ISSN 10637761, Journal of Experimental and Theoretical Physics, 2009, Vol. 109, No. 4, pp. 602–608. © Pleiades Publishing, Inc., 2009. Original Russian Text © A.F. Orlov, A.B. Granovsky, L.A. Balagurov, I.V. Kulemanov, Yu.N. Parkhomenko, N.S. Perov, E.A. Gan’shina, V.T. Bublik, K.D. Shcherbachev, A.V. Kartavykh, V.I. Vdovin, A. Sapelkin, V.V. Saraikin, Yu.A. Agafonov, V.I. Zinenko, A. Rogalev, A. Smekhova, 2009, published in Zhurnal Éksperimental’noі i Teoreticheskoі Fiziki, 2009, Vol. 136, No. 4, pp. 703–710.
ORDER, DISORDER, AND PHASE TRANSITION IN CONDENSED SYSTEM
Structure, Electrical and Magnetic Properties, and the Origin of the Room Temperature Ferromagnetism in MnImplanted Si A. F. Orlova, *, A. B. Granovskyb, L. A. Balagurova, I. V. Kulemanova, Yu. N. Parkhomenkoa, N. S. Perovb, E. A. Gan’shinab, V. T. Bublikc, K. D. Shcherbachevc, A. V. Kartavykhd, V. I. Vdovind, A. Sapelkine, V. V. Saraikinf, Yu. A. Agafonovg, V. I. Zinenkog, A. Rogalevh, and A. Smekhovah a
State Institute for Rare Metals, Moscow, 119017 Russia *email:
[email protected] b Moscow State University, Moscow, 119991 Russia c Moscow Institute of Steel and Alloys, Moscow, 119049 Russia d Institute for Chemical Problems of Microelectronics, Moscow, 119017 Russia e Queen Mary University of London, E1 4NS, London, UK f State Research Institute of Physical Problems, Zelenograd, Moscow oblast, 103460 Russia g Institute of Microelectronics Technology and High Purity Materials, Chernogolovka, Moscow oblast, 142432 Russia h European Synchrotron Radiation Facility, 38043 Grenoble Cedex 9, France Received April 13, 2009
Abstract—The structure and the electrical and magnetic properties of Mnimplanted Si, which exhibits fer romagnetic ordering at room temperature, are studied. Singlecrystal n and ptype Si wafers with high and low electrical resistivities are implanted by manganese ions to a dose of 5 × 1016 cm–2. After implantation and subsequent vacuum annealing at 850°C, the implanted samples are examined by various methods. The Mn impurity that exhibits an electric activity and is incorporated into the Si lattice in interstitial sites is found to account for only a few percent of the total Mn content. The main part of Mn is fixed in Mn15Si26 nanopre cipitates in the Si matrix. The magnetization of implanted Si is found to be independent of the electrical resis tivity and the conductivity type of silicon and the type of implanted impurity. The magnetization of implanted Si increases slightly upon shortterm postimplantation annealing and disappears completely upon vacuum annealing at 1000°C for 5 h. The Mn impurity in Si is shown to have no significant magnetic moment at room temperature. These results indicate that the room temperature ferromagnetism in Mnimplanted Si is likely to be caused by implantationinduced defects in the silicon lattice rather than by a Mn impurity. PACS numbers: 61.72.uf, 72.80.Cw, 75.50.Pp DOI: 10.1134/S1063776109100069
1. INTRODUCTION Dilute magnetic semiconductors have attracted close attention because of their potential application in semiconductor spin electronic devices [1]. These materials are being extensively studied all over the world after predicting the possibility of room tempera ture ferromagnetism (RTF) in them [2]. As the best candidates for semiconductor spin electronics, wide bandgap semiconductor oxides and III–V com pounds doped with transition metals or manganese, respectively, are being considered. A possible use of semiconducting ferromagnetic silicon for these pur poses is promising, since it is compatible with standard semiconductor technologies. The possibility of ferro magnetic ordering at above room temperature in Mn doped Group IV semiconductors was noted in [2, 3]. Experimentally, RTF was first observed in vacuum deposited Si films doped with Mn [4] and in Mn implanted Si wafers [5]. A saturation magnetization of
