High-energy spectroscopic study of the III-V diluted magnetic semiconductor Ga 1 x Mn x N

Transcription

1 High-energy spectroscopic study of the III-V diluted magnetic semiconductor Ga 1 x Mn x N Master Thesis Jong-Il Hwang Department of Complexity Science and Engineering, University of Tokyo January, 2004

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3 Contents 1 Introduction 5 2 Physical properties of Ga 1 x Mn x N III-nitride semiconductors Ga 1 x T x N Principles of electron spectroscopy Photoemission spectroscopy Resonant photoemission spectroscopy X-ray absorption spectroscopy X-ray magnetic circular dichroism Configuration-interaction cluster model Experimental Experimental Crystal growth and surface treatment of Ga 1 x Mn x N Results and discussion X-ray absorption spectroscopy and magnetic circular dichroism X-ray photoemission spectroscopy Resonant photoemission spectroscopy Summary 67

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5 Chapter 1 Introduction The remarkable development currently being made in the fields of electronics and information technologies has been made possible by exploiting the properties of electron charge and spin. Integrated circuits used for data processing utilize the charge of electrons in semiconductors, while data storage media such as hard disks utilize the spin of electrons in magnetic materials. Recently, effort to control the spin degrees of freedom in semiconductors has been made. Various types of spin polarization have been realized in semiconductors, such as carrier spins, spins of doped magnetic atoms, spins in artificialsuperlattices and nuclear spins The control of these spins can lead to the advent of a new field - semiconductor spin electronics (semiconductor spintronics) - involving the using the spin states inside semiconductors. Semiconductors currently used for integrated circuits, transistors and optical devices, such as silicon and gallium arsenide, are non-magnetic, and the electron energy is almost independent of the spin direction. However, as the miniaturization of such devices is developed by the progress of nanotechnology and crystal growth technique, exchange interaction has become more pronounced effect, and the existence of the spin has become more tangible. The exchange interaction can lead to spin related phenomena not only in nanostructures but also in devices of conventional size. For instance, the breakthrough of crystal growth technique made by development of molecular beam epitaxy (MBE), which can control the rate and direction of crystal growth layer by layer, has enabled us to grow high quality crystals. The emergence of MBE method has made it possible to prepare high quality thin films of diluted magnetic semiconductors (DMS s). The MBE method has become to incorporate transition or rare-earth metal atoms into the host semiconductors. The fabrication of the III-V DMS s In 1 x Mn x As and Ga 1 x Mn x As was realized by using this MBE method [1.1, 2]. These materials thus fabricated have shown ferromagnetism with p-type conduction

6 6 Chapter 1. Introduction and has stimulated the extensive studies in this field. Furthermore, by making the heterostructures between DMS s and semiconductors using the MBE method, newly fabricated devices are realized. In practice, however, there are some problems in realizing new devices using DMS s. Up to now, the Curie temperature (T C ) of the III-V DMS s such as Ga 1 x Mn x As and In 1 x Mn x As has been well below the room temperature. The T C is 110 K for Ga 1 x Mn x As [1.3] and 50 K for In 1 x Mn x As. Therefore, one of the issue is to develop a DMS with T C above room temperature. There are many challenges to realize DMS s with T C above room temperature. One of the strategy to realize the ferromagnet with T C above room temperature is to utilize GaAs that is used extensively in present-day electronics. There are reports that the optimization of the layer thickness and the annealing of Ga 1 x Mn x As enhances the T C up to 150 K [1.4] and that Mn δ-doped GaAs in the nominal Mn concentration of cm 2 have T C as high as 172 K [1.5]. Moreover, by co-doping C to make p-type GaAs with Mn of 1-5 at % through ion implantation, T C rises up to 280K [1.6]. Another challenge is the optimization of the of between the host material and the transition or rare earth metals. A new function was proposed by incorporating Mn into chalcopyrite semiconductors including CdGeP 2 and ZnGeP 2, which shows ferromagnetism above room temperature [1.7, 8]. In a recent theoretical study, in II-VI and III-V compound semiconductors, it has been predicted that ferromagnetism with a very high T C occurs in systems such as p-type Ga 1 x Mn x N and Zn 1 x Mn x O [1.9]. Figure 1.1(a) shows the schematic diagram indicating the T C of Mn-doped semiconductors calculated by Dietl et al.. In the calculation, 2.5% of Mn atoms in divalent charge state and holes par cm 3. The calculation predicts that the T C of Mn-doped GaN, InN, C and ZnO exceed above room temperature. The stability of ferromagnetic state has been predicted in ZnO and GaNbased DMS s [1.10, 7]. Their calculations shows that V, Cr and Mn-doped GaN as shown in Fig. 1.1(b). and transition-metal(except for Mn)-doped ZnO are promising candidates for room temperature ferromagnetic DMS s. Motivated by such material designing, the fabrication of new ferromagnetic DMS s, such as ZnO:Co [1.12] and ZnO:V [1.13], were attempted. In the III-V DMS s, too, after the successful Mn doping into GaN [1.14], several groups reported that Ga 1 x Mn x N shows an indication of ferromagnetic behavior [1.16, 17]. GaN is a key material and essential for the progress of electronics so that recent developments in growth technique for wurtzite GaN has led to the fabrication of GaN-based optical and electrical devices. However, so far, the results have been quite diverse between different reports, and the occurrence of ferromagnetism is still controversial [1.21]. To realize room-temperature ferromagnetism in DMS s, it is necessary to elu-

7 7 Figure 1.1: (a) Prediction of T C for various DMS s. 2.5 % of Mn atoms in divalent charge state and holes per cm 3 had been assumed [1.9]. (b) Stability of the ferromagnetic states in GaN-based DMS s. Positive difference means that the ferromagnetic state is more stable than the spin glass state. cidate the mechanism of the occurrence of ferromagnetism in the DMS s. Especially important is information about their electronic structure including the interaction between the 3d electrons of the transition metal and the band electrons of host material. As the electronic structure of Ga 1 x Mn x N has not been studied experimentally so far, it is strongly desired to study its electronic structure to see whtere ferromagnetism is possible or not. We have investigated the electronic structure of the Ga 1 x Mn x N using photoemission spectroscopy (PES) and subsequent configuration-interaction (CI) cluster model analysis, x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (MCD). PES and XAS are powerful tools to investigate the electronic structure of solids. In the studies of DMS s, too, PES and XAS have played important roles to investigate their electronic structures. CI approach is a powerful analytical tool to describe such systems in which the Coulomb interaction term and the hybridization term is competing.

