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Angular dependences in auger electron emission from the Ni(001) face
Surface Science 224 (1989) 235-242 North-Holland. Amsterdam
235
ANGULAR DEPENDENCES IN AUGER ELECTRON EMISSION FROM THE Ni(OO1) FACE 1. Dependence of the Auger signal from the Ni(OO1) face on the direction of the primary electron beam * S. MR6Z
and A. MR6Z
institure of Experimental Physics. PI. SO-205 Wrociaw. Poland Received
17 January
University of Wrocb~.
1989; accepted
for publication
ul. CyMskiego
11 August
36.
1989
The dependence of the Auger signals, obtained for the Ni(OO1) face by twice-performed integration of the Auger peaks in dN/dE spectra of NiM,,sVV (62 eV), Ni LsVV (848 eV) and SLVV (152 eV) transitions (sulphur was segregated to the sample surface), on the incidence angte of the primary electron beam was investigated for different azimuth angles and primary electron energies. The Auger signal contrast resulting from the channeIling of primary electrons in the sample volume was determined and discussed qualitatively in the framework of the two-beam approximation. It was noticed that for a high symmetry [0+X] direction the cantrast results from simultaneous channelling of primary electrons on different low-index planes parallel to this direction. The modulation of the backscattered electron flux by the channelling of primary electrons is supposed to be the main reason of the observed changes of the Auger signal for low-energy transitions.
1. Introduction The quantitative analysis of crystalline surfaces by AES is remarkably influenced by the so-called “crystalline effects”, i.e. anisotropy of Auger electron emission and dependence of the Auger signal on the direction of the primary electron beam. This dependence becomes strong for directions close to the high-symmetry axes in the crystal investigated owing to the channelling of primary electrons by Iow-index crystalline planes. Such channelling was discovered by Coates [l] in the scanning electron microscope and discussed by Hirsch et al. by using the two-beam approximation [2]. For the incidence angles slightly lower and slightly higher than the Bragg angle the wave function of the primary electrons appears to be concentrated on the atomic planes and between them, respectively. As a result, the number of secondary eiectrons, electrons creating the Kikuchi pattern and Auger electrons is larger * Supported
by the Research
Project CPBP 01.08.A.
0039-6028/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
in the former case. Besides, the flux of backscattered electrons inside the sample is also larger in the former case, which leads again to the increase 01 the emission of electrons mentioned above. The dependence of the Auger signal on the primary electron beam direction was described for different crystals in numerous papers, see e.g. Bishop et al. [3], Morin [4], Gardiner et al. [5], Sakai and Mogami [8]. Investigations described in these papers indicate the importance of the observed phenomena, but they are not systematic enough to compare the obtained results with the theoretical predictions. The aim of the present work was to investigate the above dependence for directions close to [OOl] axis in the Ni crystal for different azimuth angles, for low- and high-energy Auger transitions in nickel and for such a transition in sulphur adsorbed on the nickel surface. Energy of the primary electron beam (El,) varied from 700 to 2200 eV. The results obtained were qualitatively compared with the predictions of the two-beam theory.
2. Experimental Measurements were performed in a metallic UHV system equipped with a three-grid RFA analyser enabling LEED observations and AES spectra recording. AES measurements were controlled by a microcomputer. The part of the AES spectrum around the peak investigated was stored and processed in this microcomputer, including integration of the dN/d E curve, fitting and subtraction of the background in the form: NR( E) = A + BE + CE’ and integration of the obtained NA( E) curve. The result of the last integration was considered as the Auger signal. The holder of the sample investigated (Ni crystal with the surface cut parallel to the (001) face accurate to 0.5” ) was mounted on the rotary feedtrough enabling independent rotation around two axes: one lying on the plane of the sample surface and the other perpendicular to this surface. The accuracy of rotation around the former axis was 0.5” while that for rotation around the latter axis was - 2”. Tilting of the sample was also possible. The heater made from a tungsten wire was placed inside the sample holder which enabled sample heating up to 800°C. To clean the sample, simultaneous heating at 600” C and bombardment with potassium ions emitted from a zeolite source was applied. The surface subjected to such a treatment was clean but destroyed and further heating at 500°C was necessary to obtain a fair LEED pattern with a low background. In the first cycles of such a cleaning the heating mentioned above leads to sulphur segregation to the surface and formation of a c(2 x 2) sulphur layer remaining unchanged during heating and during prolongated measurements as well. The sample surface covered with such a layer was used for measurements.
