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Electronic and magnetic properties of second main-group and second sub-group metals substitution for Al in delafossite CuAlO2
Chemical Physics Letters 633 (2015) 181–185
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Electronic and magnetic properties of second main-group and second sub-group metals substitution for Al in delafossite CuAlO2 Qi-Jun Liu a,b,∗ , Fu-Sheng Liu a,b , Zheng-Tang Liu c a School of Physical Science and Technology, Southwest Jiaotong University, Key Laboratory of Advanced Technologies of Materials, Ministry of Education of China, Chengdu 610031, People’s Republic of China b Bond and Band Engineering Group, Sichuan Provincial Key Laboratory (for Universities) of High Pressure Science and Technology, Southwest Jiaotong University, Chengdu 610031, People’s Republic of China c State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 11 December 2014 In final form 18 May 2015 Available online 30 May 2015
a b s t r a c t A systematic theoretical investigation has been carried out for the structural, electronic and magnetic properties of second main-group and second sub-group metals substitution for Al in delafossite CuAlO2 in the framework of density functional theory. The structural parameters and formation energies were calculated and discussed. The appearance of enhanced p-type conductivity after doping has been analyzed. Moreover, it is shown that all dopants have relatively large magnetic moments, but their ferromagnetic states are unstable, showing that their potential application in dilute magnetic semiconductors is not applicable. © 2015 Elsevier B.V. All rights reserved.
Transparent conducting oxides (TCOs) have both exclusive transparency and conductivity [1], making them extremely useful for technological and industrial applications [2], especially in optoelectronic and electronic devices [3–5]. Among them, most of the commercial and practical applications are n-type [6,7], whose monopolarity somewhat limits their use as oxide semiconductors [8]. Moreover, most devices need p-type TCOs to develop functional p-n junctions [6,9,10], which induces considerable attention to discover available p-type TCOs [11,12]. However, the conductivity of p-type TCOs is obviously lower than that of common n-type TCOs [13,14]. Therefore many efforts have been devoted to investigate the low-resistive p-type TCOs after the relatively high mobility in p-type delafossite CuAlO2 films had been reported [15]. There are two methods to enhance the conductivity of p-type delafossite CuAlO2 . One method is O-site substitution by doping of a less electronegative atom [16–18], another method is acceptor doping such as bivalent cations substitution for Al [19], and N-doping [20]. The reduced band gap and effective mass of S-doped CuAlO2 are good for p-type conductivity [18]. The less electronegative atoms (S, Se) reduce the ‘hole capacity’ and increase the shallow antibonding density of states [17]. It can be seen that the
∗ Corresponding author at: School of Physical Science and Technology, Southwest Jiaotong University, Chengdu, Sichuan 610031, People’s Republic of China. E-mail address: [email protected] (Q.-J. Liu). http://dx.doi.org/10.1016/j.cplett.2015.05.028 0009-2614/© 2015 Elsevier B.V. All rights reserved.
equivalence substitution on O-site is a feasible way to design low-resistive p-type TCOs, so we want to know if the inequivalence substitution of acceptor doping is also doable. The NO and Ni of N-doped CuAlO2 have been studied, indicating that the former enhances p-type conductivity and the latter is not good [20]. Moreover, the unpaired electron in N caused ferromagnetic arrangements [20]. Experimentally, the resistivity of the deposited Cu Al O films was increased with the Al content due to that the incorporated Al3+ ions substituting Cu2+ ions induces the decrease of hole concentration [21]. Hence, the substituting Al3+ in the CuAlO2 films with Mg2+ [22] and the room-temperature RFsputtered CuAlOx : Ca films [23] have been studied, which favor the acceptor doping to enhance p-type conductivity. In order to understand the p-type conduction mechanism of the bivalent cations substitution for Al, the electronic properties of Be, Mg and Ca substitution for Al were investigated, which showed the enhancement of covalence in the Cu O bonds [19]. However, though it is known that these substitutions for Al introduce holes, their magnetic properties have not been reported. This is an interesting problem due to the application of CuAlO2 based dilute magnetic semiconductors [24–28], but there is no corresponding report of magnetic properties for the groups IIA and IIB metals substitution for Al in delafossite CuAlO2 . The investigations of the magnetic nature in CuAlO2 based semiconductors can explore the potential applications in spintronic devices [27], spin-valve transistors and spin light emitting diodes [26], which also attract more attention
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Q.-J. Liu et al. / Chemical Physics Letters 633 (2015) 181–185
Figure 1. The Vdopant /Vexperiment and formation energy versus atomic radius of metal atoms.
