- Email: [email protected]
Ion-induced Auger electron studies on Al(110)
Nuclear Instruments North-Holland
and Methods
in Physics
Research
B45 (1990) 637-640
637
ION-INDUCED AUGER ELECTRON STUDIES ON Al(110) L. WONG, P.F.A. ALKEMADE, W.N. LENNARD and I.V. MITCHELL Interface Science Western, Department
The yield and energy spectra MeV He+ beam in several random the electron continuum and of the elastic mean free path A, for 1.4
of Physics, The University of Western Ontario, London, Ontario, Canada N6A 3K7
of KLL Auger electrons have been measured from a clean Al(110) crystal under impact of a 1.5 and channeling directions. Under channeling conditions we observe a reduction in the intensity of KLL Auger signal. From an analysis of the spectral intensity yields we conclude that the transport keV electrons in Al is of the order of 70 A.
1. Introduction The interaction of energetic electrons with solids can be studied by using ion-induced secondary-electron emission. For example, the attenuation length of (Auger) electrons can be measured by ion-induced Auger electron spectroscopy [l]. Using channeling methods, MacDonald et al. [2] have examined Auger electron data from a Si(ll1) single crystal and obtained a value for the electron inelastic mean free path; while Kudo et al. [3] have deduced the average excitation energy in an inelastic interaction of keV electrons in single-crystalline Si. In this paper we report measurements of the energy spectra of ion-induced Auger electrons escaping from an Al(110) crystal under both random and channeling ion beam configurations. We show that these measurements can be used to determine the (transport) mean free path for elastic scattering of (1.4 keV) electrons in aluminum.
2. Theory In studies of Auger electron attenuation it is the length (i.e. the thickness of material in which the initial, or “zero-energy-loss”, Auger electron intensity is attenuated by a factor of l/e) which is usually measured. The attenuation length and inelastic mean free path (i.e. the average path length between two successive inelastic interactions) are different since electrons do not move in straight lines but scatter elastically from the nuclei of the atoms [4,5]. Knowledge of the transport of electrons in solids - in particular of the (transport) ekustic mean free path X, (i.e. the average distance between two successive elastic deflections) - is then needed to relate the measured attenuation length to the theoretical inelastic mean free path [6]. The attenuation
0168-583X/90/$03.50 0 Elsevier Science Publishers B.V. (North-Holland)
transport elastic mean free path is of the order of several tens to hundreds of A. A simplified model for the transport of electrons in solids is described by Alkemade et al. [7]; it is related to the theoretical models employed by Sigmund and Tougaard [4]. Basically, Auger electrons that emerge from the solid surface can be considered to come from either the so-called direct-transport or the so-called diffusion region. Electrons created in the near-surface region have a high probability of escaping directly, i.e. without being deflected: as a result, they have an average energy loss E, that is proportional to the depth z where they are initially created. This region of the solid is the direct-transport region. Most of the electrons that escape from deeper regions have been elastically scattered one or more times. Consequently, the transport of the electrons to the surface resembles a diffusion process in which the total path length and average energy loss E, are proportional to z2. This region, where z is greater than the elastic mean free path X,, is the diffusion region. It is shown theoretically [4,7,8] that a homogeneous source of monoenergetic electrons results in a spectrum that is constant in height for small energy losses and falls off as 1/ fi for larger losses. By demanding continuity between the direct-transport and the diffusion regions in the spectrum and assuming that the energy loss is proportional to the path length, the following relation is obtained [7]: E, = ~
h*S
4 cos2e ’
in which E, is the transition energy loss between the direct-transport and diffusion regions in the spectrum, S is the stopping power for the electrons and 0 the angle between the direction of escape of the electrons and the surface normal of the solid. By determining the transition energy in a spectrum, the mean free path can be calculated. The above expression is, apart from a X. COMPLEMENTARY
TECHNIQUES
638
L.. Wong et al. / Ion-induced Auger electron studies on Al(ll0)
numerical constant that is close to unity, equal to that derived by Sigmund and Tougaard [4]. In the region of the solid where Auger electrons can escape - the outermost few hundred A - the energy degradation and directional spread of an incident MeV ion beam (H+ or He+) is negligible. Consequently, if the solid is amorphous or polycrystalline, or if the beam is incident along a random direction, it acts as a homogeneous source of monoenergetic (Auger) electrons. Channeling of the incoming ions affects both inner-shell ionization and ion backscattering probabilities the same. In a channeling geometry we limit the depth where the Auger electrons are created to a thin layer near the surface for those Auger electrons produced by the inner-shell vacancy decay. The relative depth dependence of the Auger electron production yield can then be obtained from the measured intensity of backscattered ions and/or by using Monte Carlo simulations for ion channeling (for example as in ref. [2]).
