- Email: [email protected]
Influence of RF magnetron sputtering conditions on the properties of transparent conductive gallium-doped magnesium zinc oxide thin films
Surface & Coatings Technology 231 (2013) 539–542
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Influence of RF magnetron sputtering conditions on the properties of transparent conductive gallium-doped magnesium zinc oxide thin films Hsin-Chun Lu ⁎, Jia-Chiuan Jou, Chun-Lung Chu Department of Chemical and Materials Engineering, Chang Gung University, Kwei-Shan, Tao-Yuan 333, Taiwan, ROC
a r t i c l e
i n f o
Available online 18 October 2012 Keywords: Gallium-doped magnesium zinc oxide Thin films Electrical and optical properties RF magnetron sputtering
a b s t r a c t Gallium-doped magnesium zinc oxide (GMZO) thin films were deposited onto glass substrates using RF magnetron sputtering using GMZO ceramic targets under different sputtering conditions. The substrate temperature and deposition pressure effects on the structural, electrical and optical properties of the GMZO thin film were investigated. Transparent conductive GMZO thin films were demonstrated in this study to possess a wurtzite structure, average transmittance above 80% in the 400 nm and 800 nm wavelength range with a low resistivity of 1.62 × 10 −3 Ω cm when deposited onto glass substrates using RF magnetron sputtering. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Transparent conductive oxide (TCO) thin films have been widely used as transparent electrodes in flat panel displays, touch panels and solar cells. Semiconducting zinc oxide (ZnO) is one of the most promising TCO materials because ZnO is an abundant, low-cost, and non-toxic material [1]. The intrinsic n-type semi-conductivity of ZnO originates from such structural defects as oxygen vacancies and zinc interstitials [2]. To further improve its electrical conductivity, ZnO was doped with other metal elements such as boron, aluminum, gallium, etc. Among these, aluminum-doped zinc oxide (AZO) and gallium-doped zinc oxide (GZO) are the most widely explored [3,1]. The optical properties of ZnO can be adjusted by changing the ZnO energy band gap through doping ZnO with wide band gap materials. Minemoto et al. [4] indicated that the energy band gap of ZnO could be changed from 3.24 eV to 4.02 eV by doping zinc oxide with the wide band gap MgO (7.70 eV). By increasing the concentration of Mg dopants in this magnesium zinc oxide (MgxZn1 −xO, MZO) mixture and, therefore, the band gap energy of MZO, the cell conversion efficiency (of the Cu [In, Ga] Se2 [CIGS] thin film solar cells using MZO as the window layer material) was enhanced due to increased transmittance of sunlight in the near infrared region [5]. However, the resistivities of MZO thin films were over 10 5 Ω cm and nearly equivalent to the resistivity of intrinsic ZnO. To enhance the electrical conductivity of MZO films, it was suggested that extra dopant be added to the MZO thin films [6]. Among the potential candidates aluminum-doped magnesium zinc oxide (AMZO) and galliumdoped magnesium zinc oxide (GMZO) received the most attention [7,8]. Due to the slight size difference between Ga+3 (0.62 nm) and Zn+2 (0.74 nm) ions compared to that between Al+3 (0.53 nm) and Zn+2 (0.74 nm) ions, the lattice distortion of the GMZO crystals is less ⁎ Corresponding author. Tel.: +886 3 211 8800x5292; fax: +886 3 211 8668. E-mail address: [email protected] (H.-C. Lu). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.10.029
severe than that of AMZO crystals [9]. This lower lattice distortion results in higher electron mobility, higher allowable dopant concentration and higher electron concentration in the GMZO thin films and, therefore, the observed lower resistivity of GMZO thin films [10,11]. The higher electron concentration in the GMZO thin films increased the Burstein–Moss effect, resulting in the reflective behavior improvement in GMZO thin films in the near infrared region over that of AMZO thin films [11]. Although GMZO thin films possess the advantages mentioned above, few reports are found in the literature describing their preparation. Wei et al. [8] reported on MgxZn1 − xO:Ga thin films with a minimum resistivity of 1.9 × 10 −3 Ω cm using pulsed laser deposition using Ga-doped MgxZn1−xO targets. Ma et al. [10] demonstrated galliumdoped MgxZn1−xO thin film preparation with a minimum resistivity of 3 × 10 − 4 Ω cm using DC reactive magnetron sputtering using a ternary metal alloy target. Maejima et al. [11] prepared gallium-doped MgxZn1−xO thin films with a minimum resistivity of 8×10−4 Ω cm using a multi-cathode RF magnetron co-sputtering process using GZO and MgO targets simultaneously. To be mass-produced for industrial purposes, GMZO thin films must follow the industrially standardized TCO thin film production process of single cathode magnetron sputtering, which uses the oxide ceramic targets of unique designated compositions [12,13]. This paper presents GMZO thin film deposition using single cathode RF magnetron sputtering using a GMZO ceramic target of designated composition without post deposition annealing. The substrate temperature and deposition pressure effects on the structure, electrical and optical properties of RF magnetron sputtered GMZO thin films are also reported. 2. Experimental GMZO thin films were prepared on glass substrates (Corning 1737) using RF magnetron sputtering. GMZO targets, 2 in. in diameter with a
540
H.-C. Lu et al. / Surface & Coatings Technology 231 (2013) 539–542
