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Improvement of high-temperature resistance of the Ag-based multilayer films deposited by magnetron sputtering
Materials Letters 118 (2014) 62–65
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Materials Letters journal homepage: www.elsevier.com/locate/matlet
Improvement of high-temperature resistance of the Ag-based multilayer films deposited by magnetron sputtering Jingkai Yang n, Hongli Zhao, Hesong Sha, Jian Li, Liping Zhao, Jiajia Chen, Bin Yu, Fucheng Zhang State Key Laboratory of Metastable Materials Science and Technology, College of Materials Science and Engineering, Yanshan University, Qinhuangdao 066004, China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 September 2013 Accepted 7 December 2013 Available online 16 December 2013
Ag-based films with the multilayer construction of top-Si3N4/SnO2/NiCrOx/Ag/ZnO/NiCrOx/TiO2/underSi3N4 were deposited on glass substrates by magnetron sputtering at room temperature, and then heated at 200 1C, 400 1C, 600 1C, 650 1C and 700 1C for 5 min in the air. The effects of post-heated temperature on the optical and electrical properties of the advanced Ag-based Low-E glass were investigated. The results show that the multilayer films mainly contain a crystalline Ag layer with 3C structure and other amorphous layers. A transformation of the preferred orientation from (111) plane to (220) plane occurs at the temperature of 600 1C. The optimal overall performance of Ag-based Low-E films is obtained when heated at 600 1C for 5 min with the minimum sheet resistance of 5.10 Ω/□ and a lowest emissivity of 0.06 and the transmittance in the visible region of 79.40%. The high-temperature resistance of Ag-based films has been improved with this modified kind of multilayer construction. & 2013 Elsevier B.V. All rights reserved.
Keywords: Thin films Ag-based X-ray diffraction Electrical properties Optical properties
1. Introduction Silver-based multilayer films with low infrared emissivity have excellent optoelectronic properties of heat insulation, solar energy reflection and electrical conductivity. They are usually applied in flat panel displays, solar cells, architectural and vehicle glass as spectral selective filters to control sunlight incidence and reflection and preserve thermal energy [1–4]. The Low-E films are usually prepared by a magnetron sputtering method with dielectric–metal– dielectric (D/M/D) type of multilayer structures, such as ITO/Ag/ITO [1,2], CuAlO2/Ag/CuAlO2 [3], ZnO/Ag/ZnO [5], AZO/Ag/AZO (Aldoped ZnO) [4,6–8], GZO/Ag/GZO [9,10], ZnS/Ag/ZnS [11,12], TiO2/ Ag/Ti/TiO2/SiON [13], and so on. Silver is often sandwiched within dielectric layers due to its lower absorbance (o5%) in the visible light region, high spectral reflection and low thermal emissivity. Metal oxides are chosen as the dielectric layers to protect Ag layer from the chemical and mechanical damage. In addition, they act as adhesion layers, nucleation templates and diffusion barriers. Most attention is paid on how to enhance the optical and electrical properties by changing the substrate temperature [5,7], the thickness of every layer [3,6,8,13], cathode voltage [14] and the underlayer [15,16].
n
Corresponding author. Tel.: þ 86 13903339287; fax: þ86 335 8050727. E-mail addresses: [email protected] (J. Yang), [email protected] (H. Zhao). 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.12.033
It is well known that Ag-based Low-E films are not durable against mechanical abrasion, chemical agents and high temperature treatment. For many architectural applications, these glasses usually undergo various secondary processing, such as bending/vacuum forming and, most importantly, thermal toughening, to enhance the fracture behavior for avoiding breakage into large and sharp fragments. This processing demands that Ag-based Low-E films must withstand these high temperatures without trading-off functionality. Therefore, it is necessary to investigate the effects of heat treatment on the optical and electrical properties. Sahu and co-workers [5] found that the sheet resistance of ZnO/Ag/ZnO multilayer films increased when the heated temperature was higher than 300 1C in the air atmosphere, while Neghabi and co-workers [11] found that the sheet resistance of ZnS/Ag/ZnS films increased when the temperature was higher than 200 1C. Long et al. [12] reported that ZnS– SiO2/Ag/ZnS–SiO2 films exhibited a low sheet resistance of 9.7 Ω/□ and a high average transmittance of 84.1% with the heated temperature of 200 1C, and the degradation of optoelectronic performances occurred when the temperature was 300 1C. However, on the basis of guarantee the photoelectric performance, the highest temperature in current research results (300 1C) cannot meet the needs of the toughened glass (often up to 700 1C). Therefore, it is imperative to develop a new system of Ag-based multilayer films to improve the high-temperature resistance of the Ag-based films. In this study, Ag-based Low-E films with a new layer construction of top-Si3N4/SnO2/NiCrOx/Ag/ZnO/NiCrOx/TiO2/under-Si3N4 were deposited on unheated glass substrates by vacuum magnetron
J. Yang et al. / Materials Letters 118 (2014) 62–65
sputtering technique, and then heated at 200 1C, 400 1C, 600 1C, 650 1C and 700 1C for 5 min in the air. The effects of heat treatment temperature on the structural, electrical and optical properties of Ag-based Low-E films were investigated.
