- Email: [email protected]
Preparation and properties of blue-light-transmitted and yellow-light-reflected multilayer films for high-luminous-efficiency white LEDs
Optik - International Journal for Light and Electron Optics 208 (2020) 164577
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Preparation and properties of blue-light-transmitted and yellowlight-reflected multilayer films for high-luminous-efficiency white LEDs
T
Jing Jiaa, Tingting Yangb, Qin Wangc,d, Husheng Jiac,d, Aiqin Zhangc,e,* a
Instrumental Analysis Center, Taiyuan University of Technology, Taiyuan 030024, PR China Shanxi Province Key Laboratory of Microstructure Functional Materials Institute of Solid State Physics, Shanxi Datong University, Datong 037009, PR China c Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Taiyuan 030024, PR China d College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China e College of Textile Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China b
A R T IC LE I N F O
ABS TRA CT
Keywords: Magnetron sputtering Multilayer film Light transmittance High luminous efficiency
The structure design and properties of a blue-light-transmitted and yellow-light-reflected intermediate layer are presented and analyzed in this paper. The ZnS/TiO2/MgF2 multilayer thin film was prepared by means of TFC software simulation and magnetron sputtering, and its surface topography was compact and uniform with low roughness of 6.229 nm and small thickness of 912 nm. Neither blue emission from LED chips nor yellow emission from phosphors would be absorbed by this type of multilayer film during the experiment. Furthermore, for ZnS/TiO2/MgF2 multilayer films, the five-layered structure is more suitable to achieve white LEDs with higher luminous efficiency, which shows maximum transmittance of blue light and average transmittance of yellow light were found to be around 86 % and 35 %, respectively. While applying this film between conventional blue chips and YAG: Ce phosphors, backscattered photons from phosphor layer to blue chip can be significantly decreased at the expense of relatively less exciting blue light, making great contribution to the overall increase in luminous efficiency.
1. Introduction Admittedly, it is commonly believed as a fact that white LEDs, as a typical green technology [1,2], have been widely applied throughout lighting field due to a great range of advantages such as small size, long service life, high efficiency, energy conservation and pollution-free. Nowadays, there are three main approaches for the fabrication of white LEDs [3–5], one of them is to employ InGaN-based blue LED chip to excite YAG: Ce yellow phosphors, and the blend of blue emission from light-emitting diodes and yellow fluorescence from phosphor particles will generate white light. Nevertheless, a considerable number of yellow photons would be backscattered by phosphor particles and lost in the chip [6], which has a serious negative influence on the extraction efficiency of high-power white LED devices. To solve this problem, an effective blue-light-transmitted and yellow-light-reflected film is designed and used as an intermediate layer in the conventional LED-phosphor hybrid lighting system. Not only can it enable white LEDs to obtain higher luminous efficiency, as a result of high light transmittance under violet-blue excitation from 380 to 500 nm and high reflectivity in the yellow-orange range between 565 nm and 630 nm, but also it helps to reduce the actual operating temperature of
⁎
Corresponding author. E-mail address: [email protected] (A. Zhang).
https://doi.org/10.1016/j.ijleo.2020.164577 Received 23 September 2019; Accepted 16 March 2020 0030-4026/ © 2020 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 208 (2020) 164577
J. Jia, et al.
Fig. 1. The structure model of ZnS/TiO2/MgF2 multilayer film.
