Plasmon effects in electron energy loss and gain spectra in aluminium

SURFACE

SCIENCE 33 (1972) 437-444 8 North-Holland

PLASMON

EFFECTS

IN ELECTRON

AND GAIN SPECTRA B. D. POWELL

Publishing Co.

ENERGY

LOSS

IN ALUMINIUM

and D. P. WOODRUFF

Department of Physics, University of Warwick, Coventry, England

Received 10 July 1972 Oberservations of the low energy secondary and Auger electron spectra and the electron energy loss spectra from a clean aluminium surface have been made and the results are compared with other recent studies including that of Jenkins and Chung (1971). Low energy emissions at 5.7 eV and 10.3 eV are associated with the creation of single electron excitations in the valence band by plasmon decay. An apparent anomaly in the plasmon loss and gain peaks associated with the Auger spectrum is discussed.

1. Introduction The free electron nature of aluminium is well known and has made it an excellent material for the study of plasmon effects using a wide range of techniques. However, aluminium is also well known for its considerable affinity for oxygen under normal atmospheric conditions and thus the problem of producing clean surfaces might be expected to be particularly acute. Previous studies have attempted to overcome this problem either by using films evaporated in situ in the experimental vacuum chamber 1, a), or by using rather vigorously the conventional argon ion bombardment and heating cycle technique 3*4). One novel approach which has not been previously used, is to study a fracture surface of the solid produced in the vacuum chamber. Evidently the only difficulty with this approach is that the pure solid is extremely ductile and so can only be fractured by work hardening. The surfaces studied here therefore, which were produced by this method, are undoubtedly clean, but are also heavily deformed. However, there are good reasons to believe this will not seriously affect the plasmon effects as previous experiments by Powell on liquid aluminiums) suggest that there is no significant change in electron energy loss spectra (using a primary beam energy of 800 eV) on transforming pure liquid to solid. Of more significance will be the roughness59 ‘3)of our fracture, which will be expected to increase the importance of surface plasmon excitation relative to that of volume plasmons for angles of incidence of the primary beam approximately normal to the fracture 437

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438

AND

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surface. In fact the general spectra observed in these experiments were very similar to those recently reported by Jenkins and Chung4) and by Suleman and Pattinsons) and by Quint0 and Robertsons) indicating both that their surfaces were also clean and that the deformation of our surfaces has no marked effect on either loss or secondary (including Auger) features in the spectrum. We therefore wish to concentrate our remarks on the interpretation of certain of these features, and in particular on the very low energy secondary peaks at approximately 5 eV and 10 eV and on the structure associated with the Auger electron peak near 67 eV. 2. Experimental The experiments were carried out operating a conventional three-grid LEED optics assembly as a retarding field analyser. The retard voltage was applied to the second and third grids, which were electrically strapped together, and the capacitative coupling between these grids and the electron colector was neutralized using a capacitance bridge network. The specimen used was a rod of nominally 99.999% aluminium of 2 mm x x 4 mm cross sectional area, which was fractured in UHV by repeated bending and straightening, maintaining the background pressure below 5 x 10-l’ torr. This procedure caused considerable necking down of the specimen before fracture, but a sufficient area of clean surface was produced to ensure that the incident electron beam was wholly on the fracture over a fairly wide range of incident energies. 3. Results 3.1. LOSS SPECTRUM Typical

loss spectra

are shown in fig. 1. In agreement

with the many pre-

vious studies, the main features are multiple losses at approximately 11 eV and at 15.8 eV which can be almost certainly attributed to the surface and volume plasmon excitations. The theory of these spectra and the correlation with, in particular, their angular dependence as measured by Powell makes this attribution quite clear526). As expected we observe an increase in the surface loss relative to the volume loss as the primary energy is reduced. The number of multiple volume losses is quite spectacular and we have observed as many as 11 of these losses, but this would seem to be consistent with the observations of Jenkins and Chung and is only noticeably more spectacular than the earlier spectra such as those of Powell due to our ability to use the differential [d(N(E))/dE] mode of display of the results.

PLASMON

439

EFFECTS

Ircident Energy

(eV1

800 2

500 J

300 -2

150 d

1

1

0

100 Energy

Loss

(eV1

Fig. 1. Characteristic loss spectrum on pure aluminium fracture surface for incident energies between 150 eV and 800 eV. These results are shown as N(E) versus E rather than the differential display used in figs. 2 and 3.

3.2.

