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Experimental estimate of absorption length and total electron yield (TEY) probing depth in dysprosium
Journalof ElectronSpectroscopyandRelatedPhenomena, 61 (1994) 18l- 188
181
036!3-2048/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved
Experimental estimate of absorption length and total electron yield (TEY) probing depth in dysprosium Jan VogePb**, Maurizio Sacchia aLaboratoirepour l’lltilisation du Rayonnement Electromagnetique, Centre Universitaire Paris-Sud, Bat. 209d, F-91405 Orsay, France ‘Research Institute for Materials, University of Nijmegen, Toernooiveld, NL-6525 ED Nzjmegen, The Netherlands
(First received 26 May 1993; in final form 4 October 1993) Abstract Absorption spectroscopy has been performed at the A45edge of dysprosium and at the L~,J edges of nickel by collecting the total electron yield (TEY) in Dy layers of different thicknesses deposited on a Ni(l10) substrate. From the thickness dependence of the Ni intensity and from the angular dependence of the Dy intensity we have obtained an experimental estimate of the probing depth and of the minimum absorption length. Both values are found to bc very small. We discuss the implications of these results on XAS measurements in TEY mode.
1. IIltroduction
coefficient
Absorption spectroscopy in the soft X-ray range has received increasing attention in recent years, especially after the possibility of using this high energy spectroscopic technique to investigate low energy ground state properties in the solid state was clearly established [1,2]. M4,5 edges in rare earths (RE) and L2,3 edges in transition metals (TM) can indeed offer detailed information about magnetic [1,3] and crystal field effects [4,5] on the atomic ground state, as well as on angular momentum [6] and spin [7] expectation values, when the tunability in both energy and polarization of synchrotron radiation are fully exploited. Performing XAS in the energy range 100-2000eV on solid samples does not usually allow a simple transmission geometry, where incoming and transmitted intensities are measured to give the linear absorption
p(w) = Ilnw
*Corresponding author. SSDI 0368-2048(93)02034-J
where t is the sample thickness. The high crosssection for the absorption of soft X-rays would impose extremely low values for t (about 100 A) which are difficult to obtain and do not guarantee the homogenity of the sample over the illuminated area (about 1 mm*). Indirect measurement are then preferred, which involve either electrons or photons as byproducts of the absorption process. The collection of the total electron yield (TEY) from the sample by using a channel electron multiplier (CEM), or the equivalent measurement of the photo induced current, is one of the most common ways of measuring XAS, because of its simplicity and high efficiency. Unfortunately, no effort correspondent to its widespread application has been made to fully characterize the limitations, range of applicability and eventual necessary corrections
182
J. Vogel, ki. Sacchi}J. Electron Spectrosc. Relat. Phenom. 67 (1994) 181-188
related to this detection mode. In particular, the probing depth d of TEY measurements has always been estimated by calculating the diffusion length of the electrons in the material according to the so-called universal curve. Correctly including photo emitted and Auger electrons, and the cascade of secondary electrons [S], one usually obtains values for d in the range 200-5OOp\, where the dependence on the specific sample only relates to the energies of the primary electrons. A few observations contradicting these values were reported a few years ago [9,10], but apparently did not receive much attention. Recently, the renewed interest in XAS mentioned before demanded a better understanding of the TEY measurements, and it was soon realized that a simplified and unified picture as previously adopted was not able to interpret even a few pre-
liminary observations. On the one hand, one could easily measure layers buried under 125 A of a different material [I 11, supporting the previously given estimates for d. On the other hand, cases were reported where an experimental estimate of the probing depth gave values as low as 30A [12]. At the same time, a partial electron yield analysis [13] showed that there are materials where the universal curve more or less holds, and others where it breaks down, especially on the side of the low electron energies. Another parameter which plays an extremely important role in the soft X-ray range is the absorption length 1. In particular for TM and RE the cross sections can be very high (up to a few tenths of A2), reducing A to very low values (about 1001%) [lo]. The relative magnitude of I and d is of the highest importance, since only in
60
PI
6
a70
a80
Photon Energy (eV)
a90
840
Ni L,,
9
.
t
.-
860
.
850
860
870
660
890
Photon Energy (eV)
Fig. 1. Ni &,, spectra of a Ni( 110) single crystal covered with Dy layers of different thickness: (a) as collected spectra, (b) normalized to the f0 and resealed to a common value in the pre-edge region. The samples are liquid nitrogen cooled. The Dy thickness (A) is given by the side of each curve.
