Electronic properties of bivalent cations (Be, Mg and Ca) substitution for Al in delafossite CuAlO2 semiconductor by first-principles calculations

Journal of Alloys and Compounds 553 (2013) 245–252

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Electronic properties of bivalent cations (Be, Mg and Ca) substitution for Al in delafossite CuAlO2 semiconductor by first-principles calculations Haifeng Jiang a,d,⇑, Xiancai Wang a, Xueping Zang a, Weifeng Wu a, Shunping Sun b, Chao Xiong c, Weiwei Yin a, Chuanyou Gui a, Xuebin Zhu d a

Department of Mechanics and Electronic Engineering, Chizhou College, Chizhou 247000, China School of Materials Engineering, Jiangsu Teachers University of Technology, Changzhou 213001, China School of Photoelectric Engineering, Changzhou Institute of Technology, Changzhou 213001, China d Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China b c

a r t i c l e

i n f o

Article history: Received 23 July 2012 Received in revised form 13 November 2012 Accepted 17 November 2012 Available online 29 November 2012 Keywords: Delafossite CuAlO2 Charge density Band structure Density of states

a b s t r a c t Electronic properties of delafossite-type CuAlO2 doped with the bivalent cation (Be, Mg or Ca) were systematically calculated by using first-principles PAW method based on density functional theory. The calculated results show the Cu–O distance nearest to the substituted bivalent cation for Al (0.5 0.5 0.5) is decreased with the increase of atomic number from Be to Ca. Moreover, the denser energy band structures have been observed in the valence band in the substituted structures, which are related to the enhancement of covalent character of the Cu–O bond nearest to the substituted site. The contributions from the substituted bivalent cations (Be2+, Mg2+ and Ca2+) to the valence band begin at 6.5 eV, relative to 8 eV of Al3+, which could be another cause to variations in band structures. From Be to Ca, their partial densities of states show the contributions to the valence band are gradually decreased, in great agreement with the variation trend for the pauling electronegativity. The calculated defect formation energies indicate the (BeAl, 1) forms more easily than the others. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The combination of optical and electrical properties in the delafossite-type CuAlO2 semiconductor and its p-type conductivity initially reported by Kawazoe et al. [1], attracted much attention of researchers. In recent years, other properties such as thermoelectricity [2], ozone sensitivity [3], ferroelectricity [4] and catalysis [5], have also been found in its sister compounds. From literatures concerning experiments or theoretical calculations, many studies have been carried out on the delafossite-structured compounds to clarify the origin of their physical properties. For examples, Hiraga et al. [6] comparatively studied the electronic structure of the delafossite-type CuMO2 (M = Sc, Cr, Mn, Fe and Co) from both experimental measurements and first-principles calculations. Two optical transitions were found: one is associated with Cu3d + O2p ? Cu3dz2 + 4s observed in all samples, which shows an unexpected dependence on the atomic number of M.

⇑ Corresponding author at: Department of Mechanics and Electronic Engineering, Chizhou College, Chizhou 247000, China. Tel.: +86 566 2616846; fax: +86 566 2092911. E-mail address: [email protected] (H. Jiang). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.101

The other is associated with Cu3d + O2p ? M3d detected for M = Mn, Fe, and Co. Scanlon et al. [7] utilized the screened hybrid exchange functional to investigate defects in CuAlO2 and found copper vacancies and copper on aluminum antisites would dominate under Cu-poor/Al-poor conditions. And the likely defect levels were often mistaken as indirect band gaps. CuAlO2 has its unique merit because the component elements are easily accessible and hence inexpensive. The prepared CuAlO2 thin films have higher visible-light transmittance of average 70% [8] and larger p-type conductivity of approximate 2.4 S/cm [9]. Up to now, the corresponding researches mostly focused on the p-type conduction mechanism [10], electrical anisotropy [11], the nature of optical band gaps [12], effective mass on the top of the valence band [13], intrinsic or extrinsic defects’ formation energies [14,15] and so on. However, the effects of substitutions on electronic properties of CuAlO2 have rarely been under consideration except for the published article involving the S substitution [16] at an O site in CuAlO2 structure. In this paper, bivalent cation substitutions on the Al site (0.5 0.5 0.5) were systematically studied. We paid much attention to the variations in the structures, charge densities, band structures and electronic densities of states. Some novel and intriguing phenomena have been observed and the corresponding interpretations were presented.