1.3 and 0.3 emu/g at 300 K was detected in [4] and [5], respectively. The authors of [5] also revealed the effect of postimplantation annealing and the type of conduc tion on the magnetization, which indicates that the carrierassisted exchange coupling in a semiconductor is the origin of ferromagnetism. A weak magnetization at room temperature in Mnimplanted Si was also observed in [6–8]. In [9], ferromagnetism was detected at room temperature in Si:Mn films contain ing up to 35% Mn and deposited by laser ablation. However, other researchers detected ferromagnetic ordering in Mnimplanted Si only at extremely low temperatures (less than 20 K) [10, 11]; in [11, 12], this finding was related to ferromagnetic MnxSiy precipi tates in the material. Finally, Liu et al. [13] produced silicon films doped with a high Mn concentration using rf magnetron sputtering followed by shortterm annealing at 1300°C in an argon atmosphere and rapid cooling. These films consisted of a silicon matrix with
602
STRUCTURE, ELECTRICAL AND MAGNETIC PROPERTIES ρ, Ω cm 200
603
ρ, Ω cm 0.06 0.05
150 (a)
0.04
100
2
50
(b)
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3
1
0.01
0
1 0
0.2
0.4
0.6
0.8
1.0 D, μm
0
2 0
0.2
0.4
0.6
0.8
1.0 D, μm
Fig. 1. Depth profiles of the electrical resistivity in implanted Si layers. (a) Highresistivity (1) ntype and (2) ptype silicon implanted by Mn at a dose of 2 × 1016 cm–2 and then annealed, respectively, and (3) highresistivity asimplanted ptype silicon. (b) Lowresistivity (1) ntype and (2) ptype silicon implanted by Mn at a dose of 1 × 1016 cm–2 and then annealed, respectively. D is the distance from the sample surface.
2.3% at Mn dissolved in it and Mn4Si7 phase precipi tates. The temperature dependence of the magnetiza tion indicated that this material contained two ferro magnetic phases with Curie temperatures of about 50 and 250 K, which could be attributed to the Mn4Si7 phase and Mn–doped Si, respectively. However, the RTF mechanism in doped Si is still far from being understood. There is no consensus about both the nature of the magnetic moments in such materials and the mechanism of the strong exchange between these magnetic moments, which provides a longrange ferromagnetic order at room temperature. The reported experimental data are quite conflicting, even for samples fabricated under similar conditions, and fragmentary. Moreover, some authors doubt that the RTF in Si is intrinsic and is not associ ated with parasitic ferromagnetic phases. In this work, we present the results of an extended study of the structure, electrical and magnetic properties of Mn implanted Si and try to determine the origin of the RTF in this material.
and magnetooptical properties of these materials were partially presented in [14]. The depth profiles of the impurity concentrations in the implanted layers were measured by secondary ion mass spectrometry (SIMS) on an IMS4F device, and the depth profiles of the electrical resistivity in these layers were measured by spreading resistance profiling on an ASR100C device. The structural stud ies included Xray diffraction (XRD) on a D8 Dis cover diffractometer, transmission electron micros copy (TEM) and electron diffraction on a JEOL 200CX microscope, and Xray absorption (XAS) by impurity atoms. Xray absorption nearedge spectros copy (XANES) and extended Xray absorption fine structure (EXAFS) studies were performed using the Xray polarized source located at synchrotron station 7.1 of the Daresbury Laboratory (Worrington, United Kingdom). The Xray magnetic circular dichroism (XMCD) and XANES investigations were also carried out at source ID12 of the European Synchrotron Radiation Facility (Grenoble, France). The magnetic measurements were performed at room temperature on a LakeShore 7400 vibratingsample magnetometer.