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9 References [1.1] H. Munekata, H. Ohno, S. von Molnár, A. Segmuller, L.L. Chang, and L. Eisaki, Phys. Rev. Lett. 63, 1849 (1989). [1.2] H. Ohno, H. Munekata, S. von Molnár, and L.L. Chang, J. Appl. Phys. 69, 6103 (1991). [1.3] F. Matsukura, H. Ohno, A. Shen and Y. Sugawara, Phys. Rev. B. 57, R2037 (1998). [1.4] K.C. Ku, S.J. Potashnik, R.F. Wang, S.H. Chun, P. Schiffer, N. Samarth, M.J. Seong, A. Mascarenhas, E. Jonston-Harperin, R.C. Myers, A.C. Gosserd and D.D. Awschalom, Appl. Phys. Lett. 82, 2302 (2003). [1.5] A.M. Nazmul, S. Sugahara, and M. Tanaka, Phys. Rev. B. 67, (2003). [1.6] Y.D. Park, J.D. Lim, K.S. Suh, S.B. Shim, J.S. Lee, C.R. Abernathy, S.J. Pearton, Y.S. Kim, Z.G. Khim, Phys. Rev. B. 68, (2003). [1.7] G.A. Medvedkin, T. Ishibashi, T. Nishi, K. Hayata, Y. Hasegawa, and K. Sato, Jpn. J. Appl. Phys. 39, L949 (2000). [1.8] G.A. Medvedkin, K. Hirose, T. Ishibashi, T. Nishi, V.G. Voevodin, and K. Sato, J. Cryst. Growth. 236, 609 (2002). [1.9] T. Dietl, H. Ohno, F. Matukura, J. Cibert and D. Ferrand, Science 287, 1019 (2000) : T. Dietl, H. Ohno, and F. Matsukura, Phys. Rev. B. 63, (2001). [1.10] K. Sato and H. Katayama-Yoshida, Jpn. J. Appl. Phys. 39, L555 (2000) [1.11] K. Sato and H. Katayama-Yoshida, Jpn. J. Appl. Phys. 40, L485 (2001)

10 10 References [1.12] K. Ueda, H. Tabata, and T. Kawai, Appl. Phys. Lett. 79, 988 (2001). [1.13] H. Saeki, H. Tabata, and T. Kawai, Solid State Commun. 120, 439 (2001). [1.14] S. Kuwabara, T. Kondo, T. Chikyow, P. Ahmet and H. Munekata, Jpn. J. Appl. Phys. 40, L724 (2001). [1.15] S. Sonoda, S. Shimizu, T. Sasaki, Y. Yamamoto and H. Hori, J. Cryst. Growth. 237, 1358 (2002). [1.16] M.E. Overberg, C.R. Abernathy, S.J. Pearton, N.A. Theodoropoulou, K.T. McCarthy and A.F. Hebard, Appl. Phys. Lett. 79, 1312 (2001). [1.17] K. Ando, Appl. Phys. Lett. 82, 100 (2003)

11 Chapter 2 Physical properties of Ga 1 x Mn x N 2.1 III-nitride semiconductors After the development of crystal growth techniques of the III-nitride compounds including GaN and Ga 1 x Al x N [2.1, 2], the electric and optical qualities of the crystal were improved owing to the development of the crystallographic quality. Device technologies have progressed because the technique is useful not only to binary compounds such as GaN but also to ternary and quaternary compounds such as In 1 x Ga x N and In x Ga y Al 1 x y N. Ternary compounds III x III 1 x N such as In 1 x Ga x N and quaternary compounds III x III y III 1 x y N such as In x Ga y Al 1 x y N can be made using compounds having same crystal structure. All of these III-nitride semiconductors except for BN have a direct band gap and are stable in wurtzite structure at room temperature. These binary, ternary and quaternary IIInitride materials are promising candidates as optical devices because the values of the band gap, ev, range from the visible light to ultraviolet. On the other hand, these materials are expected to be useful for the production of new devices such as power devices and electron emitter because GaN has a large electron-velocity saturation compare to GaAs, and AlGaN have a negative electron affinity. Since the III-nitride semiconductors are stable physically and chemically, devices prepared using III-nitride semiconductors can work in harsh conditions compare to devices made of conventional semiconductors. Those characteristics of the III-nitride semiconductors are derived from the fact that these materials include nitrogen atoms. General physical properties of the III-nitride semiconductors are described below.

12 12 Chapter 2. Physical properties of Ga 1 x Mn x N Table 2.1: Part of the periodic table and the energies of valence electrons in free atoms (all values are negative and are given in units of ev). Let us show that the properties are much different between materials belonging to row 2 and those belonging to row 3 in the periodic table. Table. 2.1 shows a part of the periodic table and the energies of valence electrons. The number of the row in the periodic table indicates the principal quantum number. The atoms belonging low rows in the periodic table tend to have many electronic orbital. Therefore, the spatial distribution of electron in these atoms tends to become extended. Most of tetrahedrally bonded semiconductors are formed as the number of the valence electrons becomes eight per two atoms, as in Si, GaAs and ZnSe. Since the atomic radii belonging to the same row in the periodic table similar, the lattice constants of compound semiconductors consisting of atoms in the same row, such as Ge, GaAs and ZnSe, are similar to each other. On the other hand, in the III-V and II-VI compounds, one of the element is charged positively while the other one is charged negatively, namely the bonds in the compound semiconductors have the ionic nature in addition to the covalent bond. We define the ionicity f i using the bond gap E g according to Phillips [2.3]. The bond gap E g, which is the energy difference between the bonding state and anti-bonding state while the band gap is energy difference between the conduction-band minimum and valence-band maximum, can be