LED chips and promote the electro-optical conversion efficiency. This is because proposed multilayer film with a high melting point of about 1200 °C may absorb part of the heat generated by blue chips while working. Generally speaking, some considerations on selecting materials for intermediate film are essential for ensuring that film material matches with the substrate, including optical, mechanical and chemical properties of film materials, preparation techniques and sizecontrol parameters. In this experiment, apart from the mentioned factors, we also take into account light transmittance, absorption coefficient and film adhesion. All of these factors restrict the possible range of film materials as well as increase the difficulty of hierarchical structure design. MgF2 thin film shows good tensile stress, low refractive index, high mechanical strength and good adherence to the substrate [7,8]. Besides, its tensile stress increases with an increase in film thickness. ZnS is a common functional material used in the visible light range owing to large refractive index, excellent film density, good mechanical and optical properties [9]. TiO2 thin film has many benefits for optical applications such as strong adhesion, superior corrosion resistance and chemical stability, good optical and mechanical performance [10], most of all, it has high transmittance and refractive index in the visible region. Therefore, for the multilayer intermediate film, we selected MgF2, ZnS, TiO2 as the low, medium and high refractive index material, respectively. Fig. 1 displays the hierarchical structure of ZnS/TiO2/MgF2 multilayer film, in which the letter n stands for refractive index, d stands for monolayer thickness. 2. Experiments 2.1. Simulation design There is a general rule for designing multilayer film [11], which states that suitable substrate material is crucial to strengthen the adhesion between thin film and substrate, and film with fewer layers is more beneficial to avoid local stress concentration while achieving high light transmittance. According to the design principle for double effective interface and thin-film characteristic matrix, first we decided to use a three-layered film structure– G|M 2H L|A in this study, and the transmittance curve was designed with the thin film composite (TFC) software package. However, the original simulated curve for ZnS/TiO2/MgF2 three-layer film was unsatisfactory because of high transmittance between 565 nm and 630 nm (Fig. 2). Based on the needle algorithm [12] related to layer number and total thickness of the multilayer film, the simulated transmittance curve was then optimized by continuously adding objectives to obtain ideal reflectivity in the yellow and orange spectral region, as shown in Fig. 3. The corresponding varying layer thicknesses are listed in Table 1, we can see that the light transmittance for simulated ZnS/TiO2/MgF2 seven-layer film between 380 nm and 500 nm is higher than 98 %, and lower than 1% between 565 nm and 630 nm, namely its reflectivity in the yellow-
Fig. 2. The original simulated transmittance curve of ZnS/TiO2/MgF2 three-layer film. 2
Optik - International Journal for Light and Electron Optics 208 (2020) 164577
J. Jia, et al.
Fig. 3. The optimized simulated transmittance curve of ZnS/TiO2/MgF2 seven-layer film. Table 1 The relevant data for simulated ZnS/TiO2/MgF2 seven-layer film. Layer number
Material
Monolayer thickness
1 2 3 4 5 6 7
ZnS TiO2 MgF2 TiO2 MgF2 TiO2 ZnS
59.78 115.30 248.45 115.30 248.45 115.30 59.78
orange spectrum is higher than 99 %. This multilayer film structure meets requirements for blue-light-transmitted and yellow-lightreflected intermediate layer. 2.2. Preparing procedure In this paper, a series of ZnS/TiO2/MgF2 multilayer films were deposited on bare glass substrates by a radio-frequency-magnetron sputtering system, which has four planar-magnetron rf sputtering guns. Prior to preparing thin film, the vacuum pressure in the sputtering chamber was less than 5 × 10−4 Pa, which can reduce the effect of gas impurities on high-quality thin films in the sputtering environment, and the distance between target and substrate was 60 mm. In addition, microscope glass slides (10 mm ☓ 10 mm ☓ 1 mm) were thoroughly washed with deionized water, absolute alcohol and acetone in an ultrasonic cleaner for 30 min at room temperature, and then the clean substrate was put into the sputtering chamber and deposited under Ar atmosphere (purity: 99.99 %) after drying. In order to remove stains and dust from target surface, each target (size: 60 mm dia. ☓ 5 mm thick) was presputtered for 10 min in air. During deposition, the argon flow rate was fixed at 30 sccm, the chamber pressure was maintained at 1.5 Pa, and ZnS, TiO2 and MgF2 were sputtered from high-purity ceramic targets: ZnS (purity: 99.99 %), TiO2 (purity: 99.95 %) and MgF2 (purity: 99.99 %) in sequence at a target power of 150 W. Afterwards, we obtained ZnS monolayer film, ZnS/TiO2 two-layer film, ZnS/TiO2/ MgF2 three-layer film, ZnS/TiO2/MgF2/TiO2 four-layer film, ZnS/TiO2/MgF2/TiO2/MgF2 five-layer film, ZnS/TiO2/MgF2/TiO2/ MgF2/TiO2 six-layer film and ZnS/TiO2/MgF2/TiO2/MgF2/TiO2/ZnS seven-layer film in sequence. 2.3. Preparation of YAG: Ce phosphor film First, a mixture of YAG: Ce phosphor particles (0.25 g) and liquid silicone (2 g) using n-hexane (0.3 mL) as a dispersing agent was dissolved in tetrahydrofuran (THF, 0.5 mL) at room temperature. The suspension was stirred well and then shook in an ultrasonic cleaner for 10 min to completely eliminate air bubbles from the suspension. Second, the homogenized suspension was uniformly coated onto a clean glass slide and dried in the drying oven at 120℃ for 2 h. Last, we could get a yellow YAG: Ce phosphor film after peeling it away from the glass substrate. 3. Results and discussion 3.1. Morphology analysis The cross-sectional SEM image of prepared ZnS/TiO2/MgF2 seven-layer film (see Fig. 4) shows a clear hierarchy and the film thickness. However, the multilayer structure with seven stacked sputtering films is not obvious, one possible explanation might be 3