S. M&z, A. Mr& / Auger electron emission from the N~~~~l)face. I
To characterize quantitatively the influence of the primary direction on the Auger signal, the contrast C was introduced:
c = 2(‘nxix-
‘nunI/( ‘rnax+
‘min
electron
237
beam
>.
I,,, and Imin are the Auger signals for a direction parallel to the crystal planes channelling the primary beam and for a direction corresponding to the closest minimum of the signal, respectively. The primary electron beam was obtained from the electron gun placed on the axis of the RFA analyser. The incidence angle of this beam was changed by rotation of the sample around the axis parallel to its surface. For small incidence angles (measured with respect to the sample surface normal) it was assumed that changes of the measured Auger signal result from changes of the direction of the primary beam with respect to the surface and are independent of the changes of the sample orientation with respect to the RFA aperture. This assumption was confirmed [6] by the fact that the angular distribution of the Auger NiM,,,W electrons for the Ni(OO1) face covered with a c(2 x 2) sulphur layer is rather flat for polar angles ranging from 40 to 50 ‘, while the acceptance angle of our RFA was equal to about 45 ‘=‘. The desirable azimuth was chosen by rotation of the sample around the axis perpendicular to its surface. Positions of this axis protractor corresponding to the main azimuths were determined by observation of the LEED patterns for electron energies close to the Auger electron ones. Next, the sample was carefully adjusted by proper tilting. The adjustment was satisfactory when the contrast of the signal measured in the dN/dE mode by the lock-in voltmeter tuned to the maximum of the AES peak reached the highest value. The values of the contrast for the sample turned from the normal position in two opposite directions were found to be very similar in such a case. After such an adjustment, Auger signals I,,, and Imi,, were measured by using the method described in the beginning of this section and the contrast given by (1) was calculated. Next, the azimuth angle was changed and the whole procedure repeated. In this way the dependence of the contrast on the azimuth angle was obtained for the [OOl] direction and NiM,,,W Auger transition. For two main azimuths ((i.e. [lOO] and [IlO]) the dependence of the Auger signal on the incidence angle was obtained over a wide range of angles and for Ni M, y, Ni L3W and S LW transitions. Such measurements for directions far from’ the normal were appreciably influenced by the rotation of the sample with respect to the RFA aperture, nevertheless, maxima for particular channelling directions far from the normal to the surface were clearly visible. 3. Results The dependence of the contrast measured direction on the azimuth angle for the primary
for channelling in the [OOl] beam energy equal to 1500 eV
238
[I101 1 15' 11'iOl ,, , [IO01 , ,( , 60' 40' 20' 0 20' 40' 60' azimuth angle Fig. 1. Dependence of the contrast for directions of the primary electron beam close to the [OOl] direction (a) and of the incidence angle /I,,,,,, for which the minimum Auger signal is observed (b) on the azimuth angle. Circles and crosses are related to the contrast calculated from I,,,,, obtained for the “left” and “right” minimum of the Auger signal. respectively.
and for the NiM,,Y transition is shown in fig. 1. In this figure is also presented the dependence of the fI,,,,, angle formed by the sample surface normal and the direction of the primary beam for which I,,,, occurs on the azimuth angle. In fig. 2 the dependence of the contrast measured for channelling in the [OOl] direction on the primary beam energy is presented for the azimuths [loo] and [llO] for Ni M,,YV transition. The contrast values obtained for Ni M,,,VV, Ni LYV and S LVV transitions for channelling in the [OOl] direction and for [loo] and [l lo] azimuths arc given in table 1. The dependence of the Auger signal on the incidence angle for Ni L,VV transition and for [loo] azimuth is shown in fig. 3. Besides the maximum for
o~/_;“““;-:,~p~7”l, _ 1000 Fig. 2. Contrast
1500
2000
for the Ni M,,YV transition and the primary electron versus the primary electron energy.
beam close to the [OOI] axk
Table
1
Contrast different
values measured Auger transitions
and calculated by the use of relation (Er = 1500 ev)
(2) for different
azimuths
S LVV
Azimuth
Ni M a,aW h = 0.48 nm Meas.
