- Email: [email protected]
Effect of oxygen partial pressure during pulsed laser deposition on the emission of Eu doped ZnO thin films
Journal Pre-proof Effect of oxygen partial pressure during pulsed laser deposition on the emission of Eu doped ZnO thin films
Vinod Kumar, O.M. Ntwaeaborwa, H.C. Swart PII:
S0921-4526(19)30619-2
DOI:
https://doi.org/10.1016/j.physb.2019.411713
Reference:
PHYSB 411713
To appear in:
Physica B: Physics of Condensed Matter
Received Date:
24 May 2019
Accepted Date:
19 September 2019
Please cite this article as: Vinod Kumar, O.M. Ntwaeaborwa, H.C. Swart, Effect of oxygen partial pressure during pulsed laser deposition on the emission of Eu doped ZnO thin films, Physica B: Physics of Condensed Matter (2019), https://doi.org/10.1016/j.physb.2019.411713
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier.
Journal Pre-proof Effect of oxygen partial pressure during pulsed laser deposition on the emission of Eu doped ZnO thin films Vinod Kumar1,2*, O.M. Ntwaeaborwa3 and H.C. Swart2 1 Centre
for Energy Studies, Indian Institute of Technology Delhi, New Delhi 110016, India
2 Department
of Physics, University of the Free State, P.O. Box 339, Bloemfontein, ZA 9300, South Africa
3 School
of Physics, University of the Witwatersrand, Private Bag X3, Wits, 2050, South Africa
Abstract Eu3+ doped ZnO (ZnO:Eu3+) thin films were prepared by pulsed laser deposition at different oxygen partial pressures. The ZnO:Eu3+ thin films all have a hexagonal structure. The morphology and roughness of the ZnO:Eu3+ thin film were also dependent on the partial pressure of the oxygen. The strong band to band emission was observed with the weak emission of both Eu3+ and defects. The intensity of the band emission peaks has increased with an increase in the oxygen partial pressure. Keywords:- Oxygen partial pressure, ZnO:Eu3+, Pulsed laser deposition *Corresponding
Author Email:- [email protected]
Journal Pre-proof 1. Introduction Wide band gap semiconductors, such as Zinc oxide (ZnO), doped with rare earth trivalent (RE3+) ions, such as Eu3+ ions, have gained considerable attention in the field of optoelectronic devices because of their potential use in different applications including light emitting diodes (LEDs), flat panel displays (FPDs), plasma display panels (PDPs), fluorescent lamps, etc [1-3]. ZnO is a wide band gap material with advantages such as large exciton binding energy, high conductivity, high thermal stability in hydrogen plasma atmosphere and high chemical/physical stability [3-5]. Different methods have been used to synthesize RE doped ZnO nanophosphors and the emitted light can be tuned throughout the visible range [3]. Eu3+ doped ZnO presents a great opportunity to achieve intense red emission, owing to the advantage of the 4f-4f transition of Eu3+ ions and the excellent properties of ZnO as a host. Thin films create an opportunity of good adhesion between the substrate and the film material and therefore provides better luminescence properties, thermal stability and small and uniform particles, which give high image resolution [6], in some cases the emission intensity is still low and more research needed to improve the efficiency of the light output. ZnO thin films have been deposited with a number of different techniques such as pulsed laser deposition (PLD), radio frequency magnetron sputtering, sol-gel spin coating and chemical vapor deposition [7-12]. Among these techniques, the PLD is preferable and provides unique growth of oxide material due to the oxygen plasma created by the pulsed laser, is very energetic and its density is easily controllable by the oxygen pressure [13]. Doping of ZnO with selective elements has become an important route for enhancing and tuning optical, electrical, and magnetic properties, which are usually crucial for their practical applications. It is well known that rare earth (RE) ions are better candidates as luminescent centres because their special 4f intra-shell transitions usually have rich spectral lines [14]. The density of the oxygen vacancy in ZnO was easily controlled by varying the oxygen partial pressure during film deposition [15].
