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Effect of Ag-doping on microstructural, optical and electrical properties of sputtering-derived ZnS films
Journal of Alloys and Compounds 551 (2013) 430–434
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of Ag-doping on microstructural, optical and electrical properties of sputtering-derived ZnS films Xueping Song a,b,c, Shiwei Shi a,b,c, Chunbin Cao d, Xiaoshuang Chen e, Jingbiao Cui f, Gang He a,b,c, Zhaoqi Sun a,b,c,f,⇑ a
School of Physics & Material Science, Anhui University, Hefei 230039, PR China Key Laboratory of Opto-Electronic Information Acquisition & Manipulation, Ministry of Education, Hefei 230039, PR China Anhui Key Laboratory of Information Materials and Devices, Hefei 230039, PR China d School of Science, Anhui Agriculture University, Hefei 230036, PR China e National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, PR China f Department of Physics and Astronomy, University of Arkansas at Little Rock, AR 72204, USA b c
a r t i c l e
i n f o
Article history: Received 15 May 2012 Received in revised form 31 October 2012 Accepted 2 November 2012 Available online 20 November 2012 Keywords: ZnS:Ag film Microstructure Optical constant Resistivity
a b s t r a c t The ZnS:Ag nano-composite films with different Ag volume fractions (5, 10, 15 and 2 vol.%) were deposited on glass substrates by sputtering. The microstructures, optical and electrical properties of the as-deposited films were studied. XRD measurement shows that all films are polycrystalline with a microstructure of body-centered cubic phase belonged to ZnS. The samples in which Ag volume concentrations lower than 20 vol.% show a preferential orientation along the (2 2 0) direction. When Ag concentration reaches 20 vol.%, the intensity of the (2 2 0) peak belonged to ZnS decreases dramatically while the Ag (1 1 1) peak emerged. Based on our analysis, it can be noted that Ag can help the ZnS grains grow when Ag concentration is lower than 20 vol.%. However, the ZnS crystal growth is suppressed when Ag concentration is 20 vol.%. The refractive index of the samples decreases with the increase of Ag concentration while the extinction coefficient of the samples increases. The resistivity of the film reaches the minimum when Ag concentration is 20 vol.%, showing the electrical conductivity improvement of the composite film by doped Ag nano-particles. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The composite nano-ceramic thin films are deposited by inserting metal nano-particles into ceramics. They gather the advantages of both traditional composite materials and modern nano-materials [1–4]. Abeles et al. reported that, with variety of the metal concentration in the ceramic matrix, the composite films exist in the form of three regimes, namely metal, transition and dielectric regime [5]. The composite films in different regimes present different photoelectric properties, such as, low conductivity, high transmittance and quantum size effect for the films in the dielectric regime, surface plasma resonance for the films in transition regime, high conductivity and high reflection but low transmittance for the films in metal regime, and so on. ZnS, with a wide band gap, high transmittance as well as low dispersity in the visible and infrared range, has attracted more attentions for several decades due to its interesting luminescent ⇑ Corresponding author at: School of Physics & Material Science, Anhui University, Hefei 230039, PR China. Tel./fax: +86 551 5107237. E-mail address: [email protected] (Z. Sun). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.020
properties and important applications in optics and optoelectronics [6,7]. Most recently, many new preparation methods have been used to obtain ZnS material and many other elements also have been employed to prepare doped or multilayer composite ZnS films to improve their photoelectric properties [8–10]. Yi et al. reported the preparation of the Ag or Cu doped ZnS composite thin films and obtained blue emission in electroluminescence [11]. Moreover, the orange electroluminescent display devices which used ZnS:Mn films as emitting layer has developed in many fields [12,13]. Different metals or different element amounts can exhibit different influence on the properties of the composite films. So it is important to study the effect of element-doping on the structure, photo and electric properties of ZnS based composite films. In this work, ZnS:Ag composite films with different Ag concentrations (from 5 to 20 vol.%) were deposited by radio frequency (RF) magnetron sputtering. Two kinds of regime of the composite films were obtained, namely dielectric and transition regime. The microstructure, optical and electrical properties of ZnS:Ag nano-composite films were investigated. The refraction index (n) and extinction coefficient (k) were extracted from transmittance data ellipsometric spectrum (SE) technology and the results implied the potential
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use for emission wavelength tunability in electroluminescent plat display devices. The relationship between the properties of ZnS:Ag composite films was discussed in detail. 2. Experimental 2.1. Sample preparation ZnS:Ag composite films with different Ag concentrations (5, 10, 15 and 20 vol.%) were deposited on glasses by RF magnetron sputtering. The base vacuum was 8 104 Pa. The films were deposited at room temperature with a power density of 2.8 W/cm2 for 60 min at Ar pressure of 0.7 Pa. Composite target was fabricated by putting some small pieces of Ag sheet (99.99%) on ZnS target (99.99%) by elargol. Ag concentration in the films can be calculated by the follow equation [14]:
qM ¼
X¼
xmM qD SMD xmM qD SMD þ mD qM
AM AD
ð1Þ
ð2Þ
where qM is the volume concentration of Ag, mM the molar mass of Ag, mD the molar mass of ZnS, qM the density of Ag, qD the density of Ag, SMD the ratio of sputtering rate of Ag to ZnS, and x is the ratio of AM (exposed area of Ag) to AD (exposed area of ZnS). Considering the track effect caused by magnetic field in the sputtering process, silver strip was paste in symmetrically onto ZnS target by elargol. 2.2. Measurement Film thickness was measured by surface profiler meter (XP-1, Ambios, USA) using line scan model from film to substrate. The thickness of the films is about 400 nm. The microstructure of the films was analyzed by an X-ray diffractometer (XRD, MXP18AHF, Mark, Japan) employing Cu Ka radiation, accelerating voltage 40 kV, current 100 mA, scanning range 30–80°, glancing angle 2°, scanning step 0.02° and scanning rate 8°/min. The surface morphology of the films was characterized by atomic force microscope (AFM, AJ-IIIa, China). The optical transmittance was measured by a double beam spectrophotometer (UV-2550, Shimadzu, Japan). The electrical conductivity was measured by a four-probe meter (RTS-9, Probes Tech, China) which can carry out dual electrical measurement to avoid the shape affect of the film on the experimental results.
