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glass thin films deposited by sputtering
Solid State Sciences 13 (2011) 1984e1988
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Low emissivity Ag/Si/glass thin films deposited by sputtering Sun Ho Parka, Kee Sun Leea, b, *, A. Sivasankar Reddyb a b
Division of Advanced Materials Engineering, Kongju National Univ., Budaedong, Cheonan City, South Korea Green Home Energy Technology Center, Cheonan City, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 March 2011 Received in revised form 1 August 2011 Accepted 30 August 2011 Available online 7 September 2011
Si was deposited on the glass substrate as an interlayer for the 2-D growth of Ag thin films because Si has a strong binding energy against Ag and can lead to a negative surface energy change in Ag/glass. The Si interlayer induced an extremely smooth and flat Ag surface, which effectively reduced the resistance and enhanced the reflectance in the IR region. In particular, Ag (9 nm)/Si (3 nm)/glass showed 0.074 emissivity and w91% reflectance in the IR region with 67% transmittance in the visible region. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Interlayer Low emissivity Reflectance Sputtering
1. Introduction Currently, low emissivity building glazing is in the spotlight because it effectively reduces energy loss from the building. For glazing, typical glass coatings are based on multilayer silver thin films. The critical properties are low emissivity (low-e), high reflectance of solar infrared radiation and high transparency in the visible range of the spectrum [1,2]. Generally, low-e coating structures have transparent dielectric/Ag/dielectric multilayer films [3]. Silver is widely used as the reflectance layer due to its good electrical and optical properties [4-6]. The properties of silver thin films depend on whether they are grown in two-dimensions (2-D) or three-dimensions (3-D). The silver films are typically grown via a 3D VolmereWeber mechanism [7] due to their poor wettability on glass and tend to agglomerate at a relatively low temperature [8]. The agglomeration is caused by the high silver/glass interfacial energy [9]. Therefore, various routes for inhibition of agglomeration through 2-D growth have been suggested [10-13]. The growth of the thin film is determined by the change of the surface energy [14], which can be modified by an interlayer between the Ag thin film and glass substrate. In this study, Si was applied as the interlayer because Si has a strong binding energy against Ag and could lead to a negative surface energy change in the Ag/glass. In
* Corresponding author. Division of Advanced Materials Engineering, Kongju National Univ., Budaedong, Cheonan City, South Korea. Tel.: þ82 41 521 9375; fax: þ82 41 568 5776. E-mail address: [email protected] (K.S. Lee). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.08.029
addition, the effect of the Si interlayer on the electrical and optical properties of the Ag/Si/glass was investigated. 2. Experimental details Ag/Si/glass films were deposited by DC magnetron sputtering with silicon (3 inch) and silver (2 inch) targets (99.99%). The substrate was rinsed in an H2SO4:H2O2 (2:1) acid bath, then rinsed in isopropyl alcohol and dried under flowing dry nitrogen. Prior to deposition, the process chamber was evacuated until the base pressure was less than 6.6 105 Pa. The distance between target and substrate was 10 cm. The working pressure was set at 0.6 Pa and the sputtering power was 40 W and 30 W for the Si and Ag targets, respectively. The substrate was kept at room temperature. The film thickness was measured by the alpha step. The microstructures were observed using a field emission scanning electron microscope (FESEM). The surface morphology was analyzed using an atomic force microscope (AFM). An ultravioletevisibleenear infrared (UVeViseNIR) spectrometer was used to measure both the transmittance and reflectance within the range of w300e2400 nm. The electrical resistance was measured using the 4 point probe method. 3. Results and discussion 3.1. Transmittance of Si thin films with different thicknesses To use Si as an interlayer, a higher transmittance in the visible range is desirable. The transmittances of Si thin films with different
S.H. Park et al. / Solid State Sciences 13 (2011) 1984e1988
1985
thicknesses deposited on glass were evaluated, as shown in Fig. 1. Considering that soda-lime glass has a transmittance of 90% (l ¼ 550 nm), the Si interlayer (1e5 nm thick) had a fairly high transmittance of more than 80%, which provides an advantage over other metals with poor transmittance for application as the interlayer. 3.2. Nanostructure evolution of Ag/Si/glass
Fig. 1. Transmittance of Si films with various thicknesses.