2. EXPERIMENTAL As the starting materials, we used commercially available Si wafers grown by the Czochralski method. The ntype wafers had an electrical resistivity of 0.01 (doped with Sb) and 4.5 Ω cm (doped with P), and the ptype wafers had a resistivity of 0.005 and 10 Ω cm (doped with B). The wafers were implanted by 195keV 55Mn+ ions at doses ranging from 1 × 1015 to 5 × 1016 cm–2 and a temperature of 350°C. For compar ison, some wafers were implanted by 59Co+ ions under the same conditions and at a dose of 1 × 1016 cm–2. After implantation, some samples were annealed in a vac uum of 3 × 10–6 Torr at 850°C for 5 min. The magnetic
3. ELECTRICAL PROPERTIES OF THE IMPLANTED LAYERS The depth profiles of electrical resistivity ρ in Mn implanted Si layers were studied in detail in [15]. Here, we only present the main obtained results. Figure 1 shows the depth profiles of the electrical resistivity in the implanted layers in silicon of both conductivity types, with low and high levels of the initial electrical resistivity, after vacuum annealing at 850°C for 5 min. The SIMS maximum in the depth profile of the Mn concentration is located at a depth of 0.18 μm from the surface, which agrees with the calculated distribution
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AX T = 80 K
0.9
300 K 0.6
0.3 6500
6600
6700
6800
6900 7000 Energy, eV
Fig. 2. Spectra of Xray absorption by Mn atoms in a sili con sample implanted with Mn at a dose of 1 × 1016 cm–2.
of implanted Mn in Si at an ion energy of 195 keV. It is seen that the Mn impurity exhibits amphoteric behav ior and compensates for acceptors in the ptype silicon with a high initial electrical resistivity and for donors in the ntype silicon with a low initial electrical resis tivity. The compensation appears upon postimplanta tion annealing rather than immediately after implan tation. Using the value of compensation, we deter mined the positions of the appearing energy levels and found that they correspond to two wellknown states of interstitial Mn in the Si lattice, namely, (Mni)+/++ and (Mni)–/0. Using the SIMS results, we compared the manganese and maindopant concentrations at the depth where compensation disappears and found that only a small part of the total Mn content in the layer (a few percent) is incorporated into the crystal lattice of Si and exhibits an electric activity after implantation and annealing. This raises the question of the state of the main part of Mn implanted in Si.
To determine the short and longrange orders in the crystal structure of Mnimplanted Si, we per formed XRD, TEM, and XAS studies. Figure 2 shows the spectra of Xray absorption by Mn atoms in a Mn implanted Si sample measured at room temperature and 80 K. According to [12], the character of the curves in Fig. 2 corresponds to a MnxSiytype silicide. At 80 K, the magnitude of the Fourier transform of the k3weighted absorption fine structure of the absorp tion spectrum higher than the K absorption edge of Mn and the calculated curves are shown in Fig. 3. The spectrum exhibits two coordination shells around a Mn atom. The first shell consists of more than four Si atoms. Therefore, there are no Mn clusters in the material, which agrees with [12]. This composition of the first shell allows the penetration of Mn into the Si lattice, and the second shell is likely to consist of Mn rather than Si atoms. This structure of the nearest coordination shells around Mn atoms is consistent with the presence of MnSitype clusters (e.g., with a B20type structure) in the material. A simple analysis of the spectra shown in Fig. 2 without regard for mul tiple scattering gives the following results for the two nearest coordination shells around Mn: R ( Mn–Si ) = 2.37 ( 1 ) Å, N ( Si ) = 7 ( 1 ), 2
A ( Si ) = 0.017 ( 4 ) Å , R ( Mn–Mn ) = 2.95 ( 1 ) Å,
N ( Mn ) = 2.5 ( 9 ), 2
A ( Mn ) = 0.005 ( 5 ) Å , where R is the bond length, N is the number of atoms in the coordination shell, and A is the Debye–Waller temperature factor. In Fig. 3b, we compare the calcu lated and experimental curves. The more significant disorder in the first coordination shell reflects the fact that the interatomic distances in this shell are differ ent, which is consistent with the B20 structure of MnSi. However, this model describes the experimen tal results insufficiently accurately. In the MnSi sili χ(k)k3
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(b)
1 0 2 −1
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10 R, Å
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10 k, Å−1
