- Email: [email protected]
Electronic structures of (Zn, TM)O (TM: V, Cr, Mn, Fe, Co, and Ni) in the self-interaction-corrected calculations
ARTICLE IN PRESS
Physica B 376–377 (2006) 647–650 www.elsevier.com/locate/physb
Electronic structures of (Zn, TM)O (TM: V, Cr, Mn, Fe, Co, and Ni) in the self-interaction-corrected calculations M. Toyodaa,, H. Akaib, K. Satoa, H. Katayama-Yoshidaa a
Department of Condensed Matter Physics and Department of Computational Nanomaterials Design, Nanoscience and Nanotechnology Center, Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan b Department of Physics, Graduate School of Science, Osaka University, 1-16 Machikaneyama, Toyonaka, Osaka 560-0043, Japan
Abstract We calculate the electronic structures of ZnO-based dilute magnetic semiconductors within the self-interaction-corrected local density approximation. The results are compared with those calculated within the standard local density approximation. We find the differences in the band gap energy, the energetic position of the Zn 3d bands, and the description of the transition-metal d bands. r 2006 Elsevier B.V. All rights reserved. PACS: 31.15.Ar; 75.50.Pp; 71.20.b; 71.15.m Keywords: ZnO; Self-interaction correction; Dilute magnetic semiconductors; Photoemission spectroscopy
1. Introduction II–VI compound semiconductors such as TiO2 and ZnO have been attracting much interest as a host material for the dilute magnetic semiconductors (DMS) which show ferromagnetism at or above room temperature. One of their advantages is that they are wide-gap semiconductors and transparent for visible light, and thus that the DMS based on them could be used for electrooptical applications. After the first successful growth of (Zn, Mn)O by Fukumura et al. [1], many experiments on ZnO-based DMS have been performed. However, there are widely different reports of the experimental observations of, for example, the magnetic ground state and the magnetic moment [2]. This might be because the second phases or the ferromagnetic precipitates could be formed in the samples, depending delicately on the growth conditions. Careful structural characterizations thus should be necessary. In the theoretical studies, typically using first-principles calculations, the electronic, magnetic and structural propCorresponding author. Tel.: +81 6 6879 8536; fax: +81 6 6879 8539.
E-mail address: [email protected] (M. Toyoda). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.12.163
erties are efficiently investigated without such experimental difficulties. However, the standard technique of firstprinciples calculations, the density functional theory within the local density approximation (LDA), would be fairly inadequate for the wide-gap DMS for two reasons. First, LDA often underestimates the band gap energies of semiconductors. On example of this is that the band gap energy of ZnO (3.4 eV [3]) is calculated to be only about 1 eV in LDA. Secondly, LDA often gives very poor description of the systems with strong electron correlation effect. According to the analysis of the X-ray absorption spectra by using configuration-interaction cluster-model calculations, the on-site Coulomb energy U for Mn2þ , Fe2þ , and Co2þ impurities in ZnO are obtained as 5.0, 5.5, and 6.0 eV, respectively [4]. The large U values indicate that the electron correlation effect is not negligibly small in those DMS and should be taken into account in calculations. In fact, the recently measured photoemission spectra of (Zn, V)O [5], (Zn, Mn)O [6] and (Zn, Co)O [7] show considerable difference from the electronic structures calculated within the standard LDA, particularly in the description of the transition-metal 3d states. One major source of these shortcomings of LDA is the presence of self-interaction. We therefore carry out the
ARTICLE IN PRESS M. Toyoda et al. / Physica B 376–377 (2006) 647–650
648
calculations on ZnO-based DMS by using a self-interaction-corrected LDA approach. The self-interaction-corrected electronic structures of (Zn, V)O, (Zn, Mn)O and (Zn, Co)O are shown to be in very good agreement with the photoemission experiments [8]. In this paper, we present the systematic comparison of the electronic structures of ZnO-based DMS calculated within the self-interaction-corrected LDA and the standard LDA.
