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Photoemission study of amorphous and crystalline GeTe and (Ge,Mn)Te semiconductors
Radiation Physics and Chemistry 93 (2013) 77–81
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Photoemission study of amorphous and crystalline GeTe and (Ge,Mn)Te semiconductors W. Knoff a,n, M.A. Pietrzyk a, A. Reszka a, B.J. Kowalski a, B. Taliashvili a, T. Story a, R.L. Johnson b, B.A. Orłowski a a b
Institute of Physics, Polish Academy of Sciences, al. Lotników 32/46, PL-02-668 Warsaw, Poland Institute of Experimental Physics, University of Hamburg, Luruper Chaussee 149, D-22761 Hamburg, Germany
H I G H L I G H T S
Monocrystalline and amorphous layers of GeTe and (Ge,Mn)Te were grown on BaF2 (111) monocrystalline substrates by molecular beam epitaxy technique. The electronic structure of amorphous and monocrystalline GeTe and (Ge,Mn)Te layers was studied by resonant photoemission spectrosocopy (RPES) technique. Configuration interaction analysis reveals stronger p–d hybridization of electronic states in monocrystalline (Ge,Mn)Te layers.
art ic l e i nf o
a b s t r a c t
Article history: Received 20 June 2012 Accepted 29 May 2013 Available online 22 June 2013
Resonant photoemission spectroscopy was applied to compare the valence band structure of Ge0.9Mn0.1Te and GeTe semiconductor layers deposited on BaF2 substrate in monocrystalline and amorphous forms. In (Ge,Mn)Te the contribution of Mn 3d5 electronic orbitals to density of states was found in three binding energy regions: below the top of the valence band (Eb o 4.2 eV), at the binding energy range 4.2–4.4 eV, and in many-body satellite at 9–13 eV. The comparative analysis of the photoemission spectra based on configuration interaction model showed that p–d hybridization effects, important for magnetic and optical properties of (Ge,Mn)Te, are stronger in monocrystalline than in amorphous (Ge,Mn)Te layers. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Resonant photoemission Germanium telluride Diluted magnetic semiconductors.
1. Introduction GeTe is a IV–VI narrow-gap semiconductor compound which below the temperature of about 700 K has the crystal structure of rhombohedral symmetry. It can be viewed as the cubic (rock-salt) crystal lattice slightly elongated in body diagonal [111] direction with mutually shifted anion and cation sublattices (Johnston and Sestrich, 1961). It results in ferroelectric (FE) properties of rhombohedral GeTe crystals. These properties are also observed in (Ge,Mn)Te mixed crystals but the FE transition temperature decreases with the increasing Mn concentration. In (Ge,Mn)Te crystals with Mn content of about 20 at% the FE transition takes place close to room temperature. (Ge,Mn)Te is also known as a diluted magnetic (semimagnetic) semiconductor in which magnetic Mn2+ ions substitute Ge2+ ions at cation sublattice sites. This substitution is electrically isovalent but produces the local magnetic moments of 5 Bohr magnetons (3d5 electronic configuration with S ¼ 5/2 n
Corresponding author. Tel.: +48 22 843 66 01x3287. E-mail address: [email protected] (W. Knoff).
0969-806X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2013.05.059
spin state). Carrier-induced ferromagnetic (FM) transition takes place in (Ge,Mn)Te crystals with the ferromagnetic Curie temperature TC depending both on conducting hole concentration p and Mn content x. Due to very high carrier concentration, p¼1020−1021 cm−3, induced in Ge1−xMnxTe by electrically active native defects (Ge vacancies), Ge1−xMnxTe shows relatively high Curie temperatures up to TC ¼ 150 K in bulk polycrystals, (Cochrane et al., 1973) and TC ¼190 K in thin layers (Hassan et al., 2011). Similarly to ferromagnetic properties of previously studied IV–VI semiconductors with Mn, such as Sn1−xMnxTe and Pb1−x−ySnyMnxTe mixed crystals, the ferromagnetic exchange interactions in Ge1−xMnxTe are driven by the Ruderman–Kittel–Kasuya–Yosida (RKKY) indirect exchange mechanism via conducting holes (Story et al., 1992; Eggenkamp et al., 1995; Szałowski and Balcerzak, 2008). The key electronic parameter determining the strength of this interaction is the p-d exchange coupling Jpd between the spins of valence-band carriers and the localized magnetic moments of 3d5 electrons of Mn2+ ions. The experimentally observed relatively large value of this parameter in (Ge,Mn)Te (Jpd ¼0.4–0.8 eV) is believed to originate from the kinetic exchange mechanism that involves the hybridization of valence band and Mn 3d electronic states.