Journal Pre-proof In this work, the effect of oxygen partial pressure during PLD on the structural, morphological and optical properties of ZnO:Eu3+ thin films was investigated. A schematic band diagram is provided to understand the emission mechanism from ZnO:Eu3+ thin films
2. Experimental details ZnO:Eu3+ thin films were deposited on Si (100) wafers using a target in a reactive PLD process. ZnO:Eu3+ (4 mol % Eu) powder synthesized by the solution-combustion method was used for the preparation of the target. The optimization (4 mol% Eu) and characterization of ZnO powder have already been reported elsewhere [16]. A 266 nm Nd:YAG laser was used for the ablation. (It must be pointed out that the Eu concentration in the thin films was not determined again after ablation.) The silicon substrates were cleaned ultrasonically using acetone, ethanol and deionized water; after that it was dried by N2 gas. The other parameter for the thin film fabrication by PLD were already reported in our previous work [3, 17, 18]. The substrate temperature was kept at 300°C during deposition of the thin films. The laser energy and deposition time were fixed at 40 mJ and 45 min, respectively. The chamber was pumped down to a background pressure of 5×10-5 mbar. It was backfilled with O2 gas during deposition for different partial pressures, from 50 m Torr to 250 m Torr. The structural properties were analysed by using an X-ray diffractometer (XRD) PAN analytical X’pert PRO. Photoluminescence (PL) data were recorded using a He-Cd laser with a 325 nm excitation wavelength. The surface morphology of the ZnO:Eu3+ film was examined by using Shimadzu SPM-9600 atomic force microscope (AFM) in contact mode. All the characterizations were done at room temperature.
Journal Pre-proof 3. Results and discussion The XRD patterns of the ZnO:Eu3+ phosphor with different concentrations of Eu3+ are shown in Fig. 1(a). The ZnO:Eu3+ nanoparticles were highly crystalline in nature. The strong diffraction peaks at 31.7°, 34.3° and 36.2° correspond to the (100), (002) and (101) planes of the hexagonal wurtzite structure of ZnO (ICSD card no. 29272). The lattice parameter of ZnO, as calculated in a previous report, has increased with an increase in the doping concentrating due to the large ionic radius of the Eu3+ (0.95 Å) as compared with the Zn2+ (0.74 Å), indicating that the incorporated Eu3+ ions occupied the lattice sites of the Zn2+ and lead to the increase in the inter-atomic distance and the expansion in lattice constants [16]. It indicated that Eu3+ ions were properly incorporated into the lattice. A small peak observed at 29.4° (marked with an asterisk) is assigned to the cubic structure of Eu2O3 (JCPDS Database no. 43-1009) formed at higher Eu3+ concentrations. The PL spectra of ZnO:Eu3+ phosphors with different Eu3+ concentration are shown in Fig. 1(b). The broad emission was due to defects related emission of ZnO, while the sharp emission indicated the emission due to the Eu3+ ions. The defect related and band emissions in ZnO are dependent on the synthesis method, annealing temperature, and several other parameters as summarized in ref [3]. The optimization of the doping concentration of the Eu3+ in ZnO:Eu3+ powder was reported in our previous work [16]. The optimized 4 mol % Eu3+ doping concentration of Eu3+ was used for making the powder and this optimized powder was used for making the target for PLD.
Journal Pre-proof (b)
6 mol%
*
5 mol% 4 mol% 3 mol% 2 mol% 1 mol%
Intensity (arb. units)
Intensity (arb.(a.u.) units) Intensity
(a)
0 mol%
30
40 50 60 2 Theta (deg.) 2 (deg.)
70
Wavelength (nm)
Figure 1: Effect of doping concentration on (a) the XRD pattern and (b) the PL emission of ZnO:Eu3+ phosphors powder [Reproduced from ref 15 with permission].
The XRD patterns of the ZnO:Eu3+ thin films prepared by the PLD technique at different oxygen partial pressures are shown in Fig.2. The XRD reflections of the ZnO:Eu3+ films prepared by the PLD technique at different oxygen partial pressures were found to be oriented along the (002) plane. This is in line with the characteristics of the hexagonal ZnO wurtzite structure, where the c-axis is perpendicular to the substrate plane [17]. The intensity of the XRD peaks has increased (from 2000 to 18 000 counts) with an increase in the oxygen partial pressure. ZnO films usually deviated by the stoichiometry and incorporated of intrinsic defects, such as zinc interstitials (Zni) and oxygen vacancies (VO), especially when they are grown in Zn-rich or O-deficient atmospheres. The number of intrinsic defects can be reduced with an increase in the oxygen partial pressure and the film crystallinity may therefore be improved [15,19].