97 wt.% Zn0.94Mg0.06O and 3 wt.% Ga2O3 composition, were prepared by sintering the green bodies formed by mixing and compacting a mixture of ZnO powders, MgO powders and Ga2O3 powders in an argon atmosphere for 3 h at 1673 K. After the 2 cm× 2 cm glass substrates were cleaned with de-ionized water and acetone, the substrates were placed into a substrate holder at a distance of 6 cm from the GMZO target. The sputtering chamber was then pumped down from atmospheric pressure to a base pressure of 0.8×10−3 Pa. Argon gas (99.999% purity) was then introduced into the sputtering chamber. GMZO thin films were deposited onto the glass substrates using RF magnetron sputtering with the sputtering power at 80 W. The deposition pressure effect on the GMZO thin film characteristics was investigated by varying the deposition pressure from 0.4 Pa to 1.2 Pa when the substrate temperature was controlled at 473 K. After finding the deposition pressure that produced GMZO thin films with the lowest electrical resistivity the substrate temperature effect on the GMZO thin film characteristics was studied by depositing GMZO thin films at different substrate temperatures at that specific deposition pressure. The deposited GMZO thin films were characterized by studying their structural, electrical and optical properties. X-ray diffraction (XRD) was used to analyze the crystallographic structure and orientation of the deposited thin films. A Bruker's AXS D5005 X-ray diffractometer with Cu Kα radiation (λ = 1.5412 Å) was used to record the X-ray data in the 2θ range between 30° and 70°. The electrical resistivity and Hall mobility of the deposited thin films were measured by Hall measurement (Ecopia HMS-3000). The thin film thickness was measured using a profilometer (Veeco Detak ST). The optical transmittance of the deposited thin films was recorded using Perkin Elmer Lambda 900 UV–VIS-NIR double beam spectrophotometer where the uncoated glass substrate was used as a reference. The GMZO chemical bonding state information was studied using X-ray photoelectron spectroscopy (XPS) using a VG Scientific ESCA LAB 250 with monochromatic Al Kα X-ray source (1486.6 eV). All spectra were calibrated with C 1s peak at 284.5 eV. 3. Results and discussion Fig. 1 shows the XRD patterns of the RF magnetron sputtered thin films deposited at 473 K and different deposition pressures using the GMZO target with 3 wt.% Ga2O3 doping. A very strong peak corresponding to the (002) plane (JCPDS 00-036-1451) of hexagonal wurtzitestructured ZnO and a minor peak near 2θ = 30.92° also attributed to hexagonal wurtzite-structured ZnO (JCPDS 00-021-1486) were
observed in each of these XRD patterns, thereby indicating a preferred crystallite orientation in the (002) direction. The diffraction peaks in these XRD patterns were observed to shift away from the peak positions in the standard ZnO JCPDS pattern. This peak shift probably results from a combination of Zn + 2 ion replacement by the smaller Mg+2 and Ga+3 ions in the wurtzite-structured ZnO lattices [9,14] and the film stress induced during the deposition process [15]. Scherrer's formula was applied to estimate the crystallite sizes in these GMZO thin films that were estimated using the full width at half maximum (FWHM) of the (002) peak. When the deposition pressure was increased, the GMZO thin film crystal grain size decreased (Table 1). As the deposition pressure was increased, more sputtered atoms from the target surface were scattered by the increased amount of Ar atoms and Ar+ ions in the plasma, resulting in a decrease in the number and the energy of the sputtered atoms reaching the substrate surface. These decreases in both the number and energy of the sputtered atoms with the increase in deposition pressure were believed to be the cause of the decrease in the grain size of the sputter-deposited GMZO thin films [16]. Fig. 2 shows the variations in electrical resistivity, Hall mobility and the carrier concentration of the RF magnetron sputtered GMZO thin films deposited at different deposition pressures. From the Hall measurements, the deposited GMZO thin films were confirmed to be n-type semiconductors with electrons as the main charge carriers. The electrical resistivity of GMZO thin films was observed in Fig. 2 to decrease from 2.51× 10−3 Ω cm to 1.93 × 10 −3 Ω cm with the deposition pressure increasing from 0.4 Pa to 0.8 Pa, and slightly increased to 1.94× 10−3 Ω cm when the deposition pressure further increased to 1.2 Pa. With the electron concentration remaining nearly the same at around 3 × 1020 cm−3, the observed changes in electrical resistivity of the GMZO thin films deposited at different deposition pressures were due to the corresponding changes in the Hall mobilities. The Hall mobility of the GMZO thin films increased from 8.1 cm2/Vs to 11.1 cm2/Vs with the deposition pressure increasing from 0.4 Pa to 0.8 Pa, but decreased to 10.7 cm2/Vs when the deposition pressure was further increased to 1.2 Pa. Because the electrical resistivity is inversely proportional to the Hall mobility, the lowest resistivity of the GMZO thin films deposited at 0.8 Pa results from a significant increase in the Hall mobility of GMZO thin film at that deposition pressure. When the deposition pressure was increased from 0.4 Pa to 0.8 Pa, the energies of the sputtered atoms from the target surface decreased and the thin film stress induced by the bombardment of energetic sputtered atoms also decreased. The increase in Hall mobility and, thus, the decrease in GMZO thin film electrical resistivity could be attributed to the formation of less stressed GMZO thin films [15,16]. The wavelength dependence of the optical transmittance spectra of the RF magnetron sputtered GMZO thin films deposited at different deposition pressures is illustrated in Fig. 3. It can be observed that the average optical transmittance of these GMZO thin films is close to 90% in the 400 nm and 1200 nm wavelength range. In particular, the average optical transmittance in the near infrared region (800 nm–1200 nm) also exceeds 85%. This value is higher than that of sputtered GZO thin films (~75%) [11]. The optical energy band gaps of GMZO thin films deposited at different deposition pressures were calculated from these transmittance spectra [15]. The calculations showed that these GMZO Table 1 Summary of XRD analyses and optical band gap energies of the RF magnetron sputtered GMZO thin films deposited at different deposition pressures.