CalC.
Meas.
talc.
Meas.
[1Wl
0.18 0.30
0.62 0.47
0.32 0.60
0.93 0.86
0.17 0.33
Ill01
and
Ni LYV h =1.5 nm
the direction normal to the surface, other maxima appear B = 5 45”. This dependence is typical of all the investigated
for 8 = rt 20 o and Auger transitions.
4. Discussion Bishop et al. [3] proposed the following from channelling on the (I&Z) planes: c = 2~~~/~~~~~/~~
formula
for the contrast
+ 47W&J2,
resulting
(2)
where X is the inelastic mean free path for the emitted Auger electron and Ehki is the extinction distance for the Bragg reflection of the primary beam on the (hkl) planes. From among the planes parallel to the [OOI] axis in the fee crystal, the (200), (020) (220), and (220) planes should play an important role in the channelling of electrons directed close to this axis owing to their low Miller indices. Bragg angles /?s for the electron energy of 1500 eV for the (200) and (220) planes are given in table 2 together with extinction distances taken
0
20'
I LO" 60' 80" polar angle
_
Fig. 3. The Auger signal for Ni L,VV transition and the primary electron energy equal to 1500 eV versus the incidence angle of the primary electron beam oriented along the [loo] azimuth. Indices of low-index planes channelling the electrons in particular maxima are presented together with the incidence angles for which particular maxima are expected.
S. Mdz,
240
A. Mr6z / Auger electron emisswn from the N~(001) f&e. I
Table 2 Values determining
the channelling
process
Azimuth
&Cd%)
Wdes)
P ,,,=PLI+W
E(nm)
w4 v-201
5.17 7.31
2.64 1.25
7.81 X.56
3.x3 S.69
from ref. [2] and the excess of Ap of the incidence for which I,,, occurs. Ap in this table is calculated proposed by Morin [4]:
angle over the Bragg angle on the basis of the relation
where g,,, is the modulus of the reciprocal lattice vector. Let us choose the azimuth angle (p = 0 for the [OlO] direction on the sample surface. For the primary electron beam direction lying in the plane of this azimuth and forming the angle 0 with the sample normal. the angles of incidence on the particular low index planes are given by the relations: P (020) sin
=
Pc220j
03
P(2W
=
sin
=
Pc~20,
0
=
sin 45” sin 8 = 0.7 sin 8.
Now we can calculate on each plane:
the S,,,,, values corresponding
to I,,,
for channelling
fl’200’ Ill,” = j#!~’ = 7.81 O, ~~220) Ill,” = @?2,0’z pz2,0’/0.7
= 12.20,
e,,” = ( e;;lp, + flAt’n”‘)/2 = 10 O. For + = 45 o the same calculations
give:
These calculations indicate that for $I = 0 the measured contrast results from channelling on the (200) plane (with 0,,,,, = 7.8” ) and on the (220) and (220) planes (with,;, = 12.2O). The minimum of the I versus 0 curve should be found close to emin = 10” and should be rather wide owing to the difference between fI(‘O”) = 45 O. the (200) and (020) planes with Ill,” and OAtz”). For C#I in the 8III,” = 11.2O and the (220) plane with 8,,, = 8.6” should participate creation of the contrast. The minimum of the I versus 19curve is now expected to be narrower and to occur again close to e,,,,, = 10”. For other azimuth angles situation is more complicated. &,,, can be different for each of four
S. MI&Z,A. M&z / Auger elecrronemissionfrom the Ni(OOfjface. I
247
planes mentioned above and all these planes can participate in the creation of the contrast. As follows from relation (2) and from the data given in table 1, the contrast created by the (200) and (020) planes should be appreciably greater than that created by the (220) and (220) ones. It means that the greater than that contrast measured for + = 45O should be considerably were confirmed by the results measured for + = 0 O. All these expectations presented in fig. 1 and by the dependence of the AES peak maximum in the d N/d E mode on the angle 6 as mentioned in section 2. The contrast measured for NiMzs3W transition is appreciably lower than that for the NiL,W transition. This fact is in qualitative agreement with relation (I) because the mean free path for Auger electrons is smaller in the former case. Quantitative comparison of the contrast obtained