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Influence of Na and F doping on microstructures, optical and magnetic properties of ZnO films synthesized by sol-gel method
Author’s Accepted Manuscript Influence of Na & F doping on microstructures, optical and magnetic properties of ZnO films synthesized by sol-gel method Huan Yuan, Ming Xu www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(18)31345-2 https://doi.org/10.1016/j.ceramint.2018.05.214 CERI18389
To appear in: Ceramics International Received date: 1 May 2018 Revised date: 21 May 2018 Accepted date: 23 May 2018 Cite this article as: Huan Yuan and Ming Xu, Influence of Na & F doping on microstructures, optical and magnetic properties of ZnO films synthesized by solgel method, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.05.214 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 galley proof before it is published in its final citable 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.
Influence of Na & F doping on microstructures, optical and magnetic properties of ZnO films synthesized by sol-gel method Huan Yuan*, Ming Xu* Key Laboratory of Information Materials of Sichuan Province & School of Electrical and Information Engineering, Southwest Minzu University, Chengdu, China Electronic mail:[email protected], [email protected] *
Author to whom correspondence should be addressed.
Abstract Undoped, Na-doped, and Na-F codoped ZnO films were synthesized using sol-gel method. Na+ and F+ ions were used as two different dopants that yielded a synergistic doping effect. This effect was measurable using XRD, accompanied by a redshift in the optical bandgap from 3.284 to 3.261 eV in ZnO, ZnO-F, and ZnO-Na-F thin films, respectively. We then studied the resulting photoluminescent changes, which were attributed to O-related defects. Ferromagnetism measurements revealed that magnetic orderings decreased significantly with F doping. However, increased Na doping enhanced the oxygen-vacancy mediated ferromagnetic state.
Keywords: Defects; Sol-gel preparation; Thin films; Microstructure
1. Introduction Recently, researchers have focused on ZnO as a promising material with broad applications; this is primarily due to its wide room temperature bandgap (3.37 eV) and high electron binding energy (60 meV) [1]. However, accomplishing p-type doping remains a challenge due to the self-compensatory effect arising from native defects [2, 3]. Therefore, several research groups have tried to advance group-V and group-I doping to achieve p-type ZnO. For instance, Park and colleagues [4] have argued that it is theoretically possible for a group I element to substitute on Zn sites as a shallow
acceptor. This hypothesis has been proved by Fan [5]. Due in part to their small atomic radius, Group I elements tend to occupy interstitial sites. However, it is difficult to produce a similar interstitial donor impurity with Na doping due to its comparable ionic radius with Zn. Given this, research on Na-doped ZnO films requires more attention to better illustrate the possible behaviors of Na in ZnO [6]. F as an anionic dopant has a lower lattice distortion because fluorine’s ionic radius is similar to that of oxygen as compared with metal dopants [7]. Xu pointed out that F-doped ZnO film has both lower electrical resistivity and higher optical transmittance than pure ZnO film [8]. As such, it is unknown whether a combination of these two different dopants would yield a synergistic doping effect for pristine ZnO. To date, there have been many reports on F- or Na-doped ZnO. These reports have predominantly focused on the effects of doping on annealing temperature and growth orientation and on the electrical properties of doping using different concentrations of either F or Na. However, there has been little work regarding the room temperature optical properties and ferromagnetism of ZnO in cases of co-doping of the donor and acceptor. The results presented here sought to address this question, revealing some interesting results on room temperature ferromagnetism and optical properties. These results are also shown in combination with characterization studies using
X-ray
diffraction
(XRD),
scanning
electron
microscopy
(SEM),
photoluminescence spectroscopy (PL), UV–visible transmission spectrometer, and vibrating sample magnetometry (VSM).
2 Materials & Methods 2.1 Synthetic process The (F, Na)-codoped ZnO films were prepared using the sol-gel method as previously reported [9-11]. We chose zinc acetate dihydrate, ammonium fluoride, and sodium acetate trihydrate as sources for Zn, F, and Na, respectively. An appropriate amount of ethanol was added as solvent when C2H7NO was dissolved as a stabilizer. The solution was then stirred at 60C for 2 h to achieve stability and homogeneity.