13 2.1. III-nitride semiconductors 13 define using the part derived from the ionicity and the covalent bond as [2.3] E 2 g = E2 h + C2, (2.1) where E h is the energy difference between the anti- and bonding-state derived from the covalent bond, namely the bond-gap energy derived from those of atoms in the corresponding IV family. C is the energy between the anti-bonding and bonding-state derived from the ionicity. The ionicity f i is defined using as f i = C2. (2.2) Eg 2 Table 2.2 shows the ionicity of semiconductors [2.3]. The ionicity f i = 0 means a complete covalent bond. In most of the III-V semiconductors, the values of C are smaller than those of E h while in most of the II-VI and I-VII semiconductors, the values of C are larger than those of E h. However, E h and C are same in AlN, GaN and InN. The distance between the atoms strongly depend on the row which the atoms belong to. As shown in Fig. 2.1, the distance decreases rapidly in row 2. One of the characteristics of the III-nitride semiconductors is very small lattice constants compare to semiconductors consisting of atoms below row 3. The nearly atomic distances in AlN, GaN and InN are also very small as shown in Fig That the bond length is small implies a strong bonding between the atoms, namely the cohesive energy is large. The cohesive energies of row 1, 2, 3 and 4 horizontal sequences and quasi-horizontal sequences not involving row 1 are all shown in Fig Also there are large differences between crystals belonging to row 2 and below the row 3 in cohesive energies. That the bonding in the crystals belonging to row 2 is strong is thus indicted. The crystals are produced easily in order of Al>GaN>InN, as shown in Fig The strong bonding is the second characteristics of the III-nitride semiconductors. The third characteristics is that the energies of the valence electrons are low. The energies of nitrogen s and p orbitals are low as shown in Table 2.1. The s and p orbitals contribute to bonding substantially. The mean energy of these electrons E s +3E p (2.3) 4 are shown in Fig. 2.3 against the row in the periodic table. These energies of the valence electron in row 2 is extremely low while the energies of atoms below row 3 are not much different. This means that these electrons are

14 14 Chapter 2. Physical properties of Ga 1 x Mn x N Table 2.2: E h, C, and ionicity f f of various semiconductors. f i =0 means a completely covalent bond [2.3]. strongly bound to the atom. The energies of the valence electrons are lowered because the atomic bond strength increases when the distances between the atoms decreases. This tendency is conspicuous for the III-nitrides semiconductors. For instance, the conduction-band minimum of GaN is lower than the valence-band maximum of GaAs. The last characteristics worthy of special mention of the III-nitride semiconductors is that their dielectric constants are small. The dielectric constant is determined by the polarizability of constituent ions caused by external electric field. Since the nitrogen atom binds the electrons strongly, its polarizability is small. And therefore the dielectric constant is small. Coulomb interaction in the solid is large for a small dielectric constant. This is one of the reasons why the acceptor and/or donor level is relatively deep in the III-nitride compounds. Free hole is difficult to be generated because the hole is bound to acceptor through the strong Coulomb interaction. As mentioned above, the characteristics of the III-nitride semiconduc-

15 2.1. III-nitride semiconductors 15 Figure 2.1: Relationship between the distance and the ionicity f i of various semiconductors. Compounds of the same row are connected by lines. tors are considerably different from the semiconductors constituted by the atoms below row 3. Most of these remarkable characteristics are derived from nitrogen atoms. Most semiconductors have the structure of diamond type, zinc-blend type or wurtzite type. The diamond type and the zinc-blend type are the same structure basically. Figure 2.4(a) shows a part of tetrahedral bonded AB crystal. If the atoms are not charged, B 2 atoms are located at a more stable position in the middle of the A 1 atoms. On the other hand, if the atoms are charged, these A 1 and B 2 atoms attract each other, and B 2 atoms are located at just above the A 1 atoms. Figure 2.4(b) and (c) shows the wurtzite structure and zinc-blend structure, respectively. If the ionicity is stronger, each ion is six-fold coordinated because the Coulomb attractive energy is greater for the six-fold coordination, such as NaCl. Therefore, the crystal structures are determined by the ionicities of the crystal as shown in Fig. 2.5 [2.3]. All of the crystals plotted above straight solid line (f i = 0.785) in Fig. 2.5 have a rock solt structure. The zinc-blend type and the wurtzite type can be classified not by straight line, but a curve as shown in Fig Crystals near this boundary curve such as ZnSe, ZnS, AlN, GaN and InN can be crystallized not only in the wurtzite but also in the

16 16 Chapter 2. Physical properties of Ga 1 x Mn x N Figure 2.2: Gibbs free energy of atomization G s against f i [2.3]. Figure 2.3: Mean energy of sp valence electrons against the row of the periodic table.

17 2.1. III-nitride semiconductors 17 Figure 2.4: (a)part of a tetrahedral bonded AB crystal. (b)wurtzite structure. (c)zinc blend structure. zinc-blend type because of the ionicity is comparable to covalency in these semiconductors. GaN has normally wurtzite-type structure. By selecting the substrate, the cubic GaN (zinc-blend structure) can be synthesized. In this thesis, we consider only GaN in the wurtzite structure. The unit cell and the reciprocal unit cell of the wurtzite structure shown in Fig Unlike the zinc-blend structure, the structure of the reciplocal unit cell is hexagonal. The lattice constants a and c of bulk (thin film) GaN are (3.188) and (5.183), respectively [2.4]. The band structure of wurtzite GaN has been investigated theoretically and experimentally. The band structure has

18 18 Chapter 2. Physical properties of Ga 1 x Mn x N Figure 2.5: Relationship between the covalency and the ionicity of various semiconductors. The crystal structures are classified by the ionicity of the crystals [2.3]

19 2.2. Ga 1 x T x N 19 Figure 2.6: The unit cell and the reciprocal unit cell of the wurtzite structure. (a) unit cell. (b) reciprocal unit cell. been calculated using various methods such as the tight-binding method, first-principle calculation and the local-density-approximation (LDA) with self-interaction correction [2.7, 6, 8, 9]. Bulk band dispersion and surface states of the thin-film wurtzite GaN have been investigated experimentally by Dhesi et al. using angle-resolved photoemission spectroscopy [2.5]. Figure 2.7 shows the band mapping determined experimentally [2.5] and calculated [2.7]. 2.2 Ga 1 x T x N DMS s based on GaN have been extensively studied because of the intrinsic high potential of GaN and the theoretical prediction that ferromagnetism with a very high T C occurs in systems such as p-type Ga 1 x Mn x N [2.9, 7]. In this section, the physical properties of DMS s based on GaN are summarized. Dietl et al. have reported the theoretical study which predicts that ferromagnetism occurs in Ga 1 x Mn x N. for the Mn concentration of 2.5 at % per unit cell and the hole concentration of cm 3 [2.9]. It has also been predicted the T C exceeds the room temperature. On the other hand, based on a self-consistent electronic structure calculation using the local spin density approximation (LSDA), Sato et al. have studied V-, Cr-, Mn-, Fe-, Coand Ni-doped GaN-based DMS s in the ferromagnetic state. The d orbitals of the transition metal splits in to t 2g and e g states by the crystal field. The