from (2) and that measured in our work leads to wide discrepancy (see table 2). This indicates that the two-beam approximation is too simple in the case of medium energy, where intralayer scattering should be taken into account as it was done in the method proposed for medium energy electron diffraction by Pendry and Gard [9] and proved by Masud et al. [lo]. It seems to be difficult to use this method in our case owing to the role of backscattered electrons in the creation of the Auger signal. For this reason our discussion is limited to the qualitative aspects and to the two-beam appro~mation. The weak contrast for the Ni M,,,W transition results from the low value of the inelastic mean free path in comparison with the extinction distance for the primary electrons. In this case, Auger electrons are emitted only from this surface layer where the channelling of the primary electrons has not occurred yet. The observed contrast results from the influence of the channelling of the backscattered electron flux. The same occurs for the SLW transition because the sulphur atoms are placed on the surface, For the NiLYV transition this mechanism works as well, but additionally the Auger emission intensity is directly influenced by channeling of primary electrons. As a result, the contrast for this later transition is appreciably higher. The cont~bution of backscattered electrons to the creation of the contrast is discussed by Andersen and Howie in ref. [7] where the multi-beam dynamical theory is applied. They show that the backscattered electron flux should be concentrated on the atomic planes. For this reason, the role of backscattered electrons in the Auger electron emission is appreciably strengthened. This conclusion is consistent with the results obtained by Gardiner et al. [5] for the Auger signal measured for the iron film deposited onto the surface of the cylindrical tungsten crystal rotated around its axis parallel to the [110] direction. Gardiner et al. concluded that changes of the signal mentioned above can be fully explained as resulting from changes of the backscattering factor caused by the crystal rotation. A weak dependence of the contrast on the primary electron energy presented in fig. 2 is in agreement with such a dependence obtained by Morin f4]
for the Si KLL transition and for primary electron energy ranging from 10 to 60 keV. An increase of the primary electron energy enlarges both the range of depth at which channelling occurs and the backscattering factor. These two effects limit each other and the resulting dependence of the contrast on the primary electron energy is found to he weak in a wide energy range. The dependence of Auger signals on the incidence angle of the primary electron beam for different Auger transitions and different azimuth angles similar to that shown in fig. 3 can be easily interpreted as a result of the channelling of primary electrons on subsequent low-index planes reached by the primary beam during increasing of the incidence angle. Miller indices of these planes are given in this figure together with the incidence angles for which particular maxima are expected. Similar dependences were observed by De Bersuder et al. [ll] for the secondary electrons and the elastically scattered ones for the Al(001) face and were interpreted in the same way.
Acknowledgements The authors would like to thank M. SC. B. Stachnik for her assistance in measurements and in preparation of the nlanuscript and to M. SC. A. Wieczorek for the computer programme.
References (11 D.G. Coates. 121 P.B. Hirsch, [3] 141 [5] [6]
Phil. Mag. 16 (1967) 1179. A. Howie. R.B. Nicholson, D.W. Paschley and M.J. Whelan, Electron Microscopy of Thin Crystals (Butterworths, London. 1965). H.E. Bishop, B. Chornik, C. Le Gressus and A. Le Moe], Surface Interface Anal. 6 (1484) 116. P. M&n, Surface Sci. 164 (1985) 127. ‘T.M. Gardiner, H.M. Kramer and E. Bauer. Surface Sci. 121 (1982) 116. S. MF& and A. Mritz, in: Proc. 2nd Conf. on Surface Physics, Wreclaw. 9-10 Decemhu 1987. Vol. 3, p. 12.5.
