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Reflectance changes of Fe and Cr doped Ag thin films deposited by magnetron sputtering
TSF-35732; No of Pages 6 Thin Solid Films xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Reflectance changes of Fe and Cr doped Ag thin films deposited by magnetron sputtering Chadrasekhar Loka, Kee-Sun Lee ⁎ Department of Advanced Materials Engineering & Smart Natural Space Research Center, Kongju National University, Cheonan 330717, South Korea.
a r t i c l e
i n f o
Article history: Received 30 September 2016 Received in revised form 2 January 2017 Accepted 11 January 2017 Available online xxxx Keywords: Silver-alloy Thin films Sputtering Optical reflectance Atomic force microscopy
a b s t r a c t Silver-based highly-reflective coatings are extensively used in the optoelectronic devices. However, it is still difficult to accomplish both high reflectivity and long-term stability of the protected silver-based reflectors simultaneously. In this report, we have investigated the optical properties of sputter deposited FeCr-doped Silver alloy thin films on glass substrates at room temperature. We observed that the alloying FeCr with silver can significantly improve the silver surface morphology while keeping its reflectance same as that of pure silver in the nearinfrared region of the electromagnetic spectrum. However, at the same time, in the visible region about 400 nm of the wavelength, the reflectance was decreased by increasing the FeCr content. The less amount of FeCr-doped silver films exhibited high reflectivity as similar to that of pure silver. In this study, we have interpreted the simple model describing the trend of the reflectance of silver-alloy films due to FeCr-doping. We also investigated the thermal stability of the pure silver and FeCr-doped silver films and the corresponding thermally induced grain growth results revealed that, the FeCr-doping is effective to restrain the severe grain growth of silver films. © 2017 Elsevier B.V. All rights reserved.
1. Introduction High reflectivity coatings are extensively exploited in optoelectronic devices, astronomical telescopes, and space sensors in a predetermined spectral rage of the electromagnetic spectrum [1–3]. Therefore, extensive efforts are invested from the past decade to develop and improve the reflective coatings. It has been reported that only free electron-like metals which obey the Drude model [4] are suitable to be used as reflectors. Among the Drude metals, silver and aluminum are recognized as the best solar reflectors [5] with a hemispherical solar reflectance of about 96% and 92%, respectively. Due to their high spectral reflectance (97.7% at λ = 500 nm), low resistivity (1.6 μΩ cm), low polarization effects, and low IR emissivity, the thin silver films attracted increasing scientific interest in the field of electronic and optical devices [6,7]. It is known that, the working environment has great influence on silverbased optoelectronic devices which has received considerable attention in recent years. Despite, the silver-based reflectors exhibit poor environmental resistance by losing their optical performance when exposed to the atmosphere and also Ag provides poor adhesion to glass substrates [3,8–11]. Therefore, preparation of durable silver coatings with high reflectivity has proven to be challenging task in recent years, mainly in light-emitting diode industry as a potential back reflector. Recently, extensive studies have developing high efficiency durable silver-based ⁎ Corresponding author. E-mail address: [email protected] (K.-S. Lee).
reflective coatings by adopting protective over-layers and appropriate binder materials. However, a very few materials such as Al2O3, Ta2O5, Y2O3, SiO2, and MgF2, are suitable as protective layers [12]; similarly, metal-oxide or several metallic materials were exploited as a binder films [13–15]. Nevertheless, the use of thin alloy layers has also been reported, which results in reduction of the chemical reactivity while maintaining the optical properties [7,16–19]. Based on their superior performance over traditional reflective materials, Ag-alloy thin films exhibit significant potential for future commercialization. Herein, we investigate the effect of FeCr addition on microstructural and optical properties of FeCr-doped Ag thin films. In this study, small amounts of Fe-Cr (b 2 at.%) are used to improve the surface morphology and thermal durability of the alloy thin films. We report on the effects of change in reflectivity, surface roughness, and thermally induced grain growth that arise from alloying Fe-Cr. 2. Experimental procedure The thin films of pure Ag and FeCr-doped Ag were deposited on glass substrates at room temperature by DC magnetron sputtering. In order to remove impurities and minimize contamination, the substrates were ultrasonically cleaned with distillated water, ethanol and acetone, during 15 min in each solution, before deposition. The mosaic type FeCr chips embedded Ag target was prepared under laboratory circumstances, FeCr chips were prepared through induction melting of 99.9% pure Fe and Cr of 80 and 20 wt%. The films were deposited using a target
http://dx.doi.org/10.1016/j.tsf.2017.01.020 0040-6090/© 2017 Elsevier B.V. All rights reserved.