Optik - International Journal for Light and Electron Optics 208 (2020) 164577
J. Jia, et al.
Fig. 4. A SEM image of the cross section of ZnS/TiO2/MgF2 seven-layer film.
that an external force results in partial desquamation or film damage when preparing SEM samples. Besides, the interfacial permeation also causes an unclear line between adjacent layers during magnetron sputtering. Based on the length-scale, the total thickness of prepared seven-layer film is about 915 nm, which means there is a value deviation between 915 nm and 961 nm simulated by the TFC software, this is because the sputtering time control lacks precision as well as quartz crystal control unit for membrane thickness. The surface topographies of different rf sputtered films were investigated using an atomic force microscopy (AFM), as shown in Fig. 5, different numbers represented different layers in turn. The increase in particle size of ZnS/TiO2/MgF2 multilayer film due to rise in layer numbers can be attributed to the particle agglomeration between adjacent layers, this is particularly true in the first three layers. As can be seen from Fig. 5b, the root-mean-square (RMS) roughness of prepared ZnS/TiO2/MgF2 seven-layer film is found to be 6.229 nm, indicating that its surface topography is smooth, uniform and dense. In addition, it has even particle size, good adhesion to the glass substrate. 3.2. Analysis of membrane thickness The layer number-thickness curves for simulation and experimental results are displayed in Fig. 6. It is quite clear that the trend curve of experimental result is basically in accordance with that of simulation result, a small difference between them is due to a lack of high accuracy of film thickness control during sputtering. Moreover, the thickness of ZnS/TiO2/MgF2 seven-layer film measured using a Dektak-XT step profiler is around 912 nm, which is consistent with the estimated thickness of 915 nm in the SEM image. 3.3. Optical properties The light-transmittance spectra of prepared ZnS/TiO2/MgF2 multilayer films in visible wavelength region are presented in Fig. 7, completely transparent glass slide is as the blank control sample. Compared with other curves, the five-layer film has reached optimum balance between luminous transmittance of blue-light and yellow-light. Its highest transmittance is 86 % near the wavelength of 430 nm, and the average transmittance at the range between 565 and 630 nm is only 35 %. In addition, the visible light transmittances of ZnS/TiO2/MgF2 multilayer films decrease rapidly with increasing layer numbers, this kind of relationship between light transmittance and wavelength is similar to that for TFC software simulation, because the surface roughness increases with increasing film thickness, and the light scattering would be larger as the surface roughness increases, which will result in a decrease in
Fig. 5. AFM micrographs of ZnS/TiO2/MgF2 multilayer films. 4
Optik - International Journal for Light and Electron Optics 208 (2020) 164577
J. Jia, et al.
Fig. 6. Graphs of film thickness for ZnS/TiO2/MgF2 multilayer films.
Fig. 7. Light-transmittance spectra of diverse ZnS/TiO2/MgF2 films with different layers.
light transmittance. Fig. 8 exhibits the UV–vis-near IR absorption spectra of prepared ZnS/TiO2/MgF2 multilayer films, different layer numbers are represented by the colored curves in the diagram. There is no characteristic absorption peak in the whole visible light range for all of them, except for a strong peak between 300 and 330 nm, demonstrating neither blue emission from chips nor yellow emission from excited phosphors can be absorbed by ZnS/TiO2/MgF2 multilayer films. In other words, the prepared intermediate layer does not induce optical loss of LEDs. At last, a two-layer composite membrane was prepared by combining ZnS/TiO2/MgF2 five-layer film (upper layer) with YAG: Ce phosphor film (lower layer), and the corresponding photoluminescence (PL) spectrum was attained when excited at 460 nm. As shown in Fig. 9, the highest peak appears at 485 nm in blue light region due to the excitation wavelength of 460 nm, and the intensity of yellow fluorescence is nearly zero, by which we mean that the prepared ZnS/TiO2/MgF2 intermediate layer is adequate to transmit
Fig. 8. UV–vis absorption spectra of prepared ZnS/TiO2/MgF2 multilayer films. 5
Optik - International Journal for Light and Electron Optics 208 (2020) 164577
J. Jia, et al.
Fig. 9. PL spectrum of a composite membrane of ZnS/TiO2/MgF2 five-layer film and YAG: Ce phosphor film.