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W. Knoff et al. / Radiation Physics and Chemistry 93 (2013) 77–81
Amorphous a-(Ge,Mn)Te semiconductor shows disorder-induced insulating electrical properties and is a paramagnetic material due to the lack of delocalized exchange-coupling-mediating conducting holes. Applying annealing process, globally or locally, e.g. by using laser or electron beam, one can rapidly (at nanosecond scale) transform amorphous phase to polycrystalline (metallic) one. This structural transition is accompanied by a paramagnet-toferromagnet transition (Fukuma et al., 2001; Knoff et al., 2011). For microscopic understanding of magnetic, optical and electric properties of (Ge,Mn)Te semiconductors the comparative experimental study of the influence of Mn ions on density of states in (Ge,Mn)Te in crystalline and amorphous form is of key importance. Although amorphous materials reveal only short-range chemical bonding order, the valence band of amorphous GeTe (a-GeTe) with a characteristic three-peak structure due to Ge 4p, Te 5p states (0– 5 eV), Ge 4s (8.3 eV), and Te 5s (12.4 eV) is similar to the case of monocrystalline GeTe (m-GeTe). The experimentally observed slight shift of photoemission spectra of amorphous layers to higher binding energies is interpreted as resulting from a slight redistribution of Ge and Te ions due to the small change in chemical bonding (Shevchik et al., 1973; Kim et al., 2007), however a difference of the work functions or charging effect cannot be excluded. The incorporation of Mn ions in GeTe matrix markedly changes the electronic structure in the valence band region. The photoemission transitions involving Mn 3d5 states are superimposed on the three-peak structure observed in GeTe. The purpose of our work is to compare the electronic structure of amorphous and monocrystalline GeTe and (Ge,Mn)Te layers by experimentally studying the photoemission spectra in the valence band region. To experimentally reveal the contribution of Mn 3d states in a-(Ge,Mn)Te and m-(Ge,Mn)Te we apply resonant photoemission spectroscopy (RPES). In absorption region of 3p–3d transitions involving Mn ions with half-filled 3d shell, two electronic transitions caused by the incident photon flux take place: regular photoemission process, in which the final state belongs to the continuum of states: Mn3p63d5+hv-Mn3p63d4+e−, and the accompanying intra-ion excitation process: 3p63dn+hν ¼ [3p53dn+1]*, where []* indicates an excited electronic state. The quantum interference between these two processes leads to autoionization [3p53dn+1]n3p63dn−1+e− and resonant photoemission.
2. Sample preparation and experimental techniques GeTe and (Ge,Mn)Te layers with 10 at% of Mn content were grown on BaF2 (111) monocrystalline substrates by molecular
beam epitaxy (MBE) technique employing effusion cells with GeTe, Te, and Mn solid sources. The thickness of layers was about 1 mm. During the growth of monocrystalline m-GeTe and m-(Ge,Mn)Te layers the substrate temperature was kept in the range T S ¼250–300 1C. The RHEED electron diffraction in situ growth control revealed a streaky pattern characteristic of a two-dimensional mode of growth. The X-ray diffraction (XRD) measurements performed at ambient temperature showed the monocrystalline (111)-oriented rhombohedral structure of these layers and the XRD rocking curve width parameter in the range 400–600″. Amorphous a-GeTe and a-(Ge,Mn)Te layers were deposited onto BaF 2 substrate in the same MBE facility but with the substrate kept at room temperature. We achieved completely amorphous state in both a-GeTe and a-(Ge,Mn)Te layers, as confirmed by the lack of any XRD diffraction peaks. The chemical composition of samples was examined by the energy dispersive X-Ray fluorescence method (EDXRF) yielding Mn content about 10 at% for both amorphous and monocrystalline layers. Magnetic properties of (Ge,Mn)Te layers were examined by the superconducting quantum interference device (SQUID) magnetometer in the temperature range of T ¼ 5–120 K. In monocrystalline GeMnTe layers we found the ferromagnetic transition at the Curie temperature T C ¼35 K, whereas in amorphous layers only a paramagnetic phase was observed. The valence band of GeTe and (Ge,Mn)Te was studied by the photoemission spectroscopy method in synchrotron radiation facility in HASYLAB (Hamburg). We used the experimental station FLIPPER II with VUV and soft X-ray plane-grating monochromator covering the photon energy range from 10– 150 eV. Our experiments consisted of two complementary parts. The first one concerned the valence band measurements carried out on GeTe and (Ge,Mn)Te semiconductor layers in photon energy range hν ¼47–60 eV, including Mn 3p–3d Fanotype resonance photoemission. In the second part the corelevel photoemission spectroscopic studies were performed. The experimentally determined binding energies are referred to the Fermi level of a thick tantalum sample. To remove oxides and other contamination from the surface the cleaning process was performed in situ by argon (Ar +) ion sputtering. Annealing at T ¼250 1C was applied only to monocrystalline samples. This procedure was skipped for amorphous samples to avoid unintentional structural transformation to polycrystalline form. The main features of the spectra were not influenced by these procedures, as checked experimentally after each cleaning step.
Fig. 1. (a) Valence band photoemission spectra of amorphous and monocrystalline GeTe layers at photon energy hv¼ 60 eV and (b) photoemission spectra in the core absorption region of amorphous and monocrystalline GeTe layers for hv¼ 80 eV.
