Electron spectroscopy from solid surfaces in UHV: a discussion of current progress with techniques involving electron (AES) photon (ESCA) and field stimulated electron emission

Electron spectroscopy from solid surfaces in UHV: a discussion of current progress with techniques involving electron (AES) photon (ESCA) and field stimulated electron emission received 7 November C .I Todd.

1972

Post Office Research Station, Dollis Hill, London NW2 7DT. England

The present status of AES, XPS (ESCA), APS, UPS, field emission energy analysis, LEED and RHEED is briefly surveyed. Three topics are then examined in more detail: (I) electron and photon induced core level ionization, and subsequent Auger processes, (2) measurement of electron energy distributions using retarding and bandpass analysers (CMA and hemispherical) and (3) quantitative interpretation of Auger and photoelectron line intensities. Emphasis is placed on the physical relationship that exists between AES, XPS and field emission, despite the separate and sometimes isolated development of these techniques. In particular the results of recent escape depth measurements underline the surface sensitivity of XPS (ESCA). 1. Introduction A thorough understanding of the properties of solid surfaces must be based on knowledge of the crystallographic structure, the chemical composition, and the electronic character of the adsorbate-substrate surface region. The purpose here is to consider techniques which contribute to one or more of these three basic areas, by measurement of the properties of electrons scattered by or excited from the uppermost layers of a solid. Two methods of probing the surface chemical composition are Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS or ESCA). The application of XPS to solids yields the electron binding energies and chemical shifts, allowing identification of elemental types and characterization of the local environment’. Quantitative deductions from X-ray photoelectron intensities and the surface sensitivity of the technique will be taken up in Section 4. The early realization that AES could be conducted in a slightly modified LEED optics2 led to close association of AES with LEED and an early appreciation of the surface sensitivity of AES. Element identification using AES is well established. It has been suggested that appearance potential spectroscopy (APS) represents a simple approach to surface chemical analysisa. This technique relies on observation of the threshold for photon emission as the energy of a primary electron beam is raised. In Section 2.2 it will be seen that for transitions involving core states5 1500 eV below the Fermi energy, photon emission is the minority process; APS therefore represents in general a poorer measurement situation than AES. The strengths and weaknesses of APS have recently been elegantly summarized by Tracy4. Of particular concern is an apparent lack of sensitivity, compared with AES, to a wide range of common surface impurities, nontransition metals, and semiconductors. Characterization of the near Fermi energy surface electronic configuration is currently a fashionable pursuit. Steinrisser and Sickafus6 have observed low energy electron loss spectra from molecular orbitals associated with Ni(ll0) -c(2 x 2)s which were compatible with the measurements of Hagstrum and Becker’ using ion neutralization spectroscopy (INS). Fischer has suggested’ that photoemission using photon energies N 40 eV. (helium 11line) and oblique incidence should maximize sensitivity to surface electronic states. Under just these conditions Eastman8 has observed chemisorption structure. Vacuum/volume

23/number

6.

Measurements of field emission energy distribution (Section 2.3) have established this technique as an incisive method for clean surface and adsorbate-surface valence electron characterization. Field emission, and field ion microscopy (FIM) have frequently suffered from accusations of narrow application (i.e. certain refractory materials). This point of view is certainly in error in the case of field emission, and even the more stringent requirements of FIM can be met by a range of ‘soft’ materials such as Au, Fe, Ni, Cu and Ala*lo. Low energy electron diffraction (LEED) represents a unique method of obtaining surface structure information. Although the dimensions and symmetry of the 2-d surface unit mesh are simply related to the geometry of the observed LEED pattern, the content of the unit cell and the relationship between atomic layers parallel to the surface have proved difficult to obtain. However an intensive theoretical effort, during the last five years, has generated an understanding of the relationship between LEED intensity-voltage plots and the surface structure of some clean metal surfaces; in particular for Cu 11-13,Nilp-ls, AP3-l6, Be17-18and Ag13. As emphasized by Jona and co-workers18 the range of materials being subjected to theoretical scrutiny is limited by the lack of sufficient normalized experiment I-V data. Recently Anderson and Pendry18 have predicted the positions of sodium atoms in a c(2 x 2) array on Ni-( 100) deducing a spacing between the sodium and underlying top-most nickel layer of 2.9 fO.O1 A (from LEED 1-V data). Given that sufficient normalized 1-V data becomes available, the accurate determination of a variety of surface structures, using LEED, is imminent. Reflection high energy electron diffraction (RHEED) is well suited to examination of the initial stages of epitaxial growth. Although the structural interpretation of grazing incidence RHEED data from a surface layer may be simpler and more accurate than is the case for LEED [understanding of the Au (1 x 5) or (5 x 20) clean surface structure is a case in point 20]the practical problem of obtaining microscopically flat surfaces may hinder the development of this technique. The problem of surface topographical measurements on the 5-250 8, level requires no emphasis. However, Young and co-workersel have recently developed the prototype ‘Topografiner’, a remarkable instrument based on field emission, capable of lateral location of 200 A, and vertical (step height) measurements of 30 A to a precision of 3 A.