to transparent spintronics [28]. Here we address the electronic and magnetic properties of delafossite CuAlO2 with bivalent cations substitutions using the first-principles within density functional theory (the Mg-doped and Ca-doped CuAlO2 have been reported experimentally). We calculated the structural, electronic and magnetic properties of delafossite CuAlO2 with bivalent cations substitutions using the CASTEP package [29]. The local density approximation (LDA) with the Ceperley-Alder–Perdew–Zunger (CA-PZ) form [30] was used for the exchange-correlation functional. The Vanderbilt-type ultrasoft pseudopotential [31] with valence states Cu 3d10 4s1 , Al 3s2 3p1 , O 2s2 2p4 , Be 2s2 , Mg 2p6 3s2 , Ca 3s2 3p6 4s2 , Sr 4s2 4p6 5s2 , Ba 5s2 5p6 6s2 , Zn 3d10 4s2 , Cd 4d10 5s2 and Hg 5d10 6s2 was used. The plane-wave cutoff energy of 440 eV and the Brillouin-zone integration with 3 × 3 × 1 Monkhorst-Pack meshes [32] were used
here to study the doped delafossite CuAlO2 with a supercell of size 2 × 2 × 1 including 48 atoms (we substituted an Al atom by a IIA/IIB metal atom). The lattice parameters are a × 2 = b × 2 = 5.716 A˚ and c = 16.958 A˚ [33,34] for the supercell. The structural relaxations were carried out until the maximum force, maximum displace˚ 5 × 10−4 A˚ and ment and maximum stress were less than 0.01 eV/A, 0.02 GPa, respectively. Furthermore, we used the LDA + U (U = 5.2 eV for the Cu-d) approach [7] to treat bandgap correction. Before we investigate electronic and magnetic properties of the doped delafossite CuAlO2 , we first study the structural parameters and phase stability of dopants. Figure 1 shows Vdopant /Vexperiment (Vdopant is the optimized supercell volume of doped delafossite CuAlO2 and Vexperiment is the experimental supercell volume of pure CuAlO2 ) and formation energy versus atomic radius of metal atoms, respectively. The calculated volumes of IIA and IIB metals substitutions increase with the atomic radius of metal atoms ˚ < rMg (1.72 A) ˚ < rAl (1.82 A) ˚ < rCa (2.23 A) ˚ < rSr (2.45 A) ˚ < (rBe (1.40 A) ˚ rZn (1.53 A) ˚ < rCd (1.71 A) ˚ < rHg (1.76 A) ˚ < rAl (1.82 A)). ˚ HowrBa (2.78 A), ever, compared with the LDA calculation volume of pure CuAlO2 ˚ (aLDA = bLDA = 2.798 A˚ and cLDA = 16.681 A), the optimized volumes of dopants all increase. Since the ionic radii of IIA ˚ < and IIB metals are larger than that of Al3+ (rAl3+ (0.51 A) ˚ < r 2+ (0.99 A) ˚ < r 2+ (1.12 A) ˚ < r 2+ (1.34 A), ˚ r 2+ (0.66 A) Mg
Ca
Sr
Ba
˚ < r 2+ (0.74 A) ˚ < r 2+ (0.97 A) ˚ < r 2+ (1.10 A)), ˚ rAl3+ (0.51 A) the Zn Hg Cd dilations of volumes of dopants are divinable and reasonable. ˚ also induces Whereas, the small ionic radius of Be2+ (0.35 A) dilation. This may be due to the distorted BeO6 octahedron having positive charge excludes the neighboring copper positive ions, which induces the increase of lattice parameter c and total volume. The absolute value of increased volume is bigger than that of volume shrinkage mass caused by small ionic radius of Be2+ . Moreover, the large ionic radius of Ba2+ induces distorted-lattice and rapid increase of volume.
Figure 2. Total density of states of the doped CuAlO2 : (a) LDA and (b) LDA + U for group-IIA metals, (c) LDA and (d) LDA + U for group-IIB metals.
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Figure 3. Partial density of states of the doped atoms: (a) LDA and (b) LDA + U for group-IIA metals, (c) LDA and (d) LDA + U for group-IIB metals.
