Auger electron spectroscopy and its application to surface studies

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Auger electron spectroscopy and its application to surface studies

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1972 Review of Physics in Technology 3 31

(http://iopscience.iop.org/0034-6683/3/1/I02)

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T E GALLON and J A D MATTHEW Physics Department, University of York, Heslington, York

Auger electron spectroscopy and its application to surface studies

Abstract, Auger electron spectroscopy (AES) is becoming increasingly used for the chemical analysis of solid surfaces. An introduction to the atomic Auger effect is given and the modifications introduced by a solid state environment are examined. The article contains a brief discussion on instrumentation and some of the results obtained from a number of solids are reviewed. The potential of AES as a quantitative technique is considered, and the method is related to other methods of surface chemical analysis.

1. Introduction

Many important physical and chemical processes in solids such as corrosion, catalysis, thermionic emission, etc, depend critically on the nature and condition of the interface between the solid and its surroundings. That is to say these processes depend on the structure of the surface of the solid, where by the term surface we mean the top atomic layer or the top layer and a few atomic layers beneath. In recent years the improvement of vacuum technology to the point where pressures in the 10-10 Torr range are easily attainable has enabled atomically clean surfaces to be formed and maintained for sufficient time to allow detailed experimental study. Many techniques have been developed to examine surfaces but perhaps the most widely used and most productive method to date is that of LEED, low energy electron diffraction (Estrup and McRae 1971, Prutton 1971). LEED is a true surface technique, sensitive only to the condition of the top few atomic layers of the solid, but it is only applicable to single-crystal specimens; it gives only structural information and exact unambiguous interpretation is difficult, especially in the absence of information on the chemical composition of the surface. In order to complement and extend the results of LEED investigations, a method of surface chemical analysis is required. Such chemical information is difficult to obtain and many different analytical methods have been used. The ideal method would be simple, non-destructive, of high sensitivity in the surface region, would give accurate quantitative information, give information on the chemical state of the chemical species present and require no special specimen preparation. At present no technique fulfils all of these requirements but Auger electron spectroscopy (AES) is perhaps the method which at this time most nearly satisfies all the above conditions. Other methods such as electron probe microanalysis, ion microprobe analysis, ion scattering, ESCA, FIM/FEM and ion neutralization spectroscopy have been used and some of these are superior to AES in fulfilling one or more of the analytical requirements. A brief discussion of these methods will be given in order to compare AES with other current methods of surface chemical analysis.

1.1. Suvface analytical techniques

The electron microprobe (Reuter 1971) identifies chemical species present by the characteristic x rays generated in the specimen by a focused electron beam with energy up to 30 keV. This method allows high spatial resolution, better than 1 pm, and has

31

32 T E Gallon and J A D Matthew

been developed to give quantitative measurements to within an error of about 5%. However, the entire depth of penetration of the electron beam is sampled and as this is usually greater than 1 um, the electron microprobe is not really suitable for true surface analysis.

In the ion microprobe a focused beam of rare-gas ions strikes the specimen and sputters atoms from the surface into a sensitive mass spectrometer (Socha 1971). This method is extremely sensitive and can detect concentrations of a few parts in 109. It also allows isotopic separation and is capable of quantitative interpretation. The technique is sensitive to the top layer and by successive sputtering a profile of the surface region is obtained. However, this method is destructive and requires a complex instrument not readily combined with other techniques.

Electron spectroscopy for chemical analysis (ESCA) is a method developed by Siegbahn and co-workers (Siegbahn et al 1967). A beam of x rays is directed onto the specimen and the electrons emitted are collected and energy analysed. Some of the electrons detected are direct photoelectrons ejected from atoms in the solid and have well-defined energies which allow the emitting species to be identified. Information on the chemical state of the emitting atom can be obtained by measuring the shift in energy from the free-atom energy (chemical shift). Although not greatly used so far for surface analysis, ESCA is really a surface sensitive technique since the depth of sampling is determined, just as in the case of Auger emission, by the escape depth of the ejected electrons (see 54.3 below). The main drawback to ESCA is that the cross section for photoelectron emission is small and long counting times are needed with this method. The difficulty of producing a well-focused x ray beam will probably mean that the spatial resolution of this technique is poor. If uv radiation is used in place of x rays detailed information on the outer bonding electrons may be obtained.

Field emission microscopy (FEM) (Melmed 1967, Southworth 1970) and field ion microscopy (Muller 1970) are also powerful surface characterization techniques. FEM in conjunction with electron energy analysis can give information on the electronic states of adsorbed atoms (Plummer and Young 1970), while the atom probe field ion microprobe (Muller 1968) can identify single atoms. However. the specimens must be in the form of fine spherical tips, a few hundred 5ngstroms in radius: further, the surfaces must be studied under very large applied electric fields, and the methods are only applicable to refractory metals.

Ion scattering methods measure the change in energy of light ions, usually H+ or He+, scattered inelastically from the surface (Smith 1971). The energy loss can be simply related to the mass of the ion and the mass of the scattering particle and hence the scattering particle can be identified. Both high (MeV) and low (keV) energies have been used but the high energy technique requires an accelerator. The technique is destructive and is not at present capable of quantitative interpretation. Ion neutralization spectroscopy is in effect a branch of AES and will be discussed briefly in 57.

In addition to the above, a variety of techniques are in use which are sensitive to changes in the surface but which do not give direct chemical information; examples are work function measurement and ellipsometry. Mention should also be made of the promising methods of appearance potential spectroscopy (Park et a1 1970) and ionization loss spectroscopy (Bishop and Rivi&re 1970b, Gerlach et a1 1970), which have yet to be fully evaluated but may prove useful when employed in conjunction with Auger spectroscopy.

How far AES meets the requirements of the ideal method of surface analysis will be discussed below but in general AES is non-destructive, is sensitive to the surface region,

Auger electron spectroscopy 33

is capable of giving some chemical information and should be capable of a degree of quantitative development.

1 . 2 . Auger emission

An atom ionized in an inner level can relax to a state of lower energy by a process in which an electron from a higher level fills the vacancy and electromagnetic radiation is emitted to carry off the energy difference. A process which competes with photon emission is Auger emission. The Auger process is a radiationless transition in an ionized atom in which the atom relaxes to a state of lower energy and ejects an electron to carry off the energy difference, leaving the atom doubly ionized. The effect is illustrated in figure l(a). The initial vacancy in level K is filled by an electron from level LI and an Auger electron from level L2,3 is ejected with energy leaving the atom doubly ionized. The energy is characteristic of the energy levels of the ejecting atom

L , 45 eV

2T &+ Work

function

Figure 1. Schematic diagram of (a) KLILP,s Auger transition in neon and (b) L2,3M2,3V

Auger transition in titanium.

and if the electron can be detected and its energy determined, the chemical species of the emitting atom can be determined. This is the principle of Auger electron spectroscopy. The labelling of such Auger transitions, the methods of calculating energies and transition rates and the effects of a solid state environment on these will be discussed in detail below.

1 .3. Electron induced Auger emission

The Auger effect is an intrinsic property of the ionized atom and any ionization mechanism may be used to create the initial vacancy. The effect was first discovered using x ray ionization (Auger 1925) and high-quality Auger spectra have been recorded by Siegbahn and co-workers using x rays (eg Siegbahn et a1 1967). Other methods of ionization such as low and intermediate energy ions can initiate Auger transitions but the majority of work on the Auger electron spectroscopy of solids has used electrons of energy in the range 500 eV-2 keV as the source of initial ionization. Electrons have several advantages over x rays as the source of primary ionization: the cross section for ionization of a given level is greater for electrons than for x rays of comparable energy

34 T E Gallon and J A D Matthew,

so that experimental times are reduced, high-density well-focused electron beams can be produced with any required energy, and electron guns are relatively cheap and simple pieces of equipment. The energy of the primary electrons is usually chosen to produce Auger electrons below 1 keV, an energy for which the competing process of x ray emission is negligible. The escape depth of these electrons in solids is small (see 94) so that those Auger electrons produced in the bulk of the solid have small probability of escape and collection without energy losses. The combination of high ionization cross section, high probability of Auger emission and small escape depth makes electron-excited Auger spectroscopy an ideal tool for surface analysis.

The use of electrons to excite Auger spectra has one serious disadvantage; the collected Auger electrons have to be separated from the background of secondary electrons. Figure 2 shows the energy distribution of backscattered electrons produced by irradiating

I

E Er Figure 2. Energy distribution of electrons scattered from a solid. ED is the primary beam energy.

a solid with primary electrons of energy Ep. The curve is conventionally divided into three parts. The peak marked I contains electrons elastically reflected without energy loss. The broad peak 111 is produced by the true secondary electrons generated within the solid. Region IT, sometimes termed the region of rediffused primary electrons, contains inelastically scattered primary electrons and the more energetic secondary electrons. The small peaks close to the elastic peak are produced by electrons which have suffered well-defined energy losses. The most common loss process occurs when the incident electron loses energy by exciting collective oscillations of the electrons in the solid, plasma oscillations. This gives rise to a series of plasma peaks of decreasing magnitude spaced by energy hwp, where w p is the frequency of the plasma oscillation (Pines 1955).

The Auger electrons generated in the solid fall in regions I1 and 111 and those shown schematically in figure 2 are of approximately the relative size observed by Harrower (1956) and Harris (1968a). The use of Auger electrons for surface chemical analysis was first suggested by Lander (1953) but it was not until Harris (1968a) showed that electronic differentiation of the electron energy distribution greatly enhanced the sensitivity that the technique became widely used. The effect of differentiating the energy distribution of figure 2 is shown in figure 3. It may be seen that each Auger peak is now represented by a double peak and that these stand out clearly from the background. Most experiments on the AES of solids use this differential method.


I E

c

Figure 3. The differential of the energy distribution shown in figure 2.

The object of this review is to describe the various types of application of Auger spectroscopy to surface physics attempted so far, and to suggest future developments in the field. The reference coverage is not intended to be comprehensive, and readers are referred to other reviews, for example by Chang (1971) and Taylor (1971), for different emphasis on the subject.

Before describing the apparatus used in Auger spectroscopy and the results of studies on solid surfaces we will examine the Auger process in more detail.