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2. Computational details

3. Results and discussion

Delafossite-type CuAlO2 semiconductor has a rhombohedral structure with space group R-3m. In this structure, the joint of neighboring Cu+ layers along c axis is separated by an AlO2 octahedral layer, in which the octahedrons AlO6 are connected with each other by sharing their edge. The atomic positions of this structure are all fixed by symmetry except the u-value of oxygen atoms. The positions of Cu, Al and O are at 3a site (0 0 0), 3b site (0 0 0.5) and 6c site (0 0 u) [17], respectively. The initial experimental values of a = 2.858, c = 16.958 and u = 0.1099 [18] were used to construct a 2  2  1 supercell. The total energy calculations in this study were performed using the first-principles PAW pseudopotential method within Perdew, Burke, and Ernzerhof (PBE) generalized gradient approximation as implemented in the VASP code. In our calculations, the supercells containing 48 host atoms were used, and the fractional coordinate of the substituted M2+ (Be2+, Mg2+, or Ca2+) for Al3+ was (0.5 0.5 0.5). The valence electron configurations included Be s2p0, Mg s2p0, Ca p6s2d0.01, O s2p4, Al s2p1 and Cu d10s1. The energy cutoff for the plane-wave basis was 400 eV. The 5  5  5 Monkhorst–Pack k-point set has been used to sample the Brillouin zone. All atomic positions have been relaxed according to the calculated Helmann–Feynman forces. The tests for the k-mesh and energy convergences had been done before the calculations of energy bands and densities of states. This set of parameters assures a total energy convergence of 0.05 meV/ atom.

3.1. Structural properties In delafossite structure CuAlO2 (Fig. 1), each Al3+ is coordinated to six O atoms, forming the distorted AlO6 octahedron. We constructed the 48-atom supercell based on the experimental data mentioned above, and relaxed all atoms with respect to the minimum total energy. The calculated results are shown in Table 1. The bond length between Al and its nearest-neighbor O is 1.926 Å, well consistent with the calculated result of 1.928 Å by Scanlon et al. [19]. The bond angles of Al1–Al–Al2 and O1–Al–O2 are 60° and 97.09°, respectively. The former angle is the same with the experimental value, while the latter is slightly larger than 96.84° derived from the experimental measurement. The optimized crystal lattice parameters a, c and internal parameter u are 2.887, 17.066 Å and 0.1105 (fractional coordinate), respectively, which are also larger than the experimental values. From available literatures, the enhancement of these parameters is comprehensible because of the well-known overestimation caused by GGA calculations [19,20]. The Al atom at M site was substituted by bivalent cations (Be, Mg and Ca). From this table, the a parameter is increased with the ionic radii from 0.45, 0.54, 0.72 to 1.00 Å for Be2+, Al3+, Mg2+ and Ca2+, respectively. As a variable dependent on a, the c parameter is decreased orderly with the increase in radius under geometry optimization of the unchanged supercell volume. The distance of Cu–O (u3 in Å) as an independent variable, is decreased too with the enhancement of ionic radii. The variation of parameters a and u3 indicates a lattice distortion relative to CuAlO2, due to the radius mismatch between Al3+ and bivalent cations. Interestingly, taking the M site as a center, the Cu–O distances (u1 and u2) first and second nearest to that are obviously different, with smaller u1 and larger u2 than u3. The replacement of bivalent cations for Al3+ produces smaller u1 against undoped CuAlO2, suggesting that this kind of substitutions strengthens the bonding of first nearest Cu–O even if the smaller Be2+ relative to Al3+. The appearance of the larger u2 relative to u3 for each substitution may be due to the requirement of energy minimization for their supercell. The variation trends of u2 and u3 is the same with the increase of M radius, and the decrease from u2 to u3 reveals the degeneration of lattice distortion because of the further distance from the M center. 3.2. Charge densities

Fig. 1. Crystal structure for delafossite CuAlO2. Several special atoms are labeled with signs.