To study the thermodynamic stability, we calculate their formation energies (Hf ) shown in Figure 1, which is defined as the total energy change of the substitution reaction: II-group doped CuAlO2
Hf =
Etotal
II-group where Etotal
II group
O2 Cu Al − 12Ecrystal − 12Egas − 11Ecrystal − Ecrystal
48 doped CuAlO2
Cu CuAlO2 , Ecrystal ,
(1)
is the total energy of II-group doped II group
Al Ecrystal and Ecrystal are the energies at their staO2 is the energy at the gas molecule state. ble crystal phases, Egas The formation energies of delafossite CuAlO2 with bivalent cations substitutions are −3.358, −3.359, −3.344, −3.300, −3.311 eV/atom
for IIA-group Be2+ , Mg2+ , Ca2+ , Sr2+ , Ba2+ , and −3.314, −3.274, −3.214 eV/atom for IIB-group Zn2+ , Cd2+ and Hg2+ , respectively, indicating that all substitution reactions are exothermic. However, these formation energies are all bigger than that of pure CuAlO2 (−3.838 eV/atom), meaning that the dopants decrease the stability. It can be seen that the more deviation of ionic radius compared with that of Al3+ is, the more instability is (the decreased formation energy of Ba2+ substitution is due to the distorted-lattice). Then we study the influences of the bivalent cations substitutions on the electronic structures. Figures 2–4 show the total and partial density of states (TDOS and PDOS) of the doped CuAlO2
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Figure 4. Partial density of states of the neighbor atoms: (a) LDA and (b) LDA + U for Cu-3d, (c) LDA and (d) LDA + U for O-2p, near the doped atoms.
with the LDA and LDA + U calculations. The substitution of an Al atom by a bivalent atom introduces a hole, which enhances the p-type conductivity (the distorted-lattice of Ba dopant results in metallization with the LDA calculation, but the increased p-type conductivity appears with the LDA + U calculation) and causes ferromagnetism. From the TDOS in Figure 2, the Fermi level is located across the spin-down valence bands, showing empty valence bands above Ef . The electron acceptors above Ef with the LDA calculation (LDA + U calculation) are at 0.026 (0.157), 0.022 (0.119), 0.026 (0.168), 0.027 (0.172), (0.337) eV for IIA-group Be2+ , Mg2+ , Ca2+ , Sr2+ , Ba2+ , and 0.025 (0.127), 0.059 (0.120) and 0.072 (0.122) eV for IIB-group Zn2+ , Cd2+ and Hg2+ , respectively, indicating that the more deviation of ionic radius compared with that of Al3+ is, the higher location of electron acceptors for Be Mg Ca Sr Ba is. The appearance of electron acceptors above Ef means stronger p-type conductivity than that of pure CuAlO2 . Moreover, the decreased bandgaps of dopants can be obviously seen. The bandgap of pure CuAlO2 based on LDA calculation is 2.195 eV, and the bandgaps of Be Mg Ca Sr Ba, and Zn Cd Hg dopants with the LDA calculation (LDA + U calculation) are 1.150 (1.883), 1.398 (1.888), 1.305 (1.928), 0.966 (1.580), (1.098), 1.339 (1.712), 0.947 (1.465) and 0.371 (0.465) eV. This decrease is good for electron transition from the valence bands to the conduction bands. Figure 3 shows the PDOS of doped atoms and Figure 4 shows the PDOS of Cu-3d and O-2p states, which can explain the formation of electron acceptors. The contribution to electron acceptors of these II-group atoms is limited, and the main contributions come from Cu-3d and O-2p states, namely, the introduced hole by II-group metals is transferred to the neighbor hybridized Cu O bonds. The covalent nature of Cu O bonds is increased according to the charge density (LDA
calculation): the charge of Cu in pure CuAlO2 is 0.28e, and the charges of Cu near the doped atoms are 0.15e, 0.11e, 0.16e, 0.17e for Be Mg Ca Sr, respectively, which is good for hole delocalization. In a word, the effect of II-group metals substitution for Al in delafossite CuAlO2 is the same as the chalcogen substitution for O [35], which all enhance p-type conductivity. Moreover, experiments reported that the excess oxygen atoms induced the increase of p-type conductivity [36], so the relation between oxygen stoichiometry and generated holes by bivalent cations substitutions seems interesting, which should be solved by designing a special model. On the other hand, from the DOS shown in Figures 2–4, the ferromagnetism appears after the doping. The introduced hole by II-group metals causes magnetic moments that are 13.0 B for all doped supercell, indicating the effect of II-group metals substitution on magnetic properties is the same. However, the nonmagnetic elements doping contributes little to the origin of ferromagnetism. The main origin comes from Cu element, whose average spin moments are about 0.80 B . Moreover, the average spin moments of O element are about 0.15 B . Hence, the introduced hole by II-group metals is changed to the spin-down polarized hole with Cu-3d and O-2p hybridization character. Next, we calculate the total energy difference between the ferromagnetic and paramagnetic states of the doped delafossite CuAlO2 to check out the magnetic stability [37], which are shown in Figure 5. A negative value means that the paramagnetic state is more stable. These eight dopings all show negative values, indicating that their paramagnetic states are more stable than their ferromagnetic states. The more deviation of ionic radius compared with that of Al3+ is, the less energy difference is. This conclusion is disappointed, suggesting
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References
Figure 5. Total energy difference between the ferromagnetic and paramagnetic states of the doped delafossite CuAlO2 .