The nanostructures of the Ag thin films deposited on the Si interlayer were observed, as shown in Fig. 2. The Si (1, 3 nm)/glass films exhibited dense and smooth surface morphologies. Without a Si interlayer, the Ag thin film exhibited agglomerated nanocrystals with seemingly thick boundaries. However, Ag(9 nm)/Si(1, 3 nm)/glass featured an extremely dense and smooth surface morphology without agglomeration. In order to analyze the nanostructures in detail, AFM observation was performed, as shown in Fig. 3. From the histogram, the average diameter of the Ag crystals was w2e3 nm and the surface roughness was all most two times lower than the Ag/glass without an Si interlayer (Table 1), which was consistent with the SEM images. Overall, it is evident that the Si interlayer induced the formation of a dense and smooth Ag thin film. To further understand the effect of the Si interlayer, the surface and interface energies were calculated based on the inter-atomic binding energy. Table 2 shows various Ag-material binding
Fig. 2. SEM images of surface morphology of Ag thin films (a) Ag (9 nm), (b) Ag (9 nm)/Si (1 nm) (c) Ag (9 nm)/Si (3 nm), (d) Si (1 nm), (e) Si (3 nm) and AFM images (f) Si (1 nm) (g) Si (3 nm).
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S.H. Park et al. / Solid State Sciences 13 (2011) 1984e1988
Fig. 3. AFM image and histogram of (a)Ag (9 nm), (b)Ag (9 nm)/Si (1 nm) and (c)Ag (9 nm)/Si(3 nm).
energies. The binding energy of AgeSi was 185.1 9.6 kJ/mol, which was higher than the binding energy of AgeAg at 162.9 2.9 kJ/mol [15]. Bauer’s criterion [14] indicates that 2-dimensional growth (2-D growth) or 3-dimensional growth (3-D growth) of the thin film is determined by the following the surface energy formula:
Dg ¼ gmetal þ gmetalesubstrate gsubstrate
(1)
Table 1 Analysis on the surface morphology of Ag(9 nm)/Si/glass with different Si film thicknesses. Ag/Si thickness Ag(9 nm) Ag(9 nm)/Si(1 nm) Ag(9 nm)/Si(3 nm)
Average crystal diameter (nm)
Roughness (nm)
5 3 2
1.575 0.715 0.637
where gmetal, gmetalesubstrate, gsubstrate (J/m2) were the surface and interface free energies of the Ag, Ag/(Si, glass), and Sieglass substrate, respectively. For Dg > 0, 3-D island growth occurs; otherwise, 2-D layer growth occurs. The surface free energies of most metals (0.1e4.0 J/m2) [16] are usually higher than that of soda-lime glass (0.3e0.4 J/m2) [17]. The interfacial energies of AgeSi and Ageglass were calculated using the following equation [18]:
Table 2 Binding energy of Age(Si, Ag, Ge, Cu).
Peak-to-valley (nm)
Materials
Binding energy (BE298 KJ/mol)
13.961 9.763 8.239
AgeSi AgeAg AgeGe AgeCu
185.1 162.9 174.5 171.5
9.6 2.9 21 9.6
S.H. Park et al. / Solid State Sciences 13 (2011) 1984e1988 Table 3 Surface & interface energy of Ag, Si and glass.
1987
Table 4 The data analysis on the surface morphology and resistance of thin films.