2. Calculation method We use the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) code [9,10]. The form of the potentials is restricted to the muffin-tin type. The Muffin-tin radii are chosen in such a way that the ions touch each other with the radius ratio of 1. The lattice constants are fixed to the experimental values of pure wurtzite ZnO [11]. We use up to 456 k-points in the irreducible part of the first Brillouin zone. No relativistic effect is taken into account. In order to implement the self-interaction correction, we use the pseudo-SIC method [12]. In this method, the Kohn–Sham wave functions are projected onto a set of localized basis orbitals. The corrections to the effective potentials are defined on each localized orbital. They are assumed to depend linearly on the orbital occupation numbers. This occupation number dependence of the correction size is the trick of the method. The orbital occupation numbers reflect the degree of localization on the orbitals, i.e. a state localized on an orbital has a large (close to 1) occupation number on the orbital, while a 10 Total V-d
(Zn,V)O
(Zn,Cr)O
Density of states (/eV/Unit cell or Atom)
5
delocalized state has small occupation numbers. Therefore, a large amount of correction is made to a localized state, and a small amount of correction is made for a delocalized state. Furthermore, no correction is correctly assured to be made for unoccupied states since their occupation numbers are 0. Since the amount of correction for each orbital is determined self-consistently, any additional parameter is not required. In the original pseudo-SIC method, they use the pseudoatomic wave functions as the localized orbital basis set since they use a pseudopotential planewave code. In our implementation to the KKR-CPA code, we use the spherical wave functions defined inside the muffin-tin spheres as the basis set.
3. Results and discussion Fig. 1 shows the density-of-states (DOS) of ZnO-based DMS calculated within the standard LDA. The transitionmetal 3d electrons give rise to the highly localized impurity states in the band gap. The hybridization with the valence band is relatively weak. The electron configuration is highspin type with 1–3 eV of exchange splitting. The DOS show the clear half-metallic features. Fig. 2 shows the DOS of ZnO-based DMS calculated within the pseudo-SIC method. The apparent differences from those within LDA are found in the band gap energy, the energetic position of the Zn 3d bands, and the description of the transition-metal 3d states. In the case of (Zn, Mn)O, for example, the top of the valence band is located at about 3 eV below the Fermi level, whereas it is located at about 2 eV below the Fermi level in LDA. The Total Cr-d
Total Mn-d
(Zn,Mn)O
majority spin
majority spin
majority spin
0 -5 minority spin
(a) -10 10 (Zn,Fe)O
(b) (Zn,Co)O
Total Fe-d
minority spin
minority spin
(c) (Zn,Ni)O
Total Co-d
Total Ni-d
5 majority spin
majority spin
majority spin 0 -5 minority spin
(d)
(e)
minority spin
minority spin
(f)
-10 -10
-5
0
5
-10 -5 0 Energy relative to the Fermi level (eV)
5
-10
-5
0
5
Fig. 1. DOS of (a) (Zn, V)O, (b) (Zn, Cr)O, (c) (Zn, Mn)O, (d) (Zn, Fe)O, (e) (Zn, Co)O, and (f) (Zn, Ni)O calculated within the standard LDA. Total DOS per unit cell (solid lines) and partial DOS of transition-metal 3d orbitals per atom (dashed lines) are plotted. The experimental peak positions of V (1:8 eV [5]) and Co (3.0 and 7.0 [7]) 3d states are indicated by arrows.
ARTICLE IN PRESS M. Toyoda et al. / Physica B 376–377 (2006) 647–650
10 Total V-d
(Zn,V)O
(Zn,Cr)O
Density of states (/eV/Unit cell or Atom)
5
Total Cr-d
649
Total Mn-d
(Zn,Mn)O
majority spin
majority spin
majority spin
0 -5 -10 10 (Zn,Fe)O
(Zn,Co)O
Total Fe-d
minority spin (c)
minority spin (b)
minority spin (a)
(Zn,Ni)O
Total Co-d
Total Ni-d
5 majority spin
majority spin
majority spin
minority spin (d)
minority spin (e)
minority spin (f)
0 -5 -10 -10
-5
0
5
-10 -5 0 Energy relative to the Fermi level (eV)
5
-10
-5
0
5
Fig. 2. DOS of (a) (Zn, V)O, (b) (Zn, Cr)O, (c) (Zn, Mn)O, (d) (Zn, Fe)O, (e) (Zn, Co)O, and (f) (Zn, Ni)O calculated within the self-interaction corrected LDA. Total DOS per unit cell (solid lines) and partial DOS of transition-metal 3d orbitals per atom (dashed lines) are plotted. The experimental peak positions of V (1:8 eV [5]) and Co (3.0 and 7.0 [7]) 3d states are indicated by arrows.