2. The Auger effect

2 . I . ClassiJcation of Auger processes

2.1.1. The atomic Auger e fec t . In the atom it is possible to describe the Auger emission process by specifying the initial and final electronic states. Table 1 describes typical Auger processes in Ne. Ionization of an inner shell may be followed by emission of a

Table 1. KLL transitions in Ne. Ground state configuration ls2 2 9 2pe; after ionization 1s 2s2 2pf3

Final Classification LS coupling Observed Theoretical configuration fine-structure+ energies$ energy5

lev) ( W

ls2 2s0 2p6+e KLlLl 'S 748 757 Isz 2s' 2p5+e K L I L ~ , ~ 'P, 3P 772,782 782 ls22s22p4+e KL2,3L2,3 IS, ID, 3P7 801, 804 807

t Spectroscopic notation (Kuhn 1969), for example 3P denotes L= 1, 2 S + l = 3 , that is S = l . 2 Siegbahn et al 1967. 5 From equation (2): this does not distinguish fine structure and should be compared with some mean of the numbers in the fourth column. 7 Not observed.


photon (x ray fluorescence), or alternatively, a second electron may be excited to a continuum state, that is, an Auger electron is emitted with an energy equal to the energy difference between the initial and final configurations. It is convenient to classify the process merely by reference to the initial and final holes in conventional x ray notation: holes with principal quantum number n = I , 2. 3. 4, . . . are assigned the letters K, L, M, N, . . . , while the I and i quantum numbers of the hole appear as a suffix according to the convention of table 2. The Auger transition is then defined by three letters ABC with suffixes, the first letter defining the initial ionization.

Table 2. X ray notation

suffix -

State I J

S 0 1 1 2

3 4 5 6 7

4 8 9

1

I

i

$

P

d 2

f 3

g

For atoms of low atomic number spin-orbit splitting is small, and so it is not necessary in the case of Ne to distinguish states of the same I but different j : the KLL Auger transitions may then be classified as in column 2 of table 1. The process may also be represented by an energy level diagram (figure I(a)). Auger processes in which the initial and one of the final holes have the same principal quantum number are termed Coster-Kronig transitions.

2.1.2. The Auger effect in solids. When atoms combine to form solids the outer atomic levels broaden into energy bands; the inner levels may be shifted in energy, but they remain quasi-atomic in character and the atomic notation is an adequate description. The outer electrons may retain some of their atomic character, but a hole in a broad band is conventionally assigned the letter V: figure I(b) depicts an L2,3M2,3V transition in Ti after Coad and Riviere (1971 b).

2,2. Auger electron energies

The crudest estimate of Auger electron energies may be obtained from the differences in binding energies of the various electrons involved as obtained from x ray spectra (Bearden and Burr 1967) or ESCA (Siegbahn et a1 1967): thus for an ABC transition

(1) where €A, EB and €C are the respective binding energies of the A, B, C atomic levels. Such an estimate is conceptually wrong in that the energy required to produce a two-hole final configuration is greater than the sum of the energies required to generate the holes singly; once one electron has been removed the binding energy of the second electron is enhanced by the absence of the repulsion of the removed electron. This effect is

€ARC 2 E.4 - ER - Er

Auger electrori spectroscopy 37

allowed for approximately in the formula of Chung and Jenkins (1970) developed from earlier suggestions (see Burhop 1952). For an Auger transition of type ABC for atoms of atomic number Z

EABC(Z)"EA(Z)-~(EB(Z)+EB(Z+ l)+Ec(Z)+Ec(Z+ 1)) (2)

since intuitively the effective binding energies of the holes correspond to those of an atom with an effective charge intermediate between Z and Z + 1 . This formula has a typical accuracy of the order of 5 eV, see table 1.

Where valence levels are involved particularly in the conduction band of metals, the two holes in the conduction band may not interact very strongly owing to screening by the electron gas, and it is more satisfactory to use the formulae of Coad and Rivibre (1971a) as a first estimate:

(3) i €ABV(Z)=E-~(Z)-+(EB(Z)+€B(Z+ 1)) -€\(METAL)

EA4vv(Z) =Ea - 2Ey(METAL)

where Ev(METAL) is the most prominent conduction band energy measured relative to the vacuum level. The nature of the two-hole states in metals is, however, very complicated and will be discussed at a later stage.

It would obviously be more satisfactory to have good first principle predictions of Auger electron energies by solving the Schrodinger equation for the initial and final states; such attempts have been mainly confined to KLL processes (Siegbahn et al 1967) and may require semi-empirical estimates of some quantities (Mehlhorn and Asaad 1966). First principle Hartree-Fock calculations (Larkins 1971a, b) have been carried out, for example on the KLL spectrum of neon and the LMM spectrum of argon, with reasonable success. The accuracy of such computations is, however, limited and for the most part one must still rely on the intuitive empirical estimates discussed above. Taking account of the instrumental uncertainties (see $93 and 4), fine structure (see $2.3) and environmental effects (see $7). equations (2) and (3) provide an adequate starting point for identification.

In surface physics one is concerned with Auger energies less than about 1 keV: since for a particular transition the energy will usually tend to increase with atomic number it is instructive to look at the approximate range of Z values yielding Auger electrons less than 1 keV for particular ABC transitions (table 3). A diagram of observed Auger energies has been prepared by Strausser and Uebbing (1971), though large uncertainties may be associated with some of the data plotted.

Table 3. Approximate range of Z giving an ABC transition of energy less than 1 keV. The lower limit of Z is governed by the number of electrons in the atom

Transition ABC Range of Z

KLL LMM MMM MNN NNN NNO N O 0

4-1 1 13-33 > 21 33-73 > 33 > 50 > 50

T E Gallon and J A D Matthew

2 . 3 . Fine structure The electronic state of an atom is not uniquely specified by its electronic configuration. For light atoms holes in electron shells couple through coulomb and exchange interactions to give several energy states for each configuration; these are characterized by the labels L (the net orbital angular momentum quantum number) and S (the net spin)- the Russell-Saunders or LS coupling scheme (Kuhn 1969). States with L values 0, 1, 2, 3, . . . carry spectroscopic classifications S, P, D, F, . . . , and 2S+ 1 appears as a prefix in this nomenclature. Table 1 and figure l (a) describe the possible transitions in the KLL spectrum of Ne, and their observed energies.

For a given L, S the total angular momentum J may take values from IL-SI by integer steps to L+ S ; owing to spin-orbit coupling each J value gives a distinct energy superposing a fine structure on the main LS splitting. For Ne this fine structure remains unresolved, but the size of the splitting increases with atomic number, until spin-orbit coupling dominates (j-j coupling). It is then necessary to specify only the 1 and j quantum numbers of the holes in the initial and final states. The ABC notation with suffixes as in table 2 then uniquely defines the Auger transition, but the low-energy Auger electrons involved in surface physics rarely satisfy this condition. Fine structure in atomic Auger spectra is comprehensively reviewed by Listengarten (1 962) and Bergstrom and Nordling (1965).

Most surface Auger spectroscopy carried out so far has been at low resolution and does not resolve fine structure. In molecular spectra fine structure characteristic of chemical bonding appears (Siegbahn et a1 1969), and solids may also show characteristic fine structure (Aksela et a1 1970). The roles of multiple ionization and excitation of additional electrons in the Auger spectra of solids are not yet clear, but Moddeman et a1 (1 971) have demonstrated the importance of these processes in simple molecules.

The unambiguous classification of Auger spectra is therefore not possible except for light atoms; while it is remarkable how far Auger spectroscopy has progressed under this difficulty, further insight into the nature of the relevant electronic states is essential.

2 . 4 . Fluorescence yield and relative intensities

X ray fluorescence and Auger emission compete as de-excitation mechanisms: for inner shell ionization of levels with binding energy less than a few keV the fluorescence yield is small, that is, the fraction of inner shell ionizations which yield an x ray photon is small (Bishop and Riviire 1969a). Both experiment and theory confirm this trend for K and L ionization (Bergstrom and Nordling 1965, McGuire 1969, 1970, Dick and Lucas 1971), though detailed agreement is limited; M and N ionization have received little attention.

While it is possible to neglect x ray emission to first order in surface problems, it is important to understand the relative probabilities of various competing Auger processes following a particular ionization; this probability P is approximately given by

where Yi is the wave function of the singly ionized system, \k”f the wave function of the final state with two holes and an Auger electron and V I Z is the electrostatic interaction operator between the two electrons involved (Bergstrom and Nordling 1965). Calculations on the KLL spectra of atoms in LS coupling (McGuire 1969, 1970), in the intermediate coupling approximation (Asaad and Burhop 1958) and sophisticated extensions incor- porating configurational interaction (Asaad 1965) all achieve limited agreement with


experiment, emphasizing how sensitive transition probabilities are to the wave functions of the system (Mehlhorn and Asaad 1966). For other Auger transitions little progress has been made, though an extensive experimental study of intensities of the LMM spectrum of Cu has been attempted (Jenkins and Chung 1971a).

2 . 5 . Energy distributions of Auger electrons

In the atomic Auger effect the energy distribution of emitted electrons (the line breadth) is the sum of the lifetime broadening of the initial and final states. In special cases, such as the KL2,3L2,3 spectrum of neon or the L3M2,3Me,3 spectrum of argon, the final state is essentially the ground state of the doubly ionized species and has negligible breadth. Separation of the observed breadth from the instrumental resolution leads to a direct measure of the K hole energy breadth in Ne (0.1 5 i 0.03 eV) and the L3 hole breadth in Ar (0.16 k0.02 eV) corresponding to lifetimes of the order of 1O-ls s in moderate agreement with theoretical expectation (Mehlhorn et a1 1968). Coster-Kronig transitions tend to have very rapid transition rates leading to broad diffuse Auger spectra (Mehlhorn 1968).

The wave function of inner electrons of atoms in solids are not very different from those of the free atom, and breadths for transitions involving only inner electrons will not vary much from atom to solid. On the other hand, transitions involving valence electrons may have electron energy profiles which are highly sensitive to the environment. This feature will be discussed in 67.

2.6. Summary

Our quantitative understanding of the atomic Auger effect is limited; in applying Auger spectroscopy to surface physics a purely first principles approach is not appropriate, and one must resort to semi-empirical calibration. Theory is, however, important in giving insight into the nature of Auger processes in solids.

3. Auger electron spectrometers

Details of all the instruments referred to in this section have been published and the reader is referred to original publications for full details of construction and method of operation. The analysers used in the study of Auger emission may be divided into two types : the dispersive analysers, inherently capable of high resolution, and the retarding field analysers usually based on LEED optics. In the dispersive analysers only those electrons of energy within the transmission range, E to E+hE, of the instrument are collected, while with the retarding grid analyser all electrons with energies above the potential of the cut-off grid are accepted. This means that the dispersive instruments give the energy distribution N ( E ) of the secondary electrons directly while the retarding grid analysers give

J':""" N ( E ) dE.