Intensity (arb. units)
Journal Pre-proof
20
250 mTorr
150 mTorr (002)
50 mTorr
25
30
35
40
2 Theta (deg.)
45
50
Figure 2: XRD patterns of the ZnO:Eu3+ thin films prepared at different oxygen partial pressures.
The effect of partial pressure on the microstructure and surface morphology of the ZnO:Eu3+ films was studied by AFM. The three dimension (3D) AFM images of the ZnO:Eu3+ films deposited on the Si substrates deposited at the different oxygen partial pressure are shown in Fig.3. The root mean square (RMS) roughness of the thin film has increased from 25 to 36 nm with an increase in the partial pressure of oxygen during synthesis of the film. The surface roughness of the ZnO:Eu3+ thin film changed severely with a change in the partial pressure during the deposition of the films by PLD [20]. The roughness were higher than for example thin films obtained with spin coating, where the roughness changed from 2.3 to 10.6 nm depending on the experimental conditions [21]. A ZnO channel layer with a RMS roughness of 0.7 nm was obtained by using radio-frequency (RF) magnetron sputtering at 350°C [20]. Their XRD pattern revealed a dense columnar structure of closely packed ZnO nano grains along the c-axis [22]. At higher oxygen pressure, the ablated species from the target could undergo much more collisions with the oxygen molecules than those under lower oxygen
Journal Pre-proof pressure. These collisions decreased the kinetic energy of these species. Consequently, the deposited ad-atoms do not have the required mobility in order to diffuse uniformly on the substrate and thus they form clusters leading to an increase in the roughness of the film [23]. (a)
(b)
(c)
Figure 3: The effect of oxygen partial pressure on the morphology of ZnO:Eu3+ thin films prepared by PLD at oxygen partial pressures of (a) 50 m Torr (b) 150 mTorr and (c) 250 mTorr.
The PL spectra of ZnO:Eu3+ thin films at different oxygen partial pressure are shown in Fig.4(a). Generally, ZnO have exhibited two types of emission: UV and visible emission due to band to band emission (NBE) and deep level defects (DLE), respectively [18, 24]. Normally ZnO has six kinds of defects, namely zinc vacancies (Vzn), oxygen vacancies (Vo), zinc interstitials (Zni), oxygen interstitials (Oi), zinc antisites (OZn) and oxygen antisites (ZnO), The BBE is very strong in all samples with a small amount of defects. The intensity of the BBE has increased with an increase in the partial pressure of oxygen. The oxygen partial pressure
Journal Pre-proof influenced the structural and optical properties of ZnO due to the oxygen defects in the ZnO thin films and is directly correlated to the deposition partial pressure [22]. There is always a competition between the NBE and the defect emission recombination mechanisms [25]. Normally the UV emission intensities decreased when the deep level emission intensities increased [26]. The increase of the ambient oxygen gas pressure from 50 mTorr to 250 mTorr increased the number of reactive oxygen ions generated by the photon and electron impact dissociation during the deposition process. Subsequently, the increase of the reactive oxygen ions and the interaction of the Zn ion with the reactive oxygen ion during the transportation to the substrate lead to the reduction of oxygen vacancies. The increase in the UV emission was therefore due to the decrease of oxygen vacancies in the ZnO thin films, which have further decreased with an increase in oxygen pressure. The crystallinity was improved with deposition at higher oxygen partial pressures with less oxygen vacancies. In the powder, the concentration of defects was high, so it has shown very small NBE, while during the synthesis of the thin films by PLD, the ablated material interact with the oxygen in the system when transferred from the target to substrate reducing the oxygen vacancies in the process. The Eu3+ f-f transitions emission were also observed at 617 and 590. The peaks were observed due to