Fig. 1. X-ray patterns of RF magnetron sputtered GMZO thin films deposited at different deposition pressures: (a) 0.4 Pa, (b) 0.8 Pa, and (c) 1.2 Pa.
Deposition pressure(Pa)
Substrate temperature (K)
FWHM
2 theta
Grain size (nm)
Band gap (eV)
0.4 0.8 1.2 0.8 0.8
473 473 473 498 448
0.41 0.421 0.427 0.404 0.420
34.363 34.427 34.505 34.377 34.506
20.273 19.747 19.474 20.575 19.798
3.70 3.74 3.71 3.72 3.70
H.-C. Lu et al. / Surface & Coatings Technology 231 (2013) 539–542
541
Fig. 2. Variation of electrical resistivity, Hall mobility and carrier concentration in sputtered GMZO thin films deposited at different deposition pressures.
thin films were materials with direct forbidden band gap structure. In addition, changes in pressure did not significantly change the optical band gap of GMZO thin films and their values are about 3.7 eV (Table 1). These band gap energy values are greater than 3.4 eV reported for ZnO thin films [17]. These results are an indication of the incorporation and replacement of Zn+2 ions by Mg+2 ions in the wurtzite-structured ZnO lattices, leading to the widening band gaps and, therefore, the observed increase in both infrared optical transmittance and the band gap energies [4]. Fig. 4 shows the variations in electrical resistivity, Hall mobility and the carrier concentration of RF magnetron sputtered GMZO thin films deposited at 0.8 Pa and different substrate temperatures. These GMZO thin films deposited at different substrate temperatures were also characterized by XRD to have hexagonal ZnO wurtzite structure with a preferred crystallite orientation in the (002) direction. Again from the Hall measurements, these GMZO thin films were confirmed to be n-type semiconductors with electrons as the main charge carriers. The resistivity of these GMZO thin films increased from 1.62 × 10 − 3 Ω cm to 1.93 × 10 − 3 Ω cm when the substrate temperature increased from 448 K to 498 K. The increase in the resistivity of these GMZO thin films was attributed mainly to the corresponding decrease in the electron concentration from 100
Transmittance(%)
80
60
40
(a) (b) (c)
20
3.8 × 10 20 cm − 3 to 2.9 × 10 20 cm − 3 due to the fact that the Hall mobility of these GMZO thin films did not change significantly when the substrate temperature increased. From the XPS analyses (Table 2), the Zn content in the GMZO thin films was shown to decrease when the substrate temperature increased. Wu et al. reported an increase in Ga content in RF magnetron sputtered GZO thin films when the substrate temperature increased. This is due to the loss of Zn during sputtering deposition resulting from its higher vapor pressure [18]. A similar trend was also observed by Sans et al. [19]. Sans et al. further attributed the corresponding decrease in electron concentration to the segregation of Ga atoms from the ZnO crystallites, which prohibited the replacement of Zn+2 ions by Ga +3 ions in the ZnO lattices and the generation of conduction electrons. Because the conductivity and the electron concentration of GMZO thin films are enhanced mainly through the replacement of Zn+2 ions by Ga+3 ions in the ZnO lattices, the observed decrease in electron concentration and, thus, the observed increase in electrical resistivity in GMZO thin films when the substrate temperature increased are believed to be induced by the loss of Zn caused by its higher vapor pressure and the resulting segregation of Ga atoms from ZnO crystallites. The wavelength dependence of the optical transmittance spectra of the RF magnetron sputtered GMZO thin films deposited at different substrate temperatures is illustrated in Fig. 5. It can be observed that the average optical transmittance of these GMZO thin films is above 85% in the 400 nm and 1200 nm wavelength range. Although the average optical transmittance of the GMZO thin films in the near infrared region (800–1200 nm) still exceeded 80%, the transmittance in the near infrared region was observed to increase with the substrate temperature. The increase in the near infrared transmittance of GMZO thin films with the increase in substrate temperature is consistent with the corresponding decrease in electron concentration, which led to the shift in the IR transmission cut-off wavelength toward a higher wavelength and, thus, the increase in the near infrared transmittance [10]. By calculating the band gap energies of these GMZO thin films from their optical spectra, these GMZO thin films were again confirmed to be materials with direct forbidden band gap structure. Changes in substrate temperature also created significant changes in the band gap energies of these GMZO thin films (Table 1).