Pergamon Press LtdJPrinted in Great Britain

195

C J Todd: Electron spectroscopy

from solid surfaces

in UHV

The development and application of these surface probes has followed the rapid growth of ultra high vacuum technology. Although the principles underlying attainment of UHV conditions have been known for at least 50 years, (results obtained by Davisson and Germersla m their classic experiment of 1927 are now known to have required a UHV environment) the importance of technological improvements during the last 10 years enabling easy and reliable achievement of pressures I lo-l0 torr, and hence stable surface conditions, is clear. The ion pumped stainless steel systems which predominate in both the industrial and university surface laboratories are popular on the grounds of their reliability and compatibility with accurate alignment of internal components. However, ion pumps are notorious for their pumpingselectivity, the occurrenceof carbon monoxide and methane emission under certain loads, and of course for the presence of a large magnetic field. The traditional trapped mercury pump remains the simplest route to lo-l0 torr or better, avoiding the selectivity and magnetic field problems. Present trends away from mercury towards polyphenyl ether oil diffusion pumps, for attainment of UHV conditions, offer few apparent advantages. The purpose of the following brief examination of surface electron spectroscopies is to highlight some problems and possibilities associated with a few of the techniques that have been introduced. The importance of control of surface composition underlies the majority of academic ortechnologicalsurfacestudies. Therefore emphasis is placed on a comparison of Auger and X-ray photoelectron spectroscopies. A brief excursion into near Fermi energy regions is made through the medium of field emission energy distributions. For more details of these and related techniques, the reader is referred to a number of recent exhaustive reviews, and collections of recent research activities (e.g. LEEDZe+, AESas-za, UPS and XPS1sa7 field emissionz8 and electron energy analysersee). In Section 2 some of the principles underlying electron and X-ray photon induced electron emission processes are considered. The problem of measurement of the energy distribution of electrons within a spectrum, common to all the techniques introduced here, forms the subject of Section 3. Finally in Section 4 progress towards quantitative evaluation of Auger or photoelectron line intensities is examined, particular emphasis being placed on the importance of the escape depth and its energy dependence.

2. Emission processes Three methods of inducing electron emission from the uppermost layers of a solid involve: (1) the absorption of photons, (2) the scattering of primary electrons, (3) the application of negative fields. If a solid is subjected to an electric field of sufficient magnitude (-0.10-0.5 V/A), bound electrons may tunnel through the reduced surface barrier. While escaping the surface, the field emitted electrons may lose no energy (elastic tunnelling) or they may excite a vibrational or electronic state near the surface (inelastic tunnelling)50. Similarly if a particle beam is incident on a surface, elastic and inelastic scattering or total absorption may occur. LEED is of course concerned with the elastic portion of the low energy electron induced emitted distribution, which is frequently < 0.1 per cent of the total spectrum 17, le. As a result of either field emission or incident particle stimulation the system may be left in an excited electronic state. Internal electronic transition will then occur resulting

in either photon

or Auger electron

production.

2.1 Photon or electron stimulation. The incidence

of a photon or electron on a solid may result in the excitation of a localized (core) or relatively delocalized (band) bound electron (see Figures la and lb). As the energy of the incident particle is raised from zero, a channel for absorption of the particle energy and excitation of the electron bound at Es is suddenly opened when E,(ho) =E,, in the case of a metal, leaving a core hole. The first empty level for threshold excitation in the case of a semiconductor may be a surface state or a level in the conduction band. As the energy of the incoming particle is increased beyond threshold, excitation to higher empty states of the band structure should be possible, and where E,,(ho) &?Z, +(p, (@, is the target work function) escape of the excited electron from the

Figure 1. Scattering and stimulation processes: (a) photoemission from a core level (I&), (b) electron impact ionization of E3core level. Both processes result in a core hole (c) to be filled by Auger type processes, leaving the atom doubly ionized; (d) two possible single particle mechanisms for energy loss before escape of a stimulated electron i.e. plasmon excitation (a@ and intcrband transition; (e) model resonance tunnelling of field emitted electrons through a single adsorbate level, yielding a (exagerated) perturbed energy distribution fi(E). In (a)-(c) the Fermi level is taken as the energy zero.

solid may occur. XPS typically employs incident photons with ho > 1 keV yielding a series of excited electron, or photoelectron, lines from bound states in the solid at kinetic energies Epe=ho - E,. However in the analogous case of electron impact ionization, Figure lb, experimental evidence exists to suggest that even when E,,> Es there is still a large probability for threshold energy los@. The mechanism is unclear but in the case of metals a high density of vacant states in the vicinity of the Fermi energy may be significant. Experimentally the threshold energy losses should appear as distinct features in the measured electron spectrum. Figures 2 and 3 display schematically the spectrum of electron and photon induced electron emission from a contaminated GaAs surface. The characteristic losses

C J Todd: Electron spectroscopy

from solid surfaces in UHV

lit? empirically adjusted the high energy model for a,, to account for the proximity of threshold ionization, and obtained:

(1) where B=[1.65 +2.35 exp (1 -E,/&)]

EC.