the Eu3+ f-f transitions at 590 and 617 nm attributed to the 5D0→7F2 and 5D0→7F1 radiative transitions, respectively [27]. In the PL curve of the ZnO:Eu3+ phosphor [Fig.1(b)] all transitions of Eu3+ ions were observed although the 5D0→7F0 ,5D0→7F3 and 5D0→7F4 were less intense compared to the two 5D0→7F1 and 5D0→7F2 transitions, but in the PLD films only a low intensity of the two 5D0→7F1 and 5D0→7F2 transitions were observed. Bär et al. [28] monitored the effect of an amorphous interface layer on the luminescence of Eu3+-doped GdVO4 PLD films, where a broadening of the Eu3+ luminescence peaks was observed. They found additionally, at the position of the 5D0 →7F1 transition two emission peaks at 593 nm and 595 nm. These could be explained by the two Stark levels of the 7F1 level, which were separated by 39 cm−1. Some of
Journal Pre-proof the PLD films did not show these sharp emission lines due to inhomogeneous broadening. In the corresponding excitation spectrum the peaks were strongly broadened and the 7F0 → 5D3 and 7F0 → 5D1 transitions barely exceed the noise level. The schematic band diagram of the emissions in the ZnO:Eu3+ was constructed from the data in Fig.4. The gap between valance and conduction band is ~ 3.28 eV, constructed from Pl data. The emission peak of Eu3+ is constructed from PL emission curve. The position of the theoretical value of the Zni level is at 0.22 eV below the conduction band [29,30]. The band transitions from Zni to Oi, Zni to Vo and the conduction band to Vo are at ~ 2.06, 1.63, and 1.85 eV, respectively. A non-radiative decay is possible from the conduction band of ZnO to the higher state of the Eu3+ and then recombined and provided different transition emissions. The emission intensity of the Eu3+ ions related peak was less in the thin films with respect to the phosphor powder.
Figure 4: (a) Effect of oxygen partial pressures on the Photoluminescence properties of ZnO:Eu3+ thin films prepared by PLD and (b) the energy band diagram of ZnO:Eu3+ thin films.
4. Conclusion Eu3+ doped ZnO thin films at different partial pressures of oxygen were successfully prepared by pulsed laser deposition. The XRD results confirmed the (002) preferential growth hexagonal wurtzite structure of ZnO. Band to band and Eu3+ related emission were observed in the PL
Journal Pre-proof results. Both band to band and Eu3+ emission increased with an increase in oxygen partial pressure, together with an increase in surface roughness. A big differences between the PL and XRD of the powder and thin films were observed. The concentration of defects was high in the case of the powder, so it has shown very small NBE with higher defect (DLE) and Eu3+ emission. While during the synthesis of the thin films by PLD, the ablated material interact with the oxygen in the system when transferred from the target to substrate reducing the oxygen vacancies in the process. With the consequent increase in NBE and less DLE emission. The Eu3+ emission was also severely quenched for the thin films. The reasons for the quenching of the Eu3+ emission still need more attention. The question may arise if the energy transfer to Eu3+ is either more effective from the defect levels or from the conduction band. It surely appeared that with less defects, less energy transfer has occurred. Many more experimental investigations to optimise the Eu3+ emission with different Eu3+ concentration are needed and the study on the effect of the preferential orientated ZnO also needs to be expanded.
Acknowledgments One of the authors (V.K.) is thankful to DST, New Delhi, India, for support through DSTInspire faculty award [DST/INSPIRE/04/2015/001497]. This work is based on the research supported by the National Research Foundation of South Africa (Grant Numbers 115126). This work is also based on the research supported by the South African Research Chairs Initiative of Department of Science and Technology and National Research Foundation of South Africa (84415).
