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Photoemission study of EuS layers buried in PbS
Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 763–767
Photoemission study of EuS layers buried in PbS B.A. Orlowski a,∗ , S. Mickeviˇcius b , M. Chernyshova a , I. Demchenko a , A.Y. Sipatov c , T. Story a , V. Medicherla d , W. Drube d a
d
Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland b Semiconductor Physics Institute, A.Gostauto 11, 2600 Vilnius, Lithuania c Kharkov State Technical University, Frunze 21, 310002 Kharkov, Ukraine Hamburger SynchrotronstrahlungslaborHASYLAB am Deutschen Elektronen-Synchrotron DESY, Notkestr. 85, D-22603 Hamburg, Germany Available online 9 April 2004
Abstract High-energy photoemission spectroscopy (hν = 3510 eV) was used to study the electronic structure of EuS buried layers deposited on a thick PbS layer and covered by about 30 Å of PbS layer. The study of Eu 3d, 4d and valence band photoemission spectra showed the existence of only Eu2+ ions in the as-grown buried EuS layer. The angular behavior of the Eu 3d spectra revealed that Ar ion bombardment of the sample surface, i.e. initially the PbS cap layer, leads to a change of stoichiometry in the near surface region of the EuS layer leading to depletion of S and concomittant accumulation of Eu3+ at the surface. In addition, prolonged X-ray exposure is found to gradually increase the Eu3+ concentration in the surface region. The contribution of Eu2+ 4f electrons to the electronic structure of the PbS/EuS/PbS multilayer is found at the top of the valence band. © 2004 Elsevier B.V. All rights reserved. Keywords: Semiconductor buried layer; Lead and europium monosulfide; Synchrotron radiation; Photoemission spectroscopy
1. Introduction The problem of interlayer exchange coupling (IEC) between magnetic layers across non-magnetic layers is becoming important for potential application in spintronics, a new area of solid-state electronics [1]. Spin valves and spin injectors are the first practical applications of spintronics in which the key element is IEC in magnetic superlattices (SL). In the past several years IEC was observed in e.g. EuTe/PbTe, MnTe/ZnTe, MnTe/CdTe, GaMnAs/AlGaAs and EuS/PbS superlattices by means of neutron diffraction, neutron reflectivity and magnetization measurements [2–5]. The latter belong to an IV–VI narrow-gap semiconductor magnetic SL, where EuS is a large-gap magnetic semiconductor (Eg = 1.6 eV) with Curie temperature Tc = 16.5 K and PbS is narrow-gap (Eg = 0.30 eV) degenerated semiconductor. Both of them crystallize in the rock salt crystal structure and the lattice mismatch is about 0.6%. In the EuS/PbS multilayer system EuS layers correspond to barriers and the PbS layers to quantum well material.
∗
Corresponding author. Fax: +48-22-8430926. E-mail address: [email protected] (B.A. Orlowski).
0368-2048/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2004.02.096
It is well known that the surface electronic structure often differs from that in the bulk. The change in dimensionality sometimes induces a surface reconstruction which affects the local geometry and atomic coordination leading to changes in the valence band structure and producing surface core level shifts as observed by surface sensitive photoelectron spectroscopy [6]. Application of synchrotron radiation gives the possibility to tune the photoelectron escape depth via the kinetic energy allowing for a surface and bulk-sensitive detection. A minimum depth is obtained for electrons with a kinetic energy around 100 eV, where surface related effects dominate the spectra. Combined with high-energy resolution it enables detailed studies of surface valence and shallow core levels. In order to probe bulk properties or to access buried structures, high-energy photoemission can be effectively used. It offers additional advantages to distinguish features of the surface electronic structure from those related to the volume or originating from buried species. Because at high kinetic energies the electron mean free path is large, the probing depth can be varied by measuring EDCs versus the photoelectron exit angle. This can also be done using laboratory X-ray sources (hν <1500 eV), although the energies are often too low to reach true bulk sensitivity. In this study,
B.A. Orlowski et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 763–767
synchrotron X-rays at much higher energies (3510 eV) are used to significantly enhance the volume sensitivity for shallow core levels. At the same time, also deeper atomic levels can be accessed to obtain additional information on the electronic structure. Using synchrotron X-rays, the excitation energy may be varied over a large range. This is also advantageous in the study of high-Z materials, like Eu and Pb, because it allows to discriminate the core level photoemission from the rich structure of lines originating from Auger cascades. In this paper we utilize the volume sensitivity of highenergy X-ray photoelectron spectroscopy to study EuS layers buried in PbS in the EuS/PbS superlattice.