3. Experiment description The experiments are conducted in a UHV chamber (base vacuum - 10K9 Torr) that is connected to the University of Western Ontario 2.5 MV Van de Graaff accelerator by a differentially pumped beam line. The ion beam is monoenergetic with an energy width of - 1 keV. The chamber is equipped with an ion-sputtering gun, two surface barrier detectors (SBD’s) at about 120° and 150’ relative to the incident beam, an electron gun and a hemispherical electron analyzer that is coupled to a channeltron and positioned at about 90° relative to the incident ion beam. The energy resolution of the analyzer is 12 eV (FWHM) while each SBD has - 15 keV resolution for 1 MeV He+ ions. The solid angle of the analyzer is about 20-30 msr. A rotating Faraday cup periodically interrupts the beam to measure the beam intensity. The target, an Al(110) crystal, is mounted in a fiveaxis (two rotations and three translations) goniometer. The target is heated by a tantalum filament from the back; and on the front side, a thermocouple measures the temperature. A Si(Bi) standard, containing 4.83(*0.25) x lOI Bi atoms/cm2, is mounted to one side of the goniometer. Cleaning of the crystal surface consists of a combination of sputtering with 1 keV Ar+ and annealing at - 400 ’ C until no surface impurities (C and 0) are detected by electron-induced AES. We have chosen 1.5 MeV He+ ions for the measurements since at this beam energy the Auger electron yield, Auger signal-to-background ratio and channeling conditions are optimal. The beam current is about 100 nA and the beam diameter is 0.5 mm. Electron spectra over the energy range 800-1800 eV and RBS spectra are measured simultaneously in random as well as in the
[OH], [OlO] and [ill] crystallographic directions. A random spectrum is collected by measuring at different azimuthal angles and at a fixed polar angle f3 of the crystal, where 0 is the angle between the surface normal and the electron analyzer (19= 30 O, 45 o and 54.7 o respectively). RBS spectra of the Si(Bi) standard are collected in the same geometries as that for the Al crystal. All electron spectra are corrected for the relative detection efficiency of the electron analyzer and channeltron which was measured independently. Furthermore, the spectra are corrected for counting losses due to the deadtime of the electronics; this correction is at most 5% (at 800 eV, where the electron intensity is the highest). The typical beam dose per spectrum is 100 PC.
4. Results and spectra analysis Secondary-electron energy spectra from the three random and channeling orientations are shown in fig. 1. The random and channeling spectra differ in their total yields of electrons (background and Auger), the random yield being a factor of 2 to 3 higher than the aligned yield. For both random and channeling spectra the continuous background of electrons (due to Coulomb ionization) falls off sharply as the detected electron energy E increases. Note the difference in shape between the random and aligned Auger spectra period (especially in the “tail” on the low-energy side of the Auger peak). This difference is largest for the [Oil] (the most prominent channeling direction) and smallest for the [ill] direction. For further analysis of the Auger spectra, the background must be subtracted from the measured spectra. The measured background Y,,(E) appears as a straight line in the double logarithmic plots of fig. 1 and there-
104
z F 103
1.0
1.4
1.0
Electron
energy
1.8
(ke$
Fig. 1. Secondary-electron energy spectra from Al(110) in the channeling directions [Oil], [OlO] and [ill], and the corresponding spectively
random orientations; B = 30 O, 45 o and 54.7 O, re(note the doublelog scale). Dashed lines are fits for the direct Coulomb ionization background.