3. Results and discussion 3.1. XRD analysis XRD patterns in Fig. 1 show that all films are polycrystalline with a microstructure of body-centered cubic phase belonged to the ZnS. XRD pattern of Ag foil, the topmost XRD plot in Fig. 1, shows that the small peaks in the XRD patterns of the composite
films belonged to Ag. The samples in which Ag volume concentrations lower than 20% present a preferential ZnS (2 2 0) orientation. The intensity of ZnS (2 2 0) peak increases with the increase of Ag concentration, and the small peak around 2h = 65o belonged to Ag (2 2 0) suggests tiny crystal of Ag in the film containing 15 vol.% Ag. It is interesting that the intensity of ZnS (2 2 0) peak decreases significantly while Ag (1 1 1) emerged when Ag concentration is 20 vol.%. The average crystallite size can be calculated by Scherrer formula [15]
D¼
0:9k b cosh
ð3Þ
where k, h and b are the Cu Ka X-ray wavelength (1.5406 Å), diffraction angle and FWHM of the ZnS (2 2 0) peak, respectively. The interplanar spacing d and corresponding lattice constant a can be calculated by Bragg equation ð2d sin h ¼ kÞ as shown in Table 1. Based on Table 1, it can be seen that the lattice constant a for each film is almost the same but above the standard value (0.5414 nm) slightly [16]. The crystal boundaries take up more space because of the small size of the grains, so that the distance among the atoms in the grain boundary becomes larger and the diffraction angle deviates smaller, which leads to the bigger a values calculated by previous equations. Moreover, because of the greater radius of Ag+ (1.26 nm), it cannot replace the Zn2+ (0.074 nm) to become a codissolved structure but an embedded structure in which Ag exists in the form of atom clusters or nanoparticles [6,17,18]. The crystallite size of ZnS increases with the increase of Ag content when Ag concentration is less than 20 vol.%, while the crystallite size of ZnS decreases significantly when Ag concentration is 20 vol.%. Analysis say as for the films with the smaller concentrations of Ag (5, 10, and 15 vol.%), ZnS is predominant in the composite films and the small amount of Ag exists in the ZnS crystal boundary and on the film surface in the form of atomic clusters or tiny particles. During the deposition process, silver atoms, with strong kinetic energy, collide with the other atoms to provide enough activation energy to make chemical bonds restructure, which contributes to the growth of ZnS crystallite. Moreover, when Ag concentration achieved some certain value (e.g. 20 vol.%), the silver crystals are formed and absorb energy from surrounding atoms, which inhibits the growth of ZnS crystallite grains. It can be concluded that the smaller amount of Ag that is not or weakly crystallized would be helpful for the crystal growth of ZnS due to their kinetic energy contributed to the ZnS crystallization while the crystal growth of Ag would absorb energy from surroundings and inhibit the growth of ZnS crystal when Ag content becomes large enough. 3.2. Surface morphology As shown in Fig. 2(a–c), it can be intuitively found that the surface grain size increases with the increase of Ag concentration while it cannot be distinguished ZnS grains from Ag ones. The surface roughness are 2.655, 5.035 and 10.368 nm for the films containing 5, 10 and 15 vol.% Ag, respectively. This is in agreement with previous XRD analysis. Ag element disperses in ZnS matrix
Table 1 Parameters of the (2 2 0) diffraction peak of ZnS:Ag films.
Fig. 1. XRD spectra of the ZnS:Ag films.
Samples
2h (deg.)
d (Å)
b (deg.)
a (Å)
D (nm)
ZnS:5%Ag ZnS:10%Ag ZnS:15%Ag ZnS:20%Ag
47.29 47.34 47.34 47.34
1.92 1.91 1.91 1.91
0.84 0.70 0.58 1.03
5.43 5.42 5.42 5.42
10.3 12.3 14.9 8.40
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Fig. 2. AFM images of ZnS:Ag films, (a) ZnS:5%Ag, (b) ZnS:10%Ag, (c) ZnS:15%Ag, (d) ZnS:20%Ag.
in the form of atom clusters in the case of low Ag contents which can provide collision energy to the other atoms to improve the crystallization of ZnS in the sputtering process, which contributes to the increase of ZnS crystal size and the surface roughness. But the composite films are still in the dielectric regime because the network of Ag cannot be found on the film surface. In Fig. 2(d), it’s worth noting that Ag element formed the discontinuous network on the relatively flat ZnS matrix surface. In previous XRD analysis, ZnS almost turned to the amorphous structure while Ag (1 1 0) peak emerged obviously when Ag concentration is 20 vol.%. As shown in Fig. 2(d), it can be found that ZnS matrix shows the relatively plat surface due to its small grain size which is caused by the energy absorption of Ag crystallization during sputtering process.