20 20 Chapter 2. Physical properties of Ga 1 x Mn x N Figure 2.7: Band mapping of the wurtzite GaN. The solid line and dots indicate band dispersion decided by the calculation [2.7] and experiment [2.5]. 3d t2g orbitals, which have the symmetry as functions of xy, yz and zx, hybridize well with p orbitals of valence band, so that they form bonding states t b and their anti-bonding counterpart t a as schematically shown in Fig. 2.8 [2.7]. Thus it has been predicted that the anti-bonding t a states and nonbonding e states appear in the band gap of the host semiconductors and are partially occupied. They have also reported that the V-, Cr- and Mn-doped GaN is a promising candidate to realize room temperature ferromagnetic DMS s because the ferromagnetic state is stable in these materials. There is many attempts to produce GaN-based DMS s to realize room temperature ferromagnetic DMS s. So far, it has been reported that GaN doped with Cr [2.11], Mn [2.15, 16, 17], Fe [2.12, 13, 14] Gd [2.18], Tb [2.19] were synthesized. Ferromagnetism in the GaN-based DMS s has been reported for Cr-doped [2.11], Mn-doped [2.16, 17] and Gd-doped [2.18]. Below, we shall overview GaN doped with transition-metal ions. Sonoda et al. have reported that Ga 1 x Mn x N prepared by NH 3 -MBE with Mn concentration of 9 at % has T C exceeding the room temperature [2.16]. Surprisingly, the estimated value of the T C is 940K as shown in Fig In that report, the coercivity H c and residual magnetization M r of Ga 0.91 Mn 0.09 N at 300K have been estimated to be 85 Oe and 0.77 emu/g, respectively. However, Ando have reported using MCD that Ga 1 x Mn x N prepared by NH 3 -MBE

21 2.2. Ga 1 x T x N 21 Figure 2.8: Schematic electronic structure of the transition metal atom substituting the Ga site in GaN [2.7]. Figure 2.9: Temperature dependence of the magnetization at 0.1 T of Ga 0.91 Mn 0.09 N prepared by NH 3 -MBE [2.16]

22 22 Chapter 2. Physical properties of Ga 1 x Mn x N Figure 2.10: Temperature dependence of sheet resistance (a) and Hall resistance (b) of Ga 0.93 Mn 0.07 N prepared by RF-plasma-MBE [2.17]. [2.22]is a paramagnetic DMS and the ferromagnetism of the sample arises from an unidentified material that is not detected by the x-ray diffraction [2.21]. Overberg et al. have prepared Ga 1 x Mn x N with Mn concentration of 7 at % [2.17] and reported its magnetic and magnetotransport properties as shown in Fig Based on the anomalous Hall effect, negative magnetoresistance and magnetic hysteresis at 10 K, they have concluded that Ga 1 x Mn x N have T C between 10 and 25 K because the anomalous Hall term vanishes at 25 K as shown in Fig On the other hand, Kondo et al. have reported that Ga 1-xMn xn grown by RF-plasma-assisted-MBE shows primarily paramagnetic behavior [2.13]. They have systematically investigated the properties of epilayers that the electron concentration decreases and the resistivity increases with increasing Mn concentration. They have estimated the effective spin number of the paramagnetic component as S 2.5. This implies that Mn atoms becomes Mn 2+ ions due to compensation. Probably the compensation is caused by defects which provide the electrons, consistent with the previous report [2.20]. Also, they have reported that epilayers with high Mn concentrations ( cm 3 ) have a positive paramagnetic Curie temperature as shown in Fig The positive paramagnetic Curie temperature suggests the presence of ferromagnetic spin exchange between Mn ions. From these measurements, they have proposed that the conduction type of Ga 1 x Mn x Nin the high Mn concentration region is p-type with low hole concentration while in general, the paramagnetic Curie temperature become negative in high Mn concentration region because of the presence of the anti-ferromagnetic exchange interaction between the Mn ions. Thus, so far, the magnetic property of Ga 1 x Mn x N have been quite

23 2.2. Ga 1 x T x N 23 Figure 2.11: Curie-Weiss plot for Ga 0.98 Mn 0.02 N [2.15]. diverse between the difference reports, and the occurrence of the ferromagnetism is still controversial. As the electronic structure of Ga 1 x Mn x N has not been studied experimentally so far, it is desirable to study its electronic structure to see whether there is a possibilities of the ferromagnetism in this system.

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25 References [2.1] H. Amano, N. Sawaki, I. Akasaki and Y. Toyoda, Appl. Phys. Lett. 48, 353 (1986). [2.2] I. Akasaki, H. Amano, Y. Koide, K. HIramatsu and N. Suwaki, J. Cryst. Growth. 98, 209, (1989). [2.3] J.C. Phillips : Bonds and Bands in Semiconductors (Academic, New York) (1973). [2.4] M. Leszczynski, H. Teisseyre, T. Suski, I. Grzegory, M. Bockowski, J. Jun, K. Pakula, J.M. Baranowski, C.T. Foxon and T.S. Cheng, Appl. Phys. Lett. 69, 73 (1996). [2.5] S.S. Dhesi, C.B. Stagarescu and K.E. Smith, Phys. Rev. B. 56, (1997). [2.6] K. Miwa and A. Fukumoto, Phys. Rev. B. 48, 7897 (1993). [2.7] A. Rubio, J.L. Corkill, M.L. Cohen, E.L. Shirley and S.G. Louie, Phys. Rev. B. 48, (1993). [2.8] T. Yang, S. Nakajima and S. Sasaki, Jpn. J. Apll. Phys. 34, 5912 (1995). [2.9] C. Persson and A. Zunger, Phys. Rev. B. 68, (2003). [2.10] T. Dietl, H. Ohno, F. Matukura, J. Cibert and D. Ferrand, Science 287, 1019 (2000) : T. Dietl, H. Ohno, and F. Matsukura, Phys. Rev. B. 63, (2001). [2.11] M. Hashimoto, Y.K. Zhou, M. Kanamura, H. Katayama-Yoshida and H. Asahi, J. Cryst. Growth. 251, 327 (2003). [2.12] H. Akinaga, S.Németh, J.D. Boeck, L.Nistor, H. Bender, G. Borghs, H. Ofuchi and M. Oshima, J. Appl. Lett. 77, 4377 (2000).

