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In2O3–ZnO transparent conductive oxide film deposition on polycarbonate substrates
Vacuum 83 (2009) 557–560
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
In2O3–ZnO transparent conductive oxide film deposition on polycarbonate substrates Toshihiro Moriga a, b, *, Koji Shimomura c, Daisuke Takada c, Hiroshi Suketa b, Keisuke Takita b, Kei-ichiro Murai a, b, Kikuo Tominaga a, c a
Department of Advanced Materials, Institute of Technology and Science, The University of Tokushima, 2-1 Minami-Josanjima, Tokushima 770-8506, Japan Department of Chemical Science and Technology, Graduate School of Advanced Technology and Science, The University of Tokushima, 2-1 Minami-Josanjima, Tokushima 770-8506, Japan c Department of Electric and Electronic Engineering, Graduate School of Advanced Technology and Science, The University of Tokushima, 2-1 Minami-Josanjima, Tokushima 770-8506, Japan b
a b s t r a c t Keywords: TCO PC substrate Ga2O3 addition Al2O3 addition Amorphous
Amorphous transparent conductive oxide films in the In–Zn–O system were deposited on polycarbonate (PC) substrates by simultaneous DC sputtering of an In2O3 target and a ZnO target with either 4 wt% Al2O3 or 7.5 wt% Ga2O3 impurities. Although the resistivity of the amorphous, non-doped In–Zn–O film on PC was about one order of magnitude higher than that on the glass substrate, the resistivity of the In–Zn–O films with Ga2O3 impurities on PC substrates was reduced to the level of the non-doped In–Zn–O films on glass substrates. The addition of Al2O3 or Ga2O3 to the In–Zn–O films also induced the widening of the optical band gap, which would improve transparency at blue wavelengths. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Light-weight, flexible display technologies have received much attention in the display industry in electronic devices such as personal digital assistants, smart cards, electronic papers and so on. In these applications, transparent conductive oxide (TCO) films on plastic substrates will be used in many parts [1]. The displays on the plastic substrate are strong in mechanical shock, light in weight, flexible to distortion and so on. Low-temperature deposition of TCO films onto the plastic substrates is necessary. Polycarbonate (PC) will be one of the candidates of substrate materials for the above applications [2]. PC is a particular group of thermoplastics and possesses a useful operating range from 100 C to 135 C due to its glass transition temperature of 150 C [3]. However, the as-deposited ITO films which are commercially used as TCO films deteriorate severely in low-temperature deposition [1]. Recently, our research group reported that amorphous In2O3–ZnO films deposited at substrate temperatures below 150 C by a simultaneous DC sputtering apparatus with a facing-target system showed electrical resistivity as low as 2.3 104 U cm with high transparency in the visible region [4,5]. Inoue et al. also reported amorphous IZO films (In2O3:ZnO ¼ 89.3:10.7 by weight
* Corresponding author. Department of Chemical Science and Technology, Graduate School of Advanced Technology and Science, The University of Tokushima, 2-1 Minami-Josanjima, Tokushima 770-8506, Japan. E-mail address: [email protected] (T. Moriga). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.04.051
percent) deposited by DC sputtering without heating glass substrates showed 4–5 104 U cm with good reproducibility [6,7]. Amorphous indium zinc-oxide films are a good candidate as an alternative to ITO films because of their suitability for lowtemperature deposition. In this paper, we will describe electrical and optical properties of amorphous In–Zn–O films deposited onto PC substrates (In–Zn–O/PC) without heating. 2. Experimental Thin films in the In2O3–ZnO system were deposited by a DC magnetron sputtering apparatus with targets facing each other as shown in Fig. 1. The details of the apparatus were reported in our previous paper [4,5]. This apparatus has two targets for strongly unbalanced planar magnetron sputtering. A zinc-oxide-based target was placed at the 12 o’clock position, an indium-oxide-based target was at the 6 o’clock position and a PC substrate was at the 9 o’clock position. Since the substrate is placed in an off-axis position, film degradation by energetic neutral or charged species could be eliminated. Therefore, film growth can be carried out under damage-free conditions. An In2O3 target and a ZnO target containing either 7.5 wt% Gal2O3 or 4.0 wt% Al2O3 impurities were set at a distance of 10 cm. The amounts by weight of the impurities correspond to ca. 6.4 mol% relative to zinc oxide. Sputtering was performed at a pressure of 0.133 Pa (1 mm Torr) in pure Ar gas for 2 h. A Corning #1737 glass substrate or a polycarbonate (PC) substrate having
558
T. Moriga et al. / Vacuum 83 (2009) 557–560
cooling water mA
heater
M
S
N
S
M
shutter ZnO
target diffusion pump
Plasma In2O3 target substrate M
N
vacuum gauge
M
N
S
gas valve mA cooling water Ar gas
Fig. 1. Schematic drawing of facing-target planar DC magnetron sputtering system.