3.3. Optical properties 3.3.1. Transmittance spectroscopy The transmission spectra of composite films and the fitting results were showed in Fig. 3. The fitted data are almost the same with the experimental ones, indicating the perfect fitting result. Root mean square error (RMSE) as well as fitting parameters was listed in Table 2. The transmittance of ZnS:Ag films decreases from 31.4% to 6.7% when Ag concentration increases from 5 to 20 vol.%,
Fig. 3. The transmittance spectra of ZnS:Ag films and its fitting results.
showing the influence of silver concentration on the transmittance of composite films. The peak in the transmittance curves of 5 and 10 vol.% Ag in the ZnS film, around 700–800 nm, is caused by the interference of the film.
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X. Song et al. / Journal of Alloys and Compounds 551 (2013) 430–434 Table 2 Fitting results of the ZnS:Ag films.
Table 3 Sheet resistance of ZnS:Ag films.
Samples
5%Ag
10%Ag
15%Ag
20%Ag
Samples
Sheet resistance (kX/h)
RMSE Thickness (nm) HF dielectric constant Plasma frequence (eV) Collision frequence (eV) Center energy (eV) Amplitude (eV) Vibrational frequence (eV)
0.1222 393.6715 1.2737 3.5124 11.0333 24.9493 13.2901 20.9006
0.1022 394.6463 3.5674 3.3963 3.9665 3.9078 2.6231 3.2142
0.0120 395.216 3.5891 1.1341 7.5632 2.6734 2.8474 3.1504
0.0588 394.8536 16.4019 1.3974 9.7465 2.4848 3.2333 3.7802
ZnS:5%Ag ZnS:10%Ag ZnS:15%Ag ZnS:20%Ag
39.84 19.17 6.22 0.8
3.3.2. Optical constants The transmittance spectra of ZnS:Ag films were analyzed by ellipsometric spectrum technology. Concerning both free electrons and lattice scattering contributing to the transparent conductive properties of ZnS:Ag thin films, ‘‘Drude + Lorentz Oscillator’’ model was used to calculate the optical constant [19,20]. The Dispersion equation is as follow:
e ¼ e1 1 þ
m X
A2j
j¼1
ðEcenter Þ2j EðE imÞ
x2p EðE þ imÞ
! ð4Þ
where E is the energy of incident photon, j the number of harmonicoscillator (here j = 1), e the dielectric constant, e1 high frequency dielectric constant, Ecenter the centre energy, Aj the oscillator amplitude, xp the resonant frequency of plasma, m the collision frequency. The refractive index n and extinction coefficient k were extracted from transmittance data by SE method. It can be seen from Fig. 4(a) that with the increase of the Ag concentration the refractive index n of films decreases significantly when Ag doping concentration is lower than 20 vol.%. The refractive index n of the films can be presented as [17]:
n2 ¼ eopt
4pNe2 m x2p
ð5Þ
where n is the refractive index, eopt the optic frequency dielectric, N the carrier concentration, m⁄ the effective mass of carriers and xp is the oscillation frequency of plasma. From above equation, the refractive index n decreases with the increase of the carrier concentration. But when Ag concentration is equal to 20 vol.%, the refractive index n significantly increases; we suppose that the increase of the film density and the large radius of Ag+ (1.26 nm) are contributed to this result. The composite film with high refractive index could be used for high reflective or antireflective multilayer coating system [21].
Fig. 5. The sheet resistance (Rs) of ZnS:Ag films.
It is observed from Fig. 4(b) that extinction coefficient increases with the increase of Ag concentration and the film density. An absorption peak emerged at 323.7 nm when Ag concentration is 10 vol.%, and it red shifts to 444.4 and 468.5 nm when Ag concentration is 15, 20 vol.%, respectively. According to previous reports [22], the absorption peak is caused by surface plasma resonance from Ag particles. As shown in Fig. 4(b), the peak intensity increases and peak position shifts to the longer wavelength due to the increase of electron free path with the increase of Ag crystal size. The tunable absorption peak position by different Ag particle sizes can be used for modifying the emission wavelength range, implying the potential application in electroluminescent plat display devices. 3.4. Electrical conductivity The sheet resistance (Rs) of the composite films was shown in Table 3 and Fig. 5. It is shown that the sheet resistance decreases with the increase of Ag concentration. The film resistivity reaches
Fig. 4. The optical constants of ZnS:Ag films, (a) refractive index and (b) extinction coefficient.
