- Email: [email protected]
First-principles study on the bandgap modulation of Be and Mg co-doped ZnO systems
ARTICLE IN PRESS Physica B 404 (2009) 1794–1798
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
First-principles study on the bandgap modulation of Be and Mg co-doped ZnO systems Xiyu Su a,, Panpan Si a, Qinying Hou a,b, Xianglan Kong a, Wei Cheng a a b
College of Physics and Engineering, Qufu Normal University, Qufu, Shandong 273165, China Library, Qufu Normal University, Qufu, Shandong 273165, China
a r t i c l e in f o
a b s t r a c t
Article history: Received 12 January 2009 Received in revised form 18 February 2009 Accepted 18 February 2009
Using the first-principles approach based upon the density functional theory, the electronic structure of Be and Mg co-doped wurtzite ZnO systems are studied. Compared with Be-doped ZnO systems, Be and Mg co-doped systems are more stable. For certain doping concentration a solar-blind region bandgap can be achieved. & 2009 Elsevier B.V. All rights reserved.
PACS: 63.20.dk 73.20.At 71.15.Mb 71.20.Be Keywords: First-principles BeMgZnO Electronic structure Bandgap
1. Introduction ZnO has been regarded as a promising wide-bandgap semiconductor for application in ultraviolet region optoelectronic devices because of its excellent optical and electrical properties, such as the wide direct bandgap of 3.37 eV and the high exciton binding energy of 60 meV at room temperature [1,2]. Recently, various applications of ZnO-based optoelectronic devices, such as UV detectors and light emitting diodes (LED) [3,4], have been reported. UV detectors generally work in the solar-blind region in which the strong atmospheric absorption of sunlight within a narrow UV band from 220 to 280 nm [5,6] appears. The cut-off wavelength of solar-blind UV detectors should be shorter than 280 nm (4.5 eV), a value much larger than the intrinsic bandgap (3.37 eV) of ZnO. So the ZnO-based films with wider bandgap is desired. Actually, the II–VI oxide semiconductor system Zn1xMgxO has been widely investigated [7–11] for tuning the bandgap from 3.37 eV for wurtzite ZnO to 7.8 eV for rock salt MgO. Wide-range solubility of Mg in ZnMgO is expected because of the similarity of the ionic radius of Mg2+ (0.57 A˚) and Zn2+ (0.60 A˚) [8].
Corresponding author. Tel.: +86 5374456096x415.
E-mail address: [email protected] (X. Su). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.02.021
Nowadays, the obtained maximum of the bandgap is only 3.9 eV for the doped wurtzite ZnMgO system with Mg composition of 33% [7]. For the concentration of Mg higher than 33%, phase segregation can be observed in ZnMgO films because of the structure difference and large lattice mismatch between ZnO (wurtzite structure, a ¼ 3.25 A˚, c ¼ 5.25 A˚) and MgO (rock salt, 4.22 A˚) [7–10]. Here, a axis orientation is considered. For a ZnMgO system, the well-defined bandgap between 4.2 and 5.4 eV has not been obtained [11]. For BexZn1xO systems, the bandgap can be modulated from 3.37 to 10.8 eV without phase segregation [12,13], but the large ionic radius difference between Be2+ (0.27 A˚) and Zn2+ (0.60 A˚) will lead to large lattice mismatch when bandgap offset between BeZnO and ZnO is large, and the crystal quality of BeZnO will decline as the Be content increased. In order to obtain high-quality crystalline ZnO-based films with bandgap modulated to the solar-blind region, Yang et al. [14] incorporated appropriate amounts of Be into ZnMgO by pulsed laser deposition (PLD), and a solar-blind region bandgap of 4.5 eV was obtained. The large lattice mismatches of ZnO/BeO and ZnO/MgO are counteracted by each other [12]. In this paper, we carry out theoretical study on BexMgyZn1xyO systems, the electronic structure is calculated, and the mechanism for bandgap broadening is discussed. The obtained results show that Be and Mg co-doped systems are more stable compared with the Be-doped ZnO systems. For certain
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Fig. 1. 2 2 1 Supercell of ZnO. The 16-atom supercell is used in our calculation and the ligand of ZnO is a triangular cone.
doping concentration a solar-blind region bandgap can be achieved.