Unit
Glass
Ag
Si
Ag/Si
Ag/glass
J/m2
0.325
1.172
1.721
0.433
0.244
Parameters
Ag (9 nm)/ glass
Ag (9 nm)/Si (1 nm)/glass
Ag (9 nm)/Si (3 nm)/glass
RMS roughness, Rrms Peak-to-valley Sheet resistance, R,
1.575 nm 13.961 nm 11.7 U/,
0.715 nm 9.763 nm 6.9 U/,
0.637 nm 8.239 nm 5.92 U/,
negative over the critical Si coverage, leading to a transition from 3D to 2-D growth. Considering that the atomic size of Si is 0.146 nm, a w1e3 nm thick Si layer corresponds to a w6.8e20.5 mono atomic layer. Based on SEM and AFM images of Si (1, 3 nm)/glass films, the glass substrate was presumed to be covered by silicon layer. This result confirmed that 2-D growth of the Ag thin film was more favorable on the Si interlayer as compared to on a glass substrate. These results were comparable to those for Ag/Ge [11]. 3.3. Electrical and optical properties
Fig. 4. Surface energy changes versus Si coverage on the substrate.
gSolid=Susbtrate ¼ ggeometrical þ gchemical Solid=Susbtrate Solid=Susbtrate
geometrical geometrical ¼ 0:15 gSolidðAgÞ=Vapor þ gSolidðglass; SiÞ=Vapor þ gchemical SolidðAgÞ=Substrateðglass; SiÞ
ð2Þ
where ggeometrical and ggeometrical are the surface enerSolidðAgÞ=Vapor Solidðglass; SiÞ=Vapor gies of the metal Ag/vapor and the glass, Si/vapor at 0 K (273.15 C). The chemical contribution, gchemical , is SolidðAgÞ=Substrateðglass; SiÞ very small compared to ggeometrical [18,19]. The surface SolidðAgÞ=Substrateðglass; SiÞ
The electrical and emissivity of Ag/glass, Ag/Si(1, 3 nm)/glass were evaluated, as shown in Fig. 5. The sheet resistance of Si(1, 3 nm)/glass films was above the measurable range. The resistance abruptly reduced at the thicknesses ranging from 6 to 9 nm and slowly reduced after that. In Ag(9 nm)/Si(1, 3 nm)/glass, the resistances were 36.9, 20.6 U/,, respectively, compared to 151 U/, for Ag(6 nm)/glass, which confirmed that Si was effective at the initial stage of the thin film growth. Considering that a 25 nm thick poly-Si thin film had a resistance of w3 106 U/, [21], the resistance of Ag/Si/glass was presumed not to be influenced by the resistance of the Si thin film. Such a difference in the resistance indicates that the Si interlayer contributed to the densification at the initial stage of the thin film growth, which is consistent with the result that the Si interlayer induced 2-D growth of the thin film to produce dense Ag films. The resistivity of the thin film is determined using the following resistance factors [22]:
r ¼ r0 þ rGB þ rSS þ rSR
(3)
energy of Si is 1.721 J/m2 [20]. The interfacial energies of Ag/Si and Ag/glass were 0.433 J/m2 and 0.244 J/m2, respectively (Table 3), and then Dg was calculated by substituting their values into Bauer’s criterion. Fig. 4 shows the change in Dg as a function of the Si coverage. Dg was positive until Si coverage reached 87%, which indicates 3-D growth of the Ag thin films. However, Dg was
where r0 is the bulk resistivity, rGB is the grain boundary resistivity, rSS is the resistivity due to surface scattering, and rSR is the resistivity due to surface roughness. An analysis of the surface morphology and resistance are summarized in Table 4. The Si interlayer significantly reduced the surface roughness of the
Fig. 5. Resistanceeemissivity versus the thickness of Ag in Ag/glass and Ag/Si/glass.
Fig. 6. Changes in the transmittance and reflectance as a function of wavelength in Ag/ glass and Ag/Si/glass.
1988
S.H. Park et al. / Solid State Sciences 13 (2011) 1984e1988
overlaid Ag film, indicating a dense grain boundary and smooth surface. These factors resulted in the lower resistance of the thin film. Emissivity depends on the resistance and can be calculated using the following equation [19]:
e ¼ 0:029$R, 6:7 105 R2
Ag thin film, which was due to 2-D growth as a result of the negative surface free energy change. Ag(9 nm)/Si(3 nm)/glass showed a lower resistance of 5.92 U/,, leading to a lower emissivity of 0.074 and w91% reflectance in the IR region with 67% transmittance in the visible region, which are promising properties compared to Ag/glass without an Si interlayer.