EC is the core state binding energy, and b is a constant depending

on the identity and screening of the core state. Stobbeas made analytical improvements to the model of 3 allowing a close fit to experiment for (ho -EC) 250 eV34. 1.1 x lo-l6 0”)

where f(H)=2fl (Ec/~w)1~2 {exp (-4” arc.cotH)/[l -exp (-2

Figure 2. Electron impact spectrum from a contaminated GaAs surface, displaying the energy distribution H(E), and its derivative (retarding

grid spectrum).

II”)]}

and H=[Ec/(ho-E,)]1’2

tlectron klnetlc energy

e(E)

(2)

For clarity the As, and some low

energy Ga features have been omitted. Characteristic losses for carbon (C-EK) and oxygen (O-EK) are shown, together with Auger lines from carbon, oxygen and gallium.

associated with the impurity oxygen and carbon (Figure 2) are of low intensity. This might be expected since observation would in general involve either threshold ionization followed by elastic back reflection, or large momentum transfer associated with the threshold process. Gerlach and DuCharmea2 have recently exploited characteristic ionization losses (ionization _loss spectroscopy-ILS) for measurement of ionization crosssections of a selection of elements. The cross-section for core level ionization by electron impact, &, or by photon absorption, Qp, becomes of major concern in any quantitative application of AES or XPS (see Section 4). Unfortunately too few experimental cross-sections are currently available. Theoretical predictions of the appropriate crosssection are based on calculation dating from the early days of quantum mechanics33, 34. The theories relied on the Born approximation to describe core ionization behaviour at high incident particle energies (lo4 to lo5 eV), and hydrogenic models where complexities associated with real atoms having Z> 2 were introduced via screening parameters. Worthington and Tom-

and Z* is the atomic number modified to account for screening effects. Equation (1) predicts a maximum in QDefor Ep/Ec-337, while equation (2) implies a monotonic decrease in Qp with increasing photon energy, iiw, above threshold. As a guide to the magnitude of the ionization cross-sections consider L1 ionization in gallium. Assuming a primary electron beam energy yielding me (max), EL,-1 118 eV, bLd.2Y7, the cross-section for electron impact ionization of L, (Ga) is then 3 [L, (Ga)] N 2 x 10-*0cm2 . Assuming a typical Al Ka photon source (hw=1487 eV), and taking Z* =Z-5 for L, (Ga) (this simple shielding estimate follows from assuming both K electrons lie between the L electrons and the nucleus, and that six L electrons half shield each other) the cross-section for X-ray photo electron ejection from L, (Ga) becomes Qp [L, (Ga)] N 8 x 10m21 cm2 . Bearing in mind the approximations involved in reaching equations (1) and (2), Qp=% for L, (Ga). Under the same conditions ionization of the M levels of Ga could involve %/Qp N lO-102; however values of b in equation (1) are not known for M shell electrons. It has been said that lower output signals from XPS compared with AES implies lower cross-sections for photo electron ejection. Within the context of equations (1) and (2) it can be shown that differences in primary flux are a more likely cause.

O(K)

I

I

4OJ

I

635

I I,.W,

I 83’1 Electron

klnetlc

I IL130

I

I

1400

energy

Figure 3. Photon stimulated electron spectrum from a contaminated GaAs surface. As features have been suppressed for clarity. The photon energy (Al Kcc) was sufficient to excite core levels with EC --
c J Todd: Electron spectroscopy

from solid surfaces in UHV

2.2 Auger processes. It is irrelevant whether a core state hole is produced by photon adsorption or electron impact ionization; electronic rearrangement will occur within - lo-l5 s as depicted in Figure lc. Auger emission dominates over photon emission, where the transitions are into relatively shallow core holes, characterised by filled binding energies5 1500 eV3’. Auger processes may involve two or three levels, as shown in Figure lc, or the near Fermi energy bands (not shown). A cascade of Auger transitions will in general be expected, since each Auger transition increases the number of holes, until the holes appear in the conduction band. Charge neutrality will be re-established by electron flow from ground or by return of low energy secondaries to the sample surface. In a complex situation where Auger transitions can in principle occur from a number of shallow core states and energy bands, competing probabilities will decide the outcome. The evaluation of these probabilities in solids remains unsolvedzB. It is clear from Figure 3 that as a result of photo ejection from L, and L, gallium core levels, Auger transitions involving three M levels could in principle occur. However only L,M,,,M,,, transitions do occur with any appreciable strength. The electron stimulated Auger transitions (Figure 2) form a similar pattern, except for the presence of a moderately strong L,M,M, Auger line, not seen in the photon stimulated spectrum. A serious problem follows from the non-neutral character of both initial and final system states; accurate prediction of the kinetic energies of possible Auger lines is prevented. Chung and Jenkin9 assumed the effective binding energy of the core holes, in an atom of atomic number Z, lay midway between the binding energy of filled levels in atoms characterized by Z and (Z-t 1). This leads to a predicted Auger line kinetic energy, involving three core states A, B, C, given by EABC=EA