Table 1 Theoretical results and experimental data [15] of lattice parameters and bandgap of ZnO.
2. Model and calculation method The ideal ZnO has a hexagonal wurtzite structure with the space group P63/mc and C6v-4 symmetry. The geometry constants are a ¼ b ¼ 0.3249 nm, c ¼ 0.5206 nm, a ¼ b ¼ 901, and g ¼ 1201. The 16-atom supercell (2 2 1) of the wurtzite ZnO used in our calculation is shown in Fig. 1. It can be seen that the ligand of ZnO is a triangular cone, the side arris is shorter than the underside arris, and the length of bond between the center atom and the cone-top atom is slightly longer than that of the bond between the center atom and the underside atom. The O2 coordination polyhedron is O–Zn4 tetrahedron, so is the Zn2+ coordination polyhedron. The calculation is carried out using CASTEP package provided by the Material Studios 4.1 by Accelrys Company. The package is an ab initio quantum mechanic codes based on density functional theory. In the package, the ionic potential is substituted by a pseudo-potential, the electronic wave function is expanded by the plane wave, and the exchange and correlative potential of electronic–electronic interaction is emendated by the generalized gradient approximation (GGA). In our calculation, the energy cutoff is set to be 450 eV, the total energy is converged to less than 2 105 eV/atom, and a 4 4 4 k-point mesh in the brillouin zone is used.
3. Results and discussion 3.1. Geometry optimization and electronic structure of ZnO For comparison, the pure ZnO system is firstly studied. The optimized supercell lattice parameters (a and c) and the calculated bandgap (Eg) are shown in Table 1. As can be seen, the calculated lattice parameters are in good agreement with the experimental data [15], which assure the reliability of our method. The calculated bandgap is smaller than the experimental data. In fact, the underestimation of the bandgap is a well-known drawback of using standard DFT calculations, but the results, which can be corrected by the scissors approximation, are also advisable for the qualitative analysis. The calculated band and density of states (DOS) are shown in Fig. 2, and the partial density of states (PDOS) of Zn and O atoms are shown in Fig. 3. It is evident that the bottom of the conduction band and the top of the valence band are in the same k-point (G), i.e. ZnO is a typical direct bandgap semiconductor. The valence band can generally be divided into two regions, the lower valence band within 6.6 to 2.6 eV and the upper valence band within 2.6 to 0 eV. The upper valence band is mainly contributed by O
Calculation value Experimental data [15]
a (nm)
c (nm)
c (a)
Eg (eV)
0.3295 0.3249
0.5317 0.5206
1.614 1.602
0.97 3.37
The calculated lattice parameters are in good agreement with the experimental data, which assured the reliability of our method.