[7] S.K. Anderson
(X] [S] (101 [I 11
and A. Howie, Surface Sci. 50 (1975) 197. Y. Sakai and A. Mogami, J. Vacuum Sci. Technol. A 5 (1987) 1222. J.B. Pendry and P. Card, J. Phys. C 8 (1975) 2048. N. Masud, C.G. Kinningurgh and J.B. Pendry. J. Phys. c‘ 10 (1977) 1. 1,. De Bersuder, C. Corotte, P. Ducros and D. Lafeuille. in: The Structure Solid Surfaces, Ed. G.A. Somorjai (Wiley. New York. 1969) p. 30-l.
and Chemistry
of
235
ANGULAR DEPENDENCES IN AUGER ELECTRON EMISSION FROM THE Ni(OO1) FACE 1. Dependence of the Auger signal from the Ni(OO1) face on the direction of the primary electron beam * S. MR6Z
and A. MR6Z
institure of Experimental Physics. PI. SO-205 Wrociaw. Poland Received
17 January
University of Wrocb~.
1989; accepted
for publication
ul. CyMskiego
11 August
36.
1989
The dependence of the Auger signals, obtained for the Ni(OO1) face by twice-performed integration of the Auger peaks in dN/dE spectra of NiM,,sVV (62 eV), Ni LsVV (848 eV) and SLVV (152 eV) transitions (sulphur was segregated to the sample surface), on the incidence angte of the primary electron beam was investigated for different azimuth angles and primary electron energies. The Auger signal contrast resulting from the channeIling of primary electrons in the sample volume was determined and discussed qualitatively in the framework of the two-beam approximation. It was noticed that for a high symmetry [0+X] direction the cantrast results from simultaneous channelling of primary electrons on different low-index planes parallel to this direction. The modulation of the backscattered electron flux by the channelling of primary electrons is supposed to be the main reason of the observed changes of the Auger signal for low-energy transitions.
1. Introduction The quantitative analysis of crystalline surfaces by AES is remarkably influenced by the so-called “crystalline effects”, i.e. anisotropy of Auger electron emission and dependence of the Auger signal on the direction of the primary electron beam. This dependence becomes strong for directions close to the high-symmetry axes in the crystal investigated owing to the channelling of primary electrons by Iow-index crystalline planes. Such channelling was discovered by Coates [l] in the scanning electron microscope and discussed by Hirsch et al. by using the two-beam approximation [2]. For the incidence angles slightly lower and slightly higher than the Bragg angle the wave function of the primary electrons appears to be concentrated on the atomic planes and between them, respectively. As a result, the number of secondary eiectrons, electrons creating the Kikuchi pattern and Auger electrons is larger * Supported
by the Research
Project CPBP 01.08.A.
0039-6028/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
in the former case. Besides, the flux of backscattered electrons inside the sample is also larger in the former case, which leads again to the increase 01 the emission of electrons mentioned above. The dependence of the Auger signal on the primary electron beam direction was described for different crystals in numerous papers, see e.g. Bishop et al. [3], Morin [4], Gardiner et al. [5], Sakai and Mogami [8]. Investigations described in these papers indicate the importance of the observed phenomena, but they are not systematic enough to compare the obtained results with the theoretical predictions. The aim of the present work was to investigate the above dependence for directions close to [OOl] axis in the Ni crystal for different azimuth angles, for low- and high-energy Auger transitions in nickel and for such a transition in sulphur adsorbed on the nickel surface. Energy of the primary electron beam (El,) varied from 700 to 2200 eV. The results obtained were qualitatively compared with the predictions of the two-beam theory.
2. Experimental Measurements were performed in a metallic UHV system equipped with a three-grid RFA analyser enabling LEED observations and AES spectra recording. AES measurements were controlled by a microcomputer. The part of the AES spectrum around the peak investigated was stored and processed in this microcomputer, including integration of the dN/d E curve, fitting and subtraction of the background in the form: NR( E) = A + BE + CE’ and integration of the obtained NA( E) curve. The result of the last integration was considered as the Auger signal. The holder of the sample investigated (Ni crystal with the surface cut parallel to the (001) face accurate to 0.5” ) was mounted on the rotary feedtrough enabling independent rotation around two axes: one lying on the plane of the sample surface and the other perpendicular to this surface. The accuracy of rotation around the former axis was 0.5” while that for rotation around the latter axis was - 2”. Tilting of the sample was also possible. The heater made from a tungsten wire was placed inside the sample holder which enabled sample heating up to 800°C. To clean the sample, simultaneous heating at 600” C and bombardment with potassium ions emitted from a zeolite source was applied. The surface subjected to such a treatment was clean but destroyed and further heating at 500°C was necessary to obtain a fair LEED pattern with a low background. In the first cycles of such a cleaning the heating mentioned above leads to sulphur segregation to the surface and formation of a c(2 x 2) sulphur layer remaining unchanged during heating and during prolongated measurements as well. The sample surface covered with such a layer was used for measurements.