The precursor solution was adjusted with glacial acetic acid and ammonia to give a final pH value of 8.5. The coating solution was then deposited using a spin coater (KW-4A). All films were grown on a quartz glass substrate at 500°C for 2.5 h in a muffle furnace. To obtain the desired film thickness, the spinning-preheating procedure was repeated 10 times to achieve an approximately 200-nm thick film. The three samples were dubbed ZnO, ZnO-F (Zn0.975F0.025O), and ZnO-F-Na (Zn0.975F0.0125Na0.0125O).
2.2 Characterization X-ray powder diffraction with Cu-Ka (k =1.5406 A) radiation (DX-2000 X-ray Automatic Diffractometer) was used to characterize film microstructure using a step size of 0.02. The surface morphologies of the films were examined using a field emission scanning electron microscope (FESEM, 6701F, JEOL, Japan) equipped with EDX. Film thickness was measured using an XP-Plus Stylus Profilometer and determined to be approximately 200 nm. Optical transmittance and UV absorption spectra
were
obtained
and
recorded
using
double
beam
UV–Vis–NIR
spectrophotometry on a spectrophotometer by Shimadzu. To determine the films’ magnetic properties, magnetic measurements were made using a vibrating sample magnetometer.
3 Results and Discussion 3.1 Structural studies Fig. 1 (a) shows the obtained XRD patterns of the sol-gel deposited on the three thin films on a quartz glass substrate. Fig. 2 (a) shows the XRD patterns of Na and F doping. Results indicated that all samples contained hexagonal wurtzite structures with no stray peaks corresponding to impurity phases (JPCDS 3614151). The XRD patterns also revealed that ZnO doped with F and Na peaked at (100), (002), and (101) orientation with preferential orientation along the c-axis. Moreover, the relative intensity of the (002) peak became more intense and sharp. The full width at half maximum (FWHM) of the XRD peaks and grain size were shown in Fig. 1 (b). It is
worth noting that Bu [12] argued FWHM is correlated with thin film quality. Moreover, it has been shown that crystallite size can be estimated using Scherrer’s equation. As shown in Fig. 1 (b), FWHM initially decreased with increasing Na and F codoping. This decrease was an indication of improved crystallinity. When compared with the surface morphology of the samples in Fig. 2, the surface of ZnO-F-Na is more compact and uniform. However, the grain size was not remarkably different within a range of approximately 43-48 nm (Fig. 2). Energy-dispersive X-ray spectroscopy (EDS) was also used to validate the presence of F and Na and to provide further characterization (Fig. 2 (d)).
3.2 Optical studies Variations in samples’ crystalline quality were also supported by the obtained PL spectra (Fig. 3). As shown in the inset of Fig. 3, the emission peak of the pure ZnO film was resolved into individual components. The UV band peak around 385 nm was attributed to the recombination of free excitons and is commonly known as near band edge (NBE) transition of ZnO [13]. Qin [14] proposed that the peak at around 420 nm arose from the transition of electrons from shallow donor levels (Zni) to the valence band. According to Fan’s report [15], the energy interval from the shallow acceptor (Oi) levels to the bottom of the conduction band was 2.94. This calculation was close to the peak seen near 420 nm. The band at around 440-460 nm corresponded to shallow-level trap emissions ascribed to zinc vacancies [16]. Finally, the peak at 486 nm in the ZnO material was attributed to oxygen-related defects, like VO or Oi. F and Na impurities introduced new energy levels into the bandgap of the ZnO matrix, and at around 510 nm a very weak green emission was observed. For the PL spectra, the emissions’ intensity decreased with F doping. This would decrease Zn-related defects and cause the crystalline quality of the pure ZnO film to aggrade. This is supported by our XRD results. However, the emissions related to O-related defects increased with Na doping. Concurrently, ZnO-F-Na emissions became strong. Taken together, we can conclude that moderate (F, Na)-codoping improved the crystal quality of ZnO films. To distinguish the relationship of the bandgap change with variation in F- or