Zn 3d bands which are located immediately below the valence band in LDA show the shift to the lower-energy side, being apart from the valence band by 2:5 eV. As the consequence, the hybridization with the valence band is significantly reduced. The transition-metal 3d bands are changed more drastically and a chemical trend is seen in the shapes of partial DOS of them. In (Zn, V)O and (Zn, Cr)O, the 3d bands shift downward in energy by 1 eV, but still appear within the band gap as they are in LDA. While in (Zn, Mn)O and (Zn, Fe)O, the fully occupied majority spin states of the 3d electrons shift more, showing the strong hybridization with the valence band. In (Zn, Co)O and (Zn, Ni)O, they shift further, making localized states around the bottom of the valence band. In the early studies by using the standard LDA, the transition-metal d density of states at the Fermi level has been pointed out to be playing an essential role for the ferromagnetic ordering in the ZnO-based DMS, because the double exchange interaction strongly stabilizes the ferromagnetic ground state through the hopping of d electrons. Except for the (Zn, V)O, the self-interactioncorrected electronic structures are insulating and no transition-metal d density of states is found at the Fermi level. Therefore, strong double exchange interaction is not expected to work. Take (Zn, Co)O as an example, which has been investigated intensively after the observation of room temperature ferromagnetism by Ueda et al. [13]. In LDA, the Fermi level lies in the gap of the tetrahedral-like crystal field splitting of the d states but due to the band broadening, d density of states exists at the Fermi level.
On the other hand, in pseudo-SIC, there is no d density of states at the Fermi level because the crystal field splitting is strongly enhanced. In experiments, Ueda et al. reported that ferromagnetic samples are conductive. However, there are also some reports that the insulating samples show ferromagnetism [14]. The origin of the ferromagnetism is still controversial. It should be also noted that the presenting results are in the case in which no additional carrier is introduced. Since ZnO is a native n-type semiconductor, some amounts of electrons would be introduced in ZnO-based DMS because of the O vacancy or the Zn interstitials [15]. The presence of those additional electrons would change the occupancy of the d states at the Fermi level. 4. Summary We have calculated the self-interaction-corrected electronic structures of ZnO-based DMS. Compared to those calculated within the standard LDA, (1) the larger band gap energy, (2) the Zn 3d bands correctly separated from the valence band, and (3) the qualitatively different description of the transition-metal 3d states are obtained. These features, especially the description of the transitionmetal 3d states could affect the predicted magnetic properties. Acknowledgments A part of this research is supported by JST-CREST, NEDO-nanotech, a Grant-in-Aid for Scientific Research in Priority Areas ‘‘Semiconductor Nanospintronics’’, a
ARTICLE IN PRESS 650
M. Toyoda et al. / Physica B 376–377 (2006) 647–650
Grant-in-Aid for Scientific Research for young researchers and 21st Century COE. References [1] T. Fukumura, Z. Jin, A. Ohtomo, H. Koinuma, M. Kawasaki, Appl. Phys. Lett. 75 (1999) 3366. [2] Recent studies on ZnO-based DMS are thoroughly reviewed in T. Fukumura, Y. Yamada, H. Toyosaki, T. Hasegawa, H. Koinuma, M. Kawasaki, Appl. Surf. Sci. 223 (2004) 62; R. Janisch, P. Gopal, N.A. Spaldin, J. Phys. Condens. Matter. 17 (2005) R657. [3] Y. Chen, D.M. Bagnall, Z. Zhu, T. Sekiuchi, K.-T. Park, K. Hiraga, T. Yao, S. Koyama, M.Y. Shen, T. Goto, J. Cryst. Growth 181 (1997) 165. [4] J. Okabayashi, K. Ono, M. Mizuguchi, M. Oshima, S.S. Gupta, D.D. Sarma, T. Mizokawa, A. Fujimori, M. Yuri, C.T. Chen, T. Fukumura, M. Kawasaki, H. Koinuma, J. Appl. Phys. 95 (2004) 3573. [5] Y. Ishida, J.I. Hwang, M. Kobayashi, A. Fujimori, H. Saeki, H. Tabata, T. Kawai, Physica B 351 (2004) 304. [6] T. Mizokawa, T. Nambu, A. Fujimori, T. Fukumura, M. Kawasaki, Phys. Rev. B 65 (2001) 085209.