blue light and reflect yellow light, and has potential application in the enhancement of luminous efficiency of white LEDs. Compared with transparent zinc oxide monofilm proposed by Zhennan Yu [13] in 2007, obviously the ZnS/TiO2/MgF2 five-layer film has better optical properties in this paper. 4. Conclusions In this paper, a seven-layered structure comprising ZnS, TiO2 and MgF2 was designed using TFC simulation software, and then deposited on glass substrate by rf magnetron sputtering. The prepared ZnS/TiO2/MgF2 multilayer film can be used as an intermediate layer and its surface is smooth and dense with RMS roughness of 6.229 nm. Additionally, it does not absorb emission from both LED chips and phosphors, especially for the five-layered film structure, its light transmittance in blue region and yellow region are found to be 86 % and 35 % respectively. While proposed ZnS/TiO2/MgF2 multilayer film was combined with YAG: Ce phosphor film, this type of composite membrane has potential application in high-luminous-efficiency white LED devices because of good blue-light transmittance and yellow-light reflection. However, the yellow-light reflectivity of proposed ZnS/TiO2/MgF2 multilayer film is not ideal, which need to be increased to over 90 % by means of utilizing modified preparation method or adjusting the operating parameters in our future work. Funding Key Research and Development (R&D) Projects of Shanxi Province (201803D31042, 201803D421079). References [1] J.M. Phillips, M.E. Coltrin, M.H. Crawford, et al., Research challenges to ultra-efficient inorganic solid-state lighting, Laser Photon. Rev. 1 (4) (2007) 307–333. [2] A. Zukaurkas, M.S. Shur, R. Gaska, Introduction to Solid State Lighting, John Wiley & Sons Inc, New York, 2002, pp. 1–10. [3] Yu Zorenko, V. Gorbenko, I. Konstanuerych, et al., Single-crystalline films of Ce-doped YAG and LuAG phosphors: advantages over bulk crystals analogues, J. Lumin. 114 (2) (2005) 85–94. [4] R. Kasuya, T. Isobe, H. Kuma, et al., Glyeothermal synthesis and photoluminescence of YAG: Ce3+ nanophosphors, J. Alloys. Compd. 408 (2006) 820–823. [5] Y.J. Tang, Y.G. Yang, Z.J. Yang, et al., Fabrication and properties of white luminescence conversion LEDs, Chinese J. Lumin. 22 (10) (2001) 91–94. [6] J.K. Sheu, C.M. Tsai, M.L. Lee, et al., InGaN light-emitting diodes with naturally formed truncated micropyramids on top surface, Appl. Phys. Lett. 88 (11) (2006) 3505–3507. [7] S. Kerstin, K. Erhard, G.J. PlacidO, et al., Porous MgF2 antireflective λ/4 films prepared by sol–gel processing: comparison of synthesis approaches, J. Solgel Sci. Technol. 76 (1) (2015) 82–89. [8] K.C. Kim, Effective graded refractive-index anti-reflection coating for high refractive-index polymer ophthalmic lenses, Mater. Lett. 160 (2015) 158–161. [9] A. Goudarzi, A.D. Namghi, C.S. Ha, Fabrication and characterization of nano-structured ZnS thin films as the buffer layers in solar cells, RSC Adv. 4 (104) (2014) 59764–59771. [10] T. Alig, S. GunsteR, D. Ristau, Etching behavior of optical thin films for different deposition techniques, Thin Solid Films 592 (2015) 237–239. [11] J. Hyodo, S. Ida, J.A. Kilner, et al., Electronic and oxide ion conductivity in Pr2Ni0.71Cu0.24Ga0.05O4/Ce0.8Sm0.2O2 laminated film, Solid State Ion. 230 (2013) 16–20. [12] A.V. Tikhonravov, M.K. Trubetskov, G.W. Debell, Application of the needle optimization technique to the design of optical coatings, Appl. Opt. 35 (28) (1996) 5493–5508. [13] Z.N. Yu, L. Jiang, F.Y. Jiang, et al., Preparation of ZnO transparent films by magnetron sputtering, J. Nanchang Univ. 31 (5) (2007) 452–455.