(4)
The thickness of the Ag thin film that corresponds to an emissivity of 0.1 [23], which is known to be a generally good emissivity level, was reduced from 10.5 to 8.5 nm with a 3 nm thick Si interlayer (Fig. 5). The emissivity of Ag(9 nm)/Si(1 nm) and Ag(9 nm)/ Si(3 nm) were 0.085 and 0.074, respectively, compared to 0.14 for Ag(9 nm)/glass. With an Ag thickness greater than 12 nm, the emissivity did not absolutely depend on the Si interlayer. The thicker Ag thin film had a lower emissivity at the expense of the transmittance. An Ag thin film with a smaller thickness is desirable. The Si interlayer significantly influenced the emissivity as well as the resistance at the initial stage of the thin film growth, which originated from the nanostructural differences. Therefore, it is evident that the Si interlayer effectively changed the nanostructure of the thin films and reduced the resistance and emissivity. Likewise, the Si interlayer was expected to influence the transmittance. Fig. 6 shows the transmittance and reflectance of Ag/Si/ glass as a function of the wavelength of incident light. In the visible region (l ¼ 550 nm), the transmittance was w48% for Ag(9 nm)/ glass and w67% for Ag(9 nm)/Si(3 nm)/glass, which was attributed to the induced formation of a dense Ag thin film by the Si interlayer. In general, a transmittance higher than 60% in the visible region and a lower transmittance in the infrared region are required [24]. In addition, the higher reflectance (the reciprocal of transmittance) in the range of w800e2400 nm could effectively screen solar radiation. The reflectance at wavelengths of w1000e2400 nm was w71e86% for Ag(9 nm)/glass and w60e91% for Ag(9 nm)/Si(3 nm)/ glass. To obtain good reflectance in the infrared region, the resistance should be lower. Therefore, the reduction of the resistance induced by the Si interlayer resulted in the high reflectance. In summary, the Si interlayer induced the formation of a dense Ag thin film by 2-D growth, leading to high transmittance in the visible region and high reflectance in the infrared region. 4. Conclusions The effect of the Si interlayer on Ag thin film growth in Ag/Si/ glass films was investigated. The Si interlayer both significantly reduced the surface roughness and induced the densification of the
Acknowledgments This work was supported by the Grant of the Korean Ministry of Education, Science and Technology (The Regional Core Research Program/Zero Energy Green Village Technology Center). This work was supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0028289). References [1] J.C.C. Fan, Appl. Phys. Lett. 25 (1974) 693e695. [2] R.J. Martin-Palma, L. Vazques, J.M. Martinez-Duart, Sol. Energy Mater. Sol. Cells 53 (1998) 55e66. [3] M. Arbab, MRS Bull. 22 (1997) 27e35. [4] T. Suzuki, Y. Abe, M. Kawamura, K. Sasaki, T. Shouzu, T. Kawamata, Vaccum 66 (2002) 501e504. [5] P. Taneja, P. Ayyub, R. Chandra, Phys. Rev. B. 65 (2002) 245412e245416. [6] M. Del Re, R. Gouttebaron, J.P. Dauchot, P. Leclere, R. Lazzaroni, M. Wautelet, M. Hecq, Sur. Coat. Technol. 151e152 (2002) 86e90. [7] C.T. Campbell, Surf. Sci. Rep. 27 (1997) 1. [8] H.C. Kim, T.L. Alford, D.R. Allee, Appl. Phys. Lett. 81 (2002) 22e24. [9] H.C. Kim, T.L. Alford, Appl. Phys. Lett. 81 (2002) 4287e4289. [10] A. Anders, E. Byon, D.H. Kim, K. Fukudaa, S.H.N. Lima, Solid State Commun. 