As noted above the sensitivity of the technique as a method of surface analysis is increased by measuring dN(E)/dE directly and this is done electronically by super- imposing a modulating voltage at frequency w on the analysing energy and detecting the first harmonic w in the case of the dispersive analysers or the second harmonic 2 w


in the case of the retarding grid analyser, using a phase sensitive detector (see, for example, Taylor 1969a).

3 . I . The LEEDIAuger spectrometer

In the retarding field analyser based on LEED optics (Scheibner and Tharp 1967, Weber and Peria 1967) a small modulating voltage is applied to the second grid and the signal is collected on the fluorescent screen. A block diagram of the retarding field analyser constructed in the authors' laboratory is shown in figure 4 (Gallon et a1 1969). This differs slightly from most retarding grid analysers in that it uses a tuned load and a head amplifier instead of the simple resistive load usually employed. This arrangement permits a higher modulation frequency to be used and allows long connecting cables to be used without shunting the load. Excitation of the Auger electron is either by the LEED gun at normal incidence or by a high current glancing incidence gun (see 54.4).

5 kV

270 V

- - Figure 4. Block diagram of a LEED/AUger spectrometer (from Gallon et al 1969).

Taylor (1969a) has given a full discussion of the use of LEED optics as an Auger spectrometer. He derives expressions for the current in the first and second harmonics when a modulating voltage k sin wt is used to differentiate a gaussian Auger peak of standard deviation a

i ( E ) = iA/(27ru)11' exp ( - E2/2u')

and he finds that the peak amplitude of the second harmonic is 0.06(k2/la2) ia. This expression holds for k/a<0.5 and for modulation amplitudes greater than a/2 the amplitude of the second harmonic increases more slowly with k up to a maximum value of 0.051;~ (Bishop and Riviere 1969a); however, large modulation amplitudes may degrade the resolution of the instrument. The resolution of a typical LEED/Auger spectrometer was measured by Taylor as about 2.5 % for three grid optics with grids 1 and 3 earthed and about 0.5% for 4 grid optics with grids 2 and 3 connected together. The major disadvantage of the retarding grid analyser is that all electrons with energies greater than the potential of the retarding grid are accepted so that the collected current


IC, including the elastic peak, is several orders of magnitude greater than the Auger current ZA being analysed. The shot noise in a bandwidth Afproduced by the collected current is proportional to (ZcAf)l,Z and while this may be reduced by reducing the acceptance bandwidth (increasing the time constant) of the phase sensitive detector this will eventually lead to unacceptably long scan rates. The signal-to-noise ratio, proportional to Z~/(ZcAj')1,2, is much smaller for the same l a than that of the dispersive analysers but this is offset to some extent by the very much larger collection angle which means that the retarding grid analyser collects larger values of ZA for a given resolution. Taylor compares the performance of the retarding grid and I27 ' electrostatic sector spectrometer and concludes that they are not greatly different.

Mularie and Peria (1971) have shown that it is possible to recover the intrinsic Auger energy distribution from the measured distribution in a LEED/AUger spectrometer by a deconvolution technique. They recorded both the Auger distribution and the energy distribution obtained for an elastic peak of the same energy as the Auger line. From the broadening of the elastic peak they were able to obtain the instrumental broadening function and from measurement of the low-energy tail of the elastic peak they were able to estimate the broadening produced by inelastic scattering in the solid. With a knowledge of these two functions they were able to deconvolute the true Auger electron distribution from the measured distribution. They publish deconvoluted spectra of a quality comparable with those obtained by the ESCA methods of Siegbahn and co-workers who used high-resolution analysers.

Huchital and Rigden (1970) have improved the poor signal-to-noise performance of the retarding grid analyser. They replaced the conventional large hemispherical collector by a small electrode shielded from direct secondary impingement and added a further grid system after the analyser which focused only those electrons which just pass through the analysing grid onto the collector. This means that only those electrons with energies in or close to the region of interest are collected and the more energetic electrons pass on and do not contribute to the shot noise. A further advantage of this configuration is that the removal of the large collector from the close proximity of the analysing grid seems to reduce field penetration effects, and Huchital and Rigden claim a resolution of 0.05%. This analyser would seem to combine the large transmission factor of the retarding field analysers with the resolution of the best dispersive analyser and to have very good intrinsic signal-to-noise ratio. However, this instrument has not yet been used for Auger spectroscopy.

3 . 2 . Dispersive analysers

The highest precision dispersive analysers in current use are those magnetic and electrostatic instruments developed by Siegbahn et a1 for use in ESCA (Siegbahn et a1 1967). Their resolutions range from 0.012 :i to 0.05 % but they are large specialized instruments not particularly suited to surface analysis and other dispersive electrostatic analysers have been developed for use in AES. Harris (1968a) used a 127" electrostatic analyser and this gave results of similar quality to those obtained later with retarding grid analysers, but it has the additional advantage that the very small acceptance angle allows experiments on the angular distribution of Auger electrons (Harris 1969). Mehlhorn (1968) and Pessa et a1 (1969) used cylindrical mirror spectrometers and this is a particularly advantageous configuration (Heddle 1971). The resolution of the Pessa et a1 instrument is in the range 0.01 to 0.26% depending on the slit Ridth. Palmberg et a1 (1969) have shown that by opening up the input slit and sacrificing the inherent resolution of the


cylindrical mirror spectrometer large currents and hence high signal-to-noise ratios are obtained so that the analysing voltage can be scanned fast enough to allow display of the Auger trace on an oscilloscope. By increasing the bandwidth of the phase sensitive detector to 2.5 kHz, Palmberg et a1 were able to use a scanning rate of 20 000 V s-l whch compares with a typical scan rate in the retarding grid spectrometer of about 5 Vs-1 for a spectrum with comparable noise. Other potential advantages of the favourable signal-to-noise ratio of this analyser are greatly increased sensitivity for the same scan time as a retarding grid analyser, or the use of reduced primary current for similar performance.

The cylindrical mirror and similar analysers will come to be used increasingly in Auger spectroscopy either as a sensitive analytical tool with immediate visual display or to obtain high resolution N ( E ) distributions to provide environmental information (see $7). Most work on the application of AES to solids up to the present has been performed with the LEED/AUger apparatus and in spite of the development of better spectrometers this should remain a valuable tool for surface studies. The retarding grid spectrometer has adequate resolution for most analytical purposes, it is simple, relatively easy to construct and has the advantage that in the LEED mode it gives structural information on single-crystal samples.

4. Auger emission from solids

Three stages have been distinguished in the production and collection of Auger electrons, namely: the production of the initial ionized state, the decay of the excited state by Auger emission and the escape and collection of these Auger electrons. All three processes will be affected if the atom is located in a solid. The changes in atomic Auger lines induced by a solid-state environment contain information on the nature of the electronic states of the solid and this subject will be dealt with in detail in $7.

Bishop and Rivi6re (1969a) considered the case of Auger emission from a monolayer film adsorbed on the surface of a uniform substrate. They derived an expression for the Auger current iA collected in a solid angle R when a primary beam current i o of energy E is incident at an angle 4 with the surface

sz 4iT

iA=ioN-- (1-w)rcosec4@(E,EA)

where N is the number of adsorbed atoms/cmZ of the substrate, w is the fluorescence yield, r is a backscattering factor and @(E, EA) is the cross section for ionization of the level initiating the transition (binding energy EA) by electrons of energy E. Generalization of this model to many layers has been attempted (Jacobi and Holzl 1971).

4 . I . Ionization cross section

The cross section for single ionization by electron impact @ (E , EA) is in general uncertain. Both the empirically modified Born approximation cross sections of Worthington and Tomlin (1956) and the somewhat controversial classical cross sections (Gryzinski 1959, 1965, Vriens 1969) predict simple universal formulae of the form


where U = EIE-4, the dependence of the cross section on incident energy being contained in the functionf( U ) . y is a constant in the Grysinski model and varies with the principal quantum number of the ionized level in the Worthington-Tomlin model. For either model the cross section rises sharply from threshold to a maximum at U-2.60 (Born model), 3.80 (classical) and then decreases more slowly. The function f(U) is shown in figure 5 . Such theoretical expressions should be regarded only as a general guide. Unambiguous experimental measurements are rare (Glupe and Mehlhorn 1967) but tend to peak generally in the range 3 < U < 4 (Luyken et a1 1969).

Figure5 The functionf(U); A, Gryzinski(l965)andB, Worthingtonand Tomlin (1956).

4 , 2 . The backscattering factor

The backscattering factor r is a function of the substrate. It allows for the fact that electrons entering a solid are scattered in the solid and some of these will pass back through the surface region where the more energetic can give rise to additional ionization and consequent enhancement of the Auger yield. The value of r will depend on the material of the substrate and on the value of the primary energy. Bishop (1967) has calculated the value for the backscattering coefficient for 10 keV electrons in an x ray microprobe and Bishop and Riviere used these values in their expression for i-4. Gallon ( 1 972) has measured the backscattering factor for electrons incident normally on the surface of bulk silver and silicon and found that for E/EA=3 about 30% of the Auger electrons from silver and 2070 of those from silicon were produced by backscattered electrons. Jacobi and Holzl (1971) have measured @(E, EA) for the K shell in thin carbon foils by correcting the observed energy dependence of the Auger signal for secondary electron effects and obtain reasonable agreement with @.(E, EA) obtained for carbon in gaseous CHd. Backscattering of the Auger electrons themselves should also be considered ; this will give an additional factor which is independent of incident electron energy. The combination of the two factors r and (D suggests that the primary energy should lie between three and five times the ionization potential for optimum Auger yield. The analysis of Bishop and Rivi6re concluded that the collected current from a monolayer of absorbed oxygen in a LEEDIAUger retarding grid analyser should be about 2 x A/pA incident current at optimum energy, a value in good agreement with experiment.


4 . 3 . The escape depth of Auger electrons

The primary beam energy normally used in the Auger emission spectroscopy of solids usually lies in the range 1-3 keV and Auger electrons will be generated at depths up to the limit of penetration of these beams, perhaps 500-1000 A. However, the escape depth of the Auger electrons will be much smaller than this since all Auger electrons suffering an energy loss of more than a few eV will be scattered out of the Auger peak into the general inelastic background and will not be detected as Auger electrons. The escape depth will be roughly the mean free path for the most probable inelastic electron-electron collision. No accurate values for these mean free paths exist although it is known that the inelastic cross sections at these relatively low energies are high and the escape depths will be small, making Auger spectroscopy a sensitive technique for the examination of surfaces.