6
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Preparation and properties of blue-light-transmitted and yellowlight-reflected multilayer films for high-luminous-efficiency white LEDs
T
Jing Jiaa, Tingting Yangb, Qin Wangc,d, Husheng Jiac,d, Aiqin Zhangc,e,* a
Instrumental Analysis Center, Taiyuan University of Technology, Taiyuan 030024, PR China Shanxi Province Key Laboratory of Microstructure Functional Materials Institute of Solid State Physics, Shanxi Datong University, Datong 037009, PR China c Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Taiyuan 030024, PR China d College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China e College of Textile Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China b
A R T IC LE I N F O
ABS TRA CT
Keywords: Magnetron sputtering Multilayer film Light transmittance High luminous efficiency
The structure design and properties of a blue-light-transmitted and yellow-light-reflected intermediate layer are presented and analyzed in this paper. The ZnS/TiO2/MgF2 multilayer thin film was prepared by means of TFC software simulation and magnetron sputtering, and its surface topography was compact and uniform with low roughness of 6.229 nm and small thickness of 912 nm. Neither blue emission from LED chips nor yellow emission from phosphors would be absorbed by this type of multilayer film during the experiment. Furthermore, for ZnS/TiO2/MgF2 multilayer films, the five-layered structure is more suitable to achieve white LEDs with higher luminous efficiency, which shows maximum transmittance of blue light and average transmittance of yellow light were found to be around 86 % and 35 %, respectively. While applying this film between conventional blue chips and YAG: Ce phosphors, backscattered photons from phosphor layer to blue chip can be significantly decreased at the expense of relatively less exciting blue light, making great contribution to the overall increase in luminous efficiency.
1. Introduction Admittedly, it is commonly believed as a fact that white LEDs, as a typical green technology [1,2], have been widely applied throughout lighting field due to a great range of advantages such as small size, long service life, high efficiency, energy conservation and pollution-free. Nowadays, there are three main approaches for the fabrication of white LEDs [3–5], one of them is to employ InGaN-based blue LED chip to excite YAG: Ce yellow phosphors, and the blend of blue emission from light-emitting diodes and yellow fluorescence from phosphor particles will generate white light. Nevertheless, a considerable number of yellow photons would be backscattered by phosphor particles and lost in the chip [6], which has a serious negative influence on the extraction efficiency of high-power white LED devices. To solve this problem, an effective blue-light-transmitted and yellow-light-reflected film is designed and used as an intermediate layer in the conventional LED-phosphor hybrid lighting system. Not only can it enable white LEDs to obtain higher luminous efficiency, as a result of high light transmittance under violet-blue excitation from 380 to 500 nm and high reflectivity in the yellow-orange range between 565 nm and 630 nm, but also it helps to reduce the actual operating temperature of
⁎
Corresponding author. E-mail address: [email protected] (A. Zhang).
https://doi.org/10.1016/j.ijleo.2020.164577 Received 23 September 2019; Accepted 16 March 2020 0030-4026/ © 2020 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 208 (2020) 164577
J. Jia, et al.
Fig. 1. The structure model of ZnS/TiO2/MgF2 multilayer film.