Please cite this article as: C. Loka, K.-S. Lee, Reflectance changes of Fe and Cr doped Ag thin films deposited by magnetron sputtering, Thin Solid Films (2017), http://dx.doi.org/10.1016/j.tsf.2017.01.020
2
C. Loka, K.-S. Lee / Thin Solid Films xxx (2017) xxx–xxx
power of 40 W, the sputtering atmosphere was consisted of a constant argon flow (90 sccm), the total working pressure inside the chamber was 1.079 Pa. Prior to the deposition, the deposition chamber was pumped down to a base pressure of 1.33 × 10− 4 Pa. The substrates were placed in a substrate holder located at 10 cm away from the target and rotating at a constant velocity of 23 rpm. The deposition time was kept constant for 3 min, and the FeCr concentration varied by changing the number of FeCr chips (1-chip and 3-chips) on the Ag target. In order to investigate the grain growth upon annealing time, the deposited samples are annealed at 300 °C for 10, 30, and 60 min under vacuum ambient. The surface microstructure and thickness of the coatings were evaluated by field-emission scanning electron microscopy (FE-SEM; TESCAN MIRAH). The Fe-Cr concentrations in the FeCr-doped Ag films were determined by Rutherford Backscattering Spectrometry (RBS). RBS analysis was performed by using a 2 MV Pelletron Accelerator (6SDH2, NEC) with a beam of He2 + ions with average energy of 2.009 MeV, the sample was analyzed at a tilt angle of 5°. The optical reflectance was measured in the wavelength range from 300 to 850 nm using UV–vis-NIR spectrophotometer (Shimadzu UV–3700). The optical constants of the films were measured by using spectroscopic ellipsometry (SE; J.A. Wollam Co. Inc., M-2000). Atomic force microscopy (AFM; XE-70, Park Systems) was used in non-contact mode to reveal the roughness and topography of the films.
chips and the 1-chip and 3-chips RBS spectra are shown in Fig. 1(a) and (b), respectively. The Ag and Fe peaks obtained in both the films, the Cr peak and Ag peak are seemingly overlapped. The Fe and Cr concentrations of the thin films are shown in the respective images; the areal density of the films was also given by the RUMP computer simulations. FE-SEM images of the Ag and FeCr-doped Ag films deposited on glass substrates at room temperature were shown in Fig. 2. The Ag thin film show agglomeration on its surface comprised of a few randomly distributed particles (Fig. 2(a)), which could be due to the strong adatom-adatom cohesive forces during the initial stage of Ag thin film growth to form agglomerated microstructure that continuous in the thick films. While the FeCr-doped Ag films exhibit smooth and flat
3. Results and discussion Fig. 1 shows the Rutherford Backscattering Spectra of the FeCrdoped Ag film. RBS analysis allowed to determine the Fe and Cr concentrations in the films. The Ag target surface was decorated with FeCr
Fig. 1. Rutherford Backscattering Spectrometry of the FeCr-doped Ag films: (a) FeCr(1 at.%)-doped Ag, and (b) FeCr(1.7 at.%)-doped Ag.
Fig. 2. SEM images of Ag and FeCr-doped Ag films: (a) Ag, (b) FeCr(1 at.%)-doped Ag, and (c) FeCr(1.7 at.%)-doped Ag.
Please cite this article as: C. Loka, K.-S. Lee, Reflectance changes of Fe and Cr doped Ag thin films deposited by magnetron sputtering, Thin Solid Films (2017), http://dx.doi.org/10.1016/j.tsf.2017.01.020
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surface microstructure as shown in Fig. 2(b) and (c), which could be due to the random distribution of the intermetallic FeCr phases at the boundaries of Ag to initiate a smooth and dense microstructure. Fig. 3(a) shows the reflectance spectra of the as-deposited Ag and Ag-alloy films with different FeCr concentrations. It clearly shows that the Ag films exhibit high reflectance in the entire visible region and nearinfrared region. However, the reflectance was abruptly decreased in the ultraviolet region (~300 nm) that is associated with the bulk plasma frequency of silver, which is due to the d-like valance electron absorption. The plasmon frequency (ωp) is given by the following equation, which depends on the carrier concentration N, the effective mass of a free electron me, and the permittivity of vacuum ε0. sffiffiffiffiffiffiffiffiffiffiffi Ne2 ωp ¼ me ε0
ð1Þ
The reflectance spectra (Fig. 3(a)) of the films exhibit that the FeCrdoped Ag films show slightly lower reflectance with increase in the FeCr concentration as compared to the pure Ag, particularly about 400–500 nm. However, FeCr(1 at.%)-doped Ag films showed almost comparable reflectance with pure Ag. Such an observation indicates that the alloying the FeCr in high proportions to the Ag results increase in the optical loss i.e. increase in imaginary part of the dielectric constant in that particular wavelength region of the electromagnetic spectrum.
Fig. 4. Refractive index (n) and extinction coefficient (k) of FeCr-doped Ag films.
