Transparent conductivity modulation of ZnO by group-IVA doping

Accepted Manuscript Title: Transparent Conductivity Modulation of ZnO by Group-IVA Doping Author: J. Liu X.F. Fan C.Q. Sun W. Zhu PII: DOI: Reference:

S0009-2614(16)30051-3 http://dx.doi.org/doi:10.1016/j.cplett.2016.02.033 CPLETT 33644

To appear in: Received date: Revised date: Accepted date:

15-12-2015 11-2-2016 14-2-2016

Please cite this article as: J. Liu, X.F. Fan, C.Q. Sun, W. Zhu, Transparent Conductivity Modulation of ZnO by Group-IVA Doping, Chem. Phys. Lett. (2016), http://dx.doi.org/10.1016/j.cplett.2016.02.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Highlights Group-IVA doped ZnO were investigated by first principle calculations. For group-IVA doped ZnO, Si doped ZnO has higher transmittance in visible region and UV region.

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Intraband transition causes the reduction of visible region transmittance.

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J. Liua, X. F. Fana,b, C. Q. Suna, and W. Zhua,*

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Transparent Conductivity Modulation of ZnO by Group-IVA Doping

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a. NOVITAS, School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Republic of Singapore

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b. College of Materials Science and Engineering, Jilin University, Changchun, 130012, China

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* To whom correspondence should be addressed: [email protected]

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Abstract

We examined the effect of group-IVA doping on the electronic structure and transmittance of ZnO using first-principle calculations. All these doped ZnO materials are found to perform n-

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type conductive behavior. Si-doped ZnO and Pb-doped ZnO are found to have larger optical

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band gap than those of Ge-doped ZnO and Sn-doped ZnO. The transmittance of Si-doped ZnO is found to be high in both UV and visible region. The enhancement of UV region transmittance

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can be attributed to the enhanced optical band gap, while the reduction of visible region transmittance is due to the intraband optical transition.

Key words: first-principle calculation, doping, group-IVA doped ZnO, electronic structure, optical property

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1. Introduction Zinc oxide (ZnO) is an important II-IV compound semiconductor for optoelectronic devices1-6.

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The band gap is approximately 3.37eV with a large exciton binding energy of about 60 meV7-9. Recently, ZnO as a potential excellent transparent conductive oxide (TCO) to replace tin-doped

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indium oxide (ITO) has drawn intensive attention10-12, since ITO is expensive and causes

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environmental issues13-15. The experimentally-obtained ZnO is usually an n-type semiconductor due to the intrinsic point defects, such as Zni and OV. Previous study pointed out that pristine

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ZnO is not suitable for forming TCO due to the large resistivity16, 17. In recent years, many experiments to enhance the conductivity of ZnO have been attempted. Doping ZnO with proper

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dopants can possibly improve the performance of ZnO significantly. Mono-dopant, such as C, Ga, In, B, etc.18-21 and co-dopant combinations, such as Al-N, Al-C, etc.22-25, have been reported.

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Recently, silicon as a dopant is attempted and the results show that silicon doped ZnO can

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achieve low resistivity26, 27. The highly transparent and conducting silicon doped ZnO thin film

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has been tried to fabricate. The optical properties of silicon doped ZnO and the effect of silicon concentration on the transparence and conductivity have been reported26. Similar researches on germanium doped ZnO have also been reported28. However, the relative microscopic mechanism about the electronic and optical properties is still not very clear, as we have known. In the theoretical part, some works about the electronic and optical properties of doped ZnO have been taken recently. Zhou and his coworkers calculated the defect energy level and the conductivity of In-doped ZnO under different defect concentrations29. The study shows that the conductivity is increased by following the increase of the concentration of In. Peng et al. studied the effects of intrinsic defects on the electrical and optical properties of B-doped ZnO30. Lyons et

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al. reported the electronic and structural properties of Si and Ge doped ZnO with the lowconcentration doping and found Zn replaced by Si or Ge was popular in ZnO31. While preliminary experimental results show the Si-doping ZnO may exhibit low resistivity and high

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transmittance which is promising and attractive, the theoretic analysis about the interrelated

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mechanism is limited, to the best of our knowledge.