LED chips and promote the electro-optical conversion efficiency. This is because proposed multilayer film with a high melting point of about 1200 °C may absorb part of the heat generated by blue chips while working. Generally speaking, some considerations on selecting materials for intermediate film are essential for ensuring that film material matches with the substrate, including optical, mechanical and chemical properties of film materials, preparation techniques and sizecontrol parameters. In this experiment, apart from the mentioned factors, we also take into account light transmittance, absorption coefficient and film adhesion. All of these factors restrict the possible range of film materials as well as increase the difficulty of hierarchical structure design. MgF2 thin film shows good tensile stress, low refractive index, high mechanical strength and good adherence to the substrate [7,8]. Besides, its tensile stress increases with an increase in film thickness. ZnS is a common functional material used in the visible light range owing to large refractive index, excellent film density, good mechanical and optical properties [9]. TiO2 thin film has many benefits for optical applications such as strong adhesion, superior corrosion resistance and chemical stability, good optical and mechanical performance [10], most of all, it has high transmittance and refractive index in the visible region. Therefore, for the multilayer intermediate film, we selected MgF2, ZnS, TiO2 as the low, medium and high refractive index material, respectively. Fig. 1 displays the hierarchical structure of ZnS/TiO2/MgF2 multilayer film, in which the letter n stands for refractive index, d stands for monolayer thickness. 2. Experiments 2.1. Simulation design There is a general rule for designing multilayer film [11], which states that suitable substrate material is crucial to strengthen the adhesion between thin film and substrate, and film with fewer layers is more beneficial to avoid local stress concentration while achieving high light transmittance. According to the design principle for double effective interface and thin-film characteristic matrix, first we decided to use a three-layered film structure– G|M 2H L|A in this study, and the transmittance curve was designed with the thin film composite (TFC) software package. However, the original simulated curve for ZnS/TiO2/MgF2 three-layer film was unsatisfactory because of high transmittance between 565 nm and 630 nm (Fig. 2). Based on the needle algorithm [12] related to layer number and total thickness of the multilayer film, the simulated transmittance curve was then optimized by continuously adding objectives to obtain ideal reflectivity in the yellow and orange spectral region, as shown in Fig. 3. The corresponding varying layer thicknesses are listed in Table 1, we can see that the light transmittance for simulated ZnS/TiO2/MgF2 seven-layer film between 380 nm and 500 nm is higher than 98 %, and lower than 1% between 565 nm and 630 nm, namely its reflectivity in the yellow-
Fig. 2. The original simulated transmittance curve of ZnS/TiO2/MgF2 three-layer film. 2
Optik - International Journal for Light and Electron Optics 208 (2020) 164577
J. Jia, et al.
Fig. 3. The optimized simulated transmittance curve of ZnS/TiO2/MgF2 seven-layer film. Table 1 The relevant data for simulated ZnS/TiO2/MgF2 seven-layer film. Layer number
Material
Monolayer thickness
1 2 3 4 5 6 7
ZnS TiO2 MgF2 TiO2 MgF2 TiO2 ZnS
59.78 115.30 248.45 115.30 248.45 115.30 59.78
orange spectrum is higher than 99 %. This multilayer film structure meets requirements for blue-light-transmitted and yellow-lightreflected intermediate layer. 2.2. Preparing procedure In this paper, a series of ZnS/TiO2/MgF2 multilayer films were deposited on bare glass substrates by a radio-frequency-magnetron sputtering system, which has four planar-magnetron rf sputtering guns. Prior to preparing thin film, the vacuum pressure in the sputtering chamber was less than 5 × 10−4 Pa, which can reduce the effect of gas impurities on high-quality thin films in the sputtering environment, and the distance between target and substrate was 60 mm. In addition, microscope glass slides (10 mm ☓ 10 mm ☓ 1 mm) were thoroughly washed with deionized water, absolute alcohol and acetone in an ultrasonic cleaner for 30 min at room temperature, and then the clean substrate was put into the sputtering chamber and deposited under Ar atmosphere (purity: 99.99 %) after drying. In order to remove stains and dust from target surface, each target (size: 60 mm dia. ☓ 5 mm thick) was presputtered for 10 min in air. During deposition, the argon flow rate was fixed at 30 sccm, the chamber pressure was maintained at 1.5 Pa, and ZnS, TiO2 and MgF2 were sputtered from high-purity ceramic targets: ZnS (purity: 99.99 %), TiO2 (purity: 99.95 %) and MgF2 (purity: 99.99 %) in sequence at a target power of 150 W. Afterwards, we obtained ZnS monolayer film, ZnS/TiO2 two-layer film, ZnS/TiO2/ MgF2 three-layer film, ZnS/TiO2/MgF2/TiO2 four-layer film, ZnS/TiO2/MgF2/TiO2/MgF2 five-layer film, ZnS/TiO2/MgF2/TiO2/ MgF2/TiO2 six-layer film and ZnS/TiO2/MgF2/TiO2/MgF2/TiO2/ZnS seven-layer film in sequence. 2.3. Preparation of YAG: Ce phosphor film First, a mixture of YAG: Ce phosphor particles (0.25 g) and liquid silicone (2 g) using n-hexane (0.3 mL) as a dispersing agent was dissolved in tetrahydrofuran (THF, 0.5 mL) at room temperature. The suspension was stirred well and then shook in an ultrasonic cleaner for 10 min to completely eliminate air bubbles from the suspension. Second, the homogenized suspension was uniformly coated onto a clean glass slide and dried in the drying oven at 120℃ for 2 h. Last, we could get a yellow YAG: Ce phosphor film after peeling it away from the glass substrate. 3. Results and discussion 3.1. Morphology analysis The cross-sectional SEM image of prepared ZnS/TiO2/MgF2 seven-layer film (see Fig. 4) shows a clear hierarchy and the film thickness. However, the multilayer structure with seven stacked sputtering films is not obvious, one possible explanation might be 3
Optik - International Journal for Light and Electron Optics 208 (2020) 164577
J. Jia, et al.
Fig. 4. A SEM image of the cross section of ZnS/TiO2/MgF2 seven-layer film.