26 26 References [2.13] S. Kuwabara, T. Kondo, T. Chikyow, P. Ahmet and H. Munekata, Jpn. J. Apll. Phys. 40, L724 (2001). [2.14] J. Gosk, M. Zajac, M. Byszewski, M. Kaminska, J. Szczytko, A. Twardowski, B. Strojek and S. Podsiadlo, J. Superconductivity. 16, 79 (2003) [2.15] T. Kondo, S. Kuwabara, H. Owa and H. Munekata, J. Cryst. Growth. 237, 1353 (2002). [2.16] S. Snoda, S. Shimizu, T. Sasaki, Y. Yamamoto, and H. Hori,J. Cryst. Growth. 237, 1358 (2002). [2.17] M.E. Overberg, C.R. Abernathy, S.J. Pearton, N.A. Theodoropoulou, K.T. McCarthy and A.F. Hebard, Appl. Phys. Lett. 79, 1312 (2001). [2.18] N. Teraguchi, A. Suzuki, Y. Nanishi, Y.K. Zhou, M. Hashimoto and H. Asahi, Solid. Stat. Comm. 216, 625 (1999). [2.19] K. Hara, N. Ohtake and K. Ishii, Phys. Stat. Sol. 216, 625 (1999). [2.20] Y.L. Soo, G. Kioseoglou, S. Kim, S. Huang, Y.H. Kao, S. Kuwabara, S. Owa, T. Kondo and H. Munekata, Appl. Phys. Lett. 79, 3926 (2001). [2.21] K. Ando, Appl. Phys. Lett. 82, 100 (2003). [2.22] S. Sonoda, H. Hori, Y. Yamamoto, T. sasaki, M. Sato, S. Shimizu, K. Suga and K. Kindo, IEEE Trans. Magn. 38, 2859 (2002).

27 Chapter 3 Principles of electron spectroscopy 3.1 Photoemission spectroscopy Photoemission spectroscopy (PES) is a powerful tool to directly investigate the electronic structure of materials. Photoemission is a phenomenon that a material irradiated by light emits electrons to outside. By absorbing the light, the an electron is excited from occupied state to the vacuum (unoccupied) state. PES measures the distribution of the kinetic energy of these electrons. Knowing the kinetic energy E k of the emitted electrons, one can deduce how strong the electron was bound to the material. Owing to the energy conservation law, Ekin V = hν E B Φ, (3.1) where hν is the energy of the incident photon, E B is the binding energy of the electron relative to the Fermi level E F and Φ is the work function which is the energy required for an electron at E F to escape from solid through the surface and to reach the vacuum level E vac, that is, Φ = E vac - E F. Here, let us ignore the correlation effects between the electrons and assume that according to Fermi distribution function these electrons occupy energy level up to E F, namely we assume the one-electron approximation. In actual PES measurements, since both the sample and the electron energy analyzer are grounded, the measured kinetic energy E kin of the photoelectron is referred to E F. We obtain, then, E kin = hν E B. (3.2) The energy -E B is approximately regarded as the energy of the electron inside the sample material before the photoemission. Therefore, the energy distribution inside the material can be directly mapped by the distribution of

28 28 Chapter 3. Principles of electron spectroscopy the kinetic energies of photoelectrons emitted with monochromatic incident photons. Figure 3.1: Schematic diagram of photoemission spectroscopy. The density of states N(E) can be obtained by measuring the photoemission spectra I(E). Figure 3.1 schematically shows how the electronic density of states (DOS) is measured by the electron distribution curve (EDC), that is the photoemission spectrum (I(E)). In this thesis, photoemission spectra will be displayed with the binding energy E B for the horizontal axis and the density of states of photoelectrons for vertical axis. In real systems, ignoring the correlation effect described above is inadequate to interpret the photoemission spectra For instance, photoemission

29 3.2. Resonant photoemission spectroscopy 29 spectra are affected by the relaxation of the entire electron system in the photoemission final state, in addition to the one-electron energy. In response the hole produced by the photoemission process, the surrounding electrons tend to screen the hole to lower the total energy of the system. Therefore, considering the entire electron system, the binding energy E B is given by the energy difference between the N-electron initial state Ei N and the (N 1)-electron final state E N 1 f,as E B Φ=E N 1 f E N i. (3.3) That is, E B is the energy that is required to produce the hole with photoemission process, including the relaxation energy of the total electron system. Thus, the consideration of correlation effect makes the interpretation of photoemission spectra complicated. However, this helps to obtain the information about the electron correlation in the materials. 3.2 Resonant photoemission spectroscopy Resonant photoemission spectroscopy (RPES) is a powerful technique to extract the impurity atom derived photoemission spectrum in the valence band. The capability of synchrotron radiation that one can continuously vary the photon energy is exemplified in RPES measurements. Figure 3.2 shows a schematic diagram of RPES. The direct photoemission process of a valence d electron is denoted as p 6 d N + hν p 6 d N 1 + e, (3.4) where e denotes the photoelectron. Here, we assume that the p core level is completely filled by six electrons. On the other hand, the absorption from the p core level to the valence d state occurs with the tunable photon energy. After the subsequent super-coster-krö nig decay, the final electronic configuration p 6 d N 1 is reached through p 6 d N + hν p 5 d N+1 p 6 d N 1 + e. (3.5) The energy level in the final states created by these two process have the same energy and the same electronic configuration. Therefore, these processes can interfere, resulting in a resonantly enhancement of the photoemission intensity from the d orbitals, and hence in the so-called Fano profile [3.1]. Since the enhancement occurs only for d orbitals, one can obtain the information about the d partial DOS. Using this method, one can selectively extract a orbital component from the valence-band spectrum because

30 30 Chapter 3. Principles of electron spectroscopy Figure 3.2: Schematic diagram of resonant photoemission process. the energy of the absorption where the enhancement occurs is different between the elements. This method is suitable for transition metal impurity systems because weak signals in normal photoemission measurements can be enhanced by RPES. 3.3 X-ray absorption spectroscopy The measurements of photo-absorption by excitation of a core-level electron into unoccupied states as a function of photon energy is called x-ray absorption spectroscopy (XAS). The probability of the excitation is proportional to the product between the DOS of the unoccupied states the transition