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cide (B20 structure type), the interatomic distances in the first coordination shell (2.31–2.54 Å) agree with the value calculated from the experimental spectrum, whereas the interatomic distance (2.796 Å) calculated for the second coordination shell does not agree with the experimental value (2.95 Å). These results demon strate the absence of Mn clusters in the implanted sil icon and imply the presence of MnxSiytype silicide precipitates in the Si matrix. TEM and electron diffraction examination of the implanted samples shows that the electron diffraction patterns of the samples annealed after implantation contain extra reflections in addition to the reflections of the Si matrix (indexed). These extra reflections indicate secondphase precipitates in the implanted layers (Fig. 4) and form a network with hexagonal symmetry that makes an angle of 30° with the network of the fundamental reflections of Si. The darkfield images taken with the extra reflexes contain bright spherical spots from 20nm secondphase precipi tates. The calculation of the interplanar spacings in these precipitates gives d = 3.266(2) Å, which is close to the tabulated value of the bodycentered tetragonal Mn15Si26 phase. The XRD examination of the implanted silicon samples also indicates the presence of the tetragonal Mn15Si26 phase (or similar tetragonal Mn4Si7 and Mn27Si47 phases with the ratio Si:Mn = 1.74(1)) in them. As shown above, this phase was also detected by electron diffraction. The MnSi1.7 phase was observed in [11]. No diffraction lines of pure manganese were detected in the Xray diffraction patterns. Thus, with the XRD data, we can state that the manganese impurity in implanted silicon is mainly fixed in nanoprecipitates of a bodycentered tetrago nal Mn15Si26type silicide phase. The results of TEM and XRD investigations of Mnimplanted Si layers are described in detail in [16]. 5. MAGNETIC PROPERTIES In [14], we used a SQUID technique, found that the ferromagnetic ordering in Mnimplanted Si is present up to 400 K, and observed the magnetooptical Faraday rotation in this material above room tempera ture. These effects were shown to disappear after the implanted layer was removed by mechanical polishing. In this work, we measure the magnetic properties of Mnimplanted Si samples with a vibratingsample magnetometer at room temperature. The measure ments were carried out after implantation at low and high doses followed by vacuum annealing. For com parison, we also determined the magnetization of sili con implanted by cobalt ions. The magnetometer sen sitivity was 1 × 10–6 emu. After a measurement, the signal of the holder was subtracted from the total signal and the resulting curves were approximated by a Lan gevin function.
605
− 202 − 220
− 022
− − 242
− 242
000 − 220
− 022 − 202
Fig. 4. Electron diffraction pattern (inverted image) for a Si(111) sample implanted by Mn at a dose of 5 × 1015 cm–2 and then annealed.
Figure 5 shows the hysteresis loops of asimplanted and annealed Si:Mn samples. Shortterm vacuum annealing at 850°C for 5 min was found to decrease the coercive force, whereas saturation magnetization Ms remained almost unchanged. The decrease of the coercive force can be related to the recrystallization of the material partly amorphized upon implantation. The coercive force is 30–150 Oe, which is consistent with the data in [7] and is lower than the values given in [5]. The shape of the hysteresis loops agrees with the reported data. It should be noted that the magnetiza tion is almost saturated at 2–5 kOe, which indicates a small contribution of possible superparamagnetic clusters or phases. The value of Ms was determined in a field of 6 kOe, and the results obtained are given in Tables 1 and 2. The data in Table 1 demonstrate that the specific magnetization is independent of the conductivity type and the electrical resistivity (i.e., the carrier concen tration) of silicon and of the type of implanted impu rity (Mn, Co). Thus, the initial parameters of silicon and the type of implanted impurity do not affect the resulting magnetization of the material. The magneti Table 1. Specific saturation magnetization vs. the initial parameters of Si and the implantation dose
Dose, cm–2
ptype Si
ntype Si
ρ, Ω cm
ρ, Ω cm
10
0.005
4.5
0.01
0.2 0.4 0.2 – 0.4
0 0.3 0.4 0.2 1.2
M s, G 1 × 1015 (Mn) 5 × 1015 (Mn) 5 × 1015 (Co) 1 × 1016 (Mn) 5 × 1016 (Mn)
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−12 −6
−4
−2
0
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4
6 H, kOe
−12 −6
−4
−2
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6 H, kOe
Fig. 5. Magnetic hysteresis loops of Si samples with different initial electrical resistivities ρ after (a) implantation by Mn at a dose of 5 × 1016 cm–2 and (b) subsequent vacuum annealing at 850°C for 5 min. (1) ntype Si, ρ = 0.01 Ω cm; (2) ptype Si, ρ = 10 Ω cm; (3) ptype Si, ρ = 0.005 Ω cm; and (4) ntype Si, ρ = 4.5 Ω cm.