The films were annealed at 300 °C for 10 min, and the corresponding reflectance spectra was shown in Fig. 3(b). After annealing, the Ag film reflectance was decreased as compared to the as-deposited Ag, which could be due to the changes in the surface microstructure of the films. However, after annealing, discernible discrepancies were not observed in the reflectance spectra of the FeCr-doped Ag films. The refractive index (n) and extinction coefficient (k) for Ag and FeCr-doped Ag films are shown in Fig. 4, which are calculated by using the first principle method [20]. Based on the density-functional theory, the calculations were performed by the ultra-soft pseudopotential approach. The imaginary part of the metal dielectric function ε(ω) consists of a free electron part εf(ω) and an inter-band part εi(ω) [21]. εðωÞ ¼ ε f ðωÞ þ εi ðωÞ
ð2Þ
where ε f ðωÞ ¼ 1−
ω2p ω2
ð3Þ
þ iγω
here ωp is the free-electron plasma frequency and γ is the damping constant of the material and is inverse of the electronic scattering time τ. The dielectric function of the metal ε(ω) is dominated by electron inter-band transitions, and it can be analyzed in terms of the calculated band structure and density of state. The imaginary part εi(ω) is calculated from the momentum matrix elements between the occupied and unoccupied electronic states [22], which is ^; ω Þ ¼ εi ð q
8π2 e2 V
lim
jqj→0
1 jq j2
2 ∑ uc;kþq juv;k δ Ec;kþq −Ev;k −ħω
k;v;c
ð4Þ
where the indices c and v are restricted to the conduction and the va^ denotes the direction of q, dieleclence band states, respectively. The q q ^ ¼ jqj . The volume of the unit tric function depends on the direction q cell is denoted by V, E are the single particle energies, and the translationally invariant parts of the wave functions are u. The real
Table 1 Hall mobility, and carrier concentration of the as-deposited and annealed pure Ag and FeCr-doped Ag thin films. Thin film
Fig. 3. Optical reflectance spectra of FeCr-doped Ag films with different FeCr content: (a) As-deposited, and (b) annealed at 300 °C.
Ag FeCr(1 at.%)-Ag FeCr(1.7 at.%)-Ag
As-deposited films
Annealed at 300 °C
Hall mobility (cm2/Vs)
Carrier con. (cm−3)
Hall mobility (cm2/Vs)
Carrier con. (cm−3)
25.683 12.046 9.329
5.006 × 1022 4.889 × 1022 4.29 × 1022
32.23 13.146 9.498
4.53 × 1022 4.702 × 1022 4.16 × 1022
Please cite this article as: C. Loka, K.-S. Lee, Reflectance changes of Fe and Cr doped Ag thin films deposited by magnetron sputtering, Thin Solid Films (2017), http://dx.doi.org/10.1016/j.tsf.2017.01.020
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part εf(ω) can be derived from the imaginary part εi(ω) by using the Kramers-Kronig relationship, and the optical constants such as the refractive index and extinction coefficient and reflectivity can be derived
from εf(ω) and εi(ω). Compared to the pure Ag, the refractive index and extinction coefficient of Ag-alloy are enlarged apparently in the wavelength rang 400–500 nm by increasing FeCr content. The Hall
Fig. 5. AFM images (3 × 3 μm2) scans of Ag-alloy films annealed at 300 °C for different annealing times: pure Ag films (a, b, c, d), and FeCr(1 at.%)-doped Ag films (e, f, g, h).
Please cite this article as: C. Loka, K.-S. Lee, Reflectance changes of Fe and Cr doped Ag thin films deposited by magnetron sputtering, Thin Solid Films (2017), http://dx.doi.org/10.1016/j.tsf.2017.01.020
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quantities, which are frequently used to compare films in terms of surface roughness, are the arithmetic mean roughness (Ra) and root mean square roughness (Rrms): Ra ¼
Rrms
1 N ∑jZ i −Z m j N i¼1
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 N ∑ ðZ −Z m Þ2 ¼ N i¼1 i
ð5Þ
ð6Þ
where Zm is a reference plane and N the number of data points with relative height Zi. The variation in arithmetic mean roughness and root mean square roughness of the Ag and FeCr-doped Ag films annealed at constant temperature and different annealing time was shown in Fig. 6(a). The FeCr-doped Ag films exhibited slightly higher roughness as compared to that of pure Ag. Both Ra and Rrms of the films was increased with increase in annealing time. The Ag films shows an abrupt increase in the roughness at longer annealing time. As compared to the pure Ag, the FeCr-doped Ag films exhibited relatively lower Ra. The variation in grain sizes upon annealing at 300 °C at different annealing times was shown in Fig. 6(b). Grain size of pure Ag film increased from 9.32 nm without annealing to 82.6 nm after being annealed for 60 min. While the grain size of the FeCr-doped Ag films increased from 10.8 nm to 41.5 nm after annealing. Obviously, the grain growth of Ag has been retarded by FeCr-doping, which can be due to the FeCr segregation at the grain boundaries of Ag, and hence FeCr-doped Ag films exhibit significant thermal stability along with high reflectivity. 4. Conclusions
Fig. 6. (a) Changes in surface roughness with respect to annealing time of the films annealed at 300 °C, and (b) variation in average grain diameter of the films annealed at 300 °C.