Stoichiometric CuAlO2 has the valence states of +1, +3 and 2 for Cu, Al and O elements, respectively. Taking the outmost electron distribution into account, Cu+, Al3+ and O2 should have the electronic structures of [Ar]+10 for Cu+, [Ne] for Al3+, and [Ne] for O2. That is to say, the valence electron clouds surround the [He] cores of O atoms, with no clouds surrounding Cu and Al atoms. Fig. 2(a) presents the three-dimensional charge density distribution of 48-atom CuAlO2 lattice. The deep blue, pale blue and red spheres denote the [Ar], [Ne] and [He] cores for Cu, Al and O, respectively. The cyan shells close the cores of Cu and O atoms,

Table 1 Optimized structural parameters for CuAlO2 and Cu12Al11MO24 (M = Be, Al or Ca) with M situated at (0.5 0.5 0.5) in fractional coordinate. M

Bond length (M–O) (Å)

Bond angle (Al1–M–Al2, O1–M–O2) (°)

a (Å)

c (Å)

u1, u2, u3 (Å)

Be Al Mg Ca

1.916 1.926 2.014 2.128

60, 60, 60, 60,

2.885 2.887 2.89 2.895

17.097 17.066 17.049 16.984

1.849, 1.886, 1.837, 1.819,

96.88 97.09 97.82 98.34

1.900, 1.886, 1.886, 1.870,

u3/c 1.891 1.886 1.880 1.866

0.1106 0.1105 0.1103 0.1099

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Fig. 2. The 3-dimensional (a) and 2-dimensional (b) charge density distributions of the 48-atom supercell for CuAlO2.

which indicates the valence electron clouds are not fully distributed around the O cores, with partial clouds around Cu cores due to the selection of Cu pseudopotential with d10 electrons treated as valence electrons. No clouds surround the Al cores. This picture shows the nature of Al–O bonds is ionic while that of Cu–O maybe has some degree of covalent character. Fig. 2(b) gives a two-dimensional charge density distribution for CuAlO2 (1 0 0) plane. A density scale is located on its left side. From this figure, the charge density only exists in Cu and O neighborhoods, and at the Al sites is no density. This result is the same with the former 3-dimensional charge distribution. The charge densities for the substitutions are shown in Fig. 3. The spheres with different colors and radii at the fractional coordinate (0.5 0.5 0.5) are the cores of Be2+, Mg2+ and Ca2+, respectively. From the viewpoint of pauling electronegativity, elements Be, Mg and Ca are 1.57, 1.31 and 1.00, respectively, all lower than 1.61 of element Al. Therefore, this kind of substitutions does not change the valence charge distribution around M2+ ions, e.g. the bonding of M–O is ionic. Studies on the variations of Cu–O bonding character due to the substitutions are still under way.

3.3. Band structures The figures for band structures and DOSs were plotted with reference to Fermi level at 0 eV. The calculated band structure for CuAlO2 is shown in Fig. 4(a). The conduction band minimum is at C and a broad valence band maximum is found along C–K. The band gap is indirect and measures 1.76 eV, lower than the experimental value of 1.80–2.10 eV [21]. Fig. 4(b)–(d) presents the electronic band structures of the bivalent-cation substitutions for the

Al (0.5 0.5 0.5) site. For CuAlO2 doped with Be, Mg or Ca, the conduction band minimum is at C and the valence band maximum is along C–K, too. The indirect band gaps are 1.81, 1.88 and 1.93 eV for Be, Mg and Ca substitutions, respectively. This kind of substitutions does not change the nature of band gaps, meaning that replacements of minor bivalent cations (9.1 at.% relative to Al element) little affect the crystal structure of CuAlO2. The increase of indirect band gaps could be due to the decrease of Cu– O distances (u1), which results in the increase of Cu–O covalent character. The bivalent-cation substitutions for Al3+ were considered to introduce hole carriers in the CuAlO2 lattice. The introduction of carriers is realized by the change of Cu valence state from +1 to +2 in the hexagonal layers of Cu ions, which can be expressed by the formula Al3+ + Cu+ + 2O2 = Cu2+ + M2+ + 2O2 (M2+ = Be2+, Mg2+ or Ca2+). This kind of substitutional doping significantly improves the performance of this material, especially electrical transport property. Experimental results reported by others [22,23] can support this conclusion. From this picture again, the energy bands for the bivalent cation doped CuAlO2 cross their Fermi level, which indicates a metallic character. Moreover, the bivalent substitutions effectively make denser the top valence band, which maybe enhances the mobility of hole carriers. Therefore, it can be inferred that such substitutions can improve the electrical properties of the material indeed.

