Sanjun Wang*,
Xiaobo Shi and
Jinming Li
School of Physics and Electronic Engineering, Henan Institute of Education, Zhengzhou 450046, P. R. China. E-mail: sanjunwang6@aliyun.com
First published on 28th October 2016
The electronic, magnetic, and optical properties of Er-doped ZnO were studied using the density functional theory within the generalized gradient approximation plus U. Three defect configurations of substitution of Zn by Er with/without O or Zn vacancy were considered. The electronic and magnetic results show that Er doping has no effect on the ZnO band edge properties. The Er atom induces 3.0 μB atom-like magnetic moments by its 4f electrons and makes no contribution to the host ZnO. The O vacancy in Er-doped ZnO induces a defect state in the band gap, but shows no spin polarization. For the case of Zn vacancy in Er-doped ZnO, however, the unpaired 2p electrons at the ligand O atom induce a defect state in the conduction band edge and obtain a 1.0 μB magnetic moment. This result is consistent with the experimental data. Finally, the optical properties of Er-doped ZnO are calculated. Our results explained the experiments and show that Er-doped ZnO should be a good spin optoelectronic material and useful for understanding the mechanism of the room-temperature ferromagnetism properties in ZnO.
In this study, we investigated the electronic, magnetic and optical properties of Er-doped ZnO. Using the GGA + U framework, we calculate and analyze the band structure, partial density of state (PDOS), and their magnetic properties for the three possible Er related defects in ZnO. That is, Er substitution one of Zn atom in ZnO (ErZn) and its complex with O or Zn vacancy in ZnO (ErZn–VO or ErZn–VZn). We also examined their optical properties, which should be useful in Er-doped ZnO optoelectronic device applications. This study is expected to be useful for understanding the mechanism of the room-temperature ferromagnetism properties of ZnO and spin optoelectronic device applications.
To simulate the defect properties, a 3 × 3 × 2 primitive cell, containing 72 atoms (Zn36O36) supercell was employed in this study. We substitute one Zn atom with an Er atom to achieve substitution doping, i.e., Zn35ErO36 (ErZn), as illustrated in Fig. 1(a). We remove the O ligand with Er atom to produce a defect complex, i.e., Zn35ErO35 (ErZn–VO), as illustrated in Fig. 1(b). For the defect complex combined with a Zn vacancy, i.e., Zn34ErO36 (ErZn–VZn), we remove one of the nearest Zn atoms with an Er atom, as illustrated in Fig. 1(c). Note that Er substitution of Zn and O is considered here because the rare-earth atom substitution on cation site has been both experimentally and theoretically found to be the most possible configuration in ZnO.11–13,21
In Fig. 1, we show the relaxed three defect configurations ErZn, ErZn–VO, and ErZn–VZn in ZnO. We can see that there are some of atomic displacements for the ErZn, ErZn–VO. For ErZn defect, except the Er–O bond in the c direction is lengthened from 1.98 Å to 2.16 Å, which is caused by the large atom radius of Er. Other bond lengths have almost no change. In an ErZn–VO defect, as shown in Fig. 1(b), the remaining three Er–O bonds change from 2.03 Å to 2.05 Å showing slight lengthening. The distance between Er and the removed O atom is 2.00 Å, which is much less than 2.16 Å in an ErZn defect. The reason is, with O atom removed, the original ligand Zn–O bond should shorten because of a lack of Colum interactions by the removed O. At the same time, owing to the large atomic radius of Er, the Er–O bond tends to lengthen. With both these interactions, the Coulomb interaction by the removed O shortens the bond length, whereas the large atomic radius of Er increases the bond length; finally, the Er–O bond changes slightly from 2.03 Å to 2.05 Å. The distance between Er and the removal of an O atom change slightly. However, in the ErZn–VZn defect configurations, the structure shows great disorder after relaxation. As shown in Fig. 1(c), an Er atom moves prominently to the Zn vacancy direction and the displacement reaches 0.65 Å. The O atoms, which ligand with the Er atom and Zn vacancy in the c direction, have moved in the c direction 0.33 Å and 0.35 Å, respectively. These large displacements all show great disorder in ErZn–VZn defect configuration.
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Fig. 2 Calculated spin polarizes band structure for the three type of Er-doped ZnO defects. (a) ErZn, (b) ErZn–VO, (c) ErZn–VZn. The valence band maximum was set to zero. |
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Fig. 3 Calculated partial density of states for the three types of Er-doped ZnO defects. (a) ErZn, (b) ErZn–VO, (c) ErZn–VZn. The valence band maximum was set to zero. |
Fig. 2(a) and (c) show a band gap ∼3.25 eV for ErZn and ErZn–VZn, which is similar to that of bulk ZnO. In addition, their conduction band minimum shows similar dispersion properties, which means that they may have similar optical properties to that of the bulk ZnO. The only differences are at values near 4.8 eV above the valence band maximum. In spin down, there is one degenerate defect state in the ErZn defect; see Fig. 2(a). However, in Fig. 2(c), the ErZn–VZn defect shows split defect states near 4.2 eV and 4.8 eV. This is caused by the structural distortion and different defect states. Compared to their PDOS in Fig. 3, we can identify that one of the defect states comes from O 2p electrons and the other one comes from Er 4f electrons.