2. Experimental The EuS/PbS multilayers structures were grown on (1 0 0) surfaces of the substrate KCl, using an e-beam for the evaporation of EuS and a thermal source for PbS. The standard composition of the samples was 20–30 Å of PbS (as a cap layer) on top of 10–80 Å of EuS deposited on 700–1000 Å of a PbS buffer. The data were obtained with the Tunable High-Energy X-ray Photoelectron Spectrometer at the X-ray wiggler beam line BW2 of HASYLAB (Hamburg). The photon flux from the Si(1 1 1) double crystal monochromator typically amounts to 5 × 1012 s−1 on the sample. The electrons were measured using a hemispherical analyzer with parallel detection capability (SCIENTA SES-200). An excitation energy was chosen (3510 eV) where the kinetic energy of the Eu 3d levels is not overlapping any Pb Auger electron emission originating from the PbS layers. This was checked by EDCs covering a wide energy range. The total energy resolution at this photon energy is 0.6 eV. The binding energy scale was calibrated using the Pb 5d5/2 photoemission from PbS at 18.52 eV [7]. The angle dependent data were obtained by rotating the sample relative to the electron analyzer, which accepts electrons in the SR orbit plane at 45◦ relative to the incoming beam. Moderate Ar+ sputtering was used to obtain clean EuS or PbS top layer surface in a UHV preparation chamber. Some samples were measured “as introduced”, i.e. without any surface treatment. The base pressure in the analyzer chamber was 5 × 10−10 mbar.
3. Results and discussion The Pb 5d core level and valence band spectra of PbS/EuS multilayers without sputtering taken for normal emission angle (0◦ ) are shown in Fig. 1. The inset shows the enlarged valence band region (solid line) compared to a spectrum recorded at a grazing emission angle (70◦ relative to the surface normal, circles). The spectra taken for grazing emission show features closely resembling spectra of galena crystals taken by UPS and XPS [7–9]. At normal emission
2.5
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764
PbS/EuS
PbS/EuS
normal emission
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grazing emission 1.5
Pb 5d 1.0
0.5 0.5 difference (EuS) 0.0 −15
−10 −5 Binding energy (eV)
0
0.0 25
20
15
10
5
0
Binding energy (eV)
Fig. 1. Pb 5d core level and valence band spectra of a PbS/EuS multilayer obtained for an as-introduced sample (no surface treatment, normal emission geometry, hν = 3510 eV). The inset compares the valence band region recorded at normal emission (solid line) and grazing emission angles (circles). The difference curve reveals the contribution of the buried EuS layer to the valence band structure.
(solid line) the signal of EuS buried in galena is enhanced because of the increased bulk sensitivity. The spectra have been normalized to the galena emission below 5 eV binding energy. The numerical difference of these two spectra (bottom curve in the inset of Fig. 1) can be expected to reflect the buried EuS layer in PbS. The pronounced peak located around 1 eV below the Fermi level can be associated with the Eu2+ 4f final state multiplet contribution to the valence band. Usually Eu in bulk EuS is divalent [10–13] and this structure is observed near the valence band maximum, while the Eu3+ multiplet structure in Eu compounds normally is located between 5 and 11 eV. In the difference spectra revealing the EuS buried layer no significant structure in this region is observed (see inset in Fig. 1). This result is in good agreement with resonant PES investigation of PbS/EuS multilayers [11]. The width of the Eu2+ derived maximum is strongly dependent on the final state multiplet splitting as well as possible 4f ligand hybridization. The electronic structure up to 4.5 eV binding energy can be attributed to the valence band because of overlapping Eu2+ 4f and S 3p occupied electronic states. The conclusion that Eu in the EuS layer buried in PbS is in the divalent state is also confirmed by inspection of the Eu 3d and 4d core level spectra. Fig. 2 shows the Eu 3d core level spectra from the buried layer of an unsputtered, i.e. as-introduced, sample taken at normal emission (circles) compared to the divalent Eu 3d multiplet structure of the 3d9 4f7 final state (solid line). This was derived from spectra of the sputtered sample which exhibits both 2+ and 3+ valences (see Figs. 3 and 4). The Eu 3d spectra in Fig. 2 can be divided into 3d5/2 and 3d3/2 structures due to the large spin–orbit interaction of the Eu 3d core level. The Eu 3d9 4f7 final state multiplet structure appears as a double-peak structure with a strong main line and a small separate peak. The latter appears roughly at 6.8 eV higher binding energy
B.A. Orlowski et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 763–767
765
2.0
Normalized intensity (arb. units)
Eu 3+
Fig. 2. Eu 3d core level spectra from the buried EuS layer: unsputtered PbS/EuS multilayer sample measured at normal emission (circles) and “pure” divalent Eu 3d multiplet structure (solid line, cf. Fig. 4). Excitation energy hν = 3510 eV.