W. Knoff et al. / Radiation Physics and Chemistry 93 (2013) 77–81
3. Results and discussion In order to experimentally study the valence band region (Eb ¼ 0–15 eV) in GeTe and (Ge,Mn)Te, the photoemission spectra were acquired in photon energy range hν ¼47–60 eV. Photoemission signal intensity was normalized to photon flux and the secondary electron background was subtracted by means of the Shirley method (Shirley, 1972). In GeTe, the three-peak structure (Fig. 1a) is observed in the valence band region. The maxima of the energy distribution curve were found in m-GeTe at the binding energies of 1.7, 7.0, and 11.9 eV. In a-GeTe, similar density of states spectrum was experimentally observed, with a small shift to higher binding energies of the valence band near the Fermi level. Photoemission measurements in the core absorption region performed at photon excitation energy hν ¼80 eV revealed the Ge 3d and Te 4d core levels in a-GeTe and m-GeTe (Fig. 1b). Comparing to monocrystalline layers, the small shift (up to +0.2 eV) of Ge and Te peaks was observed in amorphous materials, but the spectral structures of Ge and Te core-peaks are qualitatively similar. This experimental finding was discussed in terms of differences between amorphous and monocrystalline materials caused by the redistribution of Ge and Te atoms (Shevchik et al., 1973). However, charging of amorphous sample and accompanying shift of spectra due to its insulating properties cannot be excluded.
Due to the incorporation of Mn ions in GeTe lattice, additional contribution of Mn 3d states appears in the electronic structure (Fig. 2a and b), as experimentally found in (Ge,Mn)Te bulk polycrystals (Pietrzyk et al., 2009). The constant-initial-state (CIS) spectrum (derived from the spectra of Fig. 2a and b) in Mn 3p–3d absorption region (hν¼47–60 eV) is shown in Fig. 2c. The constant initial energy Ei corresponds to the main peak of Mn 3d states. The CIS spectra uncover resonant photoemission process with the characteristic Fano-type profile. With the photon energy increasing in the range hν¼ 47–60 eV, the intensity of the main peak reveals minimum related to the anti-resonance of Mn2+ ion transitions at hv¼48 eV in amorphous and monocrystalline (Ge,Mn)Te layers. Above this energy, the intensity of the main peak reaches the maximum at hv¼ 51 eV related to the resonance due to intra-ion Mn 3p–3d transition and then gradually decreases with further increase of photon energy. The Mn states contribution covers the whole valence band region and the main maximum is observed at Eb ¼4.2 eV in the monocrystalline sample and at Eb ¼4.4 eV in a-(Ge,Mn)Te (Fig. 2d). The feature at higher binding energies in amorphous layer is rather broad in the binding energy range Eb ¼9.3–13.6 eV, as compared to monocrystalline layer (Eb ¼10.2–13.6 eV). The changes in the spectral maximum intensity (at 4.2 eV in monocrystalline layer and at 4.4 eV in amorphous layer) enabled us to clearly identify the curves taken at the resonance and the anti-resonance energies (51 and 48 eV, respectively).
m -(Ge,Mn)Te
a - (Ge,Mn)Te
h ν (eV):
h ν (eV) : 60
60
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Intensity (arb.u.)
Intensity (arb.u.)
57 54 53 52 51
54 53 52 51 50
50 49
49
48
48
47
47
0
20 18
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4 6 8 10 12 14 Binding Energy (eV)
0
2 4 6 8 10 12 14 Binding Energy (eV)
1.2 m - (Ge,Mn)Te a - (Ge,Mn)Te
m - (Ge,Mn)Te a - (Ge,Mn)Te
1.0
hv= 51 eV
14 12 Ei = 4.2 eV
8 6
Ei = 4.37 eV
4 2
Intensity (arb.u.)
Intensity (arb.u.)
16
10
79
0.8 0.6 0.4 0.2 0.0
46 48 50 52 54 56 58 60 62 Photon Energy hν (eV)
0
2 4 6 8 10 12 14 Binding Energy (eV)
Fig. 2. Photoemission spectra of amorphous (a) and monocrystalline (b) (Ge,Mn)Te layers obtained in photon energy range 47–60 eV corresponding to Mn 3p–3d core absorption region, (c) CIS spectrum for the initial state energy Ei ¼4.2 eV for monocrystalline and Ei ¼ 4.37 eV for amorphous layers, and (d) comparison of the two spectra obtained at Mn on-resonance photon energy.
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14
qualitative conclusions based on intensity ratios are valid and clearly point that in monocrystalline (Ge,Mn)Te layer the p–d hybridization is stronger than in amorphous one. This difference in hybridization strength is of importance, e.g. for Mn-carrier exchange integral Jpd and ferromagnetic Curie temperature of (Ge,Mn)Te. One has to note, however, that the dramatic difference in magnetic properties of crystalline and amorphous (Ge,Mn)Te layers is mainly due to their qualitatively different electric properties, as discussed in the introduction.
a - (Ge,Mn)Te
ΔEDC (51 - 48 eV)
12 10 8 M
6
S
4 2
V
0 0
2
4. Conclusions 4
6
8
10
12
14
We experimentally studied the electronic structure of GeTe and (Ge,Mn)Te layers grown on BaF2 (111) substrate in monocrystalline and in amorphous forms. Applying the resonant photoemission method at photon energy range corresponding to 3p–3d Fano-type resonance we identified the main contributions of Mn 3d5 electrons to electronic states in the valence band region. Our analysis of the integrated intensities of three Mn-related spectral features observed experimentally in amorphous and in monocrystalline samples confirmed that the hybridization of Mn 3d states and the valence band states is in monocrystalline layer stronger than in amorphous one.