3.4. Densities of states The calculated total and partial (ion decomposed) electronic densities of states (TDOS/PDOS) for CuAlO2 are shown in Fig. 5.

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Fig. 3. Charge densities of substitutions with bivalent cations for Al3+ at the site (0.5 0.5 0.5).

Fig. 4. The electronic band structures of CuAlO2 and Cu12Al11MO24 (M = Be, Mg or Ca) along the high-symmetry points, plotted with reference to Fermi level at 0 eV.

The PDOS were calculated by projecting the wave function onto atom centered spherical harmonics. Four peaks are distinguishable in the valence band at 7, 4, 1.5 and 0.3 eV, respectively, which is basically consistent with Scanlon et al. calculated results [19]. The conduction band displays a narrow peak between +2 and +4 eV and a second broad peak above 6 eV, labeled V and VI respectively.

The O PDOS is dominated by 2p states between 8 and 2 eV, corresponding to peaks I and II in the TDOS. Minor O 2p states are distributed between 2 and 0 eV. The Al PDOS shows Al 3s and 3p states contribute very little to the valence band because their quantities are approximately one order lower than that for the O or Cu PDOS, which indicates ionic bonding of Al–O. The Cu PDOS presents the contribution of Cu atoms to the valence band is

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Fig. 5. The electronic density of states for CuAlO2.

Fig. 6. The partial density of states for Be substitution in CuAlO2.

derived from the Cu 3d states which are distributed between 3 and 0 eV, corresponding to peaks III and IV in the TDOS. The overlap of Cu 3d and minor O 2p states reveals the covalent character of

Cu–O in the top of valence band. As far as the conduction band is concerned, the peak V is formed mostly from Cu 3d and O 2p states. In the peak VI, the contributions from Al atoms are equally signif-

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Fig. 7. The partial density of states for Mg substituted CuAlO2.

icant, relative to those of O atoms. In addition, the second valence band situated between 20 and 18 eV (not shown here), is exclusively dominated by O 2s states. Fig. 6 shows the PDOS for Be doped CuAlO2. From the patterns, the substituted Be2+ cation unobviously influences the Cu and O PDOSs. The atomic ratio of Al:Be equals 11:1 and the Be PDOS displays the contributions of Be s and p states to the valence band are approximately 10% compared to those of 11 Al atoms, which indicates the bonding of Be–O is also ionic and the pauling electronegativity of Be is very close to that of Al, well consistent with the above referring data (Section 2). Moreover, the onset of this contribution is situated at 6.5 eV, higher than 8 eV of Al PDOS, which could be one cause for the denser valence band structure of the Bedoped CuAlO2. Another interesting phenomenon is that in the conduction band above +7.5 eV, the contribution of Be 2p states is superior to that of its 2s states, contrary to the case of Al. The reason for that is under research. The partial electronic density of states for Mg-doped CuAlO2 is shown in Fig. 7. The contributions of Cu and O to the valence band and conduction band are almost unchanged because of a little amount of Mg replaced for Al. From the Mg PDOS pattern, the contribution of Mg s and p states is almost a half relative to Be doping, which suggests the pauling electronegativity of Mg is smaller than that of Be and hence the bonding of Mg–O is more ionic. Its onset is the same with that for Be and the energy level is shifted to higher one against that for Al, which indicates again this kind of substitution is responsible for the denser valence band structure. From Fig. 8, the calculated PDOSs for O, Al and Cu are almost the same with those in the cases of Be and Mg substitutions. The emergence of Ca d states is related to the electronic configuration of Ca p6s2d0.01 in the selected pseudopotential. With respect to the valence band, the contribution of Ca s states is very little, even lower than that of Mg s states, which implies the great consistence with the trend for pauling electronegativity. Due to p6d0.01 treated as

valence electrons, their contributions to the valence and conduction bands become significant. Ca p states also contribute to the second valence band between 20 and 16 eV except for O 2s states and its d states to the conduction band at about +7.5 eV. 3.5. Defect formation energies In the charged defect system, the defect formation energy can be written as follows [14]:

DHf ða; qÞ ¼ DEða; qÞ þ nCu lCu þ nAl lAl þ nO lO þ nA lA þ qEF ; where

DEða; qÞ¼ Eða; qÞ  EðhostÞ þ nCu l0Cu þ nAl l0Al þ nO l0O þ nA l0A þ qEVBM : In this notation, E(a,q) is the total energy of the supercell containing the defect a in charge state q. E(host) is the total energy of the same supercell free of any defect. n is the number of atoms and q is the number of electrons, transferred from the supercell to the reservoirs in forming the defect cell. l0i and li are the standard and relative chemical potentials of constituent i, respectively. EVBM is the energy of the valence band maximum (VBM). EF is the Fermi energy measured from VBM and is bound between EVBM and ECBM . In this study, we take a ¼ BeAl , MgAl and CaAl and q = 1. And the relative chemical potentials must satisfy some thermodynamic limits based on the experimental growth conditions, i.e., (1) maintaining a stable CuAlO2 compound,

lCu þ lAl þ 2lO ¼ DHf ðCuAlO2 Þ ¼ 8:59 eV; (2) avoiding the precipitation of solid elements,

lCu 6 0; lCu 6 0; lCu 6 0; (3) avoiding the formation of secondary phases,

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Fig. 8. The partial density of states for Ca substituted CuAlO2.

Fig. 9. The chemical potential region of the three components in CuAlO2.

2lCu þ lO 6 DHðCu2 OÞ ¼ 1:47 eV

lCu þ lO 6 DHðCuOÞ ¼ 1:22 eV 2lAl þ 3lO 6 DHðAl2 O3 Þ ¼ 15:44 eV

lBe þ lO 6 DHðBeOÞ ¼ 6:17 eV lMg þ lO 6 DHðMgOÞ ¼ 7:05 eV lCa þ lO 6 DHðCaOÞ ¼ 7:18 eV

Thus, we can deduce the chemical potential region of the three components in CuAlO2 (shown in Fig. 9). For simplicity, here we consider only the O-rich condition (at point A, lCu ¼ 0:52 eV, lAl ¼ 6:65 eV, lO ¼ 1:42 eV, then lBe ¼ 4:75 eV, lMg ¼ 5:63 eV, lCa ¼ 5:76 eV). According to the above notation and calculations for the total energies, the defect formation energies of DHf ðBeAl Þ ¼ 0:87 eV, DHf ðMgAl Þ ¼ 1:53 eV, and DHf ðCaAl Þ ¼ 1:89 eV are obtained. It should be noted that such calculations are rough because of ignorance for the potential align-

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ment and image charge correction [15]. From the calculated results, the substitution of Be2+ cation for Al3+ has the lowest defect formation energy, which means it forms more easily in CuAlO2. 4. Conclusions In this study, we systematically calculated the electronic properties of bivalent cation substitutions including the structural properties, charge densities, energy band structures, electronic densities of states and defect formation energies. Through comparing the M2+ substituted CuAlO2 to defect-free one, some intriguing phenomena have been observed, such as the decrease of the u1 parameter with the atomic number from Be to Ca, the denser energy band structure in the valence band and the contribution onset of the substituted cations shifting to higher energy. Internal correlations of these features have been expatiated in this paper. Among these defects, the (BeAl, 1) has the lowest defect formation energy. Acknowledgments Part of the CPU-time of this work was supplied by Supercomputing Center, Central South University. The authors acknowledge the financial supports from the Key Project of the year of Anhui Province under Contract No. 11070203010, the Project of Anhui Province for College Excellent Young Talents under Grant Nos. 2010SQRL132 and 2011SQRL159, the Launched Project of Chizhou College for Introducing Postgraduates under Contract No. 2011RC030, and Natural Science Foundation of Changzhou Institute of Technology under Grant No. YN1105.

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