relative to the 3d5/2 main peak and is labeled as “separ.” in Fig. 2. The comparison of the two spectra clearly shows that Eu in the buried EuS layer is divalent. The broad extra “bumps” marked by arrows (located 15.2 eV below the Eu 3d peaks) are likely due to PbS bulk plasmon excitation since the fast Eu 3d electrons have to penetrate the PbS top layer. Fig. 3 shows two Eu 3d core level spectra of a sputtered sample of PbS/EuS multilayers: “fresh”, i.e. recorded immediately after sputtering (solid line) and after X-ray irradiation of about 20 h (circles). Both spectra were taken at grazing emission (80◦ from the surface normal) to enhance the surface sensitivity. The initial PbS cap layer is likely removed to a large extent after sputtering, so that the near surface region consists mainly of EuS. Clearly, two multiplet structures are
Fig. 3. Eu 3d core level spectra of the sputtered PbS/EuS multilayer: “fresh”: measured immediately after sputtering (solid line) and after X-ray irradiation of about 20 h (circles). Excitation energy hν = 3510 eV. Both spectra were taken at grazing emission angle (80◦ ).
1.5
1.0
0.5
0.0 1180
3d 3/2
3d 5/2
Eu 2+
1160
1140
1120
Binding energy (eV) Fig. 4. Eu 3d core level spectra of the sputtered PbS/EuS multilayer. Excitation energy hν = 3510 eV. The “pure” Eu2+ and Eu3+ 3d core level spectra were derived from linear combinations of normal emission and grazing emission (80◦ ) spectra. The middle curve is a calculation of the Eu3+ spectrum, reproduced from [14].
present which correspond to Eu2+ (3d9 4f7 final state) and Eu3+ (3d9 4f6 final state). This implies that the sputtering of the PbS/EuS sample “generates” the trivalent Eu species which is then steadily increasing upon X-ray exposure. It is mentioned that no radiation effect was observed for the buried EuS layer, at least for the time of the measurement (several hours). Fig. 4 presents the Eu 3d core level spectra recorded from the sputtered sample of PbS/EuS (like in Fig. 3) which are separated into the “pure” Eu2+ and “pure” Eu3+ components. This was achieved by a numerical linear combination of normal (0◦ ) and grazing (80◦ ) emission spectra which exhibit a significantly different weighting of the 2+ and 3+ contribution because 3+ is accumulated in the surface region. The middle curve (line) corresponds to a calculated Eu3+ spectrum, reproduced from ref. [14]. The overall agreement between the experimental and calculated multiplet structure clearly is very good. The Eu3+ 3d multiplet structure consists of two peaks like in the Eu2+ case (see Figs. 2–4) in which the separate peak appears approximately at 8.5 eV below the 3d5/2 main peak. A comparison between the divalent and trivalent components shows that the separate peak of Eu2+ partly overlaps the Eu3+ main peak (see Figs. 3 and 4). Both the Eu2+ and Eu3+ 3d multiplet peak structures in terms of line shape and energy position are in good agreement with theoretical calculations and other experimental results of the compounds with divalent and trivalent europium configurations [12–16]. The relevant process creating this double-peak structure is the exchange of 4f electrons with 3d electrons in the final state due to the electrostatic interaction, since the spin–orbit interaction of the 4f electrons is rather small.
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Eu
2+
4d
1.0 9 7
0.8
D
D
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5
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Normalized intensity
2
6
130 Binding energy (eV)
80
o
70 o 60 o 1
30
2
o
+
0o
125
Fig. 5. Eu2+ 4d core level spectra of the sputtered PbS/EuS multilayer (circles). Excitation energy hν = 3510 eV. For comparison, the solid line shows high-resolution Eu 4d core level spectra of atomic Eu, obtained at 165 eV (reproduced from [17]).
The Eu 4d core level shows very complicated multiplet structures (Fig. 5) due to the strong 4d–4f exchange interaction and the much weaker 4d spin–orbit interaction. As seen in Fig. 5, the Eu 4d spectra cannot be separated into 4d5/2 and 4d3/2 components. The 7 D final state multiplet splitting is not resolved, while in the case of the 9 D state the J = 2–6 components are easily recognized in the spectrum. For comparison, a high-resolution Eu 4d spectrum (hν = 165 eV) for atomic europium is shown (line), from ref. [17]. As mentioned above, the Eu 3d core level spectra of sputtered sample (Fig. 3) were taken at grazing emission (80◦ ), i.e. they are very surface sensitive. For this sputtered sample the surface to bulk sensitivity was varied by changing the electron emission angle. Eu 3d5/2 core level spectra of the sputtered PbS/EuS multilayer for different emission angles are shown in Fig. 6. The Eu3+ component strongly increases with increasing emission angle (angles are referred to the surface normal), but it still exists at normal emission (0◦ ). The angular behavior shows that Eu3+ is dominant near the outermost EuS surface/interface of the sample marking the europium valence change as a surface effect. Apparently, the sputtering of the sample changes the surface stoichiometry and leads to the transformation of the Eu2+ ions to Eu3+ at the sample surface. The high sensitivity of the surface stoichiometry on surface treatment (Ar ion sputtering, sample annealing) was also noticed in resonant PES investigations of EuS buried in galena [11], Pb1−x Eux Te layers [18], Eu2 Nb5 O9 and comparable metallic oxides [19]. In all these compounds the divalent europium is in the bulk and the trivalent species appear at the surface. This effect is opposite for trivalent europium-transition metal compounds where it was found that the surface valence changes from Eu3+ to Eu2+ [20].