Attempts have been made to measure the mean escape depth of Auger electrons by measuring the growth and decay of Auger peaks when one material is deposited in a layer by layer fashion on a substrate of a different material. From experiments on the growth of Ag on Au Palmberg and Rhodin (1968a) deduced that the mean escape depths of 70 and 350 eV electrons in silver were two and four layers respectively (4 and 8 A). Ridgway and Haneman (1971) have performed similar experiments for the growth of Fe layers on Si and from their results the mean escape depth of 46 eV electrons in Fe may be deduced as 2.5 layers.

An alternative method (Jacobi and Holzl 1971) is to measure the absolute Auger current, use reasonably reliable absolute values of the ionization cross section and to allow for backscattering: the mean free path of 262 eV Auger electrons in amorphous carbon has been estimated as 7.5 A.

Since the limit on the depth of emission is the scattering of the emitted electrons and not the penetration depth of the primary ionizing radiation, ESCA, applied to solids, is also a technique for surface analysis. The energies of the electrons emitted in ESCA are usually higher than in AES so that the depth of the specimen sampled will be greater. Baer et al( l970) estimated the escape depth of 1.2 keV electrons in gold as 22 A.

A knowledge of the escape depth of low-energy Auger electrons in solids is of great importance in any future quantitative application of AES. Unfortunately the number of systems which grow in a layer by layer fashion is limited. However, if enough data on escape depths could be obtained from systems which have this morphology, an empirical relationship between escape depth and energy might be obtained similar to those which give fair agreement in the intermediate and high-energy ranges (eg Kobetich and Katz 1968).

4 . 4 . The dependence of Auger yield on the angle of incidence of the primary beam

The above expression for i.4 contains the term cosec 4 to describe the variation in yield with angle of incidence. This term will arise in the case of primary ionization assuming a fixed geometrical cross section. The form of the Bishop and Rivibre equation implies that the ionization produced by backscattered electrons will also vary as cosec4 but there would seem to be no reason for this to be the case. Indeed, if the backscattered electrons suffer multiple scattering in the substrate before passing back through the surface it would seem reasonable to assume that the backscattered contribution will be independent of the angle of incidence of the primary radiation. However, it is clear that iA will increase with 4 and Palmberg (1968) has observed such an increase. Palmberg found a cosec 4 dependence for Auger emission from elements on the surface of a Pd


specimen and a smaller variation for signals from the substrate. Most Auger spectrometers now incorporate a glancing incidence gun to benefit from this increase in iA with 4.

4 . 5 . The use of AEsfor chemicalanalysis

The identification and labelling of Auger peaks arising from solid specimens is not a trivial problem. An Auger spectrum may contain a number of Auger peaks from an unknown number of elements and in addition, plasma loss peaks associated with both the elastic peak and with the stronger Auger peaks; that is, the electrons may excite oscillations in the electron gas and lose energy by discrete multiples of the plasmon energy hwp. It may therefore be difficult to identify unambiguously all the peaks in a complex spectrum. Two problems arise, firstly the separation of the Auger peaks and identification of the associated chemical species and secondly, the correct classification of the peaks.

From the point of view of surface chemical analysis the first problem is the more important and is relatively easy to solve. Plasma loss peaks associated with the elastic peak may be detected either by applying a potential to the specimen, in which case the Auger peaks with associated loss peaks will shift in energy, or by altering the primary beam energy, in which case the elastic peak and its plasma peaks will shift. The latter method is easier to apply and is applicable to insulators. Mularie and Rusch (1970) have shown that it is possible to decide with some degree of confidence whether small peaks on the low-energy side of large Auger peaks are associated plasma losses or additional Auger peaks. Their method is to reduce the primary incident energy to that of the principal Auger peak in question and to examine the position and relative magnitude of the elastic peak and its plasma loss peaks. If the position and magnitude of the satellite peaks relative to the Auger peak is similar to that of the plasma loss peaks to the elastic peak then the satellite peaks may be identified as being produced by Auger electrons suffering one, two. three. etc plasmon losses.

This philosophy is used in a deconvolution technique for spectra obtained from retarding grid analysers (Mularie and Peria 1971). However, the probability of an Auger electron emitting plasmons may not necessarily be the same as that of a primary electron of the same energy since the excitation process is different. Indeed, Auger electrons may even gain energy by destroying plasmons, a process not observed in the case of reflected primary electrons (Jenkins and Chung 1971 b, Suleman and Pattinson 1971 a).

In addition, there may be a large probability of an incident electron losing an amount of energy just in excess of the binding energy of the electron it ejects. These events give rise to ionization loss peaks (Bishop and R i d r e 1970b) which are at a fixed energy relative to the elastic peak and can be readily distinguished from Auger peaks. Additional information may be obtained for any unidentified peaks by examining the variation of peak height with primary energy and hence obtaining the ionization threshold for the transition. It is then usually possible to identify the emitting species with the aid of tabulated Auger energies as in S2.

The characteristic low-energy Auger spectra of most elements is known but very few reliable data exist concerning the exact energies of the peaks and even less exist concerning relative intensities and absolute widths. In the absence of any selection rules considerable doubts exist concerning the labelling of peaks even when the emitting element is known unambiguously. The lowest level participating in the transition is generally known from ionization data but the other two levels involved are usually obtained by choosing atomic levels to give the best energy fit. Corrections for residual


hole-hole interaction, inner potential and work function differences may or may not be applied and one or more electrons may be designated to come from the valence band of the solid, generally with little regard for the line broadening expected to be associated with band transitions. The situation is not helped by the convention of selecting the high-energy turning point in the " ( E ) curve as the position of the Auger peak. This method gives a well-defined energy but this energy corresponds to the point of inflection in the N ( E ) curve and its value will depend on instrumental broadening effects in low- resolution spectrometers and on the amplitude of the modulation voltage if this is too high. In the measurement of Auger energies it is the crossing point of the " ( E ) curve, corresponding to the maximum in the N ( E ) , which should be used; however, this is often difficult to measure accurately, especially if the Auger peak is asymmetrical or if the background is high. While deconvolution techniques may improve the effective resolution of the retarding grid analyser, field penetration and the high modulation signal needed for sensitivity limit performance, and so systematic measurements with a high-resolution analyser have an important role to play in understanding the Auger process.

5. Qualitative chemical analysis

Auger emission spectroscopy has proved an extremely useful method for surface chemical analysis and a useful complement to LEED. AES has proved useful in the monitoring of surface cleanliness and the determination of the efficiency of various cleaning methods in producing a 'perfect' surface. I t is useful at this stage to consider in some detail a case study typical of a wide variety of applications of AES for surface monitoring. Observations on Si surfaces under high temperature flashing provide a convenient example.

Figure 6(a) shows an Auger trace from the surface of n type silicon obtained after chemically cleaning by a process involving degreasing, etching in HF and further washing. This particular specimen had been subjected to bakeout at 300°C for 8 h in order to reduce the chamber base pressure to below 5 x 10-lOTorr. The largest peak is from carbon with smaller peaks indicating the presence of oxygen, sulphur and phosphorus on the surface. A small silicon peak can also be seen. Similar results have been found from unbaked Si specimens except that the presence of sulphur is not usually observed, indicating that the sulphur may have diffused over the silicon from the tantalum holder during bakeout. It is obvious that this particular chemical treatment does not produce a clean silicon surface.

The specimen was then flashed to 1100 "C for about 15 s and annealed at about 600 "C for 10 min. This process was repeated several times and Auger traces taken after each anneal. The sulphur and oxygen were found to disappear after the first flash and the carbon was considerably reduced, presumably by diffusion into the bulk silicon, with consequent increase in the silicon peak. Figure 6(b) shows the Auger trace after the first flash and anneal. The carbon peak is very much reduced and the oxygen and sulphur peaks are no longer present: however, the phosphorus peak is still clearly visible. The 91 eV Si peak is now by far the most prominent feature in the spectrum and lower energy Si peaks may now be seen. The Auger spectrum obtained after six cleaning cycles is shown in figure 6(c) . This is similar to figure 6(b) except that the phosphorus has been significantly reduced, although it is still present, the carbon has further decreased and the lower energy silicon peaks are more distinct. These changes can be correlated with LEED observations (Gallon et a1 1972).

Similar observations of the behaviour of the carbon impurity on silicon surfaces have been reported previously (see Chang 197 1 for a list of references). Auger spectroscopy


!xi I 0 100 200 300 400 500 600 0 50

E (eV 1

(0)

SI E.=700 V M"od 2 .2 V rms

~

P L X 4 C SYL

100 150 200 250 300 , ax:

EteV)

( 6 )

h=aso v Mod 3 V rms

, 'J

0 50 100 150 200 250 300 €(el')

(c)

Figure 6. Auger electron spectrum from silicon. (a) after chemical cleaning, (b) after a 15 s flash at 1100 'C followed by an anneal at 600 "C for 10 min and (c) after repeated flashings and annealings.

has provided useful information on the nature of the Si(l11)7 LEED pattern obtainea after cleaning and annealing silicon. This pattern has usually been assumed to arise from the pure silicon surface but suggestions have been proposed that i t is due to a second phase such as iron or nickel being present in submonolayer quantities at the surface. Taylor ( I 969b) found that the amount of Fe and Ni present on a surface giving a Si(ll1)7 pattern was below the detection limit by AES and therefore could not give rise to the observed 7 pattern. He did, however, find that the presence of the S i ( l l l ) d l 9 pattern was associated with nickel being present on the surface.

Already single-crystal surfaces of most simple metals, semiconductors and insulators, as well as thin films on various substrates, have come under similar scrutiny from the AES technique. A limited range of more complex technologically important materials has also been considered; steel (Marcus and Palmberg 1969), graphite fibres (Connell 1971) and polypeptide films (Bishop and Rivi6re 1970a) are interesting examples.

5 . I . Segregation at surjaces and grain boundaries

The bulk concentration of phosphorus in the n type Si from which the Auger spectra shown in figure 6 were recorded was approximately 1 part in 2 x IO3. It is not yet possible


to estimate accurately the relative concentrations of detected species from Auger spectra but the detection limit for the apparatus used is about 1 % of a monolayer and the phosphorus concentration is considerably greater than this. Thus the concentration of phosphorus at the Si surface is much greater than that in the bulk so that phosphorus has diffused from the bulk to the surface. This is an example of the quite common pheno- menon of surface segregation observed by Harris (1968b) using Auger spectroscopy. Harris examined the Auger spectrum of Ni before and after heating and he found that a large peak had appeared at 150 eV arising from sulphur which had segregated to the surface from the bulk. Harris estimated the amount of sulphur present in the bulk of his specimen as between 10 and 80 ppm. A similar experiment has been performed by Sickafus (1 970) who observed sulphur segregation even though the initial bulk concentration in his specimen was below 1 ppm. Harris (1968b) also reported observation of the segregation of chromium and antimony to the surface of steel containing small amounts of these elements.