31 3.4. X-ray magnetic circular dichroism 31 probabilities. XAS, which measure the photo-absorption intensity including the excitation probability, therefore, takes mainly the information about the density of unoccupied states, because the DOS of the core is relatively sharp compare to the unoccupied states in energy. The photo-absorption intensity is given by I µ (hν) = f T µ I 2 δ(e i E f hν). (3.6) f Here, T is the dipole transition operator and µ is the index of light polarization. The 2p core-level XAS spectra of transition metal compounds well reflect the 3d electronic states in the 3d transition metal compounds including the symmetry and the crystal field splitting of the 3d orbitals. XAS is also selective in a elemental because the energy of excitation is adjusted to the energy proper to element. There are two measurement modes for XAS, namely the transmission mode and the yield mode. In the transmission mode, the intensity of the x- ray is measured before and after the samples and the ratio of the transmitted photons is recorded. Alternatively, one can obtain the absorption cross section by measuring decay products of the core hole which is created in the absorption process. This is the yield mode measurement and is standard for soft x-rays. In this thesis, the total electron yield method is adopted. 3.4 X-ray magnetic circular dichroism Using circularly polarized light in XAS, the absorption intensity depends on the helicity of the incident light. This method is called x-ray magnetic circular dichroism (XMCD). XMCD is defined as the difference between the absorption intensities for right- and left-handed circularly polarized light when the polarized light is parallel and antiparallel to the magnetization direction of the magnetic materials in a magnetic field. One of the advantages of XMCD measurement is also that it is an element specific measurement method like RPES and XAS. Another advantage is that XMCD measurements reflect the spin and orbital polarization of local electronic states. The value of the spin and orbital moments can be separately estimated by using magneto-optical sum rules [3.2, 3]. For the 2p-3d MCD analysis, the value of the spin m spin and orbital m orb magnetic quantum numbers are given by m spin = 6 L 3 (I + I )dω 4 L 3 +L 2 (I + I )dω, (3.7) L 3 +L 2 (I + + I )dω m orb = 4 L 3 +L 2 (I + I )dω 3 L 3 +L 2 (I + + I )dω (10 n 3d) ( 1+ 7 T ) Z 1, (3.8) 2 S Z

32 32 Chapter 3. Principles of electron spectroscopy where I ± is the absorption intensity for the positive (negative) helicity, T Z is the expectation value of the magnetic dipole operator and S Z is equal to half m spin. Thus, XMCD measurement is a powerful tool to investigate the magnetic moments in materials [3.4, 5]. 3.5 Configuration-interaction cluster model In order to consider the correlation interaction between electrons, we consider the hybridization between the Slater determinants instead of a single Slater determinant, The hybridizing between the Slater determinants leads to so-called configuration-interaction (CI). In this thesis, core-level and valence-band spectra will be analyzed using CI calculation on a MnN 4 cluster. The CI cluster-model analysis has been a useful framework for understanding the electronic structure of DMS [3.6, 7, 8]. In the CI picture, we consider a tetrahedral MnN 4 cluster with the central Mn atom. Here, the distortion from the T d symmetry tetrahedron in the wurtzite structure is ignored because the magnitude of the distortion is small. The wave function of the ground state ψ g, which we call N-electron state, is spanned by linear combinations of charge transfer state as ψ g = a 0 d n + a 1 d n+1 L + a 2 d n+2 L 2 +. (3.9) The final state wave functions of Mn 2p core-level photoemission ψ c and Mn 3d valence-band ψ v are also spanned by linear combinations of charge transfer state as, ψ c = b 0 cd n + b 1 cd n+1 L + b 2 cd n+2 L 2 +, (3.10) ψ v = c 0 d n + c 1 d n+1 L + c 2 d n+2 L 2 +, (3.11) where c and L denotes holes in the valence band and ligand p orbitals, respectively, and n = 5 for the ground state of the Mn +2. The anion-to-3d orbital charge-transfer energy is defined by E(d n+1 ) E(d n ), and the 3d-3d Coulomb interaction energy is defined by U E(d n+1 )+E(d n 1 ) 2E(d n ), where E(d l k) is the center of gravity of the d l L k multiplet. The multiplet splitting is expressed using Racah parameters B and C, which are fixed at the values of the free Mn 2+ ion (B = ev, C = ev) [3.9]. The average Coulomb interaction Q between the Mn 3d electron and the Mn 2p core hole is fixed at U/Q = 0.8. In the tetrahedral cluster model, one-electron transfer integrals between the 3d and ligand p orbitals are given by Slater-Koster parameters (pdσ) and (pdπ). We have utilized the relationship (pdσ)/(pdσ) = according to Harrison [3.10]. One can

33 3.5. Configuration-interaction cluster model 33 also define the charge-transfer energy eff and 3d-3d Coulomb interaction energy U eff with the respect to the lowest term each multiplet, are given by eff = + (70B 35C)/9+7C, (3.12) U eff = U + (14B 7C)/9+14B +14C. (3.13) Figure 3.3 schematically shows the energy diagram of a cluster in the neutral (N-electron system) and positively ionized ((N-1)-electron system) and negatively ionized ((N +1)-electron system) state. Photoemission process corresponds to the process from the N-electron system to the (N-1) electron system and inverse photoemission process corresponds to the process from the N-electron system to the (N-1) electron system. In practice, the, U and (pdσ) are parameterized, which are called electronic structure parameter, and are so chosen as reproduce the photoemission spectrum. Figure 3.3: Schematic energy-level diagram from a d n transition metal impurity in a host semiconductor. Using the fitted parameters, U and (pdσ), one can estimate the exchange constant Nβ between the 3d electron and the electron at the top of the valence band of host semiconductor [3.6] by the second-order perturbation with respect to charge transfer as Nβ = 16 ( )( 1 S δ eff + U eff δ eff 3 (pdσ) 2 ) (pdπ). (3.14)