Na-doping, the optical bandgap (Eg) of the films was obtained by plotting (ahv)2 versus hv using UV-Vis spectroscopy. Note that the optical bandgap values of 3.284, 3.276, and 3.261 eV were obtained for pure ZnO, ZnO-F, and ZnO-F-Na, respectively (Fig. 4). Deng[17] argued that given the extremely strong electronegativity of fluorine, it should theoretically weaken the interaction between Zn and O in the ZnO system. This weakened interaction should further reduce the optical bandgap. Also, the d-state electron localization of Zn increased with Na doping in ZnO, strengthening the overall electron movement of the s-state electrons of Na and the p-state electrons of O. Na-doping in ZnO:F thin films enhanced the hybrid coupling between p-state electrons in F and d-state electrons in Zn, thus improving acceptor doping concentration. It is evident that Na and F atoms substituted for Zn and O ions on neighboring positive ion sites. This is supported by both our XRD results and PL spectra. Collectively, these findings show that regulating F and Na codoping can effectively modulate the bandgap for zinc oxide-based semiconductor devices.
3.2 Ferromagnetic studies To investigate the effects of Na and F ionic treatment on ZnO thin films, magnetic measurements of the samples were obtained using a vibrating sample magnetometer. As shown in Fig. 5, the saturation magnetizations (Ms) of pure ZnO, ZnO-F, and ZnO-F-Na were 6.27×10-4, 8.29×10-5, and 3.02×10-4 electromagnetic units per gram (emu/g), respectively. Also, the coercive fields are 101, 335 and 130 Oe for pure ZnO, ZnO-F, and ZnO-F-Na, respectively. To explain the magnetic properties of these samples, one needs to know about the mechanisms based on carrier-mediated FM, the super exchange model and the percolation of bound magnetic polarons. To this end, Jayakumar and colleagues found that a free-carrier-mediated mechanism was responsible for ferromagnetism [18]. However, the electrical resistivity of our films was too large to measure. Instead, this led us to propose a supercoupling mechanism based on the bound magnetic polarons BMP model [19], where the electron occupies an extended orbital state that overlaps with the d shells of several nearby metal atoms. Based on this, the magnetism of our films can be understood as follows: The
samples’ room temperature ferromagnetism can be attributed to F and Na ions, rather than any precipitates such as clusters. This possibility has been ruled out by our XRD data. Our experimental results show that enhanced ferromagnetic couplings exist in the ZnO:F sample after Na co-doping, and there is less magnetic ordering in the ZnO:F sample than in pure ZnO. This ordering was significantly decreased with increasing F concentration. We found that the RT ferromagnetism observed in our thin films was partially related with defects like oxygen vacancies, as evidenced by the film PL data (Fig. 3). Emissions related to O-related defects (around 420-460 nm) become weaker in the ZnO sample after F doping. However, the ZnO-F-Na PL intensity was substantially stronger than that of the ZnO-F thin film. This is predominantly due to the increased VO concentration induced by Na-doping. As is well known, Na+ ionic radius is larger than that of Zn2+. Wu [20] argued that, “When Na dopant substitutes Zn sites, the lattice has a distortion, and the oxygen ion space shrinks.” As such, the formation energy of VO decreases, and the concentration of VO increases. These results are in good agreement with the findings of Gu et al. who showed that the introduction of Na+ effectively mediated the F-center (bound magnetic polaron) in a ferromagnetic exchange interaction [21]. When taken together with our results, the magnetism of (Na, F)-codoped ZnO films can be understood as follows: Ferromagnetism was strongly correlated with the increase of oxygen vacancies in ZnO. As such, it is likely that diluted magnetic semiconductor devices (DMSs) with enhanced magnetic properties can be fabricated by carefully controlling the Na doping in ZnFO.