In this work, with first-principle methods, we explored the electronic and optical properties of

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heavily-doped ZnO with group-IVA elements. Si, Ge, Sn and Pb have been selected to be the

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donor dopants. Firstly, we studied the change of electronic properties of ZnO including band structure and density of states by following the doping with different dopants. Then the dielectric

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function, reflectivity and transmittance spectra and absorption coefficients of the doped ZnO are analyzed in order to shed some light on the optoelectronic properties of doped ZnO as a potential

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TCO material.

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2. Computation Method and Theoretical Model

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The ideal ZnO is with hexagonal wurtzite structure, containing two zinc atoms and two oxygen atoms in the primitive cell. The lattice constants a and c are 3.249 Å and 5.206 Å, respectively4. In this study, we have constructed a 222 supercell with 16 cations and 16 oxygen ions. In the previous studies, it has been demonstrated that the silicon atom will substitute for zinc atom instead of oxygen atom in ZnO32. Here, the doping models are made by substituting the zinc atom with one of the Group-IV elements (Si, Ge, Sn and Pb) in supercell 222, as shown in Figure 1. In these doping models, the defect concentration is 6.25% and it is considered to be the case of heavy doping for the application of TCO. Usually, such a high doping concentration may induce the formation of the interstitial Si (Sii) and/or small Si cluster

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in the lattice of ZnO, which result in the distortion of ZnO lattice. In the present calculations, we just consider the incorporation of Si (Ge, Sn and Pb) on the Zn site (SiZn), since this kind of

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doping has important effect on the electronic and optical properties of ZnO. In the present study, all the calculations were performed on the basis of density functional

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theory using the CASTEP package33. We used gradient approximation (GGA) with the parameterization of Perdew-Burke-Ernzerhof (PBE)34. Before calculating the electronic and

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optical properties, the geometry optimization was fully performed for each model. The

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convergence energy, maximum force, maximum stress and maximum displacement were set to be 5x10-6 eV/atom, 0.01e V/Å, 0.02 GPa and 5x10-4 Å, respectively. The valence electrons

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configurations are 4s23d10 for Zn, 2s22p4 for O, 3s23p2 for Si, 4s24p2 for Ge, 5s25p2 for Sn, 5d106s26p2 for Pb, respectively. The plane-wave expansion kinetic energy cutoff was set to be

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420 eV. Brillouin zones were sampled using 4x4x2 k-point grid with Monkhorst method35 and

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the self-consistent iterations (SCF) tolerance was set to be 5x10-7 eV/atom. Ultrasoft

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pseudopotentials in the Vanderbit form were used to describe the ionic potentials36. Due to the well-known limitations of usual GGA in density functional theory, the calculated band gap of ZnO is underestimated and much less than the experimental value37, 38. Recently, Sheetz et al. reported that the band gap can be calculated accurately using DFT+U method39. The calculated band gap of ZnO nanowire in their work is 3.72 eV. Other calculations based on the DFT+U method have obtained similar results24, 40. Therefore in this study, we have adopted the DFT+U method and reset the values of p-orbital electrons for oxygen atom and d-orbital electrons for zinc atom. Following the reports in literatures, the Up is set 7eV24, 39. For the Ud, the setting range in present reports varies from 9.5eV to 10.5eV20, 23, 24, 30. Considering this, we have performed the calculations for the band structure of pristine ZnO under different Ud values. 6 Page 6 of 21

According to the results, the combination of Up = 7 eV and Ud = 9.6 eV is better for that the calculated band gap is exactly 3.37 eV and is adopted in the study of doping models. Figure 2 shows the calculated band gaps of pristine ZnO with traditional DFT method and +U method.