34

35 References [3.1] U. Fano, Phys. Rev. 124, 1866 (1961). [3.2] B.T. Thole, P. Carra, R. Sette and G. ven der Laan, Phys. Rev. Lett. 68, 1943 (1992). [3.3] P. Carra, B.T. Thole, M. Altarelli and X. Wang, Phys. Rev. Lett. 70, 694 (1993). [3.4] C.T. Chen, Y.U. Idzerda, H.J. Lim, N.V. Smith, G.H. Ho, E. Pellegrion and F. Sette, Phys. Rev. Lett. 75, 152 (1995). [3.5] J. Stöhr, J. Elect. Spectrosc. Relat. Phenom. 75, 253 (1995). [3.6] T. Mizokawa and A. Fujimori, Phys. Rev. B. 48, (1993). [3.7] J. Okabayashi, A. Kimura, T. Mizokawa, A. Fujimori T. Hayashi and M. Tanaka, Phys. Rev. B. 59, 2486 (1999). [3.8] T. Mizokawa, T. Nambu, A. Fujimori, T. Fukumura and M. Kawasaki Phys. Rev. B. 65, (2002). [3.9] S. Sugano, Y. Tanabe and H. Kamimura, Multiplets of Transition Metal Ions in Crystals (Academic, New York, 1970). [3.10] W.A. Harrison, Electronic structure and the Properties of Solids (Dover, New York 1989).

36

37 Chapter 4 Experimental 4.1 Experimental The photoemission experiments were performed at BL-18A of the Photon Factory, High Energy Accelerator Research Organization. The measurements were made in an ultra high vacuum below Torr at room temperature. Photoelectrons were collected using a VG CLAM hemispherical analyzer in the angle-integrated mode. The total energy resolution including the monochromator, the electron analyzer and the temperature broadening was estimated to be 200meV from the Fermi edge of a metal. Core-level photoemission spectra were taken using a MgKα source (hν = ev). The resolution was estimated to be 0.8 ev from the Au 4f core-level spectrum. Satellite emission of the Kα 1,2,Kα 3,4 and Kα 5,6 has been numerically subtracted. The photoemission spectra were referenced to the Fermi edge of a metal in electrical contact with the sample. Also, the Au 4f 7/2 core-level binding energy set to 84.0 ev. In the electron energy analyzer, the ejected electrons are retarded by an amount V R before entering the analyzer. The analyzer is a band pass filter only transmitting electrons with energy equal to the pass energy E pass before reaching the detector as shown in Fig Then we obtain the relation, E V kin +Φ=E kin = ev R + E pass +Φ A = hν + E B, (4.1) where Ekin V and Φ are kinetic energy of photoelectron and a work function of the sample, respectively. E kin is kinetic energy of photoelectron measured from Fermi level E F. V R and Φ are the retarding voltage and the work function of the electron energy analyzer. The voltages of the outer hemisphere, the inner hemisphere and the entrance slit are shown in Fig In the hemispherical electron energy analyzer, two concentric hemispheres of mean radius R 0 are mounted. The potential V is applied between them so that

38 38 Chapter 4. Experimental Figure 4.1: Relation ship between the kinetic energy of photoelectron E kin, the pass energy E P ass and the retarding voltage V R. the outer hemisphere is negative and the inner hemisphere is positive with respect to V (R 0 )=V R which is the equipotential surface in the middle of hemispheres. The entrance and exit slits are both centered on the sphere of radius R 0. The relationship between E pass and V is given by ( R2 ev = E pass R ) 1. (4.2) R 1 R 2 The voltage on the inner and outer hemispheres, V in and V out, are given by ( ) R0 ev in = ev R +2E pass 1, (4.3) R 1 ( ) R0 ev out = ev R +2E pass 1. (4.4) R 2 Then one can calculate V R and E pass from the voltages of the outer hemisphere (V out ), the inner (V in ) and the entrance slit (V R ) using these expressions because R 0, R 1 and R 2 are constants which depend on the size of the analyzer.

39 4.2. Crystal growth and surface treatment of Ga 1 x Mn x N 39 Figure 4.2: Sketch of the hemispherical energy analyzer. 4.2 Crystal growth and surface treatment of Ga 1 x Mn x N Ga 1 x Mn x N (0001) thin films with x = 0.0, 0.02 and x = used in this study were grown by molecular beam epitaxiy with an RF-plasma nitrogen source and elemental sources of Ga and Mn on a sapphire (0001) substrate [4.1]. Figure 4.3 shows the layer structure of the sample. After nitridation of the substrate, a 3-nm-thick AlN buffer layer was grown on it followed by the growth of a 100-nm-thick GaN buffer layer. On top of the GaN layer,

40 40 Chapter 4. Experimental Sn-doped 100-nm-thick n-gan layer was grown to secure the conduction of the sample. Finally, a 110-nm-thick Ga 1 x Mn x N epitaxial layer was deposited on top of it at substrate temperature 550 C. The crystal polarity of the Ga 1 x Mn x N is determined to be Ga-terminated hexagonal surface by reflection high-energy electron diffraction (RHEED) [4.1]. All the samples thus prepared were paramagnetic from room temperature down to 4K. Figure 4.3: Sample layer structure of the Ga 1 x Mn x N. For surface cleaning, we made N + 2 ion sputtering followed by annealing up to 500 C. Sputtering of the GaN (0001) surface by Ar + and N + 2 ion beams have been investigated by Lai et al. using synchrotron-radiation photoemission spectroscopy [4.2]. For Ar + ion sputtering, the N atoms are preferentially removed and a Ga-enriched GaN surface is produced. The excess Ga atoms on the Ar + ion sputtered surface aggregate to form metallic Ga clusters at temperatures above 350 C as shown in Fig. 4.4(a). On the other hand, a well-ordered GaN (0001) 1 1 surface can be obtained by N + 2 sputtering instead of Ar +. In addition to acting as a sputtering ion, the N + 2 ion serves as a reactant which compensates for the preferential loss of N atoms caused by the physical bombardment, resulting in the reduction of deficiency of nitrogen atoms as shown in Fig. 4.4(b). The difference between the effects of the N + 2 and Ar + ion sputtering also appears in the valence-band photoemission spectra as shown in Fig For Ar + sputtering, a shoulder band appears just below the Fermi level upon annealing the sample above 350 C. This new band is attributed to the 4s and 4p states of metallic Ga. The effectiveness of the N + 2 ion sputtering for nitride compounds such as CrN is also reported [4.3]. In our measurements of Ga 1 x Mn x N, the diff-