2p states, and the lower valence band is chiefly contributed by the Zn 3d states, the conduction band is primarily contributed by Zn 4s states and O 2p states. Here, the bandgap is 0.97 eV, which is in agreement with the theoretical results by others [16]. 3.2. Geometry optimization and electronic structure of BeMgZnO The formation energy Ef(x, y) of the alloys is defined as [17,18] Ef ðx; yÞ ¼ Etot ðBex Mgy Zn1xy OÞ xEtot ðBeOÞ yEtot ðMgOÞ ð1 x yÞEtot ðZnOÞ
(1)
where Etot(BexMgyZn1xyO), Etot(BeO), Etot(MgO) and Etot(ZnO) are the total energies of the corresponding systems. Here, the total system, a quaternary compound, is composed of three binary compounds. When Ef(x, y) is negative, the quaternary compound is more steady than the binary compound; when Ef(x, y) is positive, the quaternary compound is less steady than the binary compound; larger Ef(x, y) is unfavorable for the stability of the total system. The optimized lattice parameters and formation energy for different systems are listed in Table 2. For BexZn1xO, the lattice constants and primitive cell volume V0 decrease with the increase in Be concentration; for MgxZn1xO, they increase with the increase in Mg composition; for BexMgyZn1xyO, their values lie between the values of BexZn1xO and MgxZn1xO, but are still smaller than that of ZnO. For the Be concentration of 0.125, and the Mg concentrations 0, 0.125, 0.25, 0.375 and 0.5, the parameter a increases from 3.197 to 3.214, which approaches the value of pure ZnO. It is predicted by Vegard’s law that the lattice constants of the alloys change linearly with the concentration of Be or Mg. For both Be and Mg concentration 0.25, the parameter a becomes much smaller than that of Be0.125Mg0.25Zn0.625O, and the formation energy is much larger, i.e. the alloy is less stable. So we can say that when the Be concentration is 0.125, the system is relatively stable. Thus, in the following discussion we just investigate the case of Be concentration 0.125 and Mg concentration 0, 0.125, 0.25, 0.375 and 0.5. The formation energy relation with Mg concentration for BexMgyZn1xyO (x ¼ 0.125) is shown in Fig. 4. It is obvious that the formation energy increases linearly with increase in Mg
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Fig. 2. Band and DOS of ZnO. The valence band can generally be divided into two regions, the lower valence band within 6.6 to 2.6 eV and the upper valence band within 2.6 to 0 eV. Here, the bandgap is 0.97 eV.
Table 2 Optimized lattice parameters and formation energy.
ZnO Be0.125Zn0.875O Be0.25Zn0.75O Mg0.125Zn0.875O Mg0.25Zn0.75O Be0.125Mg0.125Zn0.75O Be0.125Mg0.25Zn0.625O Be0.125Mg0.375Zn0.5O Be0.125Mg0.5Zn0.375O Be0.25Mg0.25Zn0.5O
a (A˚)
c (A˚)
V0 (A˚3)
Ef (eV)
3.259 3.191 3.115 3.264 3.270 3.197 3.204 3.209 3.214 3.150
5.252 5.147 5.072 5.243 5.238 5.137 5.127 5.120 5.109 5.043
48.299 45.385 42.629 48.390 48.502 45.468 45.667 45.754 45.918 43.126
– 0.498 0.545 0.092 0.183 0.617 0.751 0.887 1.029 1.378
For the Be concentration 0.125 and the Mg concentrations 0, 0.125, 0.25, 0.375 and 0.5, the parameter a increases from 3.197 to 3.214, trending to the value of pure ZnO.
Fig. 3. PDOS of O and Zn of ZnO. The upper valence band is mainly contributed by O 2p states, and the lower valence band is chiefly contributed by the Zn 3d states, the conduction band is primarily contributed by Zn 4s states and O 2p states. Fig. 4. Formation energies of BexMgyZn1xyO (x ¼ 0.125). The formation energy increases linearly with increase in Mg concentration.
concentration. Another point to be noted is that the formation energy of the BexMgyZn1xyO system is very small compared with those of the AlxGa1xN system calculated by de Paiva [19], indicating that such BexMgyZn1xyO alloys are very stable. We take Be0.125Mg0.125Zn0.75O as an example to discuss the bandgap broadening mechanism. The band and DOS of the system are shown in Fig. 5, the partial density of states is shown in Figs. 6 and 7. Compared with pure ZnO the bottom of the conduction band moves to the high-energy region, and the conduction band
becomes narrower, the valence band changes little, the bandgap becomes larger than that of pure ZnO. As a matter of fact, the bottom of the conduction band is decided by Zn 4s states, and the top of the valence band is decided by O 2p states; the bandgap is decided by O 2p and Zn 4s states. The Zn 4s states at the bottom of the conduction band move to the high-energy region since the
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Fig. 5. Band and DOS of Be0.125Mg0.125Zn0.75O. The conduction band moves to the high-energy region, so the bandgap becomes larger than that of pure ZnO.