3. Vogel, M. SttcchiiJ. El&won Spectrasc. Relat. Phmom. 67(1994) 181-188
the condition d < 2 (weak absorption in the measured volume) cam one claim the propo~io~~ity of the TEY to the absorption coefficient. The relevant parameter is actually the effective absorption length &.>i.e. projected along the surface normal: 1, = lsin a, where a is the angie between the propagation vector of the light and the sample surface. The condition 1>> d at normal incidence can turn into Isin o’ M d at grazing incidence, outdating an angular dependent saturation. This effect, which is always undesirable, becomes extremely disturbing in certain experiments, such as most of the polarization dependent XAS. In this paper we will show with an example how important these effects can be, and, at the same time, we will obtain q~ntitative estimates for the values of d and 1. Values are obviously specific to the chosen element (Dy), but can also be considered representative for all the RE ions.
183
2. Experimeutal The system chosen for this experiment is constituted of Dy layers evaporated on a Ni(ll0) substrate, in analogy with a previous study [12]. Particular care has been devoted to the sample preparation. The Dy was evaporated by e-bombardment from a high purity Dy ingot onto the clean and r~onstructed Ni( 110) surface, while the substrate was constantly kept at low temperature (85-90 K) by flowing liquid nitrogen through the cryostat, The evaporation rate was first calibrated versus the parameters of the evaporator (filament current, ingot voltage and current) using a quartz balance, and then monitored before and after each deposition to correct for eventual changes of the evaporation rate versus time. The overall accuracy of the method, estimated over a large number of experiments using different RE, is 60
I (I)
I lb)
:’
I
I
I
:
t
R.T.
850
860
870
680
Photon Energy (eV)
..* .. *
_r
850
860
670
880
Photon Energy (eV)
Fig. 2. (a) Spectra of Fig. 1 normalized to a common peak height. (b) Ni spectra far the thickest Dy coverage taken at liquid nitrogen (LB.) and room temperature (R.T.).
184
J. Vogel, hf. SacchiJJ. Electron Specirosc. Relat. Phenom. 67 (1994) 181-188
of about f20%, with a reproducibility better than 10%. The data reported in this paper have been collected in a single run of 8 h (unless otherwise specified) by making subsequent evaporations of Dy in a vacuum of about 2 x 10-‘mbar (base pressure about 2 x 10-*” mbar). We do not believe that an eventual contamination of the Dy layer over the duration of the experiment might alter our conclusions. The spectra were collected using the synchrotron radiation from the SuperACO storage ring at LURE (Orsay). Beryl (1010) crystals were used to monochromatize in the 800- 1500 eV range, cover-
ing both Ni &,s and Dy i&s edges. The total resolving power is about 3 x IO3 in this energy range. A CEM biased with +lOO and +25OOV at the front and back ends respectively, was used to collect the TEY from the sample. Light-tight protections were used for all the window ports on the monochromator, beam line and experimental chamber, since we observed extremely intense background count rates on the CEM (up to 5 x lo4 counts s-‘) induced by visible light after Dy evaporation. Different horizontal apertures for the incoming beam were used during the different acquisition scans, in order to keep the
100
Normalized Ni L,,, intensity
80
60
Dy thickness Fig. 3. Ni intensity versus Dy thickness: w, (P - El/B;
l
80
(A)
area (see text). The lines are best fits to the data with an c-“~ functional form.