mobility, and carrier concentration of the as-deposited and annealed films were listed in the Table 1. In the as-deposited films, the Hall mobility (μ) and carrier concentration decreases with increase in FeCr content in the FeCr-doped Ag films is due to the impurity scattering effect. After annealing, the FeCr-doped Ag films exhibited almost similar electrical properties. The relationship between the electron scattering time and mobility is given by: τ¼
me μ e
Acknowledgment ð5Þ
As the damping constant is inverse of electronic scattering time, obviously, the decrease in mobility will indicate increase in the damping constant, which indicate greater interaction of the conduction electrons with the impurity atoms. The γ is frequency dependent, for the metal alloys it is defined as: γðωÞ ¼ γ p ðωÞ þ xγi ðωÞ
In conclusion, we prepared pure silver and FeCr-doped silver films by DC magnetron sputtering technique on glass substrates at room temperature, and their optical reflectivity, electrical mobility, and the thermal stability were investigated. It was demonstrated that alloying FeCr with silver results in smooth morphology of the films. The less amount of FeCr-doped Ag films exhibits significant high reflectance as that of pure silver, by further increasing the doping concentration lead to lower the reflectance in the visible part (about 400 nm) of the electromagnetic spectrum. Ag-alloy films retard the grain growth of pure silver due to the FeCr segregation at the grain boundaries of Ag, and hence FeCr-doped Ag films exhibit significant thermal stability along with high reflectivity. These results indicate that the FeCr-doped Ag could be a promising candidate for the fabrication of high-performance optoelectronic devices.
ð6Þ
where γp is the damping constant for pure metals, x is the FeCr alloy concentration, and γi is the frequency dependent damping constant the charge carriers experience due to the FeCr alloying. The larger γ in metal alloy increases the dielectric constant contribution due to the free electrons. Hence, the reflectance at long wavelength side of the electromagnetic spectrum is seemingly similar without any significant reduction. AFM images of the surface morphology of the films annealed at 300 °C for different annealing time were shown in Fig. 5. Statistical
The research was supported by the International Science and Business Belt Program through the Ministry of Science, ICT and Future Planning (2016K000298). This work was supported by the Technology Innovation Program, 10048568, Development of Manufacturing Technology of 1500 N Grade Sapphire for Optic and Display Component Applications funded by the Ministry of Trade, Industry & Energy. References [1] W.S. Yum, J.W. Jeon, J.S. Sung, T.Y. Seong, Highly reliable Ag/Zn/Ag ohmic reflector for high-power GaN-based vertical light-emitting, Opt. Express 20 (2012) 19194–19199. [2] M. Boccas, T. Vucina, C. Araya, E. Vera, C. Ahhee, Protected-silver coatings for the 8m Gemini telescope mirrors, Thin Solid Films 502 (2006) 275–280. [3] L. Wang, Z. Shen, G. Du, P. Wang, The thermal stability of silver-based high reflectance coatings, Thin Solid Films 616 (2016) 122–125. [4] P. Drude, L. der Optik, V.v.S. Hirzel, Leipzig, 1906 (in German). [5] M. Brogren, B. Karlsson, A. Roos, A. Werner, Analysis of the effects of outdoor and accelerated ageing on the optical properties of reflector materials for solar energy applications, Sol. Energy Mater. Sol. Cells 82 (2004) 491–515. [6] M. Viswanathan, C.L. Nagendra, G.K.M. Thutupalli, High efficiency front and rear surface silver reflectors, Thin Solid Films 167 (1988) 291–297.
Please cite this article as: C. Loka, K.-S. Lee, Reflectance changes of Fe and Cr doped Ag thin films deposited by magnetron sputtering, Thin Solid Films (2017), http://dx.doi.org/10.1016/j.tsf.2017.01.020
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[7] T. Suzuki, Y. Abe, M. Kawamura, K. Sasaki, T. Shouzu, K. Kawamata, Optical and electrical properties of pure Ag and Ag-based alloy thin films prepared by RF magnetron sputtering, Vacuum 66 (2002) 501–504. [8] T.L. Alford, D. Adams, T. Laursen, B.M. Ullrich, Encapsulation of Ag films on SiO2 by Ti reactions using Ag–Ti alloy/bilayer structures and an NH3 ambient, Appl. Phys. Lett. 68 (1996) 3251–3253. [9] M.J. Zhao, F.Q. Si, C. Liu, Y.H. Lu, Y. Wang, S.M. Wang, Y. Zeng, Y. Jiang, H.J. Zhou, W.Q. Liu, Effect of AO/UV/RD exposure on spaceborne diffusers: a comparative experiment, Appl. Opt. 54 (2015) 9157–9166. [10] A.W. Czanderna, Stability of interfaces in solar-energy materials, Solar Energy Mater. 5 (1981) 349–377. [11] A. Roos, C.G. Ribbing, B. Karlsson, Stainless-steel solar mirrors – a material feasibility study, Solar Energy Mater. 18 (1989) 233–240. [12] E. Širbegović, I. Tucak, M. Čevro, An investigation of the durability of front surface Ag mirrors using oxide protecting layers, Vacuum 43 (1992) 763–765. [13] C.E. Kennedy, R.V. Smilgys, D.A. Kirkpatrick, J.S. Ross, Optical performance and durability of solar reflectors protected by an alumina coating, Thin Solid Films 304 (1997) 303–309. [14] Y.J. Xu, Q.W. Cai, X.X. Yang, Y.Z. Zuo, H. Song, Z.M. Liu, Y.P. Hang, Preparation of novel SiO2 protected Ag thin films with high reflectivity by magnetron sputtering for solar front reflectors, Sol. Energy Mater. Sol. Cells 107 (2012) 316–321.