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Journal Pre-proof [2].Y. Liu, W. Luo, R. Li, G. Liu, M. R. Antonio, X. Chen, J. Phys. Chem. C, 112 (2008) 686-694. [3].V. Kumar, O.M. Ntwaeaborwa, T. Soga, V. Dutta, H.C. Swart, ACS Photonics, 4 (2017) 2613. [4].J. Wang, R. Chen, L. Xiang, S. Komarneni, Ceramics International, 44 (7) (2018) 73577377. [5].Anderson Janotti and Chris G. Van de Walle, Appl. Phys. Lett., 87 (2005) 122102. [6].J.S. Baea, K.S. Shim, S.B. Kim, J.H. Jeong, S.S. Yi and J.C. Park, J. Cryst. Growth, 264 (2004) 290, 2004. [7].Vinod Kumar, N. Singh, A. Kapoor, O.M. Ntwaeaborwa, H.C. Swart; Materials Research Bulletin, 48 (2013) 4596. [8].Vinod Kumar, O.M. Ntwaeaborwa, E. Coetsee, H.C. Swart, J. of Coll. and Interf. Sci., 474 (2016) 129-136. [9].M.R. Alfaro Cruz, O. Ceballos-Sanchez, E. Luevano-Hipolito, L.M. Torres-Martınez, International Journal of Hydrogen Energy, 43(22) (2018) 10301-10310. [10]. Vinod Kumar, N. Singh, R.M. Mehra, A. Kapoor, L.P. Purohit, H.C. Swart, Thin Solid Films, 539 (2013) 161. [11]. J. Hu and R.G. Gordon, Textured aluminum-doped zinc oxide thin films from atmospheric pressure chemical-vapor deposition, J. Appl. Phys., 71 (1992) 880-890. [12]. H. Rotella, Y. Mazel, S. Brochen, A. Valla, A. Pautrat, C. Licitra, N. Rochat, C. Sabbione, G. Rodriguez and E. Nolo, J. Phys. D: Appl. Phys., 50 (2017) 485106 (7pp). [13]. Y.J. Shin, L. Wang, Y. Kim, H.H. Nahm, D. Lee, J. Ra. Kim, Sang, M. Yang, J.G. Yoon, J. S. Chung, M. Kim, S. H. Chang and T. Won Noh, ACS Appl. Mater. Interf., 9 (2017) 27305. [14]. K. Suzuki, K. Murayam and N. Tanak, Appl. Phys. Lett., 107 (3025) 031902.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
NO
Vinod Kumar, O.M. Ntwaeaborwa, H.C. Swart PII:
S0921-4526(19)30619-2
DOI:
https://doi.org/10.1016/j.physb.2019.411713
Reference:
PHYSB 411713
To appear in:
Physica B: Physics of Condensed Matter
Received Date:
24 May 2019
Accepted Date:
19 September 2019
Please cite this article as: Vinod Kumar, O.M. Ntwaeaborwa, H.C. Swart, Effect of oxygen partial pressure during pulsed laser deposition on the emission of Eu doped ZnO thin films, Physica B: Physics of Condensed Matter (2019), https://doi.org/10.1016/j.physb.2019.411713
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier.
Journal Pre-proof Effect of oxygen partial pressure during pulsed laser deposition on the emission of Eu doped ZnO thin films Vinod Kumar1,2*, O.M. Ntwaeaborwa3 and H.C. Swart2 1 Centre
for Energy Studies, Indian Institute of Technology Delhi, New Delhi 110016, India
2 Department
of Physics, University of the Free State, P.O. Box 339, Bloemfontein, ZA 9300, South Africa
3 School
of Physics, University of the Witwatersrand, Private Bag X3, Wits, 2050, South Africa
Abstract Eu3+ doped ZnO (ZnO:Eu3+) thin films were prepared by pulsed laser deposition at different oxygen partial pressures. The ZnO:Eu3+ thin films all have a hexagonal structure. The morphology and roughness of the ZnO:Eu3+ thin film were also dependent on the partial pressure of the oxygen. The strong band to band emission was observed with the weak emission of both Eu3+ and defects. The intensity of the band emission peaks has increased with an increase in the oxygen partial pressure. Keywords:- Oxygen partial pressure, ZnO:Eu3+, Pulsed laser deposition *Corresponding
Author Email:- [email protected]
Journal Pre-proof 1. Introduction Wide band gap semiconductors, such as Zinc oxide (ZnO), doped with rare earth trivalent (RE3+) ions, such as Eu3+ ions, have gained considerable attention in the field of optoelectronic devices because of their potential use in different applications including light emitting diodes (LEDs), flat panel displays (FPDs), plasma display panels (PDPs), fluorescent lamps, etc [1-3]. ZnO is a wide band gap material with advantages such as large exciton binding energy, high conductivity, high thermal stability in hydrogen plasma atmosphere and high chemical/physical stability [3-5]. Different methods have been used to synthesize RE doped ZnO nanophosphors and the emitted light can be tuned throughout the visible range [3]. Eu3+ doped ZnO presents a great opportunity to achieve intense red emission, owing to the advantage of the 4f-4f transition of Eu3+ ions and the excellent properties of ZnO as a host. Thin films create an opportunity of good adhesion between the substrate and the film material and therefore provides better luminescence properties, thermal stability and small and uniform particles, which give high image resolution [6], in some cases the emission intensity is still low and more research needed to improve the efficiency of the light output. ZnO thin films have been deposited with a number of different techniques such as pulsed laser deposition (PLD), radio frequency magnetron sputtering, sol-gel spin coating and chemical vapor deposition [7-12]. Among these techniques, the PLD is preferable and provides unique growth of oxide material due to the oxygen plasma created by the pulsed laser, is very energetic and its density is easily controllable by the oxygen pressure [13]. Doping of ZnO with selective elements has become an important route for enhancing and tuning optical, electrical, and magnetic properties, which are usually crucial for their practical applications. It is well known that rare earth (RE) ions are better candidates as luminescent centres because their special 4f intra-shell transitions usually have rich spectral lines [14]. The density of the oxygen vacancy in ZnO was easily controlled by varying the oxygen partial pressure during film deposition [15].