2. Experimental The layer construction of Ag-based Low-E glass is top-Si3N4/ SnO2/NiCrOx/Ag/ZnO/NiCrOx/TiO2/Si3N4/glass. The functional Ag layer of 12.2 nm was deposited in Ar gas with a pure Ag target. Si3N4 layer was deposited in N2 gas using a Si target, while Sn, NiCr and Zn were used as the cathode materials for the corresponding oxide layers in O2 gas, and TiO2 layer were deposited from two tubular targets made of TiOx ceramic in a working gas consisting of an Ar/O2 mixture. The base pressure and gas pressure were 5 10-4 Pa and 1.5 105 Pa, respectively. Argon with purity of 99.996% was used as working gas, while nitrogen and oxygen with purity of 99.99% were used as reactive gas. The thickness of each layer was adjusted by the substrate conveyance speed. Finally, Ag-based Low-E films with a layer construction of top-22 nm Si3N4/13 nm SnO2/1.1 nm NiCrOx/12.2 nm Ag/4.6 nm ZnO /1.1 nm NiCrOx/ 16.5 nm TiO2/ under-12 nm Si3N4 were deposited on glass by vacuum magnetron sputtering technique with the total film thickness of 82.5 nm. Heat treatment of the as-deposited Ag-based Low-E glass was carried out in resistance furnace (KLX-12B) at 200 1C, 400 1C, 600 1C and 700 1C for 5 min, respectively. Then the samples were cooled to room temperature in the furnace. The structure of as-deposited and post-heat treated Low-E glass was studied by X-ray Diffraction (D/MAX-2500HBþ/PC) using the Cu Kα radiation (λ ¼0.15406 nm). Sheet resistance was determined using the four-point probe method. The optical transmittance spectra in the visible range of 190–900 nm at different heat treatment temperatures were measured by a double-beam UV–vis spectrophotometer (WFZ-26 A).
3. Results and discussion The X-ray diffraction patterns of the as-deposited and posttreated Ag-based films at different temperatures for 5 min are shown in Fig. 1. It can be seen that there are only two main diffraction peaks at about 38.21 and 64.61, corresponding to (111) and (220) plane of silver with 3C structure. The broad peak around 24.51 is related to the glass substrate. There was no XRD peak corresponding to other layers of the Si3N4, SnO2, NiCrOx, ZnO and
63
TiO2 layers observed, indicating that the as-grown other films are amorphous. Therefore, the multilayer films mainly containing a crystalline Ag layer and other amorphous layers when deposited on glass substrates by vacuum magnetron sputtering technique. When the temperature is high to 600 1C, 650 1C and 700 1C, two very weak peaks at 48.11 and 52.21 occur and correspond to the diffractions of Ag3O4, which indicates that a little oxidation happens to the functional Ag layer at such high temperatures. As seen in Fig. 1, it is obvious that the relative intensity of (111) plane decreases, while that of (220) plane increases, with the increasing temperature. In order to analyze the preferred orientation more quantitatively, the integrated intensity ratio η(111) of I(111)/[(I(111) þI(220))] was introduced, where I(111) and I(220) is the integrated intensities of (111) and (220) planes, respectively. η(111) according to the standard intensity given in the ASTM cards (No. 04-0783) is 0.71. If η(111) is greater than 0.71, (111) plane is the preferred orientation, otherwise, (220) plane is the preferred orientation. The integrated intensity ratio η(111), as listed in Table 1, is larger than 0.71 when the temperature is lower than 400 1C, indicating Ag grains in the multilayer films possess the (111) preferred orientation. When the temperature is equal to or higher than 600 1C, η(111) is smaller than 0.71, in addition, the values of η(111) decrease sharply with the increasing temperature (higher than 600 1C). Therefore, Ag grains in the multilayer films heated at higher temperatures possess an obviously (220) preferred orientation. The average crystallite sizes for the as-deposited and postheated films are calculated in planes with Miller indices (111) and (220) according to the Scherrer equation [17] and listed in Table 1. The crystallite size of the as-deposited