J. Vogel, M. SacchilJ.
Electron Spectrosc. Relat. Phenom. 67(1994) 181-188
maximum count rates on the CEM approximately at the same value (4 x lo4 to 6 x lo4 countss-‘). Saturation effects induced by the counting chain itself were observed above 1.5 x 10’ counts s-’ . 3. Results and discussion In Fig. l(a) the raw Ni L2,3 spectra are reported as a function of the Dy coverage in BngstrGms. In (b) they are normalized to the incoming intensity and resealed to a common value in the pre-edge region. Figure 2(a) reports the same spectra on an intensity scale which is more suitable for observation (all spectra are normalized between 0 and 100). We want to point out that even for the thickest Dy coverage, the Ni spectrum (hardly visible in Fig. 1 and very noisy in Fig. 2) does not show any clear variation in the peak positions with respect to the clean substrate spectrum. In our opinion this confirms the non-reactivity of the Dy/Ni interface, an effect that could undermine any attempt at a quantitative evaluation of d. The importance of preparing and keeping the samples at low temperature is illustrated in Fig. 2(b), where we report the Ni spectra for the thickest Dy overlayer taken at liquid nitrogen temperature and after stopping the liquid nitrogen flow and letting the sample warm up to room temperature . The clear changes in line shape and position of the peaks correspond to what was reported for RE-Ni alloys and RE/Ni(llO) annealed layers [14], showing that we are now measuring Ni atoms interdiffused with the Dy of the overlayer. To reach a quantitative estimate for d through the Dy layer, we have plotted the intensity of the Ni absorption versus the layer thickness. Two values are plotted in Fig. 3. One refers to the ratio (P - B)/B, where P is the peak height at the L3 maximum, and B is the background value just before the L3 edge. The other value is the area under the spectrum obtained by extrapolating the pre-edge with a straight line. In both cases we used the spectra of Fig. 1(b), and the value obtained for pure Ni was set to 100. As it is seen in Fig. 3, it does not really matter which definition we chose for the
185
Ni intensity. In both cases we obtain a continuous decrease versus Dy thickness, with a functional form which is compatible with an exponential. Fitting the curves in Fig. 3 with an e-*/d curve, one obtains d = 11 f 2 A, a value much shorter than previously reported in a Tb/Ni experiment [12]. To clarify this discrepancy, one should go back to Fig. 2(b): it is clear that the same experiment performed at room temperature (as the Tb/Ni one was) would have led to a much higher but wrong value for d. Let us turn now to the MS absorption edge of Dy (3d -+ 4f transitions with 3dsj2 core hole). The spectra taken at normal incidence of the photons do not show any thickness dependence above about 2OA and correspond to the spectra of Dy in various reference samples, The variations below 20 A can be attributed to polarization dependent absorption of the Dy ions in the surface crystal field [5]. Of particular interest for our
I
I 1290
I
I
I
I
1295
1300
1305
Photon Energy (eV) Fig. 4. Dy MS edge versus a in the 60A Dy/Ni(l 10) sample at liquid nitrogen temperature. The spectra are normalized to the height of the first peak.
J. Vogel, hf. Sacchi/J. Electron Spectrosc. Relat. Phenom. 67 (1994) 181-188
186
100 -
95 -
90 )r .= cn F’ -
85-
80 -
75 -
70 1
I
I
I
I
I
0
20
40
60
80
TOO
Anglea(deg.) Fig. 5. Dy intensity versus the angle CCn , area; 0, peak height. The continuous line is the best fit (see text).
purposes is the angular dependence of the absorption, shown in Fig. 4: the normalization of the spectra to the first peak of lowest intensity (about 1295 eV) allows us to observe a clear angular dependent saturation of the other more intense structures. The intensity of the TEY as a function of the absorption length can be expressed as
L(hv) =(1(kv)/d)$n
where p(h~) is the linear absorption coefficient. The 1, function scales as l/ sin (o) to account for the different photon path length into the active thickness d when the angle of incidence is changed. To have an intensity function which is normalized per absorbing atom, one can multiply by sin (a), to obtain IZ(hv) = &(hv) sin Q = (I,d) +A cosec a:
(cy) + 1
where A represents the number of counts measured per absorption event, d and (y:are as previously defined. For 1sin (Y> d, one has
This expression gives an a-independent intensity as long as the condition /sin Q >> d is satisfied (i.e. for I>> d and angles greater than about 10”). In Fig. 5 the values of the maximum intensity at the Dy MS absorption edge (i.e. corresponding to the shortest I) are plotted as a function of the angle o. The data
J. Vogel,M, Socchi{J.Electran~peckrosc. Re!ar.Phmom,6ff1994) H-188
1292
1296
1300
Photon Energy(eV) Fig. 6. Dy Ms edge in a SO08, DySi,_, layer epitaxially grown ou Si(l1 l), as a function of the angle between the elation vector andthe surface normal. Continuouslines are calculations performed with parameters optimized on the normal incidence spectrum.