In Fig. 2(b), at about 2.2 eV, there is one defect level in the forbidden band gap both in the spin up and spin down. Combined with PDOS in Fig. 3(b), we can identify that this defect level is caused by the Zn 4s and Er 4p electrons. This defect level comes from the dangling bonds of Zn and Er atoms, which is caused by the removal of an O atom.
Fig. 3 shows the calculated total and PDOS of individual O, Zn, and Er elements in ErZn, ErZn–VO, ErZn–VZn defects in ZnO. From Fig. 3 displaying the total PDOS of three defects, one can see that there are three zones for the total density of states, which mainly show the bulk ZnO properties. The −10 eV to −6 eV are contributed by the Zn 3d states. The −6 eV to 0 eV come from the O 2p electrons. Above the Fermi level, Zn 4s electrons contributed the major states. In −6.0 eV to 0 eV, there are also some Zn 3d states, which possess hybridization with O 2p and aid in the construction of the valence band edge of bulk ZnO. In Fig. 3, Er atoms show evident spin polarized properties in all three Er-related doping. In spin up, the Er 4f electrons are distributed in −10 eV to −6 eV. In spin down, they are distributed near −2 eV and 4.8 eV. These asymmetrically distributed electronic states caused a spin magnetic Er atom.
Fig. 3(b) presents the PDOS of ErZn–VO, which shows similar properties with ErZn in Fig. 3(a) except at about 2.0 eV, where it shows a small peak in both spin up and spin down. We can see that this small peak comes from the Zn 4s and Er 4p electrons. These states show nonmagnetic properties and are consistent with the Fig. 2(b) band structure of ErZn–VO.
In Fig. 3(c), except for the Er induced 4f state peak near 4.8 eV, there is an O 2p induced state peak at approximately 4.2 eV in spin down. This state peak is consistent with Fig. 2(c), which shows a defect level at about 4.2 eV in spin down. Clearly, this defect state will induce a net magnetic moment in ZnO, which will be shown in more detail below.
ErZn | ErZn–VO | ErZn–VZn | |
---|---|---|---|
Er atom | 2.94 | 2.93 | 3.01 |
System | 2.83 | 2.91 | 4.00 |
With only the ErZn–VZn defect, Table 1 shows a magnetic moment of about 1.0 μB except for Er 3.0 μB magnetic moment. This is consistent with Fig. 3(c) that O 2p shows a small peak in about 4.2 eV in spin down. To show this clearly, we draw out the isosurface of the magnetic moments for ErZn–VZn defect, as shown in Fig. 4. We can clearly see the magnetic moment isosurface around an Er atom and one around the O atom. The isosurface around the Er atom shows an atomic-like magnetic. The isosurface around of the Er atom shows an atomic-like magnetic property. Our isosurface results show that the 1.0 μB magnetic moment clearly comes from the unpaired 2p electrons at the ligand O atom. These magnetic results are consistent with the recent experiment by Chen H. M. and Liu X. C. et al.12,13 This Zn vacancy-induced ferromagnetism is also consistent with the observed ferromagnetism in pulsed laser deposition ZnO films.22,23
Fig. 5 shows the complex dielectric functions of ErZn, ErZn–VO, ErZn–VZn three defects in ZnO. For the optical device, the absorption coefficient is important. The absorption coefficient is shown in Fig. 6. We can see the absorption coefficient of three defects located mainly at 10 eV same as the bulk ZnO.
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Fig. 5 Calculated real (a) and imaginary (b) part of the dielectric functions of ErZn, ErZn–VO, and ErZn–VZn three defects in ZnO. |
We first discuss the imaginary part, ε2, and then real part, ε1. The transitions obey the selection rule and electronic transitions between occupied and unoccupied states. For the defect of ErZn and ErZn–VZn, we can see in Fig. 5 that there are three main peaks of the imaginary part ε2, which are at around 5.2 eV, 9.0 eV and 15 eV. These results are consistent with the ZnO bulk optical properties and other theoretical results.20,24 For the ErZn–VO defect, there is a high peak at about 1.3 eV caused mainly by O vacancy defects.
For the real part ε1, we can see that the ErZn and ErZn–VZn still show similar results, however ErZn–VO defect shows a peak near 1.3 eV. Above 6.0 eV, three defects show a similar real part of the dielectric function. This similar profile means that the effects of Er and single intrinsic vacancy are mainly in lower energy optical properties and few on the high energy.
From Fig. 6, we can see that the absorption coefficients of all three defects of Er doping are located mainly at 10 eV, which falls in the vacuum ultraviolet region and show main bulk ZnO properties. Only the ErZn–VO defect shows a peak at about 1.3 eV, which is caused by O vacancy defects and is consistent with the previous complex dielectric function and band structure results.
From the optical properties results, we can see that although Er-doped ZnO induces some defect states, it still shows mainly ZnO optical properties. At a bandgap of about 3.26 eV, ZnO can be an excellent host material for Er ion doping and emission in the infrared region at 1.54 μm. Recently, Yang et al.6 realized red, green and blue multicolor electroluminescence using rare-earth Eu, Er, and Tm doped ZnO films acting as the lighting-emitting layer, respectively. In addition, Er doping can induce ferromagnetic properties in ZnO as shown above. This magnetic property for Er or other rare-earth-doped ZnO makes rare-earth-doped ZnO serve as a good candidate for spin optoelectronic studies.
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