0 1150
1140
1130
1120
Binding energy (eV) Fig. 6. Eu 3d5/2 core level spectra of the sputtered PbS/EuS multiplayer versus electron exit angle. Excitation energy hν = 3510 eV. After sputtering, the surface was exposed to the X-ray beam for about 4 h. The intensities have been normalized to the Eu2+ multiplet. The angular behavior clearly shows that Eu3+ is dominant near the outermost surface/ interface of the sample.
4. Summary The electronic structure of EuS buried in PbS was investigated by high-energy XPS using synchrotron X-rays at 3510 eV. The data obtained for the valence band, the Eu 3d and 4d core levels indicate that Eu is divalent in the EuS layer buried in galena. The sputtering by Ar ions as well as prolonged X-ray irradiation leads to the appearance of Eu3+ species. This trivalent europium contribution is mostly present at the outermost surface/interface of the sample. The divalent and trivalent Eu 3d spectra are in good agreement with calculated multiplet structures.
Acknowledgements This work was supported in part by: European Commission IHP-Contract HPRI CT199900040/2001-00140; and KBN SPUB-M/DESY/P-03/DZ-213/2000; and European Community programs: G1MA-CT-2002-4017 (Center of Excellence CEPHEUS) and ICAI-CT-2000-70018 (Center of Excellence CELDIS).
References [1] G.A. Prinz, Science 282 (1998) 1660. [2] H. Kˆepa, J. Kutner-Pielaszek, J. Blinowski, A. Twardowski, C.F. Majkrzak, T. Story, P. Kacman, R.R. Gal˛azka, K. Ha, H.J.M. de Jonge, A.Y. Sipatov, V. Volobuev, T.M. Giebutowicz, Europhys. Lett. 56 (1) (2001) 54. [3] H. Kˆepa, K.I. Goldman, T.M. Giebutowicz, C.F. Majkrzak, G. Springholz, H. Krenn, S. Holl, F. Schinagl, G. Bauer, Physica E 2 (1998) 399.
B.A. Orlowski et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 763–767 [4] J.J. Rhyne, J. Lin, J.K. Furdyna, T.M. Giebutowicz, J. Magn. Magn. Mater. 177–181 (1998) 1195. [5] V. Nunez, T.M. Giebutowicz, W. Faschinger, G. Bauer, H. Sitter, J.K. Furdyna, J. Magn. Magn. Mater. 140–144 (1995) 633. [6] E.-J. Cho, S.-J. Oh, Phys. Rev. B 59 (1999) R15613. [7] McFeely et al., Phys. Rev. B 7 (1973) 5228. [8] T. Grandke, L. Ley, M. Cordona, Phys. Rev. Lett. 38 (1977) 1033. [9] J. Muscat, C. Klauber, Surf. Sci. 491 (2001) 226. [10] R. Vercaemst, D. Poelman, L. Fiermans, R.L. Van Meirhaeghe, W.H. Lafrere, F. Cardon, J. Electron Spectrosc. Relat. Phenom. 74 (1995) 45. [11] B.A. Orlowski, E. Guziewicz, B.J. Kowalski, T. Story, S. Mickevièius, A.Y. Sipatov, M. Chernyshova, I. Demchenko, N. Barett, M. Taniguchi, A. Kimura, H. Sato, C.A. Sebenne, J.P. Lacharme, R. Medicherla, W. Drube, J. Alloys Comp. 362 (2004) 202. [12] Y. Ohno, T. Urata, J. Electron Spectrosc. Relat. Phenom. 125 (2002) 171.
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[13] K.H.J. Buscow, M. Campagna, G.K. Wertheim, Solid State Commun. 24 (1977) 253. [14] E.-J. Cho, S.-J. Oh, Phys. Rev. B 59 (1999) R15613. [15] E.-J. Cho, S.-J. Oh, S. Imada, S. Suga, T. Suzuki, T. Kasuya, Phys. Rev. B 51 (1995) 10146. [16] A. Kotani, H. Ogasawara, J. Electron Spectrosc. Relat. Phenom. 60 (1992) 257. [17] Ch. Gerth, K. Godehusen, M. Richter, P. Zimmermann, J. Schulz, Ph. Wernet, B. Sonntag, A.G. Kochur, I.D. Petrov, Phys. Rev. A 61 (2000) 022713. [18] R. Denecke, L. Ley, G. Springholz, G. Bauer, Phys. Rev. B 53 (1996) 4534. [19] C. Felser, J. Kõhler, A. Simon, O. Jepsen, G. Svensson, S. Cramm, W. Eberhardt, Phys. Rev. B 57 (1998) 1510. [20] W.D. Schneider, C. Laubschat, G. Kalkowski, J. Haase, A. Puschmann, Phys. Rev. B 28 (1983) 2017.