Journal Pre-proof In this work, the effect of oxygen partial pressure during PLD on the structural, morphological and optical properties of ZnO:Eu3+ thin films was investigated. A schematic band diagram is provided to understand the emission mechanism from ZnO:Eu3+ thin films
2. Experimental details ZnO:Eu3+ thin films were deposited on Si (100) wafers using a target in a reactive PLD process. ZnO:Eu3+ (4 mol % Eu) powder synthesized by the solution-combustion method was used for the preparation of the target. The optimization (4 mol% Eu) and characterization of ZnO powder have already been reported elsewhere [16]. A 266 nm Nd:YAG laser was used for the ablation. (It must be pointed out that the Eu concentration in the thin films was not determined again after ablation.) The silicon substrates were cleaned ultrasonically using acetone, ethanol and deionized water; after that it was dried by N2 gas. The other parameter for the thin film fabrication by PLD were already reported in our previous work [3, 17, 18]. The substrate temperature was kept at 300°C during deposition of the thin films. The laser energy and deposition time were fixed at 40 mJ and 45 min, respectively. The chamber was pumped down to a background pressure of 5×10-5 mbar. It was backfilled with O2 gas during deposition for different partial pressures, from 50 m Torr to 250 m Torr. The structural properties were analysed by using an X-ray diffractometer (XRD) PAN analytical X’pert PRO. Photoluminescence (PL) data were recorded using a He-Cd laser with a 325 nm excitation wavelength. The surface morphology of the ZnO:Eu3+ film was examined by using Shimadzu SPM-9600 atomic force microscope (AFM) in contact mode. All the characterizations were done at room temperature.
Journal Pre-proof 3. Results and discussion The XRD patterns of the ZnO:Eu3+ phosphor with different concentrations of Eu3+ are shown in Fig. 1(a). The ZnO:Eu3+ nanoparticles were highly crystalline in nature. The strong diffraction peaks at 31.7°, 34.3° and 36.2° correspond to the (100), (002) and (101) planes of the hexagonal wurtzite structure of ZnO (ICSD card no. 29272). The lattice parameter of ZnO, as calculated in a previous report, has increased with an increase in the doping concentrating due to the large ionic radius of the Eu3+ (0.95 Å) as compared with the Zn2+ (0.74 Å), indicating that the incorporated Eu3+ ions occupied the lattice sites of the Zn2+ and lead to the increase in the inter-atomic distance and the expansion in lattice constants [16]. It indicated that Eu3+ ions were properly incorporated into the lattice. A small peak observed at 29.4° (marked with an asterisk) is assigned to the cubic structure of Eu2O3 (JCPDS Database no. 43-1009) formed at higher Eu3+ concentrations. The PL spectra of ZnO:Eu3+ phosphors with different Eu3+ concentration are shown in Fig. 1(b). The broad emission was due to defects related emission of ZnO, while the sharp emission indicated the emission due to the Eu3+ ions. The defect related and band emissions in ZnO are dependent on the synthesis method, annealing temperature, and several other parameters as summarized in ref [3]. The optimization of the doping concentration of the Eu3+ in ZnO:Eu3+ powder was reported in our previous work [16]. The optimized 4 mol % Eu3+ doping concentration of Eu3+ was used for making the powder and this optimized powder was used for making the target for PLD.
Journal Pre-proof (b)
6 mol%
*
5 mol% 4 mol% 3 mol% 2 mol% 1 mol%
Intensity (arb. units)
Intensity (arb.(a.u.) units) Intensity
(a)
0 mol%
30
40 50 60 2 Theta (deg.) 2 (deg.)