0 400
600
800
1000
1200
Wavelength (nm) Fig. 3. Optical transmittance of RF magnetron sputtered GMZO thin films deposited at different deposition pressures: (a) 0.4 Pa, (b) 0.8 Pa, and (c) 1.2 Pa.
4. Conclusion Transparent conductive GMZO thin films with resistivity as low as 1.62 × 10 −3 Ω cm and good near infrared transmittance (>85%) were
542
H.-C. Lu et al. / Surface & Coatings Technology 231 (2013) 539–542
Fig. 4. Variation of electrical resistivity, Hall mobility and carrier concentration in sputtered GMZO thin films deposited at different substrate temperatures.
Table 2 XPS of the RF magnetron sputtered GMZO thin films deposited at different substrate temperatures. Substrate temperature (K)
Ga (at.%)
Zn (at.%)
Mg (at.%)
Ga/Ga + Zn (at.%)
448 473 498
20.64 24.64 33.3
74.97 71.97 49.27
4.39 3.36 17.42
21.59 25.52 40.24
from 0.4 Pa to 1.2 Pa without significant change in the average optical transmittance. However, noticeable changes in both the resistivity and the near infrared transmittance were observed in GMZO thin films deposited at different substrate temperatures due to the loss of Zn and the resulting decrease in the electron concentration with the increasing substrate temperature. Acknowledgments This work was supported by the National Science Council, Taiwan, ROC, through grant NSC 100-3113-E-002-011.
100
References
Transmittance(%)
80
[1] [2] [3] [4]
60
[5]
40
[6] [7] [8] [9]
(a) (b) (c)
20
[10] [11]
0 400
600
800
1000
1200
Wavelength (nm) Fig. 5. Optical transmittance of RF magnetron sputtered GMZO thin films deposited at different substrate temperatures: (a) 448 K, (b) 473 K, and (c) 498 K.
successfully prepared on glass substrates using single cathode RF magnetron sputtering using a GMZO ceramic target with a 97 wt.% Zn0.94Mg0.06O and 3 wt.% Ga2O3 composition without post deposition annealing. It was also found that the resistivity of the GMZO thin films decreased when the deposition pressure was increased
[12] [13] [14] [15] [16] [17] [18] [19]
C.F. Yu, S.H. Chen, S.J. Sun, H. Chou, Appl. Surf. Sci. 257 (2011) 6498. X.L. Chen, X.H. Geng, J.M. Xue, L.N. Li, J. Cryst. Growth 299 (2007) 77. K.H. Kim, K.C. Park, D.Y. Ma, J. Appl. Phys. 81 (1997) 7764. T. Minemoto, T. Negami, S. Nishiwaki, H. Takakura, Y. Hamakawa, Thin Solid Films 372 (2000) 173. T. Minemoto, Y. Hashimoto, T. Satoh, T. Negami, H. Takakura, Y. Hamakawa, J. Appl. Phys. 89 (2001) 8327. D.J. Cohen, K.C. Ruthe, S.A. Barnett, J. Appl. Phys. 96 (2004) 459. R.K. Gupta, K. Ghosh, R. Patel, P.K. Kahol, Mater. Sci. Eng. B 156 (2009) 1. W. Wei, C. Jin, J. Narayan, R.J. Narayan, Solid State Commun. 149 (2009) 1670. S.D. Shinde, A.V. Deshmukh, S.K. Date, V.G. Sathe, K.P. Adhi, Thin Solid Films 520 (2011) 1212. Q.B. Ma, H.P. He, Z.Z. Ye, L.P. Zhu, J.Y. Huang, Y.Z. Zhang, B.H. Zhao, J. Solid State Chem. 181 (2008) 525. K. Maejima, H. Shibata, H. Tampo, K. Matsubara, S. Niki, Thin Solid Films 518 (2010) 2949. M. Bender, W. Seelig, C. Daube, H. Frankenberger, B. Ocker, J. Stollenwerk, Thin Solid Films 326 (1998) 72. C. May, Y. Tomita, M. Toerker, M. Eritt, F. Loeffler, J. Amelung, K. Leo, Thin Solid Films 516 (2008) 4609. R. Ghosh, D. Basak, Appl. Surf. Sci. 255 (2009) 7238. L. Li, L. Fang, X.M. Chen, J. Liu, F.F. Yang, Q.J. Li, G.B. Liu, S.J. Feng, Physica E 41 (2008) 169. T. Tohsophon, N. Wattanasupinyo, B. Silskulsuk, N. Sirikulrat, Thin Solid Films 520 (2011) 726. C. Klingshirn, Phys. Status Solidi B 244 (2007) 3027. F. Wu, L. Fang, Y.J. Pan, K. Zhou, Q.L. Huang, C.Y. Kong, Physica E (2010) 228. J.A. Sans, A. Segura, J.F. Sánchez-Royo, V. Barber, M.A. Hernández-Fenollosa, B. Marí, Superlattices Microstruct. 39 (2006) 285.