ΔEDC (51-48 eV)
8 m - (Ge,Mn)Te
6
M
4
S
2 V 0 0
2
4
6 8 10 Binding Energy (eV)
12
14
Fig. 3. The resonance-antiresonance difference curve of amorphous (a) and monocrystalline (b) (Ge,Mn)Te layer. Dashed line corresponds to fitted Gaussian peaks related to electron emission from 3d states to d5L final states (main peak denoted as M). Dotted and dash-dotted lines corresponds to fitted Gaussian functions related to valence band (V) and satellite (S) contribution, respectively. The sum of all Gaussian functions is shown by the solid line.
The difference photoemission spectrum ΔEDC obtained by subtracting the anti-resonance spectrum from its resonance counterpart is shown in Fig. 3. In order to study the strength of p–d hybridization from ΔEDC spectra we used the configurationinteraction (CI) cluster model developed for MnTe6 cluster with central Mn ion and six Te ligands (Kaznacheyev et al., 1998). Due to similarities between the spectra in 3p–3d excitation region obtained from calculated differential ΔEDC spectra of MnTe6 cluster and experimentally studied m-Ge1−xMnxTe (Senba et al., 2005), one can suggest that the main Mn-related structure (M) originates from electron emission from 3d states with d5L final states (here L denotes ligand hole). At lower binding energies Eb o4.2 eV (referred as valence band contribution V), electron final states were found to be in d5L and d6L2 electronic configurations. The satellite peak (S) located at higher binding energies (Eb ¼6–12 eV) is related to photoemission with d4 final state (Kaznacheyev et al., 1998; Pietrzyk et al., 2007; Pietrzyk et al., 2009). To semi-quantitatively analyze the experimentally observed photoemission difference ΔEDC spectra we carried out fitting procedure that reproduced well the whole spectrum by taking into account several contribution of Gaussian shape (Fig. 3). We identified the main V, M and S spectral features discussed above and determined their integrated intensities IV, IM, and IS. In the spirit of the CI model we analyze the experimentally observed integrated intensity ratios IV/IM and IS/IM. It is expected that with the increasing strength of 3p–3d hybridization the ratio IV/IM increases whereas the ratio IS/IM decreases. In amorphous (Ge, Mn)Te layer we found experimentally: IV/IM ¼0.15 and IS/IM ¼1.74, whereas for monocrystalline layer: IV/IM ¼0.23 and IS/IM ¼1.30. Although such quantitative analysis of spectral intensities is subject to rather large experimental uncertainty, we expect that
Acknowledgments The authors acknowledge the support by the Ministry of Science and Higher Education (Poland) research project DESY/68/ 2007 and by the European Community via the Research Infrastructure Action under the FP6 Structuring the European Research Area” Programme (through the Integrated Infrastructure Initiative “Integrating Activity on Synchrotron and Free Electron Laser Science”) at DESY. This work was also supported by the European Union within the European Regional Development Fund, through an Innovative Economy grants POIG.01.01.02-00-108/09 and POIG.01.01.02-00-008/08 and by the National Science Center NCN (Poland) research project UMO 2011/01/B/ST3/02486. References Cochrane, R.W., Hedgcock, F.T., Ström-Olsen, J.O., 1973. Exchange scattering in ferromagnetic semiconductor. Phys. Rev. B 8, 4262. Eggenkamp, P.J.T., Swagten, H.J.M., Story, T., Litvinov, V.I., de Jonge, W.J.M., 1995. Calculation of the ferromagnet-to-spin-glass transition in diluted magnetic systems with an RKKY mechanism. Phys. Rev. B 51, 15250. Fukuma, Y., Nishimura, N., Asada, H., Koyanagi, T., 2001. Appearance of ferromagnetism by crystallizing a-Ge1−xMnxTe. Physica E 10, 268. Hassan, M., Springholz, G., Lechner, R.T., Groiss, H., Kirchschlager, R., Bauer, G., 2011. Molecular beam epitaxy of single phase GeMnTe with high ferromagnetic transition temperature. J. Cryst. Growth 323, 363–367. Johnston, W.D., Sestrich, D.E., 1961. The MnTe–GeTe phase diagram. J. Inorg. Nucl. Chem 19, 229. Kaznacheyev, K.V., Muro, T., Matsushita, T., Iwasaki, T., Kuwata, Y., Harada, H., Suga, S., Ishii, H., Miyahara, T., Mizokawa, T., Fujimori, A., Harada, T., Kanomata, T., 1998. Electronic structure of pyrite-type MnTe2 studied by electron spectroscopy. Phys. Rev. B 58, 13491. Kim, J.