140 (2006) 225e229. [11] V.J. Logeeswaran, N.P. Kobayashi, M.S. Islam, W. Wu, P. Chaturvedi, N.X. Fang, S.Y. Wang, R.S. Williams, Nano Lett. 9 (2009) 178e182. [12] M. Pivetta, F. Patthey, W.D. Schneider, Surf. Sci. 532e535 (2003) 58e62. [13] G. Leftheriotis, S. Papaefthimiou, P. Yianoulis, Solid State Ionics 136e137 (2000) 655e661. [14] E. Bauer, Z. Kristallogr 110 (1958) 423. [15] D.R. Lide, CRC Handbook of Chemistry and Physics. CRC Press, Boca Raton, FL, 2008. [16] L. Vitos, A. Ruban, H. Skriver, J. Kollar, Surf. Sci. 411 (1998) 186e202. [17] M.O. Prado, C. Fredericci, E.D. Zanotto, J. Non-Crystalline Solids 331 (2003) 145e156. [18] M. Zhao, B.X. Liu, Metall. Mater. Trans. A 41 (2010) 2484. [19] A.R. Miedema, Z. Metallkd 69 (1978) 455e461. [20] J. Szczyrbowski, A. Dietrich, K. Hartig, Solar Energy Mater. 16 (1987) 103e111. [21] T. Ueda, K. Kuribayashi, S. Hasegawa, International Symposium on Micromechatronics and Human Science, 1999. [22] P. Wissmann, H.-U. Finzel, Electrical Resistivity of Thin Metal Films. SpringerVerlag, Berlin and Heidelberg, 2007. [23] J. Szczyrbowski, G. Braèuer, M. Ruske, H. Schilling, A. Zmelty, Thin Solid Films 351 (1999) 254e259. [24] H.J. Gläser, Opt. Soc. America 47 (2008) C193eC199.
Contents lists available at SciVerse ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Low emissivity Ag/Si/glass thin films deposited by sputtering Sun Ho Parka, Kee Sun Leea, b, *, A. Sivasankar Reddyb a b
Division of Advanced Materials Engineering, Kongju National Univ., Budaedong, Cheonan City, South Korea Green Home Energy Technology Center, Cheonan City, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 March 2011 Received in revised form 1 August 2011 Accepted 30 August 2011 Available online 7 September 2011
Si was deposited on the glass substrate as an interlayer for the 2-D growth of Ag thin films because Si has a strong binding energy against Ag and can lead to a negative surface energy change in Ag/glass. The Si interlayer induced an extremely smooth and flat Ag surface, which effectively reduced the resistance and enhanced the reflectance in the IR region. In particular, Ag (9 nm)/Si (3 nm)/glass showed 0.074 emissivity and w91% reflectance in the IR region with 67% transmittance in the visible region. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Interlayer Low emissivity Reflectance Sputtering
1. Introduction Currently, low emissivity building glazing is in the spotlight because it effectively reduces energy loss from the building. For glazing, typical glass coatings are based on multilayer silver thin films. The critical properties are low emissivity (low-e), high reflectance of solar infrared radiation and high transparency in the visible range of the spectrum [1,2]. Generally, low-e coating structures have transparent dielectric/Ag/dielectric multilayer films [3]. Silver is widely used as the reflectance layer due to its good electrical and optical properties [4-6]. The properties of silver thin films depend on whether they are grown in two-dimensions (2-D) or three-dimensions (3-D). The silver films are typically grown via a 3D VolmereWeber mechanism [7] due to their poor wettability on glass and tend to agglomerate at a relatively low temperature [8]. The agglomeration is caused by the high silver/glass interfacial energy [9]. Therefore, various routes for inhibition of agglomeration through 2-D growth have been suggested [10-13]. The growth of the thin film is determined by the change of the surface energy [14], which can be modified by an interlayer between the Ag thin film and glass substrate. In this study, Si was applied as the interlayer because Si has a strong binding energy against Ag and could lead to a negative surface energy change in the Ag/glass. In