4. Conclusion To summarize, room temperature optical properties and ferromagnetism were observed in ZnO films and tuned using either Na or F doping. Surface SEM photographs revealed that Na doping to F-doped ZnO achieved a uniform and compact ZnO film. Our XRD results showed that film crystallinity with wurtzite structures improved with F and Na codoping. The optical bandgap decreased from
3.284 to 3.261 eV in pure ZnO, ZnO-F, and ZnO-F-Na, respectively. Finally, microstructure properties and PL spectra revealed that this ferromagnetism originated from surface defects like the occupied oxygen vacancy.
Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities, Southwest Minzu University (Grant No. 2018NQN11). The authors also greatly acknowledge the financial support by Sichuan Province academic and technical leader training fund (Grant No.25727502), Foundation of Science and Technology Bureau of Sichuan Province (Grant No. 2017JY0349) and National Laboratory of Solid State Microstructures, Nanjing University (Grant No. M30016), China.
References [1] R. Vettumperumal, S. Kalyanaraman, R. Thangavel, Effect of Er concentration on surface and optical properties of K doped ZnO solegel thin films, Superlattices Microstruct. 83 (2015) 237-250. [2] H. Yuan, M. Xu, X. S. Du, Effects of Co doping on the structural and optical properties of ZnCuO thin films, Mater. Lett. 154 (2015) 94-97. [3] A. Chelouche, T. Touam, F. Boudjouan, D. Djouadi, R. Mahiou, A. Bouloufa, G. Chadeyron, Z. Hadjoub, Na doping effects on the structural, conduction type and optical properties of sol-gel ZnO thin films, J. Mater. Sci: Mater Electron. 28 (2017) 1546-1554. [4] C. H. Park, S. B. Zhang, S. H. Wei, Origin of p-type doping difficulty in ZnO:The impurity perspective, Phys. Rev. B 66 (2004) 073202. [5] J. C. Fan, K. M. Sreekanth, Z. Xie, S.L. Chang, K.V. Rao, p-Type ZnO materials: theory, growth, properties and devices, Prog. Mater. Sci. 58 (2013) 874-985. [6] M. A. Basyooni, M. Shaban, A. l. M. E. Sayed, Enhanced gas sensing properties of Spin-coated Na-doped ZnO nanostructured films, Sci. Rep. 7 (2017) 41716.
[7] L. Cao, L. Zhu, J. Jiang, R. Zhao, Z. Ye, B. Zhao, Highly transparent and conducting fluorine-doped ZnO thin films prepared by pulsed laser deposition, Sol. Energ. Mat. Sol. C. 95 (2011) 894-898. 8. H. Y. Xu, Y. C. Liu, R. Mu, C. L. Shao, Y. M. Lu, D .Z. Shen, X .W. Fan, F-doping effects on electrical and optical properties of ZnO nanocrystalline films, Appl. Phys. Lett. 86 (2005) 123107. [9] H. Yuan, L. Zhang, M. Xu, X. S. Du, Effect of sol pH on microstructures, optical and magnetic properties of (Co,Fe)-codoped ZnO films synthesized by solegel method, J. Alloy. Compd. 651 (2015) 571-577. [10] H. Yuan, M. Xu, Q. Z. Huang, Effects of pH of the precursor sol on structural and optical properties of Cu-doped ZnO thin films, J. Alloy. Compd. 616 (2014) 401-407. [11] H. Yuan, X. S. Du, M. Xu, Ferromagnetic mechanism of (Co, Cu)-codoped ZnO films with different Co concentrations investigated by X-ray photoelectron spectroscopy, Physica E 79 (2016)119-126. [12] I. Y. Y. Bu, Sol-gel production of p-type ZnO thin film by using sodium doping[J]. Superlattices Microstruct. 