639
L. Wong et al. / Ion-induced Auger electron studies on AI(II0)
n ED -1000
600
1000
1200
1400
Electron
-25Ol 600
1600
1600
energy
(eV)
Fig. 2. The Al KLL and KLM Auger electron spectra for the [Oil] channeling and random directions (0 = 30 o ) after background subtraction. The solid curve is the result of a fit to the data. The corresponding value for Et is 52 eV. The dashed curves show the results for E, of 62 eV (top) and 42 eV (bottom).
fore it is fairly well described Ybs, (E)
= cE”,
by a function
of the form (2)
where c is a normalization constant. Functions of this form have been applied before [9,10] in estimates of secondary-electron spectra. We assume that cr has the form o(E)
= a,, + a,E,
' 1000
(3)
where (Y,,, (it are constants. We have fitted eqs. (2) and (3) to the measured spectra and found that a(E) is of the order of -5 at the investigated energy range. With the background subtracted, the shape of the Auger spectrum becomes more apparent (fig. 2). The effect of ion channeling on the production of the Auger electrons is evident. As expected, since they are created in a region close to the surface, most Auger electrons have an energy close to their initial energy. In the random orientation, the Auger spectrum extends over many hundreds of eV: the contribution to the Auger spectrum comes from all depths. We have fitted the Auger peak in the random spectrum according to our model [7]: a constant level for low energy loss and a l/G behaviour for larger losses. Figs. 2 and 3 show the results for B = 30 o and 45 O, respectively. The contribution to the spectrum of the individual Auger transitions is indicated in fig. 3: the KL,L, at 1345 eV, KL,L, at 1396 eV and KL,M at 1487 eV. The ratio of their respective intensities is 0.32 : 1 : 0.17. From ref. [ll] the ratio for the same, electron-induced, Auger transition intensities is 0.33 : 1 : 0.05. The intensity of the ion-induced KL,M transition is relatively high due to the higher probability for multiple ionization (by ions as compared to electrons) which depletes the L-shell and decreases the probability for KLL transitions. The appearance of the Auger spectrum is quali-
I
' 1200
1400
1600
Electron
energy
1800
(eV)
Fig. 3. The Auger electron spectrum for a random orientation (6’ = 45 o ). The separate contributions to the Auger peak from the three Auger transitions are indicated. For small energy losses El the spectrum height is constant, and it drops off as l/fi for larger losses.
tatively consistent with what was expected. For small energy losses the spectrum height is constant and drops off as l/R for larger losses, where E, is the energy loss of the detected Auger electron. Under channeling conditions the expected shape of the Auger peak would be narrow and high. However, the measured channeling Auger peak is still surprisingly broad (about 100 eV at FWHM). We can attribute this to multiple ionizations which shift the energy of subsequently emitted Auger electrons over about several tens of eV to a lower energy [12] and also to the discrete, instead of continuous, character of the energy loss. Other effects that contribute to the tail of the channeling Auger peak are the backscattering of electrons and the finite probability for the creation of Auger electrons at greater depths. An example of the corresponding ion backscattering spectra is shown in fig. 4. The minimum yield xmill, measured in all channeling directions, and the areas of the measured surface peak (SP),.., are listed in table 1 as are, for comparison, values calculated for the surface
5oo-
*
I
I
100
0 600
700
600
900
1000
Ion energy
(keV)
Fig. 4. Backscattering spectra for 1.5 MeV He+ ions incident on single-crystalline Al in the [OlO] channeling and the corresponding random direction. X. COMPLEMENTARY
TECHNIQUES
L. Wong et al. / Ion-induced Auger electron studies on Al(110)
640
Table 1 Results for the analysis of the random Auger spectra and the corresponding channeling RBS spectra; the uncertainty in E, and X, is 20% Random 0
E,
results &I
Channeling ]hkll
Xmin (SP),,, [W] [X10’5cm-2]
tsp),h
[OH] [OlO] [ill]
6.0 6.5 12.0
1.4 11.2 17.7
k&l WI @I 30 45 54.7
52 50 46
87 56 34
results
10.5 +0.8 12.0+1.0 18.5k1.4
[X10’5cm-2]
using Monte Carlo simulations (based on (S%, the surface model by Noonan and Davis [13]). It is worthwhile to note the similarity between the ion backscattering and the Auger electron spectra: in both cases, the intensities are much lower in an ion channeling geometry than in the random. Furthermore, both spectra show a surface peak. In principle, since we know that the thickness of the layer in which Auger electrons are produced is the same as that in which the ions are backscattered, this similarity enables us to calibrate the Auger electron detection yield. This is discussed in more detail in ref. [7]. From the fit to the Auger peak in the random spectrum we obtain values for E, which are listed in table 1. The stopping power for 1.4 keV electrons in Al is S = 1.8 eV/A, from ref. [14]. Consequently, we calculate from eq. (1) values for the transport elastic mean free path h, as shown in table 1. The calculated values for h, do not agree with each other. Moreover, the theoretical value is higher - approximately 250 A [15]. The discrepancy is probably due to the simplicity of our model. However, the relation between h, and E,, according to the model by Sigmund and Tougaard [4], leads to an even lower value A, (16 A for E, = 50 eV, independent of 0). peak