(~)-~[EB(~)+~B(Z+~)+EC(Z)+EC(Z+~)I

This approach is least likely to succeed where B or C lie within the near Fermi energy bands; a modified formula for this situation has been proposed by Coad and Rivi6rea9 : EAVY(Z)EEA -2Ev(metal) where Ev (metal) is taken at the maximum in the density of states. Experience suggests that these formulas predict within 5-10 eV of measurement. However the accumulation of a large quantity of standard spectra during the last few years has adequately supplemented predictive errorsz3, 24, 40. The identification of the constituents of solid surfaces, using AES, is fast becoming a reliable and straightforward procedure. Unfortunately interpretation of both Auger and photoelectron line intensities in terms of surface elemental concentration remains a research problem. 2.3 Field emission processes. The phenomenon of field emission has been studied and used in surface investigations for more than 40 years. The facility to detect and characterize surface states and adsorbate-substrate valence states, in the near Fermi energy region using field emission, has only recently been demonstrated; in particular by Swanson and Crouser41 and by Plummer, Young and Gadzuk”Z-44. Under an applied field F, a current j’, (E’) of electrons bound at E’ =E-EF below the Fermi energy EF tunnel through the reduced surface barrier of a free electron-like emitter, where j.’ (E’) =(J&)f(E’)

exp W/d)

and Jo=

dE’, d=h eF [2(2m(p)k] ml

198

“jto (E’) s 0

(3)

f(E) is the Fermi function and cp the work function. Under usual field emission conditions J,, c lo3 A/cm2 and 0.155 d5 0.25 eV, implying that j’O(E’) becomesunmeasurable for E< -5 eV. Deviations from the free electron like emission behaviour [described by equation (3)], may introduce marked structure into the energy distribution. Swanson and Grouse? observed a pronounced peak in j’,, (E) from (100) tungsten, 0.4 eV below the Fermi energy. Plummer and Gadzuk4d improved the earlier measurements and showed that the peak was compatible with emission from a surface state resonance; thus obtaining the first direct identification of a surface state by electron emission spectroscopy. Recently the same surface state has been identified by UV photoemissiot?a again 0.4 eV below EF. That the high field did not significantly perturb the surface state is very encouraging for the field emission energy analysis technique. A second situation resulting in the breakdown of equation (3) may occur in the presence of an adsorbate having a relatively narrow virtual level (- 1 eV in width)4’; this situation is depicted schematically in Figure le. Duke and Alferieffa5 showed that electrons in anappropriate energy range tunnelling through this system would undergo a resonance. Gadzuk has clearly shown43 how the observed resonance structures may be interpreted directly in terms of the position and width of the adsorbate-substrate valence levels; crucial information in understanding the mechanism of surface binding. Resonance tunnelling is an elastic process: the energy of the tunnelling electron remains unperturbed. The presence of the adsorbate valence level has effectively narrowed the barrier to tunnelling for electrons in a small energy interval. Inelastic tunnelling will also occur when E’= -liw where hw is the threshold excitation energy of a vibrational or electronic process. Inelastic losses were observed by Plummer and Bella during field emission from a system of adsorbed hydrogen and deuterium on single crystal planes of tungsten. Plummer and Bell were able to extract vibrational excitation energies for these adsorbates. Finally it is perhaps interesting to note that the removal of electrons by field emission leaves holes in the bands, which may decay by Auger type processesI’. 3. Measurement techniques in electron spectroscopies. The accurate measurement of the electron energy distribution is a problem common to field, electron or photon induced electron emission from solid surfaces. In general, with any energy analyser, the measured energy distribution N(E),, is not identical with the real energy distribution, N(E), that is N(E),. -:f[N(E)] Attempts to establish the closest agreement between the shape of N(E),n (peak position and profile) and that of the real distribution will be accompanied by a loss of sensitivity, unless the point integration time is increased. In choosing the appropriate analyser for a particular system consideration has to be given to: (1) peak widths and separation in the kinetic energy range i.e. resolution (core level photo-electron lines are of width ~0.5 to 2 eV, Auger lines are frequently wider); (2) expected peak magnitudes and the noise content of the measured signal i.e. sensitivity; (3) what constitutes a reasonable time for data collection. Taylor’” has discussed the solution of these problems in the case of the retarding and 127” analysers. Perhaps as a result of increasing interest in solid surfaces, analyser technology is experiencing a period of rapid growth. Here, three analysers are chosen for scrutiny and comparison for the reason that they are all commercially available, and can all be home constructed. They are the retarding grid LEED/