96 (2016) 59-66. [13] M. Ashokkumar, S. Muthukumaran, Effect of Ni doping on electrical, photoluminescence and magnetic behavior of Cu doped ZnO nanoparticles, J. Lumin.162 (2015) 97-103. [14] W. J. Qin, J. Sun, J. Yang, X. W. Du, Control of Cu-doping and optical properties of ZnO quantum dots by laser ablation of composite targets, Mater. Chem. Phys. 130 (2011) 425-430. [15] X. M. Fan, J. S. Lian, Z. X. Guo, H. J. Lu, Microstructure and photoluminescence properties of ZnO thin films grown by PLD on Si (111) substrates, Appl. Surf. Sci. 239 (2005) 176-181. [16] K. T. Roro, J. K. Dangbegnon, S. Sivaraya, A. W. R. Leitch, J. R. Botha, Influence of metal organic chemical vapor deposition growth parameters on the luminescent properties of ZnO thin films deposited on glass substrates, J. Appl. Phys. 103 (2008) 053516. [17] S. H. Deng and Z. L. Jiang, First-principles study on p-type ZnO codoped with F
and Na, Acta Phys. Sin. 63 (2014) 077101. [18] O. D. Jayakumar, I. K. Gopalakrishnan, S. K. Kulshreshtha, The structural and magnetization studies of Co-doped ZnO co-doped with Cu: Synthesized by co-precipitation method, J. Mater. Chem. 15 (2005) 3514-3518. [19] A. Kaminski, S. Das Sarma, Polaron percolation in diluted magnetic semiconductors, Phys. Rev. Lett. 88 (2002) 247202. [20] K. Wu, X. G. Xu, H. L. Yang, J. L. Zhang, J. Miao, Y. Jiang, Effect of annealing atmosphere on magnetic properties of pure ZnO and Na: ZnO films, Rare. Metals. 31 (2012) 27-30. [21] H. Gu, Y. Z. Jiang, M. Yan, Defect-induced room temperature ferromagnetism in Fe and Na co-doped ZnO nanoparticles, J. Alloy. Compd. 521 (2012) 90-94.
Figure captions Figure 1 (a) XRD spectra of the pure ZnO, ZnO-F and ZnO-F-Na. (b) the extracted FWHM and grain size. Figure 2 SEM images of (a) the pure ZnO, (b) ZnO-F and (c) ZnO-F-Na, (d) EDS spectra of ZnO-F-Na film. Figure 3 PL spectra of pure ZnO, ZnO-F, ZnO-F-Na films at room temperature and Gaussian decomposed PL spectra of pure ZnO film. Figure 4 Plots of (αhv)2 versus hv for the samples. Figure 5 Hysteresis curves measured at room temperature for the samples.
PII: DOI: Reference:
S0272-8842(18)31345-2 https://doi.org/10.1016/j.ceramint.2018.05.214 CERI18389
To appear in: Ceramics International Received date: 1 May 2018 Revised date: 21 May 2018 Accepted date: 23 May 2018 Cite this article as: Huan Yuan and Ming Xu, Influence of Na & F doping on microstructures, optical and magnetic properties of ZnO films synthesized by solgel method, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.05.214 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 galley proof before it is published in its final citable 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.
Influence of Na & F doping on microstructures, optical and magnetic properties of ZnO films synthesized by sol-gel method Huan Yuan*, Ming Xu* Key Laboratory of Information Materials of Sichuan Province & School of Electrical and Information Engineering, Southwest Minzu University, Chengdu, China Electronic mail:[email protected], [email protected] *
Author to whom correspondence should be addressed.
Abstract Undoped, Na-doped, and Na-F codoped ZnO films were synthesized using sol-gel method. Na+ and F+ ions were used as two different dopants that yielded a synergistic doping effect. This effect was measurable using XRD, accompanied by a redshift in the optical bandgap from 3.284 to 3.261 eV in ZnO, ZnO-F, and ZnO-Na-F thin films, respectively. We then studied the resulting photoluminescent changes, which were attributed to O-related defects. Ferromagnetism measurements revealed that magnetic orderings decreased significantly with F doping. However, increased Na doping enhanced the oxygen-vacancy mediated ferromagnetic state.