5. Summary and conclusions We have looked at the production of secondary electrons from an Al(110) crystal by impact of 1.5 MeV He+ ions. The measurements have been taken under both random and channeling ([Oil], [OlO], [ill]) incidence conditions. The Al KL,L,, KL,L, and KL,M Auger electrons are clearly visible, superimposed upon a continuous background of electrons emitted by Coulomb ionization. The background is very well described by a
function of the form E” in which (Y is of the order of -5. It is concluded that the shape of the measured Auger spectrum is qualitatively in agreement with the (simple) model by Alkemade et al. [7], and with the more sophisticated models of Sigmund and Tougaard [4] and of Dwyer et al. [8]. Using the simple model [7] we obtain three values for the transport elastic mean free path X, for three different exit angles. The values do not agree with each other but show a relative dependence on 8, probably due to the simplicity of the model. The shape of the channeling Auger peak has been discussed: it is influenced by multiple ionization effects and the backscattering of electrons in the solid. It is noted that these effects must be present in the random case as well, which emphasizes the need to improve this simple model, for example by using Monte Carlo simulations for the electron transport. The authors are grateful for the support of NSERC (Canada) and for the continued interest of J.R. MacDonald.
References [l] C.J. Powell, R.J. Stein, P.B. Needham and T.J. Driscoll, Phys. Rev. B16 (1977) 1370. [2] Jack R. MacDonald, L.C. Feldman, P.J. Silverman, J.A. Davies and T.E. Jackman, Surf. Sci. 157 (1985) L335. [3] H. Kudo, D. Schneider, E.P. Kanter, P.W. Arcuni and E.A. Johnson, Phys. Rev. B30 (1984) 4899. [4] P. Sigmund and S. Tougaard, Phys. Rev. B25 (1982) 4452. [5] C.J. Powell, Surf. Sci. 44 (1979) 29. [6] C.J. Powell, Scanning Electron Microsc. 4 (1984) 1649. [7] P.F.A. Alkemade, W.N. Leonard and I.V. Mitchell, to be published in Surf. Sci. [8] V.M. Dwyer and J.A.D. Matthew, Surf. Sci. 193 (1988) 549. [9] E.N. Sickafus, Phys. Rev. B16 (1977) 1436. [lo] M. Prutton and M.M. El Gomati, Surf. and Interf. Anal. 9 (1986) 99. [ll] L.E. Davis et al., Handbook of Auger Electron Spectroscopy, 2nd ed. (Physical Electronics Industries Inc., Minnesota, 1976). 1121 M.O. Krause, in: Atomic Inner-Shell Processes, vol. II, ed. B. Crasemann (Academic Press, New York, 1975) p. 65. [13] J.R. Noonan and H.L. Davies, Phys. Rev. B29 (1984) 4349. [14] C.J. Tung, J.C. Ashley and R.H. Ritchie, Surf. Sci. 81 (1979) 427. [15] A.L. Tofterup, Phys. Rev. B32 (1985) 2808.
and Methods
in Physics
Research
B45 (1990) 637-640
637
ION-INDUCED AUGER ELECTRON STUDIES ON Al(110) L. WONG, P.F.A. ALKEMADE, W.N. LENNARD and I.V. MITCHELL Interface Science Western, Department
The yield and energy spectra MeV He+ beam in several random the electron continuum and of the elastic mean free path A, for 1.4
of Physics, The University of Western Ontario, London, Ontario, Canada N6A 3K7
of KLL Auger electrons have been measured from a clean Al(110) crystal under impact of a 1.5 and channeling directions. Under channeling conditions we observe a reduction in the intensity of KLL Auger signal. From an analysis of the spectral intensity yields we conclude that the transport keV electrons in Al is of the order of 70 A.
1. Introduction The interaction of energetic electrons with solids can be studied by using ion-induced secondary-electron emission. For example, the attenuation length of (Auger) electrons can be measured by ion-induced Auger electron spectroscopy [l]. Using channeling methods, MacDonald et al. [2] have examined Auger electron data from a Si(ll1) single crystal and obtained a value for the electron inelastic mean free path; while Kudo et al. [3] have deduced the average excitation energy in an inelastic interaction of keV electrons in single-crystalline Si. In this paper we report measurements of the energy spectra of ion-induced Auger electrons escaping from an Al(110) crystal under both random and channeling ion beam configurations. We show that these measurements can be used to determine the (transport) mean free path for elastic scattering of (1.4 keV) electrons in aluminum.