[7] S.C. Wi, J.-S. Kang, J.H. Kim, S.S. Lee, S.-B. Cho, B.J. Kim, S. Yoon, B.J. Suh, S.W. Han, K.H. Kim, K.J. Kim, B.S. Kim, H.J. Song, H.J. Shin, J.H. Sim, B.I. Min, Phys. Stat. Sol. (b) 241 (2004) 1529; M. Kobayashi, Y. Ishida, J.I. Hwang, T. Mizokawa, A. Fujimori, K. Mamiya, J. Okamoto, Y. Takeda, T. Okane, Y. Saitoh, Y. Muramatsu, A. Tanaka, H. Saeki, H. Tabata, T. Kawai, Phys. Rev. B 72 (2005) 201201(R). [8] M. Toyoda, H.Akai, K. Sato, H. Katayama-Yoshida, in preparation. [9] H. Akai, P.H. Dederichs, Phys. Rev. B 47 (1993) 8739. [10] H. Akai, hhttp://www.sham.phys.sci.osaka-u.ac.jp/ kkr/i. [11] K. Kihara, G. Donnay, Can. Mineral. 23 (1985) 647. [12] A. Filippetti, N.A. Spaldin, Phys. Rev. B 67 (2003) 125109. [13] K. Ueda, H. Tabata, T. Kawai, Appl. Phys. Lett. 79 (2001) 988. [14] Z. Yin, N. Chen, C. Chai, F. Yang, J. Appl. Phys. 96 (2004) 5093; M. Venkatesan, C.B. Fitzgerald, J.G. Lunney, J.M.D. Coey, Phys. Rev. Lett. 93 (2004) 177206; H.-J. Lee, S.-Y. Jeong, C.R. Cho, C.H. Park, Appl. Phys. Lett. 81 (2002) 4020. [15] S.B. Zhang, S.-H. Wei, A. Zunger, Phys. Rev. B 63 (2001) 075205; A.P. Roth, J.B. Web, D.F. Williams, Phys. Rev. B 25 (1982) 7836.
Physica B 376–377 (2006) 647–650 www.elsevier.com/locate/physb
Electronic structures of (Zn, TM)O (TM: V, Cr, Mn, Fe, Co, and Ni) in the self-interaction-corrected calculations M. Toyodaa,, H. Akaib, K. Satoa, H. Katayama-Yoshidaa a
Department of Condensed Matter Physics and Department of Computational Nanomaterials Design, Nanoscience and Nanotechnology Center, Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan b Department of Physics, Graduate School of Science, Osaka University, 1-16 Machikaneyama, Toyonaka, Osaka 560-0043, Japan
Abstract We calculate the electronic structures of ZnO-based dilute magnetic semiconductors within the self-interaction-corrected local density approximation. The results are compared with those calculated within the standard local density approximation. We find the differences in the band gap energy, the energetic position of the Zn 3d bands, and the description of the transition-metal d bands. r 2006 Elsevier B.V. All rights reserved. PACS: 31.15.Ar; 75.50.Pp; 71.20.b; 71.15.m Keywords: ZnO; Self-interaction correction; Dilute magnetic semiconductors; Photoemission spectroscopy
1. Introduction II–VI compound semiconductors such as TiO2 and ZnO have been attracting much interest as a host material for the dilute magnetic semiconductors (DMS) which show ferromagnetism at or above room temperature. One of their advantages is that they are wide-gap semiconductors and transparent for visible light, and thus that the DMS based on them could be used for electrooptical applications. After the first successful growth of (Zn, Mn)O by Fukumura et al. [1], many experiments on ZnO-based DMS have been performed. However, there are widely different reports of the experimental observations of, for example, the magnetic ground state and the magnetic moment [2]. This might be because the second phases or the ferromagnetic precipitates could be formed in the samples, depending delicately on the growth conditions. Careful structural characterizations thus should be necessary. In the theoretical studies, typically using first-principles calculations, the electronic, magnetic and structural propCorresponding author. Tel.: +81 6 6879 8536; fax: +81 6 6879 8539.