LOW

ENERGY

In addition

SECONDARY

to the

normal

SPECTRUM

Auger

spectrum,

two quite

sharp

peaks

at

5.1 eV and 10.3 eV are apparent in the low energy secondary spectrum in the differential mode as seen in fig. 2. Similar features have been observed by Jenkins and Chung who offer a number of explanations for these. We believe these peaks to be associated with the decay of surface and volume plasmons into single electron excitations within the valance band of the metal. There are several reasons for believing in this interpretation. Firstly, as Jenkins and Chung remark, the energy of the features corresponds to that to be expected for an electron excited from near the Fermi level in the solid. To be added to the observed energies to correlate with the normal plasmon energies is the work function of aluminium’) (variously reported but evidently close to 4 eV) plus the contact potential between the aluminium and the retarding

440

B. D. POWELL

AND

D. P. WOODRUFF

COO eV

I

dN(E) dE

4

67

Incident

0

Energy

(eV)

Energy

2.5 keV

100

Fig. 2. Secondary emission and low energy Auger electron spectrum. The low energy region (0 to 20 eV) is shown for incident electron energies of 400 eV (upper curve) and 2.5 keV. The higher energy region is for the single incident energy of 2.5 keV.

grids which was measured to be between 3 and 4 eV. While this figure seems rather large, the correlation is evidently adequate (within about 1 eV) and the difference in the observed energies is close to that between the two plasmon energies. In order to explain the apparent location of the electron close to the Fermi energy, before ejection, it is necessary to study the density of states in the valance band. In fact it is strictly necessary to decide whether the k-vectors of the decaying plasmons are large or small relative to that of electrons at the Fermi level, k,. If they are small, as is normally supposed for plasmons, then we should strictly consider only direct transitions and should therefore look at the energy distribution of the joint density of states (EDJDOS) as used in photoemission studies. However, whether we consider the bulk density of states 3, 8), the density of surface states g, or the EDJDOS lo) as published for aluminium, we see a common feature of a smooth variation (with some slight peaking) from around 10 eV below the Fermi level to a sharp cut-off at the Fermi energy. We would therefore expect such a distributiono f electrons emitted from the decaying plasmons. However, these electrons

PLASMON

are superimposed

on the large “slow”

441

EFFECIS

or “secondary”

electron

peak and so

can only easily be detected in the differential [d (N (E))/dE] mode in which the only marked feature will be the cut-off at the Fermi energy. Thus, the observed peaks may in fact be only edges in the true N(E) curve; they would not be clearly separable from peaks on the large changing background. Thus the observed features would correspond exactly to electrons emitted from the Fermi level. This interpretation is supported by the observation, somewhat less directly, of similar features in photoemission experimentsii~is) both for surface and volume plasmons. Moreover, the relative intensities of the two emissions and their dependence on the primary beam energy is consistent with this explanation. Two effects will control their relative intensities, namely the number of volume and surface plasmons excited, and the source and mean free paths of emerging electrons resulting from the decay. For all primary energies above 250 eV the volume plasmon loss is more pronounced and so the primary beam is expected to create more volume than surface plasmons. However, at higher incident energies the slow peak moves to higher energies and so there are a much greater number of secondary electrons in the energy range just below 50 eV14) where surface plasmons are most strongly produced. Thus, the large number of secondaries will dominate the plasmon creation processes and for primary energies greater than say 100 eV there will be an increasing ratio of surface to volume plasmons. This effect will be enhanced in the decay by the fact that electrons from surface plasmons decaying might be expected to originate nearer the surface, and might even be supposed to have larger mean free paths than those from volume plasmon decays, in having energies below the volume plasmon threshold. Thus the observed behaviour (see figs. 2 and 3) of the 5.7 eV peak decreasing relative to the 10.3 eV peak as the primary beam energy is decreased from 2.5 keV to 500 eV is to be expected on our model. Finally, the observation that the 5.7 eV peak is greatly suppressed relative to the 10.3 eV peak after 2 days exposure to the ambient vacuum (-2 x lo-” torr) supports the suggestion that the lower energy peak is associated with a surface plasmon. We would finally remark on our reasons for failing to support the alternative explanations mentioned by Jenkins and Chung. One suggestion is that the features are indeed edges in the N(E) curve associated with absorption of secondaries due to plasmon creation at the surface and volume plasmon threshold energies. If this explanation was valid similar edges should be observed in LEED spectra at these energies whereas there is in fact no evidence for such a clear and simple effect in LEED experiments. Finally, it is proposed (by Jenkins and Chung) that features of this kind could be explained by