film is 10.09 nm and increases to 11.34 nm after post-heated at 600 1C for 5 min. The growth rate is relatively slow. However, when heated at 700 1C for 5 min, the crystallite size increases rapidly to 15.63 nm and is bigger than the thickness of Ag layer. The sheet resistance of Ag-based Low-E films heated at different temperatures for 5 min are shown in Fig. 2. The sheet resistance Rs decreases from 6.28 Ω/□ for the as-deposited film to 5.10 Ω/□ for the films heated at 600 1C for 5 min, then increases sharply to 20.95 Ω/□ when heated at 700 1C for 5 min. The sheet resistance before 600 1C declines slightly. That is because the crystallinity and the average crystallite size increases with the increasing temperature, the defects in the films decreases, reducing the electron scattering, and the conductivity of the films is improved. The rapid increase of sheet resistance occurs when the temperature is higher than 600 1C. Combined with the XRD results, the average crystallite size (12.91 nm for 650 1C, 15.63 nm for 700 1C) is larger than the thickness of Ag layer (12.2 nm), which makes the growth of Ag grain crystal limited seriously. Moreover, the growth along the (220) preferred orientation, which is perpendicular to surface, increases sharply. Both of the variations lead a non-uniform structure of the Ag-based multilayer films and make the resistance increased. Maybe there is severe inter-diffusion of atoms which then increases the sheet resistance of the multilayer [18]. Compared with the results of
Table 1 The integrated intensity ratio η(111) and average crystalline size of Ag-based Low-E films, as-deposited and post-heated at 200 1C, 400 1C, 600 1C, 650 1C, 700 1C for 5 min, respectively.
Fig. 1. X-ray diffraction patterns of Ag-based Low-E films, as-deposited and postheated at 200 1C, 400 1C, 600 1C, 650 1C, 700 1C for 5 min, respectively.
Heat treatment temperature
η(111) Average crystalline size (nm)
Infrared reflectivity (RIR)
Calculated emissivity ε
Transmittance at 550 nm
as-deposited 200 1C 400 1C 600 1C 650 1C 700 1C
0.77 10.09 0.80 10.50 0.73 10.58 0.69 11.34 0.29 12.91 0.21 15.63
93.66% 93.93% 94.51% 94.81% 92.03% 81.01%
0.08 0.07 0.07 0.06 0.10 0.24
76.81% 78.16% 81.23% 79.40% 72.97% 65.17%
64
J. Yang et al. / Materials Letters 118 (2014) 62–65
Fig. 2. Sheet resistance of Ag-based Low-E films with a function of heated temperature.
film, decreases to 0.06 when heated at 600 1C for 4 min, and then increases to 0.24 when the heated temperature reaches 700 1C. The effect of heat preservation and heat insulation of Ag-based films has been damaged greatly when heated at 700 1C. The transmittance curves of Ag-based Low-E films is shown in Fig. 3. Compared with the as-deposited glass, the transmittance in the visible range increases as the temperature increases to 400 1C. When heated at 400 1C for 5 min, the Ag-based Low-E films have the highest transmittance of 81.23%. That is because of the high crystallinity and low grain boundary scattering of the polycrystalline films with the increasing temperature. When heated at 600 1C for 5 min, the transmittance decreases a little to 79.40%. When the temperature reaches 700 1C, the transmittance decreases sharply to 65.17%, which is due to the increased scattering resulted from the abnormal grain growth and the non-uniform structure of the films. In this work, a minimum sheet resistance of 5.10 Ω/□ and a lowest emissivity of 0.06 is achieved when heated at 600 1C for 5 min, although the transmittance in the visible region is a little lower (1.83%) than that of Ag-based films heated at 400 1C for 5 min. Therefore, the Ag-based Low-E films heated at 600 1C for 5 min possess the optimal optoelectronic property. It indicates that the temperature of high-temperature resistance of Ag-based films has been increased to 600 1C. Therefore, the top-Si3N4/SnO2/ NiCrOx/Ag/ZnO/NiCrOx/TiO2/under-Si3N4 multilayer is a promising transparent conductive film with low sheet resistance, high transmittance, and high heat stability.