Fig. 6. PDOS of O and Zn of Be0.125Mg0.125Zn0.75O. The Zn 4s states at the bottom of the conduction band move to the high-energy region.
incorporation of Mg and Be, while the O 2p states change little, which leads to the bandgap being broadened. The bandgap relation with Mg concentration for BexMgy Zn1xyO (x ¼ 0.125) is shown in Fig. 8. Bandgap increases as the concentration of Mg arises, it reaches 2.56 eV for y ¼ 0.5. As pointed out above, underestimation of the bandgap is a wellknown drawback of using standard DFT calculations, and the results can be corrected by the scissors approximation. Taking pure ZnO as the reference, the scissors correction can approximately be set to be 2.40 eV; thus the bandgap of the BexMgy Zn1xyO system can reach 4.96 eV (250 nm), which is suitable for the application of solar-blind UV detectors.
Fig. 7. PDOS of Mg and Be of Be0.125Mg0.125Zn0.75O. The 2s and 2p states of Mg and Be are shown in the figure.
4. Conclusion Using the first-principles approach based on the density functional theory, the electronic structure of wurtzite ZnO codoped with Be and Mg has been calculated, the variation of the lattice parameters of BexMgyZn1xyO with different impurity concentrations has been analyzed, and the mechanism for bandgap broadening has been discussed. The obtained results show that the Be and Mg co-doped systems are more stable compared with the Be-doped ZnO systems. For certain doping concentrations, a solar-blind region bandgap can be achieved.
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References
Fig. 8. Bandgap of BexMgyZn1xyO (x ¼ 0.125). Bandgap increases as the concentration of Mg increases, it reaches 2.56 eV for y ¼ 0.5.
Acknowledgement This study was supported financially by the National Natural Science Foundation of China (Grant no. 10775088).
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Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
First-principles study on the bandgap modulation of Be and Mg co-doped ZnO systems Xiyu Su a,, Panpan Si a, Qinying Hou a,b, Xianglan Kong a, Wei Cheng a a b
College of Physics and Engineering, Qufu Normal University, Qufu, Shandong 273165, China Library, Qufu Normal University, Qufu, Shandong 273165, China
a r t i c l e in f o
a b s t r a c t
Article history: Received 12 January 2009 Received in revised form 18 February 2009 Accepted 18 February 2009
Using the first-principles approach based upon the density functional theory, the electronic structure of Be and Mg co-doped wurtzite ZnO systems are studied. Compared with Be-doped ZnO systems, Be and Mg co-doped systems are more stable. For certain doping concentration a solar-blind region bandgap can be achieved. & 2009 Elsevier B.V. All rights reserved.
PACS: 63.20.dk 73.20.At 71.15.Mb 71.20.Be Keywords: First-principles BeMgZnO Electronic structure Bandgap
1. Introduction ZnO has been regarded as a promising wide-bandgap semiconductor for application in ultraviolet region optoelectronic devices because of its excellent optical and electrical properties, such as the wide direct bandgap of 3.37 eV and the high exciton binding energy of 60 meV at room temperature [1,2]. Recently, various applications of ZnO-based optoelectronic devices, such as UV detectors and light emitting diodes (LED) [3,4], have been reported. UV detectors generally work in the solar-blind region in which the strong atmospheric absorption of sunlight within a narrow UV band from 220 to 280 nm [5,6] appears. The cut-off wavelength of solar-blind UV detectors should be shorter than 280 nm (4.5 eV), a value much larger than the intrinsic bandgap (3.37 eV) of ZnO. So the ZnO-based films with wider bandgap is desired. Actually, the II–VI oxide semiconductor system Zn1xMgxO has been widely investigated [7–11] for tuning the bandgap from 3.37 eV for wurtzite ZnO to 7.8 eV for rock salt MgO. Wide-range solubility of Mg in ZnMgO is expected because of the similarity of the ionic radius of Mg2+ (0.57 A˚) and Zn2+ (0.60 A˚) [8].