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Influence of RF magnetron sputtering conditions on the properties of transparent conductive gallium-doped magnesium zinc oxide thin films Hsin-Chun Lu ⁎, Jia-Chiuan Jou, Chun-Lung Chu Department of Chemical and Materials Engineering, Chang Gung University, Kwei-Shan, Tao-Yuan 333, Taiwan, ROC
a r t i c l e
i n f o
Available online 18 October 2012 Keywords: Gallium-doped magnesium zinc oxide Thin films Electrical and optical properties RF magnetron sputtering
a b s t r a c t Gallium-doped magnesium zinc oxide (GMZO) thin films were deposited onto glass substrates using RF magnetron sputtering using GMZO ceramic targets under different sputtering conditions. The substrate temperature and deposition pressure effects on the structural, electrical and optical properties of the GMZO thin film were investigated. Transparent conductive GMZO thin films were demonstrated in this study to possess a wurtzite structure, average transmittance above 80% in the 400 nm and 800 nm wavelength range with a low resistivity of 1.62 × 10 −3 Ω cm when deposited onto glass substrates using RF magnetron sputtering. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Transparent conductive oxide (TCO) thin films have been widely used as transparent electrodes in flat panel displays, touch panels and solar cells. Semiconducting zinc oxide (ZnO) is one of the most promising TCO materials because ZnO is an abundant, low-cost, and non-toxic material [1]. The intrinsic n-type semi-conductivity of ZnO originates from such structural defects as oxygen vacancies and zinc interstitials [2]. To further improve its electrical conductivity, ZnO was doped with other metal elements such as boron, aluminum, gallium, etc. Among these, aluminum-doped zinc oxide (AZO) and gallium-doped zinc oxide (GZO) are the most widely explored [3,1]. The optical properties of ZnO can be adjusted by changing the ZnO energy band gap through doping ZnO with wide band gap materials. Minemoto et al. [4] indicated that the energy band gap of ZnO could be changed from 3.24 eV to 4.02 eV by doping zinc oxide with the wide band gap MgO (7.70 eV). By increasing the concentration of Mg dopants in this magnesium zinc oxide (MgxZn1 −xO, MZO) mixture and, therefore, the band gap energy of MZO, the cell conversion efficiency (of the Cu [In, Ga] Se2 [CIGS] thin film solar cells using MZO as the window layer material) was enhanced due to increased transmittance of sunlight in the near infrared region [5]. However, the resistivities of MZO thin films were over 10 5 Ω cm and nearly equivalent to the resistivity of intrinsic ZnO. To enhance the electrical conductivity of MZO films, it was suggested that extra dopant be added to the MZO thin films [6]. Among the potential candidates aluminum-doped magnesium zinc oxide (AMZO) and galliumdoped magnesium zinc oxide (GMZO) received the most attention [7,8]. Due to the slight size difference between Ga+3 (0.62 nm) and Zn+2 (0.74 nm) ions compared to that between Al+3 (0.53 nm) and Zn+2 (0.74 nm) ions, the lattice distortion of the GMZO crystals is less ⁎ Corresponding author. Tel.: +886 3 211 8800x5292; fax: +886 3 211 8668. E-mail address: [email protected] (H.-C. Lu). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.10.029
severe than that of AMZO crystals [9]. This lower lattice distortion results in higher electron mobility, higher allowable dopant concentration and higher electron concentration in the GMZO thin films and, therefore, the observed lower resistivity of GMZO thin films [10,11]. The higher electron concentration in the GMZO thin films increased the Burstein–Moss effect, resulting in the reflective behavior improvement in GMZO thin films in the near infrared region over that of AMZO thin films [11]. Although GMZO thin films possess the advantages mentioned above, few reports are found in the literature describing their preparation. Wei et al. [8] reported on MgxZn1 − xO:Ga thin films with a minimum resistivity of 1.9 × 10 −3 Ω cm using pulsed laser deposition using Ga-doped MgxZn1−xO targets. Ma et al. [10] demonstrated galliumdoped MgxZn1−xO thin film preparation with a minimum resistivity of 3 × 10 − 4 Ω cm using DC reactive magnetron sputtering using a ternary metal alloy target. Maejima et al. [11] prepared gallium-doped MgxZn1−xO thin films with a minimum resistivity of 8×10−4 Ω cm using a multi-cathode RF magnetron co-sputtering process using GZO and MgO targets simultaneously. To be mass-produced for industrial purposes, GMZO thin films must follow the industrially standardized TCO thin film production process of single cathode magnetron sputtering, which uses the oxide ceramic targets of unique designated compositions [12,13]. This paper presents GMZO thin film deposition using single cathode RF magnetron sputtering using a GMZO ceramic target of designated composition without post deposition annealing. The substrate temperature and deposition pressure effects on the structure, electrical and optical properties of RF magnetron sputtered GMZO thin films are also reported. 2. Experimental GMZO thin films were prepared on glass substrates (Corning 1737) using RF magnetron sputtering. GMZO targets, 2 in. in diameter with a
540
H.-C. Lu et al. / Surface & Coatings Technology 231 (2013) 539–542