[15] W.S. Yum, C.H. Lee, S. Jin, T.Y. Seong, Effect of an In layer on the thermal stability of Ag reflector for vertical GaN-based light-emitting diodes, Superlattice. Microst. 56 (2013) 77–85. [16] Y. Aoshima, M. Miyazaki, K. Sato, Y. Akao, S. Takaki, K. Adachi, Development of silver-based multilayer coating electrodes with low resistance for use in flat panel displays, Jpn. J. Appl. Phys. 39 (2000) 4884. [17] S. Das, T.L. Alford, Reflectance spectroscopy of functional Ag-Cu thin films: correlation of reflectivity with Cu content, J. Miner. Met. Mater. Soc. 65 (2013) 538–541. [18] J.-Y. Kim, S.-I. Na, G.-Y. Ha, M.-K. Kwon, I.-K. Park, J.-H. Lim, S.-J. Park, Thermally stable and highly reflective AgAl alloy for enhancing light extraction efficiency in GaN light-emitting diodes, Appl. Phys. Lett. 88 (2006) 043507. [19] H. Kim, K.H. Baik, J. Cho, J. Lee, S. Yoon, H. Kim, S.-N. Lee, C. Sone, Y. Park, T.-Y. Seong, High-reflectance and thermally stable AgCu alloy p-type reflectors for GaN-based light-emitting diodes, IEEE Photon. Technol. Lett. 19 (2007) 336–338. [20] D. Vanderbilt, Soft self-consistent ultrasoft pseudopotentials in a generalized eigenvalue formalism, Phys. Rev. B 41 (1990) 7892–7895. [21] R.W. Cohen, G.D. Cody, M.W. Coutts, B. Abeles, Optical properties of granular silver and gold films, Phys. Rev. B 8 (1973) 3689–3701. [22] M. Gajdoš, K. Hummer, G. Kresse, J. Furthmüller, F. Bechstedt, Linear optical properties in the projector-augmented wave methodology, Phys. Rev. B 73 (2006) 045112–045119.
Please cite this article as: C. Loka, K.-S. Lee, Reflectance changes of Fe and Cr doped Ag thin films deposited by magnetron sputtering, Thin Solid Films (2017), http://dx.doi.org/10.1016/j.tsf.2017.01.020
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Reflectance changes of Fe and Cr doped Ag thin films deposited by magnetron sputtering Chadrasekhar Loka, Kee-Sun Lee ⁎ Department of Advanced Materials Engineering & Smart Natural Space Research Center, Kongju National University, Cheonan 330717, South Korea.
a r t i c l e
i n f o
Article history: Received 30 September 2016 Received in revised form 2 January 2017 Accepted 11 January 2017 Available online xxxx Keywords: Silver-alloy Thin films Sputtering Optical reflectance Atomic force microscopy
a b s t r a c t Silver-based highly-reflective coatings are extensively used in the optoelectronic devices. However, it is still difficult to accomplish both high reflectivity and long-term stability of the protected silver-based reflectors simultaneously. In this report, we have investigated the optical properties of sputter deposited FeCr-doped Silver alloy thin films on glass substrates at room temperature. We observed that the alloying FeCr with silver can significantly improve the silver surface morphology while keeping its reflectance same as that of pure silver in the nearinfrared region of the electromagnetic spectrum. However, at the same time, in the visible region about 400 nm of the wavelength, the reflectance was decreased by increasing the FeCr content. The less amount of FeCr-doped silver films exhibited high reflectivity as similar to that of pure silver. In this study, we have interpreted the simple model describing the trend of the reflectance of silver-alloy films due to FeCr-doping. We also investigated the thermal stability of the pure silver and FeCr-doped silver films and the corresponding thermally induced grain growth results revealed that, the FeCr-doping is effective to restrain the severe grain growth of silver films. © 2017 Elsevier B.V. All rights reserved.