Corresponding author. Tel.: +86 5374456096x415.
E-mail address: [email protected] (X. Su). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.02.021
Nowadays, the obtained maximum of the bandgap is only 3.9 eV for the doped wurtzite ZnMgO system with Mg composition of 33% [7]. For the concentration of Mg higher than 33%, phase segregation can be observed in ZnMgO films because of the structure difference and large lattice mismatch between ZnO (wurtzite structure, a ¼ 3.25 A˚, c ¼ 5.25 A˚) and MgO (rock salt, 4.22 A˚) [7–10]. Here, a axis orientation is considered. For a ZnMgO system, the well-defined bandgap between 4.2 and 5.4 eV has not been obtained [11]. For BexZn1xO systems, the bandgap can be modulated from 3.37 to 10.8 eV without phase segregation [12,13], but the large ionic radius difference between Be2+ (0.27 A˚) and Zn2+ (0.60 A˚) will lead to large lattice mismatch when bandgap offset between BeZnO and ZnO is large, and the crystal quality of BeZnO will decline as the Be content increased. In order to obtain high-quality crystalline ZnO-based films with bandgap modulated to the solar-blind region, Yang et al. [14] incorporated appropriate amounts of Be into ZnMgO by pulsed laser deposition (PLD), and a solar-blind region bandgap of 4.5 eV was obtained. The large lattice mismatches of ZnO/BeO and ZnO/MgO are counteracted by each other [12]. In this paper, we carry out theoretical study on BexMgyZn1xyO systems, the electronic structure is calculated, and the mechanism for bandgap broadening is discussed. The obtained results show that Be and Mg co-doped systems are more stable compared with the Be-doped ZnO systems. For certain
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Fig. 1. 2 2 1 Supercell of ZnO. The 16-atom supercell is used in our calculation and the ligand of ZnO is a triangular cone.
doping concentration a solar-blind region bandgap can be achieved.
Table 1 Theoretical results and experimental data [15] of lattice parameters and bandgap of ZnO.
2. Model and calculation method The ideal ZnO has a hexagonal wurtzite structure with the space group P63/mc and C6v-4 symmetry. The geometry constants are a ¼ b ¼ 0.3249 nm, c ¼ 0.5206 nm, a ¼ b ¼ 901, and g ¼ 1201. The 16-atom supercell (2 2 1) of the wurtzite ZnO used in our calculation is shown in Fig. 1. It can be seen that the ligand of ZnO is a triangular cone, the side arris is shorter than the underside arris, and the length of bond between the center atom and the cone-top atom is slightly longer than that of the bond between the center atom and the underside atom. The O2 coordination polyhedron is O–Zn4 tetrahedron, so is the Zn2+ coordination polyhedron. The calculation is carried out using CASTEP package provided by the Material Studios 4.1 by Accelrys Company. The package is an ab initio quantum mechanic codes based on density functional theory. In the package, the ionic potential is substituted by a pseudo-potential, the electronic wave function is expanded by the plane wave, and the exchange and correlative potential of electronic–electronic interaction is emendated by the generalized gradient approximation (GGA). In our calculation, the energy cutoff is set to be 450 eV, the total energy is converged to less than 2 105 eV/atom, and a 4 4 4 k-point mesh in the brillouin zone is used.