are fitted with the expression 1: given before (line) with A and l/d as free parameters. The best fit gives a value of 11.5 for the ratio between absorption length and probing depth. Taking for d the value of 11 f 2 A previously derived, one gets I w 125 f 25 il. Even considering the large error bar (about f20%), this value agrees very well with the estimate of I = 121 A obtained in atomic calculations [lo]. As Fig. 4 clearly shows, an order of magnitude between I and d is not enough to prevent ~turation effects at grazing incidence. The occurrence of angular dependent saturation in bulk samples or thick layers can be a major limitation in certain experiments. As an example of these undesirable but unavoidable effects, we show in Fig. 6 the Dy M5 edge measured on an approximately 5OOA thick DySil_, layer epitaxially grown on Si(lll). The symmetry of the Dy3+
187
site in DySiZ_, is &, with the main axis of s~rnet~ along the [I 1I] direction of the substrate (i.e. normal to the film plane). This local symmetry induces different absorption spectra according to the orientation (parallel or ~~endicular) of the linear po\arization vector of the light e and the [ill] direction [5]. The (k s 90” spectrum (E I [ill]) has been calculated according to the model discussed in Ref. 5. The spectra for the other angles have been obtained without introducing further parameters, and the variation observed as a function of a is the crystal field induced linear dichroism related to the specific symmetry of the Dy ion. A comparison between experimental and calculated spectra at grazing incidence clearly shows the occurrence of an angular dependent saturation which severely limits a detailed and quantitative analysis of these data. Two remarks are worth making about the limitations of our procedure to get an estimate for d and, consequently, for I. (i) Sample quality is extremely ~mpo~ant in order to get a reasonable estimate for d. Inhomogeneities, interdiffusion and surface roughness certainly affect the final value, We cannot exclude the occurrence of such effects in our experiment, only say that interdiffusion is reduced with respect to room temperature studies (see Fig. 2(b)). Formation of islands in the Dy layer, or interdiffusion with the Ni of the substrate would act to increase the derived value of d with respect to the real one, while a higher pre-edge background due to increased surface roughness would lead to a d value which is too low. (ii) To derive I from the fitting curve of Fig. 5 we have transferred the value of d through the Dy layer obtained at the Ni edge to the experiment at the Dy edge. This implies the crude hypothesis that d does not depend on the energy of the primary excitation process (mainly on the energy of the ejected Auger electrons). We believe that the error bars of about i~20% that we associated with d and I contain this approximation as well.
IS8
J. Vogel, M. SacchilJ. Electron Spectrosc. Relat. Phenom. 67 (1994) MI-I88
4. Conclusions
Acknowledgements
The probing depth of XAS measurements in TEY mode is very short when RE layers are concerned. The value we have obtained (about 11 A) is even shorter than previously reported in room temperature experiments. This contradicts the applicability of a universal curve for the electron escape depth in condensed matter. Rather, our results support the hypothesis of a very short mean free path for the low energy electrons, at least in a class of materials including rare earths. The value we determined for d implies that a certain degree of surface sensitivity is intrinsic in the TEY measurements. The control and characterization of the sample surface is then a strict requirement even for taking bulk spectra, and surface contributions should always be taken into account. The absorption length at peak maximum is found to be of the order of 125 & 25A. This implies that angular dependent saturation effects are observable in the range (L= lo”-30” even for relatively thin RE samples of a few monolayers. This effect has been observed, for example, in thin Sm layers on Si( 111) [ 151. We have shown with an example how these phenomena can affect the data analysis: linear dichroism is certainly the most but not the only, analysis affected. Studying in-plane magnetization with circular dichroism, for example, also requires the grazing incidence geometry. As a concluding remark, we want to point out that the short absorption length in RE would pose even more stringent limitations when measuring XAS using a technique of larger probing depth, such as, for example, fluorescence. In that case the condition 1>> d would never be satisfied, regardless of the angle of incidence.
This work has been supported in part by the Stichting Scheikundig Onderzoek in Nederland (SON) with financial support from the Nederlandse Stichting voor Wetenschappelijk Onderzoek (NWO), and by the European Community under contract number SCl-CT91-0630. References
6 7 8
9
IO
11 12
13 14 I5
B.T. Thole, G. van der Laan and GA. Sawatzky, Phys. Rev. Lett., 55 (1985) 2086. J.C. Fug& and I.E. Inglesfield (Eds.), Unoccupied Electronic States. Springer-Verlag, Berlin, 1992. T. Jo and G.A. Sawatzky, Phys. Rev. B, 43 (1991) 8771. F. deGroot, J.C. Fuggle, B.T. Thole and G.A. Sawatzky, Phys. Rev. B, 41 (1990) 928; 42 (1990) 5459. M. Sacchi, Soft X-ray linear dichroism, in Y. Petroff, 1. Nenner and F. Wuilleumier (Eds.), Proc. 10th VLJV Conference, World Publishing, Singapore, 1993, p. 431. B.T. Thole, P. Carra, F. Sette and G. van der Laan, Phys. Rev. Lett., 68 (1992) 1943. P. Carra, B.T. Thole, M. Altarelli and X. Wang, Phys. Rev. Lett., 70 (1993) 694. E. Erbil, G.S. Cargill III, R. Frahm and R.F. Biihme, Phys. Rev. B, 37 (1988) 2450; M.P. Seah and W.A. Dench, Surf. Interface Anal., 1 (1979) 2. J.-M. Esteva, R.C. Karnatak and J.P. Connerade, J. Electron Spectrosc. Relat. Phenom., 31 (1983) 1; G. van der Laan and B.T. Thole, J. Electron Spectrosc. Relat. Phenom., 46 (1988) 123. B.T. Thole, G. van der Laan, J.C. Fuggle, G.A. Sawatzky, R.C. Karnatak and J.-M. Esteva, Phys. Rev. B, 32 (1985) 5107. M. Sacchi, R.J.H. Kappert, J.C. Fuggle and E.E. Marinero, Appl. Phys. L&t., 59 (1991) 872. M. Abbate, J.B. Goedkoop, F.M.F. de Groot, J.C. Fuggle, S. Hoffmann, H. Petersen and M. Sacchi, Surf. Interface Anal., 18 (1992) 65. 0. Paul, Ph.D. Thesis, ETH Zurich, 1990 (unpublished). J.B. Goedkoop, Ph.D. Thesis, University of Nijmegen, 1989 (unpublished). 0. Sakho, M. Sacchi, F. Sirotti and G. Rossi, Phys. Rev. B, 47 (1993) 3797.