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the minimum of 0.8 kX/h when Ag concentration is 20 vol.%, indicating that Ag nano-particles improve the electrical conductivity of ZnS films. As analyzed previously in the case of low metal concentration, dielectric state presents where the tunnel conduction within the spacing between isolated metal grains is predominant [5]. In transition state, the osmosis conduction in metal mesh and the tunnel conduction within isolated metal grains contribute to the conduction. When the concentration of metal is high enough to form continuous metal matrix, the film shows high conductivity and it would be in metal state. Judged by the data of the sheet resistance, all these composite films do not achieve the metal state. As for the films in which Ag concentration below 20 vol.%, they are still in dielectric state. The sheet resistance decreases by one order of magnitude when Ag concentration increases from 5 to 20 vol.% and the composite films change from the dielectric regime to transition regime accordingly. 4. Conclusions ZnS films doped with different Ag concentrations were deposited on glass substrates by means of magnetron sputtering. XRD shows there is almost no diffraction peak belonged to Ag when Ag concentration is less than 15 vol.%. It is shown that Ag can help ZnS grain growth when Ag concentration is less than 20 vol.%. But ZnS crystal grain growth would be suppressed when Ag concentration is 20 vol.%. The transmittance of ZnS:Ag films decreases with Ag concentration increase. When Ag concentration less than 20 vol.%, the refraction index decreases with the increase of Ag concentration while the extinction coefficient shows the opposite variation tendency. The absorption peak which caused by the surface plasma resonance emerges at 323.7 nm for the film with 10 vol.% Ag, and it red shifts to 444.4 and 468.5 nm when Ag concentrations are 15 and 20 vol.%, respectively. The sheet resistance decreases with Ag concentration increase. As for the composite film with 20 vol.% Ag, Ag element constructs the conductive network so that the sample shows the highest conductivity. Acknowledgements This work has been supported by National Natural Science Foundation of China (Nos. 51272001 and 51072001), National
Science Research Foundation for Scholars Return from Overseas, Ministry of Education of China, Anhui Provincial Natural Science Foundation (1208085MF99), Provincial Natural Science Research Program of Higher Education Institutions of Anhui Province (KJ2012Z102), Introduction and Stabilizing the Talent Project of Anhui Agriculture University (yj2011-40), Outstanding Young Scientific Foundation of Anhui University (KJJQ1103), Youth Science Research Foundation of Anhui University (2009QN008A) and ‘‘211 project’’ of Anhui University (KJTD004B). References [1] C.M. Huang, K.W. Cheng, Y.R. Jhan, T.W. Chung, Thin Solid Films 515 (2007) 7935–7944. [2] S.W. Choi, S.H. Hong, K.Y. Ko, Y.R. Do, J. Am. Ceram. Soc. 91 (2008) 451–455. [3] D.F. Li, B. Deng, S.W. Xue, Z.G. Wang, F. Gao, Appl. Phys. Lett. 99 (2011). 052109 (1–3). [4] H. Qu, L.X. Cao, G. Su, W. Liu, Y.G. Sun, B.H. Dong, J. Appl. Phys. 106 (2009). 093506 (1–6). [5] B. Abeles, P. Sheng, M.D. Coutts, Adv. Phys. 24 (1975) 407–461. [6] R.N. Bhargava, D. Gallagher, Phys. Rev. Lett. 72 (1994) 416–419. [7] B.L. Abramsa, L. Williams, J.S. Bang, P.H. Holloway, J. Appl. Phys. 97 (2005). 033521 (1–4). [8] F.Y. Shen, W.X. Que, X.T. Yin, Y.W. Huang, Q.Y. Jia, J. Alloys Comp. 509 (2011) 9105–9110. [9] H.Y. Zhang, L. Jin, B. Song, J.C. Han, G.G. Wang, X.P. Kuang, R. Sun, J. Alloys Comp. 539 (2012) 40–43. [10] J.S. Liu, C.B. Zhao, Z.Q. Li, J.K. Chen, J. Alloys Comp. 509 (2011) 9428–9433. [11] L.X. Yi, Y.B. Hou, X.F. Wang, Z. Xu, J. Optoelectron. Laser 11 (2000) 570–575. [12] X.J. Wang, Q.L. Zhang, B.S. Zou, A.H. Lei, P.Y. Ren, Appl. Surf. Sci. 257 (2011) 10898–10902. [13] W.J. Zhang, Y. Li, H. Zhang, X.G. Zhou, X.H. Zhong, Inorg. Chem. 50 (2011) 10432–10438. [14] Z.G. Zhou, Z.H. Lu, Y. Zhang, J. Wuhan Univ. 41 (1995) 347–350. [15] H.X. Chen, J.J. Ding, S.Y. Ma, Physica E 42 (2010) 1487–1491. [16] V. Dimitrova, J. Tate, Thin Solid Films 365 (2000) 134–138. [17] J.H. Li, X.H. Zeng, Z.H. Ji, Y.P. Hu, B. Chen, Y.P. Fan, Acta Physica Sinica 60 (2011). 057101 (1–7). [18] H. Qu, L.X. Cao, G. Su, W. Liu, D.X. Jiang, B.H. Dong, Y.G. Sun, Spectrosc. Spectral Anal. 29 (2009) 305–308. [19] A.C. Galca, M. Secu, A. Vlad, J.D. Pedarnig, Thin Solid Films 518 (2010) 4603– 4606. [20] T.C. Peng, X.H. Xiao, X.Y. Han, X.D. Zhou, W. Wu, F. Ren, C.Z. Jiang, Appl. Surf. Sci. 257 (2011) 5908–5912. [21] B.S. Chiou, J.H. Tsai, J. Mater. Sci. 10 (1999) 491–495. [22] Y.K. Mishra, S. Mohapatra, D. Kabiraj, B. Mohanta, N.P. Lalla, J.C. Pivin, D.K. Avasthi, Scripta Mater. 56 (2007) 629–632.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of Ag-doping on microstructural, optical and electrical properties of sputtering-derived ZnS films Xueping Song a,b,c, Shiwei Shi a,b,c, Chunbin Cao d, Xiaoshuang Chen e, Jingbiao Cui f, Gang He a,b,c, Zhaoqi Sun a,b,c,f,⇑ a
School of Physics & Material Science, Anhui University, Hefei 230039, PR China Key Laboratory of Opto-Electronic Information Acquisition & Manipulation, Ministry of Education, Hefei 230039, PR China Anhui Key Laboratory of Information Materials and Devices, Hefei 230039, PR China d School of Science, Anhui Agriculture University, Hefei 230036, PR China e National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, PR China f Department of Physics and Astronomy, University of Arkansas at Little Rock, AR 72204, USA b c
a r t i c l e
i n f o
Article history: Received 15 May 2012 Received in revised form 31 October 2012 Accepted 2 November 2012 Available online 20 November 2012 Keywords: ZnS:Ag film Microstructure Optical constant Resistivity