3. Results and discussion 3.1. Geometry optimization and electronic structure of ZnO For comparison, the pure ZnO system is firstly studied. The optimized supercell lattice parameters (a and c) and the calculated bandgap (Eg) are shown in Table 1. As can be seen, the calculated lattice parameters are in good agreement with the experimental data [15], which assure the reliability of our method. The calculated bandgap is smaller than the experimental data. In fact, the underestimation of the bandgap is a well-known drawback of using standard DFT calculations, but the results, which can be corrected by the scissors approximation, are also advisable for the qualitative analysis. The calculated band and density of states (DOS) are shown in Fig. 2, and the partial density of states (PDOS) of Zn and O atoms are shown in Fig. 3. It is evident that the bottom of the conduction band and the top of the valence band are in the same k-point (G), i.e. ZnO is a typical direct bandgap semiconductor. The valence band can generally be divided into two regions, the lower valence band within 6.6 to 2.6 eV and the upper valence band within 2.6 to 0 eV. The upper valence band is mainly contributed by O
Calculation value Experimental data [15]
a (nm)
c (nm)
c (a)
Eg (eV)
0.3295 0.3249
0.5317 0.5206
1.614 1.602
0.97 3.37
The calculated lattice parameters are in good agreement with the experimental data, which assured the reliability of our method.
2p states, and the lower valence band is chiefly contributed by the Zn 3d states, the conduction band is primarily contributed by Zn 4s states and O 2p states. Here, the bandgap is 0.97 eV, which is in agreement with the theoretical results by others [16]. 3.2. Geometry optimization and electronic structure of BeMgZnO The formation energy Ef(x, y) of the alloys is defined as [17,18] Ef ðx; yÞ ¼ Etot ðBex Mgy Zn1xy OÞ xEtot ðBeOÞ yEtot ðMgOÞ ð1 x yÞEtot ðZnOÞ
(1)
where Etot(BexMgyZn1xyO), Etot(BeO), Etot(MgO) and Etot(ZnO) are the total energies of the corresponding systems. Here, the total system, a quaternary compound, is composed of three binary compounds. When Ef(x, y) is negative, the quaternary compound is more steady than the binary compound; when Ef(x, y) is positive, the quaternary compound is less steady than the binary compound; larger Ef(x, y) is unfavorable for the stability of the total system. The optimized lattice parameters and formation energy for different systems are listed in Table 2. For BexZn1xO, the lattice constants and primitive cell volume V0 decrease with the increase in Be concentration; for MgxZn1xO, they increase with the increase in Mg composition; for BexMgyZn1xyO, their values lie between the values of BexZn1xO and MgxZn1xO, but are still smaller than that of ZnO. For the Be concentration of 0.125, and the Mg concentrations 0, 0.125, 0.25, 0.375 and 0.5, the parameter a increases from 3.197 to 3.214, which approaches the value of pure ZnO. It is predicted by Vegard’s law that the lattice constants of the alloys change linearly with the concentration of Be or Mg. For both Be and Mg concentration 0.25, the parameter a becomes much smaller than that of Be0.125Mg0.25Zn0.625O, and the formation energy is much larger, i.e. the alloy is less stable. So we can say that when the Be concentration is 0.125, the system is relatively stable. Thus, in the following discussion we just investigate the case of Be concentration 0.125 and Mg concentration 0, 0.125, 0.25, 0.375 and 0.5. The formation energy relation with Mg concentration for BexMgyZn1xyO (x ¼ 0.125) is shown in Fig. 4. It is obvious that the formation energy increases linearly with increase in Mg
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Fig. 2. Band and DOS of ZnO. The valence band can generally be divided into two regions, the lower valence band within 6.6 to 2.6 eV and the upper valence band within 2.6 to 0 eV. Here, the bandgap is 0.97 eV.
Table 2 Optimized lattice parameters and formation energy.
