Auger line shape measurements using positron annihilation induced Auger electron spectroscopy

Journal of Electron Spectroscopy and Related Phenomena 72 (1995) 305-309

Auger Line Shape Measurements Using Positron Annihilation Induced Auger Electron Spectroscopy A.H. Weiss, a,b S.Yang, a H.Q. Zhou, a E. Jung, a and S. Wheeler a a Physics Department, Box 19059, The University of Texas at Arhngton, Arlington, TX 76019, U.S.A b Materials Science Program, Box 19059, The University of Texas at Arlington, Arlington, TX 76019, U.S.A

ABSTRACT

Conventional methods of Auger electron spectroscopy (AES) make use of energetic electron or photon beams to create the core-hole excitations that lead to the Auger transition. The energetic beams result in a large secondary electron background in the Auger peak region. In positron annihilation induced Auger electron spectroscopy (PALES), the core holes are created by matter-antimatter annihilation and not through collisional ionization. Measurements are reviewed which indicate that PAES can eliminate the secondary electron background by the use of very low (~10 eV) positron beam energies and that PALEShas greatly increased surface selectivity due to the trapping of positrons in surface state prior to annihilation. A new PALES spectrometer has been developed in our laboratory with an energy resolution which is an order of magnitude better than previous PAES spectrometers. The high-resolution PAES system has been used to measure the Auger M2,3VV line shape from a clean polycrystalline Cu surface. The atomic-like Auger M2VV and M3VV features are clearly resolved. Differences observed between the PAES spectra and spectra obtained using electron induced Auger spectroscopy are discussed. INTRODUCTION Positron annihilation induced Auger electron spectroscopy (PAES)[1] makes use of low energy positrons to create the core ionizations necessary for Auger spectroscopy via annihilation of core electrons. This unique ionization mechanism gives PAES some significant advantages over conventional Auger methods [2,3] including: 1. the elimination of the very large secondary electron background which makes it impossible to unambiguously determine Auger lineshapes using conventional Auger techniques and 2. increased surface selectivity which permits PALES to be used to characterize the elemental content of the topmost atomic layer. The PAES mechanism, first demonstrated in 1987 [1], can be outlined as follows: 1. Positrons implanted at low energy rapidly lose energy, diffuse to and get trapped at the surface. 2. Though most annihilate with valence electrons, a few percent of the trapped positrons annihilate with core electrons leaving atoms in excited states. 3. The atoms relax via emission of Auger electrons. In conventional Auger electron spectroscopy (AES) the electron or photon beam energy must exceed the core-hole ionization energy (typically by a factor of 3 to 5 times). As a consequence, a secondary electron background is present over the entire range

of the Auger spectral features. While this background is typically not a problem for the higher energy Auger lines, the background problem is particular pronounced for the lower energy Auger lines since the secondary electron background is largest in their energy range. Although sophisticated models of the background have been developed [4-8], the Auger line shape of these lower energy lines cannot be definitely ascertained using conventional AES methods because spectral intensity due to intrinsic energy loss events (i.e. shake up and shake off) overlap with intensity due to extrinsic loss mechanisms (i.e. those associated with energy losses of the Auger electron as they emerge from the bulk) and other portions of the secondary electron background. In addition, Auger electrons originate from an excitation vohune which extends hundreds of atomic layers below the surface, the Auger signal represents an average over several atomic layers (approximately 4-20 A). This introduces an ambiguity in the determination of true surface Auger line shapes. In PAES, the core holes are created by matter-antimatter annihilation and not by collisional ionization. Therefore, incident beam energies well below the Auger electron energy may be used. The use of low energy incident beams effectively eliminates the large secondary electron background as

0368-2048/95 $09.50 © 1995 Elsevier Science B.V. All fights reserved 5"SDI 0 3 6 8 - 2 0 4 8 ( 9 4 ) 0 2 3 1 6 - 6

306 secondary electrons cannot be created through collisional processes with energies in excess of an energy E k given by: KEsec < E k = Ep - 9- + 9 +

A comparison between EAES and PAES spectra taken with our original low resolution PAES spectrometer is shown in Fig. 1, demonstrates the ability of PAES to eliminate the large secondary electron background associated with electron and photon excited Auger electron emission [9]. The EAES signal (shown in Figs 1 (a) and 1 ( c ) ) can be seen siting on top of a large secondary electron background. This background is almost entirely absent in the positron annihilation induced Auger spectra shown in Figs. 1 (b) and 1 (d). Note the rapid increase in the number of detected electrons below the energy Ek~25 eV due to the onset of colfisionally excited secondary electrons. To confirm that the secondary electron background does not extend under the Cu M23VV Auger peak a Cs overlayer was deposited on the Cu. The Cs causes a depopulation of the positron surface state by promoting the thermal desorption of Ps at room temperature. This effectively turns off the PAES signal as can be seen in Fig. 2 in which the signals form clean and Cs coated surfaces are compared. Note the very low level of background in the energy range of the Cu Auger peak. Dark counts account for approximately half of this background.