The segregation of low concentration impurities at free surfaces has been investigated for a number of metals by Haas er a1 (1970). Table 4 shows the contaminants observed by them on the surface of high purity foils after short outgassing in vacuum. They found that S, C1 and 0 could be removed (except in the case of chromium) by simply heating to 1200°C but that carbon had to be removed by heating in oxygen or hydrogen or by argon ion bombardment.

Segregation of impurities to the surface of single-crystal specimens can give rise to new ordered structures and AES has proved useful in the interpretation of LEED patterns,

Table 4. Some contaminants observed by Auger electron spectroscopy on various refractory metals after initial short outgassing in vacuum. + contaminant found; - not found; ? questionable (from Haas et a/ 1970)

Metal

sc Ti V Cr Fe CO Ni Y Zr Nb MO Ru Rh La Hf a

Ta W Ret1 Ir Pt AU

~~

a Zr also appeared at surface on heating Hf. b Traces of MO also apparently found and were removed by subsequent thermal treat men ts .


in particular in deciding whether a new structure produced on heating is due to recon- struction of a clean surface or to the ordering of an impurity which has diffused to the surface. Bishop and Riviere (1969b) doped the surface region of a silicon single crystal with gold by ion implantation and observed the changes which occurred on heating with the aid of LEED and AES. They observed a sequence of surface structures and were able to correlate these with changes in the Auger signal from gold. Similar use has been made of AES in the determination of surface structures by many workers and the combined LEED/AES apparatus will undoubtedly continue to provide a powerful tool for the examination of surfaces.

Certain metals and alloys can fail under load either by brittle fracture or by ductile fracture. In these materials the former type of fracture occurs at low temperatures while at high temperatures ductile fracture occurs. At intermediate temperatures fractures by mixed processes occur but a conventional ductile-brittle transition temperature is defined to characterize the transformation. The importance of this transition in metallurgy is that the yield stress for ductile fracture is much greater than for brittle fracture. It is found that in some metals, for example low alloy steels, heat treatment may raise the ductile-brittle transition temperature considerably so that it lies above the temperature of the ambient in which the metal is to be used. The metal is then termed temper embrittled and in this state fracture usually occurs along grain boundaries. This embrittlement has been assumed to be caused by segregation of impurities at grain boundaries during the tempering treatment but there has been no direct evidence of this segregation. Auger spectroscopy has recently been used to compare embrittled and non-brittle fracture surfaces and the segregation model has been confirmed.

Marcus and Palmberg (1969) found that antimony segregated to fracture surfaces in embrittled low alloy steel containing 0.03 at. % antimony, while no antimony could be detected at non-brittle fracture surfaces in the same steel. They were able to estimate the thickness of the antimony-rich layer by sputtering away the surface by argon ion bombardment and they found that the antimony resided in a layer a few 5ngstroms thick. MacDonald et al(l970) repeated this experiment in a scanning electron microscope using the finely focused beam to excite Auger electrons. This method enabled a SEM micrograph to be taken, the electron beam returned to a point of interest, as shown by the micrograph, and a chemical analysis at the point obtained. MacDonald et a1 were able to obtain Auger spectrum with spatial resolution 0.8 pm and they showed that the distribution of segregated antimony was non-uniform across the fracture surface. Stein et a1 (1969) found similar results for a steel containing 620 ppm Sb and they found increase in the Cr and Ni concentrations at fracture surfaces of other embrittled steels. Joshi and Stein (1 970) have examined the embrittlement of polycrystalline tungsten and they were able to relate increase in the ductile-brittle transition temperature to increases in the concentration of phosphorus found in the fracture surface. Again, by a sputtering technique the phosphorus was found to be concentrated in a thin surface layer.

Another interesting result using AES for qualitative examination of surfaces is the study of gain fatigue in channel electron multipliers by Prince and Cross (1971). It is found that reduced lead oxide glass channel multipliers, while remaining stable when used under p m r vacuum conditions. suffer a continuous loss in gain when used in UHV.

Prince and Cross examined the behaviour of reduced lead oxide glass in UHV and found that the Auger signal from oxygen diminished with time. They were able to explain qualitatively the gain fatigue and recovery of channel multipliers using a model in which the surface oxygen acted as an electron donor. They also observed the presence of a hitherto unsuspected high surface concentration of carbon and suggested that this

50 T E Gallon and J A D Matthen,

might provide an important contribution to the surface conductivity needed to specify the surface potential.

6. Quantitative analysis

If Auger spectroscopy is to realize its full potential as an analytical technique i t must be rendered capable of giving quantitative as well as qualitative information. In this context the great advantage of AES, namely its sensitivity to surface conditions, becomes a drawback. The surface of a solid is in general a region of inhomogeneity, thus the surface atomic layer may contain adsorbed gas or segregated impurities and may be quite different from the second, third and lower atomic layers; or the surface may contain a nucleated second phase so that an incoming electron beam ‘sees’ regions of pure substrate and pure nucleated species. Arrangements like these will affect the quantitative interpretation of Auger spectra and unless the distribution of atoms is known from some other source, exact quantitative interpretation of Auger spectra will not be possible. Thus if we find peaks from material A in a spectrum taken from supposedly pure material B, no quantitative interpretation will be possible unless we know, for example, that A is uniformly distributed in B or resides only in the top atomic layer or is situated in hemispherical nuclei of given density. Even if such additional information is available, in the present state of Auger spectroscopy any quantitative analysis is tenuous and open to doubt. Many of the parameters needed for accurate quantitative analysis, such as escape depths and cross sections, have yet to be measured. However, attempts have been made to quantify AES and some of these will be dealt with in this section.

6 , I. Sub-monolayer surface deposits

Quantitative interpretation of Auger spectra can be made most readily in the case where one material A is deposited on another B and grows uniformly flat, and less than a monolayer is present. In this case the Auger signal from A, ZA, should be linear with coverage 8, and if Z-il is the signal obtained with the given experimental conditions from a complete monolayer

zA=ozA1 o < w .

Most Auger spectrometers used for surface analysis measure d2ZAldE2 not ZA but, as Weber and Johnson (1969) pointed out, provided the shape of the Auger peak does not change with coverage, d2Zla/dE2 is proportional to ZA and we may use the peak-to- peak height of the second derivative as a measure of ZA. Weber and Johnson (1969) deposited up to a monolayer of K on Ge(l11) in known amounts and were able to compare the Auger current, measured from the second differential, with K coverage. Their results are shown in figure 7. They found that the peak-to-peak height of the K Auger signal was a linear function of coverage, provided, of course, that all the spectrometer parameters were kept constant. After deposition they annealed the specimen and compared the coverage obtained from AES with the observed LEED pattern and they found that the coverage predicted by LEED models agreed with the coverage measured by AES but that the LEED patterns were found to occur over a range of coverage. This suggests that LEED is not a very suitable method for estimating surface coverage. Weber and Johnson were also able to estimate the sensitivity of their LEED/AUger


0 2 0 4 06 0.8 10 Coverage e

Figure 7. The height of the principle potassium Auger peak as a function of coverage (from Weber and Johnson 1969).

spectrometer and found that about 2 % of a monolayer of Cs on Si and of K on Si could be detected.

A quantitative investigation of oxygen adsorption on W(110) using AES has been reported by Musket and Ferrante ( 1 970). They used a background nulling method which allowed N ( E ) to be measured directly with sufficient sensitivity and, by integrating the N ( E ) curve, were able to obtain a direct measurement of the current in the oxygen Auger peak. They found that the peak shape did not vary with coverage, which is in agreement with the assumption made by Weber and Johnson in their calibration of the system K on Ge. In order to calibrate the Auger signals the magnitude of the current from a monolayer of oxygen, ZA,, had to be determined and this was done by exposing the tungsten surface to large quantities of oxygen and assuming that the saturation coverage corresponded to a monolayer. Then, by measuring the Auger current after a known oxygen exposure, a graph of coverage against exposure was obtained and from this the initial sticking coefficient for oxygen on tungsten (1 10) at 330 K was found to be 0.23*0.05. Musket and Ferrante estimated that the sensitivity of their spectrometer, a LEED/AUger instrument, was better than 2% of a monolayer of oxygen. Comparison of their coverage data with LEED showed that the ‘half order’ pattern ( 2 x 1) had a maximum intensity at a coverage of 0.58.

Similar measurements have been performed by Florio and Robertson (1 969) for the system chlorine adsorbed on Si( 11 1). Again, like Musket and Ferrante, they calibrated their Auger signals by assuming saturation coverage corresponded to one monolayer of adsorbed gas; however, they used the peak-to-peak height of the differentiated Auger signal to measure IA. The adsorption coverage was measured as a function of exposure for several temperatures and graphs of sticking coefficient against coverage obtained in each case. The thermal desorption of chlorine was also studied as a function of time for the specimen at various temperatures. From the variation of coverage with time at different temperatures Florio and Robertson were able to show that the desorption reaction was second order and they measured the rate constant, 5 x 106s-1, and the activation energy, 35 kcal mol-’, for the reaction. The activation energy was also measured by flash desorption and the value obtained, 36 kcal mol-l, agreed extremely well with that obtained from the Auger measurements.


6 .2 . Layer growth Another system which has been investigated and which is capable of quantitative interpretation is that in which a material is deposited on a substrate and grows layer by layer. Palmberg and Rhodin (1968a) investigated the growth of Ag on Au where the Ag is known to grow in a monolayer fashion. Silver was evaporated at a constant rate and the time for one monolayer of silver to form was obtained by measuring the time taken for a 1 x 1 LEED pattern to appear although this calibration method is open to doubt. From this measurement and by assuming that the sticking coefficient for Ag on Ag is the same as that for Ag on Au at room temperature, they were able to measure the growth of the silver signal and the decrease of the Au signal with silver coverage. The mean escape depths of the Auger electrons in silver were estimated from these measurements and found to be 4 A (2 monolayers) for 72 eV electrons and 8 A (4 monolayers) for 362 eV electrons. Ridgway and Haneman (1971) have performed similar measurements on the growth of Fe on Si, in which they calibrated the deposit by evaporating the iron at a rate measured by a quartz crystal ratemeter. They publish a graph of the Fe 46 eV Auger signal against thickness.