* Corresponding author. Division of Advanced Materials Engineering, Kongju National Univ., Budaedong, Cheonan City, South Korea. Tel.: þ82 41 521 9375; fax: þ82 41 568 5776. E-mail address: [email protected] (K.S. Lee). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.08.029
addition, the effect of the Si interlayer on the electrical and optical properties of the Ag/Si/glass was investigated. 2. Experimental details Ag/Si/glass films were deposited by DC magnetron sputtering with silicon (3 inch) and silver (2 inch) targets (99.99%). The substrate was rinsed in an H2SO4:H2O2 (2:1) acid bath, then rinsed in isopropyl alcohol and dried under flowing dry nitrogen. Prior to deposition, the process chamber was evacuated until the base pressure was less than 6.6 105 Pa. The distance between target and substrate was 10 cm. The working pressure was set at 0.6 Pa and the sputtering power was 40 W and 30 W for the Si and Ag targets, respectively. The substrate was kept at room temperature. The film thickness was measured by the alpha step. The microstructures were observed using a field emission scanning electron microscope (FESEM). The surface morphology was analyzed using an atomic force microscope (AFM). An ultravioletevisibleenear infrared (UVeViseNIR) spectrometer was used to measure both the transmittance and reflectance within the range of w300e2400 nm. The electrical resistance was measured using the 4 point probe method. 3. Results and discussion 3.1. Transmittance of Si thin films with different thicknesses To use Si as an interlayer, a higher transmittance in the visible range is desirable. The transmittances of Si thin films with different
S.H. Park et al. / Solid State Sciences 13 (2011) 1984e1988
1985
thicknesses deposited on glass were evaluated, as shown in Fig. 1. Considering that soda-lime glass has a transmittance of 90% (l ¼ 550 nm), the Si interlayer (1e5 nm thick) had a fairly high transmittance of more than 80%, which provides an advantage over other metals with poor transmittance for application as the interlayer. 3.2. Nanostructure evolution of Ag/Si/glass
Fig. 1. Transmittance of Si films with various thicknesses.
The nanostructures of the Ag thin films deposited on the Si interlayer were observed, as shown in Fig. 2. The Si (1, 3 nm)/glass films exhibited dense and smooth surface morphologies. Without a Si interlayer, the Ag thin film exhibited agglomerated nanocrystals with seemingly thick boundaries. However, Ag(9 nm)/Si(1, 3 nm)/glass featured an extremely dense and smooth surface morphology without agglomeration. In order to analyze the nanostructures in detail, AFM observation was performed, as shown in Fig. 3. From the histogram, the average diameter of the Ag crystals was w2e3 nm and the surface roughness was all most two times lower than the Ag/glass without an Si interlayer (Table 1), which was consistent with the SEM images. Overall, it is evident that the Si interlayer induced the formation of a dense and smooth Ag thin film. To further understand the effect of the Si interlayer, the surface and interface energies were calculated based on the inter-atomic binding energy. Table 2 shows various Ag-material binding
Fig. 2. SEM images of surface morphology of Ag thin films (a) Ag (9 nm), (b) Ag (9 nm)/Si (1 nm) (c) Ag (9 nm)/Si (3 nm), (d) Si (1 nm), (e) Si (3 nm) and AFM images (f) Si (1 nm) (g) Si (3 nm).
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S.H. Park et al. / Solid State Sciences 13 (2011) 1984e1988
Fig. 3. AFM image and histogram of (a)Ag (9 nm), (b)Ag (9 nm)/Si (1 nm) and (c)Ag (9 nm)/Si(3 nm).