181
036!3-2048/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved
Experimental estimate of absorption length and total electron yield (TEY) probing depth in dysprosium Jan VogePb**, Maurizio Sacchia aLaboratoirepour l’lltilisation du Rayonnement Electromagnetique, Centre Universitaire Paris-Sud, Bat. 209d, F-91405 Orsay, France ‘Research Institute for Materials, University of Nijmegen, Toernooiveld, NL-6525 ED Nzjmegen, The Netherlands
(First received 26 May 1993; in final form 4 October 1993) Abstract Absorption spectroscopy has been performed at the A45edge of dysprosium and at the L~,J edges of nickel by collecting the total electron yield (TEY) in Dy layers of different thicknesses deposited on a Ni(l10) substrate. From the thickness dependence of the Ni intensity and from the angular dependence of the Dy intensity we have obtained an experimental estimate of the probing depth and of the minimum absorption length. Both values are found to bc very small. We discuss the implications of these results on XAS measurements in TEY mode.
1. IIltroduction
coefficient
Absorption spectroscopy in the soft X-ray range has received increasing attention in recent years, especially after the possibility of using this high energy spectroscopic technique to investigate low energy ground state properties in the solid state was clearly established [1,2]. M4,5 edges in rare earths (RE) and L2,3 edges in transition metals (TM) can indeed offer detailed information about magnetic [1,3] and crystal field effects [4,5] on the atomic ground state, as well as on angular momentum [6] and spin [7] expectation values, when the tunability in both energy and polarization of synchrotron radiation are fully exploited. Performing XAS in the energy range 100-2000eV on solid samples does not usually allow a simple transmission geometry, where incoming and transmitted intensities are measured to give the linear absorption
p(w) = Ilnw
*Corresponding author. SSDI 0368-2048(93)02034-J
where t is the sample thickness. The high crosssection for the absorption of soft X-rays would impose extremely low values for t (about 100 A) which are difficult to obtain and do not guarantee the homogenity of the sample over the illuminated area (about 1 mm*). Indirect measurement are then preferred, which involve either electrons or photons as byproducts of the absorption process. The collection of the total electron yield (TEY) from the sample by using a channel electron multiplier (CEM), or the equivalent measurement of the photo induced current, is one of the most common ways of measuring XAS, because of its simplicity and high efficiency. Unfortunately, no effort correspondent to its widespread application has been made to fully characterize the limitations, range of applicability and eventual necessary corrections
182
J. Vogel, ki. Sacchi}J. Electron Spectrosc. Relat. Phenom. 67 (1994) 181-188
related to this detection mode. In particular, the probing depth d of TEY measurements has always been estimated by calculating the diffusion length of the electrons in the material according to the so-called universal curve. Correctly including photo emitted and Auger electrons, and the cascade of secondary electrons [S], one usually obtains values for d in the range 200-5OOp\, where the dependence on the specific sample only relates to the energies of the primary electrons. A few observations contradicting these values were reported a few years ago [9,10], but apparently did not receive much attention. Recently, the renewed interest in XAS mentioned before demanded a better understanding of the TEY measurements, and it was soon realized that a simplified and unified picture as previously adopted was not able to interpret even a few pre-
liminary observations. On the one hand, one could easily measure layers buried under 125 A of a different material [I 11, supporting the previously given estimates for d. On the other hand, cases were reported where an experimental estimate of the probing depth gave values as low as 30A [12]. At the same time, a partial electron yield analysis [13] showed that there are materials where the universal curve more or less holds, and others where it breaks down, especially on the side of the low electron energies. Another parameter which plays an extremely important role in the soft X-ray range is the absorption length 1. In particular for TM and RE the cross sections can be very high (up to a few tenths of A2), reducing A to very low values (about 1001%) [lo]. The relative magnitude of I and d is of the highest importance, since only in
60
PI
6
a70
a80
Photon Energy (eV)
a90
840
Ni L,,
9
.