97 wt.% Zn0.94Mg0.06O and 3 wt.% Ga2O3 composition, were prepared by sintering the green bodies formed by mixing and compacting a mixture of ZnO powders, MgO powders and Ga2O3 powders in an argon atmosphere for 3 h at 1673 K. After the 2 cm× 2 cm glass substrates were cleaned with de-ionized water and acetone, the substrates were placed into a substrate holder at a distance of 6 cm from the GMZO target. The sputtering chamber was then pumped down from atmospheric pressure to a base pressure of 0.8×10−3 Pa. Argon gas (99.999% purity) was then introduced into the sputtering chamber. GMZO thin films were deposited onto the glass substrates using RF magnetron sputtering with the sputtering power at 80 W. The deposition pressure effect on the GMZO thin film characteristics was investigated by varying the deposition pressure from 0.4 Pa to 1.2 Pa when the substrate temperature was controlled at 473 K. After finding the deposition pressure that produced GMZO thin films with the lowest electrical resistivity the substrate temperature effect on the GMZO thin film characteristics was studied by depositing GMZO thin films at different substrate temperatures at that specific deposition pressure. The deposited GMZO thin films were characterized by studying their structural, electrical and optical properties. X-ray diffraction (XRD) was used to analyze the crystallographic structure and orientation of the deposited thin films. A Bruker's AXS D5005 X-ray diffractometer with Cu Kα radiation (λ = 1.5412 Å) was used to record the X-ray data in the 2θ range between 30° and 70°. The electrical resistivity and Hall mobility of the deposited thin films were measured by Hall measurement (Ecopia HMS-3000). The thin film thickness was measured using a profilometer (Veeco Detak ST). The optical transmittance of the deposited thin films was recorded using Perkin Elmer Lambda 900 UV–VIS-NIR double beam spectrophotometer where the uncoated glass substrate was used as a reference. The GMZO chemical bonding state information was studied using X-ray photoelectron spectroscopy (XPS) using a VG Scientific ESCA LAB 250 with monochromatic Al Kα X-ray source (1486.6 eV). All spectra were calibrated with C 1s peak at 284.5 eV. 3. Results and discussion Fig. 1 shows the XRD patterns of the RF magnetron sputtered thin films deposited at 473 K and different deposition pressures using the GMZO target with 3 wt.% Ga2O3 doping. A very strong peak corresponding to the (002) plane (JCPDS 00-036-1451) of hexagonal wurtzitestructured ZnO and a minor peak near 2θ = 30.92° also attributed to hexagonal wurtzite-structured ZnO (JCPDS 00-021-1486) were
observed in each of these XRD patterns, thereby indicating a preferred crystallite orientation in the (002) direction. The diffraction peaks in these XRD patterns were observed to shift away from the peak positions in the standard ZnO JCPDS pattern. This peak shift probably results from a combination of Zn + 2 ion replacement by the smaller Mg+2 and Ga+3 ions in the wurtzite-structured ZnO lattices [9,14] and the film stress induced during the deposition process [15]. Scherrer's formula was applied to estimate the crystallite sizes in these GMZO thin films that were estimated using the full width at half maximum (FWHM) of the (002) peak. When the deposition pressure was increased, the GMZO thin film crystal grain size decreased (Table 1). As the deposition pressure was increased, more sputtered atoms from the target surface were scattered by the increased amount of Ar atoms and Ar+ ions in the plasma, resulting in a decrease in the number and the energy of the sputtered atoms reaching the substrate surface. These decreases in both the number and energy of the sputtered atoms with the increase in deposition pressure were believed to be the cause of the decrease in the grain size of the sputter-deposited GMZO thin films [16]. Fig. 2 shows the variations in electrical resistivity, Hall mobility and the carrier concentration of the RF magnetron sputtered GMZO thin films deposited at different deposition pressures. From the Hall measurements, the deposited GMZO thin films were confirmed to be n-type semiconductors with electrons as the main charge carriers. The electrical resistivity of GMZO thin films was observed in Fig. 2 to decrease from 2.51× 10−3 Ω cm to 1.93 × 10 −3 Ω cm with the deposition pressure increasing from 0.4 Pa to 0.8 Pa, and slightly increased to 1.94× 10−3 Ω cm when the deposition pressure further increased to 1.2 Pa. With the electron concentration remaining nearly the same at around 3 × 1020 cm−3, the observed changes in electrical resistivity of the GMZO thin films deposited at different deposition pressures were due to the corresponding changes in the Hall mobilities. The Hall mobility of the GMZO thin films increased from 8.1 cm2/Vs to 11.1 cm2/Vs with the deposition pressure increasing from 0.4 Pa to 0.8 Pa, but decreased to 10.7 cm2/Vs when the deposition pressure was further increased to 1.2 Pa. Because the electrical resistivity is inversely proportional to the Hall mobility, the lowest resistivity of the GMZO thin films deposited at 0.8 Pa results from a significant increase in the Hall mobility of GMZO thin film at that deposition pressure. When the deposition pressure was increased from 0.4 Pa to 0.8 Pa, the energies of the sputtered atoms from the target surface decreased and the thin film stress induced by the bombardment of energetic sputtered atoms also decreased. The increase in Hall mobility and, thus, the decrease in GMZO thin film electrical resistivity could be attributed to the formation of less stressed GMZO thin films [15,16]. The wavelength dependence of the optical transmittance spectra of the RF magnetron sputtered GMZO thin films deposited at different deposition pressures is illustrated in Fig. 3. It can be observed that the average optical transmittance of these GMZO thin films is close to 90% in the 400 nm and 1200 nm wavelength range. In particular, the average optical transmittance in the near infrared region (800 nm–1200 nm) also exceeds 85%. This value is higher than that of sputtered GZO thin films (~75%) [11]. The optical energy band gaps of GMZO thin films deposited at different deposition pressures were calculated from these transmittance spectra [15]. The calculations showed that these GMZO Table 1 Summary of XRD analyses and optical band gap energies of the RF magnetron sputtered GMZO thin films deposited at different deposition pressures.