-J., Kobayashi, K., Ikenaga, E., Kobata, M., Ueda, S., Matsunaga, T, Kifune, K., Kojima, R., Yamada., N., 2007. Electronic structure of amorphous and crystalline (GeTe)1−x(Sb2Te3)x investigated using hard X-ray photoemission spectroscopy. Phys. Rev. B 76, 115124. Knoff, W., Świątek, K., Andrearczyk, T., Domukhovski, V., Dziawa, P., Kowalczyk, L., Łusakowska, E., Šiušys, A., Taliashvili, B., Wróbel, J., Story, T., 2011. Magnetic anisotropy of semiconductor (Ge,Mn)Te microstructures produced by laser and electron beam induced crystallization. Phys. Status Solidi (b) 248, 1605. Pietrzyk, M.A., Kowalski, B.J., Orlowski, B.A., Knoff, W., Osinniy, V., Kowalik, I.A., Story, T., Johnson, R.L., 2007. Photoemission study of Mn 3d electrons in the valence band of Mn/GeMnTe. Acta Phys. Pol. A 112, 275–281. Pietrzyk, M.A., Kowalski, B.J., Orłowski, B.A., Dziawa, P., Knoff, W., Osinniy, V., Kowalik, I.A., Dobrowolski, W., Slynko, V.E., Slynko, E.I., Johnson, R.L., 2009. Electronic structure of bulk ferromagnetic Ge0.86Mn0.14Te. Radiat. Phys. Chem. 78, S17–S21.
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Senba, S., Fujimoto, K., Sato, H., Fukuma, Y., Nakatake, M., Arita, M., Tsuji, K., Asada, H., Koyanagi, T., Namatame, H., Taniguchi, M., 2005. Mn 3p–3d resonant photoemission spectroscopy of ferromagnetic semiconductor Ge1−xMnxTe. Electron. Spectrosc. Relat. Phenom. 144-147, 629. Shevchik, N.J., Tejeda, J., Langer, D.W., Cardona, M., 1973. Similarities in the valence bands of amorphous and crystalline GeTe determined by X-ray photoemission. Phys. Rev. Lett. 30, 659–662. Shirley, D., 1972. High-resolution X-Ray photoemission spectrum of the valence bands of gold. Phys. Rev. B 12, 4709.
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Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Photoemission study of amorphous and crystalline GeTe and (Ge,Mn)Te semiconductors W. Knoff a,n, M.A. Pietrzyk a, A. Reszka a, B.J. Kowalski a, B. Taliashvili a, T. Story a, R.L. Johnson b, B.A. Orłowski a a b
Institute of Physics, Polish Academy of Sciences, al. Lotników 32/46, PL-02-668 Warsaw, Poland Institute of Experimental Physics, University of Hamburg, Luruper Chaussee 149, D-22761 Hamburg, Germany
H I G H L I G H T S
Monocrystalline and amorphous layers of GeTe and (Ge,Mn)Te were grown on BaF2 (111) monocrystalline substrates by molecular beam epitaxy technique. The electronic structure of amorphous and monocrystalline GeTe and (Ge,Mn)Te layers was studied by resonant photoemission spectrosocopy (RPES) technique. Configuration interaction analysis reveals stronger p–d hybridization of electronic states in monocrystalline (Ge,Mn)Te layers.
art ic l e i nf o
a b s t r a c t
Article history: Received 20 June 2012 Accepted 29 May 2013 Available online 22 June 2013
Resonant photoemission spectroscopy was applied to compare the valence band structure of Ge0.9Mn0.1Te and GeTe semiconductor layers deposited on BaF2 substrate in monocrystalline and amorphous forms. In (Ge,Mn)Te the contribution of Mn 3d5 electronic orbitals to density of states was found in three binding energy regions: below the top of the valence band (Eb o 4.2 eV), at the binding energy range 4.2–4.4 eV, and in many-body satellite at 9–13 eV. The comparative analysis of the photoemission spectra based on configuration interaction model showed that p–d hybridization effects, important for magnetic and optical properties of (Ge,Mn)Te, are stronger in monocrystalline than in amorphous (Ge,Mn)Te layers. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Resonant photoemission Germanium telluride Diluted magnetic semiconductors.
1. Introduction GeTe is a IV–VI narrow-gap semiconductor compound which below the temperature of about 700 K has the crystal structure of rhombohedral symmetry. It can be viewed as the cubic (rock-salt) crystal lattice slightly elongated in body diagonal [111] direction with mutually shifted anion and cation sublattices (Johnston and Sestrich, 1961). It results in ferroelectric (FE) properties of rhombohedral GeTe crystals. These properties are also observed in (Ge,Mn)Te mixed crystals but the FE transition temperature decreases with the increasing Mn concentration. In (Ge,Mn)Te crystals with Mn content of about 20 at% the FE transition takes place close to room temperature. (Ge,Mn)Te is also known as a diluted magnetic (semimagnetic) semiconductor in which magnetic Mn2+ ions substitute Ge2+ ions at cation sublattice sites. This substitution is electrically isovalent but produces the local magnetic moments of 5 Bohr magnetons (3d5 electronic configuration with S ¼ 5/2 n
Corresponding author. Tel.: +48 22 843 66 01x3287. E-mail address: [email protected] (W. Knoff).