70
Wavelength (nm)
Figure 1: Effect of doping concentration on (a) the XRD pattern and (b) the PL emission of ZnO:Eu3+ phosphors powder [Reproduced from ref 15 with permission].
The XRD patterns of the ZnO:Eu3+ thin films prepared by the PLD technique at different oxygen partial pressures are shown in Fig.2. The XRD reflections of the ZnO:Eu3+ films prepared by the PLD technique at different oxygen partial pressures were found to be oriented along the (002) plane. This is in line with the characteristics of the hexagonal ZnO wurtzite structure, where the c-axis is perpendicular to the substrate plane [17]. The intensity of the XRD peaks has increased (from 2000 to 18 000 counts) with an increase in the oxygen partial pressure. ZnO films usually deviated by the stoichiometry and incorporated of intrinsic defects, such as zinc interstitials (Zni) and oxygen vacancies (VO), especially when they are grown in Zn-rich or O-deficient atmospheres. The number of intrinsic defects can be reduced with an increase in the oxygen partial pressure and the film crystallinity may therefore be improved [15,19].
Intensity (arb. units)
Journal Pre-proof
20
250 mTorr
150 mTorr (002)
50 mTorr
25
30
35
40
2 Theta (deg.)
45
50
Figure 2: XRD patterns of the ZnO:Eu3+ thin films prepared at different oxygen partial pressures.
The effect of partial pressure on the microstructure and surface morphology of the ZnO:Eu3+ films was studied by AFM. The three dimension (3D) AFM images of the ZnO:Eu3+ films deposited on the Si substrates deposited at the different oxygen partial pressure are shown in Fig.3. The root mean square (RMS) roughness of the thin film has increased from 25 to 36 nm with an increase in the partial pressure of oxygen during synthesis of the film. The surface roughness of the ZnO:Eu3+ thin film changed severely with a change in the partial pressure during the deposition of the films by PLD [20]. The roughness were higher than for example thin films obtained with spin coating, where the roughness changed from 2.3 to 10.6 nm depending on the experimental conditions [21]. A ZnO channel layer with a RMS roughness of 0.7 nm was obtained by using radio-frequency (RF) magnetron sputtering at 350°C [20]. Their XRD pattern revealed a dense columnar structure of closely packed ZnO nano grains along the c-axis [22]. At higher oxygen pressure, the ablated species from the target could undergo much more collisions with the oxygen molecules than those under lower oxygen
Journal Pre-proof pressure. These collisions decreased the kinetic energy of these species. Consequently, the deposited ad-atoms do not have the required mobility in order to diffuse uniformly on the substrate and thus they form clusters leading to an increase in the roughness of the film [23]. (a)
(b)
(c)
Figure 3: The effect of oxygen partial pressure on the morphology of ZnO:Eu3+ thin films prepared by PLD at oxygen partial pressures of (a) 50 m Torr (b) 150 mTorr and (c) 250 mTorr.