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The calculated band gap has increased from 0.806 eV to 3.37 eV. Figure 3 shows the DOS of Zn-3d and O-2p orbitals using traditional DFT method and +U method. The maximum peak of

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Zn-3d for DFT+U is around 6.65 eV below valance band maximum (VBM), while the maximum

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peak of Zn-3d in traditional DFT method is around 4 eV below VBM. The maximum peak of O2p in both methods is located nearly at the same energy value. Therefore, the underestimation of

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the splitting between states from Zn-3d and O-2p is corrected by +U method and results in the uplift of conduction band. Table 1 shows the calculated lattice constants of pristine ZnO in this

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work and other theoretical works in literature, which have an acceptable deviation from the

Results and Discussions

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experimental results.

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3.1 Electronic properties of doped ZnO

For pristine ZnO shown in Figure 1(a), the calculated band gap is 3.37eV, which is in excellent agreement with experimental result. The VBM and CBM are both located at  point, which verifies that ZnO is with a direct band gap. The calculated band structures of ZnO with the doped Si, Ge, Sn and Pb are shown in Figure 4(a)-(d). The results show that the doped ZnO has n-type conductive characteristics. The Fermi levels have all shifted upward into the conduction band. However, the values of shift are different. The calculated optical band gaps of ZnO: Si, ZnO: Ge, ZnO: Sn and ZnO: Pb are 4.96 eV, 2.68 eV, 2.85 eV and 4.70 eV, respectively. Here the optical band gap is defined to be the energy difference between the top of the valence band

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and the Fermi level. Obviously, the values of optical band gap of ZnO: Si and ZnO: Pb are larger than the band gap of pristine ZnO, while those of ZnO: Ge and ZnO: Sn are smaller than the band gap of pristine ZnO. For the Si and Pd doped ZnO, the enhanced optical band gap can be

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proved by the Burstein-Moss blue shift of optical absorption. The extra electrons supported by ionic group-IVA elements, for example Si4+, can also increase the carrier concentration. This is

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also confirmed by experimental results.26, 41

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In order to understand the different optical band gap with different dopant, the density of states

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(DOS) and partial density of states (PDOS) of the doped ZnO are calculated and shown in Figure 5(a)-(d). For all the cases of doping, the upper valance band is contributed from O-2p orbitals

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and the dopants do not have obvious contributions. The states from p orbitals of dopants are localized mostly at the range of 3-4 eV above Fermi level and they are separated from the states

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ZnO lattice.

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from s orbitals of dopants. This may be attributed to the weak sp3 hybridization of the dopant in

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From the DOS of ZnO: Si shown in Figure 5(a), the bottom of conduction band is mostly from the contribution of Zn-4s. This is different from the other three cases including the doping of Ge, Sn and Pb. The strong interaction between O-p orbitals and Si-s orbitals results in the broadening of states from Si-s orbitals. Therefore, there is no obvious defect band or levels found in the band structure below Fermi level. From experimental results, with the increase of Si concentration, the mobility increases at first and then decreases continuously26. In our calculations, 6.25% concentration is considered to be highly doped and results in Fermi level into conduction band. Therefore, the phenomenon that the mobility is decreased is suppressed, since the other defects, such as interstitial Si and Si cluster due to the high-content doping which can induce the distortion of ZnO lattice are not considered in the present model. 8 Page 8 of 21

In Figure 4(b)-(c), the DOSs of ZnO: Ge and ZnO: Sn are shown. In both cases, the impurity band contributed from the s orbitals of dopants is formed and separated from the other conduction bands. The formation of impurity band results in the pinning of Fermi level and the

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optical band gap is reduced for Ge- and Sn-doping. For the Pb-doping shown in Figure 4(d), the coupling between O-p and Pb-s orbitals is increased. The states from the hybridized Pb-s orbitals

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are broadened and the energy is shifted up, compared with those of Ge and Sn. This may be due

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to the big size of Pb atom. In addition, the Zn-s orbitals are also contributed obviously to this broadening impurity band. The upward shift of impurity band results in the increase of optical

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band gap, following the increase of atomic size for the doping of Ge, Sn and Pb.