a dimension of 50 50 1 mm3 was set so as to be perpendicular to the dual targets. The substrate was rotated at a speed of 10 rotations/min during the deposition and the temperature was kept at less than 50 C. Discharge currents applied to both the
: ZnO
: In2O3
(002)
b
: ZnO
Zn2In2O5 (0 0 8)
Intensity (Arb. Units)
Zn2In2O5 (0 0 8)
=0.89
=1.00
Zn2In2O5 (0 0 8)
=0.80
=0.73 =0.67 =0.62
=0.53 =0.50 =0.47 =0.43
=0.57 =0.53 =0.50 =0.47 =0.43
=0.38
=0.38
=0.33
=0.33
=0.27
=0.27
=0.20
=0.20
=0.11 20
30
40
50
2 (CuK ) / deg.
=0.80
=0.67 =0.57
10
=0.89
=0.73 =0.62
(222)
: In2O3
(002)
=1.00
Intensity (Arb. Units)
a
In2O3 and ZnO targets, IIn and IZn, were varied independently from 0 mA to 80 mA, respectively. The current ratio, d ¼ IZn/(IIn þ IZn), was selected as a parameter for deposition to control the film composition.
=0.00 60 70
=0.11 (222) 10
20
30
40
50
=0.00 60 70
2 (CuK ) / deg.
Fig. 2. Variation of X-ray diffraction patterns of In–Zn–O thin films deposited on PC substrate (In–Zn–O/PC) as a function of current ratio d. (a) Impurity free and (b) 7.5 wt% Ga2O3 impurity In–Zn–O.
T. Moriga et al. / Vacuum 83 (2009) 557–560
Cationic composition ratio ([Zn]/([In] þ [Zn])) of the films was determined by X-ray fluorescence analysis using a Shimadzu Rayny EDX-800. Phase identification was carried out by X-ray diffraction apparatus (Rigaku RINT-2500VHF). Film thickness was estimated from the optical interference observed between the film and substrate, presuming the refractive indices of the amorphous In–Zn–O and PC to be 2.0 [8] and 1.6 [3], respectively. Electrical resistivity of the films was measured with a four-point probe. UV–vis transmittance spectra were collected using a spectrophotometer (JASCO V550DS).
3. Results and discussion Fig. 2(a) shows variation of the X-ray diffraction pattern of impurity-free In–Zn–O thin films of deposited on PC substrates (In–Zn–O/PC films) as a function of current ratio d. The films were deposited from the targets of In2O3 and ZnO. The amorphous phase appeared in a wide range of d-values from 0.20 to 0.67. The d-range where the amorphous phase appeared was the same as that observed for the films on Corning #1737 glass substrates (In–Zn–O/ glass). For the addition of Ga2O3 impurities, X-ray diffraction patterns of In–Zn–O/PC films are shown in Fig. 2(b) as a function of d. The films were deposited from the targets of In2O3 and ZnO containing 7.5 wt% Ga2O3 impurities. The d-range where the amorphous phase was observed was widened up to 0.2 d 0.73, just larger than the value 0.67 for the impurity-free In–Zn–O/PC films in Fig. 2(a). For the addition of Al2O3 impurities, the d-range where the amorphous phase was observed at 0.2 d 0.73 was the same as that of Ga2O3. The contribution of the Ga2O3 or Al2O3 to the structural and physical properties would be larger in the films with the higher zinc content, because the impurity levels of Ga2O3 and Al2O3 in the ZnO targets were fixed to 7.5 wt% and 4 wt%, respectively. Therefore, a fresh amorphous region appeared not in the lower-d but in the higher-d films. Fig. 3(a) indicates d-value dependence of the resistivities of the In–Zn–O/glass and In–Zn–O/PC films. Resistivity in the amorphous region increased with increasing d-value, that is, with increasing zinc content in the film. Cationic composition ratios ([Zn]/ ([In] þ [Zn])) by X-ray fluorescence analysis for the In–Zn–O/PC and In–Zn–O/glass films with no impurities are shown in Fig. 3(b). The ratios of [Zn]/([In] þ [Zn]) in the amorphous films on the In–Zn–O/ glass films contained less zinc than that anticipated from the current ratio d ¼ IZn/(IIn þ IZn). The composition ratio in the amorphous In–Zn–O/PC films increased to be almost equal to the current ratio
a
100
559