4. Conclusions
Fig. 3. Transmittance curves in the visible range of Ag-based Low-E glass, as-deposited and post-heated at 200 1C, 400 1C, 600 1C, 650 1C, 700 1C for 5 min, respectively.
Sahu et al. [5] and Neghabi et al. [11], the high-temperature resistance of the Ag-based films has been improved greatly with this modified multilayer construction of top-Si3N4/SnO2/NiCrOx/Ag/ZnO/NiCrOx/ TiO2/under-Si3N4 and the functional Ag layer obtains a high stability. In this heat-resistant Ag-based Low-E multilayer system, NiCrOx layer serves as antiscattering layer placed between the high-refringence layer TiO2 and the wetting layer ZnO. On the other hand, during the high-temperature toughening operation, the incomplete oxidation layer NiCrOx can protect the functional Ag layer from oxidation by the oxygen diffusion from the underlayers or the upperlayers and make the Ag layer more stable, which is in agreement with others' results [19]. Therefore, the high-temperature resistance of this modified Ag-based Low-E multilayer films has been improved. The infrared reflectivity RIR and the emissivity ε of the coated glass can be calculated with the sheet resistance Rs and may be written as follows: RIR ¼ ð1 þ 0:0053Rs Þ 2
ð1Þ
ε ¼ 0:0129Rs 6:7 10 5 Rs 2
ð2Þ
The error of ε calculated from Eq. (2) is below 2% for values discussed here [20]. And the values of calculated RIR and ε are listed in Table 1. The infrared reflectivity almost keeps almost invariable when the heated temperature is lower than 600 1C, and then decreases sharply to 81.01% with the heated temperature of 700 1C for 5 min. The calculated emissivity ε is 0.08 for as-deposited
High stable transparent and conductive Ag-based low emissivity films deposited on float glass by magnetron sputtering were treated at different temperatures from 200 1C to 700 1C for 5 min, and then cooled to room temperature in the resistance furnace. The layer construction of Ag-based films is top-Si3N4/SnO2/NiCrOx/ Ag/ZnO/NiCrOx/TiO2/under-Si3N4. The multilayer films contain a crystalline Ag layer and other amorphous layers with a transformation of the preferred orientation from (111) plane to (220) plane at the temperature of 600 1C, which may be responsible for the increase of the resistance. A minimum sheet resistance of 5.10 Ω/□ was achieved when heated at 600 1C for 5 min, while the highest transmittance (81.23%) was exhibited when heated at 400 1C for 5 min. The optimal overall performance of Ag-based Low-E films is obtained when heated at 600 1C for 5 min. The high-temperature resistance of the Ag-based films has been improved greatly with this modified multilayer construction.
Acknowledgment The authors would like to thank the financial supports from the National Natural Science Foundation of China (No. 50972126), National Science and technology support program of China during the 12th Five-Year Plan (No. 2011BAE14B02), the Key Project of Research Program on Applied Fundamentals of Hebei Province (No.13961106D) and the Science Foundation of Yanshan University for the Excellent Ph.D. Students (No. 201203). References [1] Oh SH, Lee S-M, Choi KC. Thin Solid Films 2012;520:3605. [2] Hong CH, Jo YJ, Kim HA, Lee I-H, Kwak JS. Thin Solid Films 2011;519:6829. [3] Oh D, No YS, Kim SY, Cho WJ, Kwack KD, Kim TW. J Alloy Compd 2011;509:2176. [4] Qi JH, Li Y, Duong TT, Choi HJ, Yoon SG. J Alloy Compd 2013;556:121. [5] Sahu DR, Chen CY, Lin SY, Huang JL. Thin Solid Films 2006;515:932.
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[6] Sutthana S, Hongsith N, Choopun S. Curr Appl Phys 2010;10:813. [7] Sahu DR, Lin SY, Huang JL. Sol Energy Mater Sol C 2007;91:851. [8] Park HK, Kang JW, Na SI, Kim DY, Kim HK. Sol Energy Mater Sol C 2009; 93: 1994. [9] Choi KH, Jeon S, Kim HK. Mater Res Bull 2012;47:2915. [10] Jung YS, Kim KH. Mater Res Bull 2012;47:2895. [11] Neghabi M, Behjat A, Ghorashi SMB, Salehi SMA. Thin Solid Films 2011;519:5662. [12] Long GY, Geng YY. Appl Surf Sci 2012;263:546. [13] Liu Y, Wang C, Diao XG, Xue YF. Vacuum 2012;86:2040.