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3.2 Optical properties of doped ZnO

The optical properties can be described by dielectric functions ()=1()+i2(). The

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imaginary part of dielectric functions is related to the optical transition between occupied states

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and unoccupied states. The real part of dielectric function1() can be obtained from 2() with

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Kramers–Kronig relations. The other optical properties such as absorption coefficient  and reflectivity R can be calculated on the basis of 1() and 2()42. Figure 6 shows the dielectric functions of pristine ZnO and doped ZnO with different dopants. From Figure 6(a), the static dielectric constants 1(0) are 1.62, 6.91, 2.25, 2.95 and 4.92 for pristine ZnO, ZnO: Si, ZnO: Ge, ZnO: Sn and ZnO: Pb, respectively. The static dielectric constants of ZnO: Si and ZnO: Pb are higher and may result in potential applications in optical devices. Figure 6(b) shows the imaginary part of dielectric function. When doped with Si or Pb, the deviation is significant compared with pristine ZnO. An added peak appears in the low energy range of doped ZnO due to the intraband transition. For ZnO: Si, ZnO: Ge, ZnO: Sn and ZnO: Pb, the peak is located at

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1.0 eV, 2.6 eV, 2.5 eV and 1.8 eV, respectively. The larger values for ZnO: Ge and ZnO: Sn are due to the separation of the impurity bands from other conduction bands as shown in Figure 4

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and 5. Figure 7 shows the relationship of absorption and reflectivity with respect to the wavelength.

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The reflectivity spectra are almost the same in 0-200 nm region for pristine ZnO and doped ZnO, which is also observed in absorption spectra. In the range of 300-800 nm, the two spectra of

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doped ZnO are obviously different from that of pristine ZnO. From Figure 7(a), the reflectivity

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of ZnO: Si show a significant increase in the range of 600-800 nm of visible region and ZnO: Pb show a significant increase in 400-600 nm. As shown in Figure 7(b), an obvious broadening

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absorption peak appears in visible region for the doped ZnO. The maximum of absorption peak is located at 830 nm, 420 nm, 435 nm and 607 nm for ZnO: Si, ZnO: Ge, ZnO: Sn and ZnO: Pb,

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respectively. This shows that ZnO: Si may be a potential candidate for optoelectronic devices in

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visible range.

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The transmittances of pristine and doped ZnO are shown in Figure 8. The transmittance T is calculated with the formula, ,

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where R and  represent the reflectivity and absorption, respectively. The thickness (d) of film is assumed to be 250 nm for the application of TCO. For pristine ZnO, the transmittance in visible range can be up to 95%. However, the application for pristine ZnO has been limited by the disadvantage of high resistivity. For the doped ZnO, the transmittance is decreased in the long wavelength region. Especially, ZnO: Si and ZnO: Pb have much low transmittances in infrared region (700-1200 nm). In visible region, the transmittance of ZnO: Ge and ZnO: Sn has a 10 Page 10 of 21

minimum at around 450 nm. ZnO: Pb has a better transmittance in 200-400 nm and ZnO: Si has a better transmittance with the relative broad range from 200 nm to 600 nm.

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The average transmittance for pristine ZnO and doped ZnO in visible region (400-800 nm) and UV region (200-400 nm) are shown in Table 2. It is obvious that the transmittance in visible

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region decreases from 95.3% for pristine ZnO to 37.8% for ZnO: Pb. The transmittance in UV region of ZnO: Si is the largest, while that of ZnO: Ge is the least. The decrease of transmittance

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in visible region can be explained by the intraband optical transitions as discussed before. The

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increase of transmittance in UV region for ZnO: Si and ZnO: Pb can be contributed to the larger optical band gap due to the doping. From Table 2, it can be found evidently that ZnO: Si has the

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best transmittance performance compared with the other three doped ZnO in visible region. For Si doped ZnO, the experimental results show that the transmittance first increases sharply

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from about 380 nm and remain a certain value

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until decreases from 800 nm and further

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decreases with a relatively high rate after 1000 nm41. Our calculations show that the