d. These results indicate that more zinc may adhere to a PC substrate than to the glass substrate. In Fig. 3(a), the resistivities of impurity-free In–Zn–O/PC films were higher by about one order of magnitude than those of impurity-free In–Zn–O/glass films over all the d-values in the amorphous region. This result will be related to the data in Fig. 3(b). Carriers of the amorphous In–Zn–O films were reported to be mainly the oxygen vacancies [4,5,9,10]. It is also reported that carrier density decreased with an increase of the composition of Zn in the amorphous In–Zn–O films [4,5]. Since the Zn content in the amorphous In–Zn–O/PC films increases even though the current ratio is identical, the number of oxygen vacancies would be reduced so that resistivity of the amorphous In–Zn–O/PC films increased. For the addition of Ga2O3 and Al2O3 impurities in ZnO targets, the data of the resistivities of In–Zn–O/PC and In–Zn–O/glass films were also indicated in Fig. 3(a). Open circles and triangles in Fig. 3(a) indicate d-value dependence of resistivity of the In–Zn–O/ PC films with Ga2O3 and Al2O3 impurities, respectively. We see the decrease of the resistivity in the In–Zn–O/PC films with Ga2O3 and Al2O3 impurities, compared with the impurity-free In–Zn–O/PC film. The addition of Al2O3 impurity decreased the resistivity at 0.4 d 0.6, whereas the addition of Ga2O3 impurity decreased at 0.2 d 0.6. The In–Zn–O/PC films with 7.5 wt% Ga2O3 impurities were reduced to the level of the impurity-free In–Zn–O/glass films. Especially in the amorphous In–Zn–O/PC films with Ga2O3 impurities, resistivities of some 104 U cm could be attained in a wide d-range. Though it is reported that dopings of cation do not work well on the resistivity in the amorphous In–Zn–O/glass, the addition of impurities in the amorphous In–Zn–O/PC was very effective in improving resistivity. A minimum resistivity as low as 3.56 104 U cm was observed for the indium-richest amorphous In–Zn–O/PC film with the 7.5 wt% Ga2O3 impurity. Fig. 4(a) and (b) shows optical transmittance spectra of the amorphous thin films with d ¼ 0.27 and 0.50, respectively. Compared with the films on the Corning glass substrates, the absorption edge of the films on PC substrates is somewhat shifted to the higher wavelength side. Larger blue shifts were observed in the In–Zn–O/PC with the addition of Ga2O3 or Al2O3 impurities for all current ratios d. The magnitude of shifts from the impurity-free In–Zn–O/PC film seemed to be larger with an increase of d-value, because the amount of Ga2O3 or Al2O3 impurities should increase. Blue shifts seen in heavily Al-doped amorphous and homologous In–Zn–O/glass films [11] or Al- or Ga-doped amorphous Sn–Zn–O/ glass films [12] have been reported in several papers, and the origin of these shifts could be ascribed to a lattice distortion due to
b
/PC:no impurity /PC:Al2O3=4.0 wt
10–1
/PC:Ga2O3=7.5 wt
Resistivity
(
cm)
/glass:no Impurity
10–2
10–3
10–4
0
value
1
Fig. 3. (a) d-Value dependence of resistivity of the films on the glass and PC substrates. (b) Cationic composition ratios ([Zn]/([In] þ [Zn])) in In–Zn–O/PC and In–Zn–O/glass thin films as a function of current ratio d.
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T. Moriga et al. / Vacuum 83 (2009) 557–560
a
b
100
100 δ =0.50
Transmittanse (%)
Transmittanse (%)
δ =0.27
10
50 5
0 200
0 300
400
350
600
400
800
10
50 5
0 200
Wavelength (nm)
0 300
400
400
800
Wavelength (nm)
Fig. 4. Optical transmittance spectra of the amorphous indium-zinc-oxide thin films for (a) d ¼ 0.27 and (b) d ¼ 0.50. , In–Zn–O/PC Ga2O3 ¼ 7.5 wt% and , In–Zn–O/PC Al2O3 ¼ 4.0 wt%. impurity free;
the substitution of constituent metal ions by Al3þ. The distortion effect by the Al impurity on the shift seen in the amorphous In– Zn–O/PC films would be larger than that by the Ga impurity.