C J Todd:

Electron

spectroscopy

from solid surfaces in UHV

AES system, the cylindrical mirror analyser (CMA) and the hemispherical analyser, shown schematically in Figure 4. The CMA and hemispherical analysers are band pass instruments; that is, for a given setting of potentials on the analyser elements, only electrons lying in some small energy range E=tAE in the distribution are allowed to pass to the collector. The retarding analyser, on the other hand, allows collection of electrons with E>E’, where E’ is the magnitude of an applied stopping potential. The retarding analyser relies on differential techniques to extract N(E),+

Retarding

N(E) is usually obtained electronically by superimposing a small ac signal (k sin OIL)upon V,, and detecting the synchronous collected signal with a phase sensitive detector tuned to o**. 5o. The usefulness of plotting the derivative of the energy distribution, N’(E),,,, is clear from Figure 2, particularly in the region where the background N(& curve is changing rapidly with energy. Tuning the phase sensitive detector to 2w, for small k yields a signal proportional toN’(E),50. The alternative practice of applying [k sin(o/2)t] to Ge, allowing the use of highly tuned output circuitry, does require the use of anextremely stable ac signal source, otherwise small changes in w lead to Q modulation in the tuned circuits, and a high detector output ‘noise’ level.

grid

Target

Slgnal

Cylmdrlcal

Electron

beam

Hemlsphertcal

Figure 4. Three electron analysers: the retarding grid analyser arranged for electron impact and LEED studies, replacement of the axial gun by an appropriate photon source allows UPS work44a,the cylindrical mirror analyser is shown with an internal electron gun, and the hemispherical analyser with single retarding stage input lens.

3.1 Retarding grid analysers. The hemispherical (or part hemispherical) retarding grid analyser probably remains the most widely used instrument for AES and UPS studies. The configuration shown in Figure 4 has been described many times2s *5.37r48v4B.Although a grazing incidence electron gun (Superior Electronics SE-3K/5U) is frequently used to increase surface sensitivity40, unless incident angles can be carefully controlled the normal incidence system is preferred for quantitative work. Electrons back scattered by the target surface and passing through a set of fine mesh grids experience a series of decelerating and accelerating fields, shown schematically in the potential diagram for the retarding system (Figure 5). It should be noted that in the case shown, i.e. VT< CpCIelectrons escaping the solid with cp& E < CpG,will not be collected. The collected currentjc is related to a measured energy distribution N(E), by Emax N

s

E = k’,t +

(E’)m dE

(PGa

where N(E’),,, 1E=djc/dE.

G,

G,

Screen/collector

Figure 5. Schematic potential diagram for the retarding grid analyser. The stopping potential on G, (Figure 4) allows electrons with energy >E to pass to the collector. Setting EF, the Fermi energy of the target, at ‘zero’, it is clear that the applied stopping potential, Vmt.. and the threshold pass energy are related by E= Vret.+(pGa, where (P& is the work function of G,.

mirror

-tiv

j,=

G,

Typical resolution figures obtained with the retarding LEED/ AES analyser are in the range 505 EIAES 200 depending on the number of grids and on the operator. General factors limiting resolution in retarding systems have been discussed by Simpsonsl. Field penetration into the retarding potential region and stray magnetic fields are frequent culprits for poor resolution. However mu-metal shielding (rarely used with LEED/AES systems) will significantly reduce the level of stray magnetic fields, and Huchital and Rigden&* and Staibsa have recently shown that optimizing the intergrid spacing can yield E/EA - 103. Effective improvement in resolution can also be achieved by controlled pre-retardation of the electrons. Plummer and Young4= have described a restricted aperture planar retarding system, for analysis of l-2 keV field emitted electrons, which achieved a E/AE- 106. There is certainly no reason why this configuration should not be equally well employed in AES or XPS work. 3.2 Band pass analysers. Assuming that extraneous noise sources can be reduced to a minimum, shott noise remains the limiting factor determining sensitivity in electron spectroscopies. Since the retarding analysers collect all electrons with energies greater than the pass energy, shott noise can be high in these systems. Band pass analysers will usually allow an improvement of this situation since only the electrons to be measured reach the collector. The cylindrical mirror analysis and the hemispherical analyser offer different approaches to a band pass system. Figure 4 shows the most compact form of the CMA, for electron impact work, with the electron gun internally mounted. Back scattered electrons pass through the first slit S1 and their trajectories are perturbed by a retarding field between the 199