a b s t r a c t The ZnS:Ag nano-composite films with different Ag volume fractions (5, 10, 15 and 2 vol.%) were deposited on glass substrates by sputtering. The microstructures, optical and electrical properties of the as-deposited films were studied. XRD measurement shows that all films are polycrystalline with a microstructure of body-centered cubic phase belonged to ZnS. The samples in which Ag volume concentrations lower than 20 vol.% show a preferential orientation along the (2 2 0) direction. When Ag concentration reaches 20 vol.%, the intensity of the (2 2 0) peak belonged to ZnS decreases dramatically while the Ag (1 1 1) peak emerged. Based on our analysis, it can be noted that Ag can help the ZnS grains grow when Ag concentration is lower than 20 vol.%. However, the ZnS crystal growth is suppressed when Ag concentration is 20 vol.%. The refractive index of the samples decreases with the increase of Ag concentration while the extinction coefficient of the samples increases. The resistivity of the film reaches the minimum when Ag concentration is 20 vol.%, showing the electrical conductivity improvement of the composite film by doped Ag nano-particles. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The composite nano-ceramic thin films are deposited by inserting metal nano-particles into ceramics. They gather the advantages of both traditional composite materials and modern nano-materials [1–4]. Abeles et al. reported that, with variety of the metal concentration in the ceramic matrix, the composite films exist in the form of three regimes, namely metal, transition and dielectric regime [5]. The composite films in different regimes present different photoelectric properties, such as, low conductivity, high transmittance and quantum size effect for the films in the dielectric regime, surface plasma resonance for the films in transition regime, high conductivity and high reflection but low transmittance for the films in metal regime, and so on. ZnS, with a wide band gap, high transmittance as well as low dispersity in the visible and infrared range, has attracted more attentions for several decades due to its interesting luminescent ⇑ Corresponding author at: School of Physics & Material Science, Anhui University, Hefei 230039, PR China. Tel./fax: +86 551 5107237. E-mail address: [email protected] (Z. Sun). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.020
properties and important applications in optics and optoelectronics [6,7]. Most recently, many new preparation methods have been used to obtain ZnS material and many other elements also have been employed to prepare doped or multilayer composite ZnS films to improve their photoelectric properties [8–10]. Yi et al. reported the preparation of the Ag or Cu doped ZnS composite thin films and obtained blue emission in electroluminescence [11]. Moreover, the orange electroluminescent display devices which used ZnS:Mn films as emitting layer has developed in many fields [12,13]. Different metals or different element amounts can exhibit different influence on the properties of the composite films. So it is important to study the effect of element-doping on the structure, photo and electric properties of ZnS based composite films. In this work, ZnS:Ag composite films with different Ag concentrations (from 5 to 20 vol.%) were deposited by radio frequency (RF) magnetron sputtering. Two kinds of regime of the composite films were obtained, namely dielectric and transition regime. The microstructure, optical and electrical properties of ZnS:Ag nano-composite films were investigated. The refraction index (n) and extinction coefficient (k) were extracted from transmittance data ellipsometric spectrum (SE) technology and the results implied the potential
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X. Song et al. / Journal of Alloys and Compounds 551 (2013) 430–434
use for emission wavelength tunability in electroluminescent plat display devices. The relationship between the properties of ZnS:Ag composite films was discussed in detail. 2. Experimental 2.1. Sample preparation ZnS:Ag composite films with different Ag concentrations (5, 10, 15 and 20 vol.%) were deposited on glasses by RF magnetron sputtering. The base vacuum was 8 104 Pa. The films were deposited at room temperature with a power density of 2.8 W/cm2 for 60 min at Ar pressure of 0.7 Pa. Composite target was fabricated by putting some small pieces of Ag sheet (99.99%) on ZnS target (99.99%) by elargol. Ag concentration in the films can be calculated by the follow equation [14]:
qM ¼
X¼
xmM qD SMD xmM qD SMD þ mD qM
AM AD
ð1Þ
ð2Þ
where qM is the volume concentration of Ag, mM the molar mass of Ag, mD the molar mass of ZnS, qM the density of Ag, qD the density of Ag, SMD the ratio of sputtering rate of Ag to ZnS, and x is the ratio of AM (exposed area of Ag) to AD (exposed area of ZnS). Considering the track effect caused by magnetic field in the sputtering process, silver strip was paste in symmetrically onto ZnS target by elargol. 2.2. Measurement Film thickness was measured by surface profiler meter (XP-1, Ambios, USA) using line scan model from film to substrate. The thickness of the films is about 400 nm. The microstructure of the films was analyzed by an X-ray diffractometer (XRD, MXP18AHF, Mark, Japan) employing Cu Ka radiation, accelerating voltage 40 kV, current 100 mA, scanning range 30–80°, glancing angle 2°, scanning step 0.02° and scanning rate 8°/min. The surface morphology of the films was characterized by atomic force microscope (AFM, AJ-IIIa, China). The optical transmittance was measured by a double beam spectrophotometer (UV-2550, Shimadzu, Japan). The electrical conductivity was measured by a four-probe meter (RTS-9, Probes Tech, China) which can carry out dual electrical measurement to avoid the shape affect of the film on the experimental results.