1. Introduction High reflectivity coatings are extensively exploited in optoelectronic devices, astronomical telescopes, and space sensors in a predetermined spectral rage of the electromagnetic spectrum [1–3]. Therefore, extensive efforts are invested from the past decade to develop and improve the reflective coatings. It has been reported that only free electron-like metals which obey the Drude model [4] are suitable to be used as reflectors. Among the Drude metals, silver and aluminum are recognized as the best solar reflectors [5] with a hemispherical solar reflectance of about 96% and 92%, respectively. Due to their high spectral reflectance (97.7% at λ = 500 nm), low resistivity (1.6 μΩ cm), low polarization effects, and low IR emissivity, the thin silver films attracted increasing scientific interest in the field of electronic and optical devices [6,7]. It is known that, the working environment has great influence on silverbased optoelectronic devices which has received considerable attention in recent years. Despite, the silver-based reflectors exhibit poor environmental resistance by losing their optical performance when exposed to the atmosphere and also Ag provides poor adhesion to glass substrates [3,8–11]. Therefore, preparation of durable silver coatings with high reflectivity has proven to be challenging task in recent years, mainly in light-emitting diode industry as a potential back reflector. Recently, extensive studies have developing high efficiency durable silver-based ⁎ Corresponding author. E-mail address: [email protected] (K.-S. Lee).
reflective coatings by adopting protective over-layers and appropriate binder materials. However, a very few materials such as Al2O3, Ta2O5, Y2O3, SiO2, and MgF2, are suitable as protective layers [12]; similarly, metal-oxide or several metallic materials were exploited as a binder films [13–15]. Nevertheless, the use of thin alloy layers has also been reported, which results in reduction of the chemical reactivity while maintaining the optical properties [7,16–19]. Based on their superior performance over traditional reflective materials, Ag-alloy thin films exhibit significant potential for future commercialization. Herein, we investigate the effect of FeCr addition on microstructural and optical properties of FeCr-doped Ag thin films. In this study, small amounts of Fe-Cr (b 2 at.%) are used to improve the surface morphology and thermal durability of the alloy thin films. We report on the effects of change in reflectivity, surface roughness, and thermally induced grain growth that arise from alloying Fe-Cr. 2. Experimental procedure The thin films of pure Ag and FeCr-doped Ag were deposited on glass substrates at room temperature by DC magnetron sputtering. In order to remove impurities and minimize contamination, the substrates were ultrasonically cleaned with distillated water, ethanol and acetone, during 15 min in each solution, before deposition. The mosaic type FeCr chips embedded Ag target was prepared under laboratory circumstances, FeCr chips were prepared through induction melting of 99.9% pure Fe and Cr of 80 and 20 wt%. The films were deposited using a target
http://dx.doi.org/10.1016/j.tsf.2017.01.020 0040-6090/© 2017 Elsevier B.V. All rights reserved.
Please cite this article as: C. Loka, K.-S. Lee, Reflectance changes of Fe and Cr doped Ag thin films deposited by magnetron sputtering, Thin Solid Films (2017), http://dx.doi.org/10.1016/j.tsf.2017.01.020
2
C. Loka, K.-S. Lee / Thin Solid Films xxx (2017) xxx–xxx
power of 40 W, the sputtering atmosphere was consisted of a constant argon flow (90 sccm), the total working pressure inside the chamber was 1.079 Pa. Prior to the deposition, the deposition chamber was pumped down to a base pressure of 1.33 × 10− 4 Pa. The substrates were placed in a substrate holder located at 10 cm away from the target and rotating at a constant velocity of 23 rpm. The deposition time was kept constant for 3 min, and the FeCr concentration varied by changing the number of FeCr chips (1-chip and 3-chips) on the Ag target. In order to investigate the grain growth upon annealing time, the deposited samples are annealed at 300 °C for 10, 30, and 60 min under vacuum ambient. The surface microstructure and thickness of the coatings were evaluated by field-emission scanning electron microscopy (FE-SEM; TESCAN MIRAH). The Fe-Cr concentrations in the FeCr-doped Ag films were determined by Rutherford Backscattering Spectrometry (RBS). RBS analysis was performed by using a 2 MV Pelletron Accelerator (6SDH2, NEC) with a beam of He2 + ions with average energy of 2.009 MeV, the sample was analyzed at a tilt angle of 5°. The optical reflectance was measured in the wavelength range from 300 to 850 nm using UV–vis-NIR spectrophotometer (Shimadzu UV–3700). The optical constants of the films were measured by using spectroscopic ellipsometry (SE; J.A. Wollam Co. Inc., M-2000). Atomic force microscopy (AFM; XE-70, Park Systems) was used in non-contact mode to reveal the roughness and topography of the films.
chips and the 1-chip and 3-chips RBS spectra are shown in Fig. 1(a) and (b), respectively. The Ag and Fe peaks obtained in both the films, the Cr peak and Ag peak are seemingly overlapped. The Fe and Cr concentrations of the thin films are shown in the respective images; the areal density of the films was also given by the RUMP computer simulations. FE-SEM images of the Ag and FeCr-doped Ag films deposited on glass substrates at room temperature were shown in Fig. 2. The Ag thin film show agglomeration on its surface comprised of a few randomly distributed particles (Fig. 2(a)), which could be due to the strong adatom-adatom cohesive forces during the initial stage of Ag thin film growth to form agglomerated microstructure that continuous in the thick films. While the FeCr-doped Ag films exhibit smooth and flat
3. Results and discussion Fig. 1 shows the Rutherford Backscattering Spectra of the FeCrdoped Ag film. RBS analysis allowed to determine the Fe and Cr concentrations in the films. The Ag target surface was decorated with FeCr
Fig. 1. Rutherford Backscattering Spectrometry of the FeCr-doped Ag films: (a) FeCr(1 at.%)-doped Ag, and (b) FeCr(1.7 at.%)-doped Ag.