Photoemission study of EuS layers buried in PbS B.A. Orlowski a,∗ , S. Mickeviˇcius b , M. Chernyshova a , I. Demchenko a , A.Y. Sipatov c , T. Story a , V. Medicherla d , W. Drube d a
d
Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland b Semiconductor Physics Institute, A.Gostauto 11, 2600 Vilnius, Lithuania c Kharkov State Technical University, Frunze 21, 310002 Kharkov, Ukraine Hamburger SynchrotronstrahlungslaborHASYLAB am Deutschen Elektronen-Synchrotron DESY, Notkestr. 85, D-22603 Hamburg, Germany Available online 9 April 2004
Abstract High-energy photoemission spectroscopy (hν = 3510 eV) was used to study the electronic structure of EuS buried layers deposited on a thick PbS layer and covered by about 30 Å of PbS layer. The study of Eu 3d, 4d and valence band photoemission spectra showed the existence of only Eu2+ ions in the as-grown buried EuS layer. The angular behavior of the Eu 3d spectra revealed that Ar ion bombardment of the sample surface, i.e. initially the PbS cap layer, leads to a change of stoichiometry in the near surface region of the EuS layer leading to depletion of S and concomittant accumulation of Eu3+ at the surface. In addition, prolonged X-ray exposure is found to gradually increase the Eu3+ concentration in the surface region. The contribution of Eu2+ 4f electrons to the electronic structure of the PbS/EuS/PbS multilayer is found at the top of the valence band. © 2004 Elsevier B.V. All rights reserved. Keywords: Semiconductor buried layer; Lead and europium monosulfide; Synchrotron radiation; Photoemission spectroscopy
1. Introduction The problem of interlayer exchange coupling (IEC) between magnetic layers across non-magnetic layers is becoming important for potential application in spintronics, a new area of solid-state electronics [1]. Spin valves and spin injectors are the first practical applications of spintronics in which the key element is IEC in magnetic superlattices (SL). In the past several years IEC was observed in e.g. EuTe/PbTe, MnTe/ZnTe, MnTe/CdTe, GaMnAs/AlGaAs and EuS/PbS superlattices by means of neutron diffraction, neutron reflectivity and magnetization measurements [2–5]. The latter belong to an IV–VI narrow-gap semiconductor magnetic SL, where EuS is a large-gap magnetic semiconductor (Eg = 1.6 eV) with Curie temperature Tc = 16.5 K and PbS is narrow-gap (Eg = 0.30 eV) degenerated semiconductor. Both of them crystallize in the rock salt crystal structure and the lattice mismatch is about 0.6%. In the EuS/PbS multilayer system EuS layers correspond to barriers and the PbS layers to quantum well material.
∗
Corresponding author. Fax: +48-22-8430926. E-mail address: [email protected] (B.A. Orlowski).
0368-2048/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2004.02.096
It is well known that the surface electronic structure often differs from that in the bulk. The change in dimensionality sometimes induces a surface reconstruction which affects the local geometry and atomic coordination leading to changes in the valence band structure and producing surface core level shifts as observed by surface sensitive photoelectron spectroscopy [6]. Application of synchrotron radiation gives the possibility to tune the photoelectron escape depth via the kinetic energy allowing for a surface and bulk-sensitive detection. A minimum depth is obtained for electrons with a kinetic energy around 100 eV, where surface related effects dominate the spectra. Combined with high-energy resolution it enables detailed studies of surface valence and shallow core levels. In order to probe bulk properties or to access buried structures, high-energy photoemission can be effectively used. It offers additional advantages to distinguish features of the surface electronic structure from those related to the volume or originating from buried species. Because at high kinetic energies the electron mean free path is large, the probing depth can be varied by measuring EDCs versus the photoelectron exit angle. This can also be done using laboratory X-ray sources (hν <1500 eV), although the energies are often too low to reach true bulk sensitivity. In this study,
B.A. Orlowski et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 763–767
synchrotron X-rays at much higher energies (3510 eV) are used to significantly enhance the volume sensitivity for shallow core levels. At the same time, also deeper atomic levels can be accessed to obtain additional information on the electronic structure. Using synchrotron X-rays, the excitation energy may be varied over a large range. This is also advantageous in the study of high-Z materials, like Eu and Pb, because it allows to discriminate the core level photoemission from the rich structure of lines originating from Auger cascades. In this paper we utilize the volume sensitivity of highenergy X-ray photoelectron spectroscopy to study EuS layers buried in PbS in the EuS/PbS superlattice.