442

B. D. POWELL

AND

D. P. WOODRUFF

hvJ xl

1Vrms

,x25

I!!% dE

Y Incident Energy 1.5 keV

0

Energy ieV1

100

Fig. 3. Low energy Auger electron spectrum with incident electron energy of 1.5 keV. Upper curves show clean surface results; lower curves show spectra after exposure to air at atmospheric pressure. Only additional detectable Auger peak after this treatment was associated with oxygen.

energy gain effects from plasmons on the slow peak electrons. Apart from difficulties in defining the “true” slow peak positions which should presumably be below the vacuum level (the observed peak being due to a cut-off at the vacuum level) the very short lifetime of plasmons would be expected to make this process far too weak to be observed. Thus no plasmon gain peaks are observed on elastic peaks. On the other hand both conduction and Auger electrons should be untroubled by this lifetime problem as the coupling of these electrons to plasmons should be strong. In the case of Auger electrons

PLASMON

EFFECTS

443

this coupling is thought to result from the disturbance by the ionised (Auger electron emitting) atom of the (conduction) electrons associated with the plasmon. Also, as Jenkins and Chung point out, there is no peak corresponding to the bulk plasmon on this explanation. 3.3. AUGERELECTRON SPECTRUM The low energy Auger electron spectrum is shown in figs. 2 and 3. The main features of these transitions and their probable assignments have been given by Quint0 and Roberton, Suleman and Pattinson and by Jenkins and Chung. We wish, however, to remark on the assignment of the peaks at approximately 51 eV and 83 eV. Quint0 and Robertson suggest that the lower of these features may be due to an Auger cross-transition in aluminium oxide on the surface and show this peak as indeed more pronounced when oxygen was adsorbed into the surface (fig. 3). However, its appearance on our clean spectra (as on other results) shows that it must, at least in part, be a real clean surface effect. The alternative assignments are that the two peaks are volume plasmon loss and gain peaks associated with the main 67 eV L, ,,VV Auger peak. Indeed, Watts ls) has recently shown that the amplitude of the gain peak is entirely consistent with this interpretation in implying a very reasonable value for the plasmon lifetime. One rather odd feature marrs this seemingly clear interpretation. It is well known that at low primary energies (i.e., below 100 eV) the surface plasmon loss is much larger than the volume loss and it is predicted that the probability of Auger electrons making similar losses should be identicala). Indeed at a primary energy of 150 eV it is very clear in our own data (fig. 1). However, the observed loss or gain peaks on the 67 eV Auger peak are being attributed to a volume plasmon and there is no evidence for 10 eV surface plasmon loss or gain. We might expect this effect to be even more pronounced by the assumption that 67 eV electrons are being seen only from atoms close to the surface where one might expect coupling to surface plasmons to be largest. We are therefore unable to explain these results other than to remark that it has been suggested that electrons can “see” surface plasmons outside a crystal surface6) and so the interaction of these processes with reflected electrons may be much stronger than with electrons coming from within the surface and only passing through this outer region briefly. References 1) 2) 3) 4)

C. J. Powell and J. B. Swan, Phys. Rev. 118(1960) 640. M. Suleman and E. B. Pattinson, J. Phys. F (Metal Phys.) 1 (1971) L21. D. T. Quint0 and W. D. Robertson, Surface Sci. 27 (1971) 645. L. H. Jenkins and M. F. Chung, Surface Sci. 28 (1971) 409.

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5) C. J. Powell, Phys. Rev. 175 (1968) 972. 6) A. A. Lucas and M. Sunjic, J. Vacuum Sci. Technol. 9 (1972) 725 ; A. A. Lucas and M. Sunjic, Phys. Rev. Letters 26 (1971) 229; M. Sunjic and A. A. Lucas, Phys. Rev. 3 (1971) 719. 7) J. C. Riviere, Solid State Surface Sci. 1 (1969) 179. 8) V. A. Fomichev, Soviet Phys.-Solid State 8 (1967) 2312. 9) V. Hoffstein, Solid State Commun. 10 (1972) 605. 10) R. Y. Koyama and N. V. Smith, Phys. Rev. B 2 (1970) 3049. 11) W. Steinmann and M. Skibowski, Phys. Rev. Letters 16 (1966) 989. 12) J. G. Endriz and W. E. Spicer, Phys. Rev. Letters 24 (1970) 64. 13) J. G. Endriz and W. E. Spicer, Phys. Rev. Letters 27 (1971) 510. 14) J. Thirlwell, J. Phys. C (Proc. Phys. Sot) 1 (1968) 979. 15) C. M. K. Watts, J. Phys. F (Metal Phys.) 2 (1972) 574.