0969-806X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2013.05.059
spin state). Carrier-induced ferromagnetic (FM) transition takes place in (Ge,Mn)Te crystals with the ferromagnetic Curie temperature TC depending both on conducting hole concentration p and Mn content x. Due to very high carrier concentration, p¼1020−1021 cm−3, induced in Ge1−xMnxTe by electrically active native defects (Ge vacancies), Ge1−xMnxTe shows relatively high Curie temperatures up to TC ¼ 150 K in bulk polycrystals, (Cochrane et al., 1973) and TC ¼190 K in thin layers (Hassan et al., 2011). Similarly to ferromagnetic properties of previously studied IV–VI semiconductors with Mn, such as Sn1−xMnxTe and Pb1−x−ySnyMnxTe mixed crystals, the ferromagnetic exchange interactions in Ge1−xMnxTe are driven by the Ruderman–Kittel–Kasuya–Yosida (RKKY) indirect exchange mechanism via conducting holes (Story et al., 1992; Eggenkamp et al., 1995; Szałowski and Balcerzak, 2008). The key electronic parameter determining the strength of this interaction is the p-d exchange coupling Jpd between the spins of valence-band carriers and the localized magnetic moments of 3d5 electrons of Mn2+ ions. The experimentally observed relatively large value of this parameter in (Ge,Mn)Te (Jpd ¼0.4–0.8 eV) is believed to originate from the kinetic exchange mechanism that involves the hybridization of valence band and Mn 3d electronic states.
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W. Knoff et al. / Radiation Physics and Chemistry 93 (2013) 77–81
Amorphous a-(Ge,Mn)Te semiconductor shows disorder-induced insulating electrical properties and is a paramagnetic material due to the lack of delocalized exchange-coupling-mediating conducting holes. Applying annealing process, globally or locally, e.g. by using laser or electron beam, one can rapidly (at nanosecond scale) transform amorphous phase to polycrystalline (metallic) one. This structural transition is accompanied by a paramagnet-toferromagnet transition (Fukuma et al., 2001; Knoff et al., 2011). For microscopic understanding of magnetic, optical and electric properties of (Ge,Mn)Te semiconductors the comparative experimental study of the influence of Mn ions on density of states in (Ge,Mn)Te in crystalline and amorphous form is of key importance. Although amorphous materials reveal only short-range chemical bonding order, the valence band of amorphous GeTe (a-GeTe) with a characteristic three-peak structure due to Ge 4p, Te 5p states (0– 5 eV), Ge 4s (8.3 eV), and Te 5s (12.4 eV) is similar to the case of monocrystalline GeTe (m-GeTe). The experimentally observed slight shift of photoemission spectra of amorphous layers to higher binding energies is interpreted as resulting from a slight redistribution of Ge and Te ions due to the small change in chemical bonding (Shevchik et al., 1973; Kim et al., 2007), however a difference of the work functions or charging effect cannot be excluded. The incorporation of Mn ions in GeTe matrix markedly changes the electronic structure in the valence band region. The photoemission transitions involving Mn 3d5 states are superimposed on the three-peak structure observed in GeTe. The purpose of our work is to compare the electronic structure of amorphous and monocrystalline GeTe and (Ge,Mn)Te layers by experimentally studying the photoemission spectra in the valence band region. To experimentally reveal the contribution of Mn 3d states in a-(Ge,Mn)Te and m-(Ge,Mn)Te we apply resonant photoemission spectroscopy (RPES). In absorption region of 3p–3d transitions involving Mn ions with half-filled 3d shell, two electronic transitions caused by the incident photon flux take place: regular photoemission process, in which the final state belongs to the continuum of states: Mn3p63d5+hv-Mn3p63d4+e−, and the accompanying intra-ion excitation process: 3p63dn+hν ¼ [3p53dn+1]*, where []* indicates an excited electronic state. The quantum interference between these two processes leads to autoionization [3p53dn+1]n3p63dn−1+e− and resonant photoemission.