(1)

where KEse c is the kinetic energy with which the secondary electrons leave the surface of the sample, Ep is the kinetic energy of the primary beam at the sample surface, and 9- and 9 + are the positron and electron work functions respectively [9]. The enhanced surface selectivity of PAES stems from the fact that positrons implanted into a metal or semiconductor at low energies have a high probability of becoming trapped in an "imagecorrelation" well before they annihilate [10]. The positrons in this well are localized at the surface and annihilate almost exclusively with atoms at the surface. As a result almost all of the Auger electrons originate from the top-most atomic layer [11], so it will be possible to separate out contributions to the Auger spectra from intrinsic energy loss processes since intensity due to extrinsic process are greatly reduced. In the remainder of this paper, we will briefly present data indicating the reduced background and enhanced surface selectivity that is possible with PAES. In addition we will present preliminary results of the first high resolution PAES measurements (currently in progress) and compare them with results obtained using electron induced Auger (EAES). SECONDARY ELECTRON ELIMINATION 1.0 12. >OC < (:3 Z 0

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BACKGROUND

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SURFACE SELECTIVITY The ability of PAES to determine the elemental content of the topmost atomic layer has been exploited in a number of investigations at University of Texas at Arlington (UTA) aimed at the study of the growth and stability of ultrathin metal layers on metal substrates [12-18]. Measurements by Lee et al. on ultrathin layers of Au on Cu [19] demonstrate the lfigh degree of surface selectivity

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Fig. 2. PAES spectra obtained (a) from a clean Cu(ll0) surface, and (b) the same surface with a saturation coverage of Cs. Both spectra have been normalized to the incident beam current. Note the large increase in the secondary electron background below the energy Ek (indicated by a dotted line). Energy conservation permits the collisional excitation of secondary electrons below this energy [from reference 9]. attainable using PAES. In Fig. 3 it may be seen that the PAES signal from Au is almost completely saturated and the Cu signal is almost zero at one monolayer Au coverage on Cu(100). This is consistent with detailed theoretical calculations which indicate that the positron wave function for the surface trapped positron does not significantly penetrate into the bulk and that as a result - 9 5 % of the annihilation induced Auger electrons are excited in the topmost atomic layer.

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0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 i.8 2.0 Amount of Au Deposited (ML) Fig. 3. PAES intensity versus deposition time of Au deposited on Cu(100). PAES measurement for Au deposition on an annealed Cu substrate taken after deposition at 173 K. Open triangles correspond to measured Au PAES intensities an Cu PAES intensities respectively. Dashed lines were obtained from a fit to a function of the form A + Be Cx. Solid triangles and squares (connected by solid lines) represent values obtained from model calculations for the PAES intensities of Au and Cu respectively.

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HIGH RESOLUTION PAES MEASUREMENTS

The experiments were performed using the newly completed high resolution PAES system at U n i v e r s i t y o f T e x a s at A r l i n g t o n , s h o w n schematically in Fig 4. [20] The PAES and EAES measurements of Cu M2,3 W Auger spectra are shown in Figs 5(a) and (b), respectively. While both spectra are dominated by a large secondary electron feature peaked at - 10 eV and another smaller feature at higher energy corresponding to the Cu M2,3 VV Auger transition, significant differences are evident. The major difference is seen on the high kinetic energy side of the secondary electron feature. For the PAES spectrum, the cut-off (kinetic edge) of the secondary

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electrons, which is determined by the positron beam energy (10 eV) and the sample bias (-2.5 V), occurs at 15 eV, therefore, the contribution of secondary electrons in the Cu M2,3VVAuger spectral region is completed eliminated. However, for the EAES measurement, due to the much higher energy (735 eV) of the electron beam used to create the coreholes, the secondary electron feature tails off slowly and contributes a large background intensity underneath the Cu M2,3VV Auger feature. It can be estimated from Fig. 5(b) that the secondary electron background intensity at Ek = 65 eV is about four times larger than the actual Cu M2,3VV Auger intensity measured. Figure 6 shows a close-up of the Cu M2,3 VV Auger spectrum measured with our high-resolution PAES system• The PAES data presented reflects a total data acquisition time of about 30 hours, and the sample was sputtered every 10 hours to ensure its cleanness. Included in Fig. 6 are the raw PAES data and a curve obtained by digital smoothing. There are two well resolved features (A and B) located at 60.8 and 63.2 eV (with a sample bias of -2.5 V), which can be easily identified as the M3VV (3P3/23d3d) and M2VV (3p 1/23d3d) Auger transitions. The atomiclike M2,3VVline shape can be attributed to the strong hole-hole interaction [21-23] in the two-hole final state configuration for a narrow-valence-band metal undergoing a core-valence-valence (CVV) Auger transition. The separation of these two feature is also consistent with our EAES measurements as well as the published data of Madden et al, [4] and Sickafus and Kukla [5].

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M2,3VV measurement with the EAES measurements [4,5] of Madden et al, and Sickafus and Kukla, indicates that the PAES spectra has significantly more intensity in the low energy tail and that unlike the EAES data, the M3 w peak is larger that the M2 w peak. A detailed analysis of these differences will be presented elsewhere [24]•

309

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