values).

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near the surface region,

similar and both show a main peak * at 216 eV (L2sMz3M2s Auger de-excitation) associated with two or three bulk plasmon loss peaks. With Si, GaP and Ti targets, besides the main peak with one or two plasmon loss peaks (which are hardly observable in the Ar’ excited Auger spectrum from Ti), the ion-excited Auger spectra exhibit an additional peak at higher energy (ca. 230 eV) probably corresponding to a (L2s)sM2sM2s transition (Auger effect with two vacancies in the initial Ar 2p level). Surface charge effects on Si and GaF targets give rise to opposite displacements of the electron- and ion-excited Auger peaks, which can be suppressed by an additional low-energy bombardment with oppositely charged particles, to cancel the total target current. 3.2. Ion energy dependence of Auger intensities As the mean free path of a 210 eV electron inside a solid is nearly equal to 6 R [9], the electron-excited argon Auger intensities are proportional to the implanted argon concentration in the first atomic layers of the targets. As shown on fig. 3, the argon concentration appears to depend very little upon the nature of the target and slightly decreases for increasing ion energy, as previously observed for silicon [5]. In contrast, the ion-excited argon Auger intensities (normalized to the same ion current) strongly depend upon the primary ion energy. The Auger intensity from a Ti target abruptly increases above 6 keV and becomes the highest one beyond 9 keV. With a scaling factor, it reproduces the * According to the usual AES convention, the “position” of a peak is taken as the point of the minimum in the high energy wing of the first derivative. A “peak” at 216 eV in dn(E)/dE thus corresponds to a peak at ca. 210 eV in the true energy spectrum n(E).

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.

Mg Al Si Ti

1O-22 _

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2

L

6

8

10

12

E i,,/ keV

Fig. 4. Ion excited argon Auger intensities versus primary ion energy, scaled to fit the theore& cal cross-section (solid curves) for 2p vacancy formation in argon after an Ar + Ar or Ar + Ti collision [4]. The scaling factors for Mg, Al, Si, Ti are respectively proportional to 1, 1.4, 1 and 4 X 1 0w3 (the Auger Intensity from Ti is In fact higher than the others by an order of magnitude above 10 keV).

cross section for 2p vacancy formation in an argon atom after an Ar + Ti collision (fig. 4), as earlier studied by Auger emission [4] and by X-ray emission [lo]. At the same ion energy, the Auger intensities from Mg and Al targets are similar to that from Ti target below 5 keV, but they reach no more than 10% of it at 10 keV; they have nearly the same values (&s = 1.41,.& and show the same energy dependence which reproduces the cross-section for 2p vacancy formation in arr argon atom after an Ar -+ Ar collision, as observed by X-ray emission [7]. The ion energy dependences of argon Auger emission from Si and GaP targets appear quite similar to each other, but very different from those from Mg, Al or Ti targets. As observed for Si in ref. [S], the Auger emission already begins at 3 keV and increases more slowly beyond 10 keV; it always remains higher than that of Mg and Al, but becomes lower than that of Ti above 9 keV. This dependence cannot~be explained either by Ar + Ar collisions or by Ar + Si collisions which .could be effective for argon at very high energy only. Besides, the other Auger peak at 230 eV does not seem to show the same energy dependence: relative to the main peak, its height is about 20% at 5 keV and 1% at 7 keV. It vanishes into the high energy side of the 216 eV peak beyond 10 keV.

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4. Discussion

In a Ti target, as in other first series transition metals [4], the ion-excited argon Auger emission is principally due to Ar -+ Ti collisions, where one or two vacancies are created in the 2p level of the argon atom, because after an asymmetric collision the vacancies are preferably created in the colliding atom with lower mass number [6]. In that case, precedently implanted Ar atoms bring only a very small contribution. This first process cannot be effective with the other targets, because a collision such as Ar -+ Al leads to a vacancy in the 2p level of the Al atom only (with a subsequent Al Auger emission [ 1,2], like after symmetric collisions Al + Al), as long as the ion energy does not exceed about 70 keV in the laboratory system [lo]. As proved by its energy dependence, the argon Auger emission from Mg and Al is only due to symmetric Ar + Ar collisions of incident Ar projectiles against pre-implanted Ar atoms. Two facts confirm each other: the low intensity of the Auger emission, due to the small argon concentration in the first layers of the target, and the absence of the further Auger peak (if two vacancies are created during a symmetric collision, they are equally shared between both colliding atoms and practically no Ar atom should have two 2p vacancies after such a collision). A third process must be added to the second one in order to explain the relatively high intensity of argon Auger emission from a Si target, especially at lower ion energies. The existence of a high energy peak in the argon Auger spectrum, like for Ti, proves that asymmetric collisions occur, after which the two vacancies possibly created remain in the Ar atom. The Ar + Si collisions are excluded, because they lead to 2p vacancies only in Si atoms in the energy range studied. It seems necessary to consider the collisions of Si* atoms (energetic recoil Si atoms with one or two 2p vacancies) against pre-implanted Ar atoms. The energies of the Ar and excited Si* M levels would be sufficiently close to allow interaction. Vacancies could therefore be present on the 3 su and 3 pn molecular orbitals, correlated to the Ar M levels in an Ar-Si collision, and they might be transferred by radial coupling into the 3 da orbital correlated to the argon 2p level (the Ar-Si system has a correlation diagram quite similar to the K-Cl system shown on fig. 8 of ref. [6]). This explanation is confirmed by the fact that argon Auger emission from GaP target shows the same behaviour as that from Si target (in GaP, only the P* + Ar collisions are effective).

5. Conclusion

The present experiments show the interest of a comparative study between ionand electron-excited Auger emissions. A possible practical application could be the measurement of the argon concentration in the first atomic layers of any target. As the cross-sections for 2p vacancy formation in an argon atom after Ar + Ar and

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Ar + Ti collisions are known, it should be possible to calibrate the AES argon peak by the ion-excited argon Auger intensities from a Ti target (to determine the instrumental transmissivity) and from an Al target (to obtain the absolute value of the argon concentration).

Acknowledgements

The authors would like to thank Professor J. Baudon for helpful discussions about the electron promotion model. They are very much indebted to Mr. J.-P. Leroux for valuable technical assistance.

References [l] J.-F. Hennequm and P. Viaris de Lesegno, Surface Sci. 42 (1974) 50. [2] L. Viel, C. Benazeth and N. Benazeth, Surface Sci. 54 (1976) 635. [3] C. Snoek, R. Geballe, W.F. van der Weg, P.K. Rol and D.J. Bierman, Physica 31 (1965) 1553. [4] P. Viaris de Lesegno and J.-F. Hennequin, J. Phys. (Paris) 3.5 (1974) 759. [5] J. Kempf and G. Kaus, Appl. Phys. 13 (1977) 261. [6] M. Barat and W. Lichten, Phys. Rev. A6 (1972) 211. [7] F.W. Saris and D. Onderdelinden, Physica 49 (1970) 441. [8] R.-L. Inglebert and P. Viaris de Lesegno, Rev. Phys. Appl. 10 (1975) 339. [9] J.C. Tracy, in: Photoelectron and Auger Spectroscopy, Ed. T.A. Carlson (Plenum, 1975). [lo] F.W. Saris, Physica 52 (1971) 290.