41 4.2. Crystal growth and surface treatment of Ga 1 x Mn x N 41 Figure 4.4: Ga 3d and N 1s photoemission spectra of the GaN (0001). (a) Sputtered by 1 kv Ar + for 10 min at room temperature and then annealed to 623, 723 and 823 K. (b) Sputtered by 1 kv N + 2 for 10 min at room temperature and then annealed to the indicated temperatures [4.2]. ences between N + 2 and Ar + ion sputtering is confirmed in the photoemission spectra and low-energy electron diffraction (LEED). Figure 4.6 shows the valence-band photoemission spectra of Ga 0.98 Mn 0.02 N after 2.0 kv N + 2 or 1.5 kv Ar + ion sputtering followed by annealing up to 500 C. As stated above, a shoulder band just below the Fermi level and a Fermi edge appear for Ar + ion sputtering while they do not appear for N + 2 ion sputtering. The spectrum for Ar + ion sputtering is broad as a whole compare to that for N + 2 ion sputtering. Figure 4.7 shows the LEED pattern of Ga 0.98 Mn 0.02 N after the N + 2 or the Ar + ion sputtering followed by annealing up to 500 C. The LEED pattern of the Ar + sputtered surface is broad and weak than that of N + 2 ion sputtered surface. In our measurements of Ga 1 x Mn x N, the cleanliness of the surface was checked by LEED and core-level XPS measurements. Figure 4.8 shows the LEED pattern of Ga 1 x Mn x N (0001). The O 1s and C 1s core-level peaks were diminished below the detectable limit by repeated N + 2 ion sputtering

42 42 Chapter 4. Experimental Figure 4.5: Valence-band photoemission spectra of the GaN (0001) surfaces sputtered by (a) 1 kv Ar + and (b) 1 kv N + 2 for 10 min at room temperature. The sputtered GaN samples are subsequently annealed to 623, 723, 823 K [4.2]. and annealing, and a clear low-energy electron diffraction (LEED) pattern was obtained, reflecting an ordered clean surface.

43 4.2. Crystal growth and surface treatment of Ga 1 x Mn x N 43 Intensity (arb.unit) Ga0.98Mn0.02N AIPES (hν = 45 ev) 2 kv N 2 + sputtering 1.5 kv Ar + sputtering Binding Energy (ev) Figure 4.6: Difference between N + 2 and Ar + ion sputtering in the valenceband photoemission spectra of Ga 0.98 Mn 0.02 N (0001). Figure 4.7: Difference of between the N + 2 and the Ar + ion sputtering in the LEED pattern of Ga 0.98 Mn 0.02 N (0001) surface.

44 44 Chapter 4. Experimental Figure 4.8: LEED pattern of the Ga 1 x Mn x N (0001) surface.

45 References [4.1] T. Kondo and H. Munakata, Oyobuturi 71, 1274 (2002). (in Japanese) [4.2] Y.H. Lai, C.T. Yeh, J.M. Hwang, H.L. Hwang, C.T. Chen and W.H. Hung, J. Phys. Chem. B (2001). [4.3] I. Bertoti, M. Mohai, P. H. Mayrhofer and C. Mitterer, Surf. Interface. Anal. 34, 740 (2002)

46

47 Chapter 5 Results and discussion 5.1 X-ray absorption spectroscopy and magnetic circular dichroism We have made an x-ray absorption spectroscopy (XAS) measurement to obtain the information about the valence state of the Mn ion in Ga 1 x Mn x N. In contrast to photoemission spectroscopy, absorption experiment provides us with a direct insight into the 3d valence electronic structure of Mn since the dipole selection rule ensures that the Mn 2p core electron is excited to these states. Figure 5.1 shows the Mn absorption spectrum of Ga Mn N. The two groups of peaks are associated with Mn 2p 3/2 and 2p 1/2 spin-orbit doublet. If the final state is delocalized, the line shape of the absorption edges is reflects the density of unoccupied states above the Fermi level. However, the rich structures of the observed spectrum are typical for localized 3d states such as those of the Ga 1 x Mn x As [5.1]. Comparing the line shape observed here with calculated absorption spectrum, one can obtain the information about the valence state of a Mn in the Ga 1 x Mn x N. Calculations for the d 3, d 4, d 5 configuration in a tetrahedral crystal field have been done by van der Laan [5.2] for various magnitude of the crystal field. Figures 5.2 and 5.3 show the experimental spectrum compared with the calculations for the d 4 and d 5 ground states in a tetrahedral crystal field, respectively. In Fig. 5.2, the calculations for all values of the crystal-field splitting 10Dq do not correspond to the experimental spectrum, particularly for the 2p 1/2 peak. In contrast, d 5 ground state as shown in Fig. 5.3, the calculation with the values of 10Dq = ev well correspond to the experimental spectrum. This leas us to conclude the experimental spectrum to be that of the d 5 ground state in the tetrahedral crystal field, namely, the valence state of Mn in the Ga 1 x Mn x N is close to 2+ and S 5/2 as

48 48 Chapter 5. Results and discussion norm. total yield (arb.unit) Mn 2p 3/2 Ga0.958Mn0.042N XAS (T = 80K) Mn 2p 1/2 experimental back ground Photon Energy (ev) Figure 5.1: Absorption spectrum of the Mn 2p in Ga Mn Nat80K. The two peaks are associated with Mn 2p 3/2 and 2p 1/2. in Ga 1 x Mn x As [5.1]. This also indicates that the Mn ion acts an acceptor in the host GaN and produces a hole, consistent with the previous report [5.3, 4]. We have also measured magnetic circular dichroism in x-ray absorption spectroscopy. This is called x-ray magnetic circular dichroism (XMCD). Its utmost strength is the element-specific, quantitative determination of the spin and orbital magnetic moments. Figure 5.4 shows the Mn 2p core absorption spectra excited with circular polarized light of either helicity at 80K in an applied field of 2 T. The spectra taken with different light helicity show differences, showing circular dichroism, especially at 2p 3/2 absorption peak. The circular dichroism is shown in the lower panel of Fig We obtain the value of 3.8% at the 2p 3/2 peak. According to the XMCD sum rules [5.5, 6] as indicated by Eq. 3.7 and 3.8 in chapter 3, one can estimate the spin and orbital magnetic moment of the Mn ion in Ga 1 x Mn x N. The sum rules are m spin = 6 L 3 (I + I )dω 4 L 3 +L 2 (I + I )dω, (5.1) L 3 +L 2 (I + + I )dω