that these dopings are not suitable to practical CuAlO2 based dilute magnetic semiconductors. Moreover, there is no meaning of Curie temperature according to the mean-field approximation [38]. In summary, we have studied the structural, electronic and magnetic properties of groups IIA and IIB metals substitution for Al in delafossite CuAlO2 . The calculated volumes are associated with the atomic/ionic radius of doped atoms, which has been discussed theoretically. The obtained formation energies show that all substitution reactions are exothermic, but the instability is increased after doping. The calculated electronic structures present new characters, including the appearance of electron acceptors (shallow acceptors) above Ef , the decreased bandgaps and the enhanced covalent nature of Cu O bonds, which are all good for p-type conductivity. This conclusion is in good agreement with the experimental results [22,23]. Moreover, the ferromagnetism with high magnetic moments have been found, but their ferromagnetic states are unstable. According to our calculated results, the favorable dopant is Mg atom which acts as shallow acceptor and is used to finish hole doping. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 11347199 and 51402244), the Specialized Research Fund for Doctoral Program of Higher Education of China (Grant No. 20130184120028), the Fundamental Research Fund for the Central Universities, China (Grant Nos. 2682014CX084 and 2682014ZT31) and the fund of the State Key Laboratory of Solidification Processing in NWPU (Grant No. SKLSP201511).
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Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Electronic and magnetic properties of second main-group and second sub-group metals substitution for Al in delafossite CuAlO2 Qi-Jun Liu a,b,∗ , Fu-Sheng Liu a,b , Zheng-Tang Liu c a School of Physical Science and Technology, Southwest Jiaotong University, Key Laboratory of Advanced Technologies of Materials, Ministry of Education of China, Chengdu 610031, People’s Republic of China b Bond and Band Engineering Group, Sichuan Provincial Key Laboratory (for Universities) of High Pressure Science and Technology, Southwest Jiaotong University, Chengdu 610031, People’s Republic of China c State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 11 December 2014 In final form 18 May 2015 Available online 30 May 2015
a b s t r a c t A systematic theoretical investigation has been carried out for the structural, electronic and magnetic properties of second main-group and second sub-group metals substitution for Al in delafossite CuAlO2 in the framework of density functional theory. The structural parameters and formation energies were calculated and discussed. The appearance of enhanced p-type conductivity after doping has been analyzed. Moreover, it is shown that all dopants have relatively large magnetic moments, but their ferromagnetic states are unstable, showing that their potential application in dilute magnetic semiconductors is not applicable. © 2015 Elsevier B.V. All rights reserved.
Transparent conducting oxides (TCOs) have both exclusive transparency and conductivity [1], making them extremely useful for technological and industrial applications [2], especially in optoelectronic and electronic devices [3–5]. Among them, most of the commercial and practical applications are n-type [6,7], whose monopolarity somewhat limits their use as oxide semiconductors [8]. Moreover, most devices need p-type TCOs to develop functional p-n junctions [6,9,10], which induces considerable attention to discover available p-type TCOs [11,12]. However, the conductivity of p-type TCOs is obviously lower than that of common n-type TCOs [13,14]. Therefore many efforts have been devoted to investigate the low-resistive p-type TCOs after the relatively high mobility in p-type delafossite CuAlO2 films had been reported [15]. There are two methods to enhance the conductivity of p-type delafossite CuAlO2 . One method is O-site substitution by doping of a less electronegative atom [16–18], another method is acceptor doping such as bivalent cations substitution for Al [19], and N-doping [20]. The reduced band gap and effective mass of S-doped CuAlO2 are good for p-type conductivity [18]. The less electronegative atoms (S, Se) reduce the ‘hole capacity’ and increase the shallow antibonding density of states [17]. It can be seen that the
∗ Corresponding author at: School of Physical Science and Technology, Southwest Jiaotong University, Chengdu, Sichuan 610031, People’s Republic of China. E-mail address: [email protected] (Q.-J. Liu). http://dx.doi.org/10.1016/j.cplett.2015.05.028 0009-2614/© 2015 Elsevier B.V. All rights reserved.