[14] [15] [16] [17] [18]
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Kato K, Omoto H, Takamatsu A. Vacuum 2010;84:587. Tsuda Y, Omoto H, Tanaka K, Ohsaki H. Thin Solid Films 2006;502:223. Arbab M. Thin Solid Films 2001;381:15. Chen CH, Kelder EM, Schoonman J. Thin Solid Films 1999;342:35. Aoshima Y, Miyazaki M, Sato K, Akao Y, Takaki S, Adachi K. Jpn J Appl Phys 2000;39:4884. [19] Ihlo L, Schmidt U, Comtesse R, Heinz S. United States, US 2009/0186213 A1, 23/ 7/2009. [20] Szczyrbowski J, Dietrich A, Hartig K. Sol Energy Mater 1989;19:43.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Improvement of high-temperature resistance of the Ag-based multilayer films deposited by magnetron sputtering Jingkai Yang n, Hongli Zhao, Hesong Sha, Jian Li, Liping Zhao, Jiajia Chen, Bin Yu, Fucheng Zhang State Key Laboratory of Metastable Materials Science and Technology, College of Materials Science and Engineering, Yanshan University, Qinhuangdao 066004, China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 September 2013 Accepted 7 December 2013 Available online 16 December 2013
Ag-based films with the multilayer construction of top-Si3N4/SnO2/NiCrOx/Ag/ZnO/NiCrOx/TiO2/underSi3N4 were deposited on glass substrates by magnetron sputtering at room temperature, and then heated at 200 1C, 400 1C, 600 1C, 650 1C and 700 1C for 5 min in the air. The effects of post-heated temperature on the optical and electrical properties of the advanced Ag-based Low-E glass were investigated. The results show that the multilayer films mainly contain a crystalline Ag layer with 3C structure and other amorphous layers. A transformation of the preferred orientation from (111) plane to (220) plane occurs at the temperature of 600 1C. The optimal overall performance of Ag-based Low-E films is obtained when heated at 600 1C for 5 min with the minimum sheet resistance of 5.10 Ω/□ and a lowest emissivity of 0.06 and the transmittance in the visible region of 79.40%. The high-temperature resistance of Ag-based films has been improved with this modified kind of multilayer construction. & 2013 Elsevier B.V. All rights reserved.
Keywords: Thin films Ag-based X-ray diffraction Electrical properties Optical properties
1. Introduction Silver-based multilayer films with low infrared emissivity have excellent optoelectronic properties of heat insulation, solar energy reflection and electrical conductivity. They are usually applied in flat panel displays, solar cells, architectural and vehicle glass as spectral selective filters to control sunlight incidence and reflection and preserve thermal energy [1–4]. The Low-E films are usually prepared by a magnetron sputtering method with dielectric–metal– dielectric (D/M/D) type of multilayer structures, such as ITO/Ag/ITO [1,2], CuAlO2/Ag/CuAlO2 [3], ZnO/Ag/ZnO [5], AZO/Ag/AZO (Aldoped ZnO) [4,6–8], GZO/Ag/GZO [9,10], ZnS/Ag/ZnS [11,12], TiO2/ Ag/Ti/TiO2/SiON [13], and so on. Silver is often sandwiched within dielectric layers due to its lower absorbance (o5%) in the visible light region, high spectral reflection and low thermal emissivity. Metal oxides are chosen as the dielectric layers to protect Ag layer from the chemical and mechanical damage. In addition, they act as adhesion layers, nucleation templates and diffusion barriers. Most attention is paid on how to enhance the optical and electrical properties by changing the substrate temperature [5,7], the thickness of every layer [3,6,8,13], cathode voltage [14] and the underlayer [15,16].