S. M&z, A. Mr& / Auger electron emission from the N~~~~l)face. I
To characterize quantitatively the influence of the primary direction on the Auger signal, the contrast C was introduced:
c = 2(‘nxix-
‘nunI/( ‘rnax+
‘min
electron
237
beam
>.
I,,, and Imin are the Auger signals for a direction parallel to the crystal planes channelling the primary beam and for a direction corresponding to the closest minimum of the signal, respectively. The primary electron beam was obtained from the electron gun placed on the axis of the RFA analyser. The incidence angle of this beam was changed by rotation of the sample around the axis parallel to its surface. For small incidence angles (measured with respect to the sample surface normal) it was assumed that changes of the measured Auger signal result from changes of the direction of the primary beam with respect to the surface and are independent of the changes of the sample orientation with respect to the RFA aperture. This assumption was confirmed [6] by the fact that the angular distribution of the Auger NiM,,,W electrons for the Ni(OO1) face covered with a c(2 x 2) sulphur layer is rather flat for polar angles ranging from 40 to 50 ‘, while the acceptance angle of our RFA was equal to about 45 ‘=‘. The desirable azimuth was chosen by rotation of the sample around the axis perpendicular to its surface. Positions of this axis protractor corresponding to the main azimuths were determined by observation of the LEED patterns for electron energies close to the Auger electron ones. Next, the sample was carefully adjusted by proper tilting. The adjustment was satisfactory when the contrast of the signal measured in the dN/dE mode by the lock-in voltmeter tuned to the maximum of the AES peak reached the highest value. The values of the contrast for the sample turned from the normal position in two opposite directions were found to be very similar in such a case. After such an adjustment, Auger signals I,,, and Imi,, were measured by using the method described in the beginning of this section and the contrast given by (1) was calculated. Next, the azimuth angle was changed and the whole procedure repeated. In this way the dependence of the contrast on the azimuth angle was obtained for the [OOl] direction and NiM,,,W Auger transition. For two main azimuths ((i.e. [lOO] and [IlO]) the dependence of the Auger signal on the incidence angle was obtained over a wide range of angles and for Ni M, y, Ni L3W and S LW transitions. Such measurements for directions far from’ the normal were appreciably influenced by the rotation of the sample with respect to the RFA aperture, nevertheless, maxima for particular channelling directions far from the normal to the surface were clearly visible. 3. Results The dependence of the contrast measured direction on the azimuth angle for the primary
for channelling in the [OOl] beam energy equal to 1500 eV
238
[I101 1 15' 11'iOl ,, , [IO01 , ,( , 60' 40' 20' 0 20' 40' 60' azimuth angle Fig. 1. Dependence of the contrast for directions of the primary electron beam close to the [OOl] direction (a) and of the incidence angle /I,,,,,, for which the minimum Auger signal is observed (b) on the azimuth angle. Circles and crosses are related to the contrast calculated from I,,,,, obtained for the “left” and “right” minimum of the Auger signal. respectively.
and for the NiM,,Y transition is shown in fig. 1. In this figure is also presented the dependence of the fI,,,,, angle formed by the sample surface normal and the direction of the primary beam for which I,,,, occurs on the azimuth angle. In fig. 2 the dependence of the contrast measured for channelling in the [OOl] direction on the primary beam energy is presented for the azimuths [loo] and [llO] for Ni M,,YV transition. The contrast values obtained for Ni M,,,VV, Ni LYV and S LVV transitions for channelling in the [OOl] direction and for [loo] and [l lo] azimuths arc given in table 1. The dependence of the Auger signal on the incidence angle for Ni L,VV transition and for [loo] azimuth is shown in fig. 3. Besides the maximum for
o~/_;“““;-:,~p~7”l, _ 1000 Fig. 2. Contrast
1500
2000
for the Ni M,,YV transition and the primary electron versus the primary electron energy.
beam close to the [OOI] axk
Table
1
Contrast different
values measured Auger transitions
and calculated by the use of relation (Er = 1500 ev)
(2) for different
azimuths
S LVV
Azimuth
Ni M a,aW h = 0.48 nm Meas.