C J Todd: Electron spectroscopy

from solid surfaces in UHV

cylinders. Only electrons in the interval E &AE pass 5, and the aperture into the electron multiplier detector. The trajectory analysis is straightforward, and has been examined in detail by several authorsS4-ss. The special second order focussing properties of the CMA have been studied by Hafner et aP. Palmberg et aP appreciated that although the resolution of the basic CMA was not great (E/AE-200) its geometrical transmission was high (_ 10 per cent) compared with many band pass analysers. The large geometrical transmission and the low shott noise permit a S/N (signal to noise) improvement, compared with the 4 grid LEED/AES retarding analyser, by a factor lying between 25 and 500 depending on the electron kinetic energy and primary energy. This large S/N combined with the convergent output into an electron multiplier (minimising Johnson noise) allows spectrum scan times of 5 x 10mz s, over 1000 eV span, with a 5 x 10m5A beam. (Scan time with LEED/ AES system is N 10 min.) Alternatively reduction of the primary beam current to -lO-8 A increases the read out time but permits the use of highly focussed primary beams, and minimixes electron beam-adsorbate effects. The resolution of the CMA may be improved in two ways. The electrons can be retarded before the first slit; a resolution approaching lo4 has been obtained in this way ir0.In general there are some difficulties in designing retarding lenses which cope well with the large input angle (=42”) required for optimum resolution. A second approach is to combine two CMA’s in series yielding a resolution of 150080 or, permitting the use of extended sources such as an X-ray photon beams’. While the intrinsic resolving power of the CMA exceeds that of the hemispherical analyser 67, the latter instrument achieves very high effective resolution by employing small angle axially symmetric retarding lenses, which have reached an advanced stage of developmentsz. A narrow cone of electrons (half angle < 0.1 rad) leaving the target (lower diagram, Figure 4) pass an entrance aperture, are retarded in a series ofstages, and focused into the entrance plane of the hemispherical analyser. Application of a suitable potential between the hemispheres produces a focused image at the exit plane of electrons passing the (virtual) entrance aperture and lying in the energy range E &AE. An accelerating output lens focuses these electrons onto an exit slit and from there a Soa lens (plus some deflection steering) produces an image of fixed size on the first dynode of the electron multiplier detector. Kuyatt and P1ummerG3 have described an extremely useful modification of this basic designa2, (two stages of retardation from 2000 or 1000 eV to 1.333eV, and an analyser employing 13Y ‘hemispheres’) having resolution E/AE- lo5 and good S/N. This compact design, intended for field emission energy analysis and operated in lo-l2 torr vacuum, should be directly applicable to very high resolution photoelectron and Auger electron measurements. Basset and co-workersB4 have designed a single retarding stage hemispherical analyser which has sacrificed some resolution (E/AE - 1500) in favour of fairly high speed scans of Auger electron spectra (1000 eV in 1 s). A useful advantage of the hemispherical analyser over the CMA lies in the target to first element distance of ~5 cm (0.5 cm in the CMA). When interpreting the output signal strength of band pass analysers of either the hemispherical or cylindrical type, a problem arises due to the energy dependence of the transmission function” (not to be confused with the geometrical transmission). As a result of the angular spread of electrons passing through the (virtual) analyser entrance aperture, at the analysed energy E,, 200

the image of the entrance aperture is not entirely contained within the exit aperture; in the case of these linear dispersive analysers the extent of overlap of the image and exit aperture is EU dependent. The output spectrum of the analyser is then related to the real energy distribution weighted by E,. Comparison of peak intensities at different energies in the output spectrum must certainly include the effect of the energy dependent transmission function. 3.3 Analyser comparison. The choice of a suitable analyser is never straightforward, particularly in this period of rapidly developing analyser technology. The three analysers discussed here are all now commercially available, and the order of discussion probably represents the order of increasing ditliculty for laboratory construction. A few guide lines are perhaps becoming clear. If LEED and electron stimulated AES are to be combined the 4 grid retarding analyser represents the simplest and most economical approach. The large acceptance angle of this analyser integrates over emission anisotropies. Until more information is forthcoming on angular effects in XPS and AES, angular integration may well be the safest course. However in the opinion of this author the CMA will probably quickly become the standard instrument for electron stimulated Auger (and possibly photoelectron) spectroscopy. The combination of high sensitivity, rapid data acquisition, sufficient resolution for most identification purposes and ease of operation will allow AES to become a routine analytic technique, compatible even with high vacuum production line technology. Combination of the CMA and the high resolution scanning microscope, to yield a 2-d map of element distribution on the submicron scale has already been accomplished by McDonald and Waldrope6. A difficulty lay in the low flux scanning beam necessitating long time constants in developing the 2-d analytic net. The advent of high brightness field emission source scanning microscopes, operating in high to ultra high vacuum may significantly reduce the analysis time. An alternative approach to the problem of analysing heterogeneous surfaces and establishing a much coarser 2-d analysis involves turning the typical AES system into a low resolution scanning microscope. Such a system has recently been developed by the simple addition of scanning electronics to drive the deflector system of the typical glancing electron gun employed for AES work6”. Scope display of the target drain current, or secondary emission current, in the usual way allows heterogeneous features to be recognised. Stopping the scan at points of interest allows realistic AES analysis of non-uniform surfaces. The hemispherical band pass analyser represents the reliable approach to exceedingly high resolution, and a useful S/N ratio, for Auger, photoelectron spectroscopy or field emission. In the research laboratory this is probably the most useful analyser for the study of angular anisotropies and other properties of photon, field or electron stimulated emission processes. 4. Quantitative Auger and photoelectron spectroscopy The kinetic energy at which photoelectron or Auger lines (however stimulated) appear in the spectrum from a solid surface allows identification of major or minor constituents in the surface region. If chemical shifts are observed information relating to the environment of a particular element may be extracted’. The claim that photoelectron lines are more sensitive to chemical shifts of the core levels than Auger lines is not entirely correct. The expressions given in Section 2.2, for determination of Auger line energies, show that only under