A model for the growth of Auger signal with thickness has been proposed by Gallon (1969) for the case of layer-by-layer growth. It assumes that the Auger current can be expressed as the sum of contributions from parallel planes and that an emitted electron has a fixed probability of penetrating each plane. The current from a deposit n layers thick is given by

Zs=Za{l - ( I -Z1/Zm)"} n > 1

where I1 is the Auger current from a monolayer and I , is the bulk Auger current, strictly with backscattering contribution appropriate to the substrate.

The form of this expression fitted to the Palmberg and Rhodin data is shown in figure 8. This equation can also be expressed for quasi-continuous growth as

where

and d is the layer spacing. In this expression SO does not represent the mean escape depth since the growth of

Z, is linear with coverage up to x = d . The correct value for the mean escape depth no, in layers, is given by

However, this does not differ significantly from In-1 {I&, - Zl)} unless I1 N I,. This analysis applied to the data of Palmberg and Rhodin gives no = 3.8 for 360 eV

electrons in silver and from the data of Ridgway and Haneman (1 971) no = 2.5 for 46 eV electrons in iron.

6.3. Homogeneous solids

The quantitative analysis of more complicated systems is difficult. The simplest approach for the case of a uniform solid containing N atoms/unit volume is to generalize the


1.0-

0.8-

B 2 0.6,- 3

0.4-

2 4 6 8 10 I2 I 4 16 n

Figure 8. Comparison of the variation of Auger signal with thickness predicted by Gallon (1969), with the data of Palmberg and Rhodin (1968a).

equation used by Bishop and Rivibre (1969a). This gives, assuming no attenuation of the primary beam in the escape region for Auger electrons (energy E),

ZA = ZpGr (Ep) @T(EP. EA) NT(E)

where G is a geometrical factor determined by the angular distribution of Auger electrons and the collection solid angle and T is a length determined by the depth of escape of electrons of energy E in the solid. A similar approach has been used by Jacobi and Holzl (1971) and they indicate how G may be calculated. The mean escape depth, nod, might be taken as a measure of T but a better value is obtained by defining the equivalent escape depth, T=(Zm/I1) d, since this gives the correct number of atoms contributing to the bulk signal. Strictly I1 is the signal from a monolayer on a substrate of the same material and if it is measured by depositing the material on a different substrate it should be corrected for the different backscattering factor.

If we now consider the case of a material composed of a uniform mixture of atoms A and B giving rise to Auger currents ZA and ZB from levels EAA and EAB, the ratio of the two Auger currents is given by

IA - rA(EP, EAA) QTANATA I B r d E p , LAB) 0)TBNBTB'

_ _ _ _ ~ _ _ ~ _ _

If we assume

The quantities r and 0 0 can, in principle, be measured in separate experiments or one of the formulae for 0 6 could be used with a corresponding degree of uncertainty (see $4 above). As more information is obtained from experiments of the growth of uniform films it might be possible to obtain an empirical expression for T . If IA/ZB is measured by taking the ratio of the peak-to-peak heights of the second derivative, correction must be made in the case where the peak widths of A and B differ (Taylor 1969a).


6.4. Inhomogeneous solids

The surface of a solid is often a region of non-uniformity and the above analysis will not apply. Some information on the nature of the distribution may be obtained from Auger spectroscopy by varying the angle of incidence of the primary beam. At high angle of incidence the primary beam can suffer appreciable attenuation in penetrating the escape depth and the ratio of Auger peak heights of a surface species to that of a uniformly distributed species will increase with angle of incidence.

A further case of interest is that of the growth of nuclei of a deposited material on a substrate. The determination of whether a deposit nucleates or grows in a monolayer fashion should be possible from measurement of the rate of increase of the Auger signals. For a deposit which grows as a monolayer the increase in the Auger signal should be linear with time, at a constant deposition rate, up to a monolayer since all deposited atoms contribute, while in the case of a nucleating deposit the growth rate will be slower since contributions to the Auger current from those atoms in the interior of nuclei will be small. Pollard (1970) has observed interesting effects in the case of thorium growing on tungsten at 1400°C. The Th Auger signal was found to increase almost linearly up to a monolayer, determined from work function minimum, then to remain constant for an equivalent coverage between 1 and 2 monolayers and finally to increase slightly with increase in Th deposition. Pollard interprets this as the Th growing initially flat up to a monolayer and then nucleating as more thorium is deposited; this leads to a slight increase in the Th surface area and consequent increase in the Auger signal.

7. Environmental effects

So far the presence of certain Auger lines has been regarded as a diagnostic of the presence of a particular atomic species on the surface. If, however, one looks with greater precision at the energy distribution of Auger electrons information about the environment of the atom in the surface region may be obtained. Qualitatively, two things may happen. The Auger process may be very similar to that in the free atom, yet the presence of other nearby atoms modifies the potential seen by the electrons. Energy shifts may occur in both the initial single-hole state and the final double-hole state, resulting in a so-called chemical shift in the Auger energy. If, however, the Auger transition involves outer electrons in the atom the atomic classification of the final state may be inappropriate and the energy distribution of the Auger electrons may reflect the bonding of the electrons or the band structure of the solid in the surface region.

7.1 . Chemical shifts

Though the inner electrons of an atom in a solid are quasi-atomic, their energies are influenced by the interaction with outer valence electrons and the remainder of the solid; this results in a chemical shift in the Auger energy which for an ABC transition is given by

AEABC = ABC - AA

where AA and ABC are the chemical shifts of the initial and final states. While AA may be found by ESCA (Siegbahn et a1 1967), ABC cannot be directly determined by alternative methods, but will be very approximately given by ABC N A B + 4 c (see figure 9).


The chemical shift may be divided into two components: (i) the chemical shift due to the electronic structure of the atom or ion itself; (ii) the shift arising from the potential due to the other atoms in the solid. The former contribution is very sensitive to the ionic charge of the atomic species in

its environment (Siegbahn et a1 1967, Coulson and Gianturco 1968). while the latter may to a first order be estimated by neglecting the electronic structure of the other atoms, and assigning to them effective point charges characteristic of the ionicity of the bonding in the crystal. The crystal field contribution to the chemical shift is then simply related to the Madelung energy at the site under consideration. Since there is overall charge neutrality in the crystal, the two effects tend to cancel. Fahlman et a1 (1966) have investigated the KLL Auger spectrum of sulphur in the thiosulphate ion. The observed difference in energy between Auger lines corresponding to the two oxidation states 6 f and 2- in the ion is 4.7 eV, which agrees well with the findings of ESCA. On clean surfaces relatively few studies of sufficient resolution have been carried out, but chemical shifts in the NNN spectrum of tantalum exposed to oxygen may be correlated with various oxide structures observed by LEED (Haas and Grant 1969).

It is difficult in general to associate unambiguously an observed chemical shift with a particular valence state and surface environment, and for the most part chemical shifts will tend to be used as empirical environmental fingerprints.

7 . 2 . Auger specrra of calence electrons

When valence or conduction electrons take part in the Auger transition, the final electronic state is quite different from the case of the atom, and even at low resolution characteristic solid-state effects may appear. These will be reviewed according to the bonding type.


7.2.1. Ionic crystals. The 2p electrons of the fluorine ion in LiF are bonding electrons which contribute to a valence band in the crystal with breadth of the order of a few eV (Kunz et al 1969) yet the KLL spectrum of fluorine closely resembles that of isoelectronic atomic neon (figure 10). In both systems the L-S coupled fine structure (table 1) is well resolved: the two holes in the final state must be highly localized to give exchange splittings characteristic of atomic systems; that is, the final state is in effect an Ff ion embedded in the crystal field due to the rest of the crystal. It is nor appropriate then to think of producing two holes in the valence band of the crystal; rather one is producing a short lived defect which is quasi-atomic in character and whose energy levels are strongly influenced by the repulsion of the two holes in the final state.

1

KL L J d L ( ' 5 )

J , 750 760 770 780 790 800 810

I L Not an Auger l ine

590 600 6io 6 2 0 6 i o 6IQo 640 660- Kinetic energy ( e V )

Figure 10. (U) KLL spectrum of neon and (b) KLL spectrum of fluorine in LiF (from Siegbahn et a1 1967). Here N ( E ) has been measured directly as in figure 2 rather than dN(E)ldE as in figure 3.

The fluorine spectrum in LiF is, however, much broader than those of atomic neon for the following reasons :

(a) The final state in the solid is very much more short lived than in the atom, that is, the two holes may fly apart to generate the lower energy configuration of two non- interacting holes in the valence band. There is therefore lifetime broadening associated with the final state as well as the initial state (Matthew 1970a).

(b) The defect is coupled to a vibrating lattice, and broadening of Auger lines will result from positional fluctuations of neighbouring nuclei (Matthew 1970b).


Environmental dependence of the breadth of Auger lines in LiF, NaF and MgFz has been observed (Albridge et a1 1968).

If the Li+ ion in LiF undergoes further ionization by an electron, the resulting Li++ ion has only one electron and can only de-excite by an Auger transition involving electrons mainly associated with neighbouring fluorine ions. Here the final state will be a quasi-molecular defect with interaction between holes again playing an important part (Gallon and Matthew 1970); molecular orbital techniques (Urch 1970, Fischer 1970) may be appropriate in classifying the observed lines.

7 . 2 . 2 . Metals. In contrast with the localized picture that has developed for the Auger effect in insulators, interpretation of Auger spectra involving conduction electrons has emphasized the relationship between observed profiles of Auger electron energies and the density of one electron states in the conduction band of the metal.

He+ ( 0 ) ( b )

Figure 11. Comparison of (a) ion neutralization Auger emission and (b) A W Auger transition.

High resolution Auger experiments on metals are rare (Aksela et a1 1970, Aksela 1971) but an alternative to ionization by an electron beam is possible-ion neutralization spectroscopy (Hagstrum 1966). Helium or neon ions are directed at clean metal surfaces; an electron tunnels through the potential barrier between the metal surface potential and the ionic potential and neutralizes the ion, its excess energy being used in ejecting another electron from the conduction band (figure 1 Ita)). This is strongly analogous to an AVV Auger process, with the rare gas ion providing the initial hole rather than an inner level of the crystal atoms (figure 1 l(b)).