equivalence substitution on O-site is a feasible way to design low-resistive p-type TCOs, so we want to know if the inequivalence substitution of acceptor doping is also doable. The NO and Ni of N-doped CuAlO2 have been studied, indicating that the former enhances p-type conductivity and the latter is not good [20]. Moreover, the unpaired electron in N caused ferromagnetic arrangements [20]. Experimentally, the resistivity of the deposited Cu Al O films was increased with the Al content due to that the incorporated Al3+ ions substituting Cu2+ ions induces the decrease of hole concentration [21]. Hence, the substituting Al3+ in the CuAlO2 films with Mg2+ [22] and the room-temperature RFsputtered CuAlOx : Ca films [23] have been studied, which favor the acceptor doping to enhance p-type conductivity. In order to understand the p-type conduction mechanism of the bivalent cations substitution for Al, the electronic properties of Be, Mg and Ca substitution for Al were investigated, which showed the enhancement of covalence in the Cu O bonds [19]. However, though it is known that these substitutions for Al introduce holes, their magnetic properties have not been reported. This is an interesting problem due to the application of CuAlO2 based dilute magnetic semiconductors [24–28], but there is no corresponding report of magnetic properties for the groups IIA and IIB metals substitution for Al in delafossite CuAlO2 . The investigations of the magnetic nature in CuAlO2 based semiconductors can explore the potential applications in spintronic devices [27], spin-valve transistors and spin light emitting diodes [26], which also attract more attention
182
Q.-J. Liu et al. / Chemical Physics Letters 633 (2015) 181–185
Figure 1. The Vdopant /Vexperiment and formation energy versus atomic radius of metal atoms.
to transparent spintronics [28]. Here we address the electronic and magnetic properties of delafossite CuAlO2 with bivalent cations substitutions using the first-principles within density functional theory (the Mg-doped and Ca-doped CuAlO2 have been reported experimentally). We calculated the structural, electronic and magnetic properties of delafossite CuAlO2 with bivalent cations substitutions using the CASTEP package [29]. The local density approximation (LDA) with the Ceperley-Alder–Perdew–Zunger (CA-PZ) form [30] was used for the exchange-correlation functional. The Vanderbilt-type ultrasoft pseudopotential [31] with valence states Cu 3d10 4s1 , Al 3s2 3p1 , O 2s2 2p4 , Be 2s2 , Mg 2p6 3s2 , Ca 3s2 3p6 4s2 , Sr 4s2 4p6 5s2 , Ba 5s2 5p6 6s2 , Zn 3d10 4s2 , Cd 4d10 5s2 and Hg 5d10 6s2 was used. The plane-wave cutoff energy of 440 eV and the Brillouin-zone integration with 3 × 3 × 1 Monkhorst-Pack meshes [32] were used
here to study the doped delafossite CuAlO2 with a supercell of size 2 × 2 × 1 including 48 atoms (we substituted an Al atom by a IIA/IIB metal atom). The lattice parameters are a × 2 = b × 2 = 5.716 A˚ and c = 16.958 A˚ [33,34] for the supercell. The structural relaxations were carried out until the maximum force, maximum displace˚ 5 × 10−4 A˚ and ment and maximum stress were less than 0.01 eV/A, 0.02 GPa, respectively. Furthermore, we used the LDA + U (U = 5.2 eV for the Cu-d) approach [7] to treat bandgap correction. Before we investigate electronic and magnetic properties of the doped delafossite CuAlO2 , we first study the structural parameters and phase