E-mail address: [email protected] (M. Toyoda). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.12.163
erties are efficiently investigated without such experimental difficulties. However, the standard technique of firstprinciples calculations, the density functional theory within the local density approximation (LDA), would be fairly inadequate for the wide-gap DMS for two reasons. First, LDA often underestimates the band gap energies of semiconductors. On example of this is that the band gap energy of ZnO (3.4 eV [3]) is calculated to be only about 1 eV in LDA. Secondly, LDA often gives very poor description of the systems with strong electron correlation effect. According to the analysis of the X-ray absorption spectra by using configuration-interaction cluster-model calculations, the on-site Coulomb energy U for Mn2þ , Fe2þ , and Co2þ impurities in ZnO are obtained as 5.0, 5.5, and 6.0 eV, respectively [4]. The large U values indicate that the electron correlation effect is not negligibly small in those DMS and should be taken into account in calculations. In fact, the recently measured photoemission spectra of (Zn, V)O [5], (Zn, Mn)O [6] and (Zn, Co)O [7] show considerable difference from the electronic structures calculated within the standard LDA, particularly in the description of the transition-metal 3d states. One major source of these shortcomings of LDA is the presence of self-interaction. We therefore carry out the
ARTICLE IN PRESS M. Toyoda et al. / Physica B 376–377 (2006) 647–650
648
calculations on ZnO-based DMS by using a self-interaction-corrected LDA approach. The self-interaction-corrected electronic structures of (Zn, V)O, (Zn, Mn)O and (Zn, Co)O are shown to be in very good agreement with the photoemission experiments [8]. In this paper, we present the systematic comparison of the electronic structures of ZnO-based DMS calculated within the self-interaction-corrected LDA and the standard LDA.
2. Calculation method We use the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) code [9,10]. The form of the potentials is restricted to the muffin-tin type. The Muffin-tin radii are chosen in such a way that the ions touch each other with the radius ratio of 1. The lattice constants are fixed to the experimental values of pure wurtzite ZnO [11]. We use up to 456 k-points in the irreducible part of the first Brillouin zone. No relativistic effect is taken into account. In order to implement the self-interaction correction, we use the pseudo-SIC method [12]. In this method, the Kohn–Sham wave functions are projected onto a set of localized basis orbitals. The corrections to the effective potentials are defined on each localized orbital. They are assumed to depend linearly on the orbital occupation numbers. This occupation number dependence of the correction size is the trick of the method. The orbital occupation numbers reflect the degree of localization on the orbitals, i.e. a state localized on an orbital has a large (close to 1) occupation number on the orbital, while a 10 Total V-d
(Zn,V)O
(Zn,Cr)O
Density of states (/eV/Unit cell or Atom)
5
delocalized state has small occupation numbers. Therefore, a large amount of correction is made to a localized state, and a small amount of correction is made for a delocalized state. Furthermore, no correction is correctly assured to be made for unoccupied states since their occupation numbers are 0. Since the amount of correction for each orbital is determined self-consistently, any additional parameter is not required. In the original pseudo-SIC method, they use the pseudoatomic wave functions as the localized orbital basis set since they use a pseudopotential planewave code. In our implementation to the KKR-CPA code, we use the spherical wave functions defined inside the muffin-tin spheres as the basis set.
3. Results and discussion Fig. 1 shows the density-of-states (DOS) of ZnO-based DMS calculated within the standard LDA. The transitionmetal 3d electrons give rise to the highly localized impurity states in the band gap. The hybridization with the valence band is relatively weak. The electron configuration is highspin type with 1–3 eV of exchange splitting. The DOS show the clear half-metallic features. Fig. 2 shows the DOS of ZnO-based DMS calculated within the pseudo-SIC method. The apparent differences from those within LDA are found in the band gap energy, the energetic position of the Zn 3d bands, and the description of the transition-metal 3d states. In the case of (Zn, Mn)O, for example, the top of the valence band is located at about 3 eV below the Fermi level, whereas it is located at about 2 eV below the Fermi level in LDA. The Total Cr-d
Total Mn-d
(Zn,Mn)O
majority spin
majority spin
majority spin
0 -5 minority spin
(a) -10 10 (Zn,Fe)O
(b) (Zn,Co)O
Total Fe-d
minority spin
minority spin
(c) (Zn,Ni)O
Total Co-d
Total Ni-d
5 majority spin
majority spin
majority spin 0 -5 minority spin
(d)
(e)
minority spin
minority spin
(f)
-10 -10
-5
0
5
-10 -5 0 Energy relative to the Fermi level (eV)
5
-10
-5
0
5
Fig. 1. DOS of (a) (Zn, V)O, (b) (Zn, Cr)O, (c) (Zn, Mn)O, (d) (Zn, Fe)O, (e) (Zn, Co)O, and (f) (Zn, Ni)O calculated within the standard LDA. Total DOS per unit cell (solid lines) and partial DOS of transition-metal 3d orbitals per atom (dashed lines) are plotted. The experimental peak positions of V (1:8 eV [5]) and Co (3.0 and 7.0 [7]) 3d states are indicated by arrows.