CalC.
Meas.
talc.
Meas.
[1Wl
0.18 0.30
0.62 0.47
0.32 0.60
0.93 0.86
0.17 0.33
Ill01
and
Ni LYV h =1.5 nm
the direction normal to the surface, other maxima appear B = 5 45”. This dependence is typical of all the investigated
for 8 = rt 20 o and Auger transitions.
4. Discussion Bishop et al. [3] proposed the following from channelling on the (I&Z) planes: c = 2~~~/~~~~~/~~
formula
for the contrast
+ 47W&J2,
resulting
(2)
where X is the inelastic mean free path for the emitted Auger electron and Ehki is the extinction distance for the Bragg reflection of the primary beam on the (hkl) planes. From among the planes parallel to the [OOI] axis in the fee crystal, the (200), (020) (220), and (220) planes should play an important role in the channelling of electrons directed close to this axis owing to their low Miller indices. Bragg angles /?s for the electron energy of 1500 eV for the (200) and (220) planes are given in table 2 together with extinction distances taken
0
20'
I LO" 60' 80" polar angle
_
Fig. 3. The Auger signal for Ni L,VV transition and the primary electron energy equal to 1500 eV versus the incidence angle of the primary electron beam oriented along the [loo] azimuth. Indices of low-index planes channelling the electrons in particular maxima are presented together with the incidence angles for which particular maxima are expected.
S. Mdz,
240
A. Mr6z / Auger electron emisswn from the N~(001) f&e. I
Table 2 Values determining
the channelling
process
Azimuth
&Cd%)
Wdes)
P ,,,=PLI+W
E(nm)
w4 v-201
5.17 7.31
2.64 1.25
7.81 X.56
3.x3 S.69
from ref. [2] and the excess of Ap of the incidence for which I,,, occurs. Ap in this table is calculated proposed by Morin [4]:
angle over the Bragg angle on the basis of the relation
where g,,, is the modulus of the reciprocal lattice vector. Let us choose the azimuth angle (p = 0 for the [OlO] direction on the sample surface. For the primary electron beam direction lying in the plane of this azimuth and forming the angle 0 with the sample normal. the angles of incidence on the particular low index planes are given by the relations: P (020) sin
=
Pc220j
03
P(2W
=
sin
=
Pc~20,
0
=
sin 45” sin 8 = 0.7 sin 8.
Now we can calculate on each plane:
the S,,,,, values corresponding
to I,,,
for channelling
fl’200’ Ill,” = j#!~’ = 7.81 O, ~~220) Ill,” = @?2,0’z pz2,0’/0.7
= 12.20,
e,,” = ( e;;lp, + flAt’n”‘)/2 = 10 O. For + = 45 o the same calculations
give:
These calculations indicate that for $I = 0 the measured contrast results from channelling on the (200) plane (with 0,,,,, = 7.8” ) and on the (220) and (220) planes (with,;, = 12.2O). The minimum of the I versus 0 curve should be found close to emin = 10” and should be rather wide owing to the difference between fI(‘O”) = 45 O. the (200) and (020) planes with Ill,” and OAtz”). For C#I in the 8III,” = 11.2O and the (220) plane with 8,,, = 8.6” should participate creation of the contrast. The minimum of the I versus 19curve is now expected to be narrower and to occur again close to e,,,,, = 10”. For other azimuth angles situation is more complicated. &,,, can be different for each of four
S. MI&Z,A. M&z / Auger elecrronemissionfrom the Ni(OOfjface. I
247
planes mentioned above and all these planes can participate in the creation of the contrast. As follows from relation (2) and from the data given in table 1, the contrast created by the (200) and (020) planes should be appreciably greater than that created by the (220) and (220) ones. It means that the greater than that contrast measured for + = 45O should be considerably were confirmed by the results measured for + = 0 O. All these expectations presented in fig. 1 and by the dependence of the AES peak maximum in the d N/d E mode on the angle 6 as mentioned in section 2. The contrast measured for NiMzs3W transition is appreciably lower than that for the NiL,W transition. This fact is in qualitative agreement with relation (I) because the mean free path for Auger electrons is smaller in the former case. Quantitative comparison of the contrast obtained from (2) and that measured in our work leads to wide discrepancy (see table 2). This indicates that the two-beam approximation is too simple in the case of medium energy, where intralayer scattering should be taken