n
Corresponding author. Tel.: þ 86 13903339287; fax: þ86 335 8050727. E-mail addresses: [email protected] (J. Yang), [email protected] (H. Zhao). 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.12.033
It is well known that Ag-based Low-E films are not durable against mechanical abrasion, chemical agents and high temperature treatment. For many architectural applications, these glasses usually undergo various secondary processing, such as bending/vacuum forming and, most importantly, thermal toughening, to enhance the fracture behavior for avoiding breakage into large and sharp fragments. This processing demands that Ag-based Low-E films must withstand these high temperatures without trading-off functionality. Therefore, it is necessary to investigate the effects of heat treatment on the optical and electrical properties. Sahu and co-workers [5] found that the sheet resistance of ZnO/Ag/ZnO multilayer films increased when the heated temperature was higher than 300 1C in the air atmosphere, while Neghabi and co-workers [11] found that the sheet resistance of ZnS/Ag/ZnS films increased when the temperature was higher than 200 1C. Long et al. [12] reported that ZnS– SiO2/Ag/ZnS–SiO2 films exhibited a low sheet resistance of 9.7 Ω/□ and a high average transmittance of 84.1% with the heated temperature of 200 1C, and the degradation of optoelectronic performances occurred when the temperature was 300 1C. However, on the basis of guarantee the photoelectric performance, the highest temperature in current research results (300 1C) cannot meet the needs of the toughened glass (often up to 700 1C). Therefore, it is imperative to develop a new system of Ag-based multilayer films to improve the high-temperature resistance of the Ag-based films. In this study, Ag-based Low-E films with a new layer construction of top-Si3N4/SnO2/NiCrOx/Ag/ZnO/NiCrOx/TiO2/under-Si3N4 were deposited on unheated glass substrates by vacuum magnetron
J. Yang et al. / Materials Letters 118 (2014) 62–65
sputtering technique, and then heated at 200 1C, 400 1C, 600 1C, 650 1C and 700 1C for 5 min in the air. The effects of heat treatment temperature on the structural, electrical and optical properties of Ag-based Low-E films were investigated.
2. Experimental The layer construction of Ag-based Low-E glass is top-Si3N4/ SnO2/NiCrOx/Ag/ZnO/NiCrOx/TiO2/Si3N4/glass. The functional Ag layer of 12.2 nm was deposited in Ar gas with a pure Ag target. Si3N4 layer was deposited in N2 gas using a Si target, while Sn, NiCr and Zn were used as the cathode materials for the corresponding oxide layers in O2 gas, and TiO2 layer were deposited from two tubular targets made of TiOx ceramic in a working gas consisting of an Ar/O2 mixture. The base pressure and gas pressure were 5 10-4 Pa and 1.5 105 Pa, respectively. Argon with purity of 99.996% was used as working gas, while nitrogen and oxygen with purity of 99.99% were used as reactive gas. The thickness of each layer was adjusted by the substrate conveyance speed. Finally, Ag-based Low-E films with a layer construction of top-22 nm Si3N4/13 nm SnO2/1.1 nm NiCrOx/12.2 nm Ag/4.6 nm ZnO /1.1 nm NiCrOx/ 16.5 nm TiO2/ under-12 nm Si3N4 were deposited on glass by vacuum magnetron sputtering technique with the total film thickness of 82.5 nm. Heat treatment of the as-deposited Ag-based Low-E glass was carried out in resistance furnace (KLX-12B) at 200 1C, 400 1C, 600 1C and 700 1C for 5 min, respectively. Then the samples were cooled to room temperature in the furnace. The structure of as-deposited and post-heat treated Low-E glass was studied by X-ray Diffraction (D/MAX-2500HBþ/PC) using the Cu Kα radiation (λ ¼0.15406 nm). Sheet resistance was determined using the four-point probe method. The optical transmittance spectra in the visible range of 190–900 nm at different heat treatment temperatures were measured by a double-beam UV–vis spectrophotometer (WFZ-26 A).