Keywords: Defects; Sol-gel preparation; Thin films; Microstructure
1. Introduction Recently, researchers have focused on ZnO as a promising material with broad applications; this is primarily due to its wide room temperature bandgap (3.37 eV) and high electron binding energy (60 meV) [1]. However, accomplishing p-type doping remains a challenge due to the self-compensatory effect arising from native defects [2, 3]. Therefore, several research groups have tried to advance group-V and group-I doping to achieve p-type ZnO. For instance, Park and colleagues [4] have argued that it is theoretically possible for a group I element to substitute on Zn sites as a shallow
acceptor. This hypothesis has been proved by Fan [5]. Due in part to their small atomic radius, Group I elements tend to occupy interstitial sites. However, it is difficult to produce a similar interstitial donor impurity with Na doping due to its comparable ionic radius with Zn. Given this, research on Na-doped ZnO films requires more attention to better illustrate the possible behaviors of Na in ZnO [6]. F as an anionic dopant has a lower lattice distortion because fluorine’s ionic radius is similar to that of oxygen as compared with metal dopants [7]. Xu pointed out that F-doped ZnO film has both lower electrical resistivity and higher optical transmittance than pure ZnO film [8]. As such, it is unknown whether a combination of these two different dopants would yield a synergistic doping effect for pristine ZnO. To date, there have been many reports on F- or Na-doped ZnO. These reports have predominantly focused on the effects of doping on annealing temperature and growth orientation and on the electrical properties of doping using different concentrations of either F or Na. However, there has been little work regarding the room temperature optical properties and ferromagnetism of ZnO in cases of co-doping of the donor and acceptor. The results presented here sought to address this question, revealing some interesting results on room temperature ferromagnetism and optical properties. These results are also shown in combination with characterization studies using
X-ray
diffraction
(XRD),
scanning
electron
microscopy
(SEM),
photoluminescence spectroscopy (PL), UV–visible transmission spectrometer, and vibrating sample magnetometry (VSM).
2 Materials & Methods 2.1 Synthetic process The (F, Na)-codoped ZnO films were prepared using the sol-gel method as previously reported [9-11]. We chose zinc acetate dihydrate, ammonium fluoride, and sodium acetate trihydrate as sources for Zn, F, and Na, respectively. An appropriate amount of ethanol was added as solvent when C2H7NO was dissolved as a stabilizer. The solution was then stirred at 60C for 2 h to achieve stability and homogeneity.
The precursor solution was adjusted with glacial acetic acid and ammonia to give a final pH value of 8.5. The coating solution was then deposited using a spin coater (KW-4A). All films were grown on a quartz glass substrate at 500°C for 2.5 h in a muffle furnace. To obtain the desired film thickness, the spinning-preheating procedure was repeated 10 times to achieve an approximately 200-nm thick film. The three samples were dubbed ZnO, ZnO-F (Zn0.975F0.025O), and ZnO-F-Na (Zn0.975F0.0125Na0.0125O).
2.2 Characterization X-ray powder diffraction with Cu-Ka (k =1.5406 A) radiation (DX-2000 X-ray Automatic Diffractometer) was used to characterize film microstructure using a step size of 0.02. The surface morphologies of the films were examined using a field emission scanning electron microscope (FESEM, 6701F, JEOL, Japan) equipped with EDX. Film thickness was measured using an XP-Plus Stylus Profilometer and determined to be approximately 200 nm. Optical transmittance and UV absorption spectra
were
obtained
and
recorded
using
double
beam
UV–Vis–NIR
spectrophotometry on a spectrophotometer by Shimadzu. To determine the films’ magnetic properties, magnetic measurements were made using a vibrating sample magnetometer.