stability of dopants. Figure 1 shows Vdopant /Vexperiment (Vdopant is the optimized supercell volume of doped delafossite CuAlO2 and Vexperiment is the experimental supercell volume of pure CuAlO2 ) and formation energy versus atomic radius of metal atoms, respectively. The calculated volumes of IIA and IIB metals substitutions increase with the atomic radius of metal atoms ˚ < rMg (1.72 A) ˚ < rAl (1.82 A) ˚ < rCa (2.23 A) ˚ < rSr (2.45 A) ˚ < (rBe (1.40 A) ˚ rZn (1.53 A) ˚ < rCd (1.71 A) ˚ < rHg (1.76 A) ˚ < rAl (1.82 A)). ˚ HowrBa (2.78 A), ever, compared with the LDA calculation volume of pure CuAlO2 ˚ (aLDA = bLDA = 2.798 A˚ and cLDA = 16.681 A), the optimized volumes of dopants all increase. Since the ionic radii of IIA ˚ < and IIB metals are larger than that of Al3+ (rAl3+ (0.51 A) ˚ < r 2+ (0.99 A) ˚ < r 2+ (1.12 A) ˚ < r 2+ (1.34 A), ˚ r 2+ (0.66 A) Mg
Ca
Sr
Ba
˚ < r 2+ (0.74 A) ˚ < r 2+ (0.97 A) ˚ < r 2+ (1.10 A)), ˚ rAl3+ (0.51 A) the Zn Hg Cd dilations of volumes of dopants are divinable and reasonable. ˚ also induces Whereas, the small ionic radius of Be2+ (0.35 A) dilation. This may be due to the distorted BeO6 octahedron having positive charge excludes the neighboring copper positive ions, which induces the increase of lattice parameter c and total volume. The absolute value of increased volume is bigger than that of volume shrinkage mass caused by small ionic radius of Be2+ . Moreover, the large ionic radius of Ba2+ induces distorted-lattice and rapid increase of volume.
Figure 2. Total density of states of the doped CuAlO2 : (a) LDA and (b) LDA + U for group-IIA metals, (c) LDA and (d) LDA + U for group-IIB metals.
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Figure 3. Partial density of states of the doped atoms: (a) LDA and (b) LDA + U for group-IIA metals, (c) LDA and (d) LDA + U for group-IIB metals.
To study the thermodynamic stability, we calculate their formation energies (Hf ) shown in Figure 1, which is defined as the total energy change of the substitution reaction: II-group doped CuAlO2
Hf =
Etotal
II-group where Etotal
II group
O2 Cu Al − 12Ecrystal − 12Egas − 11Ecrystal − Ecrystal
48 doped CuAlO2
Cu CuAlO2 , Ecrystal ,
(1)
is the total energy of II-group doped II group
Al Ecrystal and Ecrystal are the energies at their staO2 is the energy at the gas molecule state. ble crystal phases, Egas The formation energies of delafossite CuAlO2 with bivalent cations substitutions are −3.358, −3.359, −3.344, −3.300, −3.311 eV/atom
for IIA-group Be2+ , Mg2+ , Ca2+ , Sr2+ , Ba2+ , and −3.314, −3.274, −3.214 eV/atom for IIB-group Zn2+ , Cd2+ and Hg2+ , respectively, indicating that all substitution reactions are exothermic. However, these formation energies are all bigger than that of pure CuAlO2 (−3.838 eV/atom), meaning that the dopants decrease the stability. It can be seen that the more deviation of ionic radius compared with that of Al3+ is, the more instability is (the decreased formation energy of Ba2+ substitution is due to the distorted-lattice). Then we study the influences of the bivalent cations substitutions on the electronic structures. Figures 2–4 show the total and partial density of states (TDOS and PDOS) of the doped CuAlO2
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Figure 4. Partial density of states of the neighbor atoms: (a) LDA and (b) LDA + U for Cu-3d, (c) LDA and (d) LDA + U for O-2p, near the doped atoms.