3. Results and discussion The X-ray diffraction patterns of the as-deposited and posttreated Ag-based films at different temperatures for 5 min are shown in Fig. 1. It can be seen that there are only two main diffraction peaks at about 38.21 and 64.61, corresponding to (111) and (220) plane of silver with 3C structure. The broad peak around 24.51 is related to the glass substrate. There was no XRD peak corresponding to other layers of the Si3N4, SnO2, NiCrOx, ZnO and
63
TiO2 layers observed, indicating that the as-grown other films are amorphous. Therefore, the multilayer films mainly containing a crystalline Ag layer and other amorphous layers when deposited on glass substrates by vacuum magnetron sputtering technique. When the temperature is high to 600 1C, 650 1C and 700 1C, two very weak peaks at 48.11 and 52.21 occur and correspond to the diffractions of Ag3O4, which indicates that a little oxidation happens to the functional Ag layer at such high temperatures. As seen in Fig. 1, it is obvious that the relative intensity of (111) plane decreases, while that of (220) plane increases, with the increasing temperature. In order to analyze the preferred orientation more quantitatively, the integrated intensity ratio η(111) of I(111)/[(I(111) þI(220))] was introduced, where I(111) and I(220) is the integrated intensities of (111) and (220) planes, respectively. η(111) according to the standard intensity given in the ASTM cards (No. 04-0783) is 0.71. If η(111) is greater than 0.71, (111) plane is the preferred orientation, otherwise, (220) plane is the preferred orientation. The integrated intensity ratio η(111), as listed in Table 1, is larger than 0.71 when the temperature is lower than 400 1C, indicating Ag grains in the multilayer films possess the (111) preferred orientation. When the temperature is equal to or higher than 600 1C, η(111) is smaller than 0.71, in addition, the values of η(111) decrease sharply with the increasing temperature (higher than 600 1C). Therefore, Ag grains in the multilayer films heated at higher temperatures possess an obviously (220) preferred orientation. The average crystallite sizes for the as-deposited and postheated films are calculated in planes with Miller indices (111) and (220) according to the Scherrer equation [17] and listed in Table 1. The crystallite size of the as-deposited film is 10.09 nm and increases to 11.34 nm after post-heated at 600 1C for 5 min. The growth rate is relatively slow. However, when heated at 700 1C for 5 min, the crystallite size increases rapidly to 15.63 nm and is bigger than the thickness of Ag layer. The sheet resistance of Ag-based Low-E films heated at different temperatures for 5 min are shown in Fig. 2. The sheet resistance Rs decreases from 6.28 Ω/□ for the as-deposited film to 5.10 Ω/□ for the films heated at 600 1C for 5 min, then increases sharply to 20.95 Ω/□ when heated at 700 1C for 5 min. The sheet resistance before 600 1C declines slightly. That is because the crystallinity and the average crystallite size increases with the increasing temperature, the defects in the films decreases, reducing the electron scattering, and the conductivity of the films is improved. The rapid increase of sheet resistance occurs when the temperature is higher than 600 1C. Combined with the XRD results, the average crystallite size (12.91 nm for 650 1C, 15.63 nm for 700 1C) is larger than the thickness of Ag layer (12.2 nm), which makes the growth of Ag grain crystal limited seriously. Moreover, the growth along the (220) preferred orientation, which is perpendicular to surface, increases sharply. Both of the variations lead a non-uniform structure of the Ag-based multilayer films and make the resistance increased. Maybe there is severe inter-diffusion of atoms which then increases the sheet resistance of the multilayer [18]. Compared with the results of
Table 1 The integrated intensity ratio η(111) and average crystalline size of Ag-based Low-E films, as-deposited and post-heated at 200 1C, 400 1C, 600 1C, 650 1C, 700 1C for 5 min, respectively.
Fig. 1. X-ray diffraction patterns of Ag-based Low-E films, as-deposited and postheated at 200 1C, 400 1C, 600 1C, 650 1C, 700 1C for 5 min, respectively.
Heat treatment temperature
η(111) Average crystalline size (nm)
Infrared reflectivity (RIR)
Calculated emissivity ε
Transmittance at 550 nm
as-deposited 200 1C 400 1C 600 1C 650 1C 700 1C
0.77 10.09 0.80 10.50 0.73 10.58 0.69 11.34 0.29 12.91 0.21 15.63
93.66% 93.93% 94.51% 94.81% 92.03% 81.01%
0.08 0.07 0.07 0.06 0.10 0.24
76.81% 78.16% 81.23% 79.40% 72.97% 65.17%
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J. Yang et al. / Materials Letters 118 (2014) 62–65
Fig. 2. Sheet resistance of Ag-based Low-E films with a function of heated temperature.
film, decreases to 0.06 when heated at 600 1C for 4 min, and then increases to 0.24 when the heated temperature reaches 700 1C. The effect of heat preservation and heat insulation of Ag-based films has been damaged greatly when heated at 700 1C. The transmittance curves of Ag-based Low-E films is shown in Fig. 3. Compared with the as-deposited glass, the transmittance in the visible range increases as the temperature increases to 400 1C. When heated at 400 1C for 5 min, the Ag-based Low-E films have the highest transmittance of 81.23%. That is because of the high crystallinity and low grain boundary scattering of the polycrystalline films with the increasing temperature. When heated at 600 1C for 5 min, the transmittance decreases a little to 79.40%. When the temperature reaches 700 1C, the transmittance decreases sharply to 65.17%, which is due to the increased scattering resulted from the abnormal grain growth and the non-uniform structure of the films. In this work, a minimum sheet resistance of 5.10 Ω/□ and a lowest emissivity of 0.06 is achieved when heated at 600 1C for 5 min, although the transmittance in the visible region is a little lower (1.83%) than that of Ag-based films heated at 400 1C for 5 min. Therefore, the Ag-based Low-E films heated at 600 1C for 5 min possess the optimal optoelectronic property. It indicates that the temperature of high-temperature resistance of Ag-based films has been increased to 600 1C. Therefore, the top-Si3N4/SnO2/ NiCrOx/Ag/ZnO/NiCrOx/TiO2/under-Si3N4 multilayer is a promising transparent conductive film with low sheet resistance, high transmittance, and high heat stability.