t
.-
860
.
850
860
870
660
890
Photon Energy (eV)
Fig. 1. Ni &,, spectra of a Ni( 110) single crystal covered with Dy layers of different thickness: (a) as collected spectra, (b) normalized to the f0 and resealed to a common value in the pre-edge region. The samples are liquid nitrogen cooled. The Dy thickness (A) is given by the side of each curve.
3. Vogel, M. SttcchiiJ. El&won Spectrasc. Relat. Phmom. 67(1994) 181-188
the condition d < 2 (weak absorption in the measured volume) cam one claim the propo~io~~ity of the TEY to the absorption coefficient. The relevant parameter is actually the effective absorption length &.>i.e. projected along the surface normal: 1, = lsin a, where a is the angie between the propagation vector of the light and the sample surface. The condition 1>> d at normal incidence can turn into Isin o’ M d at grazing incidence, outdating an angular dependent saturation. This effect, which is always undesirable, becomes extremely disturbing in certain experiments, such as most of the polarization dependent XAS. In this paper we will show with an example how important these effects can be, and, at the same time, we will obtain q~ntitative estimates for the values of d and 1. Values are obviously specific to the chosen element (Dy), but can also be considered representative for all the RE ions.
183
2. Experimeutal The system chosen for this experiment is constituted of Dy layers evaporated on a Ni(ll0) substrate, in analogy with a previous study [12]. Particular care has been devoted to the sample preparation. The Dy was evaporated by e-bombardment from a high purity Dy ingot onto the clean and r~onstructed Ni( 110) surface, while the substrate was constantly kept at low temperature (85-90 K) by flowing liquid nitrogen through the cryostat, The evaporation rate was first calibrated versus the parameters of the evaporator (filament current, ingot voltage and current) using a quartz balance, and then monitored before and after each deposition to correct for eventual changes of the evaporation rate versus time. The overall accuracy of the method, estimated over a large number of experiments using different RE, is 60
I (I)
I lb)
:’
I
I
I
:
t
R.T.
850
860
870
680
Photon Energy (eV)
..* .. *
_r
850
860
670
880
Photon Energy (eV)
Fig. 2. (a) Spectra of Fig. 1 normalized to a common peak height. (b) Ni spectra far the thickest Dy coverage taken at liquid nitrogen (LB.) and room temperature (R.T.).
184
J. Vogel, hf. SacchiJJ. Electron Specirosc. Relat. Phenom. 67 (1994) 181-188
of about f20%, with a reproducibility better than 10%. The data reported in this paper have been collected in a single run of 8 h (unless otherwise specified) by making subsequent evaporations of Dy in a vacuum of about 2 x 10-‘mbar (base pressure about 2 x 10-*” mbar). We do not believe that an eventual contamination of the Dy layer over the duration of the experiment might alter our conclusions. The spectra were collected using the synchrotron radiation from the SuperACO storage ring at LURE (Orsay). Beryl (1010) crystals were used to monochromatize in the 800- 1500 eV range, cover-
ing both Ni &,s and Dy i&s edges. The total resolving power is about 3 x IO3 in this energy range. A CEM biased with +lOO and +25OOV at the front and back ends respectively, was used to collect the TEY from the sample. Light-tight protections were used for all the window ports on the monochromator, beam line and experimental chamber, since we observed extremely intense background count rates on the CEM (up to 5 x lo4 counts s-‘) induced by visible light after Dy evaporation. Different horizontal apertures for the incoming beam were used during the different acquisition scans, in order to keep the
100
Normalized Ni L,,, intensity
80
60
Dy thickness Fig. 3. Ni intensity versus Dy thickness: w, (P - El/B;
l
80
(A)
area (see text). The lines are best fits to the data with an c-“~ functional form.