Fig. 1. X-ray patterns of RF magnetron sputtered GMZO thin films deposited at different deposition pressures: (a) 0.4 Pa, (b) 0.8 Pa, and (c) 1.2 Pa.
Deposition pressure(Pa)
Substrate temperature (K)
FWHM
2 theta
Grain size (nm)
Band gap (eV)
0.4 0.8 1.2 0.8 0.8
473 473 473 498 448
0.41 0.421 0.427 0.404 0.420
34.363 34.427 34.505 34.377 34.506
20.273 19.747 19.474 20.575 19.798
3.70 3.74 3.71 3.72 3.70
H.-C. Lu et al. / Surface & Coatings Technology 231 (2013) 539–542
541
Fig. 2. Variation of electrical resistivity, Hall mobility and carrier concentration in sputtered GMZO thin films deposited at different deposition pressures.
thin films were materials with direct forbidden band gap structure. In addition, changes in pressure did not significantly change the optical band gap of GMZO thin films and their values are about 3.7 eV (Table 1). These band gap energy values are greater than 3.4 eV reported for ZnO thin films [17]. These results are an indication of the incorporation and replacement of Zn+2 ions by Mg+2 ions in the wurtzite-structured ZnO lattices, leading to the widening band gaps and, therefore, the observed increase in both infrared optical transmittance and the band gap energies [4]. Fig. 4 shows the variations in electrical resistivity, Hall mobility and the carrier concentration of RF magnetron sputtered GMZO thin films deposited at 0.8 Pa and different substrate temperatures. These GMZO thin films deposited at different substrate temperatures were also characterized by XRD to have hexagonal ZnO wurtzite structure with a preferred crystallite orientation in the (002) direction. Again from the Hall measurements, these GMZO thin films were confirmed to be n-type semiconductors with electrons as the main charge carriers. The resistivity of these GMZO thin films increased from 1.62 × 10 − 3 Ω cm to 1.93 × 10 − 3 Ω cm when the substrate temperature increased from 448 K to 498 K. The increase in the resistivity of these GMZO thin films was attributed mainly to the corresponding decrease in the electron concentration from 100
Transmittance(%)
80
60
40
(a) (b) (c)
20
3.8 × 10 20 cm − 3 to 2.9 × 10 20 cm − 3 due to the fact that the Hall mobility of these GMZO thin films did not change significantly when the substrate temperature increased. From the XPS analyses (Table 2), the Zn content in the GMZO thin films was shown to decrease when the substrate temperature increased. Wu et al. reported an increase in Ga content in RF magnetron sputtered GZO thin films when the substrate temperature increased. This is due to the loss of Zn during sputtering deposition resulting from its higher vapor pressure [18]. A similar trend was also observed by Sans et al. [19]. Sans et al. further attributed the corresponding decrease in electron concentration to the segregation of Ga atoms from the ZnO crystallites, which prohibited the replacement of Zn+2 ions by Ga +3 ions in the ZnO lattices and the generation of conduction electrons. Because the conductivity and the electron concentration of GMZO thin films are enhanced mainly through the replacement of Zn+2 ions by Ga+3 ions in the ZnO lattices, the observed decrease in electron concentration and, thus, the observed increase in electrical resistivity in GMZO thin films when the substrate temperature increased are believed to be induced by the loss of Zn caused by its higher vapor pressure and the resulting segregation of Ga atoms from ZnO crystallites. The wavelength dependence of the optical transmittance spectra of the RF magnetron sputtered GMZO thin films deposited at different substrate temperatures is illustrated in Fig. 5. It can be observed that the average optical transmittance of these GMZO thin films is above 85% in the 400 nm and 1200 nm wavelength range. Although the average optical transmittance of the GMZO thin films in the near infrared region (800–1200 nm) still exceeded 80%, the transmittance in the near infrared region was observed to increase with the substrate temperature. The increase in the near infrared transmittance of GMZO thin films with the increase in substrate temperature is consistent with the corresponding decrease in electron concentration, which led to the shift in the IR transmission cut-off wavelength toward a higher wavelength and, thus, the increase in the near infrared transmittance [10]. By calculating the band gap energies of these GMZO thin films from their optical spectra, these GMZO thin films were again confirmed to be materials with direct forbidden band gap structure. Changes in substrate temperature also created significant changes in the band gap energies of these GMZO thin films (Table 1).