2. Theory In studies of Auger electron attenuation it is the length (i.e. the thickness of material in which the initial, or “zero-energy-loss”, Auger electron intensity is attenuated by a factor of l/e) which is usually measured. The attenuation length and inelastic mean free path (i.e. the average path length between two successive inelastic interactions) are different since electrons do not move in straight lines but scatter elastically from the nuclei of the atoms [4,5]. Knowledge of the transport of electrons in solids - in particular of the (transport) ekustic mean free path X, (i.e. the average distance between two successive elastic deflections) - is then needed to relate the measured attenuation length to the theoretical inelastic mean free path [6]. The attenuation
0168-583X/90/$03.50 0 Elsevier Science Publishers B.V. (North-Holland)
transport elastic mean free path is of the order of several tens to hundreds of A. A simplified model for the transport of electrons in solids is described by Alkemade et al. [7]; it is related to the theoretical models employed by Sigmund and Tougaard [4]. Basically, Auger electrons that emerge from the solid surface can be considered to come from either the so-called direct-transport or the so-called diffusion region. Electrons created in the near-surface region have a high probability of escaping directly, i.e. without being deflected: as a result, they have an average energy loss E, that is proportional to the depth z where they are initially created. This region of the solid is the direct-transport region. Most of the electrons that escape from deeper regions have been elastically scattered one or more times. Consequently, the transport of the electrons to the surface resembles a diffusion process in which the total path length and average energy loss E, are proportional to z2. This region, where z is greater than the elastic mean free path X,, is the diffusion region. It is shown theoretically [4,7,8] that a homogeneous source of monoenergetic electrons results in a spectrum that is constant in height for small energy losses and falls off as 1/ fi for larger losses. By demanding continuity between the direct-transport and the diffusion regions in the spectrum and assuming that the energy loss is proportional to the path length, the following relation is obtained [7]: E, = ~
h*S
4 cos2e ’
in which E, is the transition energy loss between the direct-transport and diffusion regions in the spectrum, S is the stopping power for the electrons and 0 the angle between the direction of escape of the electrons and the surface normal of the solid. By determining the transition energy in a spectrum, the mean free path can be calculated. The above expression is, apart from a X. COMPLEMENTARY
TECHNIQUES
638
L.. Wong et al. / Ion-induced Auger electron studies on Al(ll0)
numerical constant that is close to unity, equal to that derived by Sigmund and Tougaard [4]. In the region of the solid where Auger electrons can escape - the outermost few hundred A - the energy degradation and directional spread of an incident MeV ion beam (H+ or He+) is negligible. Consequently, if the solid is amorphous or polycrystalline, or if the beam is incident along a random direction, it acts as a homogeneous source of monoenergetic (Auger) electrons. Channeling of the incoming ions affects both inner-shell ionization and ion backscattering probabilities the same. In a channeling geometry we limit the depth where the Auger electrons are created to a thin layer near the surface for those Auger electrons produced by the inner-shell vacancy decay. The relative depth dependence of the Auger electron production yield can then be obtained from the measured intensity of backscattered ions and/or by using Monte Carlo simulations for ion channeling (for example as in ref. [2]).