2. Sample preparation and experimental techniques GeTe and (Ge,Mn)Te layers with 10 at% of Mn content were grown on BaF2 (111) monocrystalline substrates by molecular
beam epitaxy (MBE) technique employing effusion cells with GeTe, Te, and Mn solid sources. The thickness of layers was about 1 mm. During the growth of monocrystalline m-GeTe and m-(Ge,Mn)Te layers the substrate temperature was kept in the range T S ¼250–300 1C. The RHEED electron diffraction in situ growth control revealed a streaky pattern characteristic of a two-dimensional mode of growth. The X-ray diffraction (XRD) measurements performed at ambient temperature showed the monocrystalline (111)-oriented rhombohedral structure of these layers and the XRD rocking curve width parameter in the range 400–600″. Amorphous a-GeTe and a-(Ge,Mn)Te layers were deposited onto BaF 2 substrate in the same MBE facility but with the substrate kept at room temperature. We achieved completely amorphous state in both a-GeTe and a-(Ge,Mn)Te layers, as confirmed by the lack of any XRD diffraction peaks. The chemical composition of samples was examined by the energy dispersive X-Ray fluorescence method (EDXRF) yielding Mn content about 10 at% for both amorphous and monocrystalline layers. Magnetic properties of (Ge,Mn)Te layers were examined by the superconducting quantum interference device (SQUID) magnetometer in the temperature range of T ¼ 5–120 K. In monocrystalline GeMnTe layers we found the ferromagnetic transition at the Curie temperature T C ¼35 K, whereas in amorphous layers only a paramagnetic phase was observed. The valence band of GeTe and (Ge,Mn)Te was studied by the photoemission spectroscopy method in synchrotron radiation facility in HASYLAB (Hamburg). We used the experimental station FLIPPER II with VUV and soft X-ray plane-grating monochromator covering the photon energy range from 10– 150 eV. Our experiments consisted of two complementary parts. The first one concerned the valence band measurements carried out on GeTe and (Ge,Mn)Te semiconductor layers in photon energy range hν ¼47–60 eV, including Mn 3p–3d Fanotype resonance photoemission. In the second part the corelevel photoemission spectroscopic studies were performed. The experimentally determined binding energies are referred to the Fermi level of a thick tantalum sample. To remove oxides and other contamination from the surface the cleaning process was performed in situ by argon (Ar +) ion sputtering. Annealing at T ¼250 1C was applied only to monocrystalline samples. This procedure was skipped for amorphous samples to avoid unintentional structural transformation to polycrystalline form. The main features of the spectra were not influenced by these procedures, as checked experimentally after each cleaning step.
Fig. 1. (a) Valence band photoemission spectra of amorphous and monocrystalline GeTe layers at photon energy hv¼ 60 eV and (b) photoemission spectra in the core absorption region of amorphous and monocrystalline GeTe layers for hv¼ 80 eV.
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3. Results and discussion In order to experimentally study the valence band region (Eb ¼ 0–15 eV) in GeTe and (Ge,Mn)Te, the photoemission spectra were acquired in photon energy range hν ¼47–60 eV. Photoemission signal intensity was normalized to photon flux and the secondary electron background was subtracted by means of the Shirley method (Shirley, 1972). In GeTe, the three-peak structure (Fig. 1a) is observed in the valence band region. The maxima of the energy distribution curve were found in m-GeTe at the binding energies of 1.7, 7.0, and 11.9 eV. In a-GeTe, similar density of states spectrum was experimentally observed, with a small shift to higher binding energies of the valence band near the Fermi level. Photoemission measurements in the core absorption region performed at photon excitation energy hν ¼80 eV revealed the Ge 3d and Te 4d core levels in a-GeTe and m-GeTe (Fig. 1b). Comparing to monocrystalline layers, the small shift (up to +0.2 eV) of Ge and Te peaks was observed in amorphous materials, but the spectral structures of Ge and Te core-peaks are qualitatively similar. This experimental finding was discussed in terms of differences between amorphous and monocrystalline materials caused by the redistribution of Ge and Te atoms (Shevchik et al., 1973). However, charging of amorphous sample and accompanying shift of spectra due to its insulating properties cannot be excluded.
Due to the incorporation of Mn ions in GeTe lattice, additional contribution of Mn 3d states appears in the electronic structure (Fig. 2a and b), as experimentally found in (Ge,Mn)Te bulk polycrystals (Pietrzyk et al., 2009). The constant-initial-state (CIS) spectrum (derived from the spectra of Fig. 2a and b) in Mn 3p–3d absorption region (hν¼47–60 eV) is shown in Fig. 2c. The constant initial energy Ei corresponds to the main peak of Mn 3d states. The CIS spectra uncover resonant photoemission process with the characteristic Fano-type profile. With the photon energy increasing in the range hν¼ 47–60 eV, the intensity of the main peak reveals minimum related to the anti-resonance of Mn2+ ion transitions at hv¼48 eV in amorphous and monocrystalline (Ge,Mn)Te layers. Above this energy, the intensity of the main peak reaches the maximum at hv¼ 51 eV related to the resonance due to intra-ion Mn 3p–3d transition and then gradually decreases with further increase of photon energy. The Mn states contribution covers the whole valence band region and the main maximum is observed at Eb ¼4.2 eV in the monocrystalline sample and at Eb ¼4.4 eV in a-(Ge,Mn)Te (Fig. 2d). The feature at higher binding energies in amorphous layer is rather broad in the binding energy range Eb ¼9.3–13.6 eV, as compared to monocrystalline layer (Eb ¼10.2–13.6 eV). The changes in the spectral maximum intensity (at 4.2 eV in monocrystalline layer and at 4.4 eV in amorphous layer) enabled us to clearly identify the curves taken at the resonance and the anti-resonance energies (51 and 48 eV, respectively).
m -(Ge,Mn)Te
a - (Ge,Mn)Te
h ν (eV):
h ν (eV) : 60
60
57
Intensity (arb.u.)