energies. The binding energy of AgeSi was 185.1 9.6 kJ/mol, which was higher than the binding energy of AgeAg at 162.9 2.9 kJ/mol [15]. Bauer’s criterion [14] indicates that 2-dimensional growth (2-D growth) or 3-dimensional growth (3-D growth) of the thin film is determined by the following the surface energy formula:
Dg ¼ gmetal þ gmetalesubstrate gsubstrate
(1)
Table 1 Analysis on the surface morphology of Ag(9 nm)/Si/glass with different Si film thicknesses. Ag/Si thickness Ag(9 nm) Ag(9 nm)/Si(1 nm) Ag(9 nm)/Si(3 nm)
Average crystal diameter (nm)
Roughness (nm)
5 3 2
1.575 0.715 0.637
where gmetal, gmetalesubstrate, gsubstrate (J/m2) were the surface and interface free energies of the Ag, Ag/(Si, glass), and Sieglass substrate, respectively. For Dg > 0, 3-D island growth occurs; otherwise, 2-D layer growth occurs. The surface free energies of most metals (0.1e4.0 J/m2) [16] are usually higher than that of soda-lime glass (0.3e0.4 J/m2) [17]. The interfacial energies of AgeSi and Ageglass were calculated using the following equation [18]:
Table 2 Binding energy of Age(Si, Ag, Ge, Cu).
Peak-to-valley (nm)
Materials
Binding energy (BE298 KJ/mol)
13.961 9.763 8.239
AgeSi AgeAg AgeGe AgeCu
185.1 162.9 174.5 171.5
9.6 2.9 21 9.6
S.H. Park et al. / Solid State Sciences 13 (2011) 1984e1988 Table 3 Surface & interface energy of Ag, Si and glass.
1987
Table 4 The data analysis on the surface morphology and resistance of thin films.
Unit
Glass
Ag
Si
Ag/Si
Ag/glass
J/m2
0.325
1.172
1.721
0.433
0.244
Parameters
Ag (9 nm)/ glass
Ag (9 nm)/Si (1 nm)/glass
Ag (9 nm)/Si (3 nm)/glass
RMS roughness, Rrms Peak-to-valley Sheet resistance, R,
1.575 nm 13.961 nm 11.7 U/,
0.715 nm 9.763 nm 6.9 U/,
0.637 nm 8.239 nm 5.92 U/,
negative over the critical Si coverage, leading to a transition from 3D to 2-D growth. Considering that the atomic size of Si is 0.146 nm, a w1e3 nm thick Si layer corresponds to a w6.8e20.5 mono atomic layer. Based on SEM and AFM images of Si (1, 3 nm)/glass films, the glass substrate was presumed to be covered by silicon layer. This result confirmed that 2-D growth of the Ag thin film was more favorable on the Si interlayer as compared to on a glass substrate. These results were comparable to those for Ag/Ge [11]. 3.3. Electrical and optical properties
Fig. 4. Surface energy changes versus Si coverage on the substrate.
gSolid=Susbtrate ¼ ggeometrical þ gchemical Solid=Susbtrate Solid=Susbtrate
geometrical geometrical ¼ 0:15 gSolidðAgÞ=Vapor þ gSolidðglass; SiÞ=Vapor þ gchemical SolidðAgÞ=Substrateðglass; SiÞ
ð2Þ
where ggeometrical and ggeometrical are the surface enerSolidðAgÞ=Vapor Solidðglass; SiÞ=Vapor gies of the metal Ag/vapor and the glass, Si/vapor at 0 K (273.15 C). The chemical contribution, gchemical , is SolidðAgÞ=Substrateðglass; SiÞ very small compared to ggeometrical [18,19]. The surface SolidðAgÞ=Substrateðglass; SiÞ
The electrical and emissivity of Ag/glass, Ag/Si(1, 3 nm)/glass were evaluated, as shown in Fig. 5. The sheet resistance of Si(1, 3 nm)/glass films was above the measurable range. The resistance abruptly reduced at the thicknesses ranging from 6 to 9 nm and slowly reduced after that. In Ag(9 nm)/Si(1, 3 nm)/glass, the resistances were 36.9, 20.6 U/,, respectively, compared to 151 U/, for Ag(6 nm)/glass, which confirmed that Si was effective at the initial stage of the thin film growth. Considering that a 25 nm thick poly-Si thin film had a resistance of w3 106 U/, [21], the resistance of Ag/Si/glass was presumed not to be influenced by the resistance of the Si thin film. Such a difference in the resistance indicates that the Si interlayer contributed to the densification at the initial stage of the thin film growth, which is consistent with the result that the Si interlayer induced 2-D growth of the thin film to produce dense Ag films. The resistivity of the thin film is determined using the following resistance factors [22]:
r ¼ r0 þ rGB þ rSS þ rSR
(3)
energy of Si is 1.721 J/m2 [20]. The interfacial energies of Ag/Si and Ag/glass were 0.433 J/m2 and 0.244 J/m2, respectively (Table 3), and then Dg was calculated by substituting their values into Bauer’s criterion. Fig. 4 shows the change in Dg as a function of the Si coverage. Dg was positive until Si coverage reached 87%, which indicates 3-D growth of the Ag thin films. However, Dg was
where r0 is the bulk resistivity, rGB is the grain boundary resistivity, rSS is the resistivity due to surface scattering, and rSR is the resistivity due to surface roughness. An analysis of the surface morphology and resistance are summarized in Table 4. The Si interlayer significantly reduced the surface roughness of the
Fig. 5. Resistanceeemissivity versus the thickness of Ag in Ag/glass and Ag/Si/glass.