into account as it was done in the method proposed for medium energy electron diffraction by Pendry and Gard [9] and proved by Masud et al. [lo]. It seems to be difficult to use this method in our case owing to the role of backscattered electrons in the creation of the Auger signal. For this reason our discussion is limited to the qualitative aspects and to the two-beam appro~mation. The weak contrast for the Ni M,,,W transition results from the low value of the inelastic mean free path in comparison with the extinction distance for the primary electrons. In this case, Auger electrons are emitted only from this surface layer where the channelling of the primary electrons has not occurred yet. The observed contrast results from the influence of the channelling of the backscattered electron flux. The same occurs for the SLW transition because the sulphur atoms are placed on the surface, For the NiLYV transition this mechanism works as well, but additionally the Auger emission intensity is directly influenced by channeling of primary electrons. As a result, the contrast for this later transition is appreciably higher. The cont~bution of backscattered electrons to the creation of the contrast is discussed by Andersen and Howie in ref. [7] where the multi-beam dynamical theory is applied. They show that the backscattered electron flux should be concentrated on the atomic planes. For this reason, the role of backscattered electrons in the Auger electron emission is appreciably strengthened. This conclusion is consistent with the results obtained by Gardiner et al. [5] for the Auger signal measured for the iron film deposited onto the surface of the cylindrical tungsten crystal rotated around its axis parallel to the [110] direction. Gardiner et al. concluded that changes of the signal mentioned above can be fully explained as resulting from changes of the backscattering factor caused by the crystal rotation. A weak dependence of the contrast on the primary electron energy presented in fig. 2 is in agreement with such a dependence obtained by Morin f4]
for the Si KLL transition and for primary electron energy ranging from 10 to 60 keV. An increase of the primary electron energy enlarges both the range of depth at which channelling occurs and the backscattering factor. These two effects limit each other and the resulting dependence of the contrast on the primary electron energy is found to he weak in a wide energy range. The dependence of Auger signals on the incidence angle of the primary electron beam for different Auger transitions and different azimuth angles similar to that shown in fig. 3 can be easily interpreted as a result of the channelling of primary electrons on subsequent low-index planes reached by the primary beam during increasing of the incidence angle. Miller indices of these planes are given in this figure together with the incidence angles for which particular maxima are expected. Similar dependences were observed by De Bersuder et al. [ll] for the secondary electrons and the elastically scattered ones for the Al(001) face and were interpreted in the same way.
Acknowledgements The authors would like to thank M. SC. B. Stachnik for her assistance in measurements and in preparation of the nlanuscript and to M. SC. A. Wieczorek for the computer programme.
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Phil. Mag. 16 (1967) 1179. A. Howie. R.B. Nicholson, D.W. Paschley and M.J. Whelan, Electron Microscopy of Thin Crystals (Butterworths, London. 1965). H.E. Bishop, B. Chornik, C. Le Gressus and A. Le Moe], Surface Interface Anal. 6 (1484) 116. P. M&n, Surface Sci. 164 (1985) 127. ‘T.M. Gardiner, H.M. Kramer and E. Bauer. Surface Sci. 121 (1982) 116. S. MF& and A. Mritz, in: Proc. 2nd Conf. on Surface Physics, Wreclaw. 9-10 Decemhu 1987. Vol. 3, p. 12.5.
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and A. Howie, Surface Sci. 50 (1975) 197. Y. Sakai and A. Mogami, J. Vacuum Sci. Technol. A 5 (1987) 1222. J.B. Pendry and P. Card, J. Phys. C 8 (1975) 2048. N. Masud, C.G. Kinningurgh and J.B. Pendry. J. Phys. c‘ 10 (1977) 1. 1,. De Bersuder, C. Corotte, P. Ducros and D. Lafeuille. in: The Structure Solid Surfaces, Ed. G.A. Somorjai (Wiley. New York. 1969) p. 30-l.
and Chemistry
of