3. Results and discussion 3.1. XRD analysis XRD patterns in Fig. 1 show that all films are polycrystalline with a microstructure of body-centered cubic phase belonged to the ZnS. XRD pattern of Ag foil, the topmost XRD plot in Fig. 1, shows that the small peaks in the XRD patterns of the composite
films belonged to Ag. The samples in which Ag volume concentrations lower than 20% present a preferential ZnS (2 2 0) orientation. The intensity of ZnS (2 2 0) peak increases with the increase of Ag concentration, and the small peak around 2h = 65o belonged to Ag (2 2 0) suggests tiny crystal of Ag in the film containing 15 vol.% Ag. It is interesting that the intensity of ZnS (2 2 0) peak decreases significantly while Ag (1 1 1) emerged when Ag concentration is 20 vol.%. The average crystallite size can be calculated by Scherrer formula [15]
D¼
0:9k b cosh
ð3Þ
where k, h and b are the Cu Ka X-ray wavelength (1.5406 Å), diffraction angle and FWHM of the ZnS (2 2 0) peak, respectively. The interplanar spacing d and corresponding lattice constant a can be calculated by Bragg equation ð2d sin h ¼ kÞ as shown in Table 1. Based on Table 1, it can be seen that the lattice constant a for each film is almost the same but above the standard value (0.5414 nm) slightly [16]. The crystal boundaries take up more space because of the small size of the grains, so that the distance among the atoms in the grain boundary becomes larger and the diffraction angle deviates smaller, which leads to the bigger a values calculated by previous equations. Moreover, because of the greater radius of Ag+ (1.26 nm), it cannot replace the Zn2+ (0.074 nm) to become a codissolved structure but an embedded structure in which Ag exists in the form of atom clusters or nanoparticles [6,17,18]. The crystallite size of ZnS increases with the increase of Ag content when Ag concentration is less than 20 vol.%, while the crystallite size of ZnS decreases significantly when Ag concentration is 20 vol.%. Analysis say as for the films with the smaller concentrations of Ag (5, 10, and 15 vol.%), ZnS is predominant in the composite films and the small amount of Ag exists in the ZnS crystal boundary and on the film surface in the form of atomic clusters or tiny particles. During the deposition process, silver atoms, with strong kinetic energy, collide with the other atoms to provide enough activation energy to make chemical bonds restructure, which contributes to the growth of ZnS crystallite. Moreover, when Ag concentration achieved some certain value (e.g. 20 vol.%), the silver crystals are formed and absorb energy from surrounding atoms, which inhibits the growth of ZnS crystallite grains. It can be concluded that the smaller amount of Ag that is not or weakly crystallized would be helpful for the crystal growth of ZnS due to their kinetic energy contributed to the ZnS crystallization while the crystal growth of Ag would absorb energy from surroundings and inhibit the growth of ZnS crystal when Ag content becomes large enough. 3.2. Surface morphology As shown in Fig. 2(a–c), it can be intuitively found that the surface grain size increases with the increase of Ag concentration while it cannot be distinguished ZnS grains from Ag ones. The surface roughness are 2.655, 5.035 and 10.368 nm for the films containing 5, 10 and 15 vol.% Ag, respectively. This is in agreement with previous XRD analysis. Ag element disperses in ZnS matrix
Table 1 Parameters of the (2 2 0) diffraction peak of ZnS:Ag films.
Fig. 1. XRD spectra of the ZnS:Ag films.
Samples
2h (deg.)
d (Å)
b (deg.)
a (Å)
D (nm)
ZnS:5%Ag ZnS:10%Ag ZnS:15%Ag ZnS:20%Ag
47.29 47.34 47.34 47.34
1.92 1.91 1.91 1.91
0.84 0.70 0.58 1.03
5.43 5.42 5.42 5.42
10.3 12.3 14.9 8.40
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Fig. 2. AFM images of ZnS:Ag films, (a) ZnS:5%Ag, (b) ZnS:10%Ag, (c) ZnS:15%Ag, (d) ZnS:20%Ag.
in the form of atom clusters in the case of low Ag contents which can provide collision energy to the other atoms to improve the crystallization of ZnS in the sputtering process, which contributes to the increase of ZnS crystal size and the surface roughness. But the composite films are still in the dielectric regime because the network of Ag cannot be found on the film surface. In Fig. 2(d), it’s worth noting that Ag element formed the discontinuous network on the relatively flat ZnS matrix surface. In previous XRD analysis, ZnS almost turned to the amorphous structure while Ag (1 1 0) peak emerged obviously when Ag concentration is 20 vol.%. As shown in Fig. 2(d), it can be found that ZnS matrix shows the relatively plat surface due to its small grain size which is caused by the energy absorption of Ag crystallization during sputtering process.