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transmittance decreases from 400 nm and reaches a stable value after 800 nm. The trend is not fully consistent with that of experimental results. The possible reason is that our model only consider the substituting case, while the experimental results point out that the Si may not be well incorporated in the ZnO lattice and the formation of Sii and cluster Si in ZnO lattice will result in the down-shifting of Fermi level which can increase the transmittance in visible region41. 4. Conclusions In this study, we have performed first-principle calculations of group-IVA doped ZnO. The electronic properties including band structure and density of states (DOS) and optical properties including dielectric functions and transmittance have been calculated successfully using DFT+U 11 Page 11 of 21

method and discussed in detail. For the electronic properties, the calculated band gap for pristine ZnO is 3.37 eV, which is in accordance with experimental data. For the doped ZnO with groupIV dopants Si, Ge, Sn and Pb, they all have n-type characteristics. ZnO: Si and ZnO: Pb have

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larger optical band gap values than those of ZnO: Ge and ZnO: Sn. In the band structure of ZnO:

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Si, there is no obvious defect band which is much different from the other three cases of doping. It is found that the optical properties of ZnO: Si stand out in both visible region (400-800nm)

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and UV region (200-400 nm). In visible region (400-800 nm), the average transmittance is

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72.2% with peak value up to 95%. In UV region (200-400 nm), the average transmittance is 85.6% which is even better than pristine ZnO. ZnO: Pb also has better transmittance up to 70.1%

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in UV region compared with that of pristine ZnO. The decrease of transmittance of doped ZnO in visible region (400-800 nm) is due to the electron intraband transitions. The increase of

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transmittance of Si- and Pb-doped ZnO in UV region can be ascribed to the enlarged optical

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band gap. From the calculation results, we can draw the conclusion that Si is the best candidate

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dopant among Si, Ge, Sn and Pb to improve the performance of pristine ZnO.

Acknowledgments

This work was supported under Grant Nos. RG97/15 and RG101/14 from Nanyang Technological University. References: 1. 2. 3.

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Previous Value LDA+U44

Present Value GGA+U

3.203 5.139 1.604

3.148 5.074 1.612

3.283 5.298 1.614

Experimental Difference Value4 (%)

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3.249 5.206 1.602

1.04 1.77 0.75

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a(Å) c(Å) c/a

Previous Value LDA43

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Table 1. The calculated lattice parameters of pristine ZnO with that from other theoretical works and experimental values.

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Table 2. The average transmittance for pristine ZnO and Group-IV doped ZnO in visible region (400-800 nm) and UV region (200-400 nm).

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Pristine ZnO ZnO: Si ZnO: Ge ZnO: Sn ZnO: Pb

Transmittance (%, visible region) 95.3 72.2 68.5 56.4 37.8

Transmittance (%, UV region) 67.2 85.6 58.6 60.6 70.1

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Figure 1

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Figure 1. Structures of (a) pristine ZnO and (b) Group-IV doped ZnO with a 222 supercell. The red atom represents oxygen, gray atom represents zinc, and the yellow atom represents group-IVA dopant.

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Figure 2

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Figure 2. Calculated band gap of pristine ZnO using (a) GGA+U method and (b) traditional GGA method. F and Q denote the k points (0, 0.5, 0) and (0, 0.5, 0.5), respectively.

Figure 3. Comparison of partial DOS of (a) Zn-3d and (b) O-2p orbital using traditional GGA method (red dash line) and GGA+U method (black solid line). 17 Page 17 of 21

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Figure 4.

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Figure 4. Band structure of ZnO doped with (a) Si, (b) Ge, (c) Sn, and (d) Pb. The dash line represents the Fermi level. F and Q denote the k points (0, 0.5, 0) and (0, 0.5, 0.5), respectively.

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Figure 5.

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Figure 5. Total DOS and partial DOS of doped with (a) Si, (b) Ge, (c) Sn, and (d) Pb. The solid line represents the Fermi Level.

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Figure 6

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Figure 7

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Figure 6. Calculated dielectric functions of pristine ZnO and doped ZnO: (a) Real part 1() (b) Imaginary part 2().

Figure 7. Calculated optical constants of pristine ZnO and doped ZnO: (a) reflectivity and (b) absorption.

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Figure 8

Figure 8. Calculated transmittance of pristine ZnO and doped ZnO.

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