0 400
600
800
1000
1200
Wavelength (nm) Fig. 3. Optical transmittance of RF magnetron sputtered GMZO thin films deposited at different deposition pressures: (a) 0.4 Pa, (b) 0.8 Pa, and (c) 1.2 Pa.
4. Conclusion Transparent conductive GMZO thin films with resistivity as low as 1.62 × 10 −3 Ω cm and good near infrared transmittance (>85%) were
542
H.-C. Lu et al. / Surface & Coatings Technology 231 (2013) 539–542
Fig. 4. Variation of electrical resistivity, Hall mobility and carrier concentration in sputtered GMZO thin films deposited at different substrate temperatures.
Table 2 XPS of the RF magnetron sputtered GMZO thin films deposited at different substrate temperatures. Substrate temperature (K)
Ga (at.%)
Zn (at.%)
Mg (at.%)
Ga/Ga + Zn (at.%)
448 473 498
20.64 24.64 33.3
74.97 71.97 49.27
4.39 3.36 17.42
21.59 25.52 40.24
from 0.4 Pa to 1.2 Pa without significant change in the average optical transmittance. However, noticeable changes in both the resistivity and the near infrared transmittance were observed in GMZO thin films deposited at different substrate temperatures due to the loss of Zn and the resulting decrease in the electron concentration with the increasing substrate temperature. Acknowledgments This work was supported by the National Science Council, Taiwan, ROC, through grant NSC 100-3113-E-002-011.
100
References
Transmittance(%)
80
[1] [2] [3] [4]
60
[5]
40
[6] [7] [8] [9]
(a) (b) (c)
20
[10] [11]
0 400
600
800
1000
1200
Wavelength (nm) Fig. 5. Optical transmittance of RF magnetron sputtered GMZO thin films deposited at different substrate temperatures: (a) 448 K, (b) 473 K, and (c) 498 K.
successfully prepared on glass substrates using single cathode RF magnetron sputtering using a GMZO ceramic target with a 97 wt.% Zn0.94Mg0.06O and 3 wt.% Ga2O3 composition without post deposition annealing. It was also found that the resistivity of the GMZO thin films decreased when the deposition pressure was increased
[12] [13] [14] [15] [16] [17] [18] [19]
C.F. Yu, S.H. Chen, S.J. Sun, H. Chou, Appl. Surf. Sci. 257 (2011) 6498. X.L. Chen, X.H. Geng, J.M. Xue, L.N. Li, J. Cryst. Growth 299 (2007) 77. K.H. Kim, K.C. Park, D.Y. Ma, J. Appl. Phys. 81 (1997) 7764. T. Minemoto, T. Negami, S. Nishiwaki, H. Takakura, Y. Hamakawa, Thin Solid Films 372 (2000) 173. T. Minemoto, Y. Hashimoto, T. Satoh, T. Negami, H. Takakura, Y. Hamakawa, J. Appl. Phys. 89 (2001) 8327. D.J. Cohen, K.C. Ruthe, S.A. Barnett, J. Appl. Phys. 96 (2004) 459. R.K. Gupta, K. Ghosh, R. Patel, P.K. Kahol, Mater. Sci. Eng. B 156 (2009) 1. W. Wei, C. Jin, J. Narayan, R.J. Narayan, Solid State Commun. 149 (2009) 1670. S.D. Shinde, A.V. Deshmukh, S.K. Date, V.G. Sathe, K.P. Adhi, Thin Solid Films 520 (2011) 1212. Q.B. Ma, H.P. He, Z.Z. Ye, L.P. Zhu, J.Y. Huang, Y.Z. Zhang, B.H. Zhao, J. Solid State Chem. 181 (2008) 525. K. Maejima, H. Shibata, H. Tampo, K. Matsubara, S. Niki, Thin Solid Films 518 (2010) 2949. M. Bender, W. Seelig, C. Daube, H. Frankenberger, B. Ocker, J. Stollenwerk, Thin Solid Films 326 (1998) 72. C. May, Y. Tomita, M. Toerker, M. Eritt, F. Loeffler, J. Amelung, K. Leo, Thin Solid Films 516 (2008) 4609. R. Ghosh, D. Basak, Appl. Surf. Sci. 255 (2009) 7238. L. Li, L. Fang, X.M. Chen, J. Liu, F.F. Yang, Q.J. Li, G.B. Liu, S.J. Feng, Physica E 41 (2008) 169. T. Tohsophon, N. Wattanasupinyo, B. Silskulsuk, N. Sirikulrat, Thin Solid Films 520 (2011) 726. C. Klingshirn, Phys. Status Solidi B 244 (2007) 3027. F. Wu, L. Fang, Y.J. Pan, K. Zhou, Q.L. Huang, C.Y. Kong, Physica E (2010) 228. J.A. Sans, A. Segura, J.F. Sánchez-Royo, V. Barber, M.A. Hernández-Fenollosa, B. Marí, Superlattices Microstruct. 39 (2006) 285.