ARTICLE IN PRESS M. Toyoda et al. / Physica B 376–377 (2006) 647–650
10 Total V-d
(Zn,V)O
(Zn,Cr)O
Density of states (/eV/Unit cell or Atom)
5
Total Cr-d
649
Total Mn-d
(Zn,Mn)O
majority spin
majority spin
majority spin
0 -5 -10 10 (Zn,Fe)O
(Zn,Co)O
Total Fe-d
minority spin (c)
minority spin (b)
minority spin (a)
(Zn,Ni)O
Total Co-d
Total Ni-d
5 majority spin
majority spin
majority spin
minority spin (d)
minority spin (e)
minority spin (f)
0 -5 -10 -10
-5
0
5
-10 -5 0 Energy relative to the Fermi level (eV)
5
-10
-5
0
5
Fig. 2. DOS of (a) (Zn, V)O, (b) (Zn, Cr)O, (c) (Zn, Mn)O, (d) (Zn, Fe)O, (e) (Zn, Co)O, and (f) (Zn, Ni)O calculated within the self-interaction corrected LDA. Total DOS per unit cell (solid lines) and partial DOS of transition-metal 3d orbitals per atom (dashed lines) are plotted. The experimental peak positions of V (1:8 eV [5]) and Co (3.0 and 7.0 [7]) 3d states are indicated by arrows.
Zn 3d bands which are located immediately below the valence band in LDA show the shift to the lower-energy side, being apart from the valence band by 2:5 eV. As the consequence, the hybridization with the valence band is significantly reduced. The transition-metal 3d bands are changed more drastically and a chemical trend is seen in the shapes of partial DOS of them. In (Zn, V)O and (Zn, Cr)O, the 3d bands shift downward in energy by 1 eV, but still appear within the band gap as they are in LDA. While in (Zn, Mn)O and (Zn, Fe)O, the fully occupied majority spin states of the 3d electrons shift more, showing the strong hybridization with the valence band. In (Zn, Co)O and (Zn, Ni)O, they shift further, making localized states around the bottom of the valence band. In the early studies by using the standard LDA, the transition-metal d density of states at the Fermi level has been pointed out to be playing an essential role for the ferromagnetic ordering in the ZnO-based DMS, because the double exchange interaction strongly stabilizes the ferromagnetic ground state through the hopping of d electrons. Except for the (Zn, V)O, the self-interactioncorrected electronic structures are insulating and no transition-metal d density of states is found at the Fermi level. Therefore, strong double exchange interaction is not expected to work. Take (Zn, Co)O as an example, which has been investigated intensively after the observation of room temperature ferromagnetism by Ueda et al. [13]. In LDA, the Fermi level lies in the gap of the tetrahedral-like crystal field splitting of the d states but due to the band broadening, d density of states exists at the Fermi level.