ZnO Be0.125Zn0.875O Be0.25Zn0.75O Mg0.125Zn0.875O Mg0.25Zn0.75O Be0.125Mg0.125Zn0.75O Be0.125Mg0.25Zn0.625O Be0.125Mg0.375Zn0.5O Be0.125Mg0.5Zn0.375O Be0.25Mg0.25Zn0.5O
a (A˚)
c (A˚)
V0 (A˚3)
Ef (eV)
3.259 3.191 3.115 3.264 3.270 3.197 3.204 3.209 3.214 3.150
5.252 5.147 5.072 5.243 5.238 5.137 5.127 5.120 5.109 5.043
48.299 45.385 42.629 48.390 48.502 45.468 45.667 45.754 45.918 43.126
– 0.498 0.545 0.092 0.183 0.617 0.751 0.887 1.029 1.378
For the Be concentration 0.125 and the Mg concentrations 0, 0.125, 0.25, 0.375 and 0.5, the parameter a increases from 3.197 to 3.214, trending to the value of pure ZnO.
Fig. 3. PDOS of O and Zn of ZnO. The upper valence band is mainly contributed by O 2p states, and the lower valence band is chiefly contributed by the Zn 3d states, the conduction band is primarily contributed by Zn 4s states and O 2p states. Fig. 4. Formation energies of BexMgyZn1xyO (x ¼ 0.125). The formation energy increases linearly with increase in Mg concentration.
concentration. Another point to be noted is that the formation energy of the BexMgyZn1xyO system is very small compared with those of the AlxGa1xN system calculated by de Paiva [19], indicating that such BexMgyZn1xyO alloys are very stable. We take Be0.125Mg0.125Zn0.75O as an example to discuss the bandgap broadening mechanism. The band and DOS of the system are shown in Fig. 5, the partial density of states is shown in Figs. 6 and 7. Compared with pure ZnO the bottom of the conduction band moves to the high-energy region, and the conduction band
becomes narrower, the valence band changes little, the bandgap becomes larger than that of pure ZnO. As a matter of fact, the bottom of the conduction band is decided by Zn 4s states, and the top of the valence band is decided by O 2p states; the bandgap is decided by O 2p and Zn 4s states. The Zn 4s states at the bottom of the conduction band move to the high-energy region since the
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Fig. 5. Band and DOS of Be0.125Mg0.125Zn0.75O. The conduction band moves to the high-energy region, so the bandgap becomes larger than that of pure ZnO.
Fig. 6. PDOS of O and Zn of Be0.125Mg0.125Zn0.75O. The Zn 4s states at the bottom of the conduction band move to the high-energy region.
incorporation of Mg and Be, while the O 2p states change little, which leads to the bandgap being broadened. The bandgap relation with Mg concentration for BexMgy Zn1xyO (x ¼ 0.125) is shown in Fig. 8. Bandgap increases as the concentration of Mg arises, it reaches 2.56 eV for y ¼ 0.5. As pointed out above, underestimation of the bandgap is a wellknown drawback of using standard DFT calculations, and the results can be corrected by the scissors approximation. Taking pure ZnO as the reference, the scissors correction can approximately be set to be 2.40 eV; thus the bandgap of the BexMgy Zn1xyO system can reach 4.96 eV (250 nm), which is suitable for the application of solar-blind UV detectors.
Fig. 7. PDOS of Mg and Be of Be0.125Mg0.125Zn0.75O. The 2s and 2p states of Mg and Be are shown in the figure.
4. Conclusion Using the first-principles approach based on the density functional theory, the electronic structure of wurtzite ZnO codoped with Be and Mg has been calculated, the variation of the lattice parameters of BexMgyZn1xyO with different impurity concentrations has been analyzed, and the mechanism for bandgap broadening has been discussed. The obtained results show that the Be and Mg co-doped systems are more stable compared with the Be-doped ZnO systems. For certain doping concentrations, a solar-blind region bandgap can be achieved.
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X. Su et al. / Physica B 404 (2009) 1794–1798
References
Fig. 8. Bandgap of BexMgyZn1xyO (x ¼ 0.125). Bandgap increases as the concentration of Mg increases, it reaches 2.56 eV for y ¼ 0.5.
Acknowledgement This study was supported financially by the National Natural Science Foundation of China (Grant no. 10775088).
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