3.3. Optical properties 3.3.1. Transmittance spectroscopy The transmission spectra of composite films and the fitting results were showed in Fig. 3. The fitted data are almost the same with the experimental ones, indicating the perfect fitting result. Root mean square error (RMSE) as well as fitting parameters was listed in Table 2. The transmittance of ZnS:Ag films decreases from 31.4% to 6.7% when Ag concentration increases from 5 to 20 vol.%,
Fig. 3. The transmittance spectra of ZnS:Ag films and its fitting results.
showing the influence of silver concentration on the transmittance of composite films. The peak in the transmittance curves of 5 and 10 vol.% Ag in the ZnS film, around 700–800 nm, is caused by the interference of the film.
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Table 3 Sheet resistance of ZnS:Ag films.
Samples
5%Ag
10%Ag
15%Ag
20%Ag
Samples
Sheet resistance (kX/h)
RMSE Thickness (nm) HF dielectric constant Plasma frequence (eV) Collision frequence (eV) Center energy (eV) Amplitude (eV) Vibrational frequence (eV)
0.1222 393.6715 1.2737 3.5124 11.0333 24.9493 13.2901 20.9006
0.1022 394.6463 3.5674 3.3963 3.9665 3.9078 2.6231 3.2142
0.0120 395.216 3.5891 1.1341 7.5632 2.6734 2.8474 3.1504
0.0588 394.8536 16.4019 1.3974 9.7465 2.4848 3.2333 3.7802
ZnS:5%Ag ZnS:10%Ag ZnS:15%Ag ZnS:20%Ag
39.84 19.17 6.22 0.8
3.3.2. Optical constants The transmittance spectra of ZnS:Ag films were analyzed by ellipsometric spectrum technology. Concerning both free electrons and lattice scattering contributing to the transparent conductive properties of ZnS:Ag thin films, ‘‘Drude + Lorentz Oscillator’’ model was used to calculate the optical constant [19,20]. The Dispersion equation is as follow:
e ¼ e1 1 þ
m X
A2j
j¼1
ðEcenter Þ2j EðE imÞ
x2p EðE þ imÞ
! ð4Þ
where E is the energy of incident photon, j the number of harmonicoscillator (here j = 1), e the dielectric constant, e1 high frequency dielectric constant, Ecenter the centre energy, Aj the oscillator amplitude, xp the resonant frequency of plasma, m the collision frequency. The refractive index n and extinction coefficient k were extracted from transmittance data by SE method. It can be seen from Fig. 4(a) that with the increase of the Ag concentration the refractive index n of films decreases significantly when Ag doping concentration is lower than 20 vol.%. The refractive index n of the films can be presented as [17]:
n2 ¼ eopt
4pNe2 m x2p
ð5Þ
where n is the refractive index, eopt the optic frequency dielectric, N the carrier concentration, m⁄ the effective mass of carriers and xp is the oscillation frequency of plasma. From above equation, the refractive index n decreases with the increase of the carrier concentration. But when Ag concentration is equal to 20 vol.%, the refractive index n significantly increases; we suppose that the increase of the film density and the large radius of Ag+ (1.26 nm) are contributed to this result. The composite film with high refractive index could be used for high reflective or antireflective multilayer coating system [21].
Fig. 5. The sheet resistance (Rs) of ZnS:Ag films.
It is observed from Fig. 4(b) that extinction coefficient increases with the increase of Ag concentration and the film density. An absorption peak emerged at 323.7 nm when Ag concentration is 10 vol.%, and it red shifts to 444.4 and 468.5 nm when Ag concentration is 15, 20 vol.%, respectively. According to previous reports [22], the absorption peak is caused by surface plasma resonance from Ag particles. As shown in Fig. 4(b), the peak intensity increases and peak position shifts to the longer wavelength due to the increase of electron free path with the increase of Ag crystal size. The tunable absorption peak position by different Ag particle sizes can be used for modifying the emission wavelength range, implying the potential application in electroluminescent plat display devices. 3.4. Electrical conductivity The sheet resistance (Rs) of the composite films was shown in Table 3 and Fig. 5. It is shown that the sheet resistance decreases with the increase of Ag concentration. The film resistivity reaches
Fig. 4. The optical constants of ZnS:Ag films, (a) refractive index and (b) extinction coefficient.