2. Experimental The EuS/PbS multilayers structures were grown on (1 0 0) surfaces of the substrate KCl, using an e-beam for the evaporation of EuS and a thermal source for PbS. The standard composition of the samples was 20–30 Å of PbS (as a cap layer) on top of 10–80 Å of EuS deposited on 700–1000 Å of a PbS buffer. The data were obtained with the Tunable High-Energy X-ray Photoelectron Spectrometer at the X-ray wiggler beam line BW2 of HASYLAB (Hamburg). The photon flux from the Si(1 1 1) double crystal monochromator typically amounts to 5 × 1012 s−1 on the sample. The electrons were measured using a hemispherical analyzer with parallel detection capability (SCIENTA SES-200). An excitation energy was chosen (3510 eV) where the kinetic energy of the Eu 3d levels is not overlapping any Pb Auger electron emission originating from the PbS layers. This was checked by EDCs covering a wide energy range. The total energy resolution at this photon energy is 0.6 eV. The binding energy scale was calibrated using the Pb 5d5/2 photoemission from PbS at 18.52 eV [7]. The angle dependent data were obtained by rotating the sample relative to the electron analyzer, which accepts electrons in the SR orbit plane at 45◦ relative to the incoming beam. Moderate Ar+ sputtering was used to obtain clean EuS or PbS top layer surface in a UHV preparation chamber. Some samples were measured “as introduced”, i.e. without any surface treatment. The base pressure in the analyzer chamber was 5 × 10−10 mbar.
3. Results and discussion The Pb 5d core level and valence band spectra of PbS/EuS multilayers without sputtering taken for normal emission angle (0◦ ) are shown in Fig. 1. The inset shows the enlarged valence band region (solid line) compared to a spectrum recorded at a grazing emission angle (70◦ relative to the surface normal, circles). The spectra taken for grazing emission show features closely resembling spectra of galena crystals taken by UPS and XPS [7–9]. At normal emission
2.5
1.0 Normalized intensity (a.u.)
764
PbS/EuS
PbS/EuS
normal emission
2.0
grazing emission 1.5
Pb 5d 1.0
0.5 0.5 difference (EuS) 0.0 −15
−10 −5 Binding energy (eV)
0
0.0 25
20
15
10
5
0
Binding energy (eV)
Fig. 1. Pb 5d core level and valence band spectra of a PbS/EuS multilayer obtained for an as-introduced sample (no surface treatment, normal emission geometry, hν = 3510 eV). The inset compares the valence band region recorded at normal emission (solid line) and grazing emission angles (circles). The difference curve reveals the contribution of the buried EuS layer to the valence band structure.
(solid line) the signal of EuS buried in galena is enhanced because of the increased bulk sensitivity. The spectra have been normalized to the galena emission below 5 eV binding energy. The numerical difference of these two spectra (bottom curve in the inset of Fig. 1) can be expected to reflect the buried EuS layer in PbS. The pronounced peak located around 1 eV below the Fermi level can be associated with the Eu2+ 4f final state multiplet contribution to the valence band. Usually Eu in bulk EuS is divalent [10–13] and this structure is observed near the valence band maximum, while the Eu3+ multiplet structure in Eu compounds normally is located between 5 and 11 eV. In the difference spectra revealing the EuS buried layer no significant structure in this region is observed (see inset in Fig. 1). This result is in good agreement with resonant PES investigation of PbS/EuS multilayers [11]. The width of the Eu2+ derived maximum is strongly dependent on the final state multiplet splitting as well as possible 4f ligand hybridization. The electronic structure up to 4.5 eV binding energy can be attributed to the valence band because of overlapping Eu2+ 4f and S 3p occupied electronic states. The conclusion that Eu in the EuS layer buried in PbS is in the divalent state is also confirmed by inspection of the Eu 3d and 4d core level spectra. Fig. 2 shows the Eu 3d core level spectra from the buried layer of an unsputtered, i.e. as-introduced, sample taken at normal emission (circles) compared to the divalent Eu 3d multiplet structure of the 3d9 4f7 final state (solid line). This was derived from spectra of the sputtered sample which exhibits both 2+ and 3+ valences (see Figs. 3 and 4). The Eu 3d spectra in Fig. 2 can be divided into 3d5/2 and 3d3/2 structures due to the large spin–orbit interaction of the Eu 3d core level. The Eu 3d9 4f7 final state multiplet structure appears as a double-peak structure with a strong main line and a small separate peak. The latter appears roughly at 6.8 eV higher binding energy
B.A. Orlowski et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 763–767
765
2.0
Normalized intensity (arb. units)
Eu 3+
Fig. 2. Eu 3d core level spectra from the buried EuS layer: unsputtered PbS/EuS multilayer sample measured at normal emission (circles) and “pure” divalent Eu 3d multiplet structure (solid line, cf. Fig. 4). Excitation energy hν = 3510 eV.