3 Results and Discussion 3.1 Structural studies Fig. 1 (a) shows the obtained XRD patterns of the sol-gel deposited on the three thin films on a quartz glass substrate. Fig. 2 (a) shows the XRD patterns of Na and F doping. Results indicated that all samples contained hexagonal wurtzite structures with no stray peaks corresponding to impurity phases (JPCDS 3614151). The XRD patterns also revealed that ZnO doped with F and Na peaked at (100), (002), and (101) orientation with preferential orientation along the c-axis. Moreover, the relative intensity of the (002) peak became more intense and sharp. The full width at half maximum (FWHM) of the XRD peaks and grain size were shown in Fig. 1 (b). It is
worth noting that Bu [12] argued FWHM is correlated with thin film quality. Moreover, it has been shown that crystallite size can be estimated using Scherrer’s equation. As shown in Fig. 1 (b), FWHM initially decreased with increasing Na and F codoping. This decrease was an indication of improved crystallinity. When compared with the surface morphology of the samples in Fig. 2, the surface of ZnO-F-Na is more compact and uniform. However, the grain size was not remarkably different within a range of approximately 43-48 nm (Fig. 2). Energy-dispersive X-ray spectroscopy (EDS) was also used to validate the presence of F and Na and to provide further characterization (Fig. 2 (d)).
3.2 Optical studies Variations in samples’ crystalline quality were also supported by the obtained PL spectra (Fig. 3). As shown in the inset of Fig. 3, the emission peak of the pure ZnO film was resolved into individual components. The UV band peak around 385 nm was attributed to the recombination of free excitons and is commonly known as near band edge (NBE) transition of ZnO [13]. Qin [14] proposed that the peak at around 420 nm arose from the transition of electrons from shallow donor levels (Zni) to the valence band. According to Fan’s report [15], the energy interval from the shallow acceptor (Oi) levels to the bottom of the conduction band was 2.94. This calculation was close to the peak seen near 420 nm. The band at around 440-460 nm corresponded to shallow-level trap emissions ascribed to zinc vacancies [16]. Finally, the peak at 486 nm in the ZnO material was attributed to oxygen-related defects, like VO or Oi. F and Na impurities introduced new energy levels into the bandgap of the ZnO matrix, and at around 510 nm a very weak green emission was observed. For the PL spectra, the emissions’ intensity decreased with F doping. This would decrease Zn-related defects and cause the crystalline quality of the pure ZnO film to aggrade. This is supported by our XRD results. However, the emissions related to O-related defects increased with Na doping. Concurrently, ZnO-F-Na emissions became strong. Taken together, we can conclude that moderate (F, Na)-codoping improved the crystal quality of ZnO films. To distinguish the relationship of the bandgap change with variation in F- or
Na-doping, the optical bandgap (Eg) of the films was obtained by plotting (ahv)2 versus hv using UV-Vis spectroscopy. Note that the optical bandgap values of 3.284, 3.276, and 3.261 eV were obtained for pure ZnO, ZnO-F, and ZnO-F-Na, respectively (Fig. 4). Deng[17] argued that given the extremely strong electronegativity of fluorine, it should theoretically weaken the interaction between Zn and O in the ZnO system. This weakened interaction should further reduce the optical bandgap. Also, the d-state electron localization of Zn increased with Na doping in ZnO, strengthening the overall electron movement of the s-state electrons of Na and the p-state electrons of O. Na-doping in ZnO:F thin films enhanced the hybrid coupling between p-state electrons in F and d-state electrons in Zn, thus improving acceptor doping concentration. It is evident that Na and F atoms substituted for Zn and O ions on neighboring positive ion sites. This is supported by both our XRD results and PL spectra. Collectively, these findings show that regulating F and Na codoping can effectively modulate the bandgap for zinc oxide-based semiconductor devices.