It is conventional to neglect the interaction between the resultant holes in the conduction band; one may then consider an Auger process involving two independent conduction electrons with energies (+ A and (- A measured from the Fermi level of the metal (work function 4); the Auger electron has energy

E= E A - 2(( + 4). where EA is the binding energy of the initially ionized level.

T E Gallon and J A D Matthew

Various transitions of the same E may have different values of A, and so the intensity of Auger electrons at energy E is of the form

F(E)cc ). TPA d h

where TPA is the transition probability for transitions of a particular A. If the transition matrix elements vary slowly with 1, TPA will be closely related to the product of the density of available states at ( + A and ( - A . The profile is then a convolution of the double density of states in the conduction band. Hagstrum (1966) and Hagstrum and Becker (1967) using ion neutralization spectroscopy, and Amelio and Sheibner (1968), using conventional Auger spectroscopy on graphite, have deduced effective one particle densities of states, which agree in some features but not in detail with those deduced from soft x ray spectroscopy (Fabian 1968) or theory.

The KVV spectrum of metallic beryllium has proved controversial. Musket and Fortner (1971) observe two lines at 92 eV and 104 eV in contrast with the single KL1L1 line expected from atomic beryllium (configuration ls2 2s"); this result is interpreted as showing two kinds of Auger transition, the lower involving two s-like conduction electrons, the higher two p-like electrons. However, Suleman and Pattinson (I971 b) suggest that the double line structure is due to oxygen contamination.

The L3M2,3V spectrum of transition metals (Aksela et a1 1970) and the MVV spectra of Ag and other elements (Aksela 1971) also reveal doublet structure, but again results are sensitive to surface condition (Coad and RiviQe 1971 b).

The shape of Auger electron energy distributions will also be modified by lifetime broadening of the initial state and many body effects as in the x ray case. Changes in the shape of the threshold (Natta and Joyes 1970) and plasmon emission are two of the possibilities.

Why is an approach so different from that for insulators appropriate? What are its limitations? In two respects the metallic Auger process differs from that in ionic crystals: (a) the presence of a hole in an inner state of a metal will be unlikely to produce highly localized bound electron states (Harrison 1968) ; (b) hole-hole interaction will be reduced by screening, and so energy splittings may not be as large as for insulators. However, the conduction electrons in both the initial and final states may differ significantly from the non-interacting perfect crystal Bloch states on which the above argument is based, and the surface environment may have an important influence. Much further work is required to properly understand the energy profile of AVV transitions in metals. It would seem that beryllium and aluminium are particularly simple systems which would benefit from very thorough experimental study at high resolution, and which might be amenable to detailed theoretical analysis.

7.2.3. Semiconductors. The Auger spectrum of clean silicon in the range 40-100eV has been variously classified in terms of atomic transitions (Grant and Haas 1970b) and in terms of the band structure of silicon (Amelio 1970, Maguire and Augustus 1971) as in the analysis of the Auger spectrum of metals. It is possible that the directional bonding character of silicon and other semiconductors might manifest itself as in the case of free molecules (Siegbahn et a1 1969); no analysis of that type has so far been attempted, and the nature of the electronic states involved is not yet clear.

7 . 2 . 4 . EfSects of adsorbed atoms. Much of the current work on Auger spectroscopy has been concerned with identification of impurities on supposedly clean surfaces and the


study of adsorbed layers. A limited amount of environmental information has so far been obtained on this problem. Hagstrum and Becker (1969, 1971) have carried out ion neutralization spectroscopy uith 0, S and Se adsorbed on Ni; characteristic adsorbate features appear in the spectra, and it is possible to describe the adsorbate lines in terms of a ‘surface molecule’ (Grimley 1969). Also carbon overlayers display characteristic structure according to whether graphite or carbides are formed (Coad and Rivikre 1971a, Grant and Haas 1970a. 1971).

The intensity of various Auger lines characteristic of the substrate is very sensitive to the presence of adsorbate (Bishop et a1 1971). An adsorbed layer will modify the escape probability of Auger electrons from the underlying metals, low-energy Auger electrons being particularly strongly affected. In addition, however, valence electrons will undergo redistribution owing to the formation of bonds with the adsorbate, and the probabilities of various Auger transitions may be severely modified. Inner levels may suffer chemical shifts and so these effects will lead to distinctive modifications of the low-energy substrate spectrum. for example on oxidation of chromium and vanadium (Coad and Rivikre 1971~) and on exposure of beryllium to oxygen (Suleman and Pattinson 1971 b). Similarly, modifications of the ‘clean’ silicon Auger spectrum are observed on exposure to oxygen (Chang 1970) and in various micas (Carriere et a1 1971). A detailed description of these phenomena w i l l require correlation Lvith other experimental techniques, for example, ESCA, and better theoretical understanding of the Auger processes.

8. Electron beam effects

It was stated in $1 that AES is a non-destructive analytical technique. This is not always the case; under some circumstances interactions can occur between the solid and the electron beam used to probe the surface, which produce changes in the solid surface. We may distinguish three effects of the beam: electron impact desorption, electron induced dissociation and electron stimulated adsorption.

8.1. Electron impact desorption

It is well known that gases bonded on a solid surface can be destroyed by electron impact and the electron beam used in AES can cause desorption of quite strongly bound gases. Desorption effects in AES have been reported by Florio and Robertson (1969) in their study of chlorine on Si and by Musket and Ferrante (1970) for the system oxygen on tungsten. A gradual reduction of the Auger signal from the adsorbed gas was observed as electron bombardment progressed. Musket (1970) has used the decay of the Auger signal from oxygen adsorbed on tungsten under electron bombardment to measure the cross section for electron desorption.

8.2. Electron induced dissociation

Some solids are readily damaged by low-energy electron irradiation and care must be taken in interpreting Auger spectra from such materials. The alkali halides are particularly susceptible to electron beam damage (Palmberg and Rhodin 1968b. Tokutaka et a1 1970). Electron beams of energy below 200 eV can cause preferential desorption of a halogen atom with the production of a colour centre in the solid (Townsend and Kelly


K .O 0

0

I I I I I 0 16 24 32 40

Figure 12. Time dependence of the principal K and Cl Auger peaks from (100) KCI under continuous irradiation with 1.5 keV electrons at room temperature (from Tokutaka et a1 1970).

Time ( m i n )

1968, Lord 1971). Auger spectroscopy shows that the surface region becomes depleted in halogen atoms as may be seen in figure 12. Metal atoms from the resultant metal rich surface have been observed to diffuse over the surface of material deposited on the damaged surface (Gallon et al 1968). Organic materials are notoriously susceptible to electron damage and this, together with the instability of some of these substances in UHV, will limit the application of AES in these materials.

8.3. Electron stirnulated adsorption

An electron beam can assist adsorption of gas molecules which would otherwise only be weakly bound to a clean surface by dissociating the molecule on or close to the surface. If one of the dissociated species binds strongly to the surface there will be an increase in the concentration of that species in the region of electron bombardment. Such an effect has been reported by Coad et a1 (1970) who observed, using AES, a build-up of oxygen on clean silicon only inside the area irradiated by the electron beam. They conclude that this arose from dissociation on or near the surface of CO from the residual vacuum. Similar results were reported by Higginbotham (1970) who found that when a freshly cleaved NaF surface was exposed to water vapour an oxygen Auger signal only appeared in those regions of the crystal surface which had been exposed to an electron beam. Figure 13 shows his measurement of the increase in oxygen Auger signal with electron beam exposure at two different partial pressures of water vapour. He suggests that the effect of the beam is to split the water into HT and OH- and that the OH- radical bonds to the surface. An alternative explanation is that the dissociation produced by the electron beam produces free sodium, which combines with the water vapour.

Electron beam interaction with the substrate can readily be detected in AES since all of the above effects give rise to an Auger spectrum which is time dependent. These effects can be reduced by using the lowest possible beam current and. in the case of the retarding

Auger electrori spectroscopy

E P L -

61

0 0 ; B

Time (m in )

Figure 13. Gro- th of the oxygen Auger signal from an NaF surface under continuous irradiation with 1.5 keV electrons. A, partial pressure of water vapour 3 x Torr and B, partial pressure of hater hapour 4 x Torr (from Higginbotham 1970).

grid spectrometer, by defocusing the beam to lower the current density. However, the best way of minimizing electron beam interaction is to reduce exposure to the electron beam and in this respect the new fast scan Auger spectrometers have a great advantage over the earlier instruments.

9 Conclusion

The most basic achievement of Auger spectroscopy in surface physics has been in showing that many surfaces, thought to be clean, were in fact strongly contaminated. This alone will ensure that in the next decade AES will be used at least as an auxiliary monitor in conjunction with other techniques-for example in photoemission of metals, early stages of epitaxial growth, catalysis, chemisorption or corrosion experiments, and studies of surface magnetism.

This review has also attempted to indicate more sophisticated applications of AES. Greater sensitivity and improved scanning rate will lead to many applications in the kinetics of surface processes. Improved spatial resolution will lead to scanning Auger spectrometers. High energy resolution will give sensitive environmental fingerprints capable of at least some theoretical interpretation. Advances in experimental technique and in both semiempirical and first principles theory should lead to reliable quantitative calibration at any rate for fairly simple physical systems.

The requirement of UHV conditions, the semidestructive nature of the technique in some ionic crystals and organic materials may limit applications, but the method has great potential in characterizing complicated metallurgical surfaces. The complexity of the physics of the Auger process may hamper detailed interpretation, but rapid progress on all fronts is expected. Uses of AES may well have analogies with the applications of infrared spectroscopy in chemistry, that is, development both as a routine diagnostic technique and as an important field of research in itself. Auger electron spectroscopy does not fully measure up to the label of the 'ideal' surface technique, but it will find many applications in both research and materials testing laboratories.


Acknowledgments

We are grateful to Dr M Prutton of the University of York for critical appraisal of the manuscript, and to other members of the surface physics group for useful discussions. We acknowledge permission from Dr R E Weber and Dr A L Johnson to reproduce figure 7, from Professor K Siegbahn for figure 10, Dr I G Higginbotham for figure 13 and Dr Haas et a1 for table 4. We wish to thank Mrs J Vanham and Mr A Gebbie for help in the preparation of the manuscript.