J. Vogel, M. SacchilJ.
Electron Spectrosc. Relat. Phenom. 67(1994) 181-188
maximum count rates on the CEM approximately at the same value (4 x lo4 to 6 x lo4 countss-‘). Saturation effects induced by the counting chain itself were observed above 1.5 x 10’ counts s-’ . 3. Results and discussion In Fig. l(a) the raw Ni L2,3 spectra are reported as a function of the Dy coverage in BngstrGms. In (b) they are normalized to the incoming intensity and resealed to a common value in the pre-edge region. Figure 2(a) reports the same spectra on an intensity scale which is more suitable for observation (all spectra are normalized between 0 and 100). We want to point out that even for the thickest Dy coverage, the Ni spectrum (hardly visible in Fig. 1 and very noisy in Fig. 2) does not show any clear variation in the peak positions with respect to the clean substrate spectrum. In our opinion this confirms the non-reactivity of the Dy/Ni interface, an effect that could undermine any attempt at a quantitative evaluation of d. The importance of preparing and keeping the samples at low temperature is illustrated in Fig. 2(b), where we report the Ni spectra for the thickest Dy overlayer taken at liquid nitrogen temperature and after stopping the liquid nitrogen flow and letting the sample warm up to room temperature . The clear changes in line shape and position of the peaks correspond to what was reported for RE-Ni alloys and RE/Ni(llO) annealed layers [14], showing that we are now measuring Ni atoms interdiffused with the Dy of the overlayer. To reach a quantitative estimate for d through the Dy layer, we have plotted the intensity of the Ni absorption versus the layer thickness. Two values are plotted in Fig. 3. One refers to the ratio (P - B)/B, where P is the peak height at the L3 maximum, and B is the background value just before the L3 edge. The other value is the area under the spectrum obtained by extrapolating the pre-edge with a straight line. In both cases we used the spectra of Fig. 1(b), and the value obtained for pure Ni was set to 100. As it is seen in Fig. 3, it does not really matter which definition we chose for the
185
Ni intensity. In both cases we obtain a continuous decrease versus Dy thickness, with a functional form which is compatible with an exponential. Fitting the curves in Fig. 3 with an e-*/d curve, one obtains d = 11 f 2 A, a value much shorter than previously reported in a Tb/Ni experiment [12]. To clarify this discrepancy, one should go back to Fig. 2(b): it is clear that the same experiment performed at room temperature (as the Tb/Ni one was) would have led to a much higher but wrong value for d. Let us turn now to the MS absorption edge of Dy (3d -+ 4f transitions with 3dsj2 core hole). The spectra taken at normal incidence of the photons do not show any thickness dependence above about 2OA and correspond to the spectra of Dy in various reference samples, The variations below 20 A can be attributed to polarization dependent absorption of the Dy ions in the surface crystal field [5]. Of particular interest for our
I
I 1290
I
I
I
I
1295
1300
1305
Photon Energy (eV) Fig. 4. Dy MS edge versus a in the 60A Dy/Ni(l 10) sample at liquid nitrogen temperature. The spectra are normalized to the height of the first peak.
J. Vogel, hf. Sacchi/J. Electron Spectrosc. Relat. Phenom. 67 (1994) 181-188
186
100 -
95 -
90 )r .= cn F’ -
85-
80 -
75 -
70 1
I
I
I
I
I
0
20
40
60
80
TOO
Anglea(deg.) Fig. 5. Dy intensity versus the angle CCn , area; 0, peak height. The continuous line is the best fit (see text).
purposes is the angular dependence of the absorption, shown in Fig. 4: the normalization of the spectra to the first peak of lowest intensity (about 1295 eV) allows us to observe a clear angular dependent saturation of the other more intense structures. The intensity of the TEY as a function of the absorption length can be expressed as
L(hv) =(1(kv)/d)$n
where p(h~) is the linear absorption coefficient. The 1, function scales as l/ sin (o) to account for the different photon path length into the active thickness d when the angle of incidence is changed. To have an intensity function which is normalized per absorbing atom, one can multiply by sin (a), to obtain IZ(hv) = &(hv) sin Q = (I,d) +A cosec a:
(cy) + 1
where A represents the number of counts measured per absorption event, d and (y:are as previously defined. For 1sin (Y> d, one has
This expression gives an a-independent intensity as long as the condition /sin Q >> d is satisfied (i.e. for I>> d and angles greater than about 10”). In Fig. 5 the values of the maximum intensity at the Dy MS absorption edge (i.e. corresponding to the shortest I) are plotted as a function of the angle o. The data
J. Vogel,M, Socchi{J.Electran~peckrosc. Re!ar.Phmom,6ff1994) H-188
1292
1296
1300
Photon Energy(eV) Fig. 6. Dy Ms edge in a SO08, DySi,_, layer epitaxially grown ou Si(l1 l), as a function of the angle between the elation vector andthe surface normal. Continuouslines are calculations performed with parameters optimized on the normal incidence spectrum.