zation increases slightly with the implantation dose: the average saturation magnetization increases approximately threefold as the implantation dose increases 50fold. Table 2 illustrates the effect of postimplantation annealing on the magnetization: shortterm vacuum annealing is seen to weakly increase the average spe cific magnetization of Mnimplanted Si. These data agree with the results in [12] and contradict those in [5]; in the latter work, annealing was performed in a nitrogen atmosphere. The magnetization of the mate rial disappears completely upon vacuum annealing at 1000°C for 5 h. These results can be compared to the magnetiza tion of silicon implanted by argon or krypton impuri ties [17]. For example, silicon implanted by manga nese at a dose of 5 × 1016 cm–2 exhibits a coercive force of 110 Oe and a specific magnetization of 1 G in a magnetic field of 1 kOe (this work), and silicon implanted by argon at a dose of 2 × 1017 cm–2 exhibits a coercive force of 100 Oe and a specific magnetization of 1.2 G in a magnetic field of 800 Oe [17]. Similar magnetization parameters were also detected in [18]
when silicon was implanted by thermal neutrons at a dose of 4 × 106 cm–2. This comparison indicates that the magnetization of implanted Si is independent of the type of implanted impurity. Finally, with a syn chrotron radiation source, we performed XMCD measurements near the Kedge absorption of Mn atoms at room temperature in order to determine the XMCD signal of Mn atoms. As follows from Fig. 6, no XMCD signal outside the background level exists in this region, which indicates the absence of any ordered magnetic moment at the Mn ions in Si at room tem perature. 6. DISCUSSION OF THE RESULTS Liu et al. [13] clearly showed that the RTF in Mn implanted Si can be caused by neither Mn silicide nanoclusters nor Mn ions incorporated into the Si lat tice. Our results also do not agree with the standard model of ferromagnetic ordering in this material due to an indirect exchange of magnetic ions through charge carriers, where manganese ions should have a magnetic moment and the exchange interaction
Table 2. Effect of postimplantation vacuum annealing on the specific saturation magnetization of Mnimplanted Si Annealing parameters 850°C, 5 min
1000°C, 5 h
Conductivity type of Si
ρ, Ω cm
Dose, cm–2
n n n p p
4.5 4.5 0.01 10 0.005
n p
4.5 0.005
M s, G after implantation
after annealing
5× (Mn) 16 5 × 10 (Mn) 5 × 1016 (Mn) 5 × 1016 (Mn) 5 × 1016 (Mn)
0.4 0.4 1.2 0.6 0.5
0.4 0.8 1.1 0.9 0.5
5 × 1016 (Mn) 5 × 1016 (Mn)
0.8 0.5
0 0
1015
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Magnetic dichroism
1.2
0.0006 0.0004 0.0002 0 −0.0002 −0.0004 −0.0006
0.8 0.4 0 6520
6540
6560 6580 Energy, eV
6600
Fig. 6. Spectrum of Xray absorption by Mn atoms and the spectrum of XMCD near the Kedge absorption of Mn atoms in a silicon sample implanted with Mn at a dose of 2 × 1016 cm–2 and then annealed.