that an external force results in partial desquamation or film damage when preparing SEM samples. Besides, the interfacial permeation also causes an unclear line between adjacent layers during magnetron sputtering. Based on the length-scale, the total thickness of prepared seven-layer film is about 915 nm, which means there is a value deviation between 915 nm and 961 nm simulated by the TFC software, this is because the sputtering time control lacks precision as well as quartz crystal control unit for membrane thickness. The surface topographies of different rf sputtered films were investigated using an atomic force microscopy (AFM), as shown in Fig. 5, different numbers represented different layers in turn. The increase in particle size of ZnS/TiO2/MgF2 multilayer film due to rise in layer numbers can be attributed to the particle agglomeration between adjacent layers, this is particularly true in the first three layers. As can be seen from Fig. 5b, the root-mean-square (RMS) roughness of prepared ZnS/TiO2/MgF2 seven-layer film is found to be 6.229 nm, indicating that its surface topography is smooth, uniform and dense. In addition, it has even particle size, good adhesion to the glass substrate. 3.2. Analysis of membrane thickness The layer number-thickness curves for simulation and experimental results are displayed in Fig. 6. It is quite clear that the trend curve of experimental result is basically in accordance with that of simulation result, a small difference between them is due to a lack of high accuracy of film thickness control during sputtering. Moreover, the thickness of ZnS/TiO2/MgF2 seven-layer film measured using a Dektak-XT step profiler is around 912 nm, which is consistent with the estimated thickness of 915 nm in the SEM image. 3.3. Optical properties The light-transmittance spectra of prepared ZnS/TiO2/MgF2 multilayer films in visible wavelength region are presented in Fig. 7, completely transparent glass slide is as the blank control sample. Compared with other curves, the five-layer film has reached optimum balance between luminous transmittance of blue-light and yellow-light. Its highest transmittance is 86 % near the wavelength of 430 nm, and the average transmittance at the range between 565 and 630 nm is only 35 %. In addition, the visible light transmittances of ZnS/TiO2/MgF2 multilayer films decrease rapidly with increasing layer numbers, this kind of relationship between light transmittance and wavelength is similar to that for TFC software simulation, because the surface roughness increases with increasing film thickness, and the light scattering would be larger as the surface roughness increases, which will result in a decrease in
Fig. 5. AFM micrographs of ZnS/TiO2/MgF2 multilayer films. 4
Optik - International Journal for Light and Electron Optics 208 (2020) 164577
J. Jia, et al.
Fig. 6. Graphs of film thickness for ZnS/TiO2/MgF2 multilayer films.
Fig. 7. Light-transmittance spectra of diverse ZnS/TiO2/MgF2 films with different layers.
light transmittance. Fig. 8 exhibits the UV–vis-near IR absorption spectra of prepared ZnS/TiO2/MgF2 multilayer films, different layer numbers are represented by the colored curves in the diagram. There is no characteristic absorption peak in the whole visible light range for all of them, except for a strong peak between 300 and 330 nm, demonstrating neither blue emission from chips nor yellow emission from excited phosphors can be absorbed by ZnS/TiO2/MgF2 multilayer films. In other words, the prepared intermediate layer does not induce optical loss of LEDs. At last, a two-layer composite membrane was prepared by combining ZnS/TiO2/MgF2 five-layer film (upper layer) with YAG: Ce phosphor film (lower layer), and the corresponding photoluminescence (PL) spectrum was attained when excited at 460 nm. As shown in Fig. 9, the highest peak appears at 485 nm in blue light region due to the excitation wavelength of 460 nm, and the intensity of yellow fluorescence is nearly zero, by which we mean that the prepared ZnS/TiO2/MgF2 intermediate layer is adequate to transmit
Fig. 8. UV–vis absorption spectra of prepared ZnS/TiO2/MgF2 multilayer films. 5
Optik - International Journal for Light and Electron Optics 208 (2020) 164577
J. Jia, et al.
Fig. 9. PL spectrum of a composite membrane of ZnS/TiO2/MgF2 five-layer film and YAG: Ce phosphor film.