Fig. 6. Changes in the transmittance and reflectance as a function of wavelength in Ag/ glass and Ag/Si/glass.
1988
S.H. Park et al. / Solid State Sciences 13 (2011) 1984e1988
overlaid Ag film, indicating a dense grain boundary and smooth surface. These factors resulted in the lower resistance of the thin film. Emissivity depends on the resistance and can be calculated using the following equation [19]:
e ¼ 0:029$R, 6:7 105 R2
Ag thin film, which was due to 2-D growth as a result of the negative surface free energy change. Ag(9 nm)/Si(3 nm)/glass showed a lower resistance of 5.92 U/,, leading to a lower emissivity of 0.074 and w91% reflectance in the IR region with 67% transmittance in the visible region, which are promising properties compared to Ag/glass without an Si interlayer.
(4)
The thickness of the Ag thin film that corresponds to an emissivity of 0.1 [23], which is known to be a generally good emissivity level, was reduced from 10.5 to 8.5 nm with a 3 nm thick Si interlayer (Fig. 5). The emissivity of Ag(9 nm)/Si(1 nm) and Ag(9 nm)/ Si(3 nm) were 0.085 and 0.074, respectively, compared to 0.14 for Ag(9 nm)/glass. With an Ag thickness greater than 12 nm, the emissivity did not absolutely depend on the Si interlayer. The thicker Ag thin film had a lower emissivity at the expense of the transmittance. An Ag thin film with a smaller thickness is desirable. The Si interlayer significantly influenced the emissivity as well as the resistance at the initial stage of the thin film growth, which originated from the nanostructural differences. Therefore, it is evident that the Si interlayer effectively changed the nanostructure of the thin films and reduced the resistance and emissivity. Likewise, the Si interlayer was expected to influence the transmittance. Fig. 6 shows the transmittance and reflectance of Ag/Si/ glass as a function of the wavelength of incident light. In the visible region (l ¼ 550 nm), the transmittance was w48% for Ag(9 nm)/ glass and w67% for Ag(9 nm)/Si(3 nm)/glass, which was attributed to the induced formation of a dense Ag thin film by the Si interlayer. In general, a transmittance higher than 60% in the visible region and a lower transmittance in the infrared region are required [24]. In addition, the higher reflectance (the reciprocal of transmittance) in the range of w800e2400 nm could effectively screen solar radiation. The reflectance at wavelengths of w1000e2400 nm was w71e86% for Ag(9 nm)/glass and w60e91% for Ag(9 nm)/Si(3 nm)/ glass. To obtain good reflectance in the infrared region, the resistance should be lower. Therefore, the reduction of the resistance induced by the Si interlayer resulted in the high reflectance. In summary, the Si interlayer induced the formation of a dense Ag thin film by 2-D growth, leading to high transmittance in the visible region and high reflectance in the infrared region. 4. Conclusions The effect of the Si interlayer on Ag thin film growth in Ag/Si/ glass films was investigated. The Si interlayer both significantly reduced the surface roughness and induced the densification of the
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