relative to the 3d5/2 main peak and is labeled as “separ.” in Fig. 2. The comparison of the two spectra clearly shows that Eu in the buried EuS layer is divalent. The broad extra “bumps” marked by arrows (located 15.2 eV below the Eu 3d peaks) are likely due to PbS bulk plasmon excitation since the fast Eu 3d electrons have to penetrate the PbS top layer. Fig. 3 shows two Eu 3d core level spectra of a sputtered sample of PbS/EuS multilayers: “fresh”, i.e. recorded immediately after sputtering (solid line) and after X-ray irradiation of about 20 h (circles). Both spectra were taken at grazing emission (80◦ from the surface normal) to enhance the surface sensitivity. The initial PbS cap layer is likely removed to a large extent after sputtering, so that the near surface region consists mainly of EuS. Clearly, two multiplet structures are
Fig. 3. Eu 3d core level spectra of the sputtered PbS/EuS multilayer: “fresh”: measured immediately after sputtering (solid line) and after X-ray irradiation of about 20 h (circles). Excitation energy hν = 3510 eV. Both spectra were taken at grazing emission angle (80◦ ).
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Binding energy (eV) Fig. 4. Eu 3d core level spectra of the sputtered PbS/EuS multilayer. Excitation energy hν = 3510 eV. The “pure” Eu2+ and Eu3+ 3d core level spectra were derived from linear combinations of normal emission and grazing emission (80◦ ) spectra. The middle curve is a calculation of the Eu3+ spectrum, reproduced from [14].
present which correspond to Eu2+ (3d9 4f7 final state) and Eu3+ (3d9 4f6 final state). This implies that the sputtering of the PbS/EuS sample “generates” the trivalent Eu species which is then steadily increasing upon X-ray exposure. It is mentioned that no radiation effect was observed for the buried EuS layer, at least for the time of the measurement (several hours). Fig. 4 presents the Eu 3d core level spectra recorded from the sputtered sample of PbS/EuS (like in Fig. 3) which are separated into the “pure” Eu2+ and “pure” Eu3+ components. This was achieved by a numerical linear combination of normal (0◦ ) and grazing (80◦ ) emission spectra which exhibit a significantly different weighting of the 2+ and 3+ contribution because 3+ is accumulated in the surface region. The middle curve (line) corresponds to a calculated Eu3+ spectrum, reproduced from ref. [14]. The overall agreement between the experimental and calculated multiplet structure clearly is very good. The Eu3+ 3d multiplet structure consists of two peaks like in the Eu2+ case (see Figs. 2–4) in which the separate peak appears approximately at 8.5 eV below the 3d5/2 main peak. A comparison between the divalent and trivalent components shows that the separate peak of Eu2+ partly overlaps the Eu3+ main peak (see Figs. 3 and 4). Both the Eu2+ and Eu3+ 3d multiplet peak structures in terms of line shape and energy position are in good agreement with theoretical calculations and other experimental results of the compounds with divalent and trivalent europium configurations [12–16]. The relevant process creating this double-peak structure is the exchange of 4f electrons with 3d electrons in the final state due to the electrostatic interaction, since the spin–orbit interaction of the 4f electrons is rather small.
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Fig. 5. Eu2+ 4d core level spectra of the sputtered PbS/EuS multilayer (circles). Excitation energy hν = 3510 eV. For comparison, the solid line shows high-resolution Eu 4d core level spectra of atomic Eu, obtained at 165 eV (reproduced from [17]).
The Eu 4d core level shows very complicated multiplet structures (Fig. 5) due to the strong 4d–4f exchange interaction and the much weaker 4d spin–orbit interaction. As seen in Fig. 5, the Eu 4d spectra cannot be separated into 4d5/2 and 4d3/2 components. The 7 D final state multiplet splitting is not resolved, while in the case of the 9 D state the J = 2–6 components are easily recognized in the spectrum. For comparison, a high-resolution Eu 4d spectrum (hν = 165 eV) for atomic europium is shown (line), from ref. [17]. As mentioned above, the Eu 3d core level spectra of sputtered sample (Fig. 3) were taken at grazing emission (80◦ ), i.e. they are very surface sensitive. For this sputtered sample the surface to bulk sensitivity was varied by changing the electron emission angle. Eu 3d5/2 core level spectra of the sputtered PbS/EuS multilayer for different emission angles are shown in Fig. 6. The Eu3+ component strongly increases with increasing emission angle (angles are referred to the surface normal), but it still exists at normal emission (0◦ ). The angular behavior shows that Eu3+ is dominant near the outermost EuS surface/interface of the sample marking the europium valence change as a surface effect. Apparently, the sputtering of the sample changes the surface stoichiometry and leads to the transformation of the Eu2+ ions to Eu3+ at the sample surface. The high sensitivity of the surface stoichiometry on surface treatment (Ar ion sputtering, sample annealing) was also noticed in resonant PES investigations of EuS buried in galena [11], Pb1−x Eux Te layers [18], Eu2 Nb5 O9 and comparable metallic oxides [19]. In all these compounds the divalent europium is in the bulk and the trivalent species appear at the surface. This effect is opposite for trivalent europium-transition metal compounds where it was found that the surface valence changes from Eu3+ to Eu2+ [20].