involves charge carriers (holes). The appearance of RTF can also be explained by itineranttype ferromag netism, where the band structure in Mndoped Si changes due to the overlapping of the d states of man ganese and s–d hybridization. However, this mecha nism can be important only for a much higher volume fraction of Mn in the material (more than 20%). According to [18], the ferromagnetism in Si that has no ferromagnetic elements can be generated by struc tural defects with uncoupled electrons. It is well known that the structural defects in crystals with dan gling or unsaturated bonds can be considered as states with unpaired electrons; hence, such states can induce a magnetic moment. Apparently, such a mechanism of appearing a magnetic moment in initially nonmag netic systems is typical of not only Sibased semicon ductors but also of other semiconductors and oxides (see, e.g., [19–21]), as well as of many allotropic mod ifications of carbon. The only problem here is to explain a strong ferromagnetic exchange between these states. We do not exclude that this can be a Zenertype interaction between carriers and the mag netic moments of dangling bonds. To answer this diffi cult question, researchers have to perform further experimental and theoretical works. 7. CONCLUSIONS Only a small part of the Mn impurity implanted into Si was found to enter into the Si lattice and to exhibit an electric activity. Direct structural measure ments of the long and shortrange orders in implanted silicon demonstrate that the main part of Mn in Si is fixed in intermetallic Mn15Si26 tetragonal phase nanoclusters. Shortterm postimplantation vac uum annealing at 850°C strongly changes the struc tural and electrical properties of the implanted layer and weakly influences its magnetization. Magnetic measurements show that the magnetization of the
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implanted silicon is independent of the initial electri cal resistivity and the initial type of conduction of sili con and of the type of implanted impurity. The ferro magnetism in the implanted silicon disappears upon vacuum annealing at 1000°C for 5 h. XMCD mea surements demonstrate that Mn in implanted Si does not have any magnetic moment at room temperature. With the results obtained, we conclude that the RTF in implanted silicon (at least, at a low impurity content) is caused by implantationinduced structural defects in silicon. This conclusion follows from our data and from their comparison with the available data on the magnetization of silicon implanted by nonmagnetic ions or irradiated by neutrons. ACKNOWLEDGMENTS This work was supported by the Russian Founda tion for Basic Research, project no. 070200327. REFERENCES 1. H. Ohno, Science (Washington) 281, 951 (1998). 2. T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Fer rand, Science (Washington) 287, 1019 (2000). 3. A. Stroppa, S. Picozzi, A. Continenza, and A. J. Free man, Phys. Rev. B: Condens. Matter 68, 155203 (2003). 4. F. M. Zhang, X. C. Liu, J. Gao, X. S. Wu, Y. W. Du, H. Zhu, J. Q. Xiao, and P. Chen, Appl. Phys. Lett. 85, 786 (2004). 5. M. Bolduc, C. AwoAffouda, A. Stollenwerk, M. B. Huang, F. G. Ramos, G. Agnello, and V. P. LaBella, Phys. Rev. B: Condens. Matter 71, 033302 (2005). 6. P. R. Bandaru, J. Park, J. S. Lee, Y. J. Tang, L.H. Chen, S. Jin, S. A. Song, and J. R. O’Brien, Appl. Phys. Lett. 89, 112502 (2006). 7. I. T. Yoon, C. J. Park, and T. W. Kang, J. Magn. Magn. Mater. 311, 693 (2007). 8. E. S. Demidov, Yu. A. Danilov, V. V. Podol’skiі, V. P. Lesnikov, M. V. Sapozhnikov, and A. I. Suchkov, Pis’ma Zh. Éksp. Teor. Fiz. 83 (12), 664 (2006) [JETP Lett. 83 (12), 568 (2006)]. 9. E. S. Demidov, B. A. Aronzon, S. N. Gusev, V. V. Kar zanov, A. S. Lagutin, V. P. Lesnikov, S. A. Levchuk, S. N. Nikolaev, N. S. Perov, V. V. Podolskii, V. V. Rylkov, M. V. Sapozhnikov, and A. V. Lashkul, J. Magn. Magn. Mater. 321, 690 (2009). 10. A. Misiuk, J. BakMisiuk, B. Surma, W. Osinniy, M. Szot, T. Story, and J. Jagielski, J. Alloys Compd. 423, 201 (2006). 11. Shengqiang Zhou, K. Potzger, Gufei Zhang, A. Mück lich, F. Eichhorn, N. Schell, R. Grötzschel, B. Schmidt, W. Skorupa, M. Helm, and J. Fassbender, Phys. Rev. B: Condens. Matter 75, 085 203 (2007). 12. A. Wolska, K. LawniczakJablonska, M. Klepka, M. S. Walczak, and A. Misiuk, Phys. Rev. B: Condens. Matter 75, 113201 (2007).
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Translated by K. Shakhlevich
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