Intensity (arb.u.)
57 54 53 52 51
54 53 52 51 50
50 49
49
48
48
47
47
0
20 18
2
4 6 8 10 12 14 Binding Energy (eV)
0
2 4 6 8 10 12 14 Binding Energy (eV)
1.2 m - (Ge,Mn)Te a - (Ge,Mn)Te
m - (Ge,Mn)Te a - (Ge,Mn)Te
1.0
hv= 51 eV
14 12 Ei = 4.2 eV
8 6
Ei = 4.37 eV
4 2
Intensity (arb.u.)
Intensity (arb.u.)
16
10
79
0.8 0.6 0.4 0.2 0.0
46 48 50 52 54 56 58 60 62 Photon Energy hν (eV)
0
2 4 6 8 10 12 14 Binding Energy (eV)
Fig. 2. Photoemission spectra of amorphous (a) and monocrystalline (b) (Ge,Mn)Te layers obtained in photon energy range 47–60 eV corresponding to Mn 3p–3d core absorption region, (c) CIS spectrum for the initial state energy Ei ¼4.2 eV for monocrystalline and Ei ¼ 4.37 eV for amorphous layers, and (d) comparison of the two spectra obtained at Mn on-resonance photon energy.
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qualitative conclusions based on intensity ratios are valid and clearly point that in monocrystalline (Ge,Mn)Te layer the p–d hybridization is stronger than in amorphous one. This difference in hybridization strength is of importance, e.g. for Mn-carrier exchange integral Jpd and ferromagnetic Curie temperature of (Ge,Mn)Te. One has to note, however, that the dramatic difference in magnetic properties of crystalline and amorphous (Ge,Mn)Te layers is mainly due to their qualitatively different electric properties, as discussed in the introduction.
a - (Ge,Mn)Te
ΔEDC (51 - 48 eV)
12 10 8 M
6
S
4 2
V
0 0
2
4. Conclusions 4
6
8
10
12
14
We experimentally studied the electronic structure of GeTe and (Ge,Mn)Te layers grown on BaF2 (111) substrate in monocrystalline and in amorphous forms. Applying the resonant photoemission method at photon energy range corresponding to 3p–3d Fano-type resonance we identified the main contributions of Mn 3d5 electrons to electronic states in the valence band region. Our analysis of the integrated intensities of three Mn-related spectral features observed experimentally in amorphous and in monocrystalline samples confirmed that the hybridization of Mn 3d states and the valence band states is in monocrystalline layer stronger than in amorphous one.
ΔEDC (51-48 eV)
8 m - (Ge,Mn)Te
6
M
4
S
2 V 0 0
2
4
6 8 10 Binding Energy (eV)
12
14
Fig. 3. The resonance-antiresonance difference curve of amorphous (a) and monocrystalline (b) (Ge,Mn)Te layer. Dashed line corresponds to fitted Gaussian peaks related to electron emission from 3d states to d5L final states (main peak denoted as M). Dotted and dash-dotted lines corresponds to fitted Gaussian functions related to valence band (V) and satellite (S) contribution, respectively. The sum of all Gaussian functions is shown by the solid line.
The difference photoemission spectrum ΔEDC obtained by subtracting the anti-resonance spectrum from its resonance counterpart is shown in Fig. 3. In order to study the strength of p–d hybridization from ΔEDC spectra we used the configurationinteraction (CI) cluster model developed for MnTe6 cluster with central Mn ion and six Te ligands (Kaznacheyev et al., 1998). Due to similarities between the spectra in 3p–3d excitation region obtained from calculated differential ΔEDC spectra of MnTe6 cluster and experimentally studied m-Ge1−xMnxTe (Senba et al., 2005), one can suggest that the main Mn-related structure (M) originates from electron emission from 3d states with d5L final states (here L denotes ligand hole). At lower binding energies Eb o4.2 eV (referred as valence band contribution V), electron final states were found to be in d5L and d6L2 electronic configurations. The satellite peak (S) located at higher binding energies (Eb ¼6–12 eV) is related to photoemission with d4 final state (Kaznacheyev et al., 1998; Pietrzyk et al., 2007; Pietrzyk et al., 2009). To semi-quantitatively analyze the experimentally observed photoemission difference ΔEDC spectra we carried out fitting procedure that reproduced well the whole spectrum by taking into account several contribution of Gaussian shape (Fig. 3). We identified the main V, M and S spectral features discussed above and determined their integrated intensities IV, IM, and IS. In the spirit of the CI model we analyze the experimentally observed integrated intensity ratios IV/IM and IS/IM. It is expected that with the increasing strength of 3p–3d hybridization the ratio IV/IM increases whereas the ratio IS/IM decreases. In amorphous (Ge, Mn)Te layer we found experimentally: IV/IM ¼0.15 and IS/IM ¼1.74, whereas for monocrystalline layer: IV/IM ¼0.23 and IS/IM ¼1.30. Although such quantitative analysis of spectral intensities is subject to rather large experimental uncertainty, we expect that
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