are fitted with the expression 1: given before (line) with A and l/d as free parameters. The best fit gives a value of 11.5 for the ratio between absorption length and probing depth. Taking for d the value of 11 f 2 A previously derived, one gets I w 125 f 25 il. Even considering the large error bar (about f20%), this value agrees very well with the estimate of I = 121 A obtained in atomic calculations [lo]. As Fig. 4 clearly shows, an order of magnitude between I and d is not enough to prevent ~turation effects at grazing incidence. The occurrence of angular dependent saturation in bulk samples or thick layers can be a major limitation in certain experiments. As an example of these undesirable but unavoidable effects, we show in Fig. 6 the Dy M5 edge measured on an approximately 5OOA thick DySil_, layer epitaxially grown on Si(lll). The symmetry of the Dy3+
187
site in DySiZ_, is &, with the main axis of s~rnet~ along the [I 1I] direction of the substrate (i.e. normal to the film plane). This local symmetry induces different absorption spectra according to the orientation (parallel or ~~endicular) of the linear po\arization vector of the light e and the [ill] direction [5]. The (k s 90” spectrum (E I [ill]) has been calculated according to the model discussed in Ref. 5. The spectra for the other angles have been obtained without introducing further parameters, and the variation observed as a function of a is the crystal field induced linear dichroism related to the specific symmetry of the Dy ion. A comparison between experimental and calculated spectra at grazing incidence clearly shows the occurrence of an angular dependent saturation which severely limits a detailed and quantitative analysis of these data. Two remarks are worth making about the limitations of our procedure to get an estimate for d and, consequently, for I. (i) Sample quality is extremely ~mpo~ant in order to get a reasonable estimate for d. Inhomogeneities, interdiffusion and surface roughness certainly affect the final value, We cannot exclude the occurrence of such effects in our experiment, only say that interdiffusion is reduced with respect to room temperature studies (see Fig. 2(b)). Formation of islands in the Dy layer, or interdiffusion with the Ni of the substrate would act to increase the derived value of d with respect to the real one, while a higher pre-edge background due to increased surface roughness would lead to a d value which is too low. (ii) To derive I from the fitting curve of Fig. 5 we have transferred the value of d through the Dy layer obtained at the Ni edge to the experiment at the Dy edge. This implies the crude hypothesis that d does not depend on the energy of the primary excitation process (mainly on the energy of the ejected Auger electrons). We believe that the error bars of about i~20% that we associated with d and I contain this approximation as well.
IS8
J. Vogel, M. SacchilJ. Electron Spectrosc. Relat. Phenom. 67 (1994) MI-I88
4. Conclusions
Acknowledgements
The probing depth of XAS measurements in TEY mode is very short when RE layers are concerned. The value we have obtained (about 11 A) is even shorter than previously reported in room temperature experiments. This contradicts the applicability of a universal curve for the electron escape depth in condensed matter. Rather, our results support the hypothesis of a very short mean free path for the low energy electrons, at least in a class of materials including rare earths. The value we determined for d implies that a certain degree of surface sensitivity is intrinsic in the TEY measurements. The control and characterization of the sample surface is then a strict requirement even for taking bulk spectra, and surface contributions should always be taken into account. The absorption length at peak maximum is found to be of the order of 125 & 25A. This implies that angular dependent saturation effects are observable in the range (L= lo”-30” even for relatively thin RE samples of a few monolayers. This effect has been observed, for example, in thin Sm layers on Si( 111) [ 151. We have shown with an example how these phenomena can affect the data analysis: linear dichroism is certainly the most but not the only, analysis affected. Studying in-plane magnetization with circular dichroism, for example, also requires the grazing incidence geometry. As a concluding remark, we want to point out that the short absorption length in RE would pose even more stringent limitations when measuring XAS using a technique of larger probing depth, such as, for example, fluorescence. In that case the condition 1>> d would never be satisfied, regardless of the angle of incidence.
This work has been supported in part by the Stichting Scheikundig Onderzoek in Nederland (SON) with financial support from the Nederlandse Stichting voor Wetenschappelijk Onderzoek (NWO), and by the European Community under contract number SCl-CT91-0630. References
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