The PL spectra of ZnO:Eu3+ thin films at different oxygen partial pressure are shown in Fig.4(a). Generally, ZnO have exhibited two types of emission: UV and visible emission due to band to band emission (NBE) and deep level defects (DLE), respectively [18, 24]. Normally ZnO has six kinds of defects, namely zinc vacancies (Vzn), oxygen vacancies (Vo), zinc interstitials (Zni), oxygen interstitials (Oi), zinc antisites (OZn) and oxygen antisites (ZnO), The BBE is very strong in all samples with a small amount of defects. The intensity of the BBE has increased with an increase in the partial pressure of oxygen. The oxygen partial pressure
Journal Pre-proof influenced the structural and optical properties of ZnO due to the oxygen defects in the ZnO thin films and is directly correlated to the deposition partial pressure [22]. There is always a competition between the NBE and the defect emission recombination mechanisms [25]. Normally the UV emission intensities decreased when the deep level emission intensities increased [26]. The increase of the ambient oxygen gas pressure from 50 mTorr to 250 mTorr increased the number of reactive oxygen ions generated by the photon and electron impact dissociation during the deposition process. Subsequently, the increase of the reactive oxygen ions and the interaction of the Zn ion with the reactive oxygen ion during the transportation to the substrate lead to the reduction of oxygen vacancies. The increase in the UV emission was therefore due to the decrease of oxygen vacancies in the ZnO thin films, which have further decreased with an increase in oxygen pressure. The crystallinity was improved with deposition at higher oxygen partial pressures with less oxygen vacancies. In the powder, the concentration of defects was high, so it has shown very small NBE, while during the synthesis of the thin films by PLD, the ablated material interact with the oxygen in the system when transferred from the target to substrate reducing the oxygen vacancies in the process. The Eu3+ f-f transitions emission were also observed at 617 and 590. The peaks were observed due to the Eu3+ f-f transitions at 590 and 617 nm attributed to the 5D0→7F2 and 5D0→7F1 radiative transitions, respectively [27]. In the PL curve of the ZnO:Eu3+ phosphor [Fig.1(b)] all transitions of Eu3+ ions were observed although the 5D0→7F0 ,5D0→7F3 and 5D0→7F4 were less intense compared to the two 5D0→7F1 and 5D0→7F2 transitions, but in the PLD films only a low intensity of the two 5D0→7F1 and 5D0→7F2 transitions were observed. Bär et al. [28] monitored the effect of an amorphous interface layer on the luminescence of Eu3+-doped GdVO4 PLD films, where a broadening of the Eu3+ luminescence peaks was observed. They found additionally, at the position of the 5D0 →7F1 transition two emission peaks at 593 nm and 595 nm. These could be explained by the two Stark levels of the 7F1 level, which were separated by 39 cm−1. Some of
Journal Pre-proof the PLD films did not show these sharp emission lines due to inhomogeneous broadening. In the corresponding excitation spectrum the peaks were strongly broadened and the 7F0 → 5D3 and 7F0 → 5D1 transitions barely exceed the noise level. The schematic band diagram of the emissions in the ZnO:Eu3+ was constructed from the data in Fig.4. The gap between valance and conduction band is ~ 3.28 eV, constructed from Pl data. The emission peak of Eu3+ is constructed from PL emission curve. The position of the theoretical value of the Zni level is at 0.22 eV below the conduction band [29,30]. The band transitions from Zni to Oi, Zni to Vo and the conduction band to Vo are at ~ 2.06, 1.63, and 1.85 eV, respectively. A non-radiative decay is possible from the conduction band of ZnO to the higher state of the Eu3+ and then recombined and provided different transition emissions. The emission intensity of the Eu3+ ions related peak was less in the thin films with respect to the phosphor powder.
Figure 4: (a) Effect of oxygen partial pressures on the Photoluminescence properties of ZnO:Eu3+ thin films prepared by PLD and (b) the energy band diagram of ZnO:Eu3+ thin films.
4. Conclusion Eu3+ doped ZnO thin films at different partial pressures of oxygen were successfully prepared by pulsed laser deposition. The XRD results confirmed the (002) preferential growth hexagonal wurtzite structure of ZnO. Band to band and Eu3+ related emission were observed in the PL
Journal Pre-proof results. Both band to band and Eu3+ emission increased with an increase in oxygen partial pressure, together with an increase in surface roughness. A big differences between the PL and XRD of the powder and thin films were observed. The concentration of defects was high in the case of the powder, so it has shown very small NBE with higher defect (DLE) and Eu3+ emission. While during the synthesis of the thin films by PLD, the ablated material interact with the oxygen in the system when transferred from the target to substrate reducing the oxygen vacancies in the process. With the consequent increase in NBE and less DLE emission. The Eu3+ emission was also severely quenched for the thin films. The reasons for the quenching of the Eu3+ emission still need more attention. The question may arise if the energy transfer to Eu3+ is either more effective from the defect levels or from the conduction band. It surely appeared that with less defects, less energy transfer has occurred. Many more experimental investigations to optimise the Eu3+ emission with different Eu3+ concentration are needed and the study on the effect of the preferential orientated ZnO also needs to be expanded.
Acknowledgments One of the authors (V.K.) is thankful to DST, New Delhi, India, for support through DSTInspire faculty award [DST/INSPIRE/04/2015/001497]. This work is based on the research supported by the National Research Foundation of South Africa (Grant Numbers 115126). This work is also based on the research supported by the South African Research Chairs Initiative of Department of Science and Technology and National Research Foundation of South Africa (84415).
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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