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the minimum of 0.8 kX/h when Ag concentration is 20 vol.%, indicating that Ag nano-particles improve the electrical conductivity of ZnS films. As analyzed previously in the case of low metal concentration, dielectric state presents where the tunnel conduction within the spacing between isolated metal grains is predominant [5]. In transition state, the osmosis conduction in metal mesh and the tunnel conduction within isolated metal grains contribute to the conduction. When the concentration of metal is high enough to form continuous metal matrix, the film shows high conductivity and it would be in metal state. Judged by the data of the sheet resistance, all these composite films do not achieve the metal state. As for the films in which Ag concentration below 20 vol.%, they are still in dielectric state. The sheet resistance decreases by one order of magnitude when Ag concentration increases from 5 to 20 vol.% and the composite films change from the dielectric regime to transition regime accordingly. 4. Conclusions ZnS films doped with different Ag concentrations were deposited on glass substrates by means of magnetron sputtering. XRD shows there is almost no diffraction peak belonged to Ag when Ag concentration is less than 15 vol.%. It is shown that Ag can help ZnS grain growth when Ag concentration is less than 20 vol.%. But ZnS crystal grain growth would be suppressed when Ag concentration is 20 vol.%. The transmittance of ZnS:Ag films decreases with Ag concentration increase. When Ag concentration less than 20 vol.%, the refraction index decreases with the increase of Ag concentration while the extinction coefficient shows the opposite variation tendency. The absorption peak which caused by the surface plasma resonance emerges at 323.7 nm for the film with 10 vol.% Ag, and it red shifts to 444.4 and 468.5 nm when Ag concentrations are 15 and 20 vol.%, respectively. The sheet resistance decreases with Ag concentration increase. As for the composite film with 20 vol.% Ag, Ag element constructs the conductive network so that the sample shows the highest conductivity. Acknowledgements This work has been supported by National Natural Science Foundation of China (Nos. 51272001 and 51072001), National
Science Research Foundation for Scholars Return from Overseas, Ministry of Education of China, Anhui Provincial Natural Science Foundation (1208085MF99), Provincial Natural Science Research Program of Higher Education Institutions of Anhui Province (KJ2012Z102), Introduction and Stabilizing the Talent Project of Anhui Agriculture University (yj2011-40), Outstanding Young Scientific Foundation of Anhui University (KJJQ1103), Youth Science Research Foundation of Anhui University (2009QN008A) and ‘‘211 project’’ of Anhui University (KJTD004B). References [1] C.M. Huang, K.W. Cheng, Y.R. Jhan, T.W. Chung, Thin Solid Films 515 (2007) 7935–7944. [2] S.W. Choi, S.H. Hong, K.Y. Ko, Y.R. Do, J. Am. Ceram. Soc. 91 (2008) 451–455. [3] D.F. Li, B. Deng, S.W. Xue, Z.G. Wang, F. Gao, Appl. Phys. Lett. 99 (2011). 052109 (1–3). [4] H. Qu, L.X. Cao, G. Su, W. Liu, Y.G. Sun, B.H. Dong, J. Appl. Phys. 106 (2009). 093506 (1–6). [5] B. Abeles, P. Sheng, M.D. Coutts, Adv. Phys. 24 (1975) 407–461. [6] R.N. Bhargava, D. Gallagher, Phys. Rev. Lett. 72 (1994) 416–419. [7] B.L. Abramsa, L. Williams, J.S. Bang, P.H. Holloway, J. Appl. Phys. 97 (2005). 033521 (1–4). [8] F.Y. Shen, W.X. Que, X.T. Yin, Y.W. Huang, Q.Y. Jia, J. Alloys Comp. 509 (2011) 9105–9110. [9] H.Y. Zhang, L. Jin, B. Song, J.C. Han, G.G. Wang, X.P. Kuang, R. Sun, J. Alloys Comp. 539 (2012) 40–43. [10] J.S. Liu, C.B. Zhao, Z.Q. Li, J.K. Chen, J. Alloys Comp. 509 (2011) 9428–9433. [11] L.X. Yi, Y.B. Hou, X.F. Wang, Z. Xu, J. Optoelectron. Laser 11 (2000) 570–575. [12] X.J. Wang, Q.L. Zhang, B.S. Zou, A.H. Lei, P.Y. Ren, Appl. Surf. Sci. 257 (2011) 10898–10902. [13] W.J. Zhang, Y. Li, H. Zhang, X.G. Zhou, X.H. Zhong, Inorg. Chem. 50 (2011) 10432–10438. [14] Z.G. Zhou, Z.H. Lu, Y. Zhang, J. Wuhan Univ. 41 (1995) 347–350. [15] H.X. Chen, J.J. Ding, S.Y. Ma, Physica E 42 (2010) 1487–1491. [16] V. Dimitrova, J. Tate, Thin Solid Films 365 (2000) 134–138. [17] J.H. Li, X.H. Zeng, Z.H. Ji, Y.P. Hu, B. Chen, Y.P. Fan, Acta Physica Sinica 60 (2011). 057101 (1–7). [18] H. Qu, L.X. Cao, G. Su, W. Liu, D.X. Jiang, B.H. Dong, Y.G. Sun, Spectrosc. Spectral Anal. 29 (2009) 305–308. [19] A.C. Galca, M. Secu, A. Vlad, J.D. Pedarnig, Thin Solid Films 518 (2010) 4603– 4606. [20] T.C. Peng, X.H. Xiao, X.Y. Han, X.D. Zhou, W. Wu, F. Ren, C.Z. Jiang, Appl. Surf. Sci. 257 (2011) 5908–5912. [21] B.S. Chiou, J.H. Tsai, J. Mater. Sci. 10 (1999) 491–495. [22] Y.K. Mishra, S. Mohapatra, D. Kabiraj, B. Mohanta, N.P. Lalla, J.C. Pivin, D.K. Avasthi, Scripta Mater. 56 (2007) 629–632.