3.2 Ferromagnetic studies To investigate the effects of Na and F ionic treatment on ZnO thin films, magnetic measurements of the samples were obtained using a vibrating sample magnetometer. As shown in Fig. 5, the saturation magnetizations (Ms) of pure ZnO, ZnO-F, and ZnO-F-Na were 6.27×10-4, 8.29×10-5, and 3.02×10-4 electromagnetic units per gram (emu/g), respectively. Also, the coercive fields are 101, 335 and 130 Oe for pure ZnO, ZnO-F, and ZnO-F-Na, respectively. To explain the magnetic properties of these samples, one needs to know about the mechanisms based on carrier-mediated FM, the super exchange model and the percolation of bound magnetic polarons. To this end, Jayakumar and colleagues found that a free-carrier-mediated mechanism was responsible for ferromagnetism [18]. However, the electrical resistivity of our films was too large to measure. Instead, this led us to propose a supercoupling mechanism based on the bound magnetic polarons BMP model [19], where the electron occupies an extended orbital state that overlaps with the d shells of several nearby metal atoms. Based on this, the magnetism of our films can be understood as follows: The
samples’ room temperature ferromagnetism can be attributed to F and Na ions, rather than any precipitates such as clusters. This possibility has been ruled out by our XRD data. Our experimental results show that enhanced ferromagnetic couplings exist in the ZnO:F sample after Na co-doping, and there is less magnetic ordering in the ZnO:F sample than in pure ZnO. This ordering was significantly decreased with increasing F concentration. We found that the RT ferromagnetism observed in our thin films was partially related with defects like oxygen vacancies, as evidenced by the film PL data (Fig. 3). Emissions related to O-related defects (around 420-460 nm) become weaker in the ZnO sample after F doping. However, the ZnO-F-Na PL intensity was substantially stronger than that of the ZnO-F thin film. This is predominantly due to the increased VO concentration induced by Na-doping. As is well known, Na+ ionic radius is larger than that of Zn2+. Wu [20] argued that, “When Na dopant substitutes Zn sites, the lattice has a distortion, and the oxygen ion space shrinks.” As such, the formation energy of VO decreases, and the concentration of VO increases. These results are in good agreement with the findings of Gu et al. who showed that the introduction of Na+ effectively mediated the F-center (bound magnetic polaron) in a ferromagnetic exchange interaction [21]. When taken together with our results, the magnetism of (Na, F)-codoped ZnO films can be understood as follows: Ferromagnetism was strongly correlated with the increase of oxygen vacancies in ZnO. As such, it is likely that diluted magnetic semiconductor devices (DMSs) with enhanced magnetic properties can be fabricated by carefully controlling the Na doping in ZnFO.
4. Conclusion To summarize, room temperature optical properties and ferromagnetism were observed in ZnO films and tuned using either Na or F doping. Surface SEM photographs revealed that Na doping to F-doped ZnO achieved a uniform and compact ZnO film. Our XRD results showed that film crystallinity with wurtzite structures improved with F and Na codoping. The optical bandgap decreased from
3.284 to 3.261 eV in pure ZnO, ZnO-F, and ZnO-F-Na, respectively. Finally, microstructure properties and PL spectra revealed that this ferromagnetism originated from surface defects like the occupied oxygen vacancy.
Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities, Southwest Minzu University (Grant No. 2018NQN11). The authors also greatly acknowledge the financial support by Sichuan Province academic and technical leader training fund (Grant No.25727502), Foundation of Science and Technology Bureau of Sichuan Province (Grant No. 2017JY0349) and National Laboratory of Solid State Microstructures, Nanjing University (Grant No. M30016), China.
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Figure captions Figure 1 (a) XRD spectra of the pure ZnO, ZnO-F and ZnO-F-Na. (b) the extracted FWHM and grain size. Figure 2 SEM images of (a) the pure ZnO, (b) ZnO-F and (c) ZnO-F-Na, (d) EDS spectra of ZnO-F-Na film. Figure 3 PL spectra of pure ZnO, ZnO-F, ZnO-F-Na films at room temperature and Gaussian decomposed PL spectra of pure ZnO film. Figure 4 Plots of (αhv)2 versus hv for the samples. Figure 5 Hysteresis curves measured at room temperature for the samples.