3. Experiment description The experiments are conducted in a UHV chamber (base vacuum - 10K9 Torr) that is connected to the University of Western Ontario 2.5 MV Van de Graaff accelerator by a differentially pumped beam line. The ion beam is monoenergetic with an energy width of - 1 keV. The chamber is equipped with an ion-sputtering gun, two surface barrier detectors (SBD’s) at about 120° and 150’ relative to the incident beam, an electron gun and a hemispherical electron analyzer that is coupled to a channeltron and positioned at about 90° relative to the incident ion beam. The energy resolution of the analyzer is 12 eV (FWHM) while each SBD has - 15 keV resolution for 1 MeV He+ ions. The solid angle of the analyzer is about 20-30 msr. A rotating Faraday cup periodically interrupts the beam to measure the beam intensity. The target, an Al(110) crystal, is mounted in a fiveaxis (two rotations and three translations) goniometer. The target is heated by a tantalum filament from the back; and on the front side, a thermocouple measures the temperature. A Si(Bi) standard, containing 4.83(*0.25) x lOI Bi atoms/cm2, is mounted to one side of the goniometer. Cleaning of the crystal surface consists of a combination of sputtering with 1 keV Ar+ and annealing at - 400 ’ C until no surface impurities (C and 0) are detected by electron-induced AES. We have chosen 1.5 MeV He+ ions for the measurements since at this beam energy the Auger electron yield, Auger signal-to-background ratio and channeling conditions are optimal. The beam current is about 100 nA and the beam diameter is 0.5 mm. Electron spectra over the energy range 800-1800 eV and RBS spectra are measured simultaneously in random as well as in the
[OH], [OlO] and [ill] crystallographic directions. A random spectrum is collected by measuring at different azimuthal angles and at a fixed polar angle f3 of the crystal, where 0 is the angle between the surface normal and the electron analyzer (19= 30 O, 45 o and 54.7 o respectively). RBS spectra of the Si(Bi) standard are collected in the same geometries as that for the Al crystal. All electron spectra are corrected for the relative detection efficiency of the electron analyzer and channeltron which was measured independently. Furthermore, the spectra are corrected for counting losses due to the deadtime of the electronics; this correction is at most 5% (at 800 eV, where the electron intensity is the highest). The typical beam dose per spectrum is 100 PC.
4. Results and spectra analysis Secondary-electron energy spectra from the three random and channeling orientations are shown in fig. 1. The random and channeling spectra differ in their total yields of electrons (background and Auger), the random yield being a factor of 2 to 3 higher than the aligned yield. For both random and channeling spectra the continuous background of electrons (due to Coulomb ionization) falls off sharply as the detected electron energy E increases. Note the difference in shape between the random and aligned Auger spectra period (especially in the “tail” on the low-energy side of the Auger peak). This difference is largest for the [Oil] (the most prominent channeling direction) and smallest for the [ill] direction. For further analysis of the Auger spectra, the background must be subtracted from the measured spectra. The measured background Y,,(E) appears as a straight line in the double logarithmic plots of fig. 1 and there-
104
z F 103
1.0
1.4
1.0
Electron
energy
1.8
(ke$
Fig. 1. Secondary-electron energy spectra from Al(110) in the channeling directions [Oil], [OlO] and [ill], and the corresponding spectively
random orientations; B = 30 O, 45 o and 54.7 O, re(note the doublelog scale). Dashed lines are fits for the direct Coulomb ionization background.
639
L. Wong et al. / Ion-induced Auger electron studies on AI(II0)
n ED -1000
600
1000
1200
1400
Electron
-25Ol 600
1600
1600
energy
(eV)
Fig. 2. The Al KLL and KLM Auger electron spectra for the [Oil] channeling and random directions (0 = 30 o ) after background subtraction. The solid curve is the result of a fit to the data. The corresponding value for Et is 52 eV. The dashed curves show the results for E, of 62 eV (top) and 42 eV (bottom).
fore it is fairly well described Ybs, (E)
= cE”,
by a function
of the form (2)
where c is a normalization constant. Functions of this form have been applied before [9,10] in estimates of secondary-electron spectra. We assume that cr has the form o(E)
= a,, + a,E,
' 1000
(3)
where (Y,,, (it are constants. We have fitted eqs. (2) and (3) to the measured spectra and found that a(E) is of the order of -5 at the investigated energy range. With the background subtracted, the shape of the Auger spectrum becomes more apparent (fig. 2). The effect of ion channeling on the production of the Auger electrons is evident. As expected, since they are created in a region close to the surface, most Auger electrons have an energy close to their initial energy. In the random orientation, the Auger spectrum extends over many hundreds of eV: the contribution to the Auger spectrum comes from all depths. We have fitted the Auger peak in the random spectrum according to our model [7]: a constant level for low energy loss and a l/G behaviour for larger losses. Figs. 2 and 3 show the results for B = 30 o and 45 O, respectively. The contribution to the spectrum of the individual Auger transitions is indicated in fig. 3: the KL,L, at 1345 eV, KL,L, at 1396 eV and KL,M at 1487 eV. The ratio of their respective intensities is 0.32 : 1 : 0.17. From ref. [ll] the ratio for the same, electron-induced, Auger transition intensities is 0.33 : 1 : 0.05. The intensity of the ion-induced KL,M transition is relatively high due to the higher probability for multiple ionization (by ions as compared to electrons) which depletes the L-shell and decreases the probability for KLL transitions. The appearance of the Auger spectrum is quali-
I
' 1200
1400
1600
Electron
energy
1800
(eV)
Fig. 3. The Auger electron spectrum for a random orientation (6’ = 45 o ). The separate contributions to the Auger peak from the three Auger transitions are indicated. For small energy losses El the spectrum height is constant, and it drops off as l/fi for larger losses.