0 1150
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Binding energy (eV) Fig. 6. Eu 3d5/2 core level spectra of the sputtered PbS/EuS multiplayer versus electron exit angle. Excitation energy hν = 3510 eV. After sputtering, the surface was exposed to the X-ray beam for about 4 h. The intensities have been normalized to the Eu2+ multiplet. The angular behavior clearly shows that Eu3+ is dominant near the outermost surface/ interface of the sample.
4. Summary The electronic structure of EuS buried in PbS was investigated by high-energy XPS using synchrotron X-rays at 3510 eV. The data obtained for the valence band, the Eu 3d and 4d core levels indicate that Eu is divalent in the EuS layer buried in galena. The sputtering by Ar ions as well as prolonged X-ray irradiation leads to the appearance of Eu3+ species. This trivalent europium contribution is mostly present at the outermost surface/interface of the sample. The divalent and trivalent Eu 3d spectra are in good agreement with calculated multiplet structures.
Acknowledgements This work was supported in part by: European Commission IHP-Contract HPRI CT199900040/2001-00140; and KBN SPUB-M/DESY/P-03/DZ-213/2000; and European Community programs: G1MA-CT-2002-4017 (Center of Excellence CEPHEUS) and ICAI-CT-2000-70018 (Center of Excellence CELDIS).
References [1] G.A. Prinz, Science 282 (1998) 1660. [2] H. Kˆepa, J. Kutner-Pielaszek, J. Blinowski, A. Twardowski, C.F. Majkrzak, T. Story, P. Kacman, R.R. Gal˛azka, K. Ha, H.J.M. de Jonge, A.Y. Sipatov, V. Volobuev, T.M. Giebutowicz, Europhys. Lett. 56 (1) (2001) 54. [3] H. Kˆepa, K.I. Goldman, T.M. Giebutowicz, C.F. Majkrzak, G. Springholz, H. Krenn, S. Holl, F. Schinagl, G. Bauer, Physica E 2 (1998) 399.
B.A. Orlowski et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 763–767 [4] J.J. Rhyne, J. Lin, J.K. Furdyna, T.M. Giebutowicz, J. Magn. Magn. Mater. 177–181 (1998) 1195. [5] V. Nunez, T.M. Giebutowicz, W. Faschinger, G. Bauer, H. Sitter, J.K. Furdyna, J. Magn. Magn. Mater. 140–144 (1995) 633. [6] E.-J. Cho, S.-J. Oh, Phys. Rev. B 59 (1999) R15613. [7] McFeely et al., Phys. Rev. B 7 (1973) 5228. [8] T. Grandke, L. Ley, M. Cordona, Phys. Rev. Lett. 38 (1977) 1033. [9] J. Muscat, C. Klauber, Surf. Sci. 491 (2001) 226. [10] R. Vercaemst, D. Poelman, L. Fiermans, R.L. Van Meirhaeghe, W.H. Lafrere, F. Cardon, J. Electron Spectrosc. Relat. Phenom. 74 (1995) 45. [11] B.A. Orlowski, E. Guziewicz, B.J. Kowalski, T. Story, S. Mickevièius, A.Y. Sipatov, M. Chernyshova, I. Demchenko, N. Barett, M. Taniguchi, A. Kimura, H. Sato, C.A. Sebenne, J.P. Lacharme, R. Medicherla, W. Drube, J. Alloys Comp. 362 (2004) 202. [12] Y. Ohno, T. Urata, J. Electron Spectrosc. Relat. Phenom. 125 (2002) 171.
767
[13] K.H.J. Buscow, M. Campagna, G.K. Wertheim, Solid State Commun. 24 (1977) 253. [14] E.-J. Cho, S.-J. Oh, Phys. Rev. B 59 (1999) R15613. [15] E.-J. Cho, S.-J. Oh, S. Imada, S. Suga, T. Suzuki, T. Kasuya, Phys. Rev. B 51 (1995) 10146. [16] A. Kotani, H. Ogasawara, J. Electron Spectrosc. Relat. Phenom. 60 (1992) 257. [17] Ch. Gerth, K. Godehusen, M. Richter, P. Zimmermann, J. Schulz, Ph. Wernet, B. Sonntag, A.G. Kochur, I.D. Petrov, Phys. Rev. A 61 (2000) 022713. [18] R. Denecke, L. Ley, G. Springholz, G. Bauer, Phys. Rev. B 53 (1996) 4534. [19] C. Felser, J. Kõhler, A. Simon, O. Jepsen, G. Svensson, S. Cramm, W. Eberhardt, Phys. Rev. B 57 (1998) 1510. [20] W.D. Schneider, C. Laubschat, G. Kalkowski, J. Haase, A. Puschmann, Phys. Rev. B 28 (1983) 2017.