with the LDA and LDA + U calculations. The substitution of an Al atom by a bivalent atom introduces a hole, which enhances the p-type conductivity (the distorted-lattice of Ba dopant results in metallization with the LDA calculation, but the increased p-type conductivity appears with the LDA + U calculation) and causes ferromagnetism. From the TDOS in Figure 2, the Fermi level is located across the spin-down valence bands, showing empty valence bands above Ef . The electron acceptors above Ef with the LDA calculation (LDA + U calculation) are at 0.026 (0.157), 0.022 (0.119), 0.026 (0.168), 0.027 (0.172), (0.337) eV for IIA-group Be2+ , Mg2+ , Ca2+ , Sr2+ , Ba2+ , and 0.025 (0.127), 0.059 (0.120) and 0.072 (0.122) eV for IIB-group Zn2+ , Cd2+ and Hg2+ , respectively, indicating that the more deviation of ionic radius compared with that of Al3+ is, the higher location of electron acceptors for Be Mg Ca Sr Ba is. The appearance of electron acceptors above Ef means stronger p-type conductivity than that of pure CuAlO2 . Moreover, the decreased bandgaps of dopants can be obviously seen. The bandgap of pure CuAlO2 based on LDA calculation is 2.195 eV, and the bandgaps of Be Mg Ca Sr Ba, and Zn Cd Hg dopants with the LDA calculation (LDA + U calculation) are 1.150 (1.883), 1.398 (1.888), 1.305 (1.928), 0.966 (1.580), (1.098), 1.339 (1.712), 0.947 (1.465) and 0.371 (0.465) eV. This decrease is good for electron transition from the valence bands to the conduction bands. Figure 3 shows the PDOS of doped atoms and Figure 4 shows the PDOS of Cu-3d and O-2p states, which can explain the formation of electron acceptors. The contribution to electron acceptors of these II-group atoms is limited, and the main contributions come from Cu-3d and O-2p states, namely, the introduced hole by II-group metals is transferred to the neighbor hybridized Cu O bonds. The covalent nature of Cu O bonds is increased according to the charge density (LDA
calculation): the charge of Cu in pure CuAlO2 is 0.28e, and the charges of Cu near the doped atoms are 0.15e, 0.11e, 0.16e, 0.17e for Be Mg Ca Sr, respectively, which is good for hole delocalization. In a word, the effect of II-group metals substitution for Al in delafossite CuAlO2 is the same as the chalcogen substitution for O [35], which all enhance p-type conductivity. Moreover, experiments reported that the excess oxygen atoms induced the increase of p-type conductivity [36], so the relation between oxygen stoichiometry and generated holes by bivalent cations substitutions seems interesting, which should be solved by designing a special model. On the other hand, from the DOS shown in Figures 2–4, the ferromagnetism appears after the doping. The introduced hole by II-group metals causes magnetic moments that are 13.0 B for all doped supercell, indicating the effect of II-group metals substitution on magnetic properties is the same. However, the nonmagnetic elements doping contributes little to the origin of ferromagnetism. The main origin comes from Cu element, whose average spin moments are about 0.80 B . Moreover, the average spin moments of O element are about 0.15 B . Hence, the introduced hole by II-group metals is changed to the spin-down polarized hole with Cu-3d and O-2p hybridization character. Next, we calculate the total energy difference between the ferromagnetic and paramagnetic states of the doped delafossite CuAlO2 to check out the magnetic stability [37], which are shown in Figure 5. A negative value means that the paramagnetic state is more stable. These eight dopings all show negative values, indicating that their paramagnetic states are more stable than their ferromagnetic states. The more deviation of ionic radius compared with that of Al3+ is, the less energy difference is. This conclusion is disappointed, suggesting
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References
Figure 5. Total energy difference between the ferromagnetic and paramagnetic states of the doped delafossite CuAlO2 .
that these dopings are not suitable to practical CuAlO2 based dilute magnetic semiconductors. Moreover, there is no meaning of Curie temperature according to the mean-field approximation [38]. In summary, we have studied the structural, electronic and magnetic properties of groups IIA and IIB metals substitution for Al in delafossite CuAlO2 . The calculated volumes are associated with the atomic/ionic radius of doped atoms, which has been discussed theoretically. The obtained formation energies show that all substitution reactions are exothermic, but the instability is increased after doping. The calculated electronic structures present new characters, including the appearance of electron acceptors (shallow acceptors) above Ef , the decreased bandgaps and the enhanced covalent nature of Cu O bonds, which are all good for p-type conductivity. This conclusion is in good agreement with the experimental results [22,23]. Moreover, the ferromagnetism with high magnetic moments have been found, but their ferromagnetic states are unstable. According to our calculated results, the favorable dopant is Mg atom which acts as shallow acceptor and is used to finish hole doping. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 11347199 and 51402244), the Specialized Research Fund for Doctoral Program of Higher Education of China (Grant No. 20130184120028), the Fundamental Research Fund for the Central Universities, China (Grant Nos. 2682014CX084 and 2682014ZT31) and the fund of the State Key Laboratory of Solidification Processing in NWPU (Grant No. SKLSP201511).
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