tatively consistent with what was expected. For small energy losses the spectrum height is constant and drops off as l/R for larger losses, where E, is the energy loss of the detected Auger electron. Under channeling conditions the expected shape of the Auger peak would be narrow and high. However, the measured channeling Auger peak is still surprisingly broad (about 100 eV at FWHM). We can attribute this to multiple ionizations which shift the energy of subsequently emitted Auger electrons over about several tens of eV to a lower energy [12] and also to the discrete, instead of continuous, character of the energy loss. Other effects that contribute to the tail of the channeling Auger peak are the backscattering of electrons and the finite probability for the creation of Auger electrons at greater depths. An example of the corresponding ion backscattering spectra is shown in fig. 4. The minimum yield xmill, measured in all channeling directions, and the areas of the measured surface peak (SP),.., are listed in table 1 as are, for comparison, values calculated for the surface
5oo-
*
I
I
100
0 600
700
600
900
1000
Ion energy
(keV)
Fig. 4. Backscattering spectra for 1.5 MeV He+ ions incident on single-crystalline Al in the [OlO] channeling and the corresponding random direction. X. COMPLEMENTARY
TECHNIQUES
L. Wong et al. / Ion-induced Auger electron studies on Al(110)
640
Table 1 Results for the analysis of the random Auger spectra and the corresponding channeling RBS spectra; the uncertainty in E, and X, is 20% Random 0
E,
results &I
Channeling ]hkll
Xmin (SP),,, [W] [X10’5cm-2]
tsp),h
[OH] [OlO] [ill]
6.0 6.5 12.0
1.4 11.2 17.7
k&l WI @I 30 45 54.7
52 50 46
87 56 34
results
10.5 +0.8 12.0+1.0 18.5k1.4
[X10’5cm-2]
using Monte Carlo simulations (based on (S%, the surface model by Noonan and Davis [13]). It is worthwhile to note the similarity between the ion backscattering and the Auger electron spectra: in both cases, the intensities are much lower in an ion channeling geometry than in the random. Furthermore, both spectra show a surface peak. In principle, since we know that the thickness of the layer in which Auger electrons are produced is the same as that in which the ions are backscattered, this similarity enables us to calibrate the Auger electron detection yield. This is discussed in more detail in ref. [7]. From the fit to the Auger peak in the random spectrum we obtain values for E, which are listed in table 1. The stopping power for 1.4 keV electrons in Al is S = 1.8 eV/A, from ref. [14]. Consequently, we calculate from eq. (1) values for the transport elastic mean free path h, as shown in table 1. The calculated values for h, do not agree with each other. Moreover, the theoretical value is higher - approximately 250 A [15]. The discrepancy is probably due to the simplicity of our model. However, the relation between h, and E,, according to the model by Sigmund and Tougaard [4], leads to an even lower value A, (16 A for E, = 50 eV, independent of 0). peak
5. Summary and conclusions We have looked at the production of secondary electrons from an Al(110) crystal by impact of 1.5 MeV He+ ions. The measurements have been taken under both random and channeling ([Oil], [OlO], [ill]) incidence conditions. The Al KL,L,, KL,L, and KL,M Auger electrons are clearly visible, superimposed upon a continuous background of electrons emitted by Coulomb ionization. The background is very well described by a
function of the form E” in which (Y is of the order of -5. It is concluded that the shape of the measured Auger spectrum is qualitatively in agreement with the (simple) model by Alkemade et al. [7], and with the more sophisticated models of Sigmund and Tougaard [4] and of Dwyer et al. [8]. Using the simple model [7] we obtain three values for the transport elastic mean free path X, for three different exit angles. The values do not agree with each other but show a relative dependence on 8, probably due to the simplicity of the model. The shape of the channeling Auger peak has been discussed: it is influenced by multiple ionization effects and the backscattering of electrons in the solid. It is noted that these effects must be present in the random case as well, which emphasizes the need to improve this simple model, for example by using Monte Carlo simulations for the electron transport. The authors are grateful for the support of NSERC (Canada) and for the continued interest of J.R. MacDonald.
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