C J Todd: Electron spectroscopy from solid surfaces in UHV

special circumstances, where the shift in level A AEA“AZ% + AEc will the Auger line be insensitive to chemical shifts. Fewer observations of chemical shifts in AES rather reflects the poorer resolution analysers used in this field. Having identified the range of surface species present, it remains to identify the absolute (or relative) quantities. In principle this information is contained in the intensity of the Auger or X-ray photoelectron line. Three common types of impurity distribution may be recognized: (1) a surface layer, (2) an impurity uniformly distributed and (3) non-uniformly distributed, throughout the analysed region. Various combinations of (l)-(3) may of course coexist. Measurement of the average thickness of a surface film is of consequence to many surface experiments. The submonolayer case has recently been examined in detail by Meyer and Vrakkinge7. Here the more general case involving absorption effects is considered. Let Zg be the flux generated by a measurable excitation (photoelectron or Auger line) involving core levels, for one atomic layer of the surface film. Then3’ I, (E,,E,,w) NT lo N cosecv Q, (Ep,Er) (5) where r is a correction to the incident flux la per cm2, to account for additional ionization by electrons back scattered from deeper layers, r is essentially unity in the case of a photoelectron line; N is the atom concentration per cm* in the layer; w relates the direction of the incident flux to the surface normal (z direction), and Q,is the relevant cross-section [equation (1) or (2)]. The flux I, escaping from the surface of the thin film consisting of n layers in a direction 8 to the surface normal may be written asB8 4, (‘Wc,E,,w) N

r

Jo

z(E,,E,,w)

exp [ -z/D(E)cose]

dz

.

(6)

A model exponential electron absorption mechanism has been assumed; D(E) is the average escape distance without measureable energy loss via complex or single particle excitation processes (Figure Id) and E is the kinetic energy of the measured line. Unfortunately in the case of electron excitation the back scattering coefficient and hence Zgis not independent of z. The back scattering coefficient will contain contributions from the substrate and the overlayer in proportions depending explicitly on the position of a given surface film layer. Despite a certain amount of work on this problemas* ‘IJa full analytic description of r(z) remains undetermined. However the error introduced at this point by assuming Zg#Zg(z) is probably no worse than errors already contained in Zg through use of relatively naive descriptions of the cross-section @. Proceeding to integrate equation (6) yields In (E&, i&w)“Z, (&%,w) D(E) cosu (1 -exp[ -na/D(E).cosO]} . The relationship between In and the appropriate put signal of an analyser I; is given by

(7) measured out-

A is the acceptance solid angle of the analyser, and T(E) is the ratio of a given signal of kinetic energy E entering the analyser, to the measured output. Thus T(E) includes all relevant analyser transmission effects. At this point it is useful to recognize the assumptions implicit in equations (7) and (8) but as yet unstated. (1) The absence of angular anisotropies in the emission has been assumed; photoemission from core states in non-single

crystal films of Au and C appears to be isotropic, however in the case of single crystal films diffraction effects have to be considered’l. Non-isotropic effects in Auger emission from solids have not been systematically investigated. (2) The cross section for core hole generation, equations (1) and (2) do not include final state effects. These may be of significance particularly in the calculation of @, the cross section for electron impact ionization, since it would appear that this is a threshold process even in cases where Ep > 1EC1.” (3) The primary beam is assumed to be essentially undegraded within the analysed volume. (4) In the case of Auger lines, Z,” must include all possible Auger transitions into the relevant core state, with filled binding energy EC. The fluorescent yield o is assumed zero; if this is not the case the correction to equation (7) is straight-foward3’. (5) The escape depth D(E) is assumed independent of the exprocess, however the production of single (photoelectron) or double hole (Auger) final states may significantly affect the dielectric constant. With these points in mind equations (7) and (8) may be used to obtain approximate film thicknesses from either X-ray photoelectron or Auger line intensities. However some simplification may be possible in practice. From equation (7) it follows that for large enough n such that exp [-m/D(E) cos 01 -C -C1 citation