4. Conclusions
Fig. 3. Transmittance curves in the visible range of Ag-based Low-E glass, as-deposited and post-heated at 200 1C, 400 1C, 600 1C, 650 1C, 700 1C for 5 min, respectively.
Sahu et al. [5] and Neghabi et al. [11], the high-temperature resistance of the Ag-based films has been improved greatly with this modified multilayer construction of top-Si3N4/SnO2/NiCrOx/Ag/ZnO/NiCrOx/ TiO2/under-Si3N4 and the functional Ag layer obtains a high stability. In this heat-resistant Ag-based Low-E multilayer system, NiCrOx layer serves as antiscattering layer placed between the high-refringence layer TiO2 and the wetting layer ZnO. On the other hand, during the high-temperature toughening operation, the incomplete oxidation layer NiCrOx can protect the functional Ag layer from oxidation by the oxygen diffusion from the underlayers or the upperlayers and make the Ag layer more stable, which is in agreement with others' results [19]. Therefore, the high-temperature resistance of this modified Ag-based Low-E multilayer films has been improved. The infrared reflectivity RIR and the emissivity ε of the coated glass can be calculated with the sheet resistance Rs and may be written as follows: RIR ¼ ð1 þ 0:0053Rs Þ 2
ð1Þ
ε ¼ 0:0129Rs 6:7 10 5 Rs 2
ð2Þ
The error of ε calculated from Eq. (2) is below 2% for values discussed here [20]. And the values of calculated RIR and ε are listed in Table 1. The infrared reflectivity almost keeps almost invariable when the heated temperature is lower than 600 1C, and then decreases sharply to 81.01% with the heated temperature of 700 1C for 5 min. The calculated emissivity ε is 0.08 for as-deposited
High stable transparent and conductive Ag-based low emissivity films deposited on float glass by magnetron sputtering were treated at different temperatures from 200 1C to 700 1C for 5 min, and then cooled to room temperature in the resistance furnace. The layer construction of Ag-based films is top-Si3N4/SnO2/NiCrOx/ Ag/ZnO/NiCrOx/TiO2/under-Si3N4. The multilayer films contain a crystalline Ag layer and other amorphous layers with a transformation of the preferred orientation from (111) plane to (220) plane at the temperature of 600 1C, which may be responsible for the increase of the resistance. A minimum sheet resistance of 5.10 Ω/□ was achieved when heated at 600 1C for 5 min, while the highest transmittance (81.23%) was exhibited when heated at 400 1C for 5 min. The optimal overall performance of Ag-based Low-E films is obtained when heated at 600 1C for 5 min. The high-temperature resistance of the Ag-based films has been improved greatly with this modified multilayer construction.
Acknowledgment The authors would like to thank the financial supports from the National Natural Science Foundation of China (No. 50972126), National Science and technology support program of China during the 12th Five-Year Plan (No. 2011BAE14B02), the Key Project of Research Program on Applied Fundamentals of Hebei Province (No.13961106D) and the Science Foundation of Yanshan University for the Excellent Ph.D. Students (No. 201203). References [1] Oh SH, Lee S-M, Choi KC. Thin Solid Films 2012;520:3605. [2] Hong CH, Jo YJ, Kim HA, Lee I-H, Kwak JS. Thin Solid Films 2011;519:6829. [3] Oh D, No YS, Kim SY, Cho WJ, Kwack KD, Kim TW. J Alloy Compd 2011;509:2176. [4] Qi JH, Li Y, Duong TT, Choi HJ, Yoon SG. J Alloy Compd 2013;556:121. [5] Sahu DR, Chen CY, Lin SY, Huang JL. Thin Solid Films 2006;515:932.
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