Fig. 2. SEM images of Ag and FeCr-doped Ag films: (a) Ag, (b) FeCr(1 at.%)-doped Ag, and (c) FeCr(1.7 at.%)-doped Ag.
Please cite this article as: C. Loka, K.-S. Lee, Reflectance changes of Fe and Cr doped Ag thin films deposited by magnetron sputtering, Thin Solid Films (2017), http://dx.doi.org/10.1016/j.tsf.2017.01.020
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surface microstructure as shown in Fig. 2(b) and (c), which could be due to the random distribution of the intermetallic FeCr phases at the boundaries of Ag to initiate a smooth and dense microstructure. Fig. 3(a) shows the reflectance spectra of the as-deposited Ag and Ag-alloy films with different FeCr concentrations. It clearly shows that the Ag films exhibit high reflectance in the entire visible region and nearinfrared region. However, the reflectance was abruptly decreased in the ultraviolet region (~300 nm) that is associated with the bulk plasma frequency of silver, which is due to the d-like valance electron absorption. The plasmon frequency (ωp) is given by the following equation, which depends on the carrier concentration N, the effective mass of a free electron me, and the permittivity of vacuum ε0. sffiffiffiffiffiffiffiffiffiffiffi Ne2 ωp ¼ me ε0
ð1Þ
The reflectance spectra (Fig. 3(a)) of the films exhibit that the FeCrdoped Ag films show slightly lower reflectance with increase in the FeCr concentration as compared to the pure Ag, particularly about 400–500 nm. However, FeCr(1 at.%)-doped Ag films showed almost comparable reflectance with pure Ag. Such an observation indicates that the alloying the FeCr in high proportions to the Ag results increase in the optical loss i.e. increase in imaginary part of the dielectric constant in that particular wavelength region of the electromagnetic spectrum.
Fig. 4. Refractive index (n) and extinction coefficient (k) of FeCr-doped Ag films.
The films were annealed at 300 °C for 10 min, and the corresponding reflectance spectra was shown in Fig. 3(b). After annealing, the Ag film reflectance was decreased as compared to the as-deposited Ag, which could be due to the changes in the surface microstructure of the films. However, after annealing, discernible discrepancies were not observed in the reflectance spectra of the FeCr-doped Ag films. The refractive index (n) and extinction coefficient (k) for Ag and FeCr-doped Ag films are shown in Fig. 4, which are calculated by using the first principle method [20]. Based on the density-functional theory, the calculations were performed by the ultra-soft pseudopotential approach. The imaginary part of the metal dielectric function ε(ω) consists of a free electron part εf(ω) and an inter-band part εi(ω) [21]. εðωÞ ¼ ε f ðωÞ þ εi ðωÞ
ð2Þ
where ε f ðωÞ ¼ 1−
ω2p ω2
ð3Þ
þ iγω
here ωp is the free-electron plasma frequency and γ is the damping constant of the material and is inverse of the electronic scattering time τ. The dielectric function of the metal ε(ω) is dominated by electron inter-band transitions, and it can be analyzed in terms of the calculated band structure and density of state. The imaginary part εi(ω) is calculated from the momentum matrix elements between the occupied and unoccupied electronic states [22], which is ^; ω Þ ¼ εi ð q
8π2 e2 V
lim
jqj→0
1 jq j2
2 ∑ uc;kþq juv;k δ Ec;kþq −Ev;k −ħω
k;v;c
ð4Þ
where the indices c and v are restricted to the conduction and the va^ denotes the direction of q, dieleclence band states, respectively. The q q ^ ¼ jqj . The volume of the unit tric function depends on the direction q cell is denoted by V, E are the single particle energies, and the translationally invariant parts of the wave functions are u. The real
Table 1 Hall mobility, and carrier concentration of the as-deposited and annealed pure Ag and FeCr-doped Ag thin films. Thin film
Fig. 3. Optical reflectance spectra of FeCr-doped Ag films with different FeCr content: (a) As-deposited, and (b) annealed at 300 °C.