On the other hand, in pseudo-SIC, there is no d density of states at the Fermi level because the crystal field splitting is strongly enhanced. In experiments, Ueda et al. reported that ferromagnetic samples are conductive. However, there are also some reports that the insulating samples show ferromagnetism [14]. The origin of the ferromagnetism is still controversial. It should be also noted that the presenting results are in the case in which no additional carrier is introduced. Since ZnO is a native n-type semiconductor, some amounts of electrons would be introduced in ZnO-based DMS because of the O vacancy or the Zn interstitials [15]. The presence of those additional electrons would change the occupancy of the d states at the Fermi level. 4. Summary We have calculated the self-interaction-corrected electronic structures of ZnO-based DMS. Compared to those calculated within the standard LDA, (1) the larger band gap energy, (2) the Zn 3d bands correctly separated from the valence band, and (3) the qualitatively different description of the transition-metal 3d states are obtained. These features, especially the description of the transitionmetal 3d states could affect the predicted magnetic properties. Acknowledgments A part of this research is supported by JST-CREST, NEDO-nanotech, a Grant-in-Aid for Scientific Research in Priority Areas ‘‘Semiconductor Nanospintronics’’, a
ARTICLE IN PRESS 650
M. Toyoda et al. / Physica B 376–377 (2006) 647–650
Grant-in-Aid for Scientific Research for young researchers and 21st Century COE. References [1] T. Fukumura, Z. Jin, A. Ohtomo, H. Koinuma, M. Kawasaki, Appl. Phys. Lett. 75 (1999) 3366. [2] Recent studies on ZnO-based DMS are thoroughly reviewed in T. Fukumura, Y. Yamada, H. Toyosaki, T. Hasegawa, H. Koinuma, M. Kawasaki, Appl. Surf. Sci. 223 (2004) 62; R. Janisch, P. Gopal, N.A. Spaldin, J. Phys. Condens. Matter. 17 (2005) R657. [3] Y. Chen, D.M. Bagnall, Z. Zhu, T. Sekiuchi, K.-T. Park, K. Hiraga, T. Yao, S. Koyama, M.Y. Shen, T. Goto, J. Cryst. Growth 181 (1997) 165. [4] J. Okabayashi, K. Ono, M. Mizuguchi, M. Oshima, S.S. Gupta, D.D. Sarma, T. Mizokawa, A. Fujimori, M. Yuri, C.T. Chen, T. Fukumura, M. Kawasaki, H. Koinuma, J. Appl. Phys. 95 (2004) 3573. [5] Y. Ishida, J.I. Hwang, M. Kobayashi, A. Fujimori, H. Saeki, H. Tabata, T. Kawai, Physica B 351 (2004) 304. [6] T. Mizokawa, T. Nambu, A. Fujimori, T. Fukumura, M. Kawasaki, Phys. Rev. B 65 (2001) 085209.
[7] S.C. Wi, J.-S. Kang, J.H. Kim, S.S. Lee, S.-B. Cho, B.J. Kim, S. Yoon, B.J. Suh, S.W. Han, K.H. Kim, K.J. Kim, B.S. Kim, H.J. Song, H.J. Shin, J.H. Sim, B.I. Min, Phys. Stat. Sol. (b) 241 (2004) 1529; M. Kobayashi, Y. Ishida, J.I. Hwang, T. Mizokawa, A. Fujimori, K. Mamiya, J. Okamoto, Y. Takeda, T. Okane, Y. Saitoh, Y. Muramatsu, A. Tanaka, H. Saeki, H. Tabata, T. Kawai, Phys. Rev. B 72 (2005) 201201(R). [8] M. Toyoda, H.Akai, K. Sato, H. Katayama-Yoshida, in preparation. [9] H. Akai, P.H. Dederichs, Phys. Rev. B 47 (1993) 8739. [10] H. Akai, hhttp://www.sham.phys.sci.osaka-u.ac.jp/ kkr/i. [11] K. Kihara, G. Donnay, Can. Mineral. 23 (1985) 647. [12] A. Filippetti, N.A. Spaldin, Phys. Rev. B 67 (2003) 125109. [13] K. Ueda, H. Tabata, T. Kawai, Appl. Phys. Lett. 79 (2001) 988. [14] Z. Yin, N. Chen, C. Chai, F. Yang, J. Appl. Phys. 96 (2004) 5093; M. Venkatesan, C.B. Fitzgerald, J.G. Lunney, J.M.D. Coey, Phys. Rev. Lett. 93 (2004) 177206; H.-J. Lee, S.-Y. Jeong, C.R. Cho, C.H. Park, Appl. Phys. Lett. 81 (2002) 4020. [15] S.B. Zhang, S.-H. Wei, A. Zunger, Phys. Rev. B 63 (2001) 075205; A.P. Roth, J.B. Web, D.F. Williams, Phys. Rev. B 25 (1982) 7836.