blue light and reflect yellow light, and has potential application in the enhancement of luminous efficiency of white LEDs. Compared with transparent zinc oxide monofilm proposed by Zhennan Yu [13] in 2007, obviously the ZnS/TiO2/MgF2 five-layer film has better optical properties in this paper. 4. Conclusions In this paper, a seven-layered structure comprising ZnS, TiO2 and MgF2 was designed using TFC simulation software, and then deposited on glass substrate by rf magnetron sputtering. The prepared ZnS/TiO2/MgF2 multilayer film can be used as an intermediate layer and its surface is smooth and dense with RMS roughness of 6.229 nm. Additionally, it does not absorb emission from both LED chips and phosphors, especially for the five-layered film structure, its light transmittance in blue region and yellow region are found to be 86 % and 35 % respectively. While proposed ZnS/TiO2/MgF2 multilayer film was combined with YAG: Ce phosphor film, this type of composite membrane has potential application in high-luminous-efficiency white LED devices because of good blue-light transmittance and yellow-light reflection. However, the yellow-light reflectivity of proposed ZnS/TiO2/MgF2 multilayer film is not ideal, which need to be increased to over 90 % by means of utilizing modified preparation method or adjusting the operating parameters in our future work. Funding Key Research and Development (R&D) Projects of Shanxi Province (201803D31042, 201803D421079). References [1] J.M. Phillips, M.E. Coltrin, M.H. Crawford, et al., Research challenges to ultra-efficient inorganic solid-state lighting, Laser Photon. Rev. 1 (4) (2007) 307–333. [2] A. Zukaurkas, M.S. Shur, R. Gaska, Introduction to Solid State Lighting, John Wiley & Sons Inc, New York, 2002, pp. 1–10. [3] Yu Zorenko, V. Gorbenko, I. Konstanuerych, et al., Single-crystalline films of Ce-doped YAG and LuAG phosphors: advantages over bulk crystals analogues, J. Lumin. 114 (2) (2005) 85–94. [4] R. Kasuya, T. Isobe, H. Kuma, et al., Glyeothermal synthesis and photoluminescence of YAG: Ce3+ nanophosphors, J. Alloys. Compd. 408 (2006) 820–823. [5] Y.J. Tang, Y.G. Yang, Z.J. Yang, et al., Fabrication and properties of white luminescence conversion LEDs, Chinese J. Lumin. 22 (10) (2001) 91–94. [6] J.K. Sheu, C.M. Tsai, M.L. Lee, et al., InGaN light-emitting diodes with naturally formed truncated micropyramids on top surface, Appl. Phys. Lett. 88 (11) (2006) 3505–3507. [7] S. Kerstin, K. Erhard, G.J. PlacidO, et al., Porous MgF2 antireflective λ/4 films prepared by sol–gel processing: comparison of synthesis approaches, J. Solgel Sci. Technol. 76 (1) (2015) 82–89. [8] K.C. Kim, Effective graded refractive-index anti-reflection coating for high refractive-index polymer ophthalmic lenses, Mater. Lett. 160 (2015) 158–161. [9] A. Goudarzi, A.D. Namghi, C.S. Ha, Fabrication and characterization of nano-structured ZnS thin films as the buffer layers in solar cells, RSC Adv. 4 (104) (2014) 59764–59771. [10] T. Alig, S. GunsteR, D. Ristau, Etching behavior of optical thin films for different deposition techniques, Thin Solid Films 592 (2015) 237–239. [11] J. Hyodo, S. Ida, J.A. Kilner, et al., Electronic and oxide ion conductivity in Pr2Ni0.71Cu0.24Ga0.05O4/Ce0.8Sm0.2O2 laminated film, Solid State Ion. 230 (2013) 16–20. [12] A.V. Tikhonravov, M.K. Trubetskov, G.W. Debell, Application of the needle optimization technique to the design of optical coatings, Appl. Opt. 35 (28) (1996) 5493–5508. [13] Z.N. Yu, L. Jiang, F.Y. Jiang, et al., Preparation of ZnO transparent films by magnetron sputtering, J. Nanchang Univ. 31 (5) (2007) 452–455.
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