Ag FeCr(1 at.%)-Ag FeCr(1.7 at.%)-Ag
As-deposited films
Annealed at 300 °C
Hall mobility (cm2/Vs)
Carrier con. (cm−3)
Hall mobility (cm2/Vs)
Carrier con. (cm−3)
25.683 12.046 9.329
5.006 × 1022 4.889 × 1022 4.29 × 1022
32.23 13.146 9.498
4.53 × 1022 4.702 × 1022 4.16 × 1022
Please cite this article as: C. Loka, K.-S. Lee, Reflectance changes of Fe and Cr doped Ag thin films deposited by magnetron sputtering, Thin Solid Films (2017), http://dx.doi.org/10.1016/j.tsf.2017.01.020
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part εf(ω) can be derived from the imaginary part εi(ω) by using the Kramers-Kronig relationship, and the optical constants such as the refractive index and extinction coefficient and reflectivity can be derived
from εf(ω) and εi(ω). Compared to the pure Ag, the refractive index and extinction coefficient of Ag-alloy are enlarged apparently in the wavelength rang 400–500 nm by increasing FeCr content. The Hall
Fig. 5. AFM images (3 × 3 μm2) scans of Ag-alloy films annealed at 300 °C for different annealing times: pure Ag films (a, b, c, d), and FeCr(1 at.%)-doped Ag films (e, f, g, h).
Please cite this article as: C. Loka, K.-S. Lee, Reflectance changes of Fe and Cr doped Ag thin films deposited by magnetron sputtering, Thin Solid Films (2017), http://dx.doi.org/10.1016/j.tsf.2017.01.020
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quantities, which are frequently used to compare films in terms of surface roughness, are the arithmetic mean roughness (Ra) and root mean square roughness (Rrms): Ra ¼
Rrms
1 N ∑jZ i −Z m j N i¼1
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 N ∑ ðZ −Z m Þ2 ¼ N i¼1 i
ð5Þ
ð6Þ
where Zm is a reference plane and N the number of data points with relative height Zi. The variation in arithmetic mean roughness and root mean square roughness of the Ag and FeCr-doped Ag films annealed at constant temperature and different annealing time was shown in Fig. 6(a). The FeCr-doped Ag films exhibited slightly higher roughness as compared to that of pure Ag. Both Ra and Rrms of the films was increased with increase in annealing time. The Ag films shows an abrupt increase in the roughness at longer annealing time. As compared to the pure Ag, the FeCr-doped Ag films exhibited relatively lower Ra. The variation in grain sizes upon annealing at 300 °C at different annealing times was shown in Fig. 6(b). Grain size of pure Ag film increased from 9.32 nm without annealing to 82.6 nm after being annealed for 60 min. While the grain size of the FeCr-doped Ag films increased from 10.8 nm to 41.5 nm after annealing. Obviously, the grain growth of Ag has been retarded by FeCr-doping, which can be due to the FeCr segregation at the grain boundaries of Ag, and hence FeCr-doped Ag films exhibit significant thermal stability along with high reflectivity. 4. Conclusions
Fig. 6. (a) Changes in surface roughness with respect to annealing time of the films annealed at 300 °C, and (b) variation in average grain diameter of the films annealed at 300 °C.
mobility, and carrier concentration of the as-deposited and annealed films were listed in the Table 1. In the as-deposited films, the Hall mobility (μ) and carrier concentration decreases with increase in FeCr content in the FeCr-doped Ag films is due to the impurity scattering effect. After annealing, the FeCr-doped Ag films exhibited almost similar electrical properties. The relationship between the electron scattering time and mobility is given by: τ¼
me μ e
Acknowledgment ð5Þ
As the damping constant is inverse of electronic scattering time, obviously, the decrease in mobility will indicate increase in the damping constant, which indicate greater interaction of the conduction electrons with the impurity atoms. The γ is frequency dependent, for the metal alloys it is defined as: γðωÞ ¼ γ p ðωÞ þ xγi ðωÞ
In conclusion, we prepared pure silver and FeCr-doped silver films by DC magnetron sputtering technique on glass substrates at room temperature, and their optical reflectivity, electrical mobility, and the thermal stability were investigated. It was demonstrated that alloying FeCr with silver results in smooth morphology of the films. The less amount of FeCr-doped Ag films exhibits significant high reflectance as that of pure silver, by further increasing the doping concentration lead to lower the reflectance in the visible part (about 400 nm) of the electromagnetic spectrum. Ag-alloy films retard the grain growth of pure silver due to the FeCr segregation at the grain boundaries of Ag, and hence FeCr-doped Ag films exhibit significant thermal stability along with high reflectivity. These results indicate that the FeCr-doped Ag could be a promising candidate for the fabrication of high-performance optoelectronic devices.
ð6Þ
where γp is the damping constant for pure metals, x is the FeCr alloy concentration, and γi is the frequency dependent damping constant the charge carriers experience due to the FeCr alloying. The larger γ in metal alloy increases the dielectric constant contribution due to the free electrons. Hence, the reflectance at long wavelength side of the electromagnetic spectrum is seemingly similar without any significant reduction. AFM images of the surface morphology of the films annealed at 300 °C for different annealing time were shown in Fig. 5. Statistical
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Please cite this article as: C. Loka, K.-S. Lee, Reflectance changes of Fe and Cr doped Ag thin films deposited by magnetron sputtering, Thin Solid Films (2017), http://dx.doi.org/10.1016/j.tsf.2017.01.020