References

Aksela S 1971 Z . Phys. 244 268-74 Aksela S , Pessa M and Karras M 1970 Z . Phys. 237 381-7 Albridge R G , Hamrin K, Johannson G and Fahlman A 1968 Z . Phys. 209 419-27 Amelio G F 1970 Surface Sri. 22 301-18 Amelio G F and Scheibner E J 1968 Surface Sri. 11 242-58 Asaad W N 1965 Nucl. Phys. 66 494-512 Asaad W N and Burhop E H S 1958 Proc. Phys. Soc. 71 369-82 Auger P 1925 J . Phys. Radium 6 205-8 Baer Y et al l970 Solid St. Commun. 8 1479-81 Bearden J A and Burr A F 1967 Rea. Mod. Phys. 39 12542 Bergstrom J and Nordling C 1965 Alpha, Beta and Gamma Spectroscopy Vol 2, ed K Siegbahn

Bishop H E 1967 Br. J . appl. Phys. 18 703-15 Bishop H E and Riviere J C 1969a J . appl. Phys. 40 1740-4 - 1969b J . Phys. D : Appl. Phys. 2 1635-42 - 1970a J . CON. In/. Sri. 33 272-7 - 1970b Appl. Phys. Lett. 16 21-3 Bishop HE, Rivikre J C and Coad J P 1971 Surface Sri. 24 1-17 Burhop E H S 1952 The Auger Effect and other Radiationless Transitions (London: Cambridge University

Carriere B, Deville J-P and Goldsztaub S 1971 C.r. Acad. Sci., Paris B272 9 5 1 4 Chang C C 1970 Surface Sri. 23 283-98 _. 1971 Surface Sri. 25 53-79 Chung M F and Jenkins L H 1970 Surface Sci. 22 479-85 Coad JP , Bishop H E and Rivikre J C 1970 Surface Sci. 21 253-64 Coad J P and Rivikre J C 1971a Z . Phys. 244 19-30 - 1971b Surface Sri. 25 609-24 __ 1971c Phys. Lett. 35A 185-6 Connell G L 1971 Nature, Lond. 230 377-8 Coulson C A and Gianturco F A 1968 J. Phys. B: Atom. molec. Phys. 1 605-13 Dick C E and Lucas A C 1971 Phys. Rea. A 2 580-6 Estrup P J and McRae E G 1971 Surface Sei. 25 1-52 Fabian D J (ed) 1968 Soft X-ray Band Spectra (New York: Academic Press) Fahlman A et a1 1966 Phys. Lett. 2OA 159-60 Fischer D W 1970 J . appl. Phys. 41 3561-9 Florio J V and Robertson W D 1969 Surface. Sri. 18 398-427 Gallon T E 1969 Surface Sci. 17 486-9

Gallon TE , Higginbotham I G and Prutton M 1969 J . Phys. E: Sri. Instrum. 2 894-6 Gallon TE , Higginbotham I G , Prutton M and Tokutaka H 1968 Thin Solid Films 2 369-73 Gallon T E and Matthew J A D 1970 Phys. Stat. Solidi. 41 343-51 Gallon TE, Prutton M and Wray L 1972 J. Vac. Sci. Technol. in the press Gerlach RL, Houston J E and Park R L 1970 Appl. Phys. Lett. 16 179-81 Glupe G and Mehlhorn W 1967 Phys. Lett. 25A 274-5

(Amsterdam: North Holland)

Press)

- 1972 J. Phys. D : Appl. Phys. 5 822-32


Grant J T and Haas T W 1970a Phys. Lett. 33A 386-7 - 1970b Surface Sei. 23 347-62 - 1971 Surface Sei. 24 3 3 2 4 Grimley T B 1969 Molecular Processes on Solid Surfaces ed E Dranglis et a1 (New York: McGraw-Hill)

Gryzinski M 1959 Phys. Rev. 115 37483 _. 1965 Phys. Rev. 138 A305-21 Haas T W and Grant J T 1969 Phys. Lett. 30A 272-3 Haas T W, Grant J T and Dooley G J 1970 J . Vac. Sei. Technol. 7 43-5 Hagstrum H D 1966 Phys. Rec. 150 495-515 Hagstrum H D and Becker G E 1967 Phys. Rev. 159 572-85 - 1969 Phys. Rec. Lett. 22 10548 - 1971 J . Chern. Phys. 54 1015-32 Harris L A 1968a J . appl. Phys. 39 1419-27 __ 1968b J . appl. Phys. 39 1428-33 _. 1969 Surface Sci. 15 77-93 Harrison W A 1968 Soft X-ray Band Spectra, ed D J Fabian (New York: Academic Press) p 227-48 Harrower G A 1956 Phys. Rec. 102 340-7 Heddle D W O 1971 J . Phys. E: Sci. Instrum. 4 589-92 Higginbotham I G 1970 D.Phil Thesis University of York Huchital D A and Rigden J D 1970 Appl. Phys. Left . 16 348-51 Jacobi K and Holzl J 1971 Surface Sei. 26 54-60 Jenkins L H and Chung M F 1971a Surface Sci. 24 125-39 - 1971 b Surface Sei. 26 15 1-64 Joshi A and Stein D F 1970 Metall. Trans. 1 2543-6 Kobetich E J and Katz R 1968 Phys. Rev. 170 391-6 Kuhn H G 1969 Atomic Spectra 2nd ed (London: Longmans Green) Kunz AB, Miyakawa 7 and Oyama S 1969 Phys. Stat. Solidi 34 581-9 Lander J J 1953 Phys. Reo. 91 1382-7 Larkins F P 1971a J . Phys. B: Atom. molec. Phys. 4 1-13 - 1971b J . Phys. B: Atom. molec. Phys. 4 14-19 Listengarten M A 1962 Bull. Acad. Sei. USSR Phys. Ser. 24 1050-83 Lord D 1971 Phys. Stat. Solidi B 43 K I 15-8 Luyken B F J, D e Heer F J and Vriens L 1969 Proc. 6th Int. Conf. on the Physics of Electronic and Atomic

Collisions (Cambridge, Mass. : MIT Press) p 25-6 MacDonald N C, Marcus H L and Palmberg P W 1970 Proc. 3rd Ann. Scanning Electron Microscope

Symp. (IIT) pp 27-31 McGuire E J 1969 Phys. Reo. 185 1-6 - 1970 Phys. Rev. 2 273-8 Maguire H and Augustus P D 1971 J . PhyJ. C : Solid St. Phys. 4 L174-9 Marcus H L and Palmberg P W 1969 Tram. Metall. Soc. AIME 245 1664-6 Matthew J A D 1970a Surface Sei. 20 183-6 - 1970b Phys. Lett. 32A 261-2 Mehlhorn W 1968 Z . Phys. 208 1-27 Mehlhorn W and Asaad W N 1966 Z . Phys. 191 231-47 Melhorn W, Stalhern D and Verbeek H 1968 Z . Naturwiss. 23a 287-94 Melmed A J 1967 Adc. Mater. Res. 1 103-43 Moddeman W E et al 1971 J . Chem. Phys. 55 2317-36 Mularie W M and Peria W T 1971 Surface Sei. 26 125-41 Mularie W M and Rusch T W 1970 Surface Sri. 19 469-74 Muller E W 1968 Molecular Processes on Solid Surfaces ed E Dranglis, R D Gretz and R J Jaffe (New

-- 1970 Modern Diffraction and Imaging Techniques in Materials Science ed S Amelinckx et al

Musket R G 1970 Surface Sci. 21 440-2 Musket R G and Ferrante J 1970 J . Vac. Sci. Technol. 7 14-7 Musket R G and Fortner R J 1971 Phys. Rec. Lett. 26 80-2 Natta M and Joyes P 1970 J . Phys. Chem. Solids 31 447-52 Palmberg P W 1968 Appl. Phys. Lett. 13 183-5

pp 299-3 16

York : McGraw-Hill) pp 67-79

(Amsterdam: North Holland) pp 701-34

64 Auger electron spectroscopy

Palmberg P W and Rhodin T N 1968a J . appl. Phys. 39 2425-32 - 1968b J . Phys. Chem. Solids 29 1917-24 Palmberg PW, Bohn G K and Tracy J C 1969 Appl. Phys. Lett. 15 254-5 Park R L , Houston J E and Schreiner D G 1970 Rev. Sci. Insfrum. 41 1810-2 Pessa M, Aksela S and Karras M 1969 Acta Polytech. Scand. Ser. Phys. 65 Pines D 1955 Solid St. Phys. 1 367450 (New York: Academic Press) Plummer E W and Young R D 1970 Phys. Rev. B 1 2088-109 Pollard J H 1970 Surface Sci. 20 269-84 Prince R H and Cross J A 1971 Rec. Sci. Instrum. 42 66-71 Prutton M 1971 ,Metall. Rec. 5 57-77 Reuter W 1971 Surface Sri. 25 80-119 Ridgway J W T and Haneman D 1971 Surface Sci. 24 451-8 Scheibner E J and Tharp L N 1967 Surface Sci. 8 247-65 Sickafus E N 1970 Surfbce Sri . 19 181-97 Siegbahn K et a1 1967 Atomic, Molecular and Solid State Structure Studied by Means of Elecfron Spec-

Siegbahn K et al 1969 ESCA Applied to Free Molecules (Amsterdam: North Holland) Smith D P 1971 Surface Sci. 25 171-91 Socha A J 1971 Surface Sci. 25 147-70 Southworth H N 1970 Contemp. Phys. 11 209-26 Stein D F, Joshi A and Laforce R P 1969 Trans. Am. Soc. Metal. 62 776-83 Strausser YE and Uebbing J J 1970 Varian Chart of Auger Energies (Varian Vacuum Division, USA) Suleman M and Pattinson E B 1971a J . Phys. F: Metal Phys. 1 L21-4

~ 1971b J . Phys. F: ,2fefal. Phys. 1 L24-7 Taylor N J 1969a Rec. Sci. Instrum. 40 792-804 - 1969b Surface Sci. 15 169-74 __ 1971 Techt2ique.c of.Mefals Research Vol 7, ed R F Bunshah (New York: Intersceience) Tokutaka H , Prutton M, Higginbotham I G and Gallon T E 1970 Surface Sci. 21 2 3 3 4 0 Townsend P D and Kelly J C 1968 Phys. Lett. 26A 138-9 Urch D S 1970 J . Phys. C: Solid St. Phys. 3 1275-91 Vriens L 1969 Case Studies in Atomic Collision Physics Vol 1 p 337-98 Weber R E and Johnson A L 1969 J . appl. Phys. 40 314-8 Weber R E and Peria W T 1967 J . appl. Phys. 38 4355-8 Worthington C R and Tomlin S C 1956 Proc. Phys. Soc. A 69 401-12

troscopy, E S C A (Uppsala: Almqvist and Wiksell)

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Auger electron spectroscopy and its application to surface studies