Z, N z, D(E) cos 8 . One may then write equation (7) as

(9

I,-I,

{l-exp[-n~/D(E)cose]} (10) since I, is an experimentally accessible quantity the errors resulting from assumptions (l)-(3) are avoided. However assuming D(E) is known, equation (10) is in error, in the case of electron excitation as a result of the z dependence of the reflection coefficient. The largest error in using equation (10) to fix the film thickness will probably arise when r (substrate) > r (surface film). Since, as n is increased, the intensity In will initially rise more quickly than predicted by equation (lo), and under certain conditions an n’ may exist such that Z,’ > I:‘*. In the case of a photoelectron line generated in the surface film these problems do not occur since the reflection coefficient should be set at unity. The purpose of this discussion has been to emphasize some major problems that currently impede progress towards qauntitative deductions from X-ray photoelectron and Auger line intensities, even in the simplest case of a uniform surface film. The more general cases of ordered element distributions throughout the analysed region present further complications. At least three areas require further attention: (1) the form of the reflection coefficient in electron stimulated AES, (2) the validity of the model exponential absorption process and (3) appropriate values for the escape depth D(E). Increasing effort is currently being applied to the measurement of Z&QBB~73-78. The usual technique involves measurement of either the decreasing substrate or the increasing surface film signal, during deposition of a calibrated homogeneous film. A technique applicable to X-ray photoelectron spectroscopy, which avoids the ’ problem of thickness measurement and uniformity of 5-15 A films has recently been developed7’. The technique is based on a comparison of the strength of photoelectron and associated Auger lines, appearing at different kinetic energies in the X-ray 201

C J Todd: Electron spectroscopy

from solid surfaces in UHV

J

L

500

1000 Klnetlc

energy.

1500

eV

Figure 7. Escape depths, D(E), vs the kinetic energy of electrons in

Al e08(0)78and GeO,(@)“, and in Au and Ag (A)6s,‘3, (A)76.

Energy

above

Fern11

level

Figure 6. Comparison of the L2, L, photoelectron signals with the associated L2M4.5M4,5 and L,M,,,M,,, Auger transitions from Ge in GeO, using Al Kcc radiation.

photon induced spectrum (see e.g. Figure 3). Fundamental to the method is the assertion that the core state hole, created by photo ejection of a core electron with binding energy EC( < 1200 eV) from a given atom in a homogeneous solid, is filled by a specific Auger transition. Thus the strength of the photoelectron signal from EC represents an upper limit to the strength of the subsequent Auger lines generated by transitions refilling the level at EC.In Figure 6, the L,and L,photoelectron lines and associated L,M,,,M,, 5Auger lines from Ge in GeO, are shown. The marked difference in strength of the associated pairs of photoelectron and Auger signals stems from the different kinetic energies of the electrons involved. Both the escape depth and the analyser transmission function are energy dependent. Correcting for transmission effects allows the relative escape depths at the two energy pairs to be directly determined. Assuming that at high energies ( > 1 keV), D(E) for Al aOs is similar to that in GeO,, absolute D(E) factors may be extracted in the low energy region ; the results are shown in Figure 7. For comparison purposes escape depth measurements for Au are also displayed. The data in Figure 7 demonstrates that X-ray photoelectron spectroscopy, employing moderate energy sources (e.g. Al K cc), probes the surface of a solid. The D(E) curves appear to follow an Ella law at the higher energies, suggesting that even with higher energy photon sources there will be a strong surface contribution. Within a given XPS or AES spectrum, the degree of surface rather than bulk sensitivity of a given line will depend on its kinetic energy. For instance, in Figures 2 and 3, the ’ =I 100 eV L,M,M, electron and photon stimulated Auger signal will be less surface sensitive than the L, or L, photoelectron lines. The absolute surface sensitivity of XPS compared with AES will depend on the appropriate cross-sections. Al202

though approximate, the cross-section calculations in Section 2. I would suggest that for equal kinetic energy lines the absolute surface sensitivities of XPS and AES may be very similar. In view of the apparent surface sensitivity it is becoming increasingly clear that quantitative photoelectron measurements on solids, even to quite high energies, should be carried out under ultra high vacuum conditions, and full use must be made of the surface cleaning and control techniques established during the last decade. Acknowledgements

The author is indebted to Dr R Heckingbottom for stimulating discussion of many aspects of the subject matter contained in this paper. Acknowledgement is made to the Senior Director of Research of the Post Office for permission to make use of the information contained in this paper.

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