350
600
, In–Zn–O/glass impurity free;
, In–Zn–O/PC
Grant-in-Aid for Scientific Research (C) (Approval No. 19560677) from Japan Society for Promotion of Science. References
4. Conclusion The resistivities of In–Zn–O/PC films with no impurities were higher than those of In–Zn–O/glass with no impurities. With the addition of Al2O3 and Ga2O3 impurities, we could obtain the amorphous In–Zn–O/PC films whose resistivities were reduced to the level of the non-doped In–Zn–O/glass films. The Ga2O3 addition was effective in decreasing resistivity in the amorphous In–Zn–O/ PC films. By the addition of either Ga2O3 or Al2O3 impurities, the optical band gap energies significantly increased. Acknowledgements The authors would like to thank Prof. J.B. Metson of University of Auckland, New Zealand for editing the manuscript. This work was supported by the research grant from Iketani Science and Technology Foundation (Approval No. 0181090A) and by the
[1] Lewis BG, Paine DC. MRS Bull 2000;25:22–7. [2] Banerjee AN, Ghosh CK, Chattopadhyay KK, Minoura H, Sarkar AK, Akiba A, et al. Thin Solid Films 2006;496:112–6. [3] Ullmann’s encyclopedia of industrial chemistry 5th completely revised edition, vol. A21. VCH Publishers Inc.; 1992. pp. 207–215. [4] Moriga T, Okamoto T, Hiruta K, Fujiwara A, Nakabayashi I, Tominaga K. J Solid State Chem 2000;155:312–9. [5] Tominaga K, Murayama K, Mori I, Okamoto T, Hiruta K, Moriga T, et al. Vacuum 2000;59:546–52. [6] Inoue K. Kino Zairyo. [Function & Materials] 1999;19:39–46 [in Japanese]. [7] Kaijio A, Inoue K, Matsuzaki S, Shigesato Y. In: Proceedings for the fourth Pacific rim international conference on advanced materials and processing (PRICM-4), vol. 2. Tokyo: JIM; 2001. p. 1787. [8] Hiramatsu H, Seo W-S, Koumoto K. Chem Mater 1998;10:3033–9. [9] Moriga T, Edwards DD, Mason TO, Palmer GB, Poeppelmeier KR, Schindler JL, et al. J Am Ceram Soc 1998;81:1310–6. [10] Ito N, Sato Y, Song PK, Kaijio A, Inoue K, Shigesato Y. Thin Solid Films 2006; 496:99–103. [11] Tominaga K, Fukumoto H, Kondo K, Hayashi Y, Murai K, Moriga T, et al. J Vac Sci Technol 2005;A23:401–7. [12] Moriga T, Nishimura Y, Suketa H, Murai K, Nogami K, Tominaga K, et al. Int J Mod Phys 2006;B20:3902–7.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
In2O3–ZnO transparent conductive oxide film deposition on polycarbonate substrates Toshihiro Moriga a, b, *, Koji Shimomura c, Daisuke Takada c, Hiroshi Suketa b, Keisuke Takita b, Kei-ichiro Murai a, b, Kikuo Tominaga a, c a
Department of Advanced Materials, Institute of Technology and Science, The University of Tokushima, 2-1 Minami-Josanjima, Tokushima 770-8506, Japan Department of Chemical Science and Technology, Graduate School of Advanced Technology and Science, The University of Tokushima, 2-1 Minami-Josanjima, Tokushima 770-8506, Japan c Department of Electric and Electronic Engineering, Graduate School of Advanced Technology and Science, The University of Tokushima, 2-1 Minami-Josanjima, Tokushima 770-8506, Japan b
a b s t r a c t Keywords: TCO PC substrate Ga2O3 addition Al2O3 addition Amorphous
Amorphous transparent conductive oxide films in the In–Zn–O system were deposited on polycarbonate (PC) substrates by simultaneous DC sputtering of an In2O3 target and a ZnO target with either 4 wt% Al2O3 or 7.5 wt% Ga2O3 impurities. Although the resistivity of the amorphous, non-doped In–Zn–O film on PC was about one order of magnitude higher than that on the glass substrate, the resistivity of the In–Zn–O films with Ga2O3 impurities on PC substrates was reduced to the level of the non-doped In–Zn–O films on glass substrates. The addition of Al2O3 or Ga2O3 to the In–Zn–O films also induced the widening of the optical band gap, which would improve transparency at blue wavelengths. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Light-weight, flexible display technologies have received much attention in the display industry in electronic devices such as personal digital assistants, smart cards, electronic papers and so on. In these applications, transparent conductive oxide (TCO) films on plastic substrates will be used in many parts [1]. The displays on the plastic substrate are strong in mechanical shock, light in weight, flexible to distortion and so on. Low-temperature deposition of TCO films onto the plastic substrates is necessary. Polycarbonate (PC) will be one of the candidates of substrate materials for the above applications [2]. PC is a particular group of thermoplastics and possesses a useful operating range from 100 C to 135 C due to its glass transition temperature of 150 C [3]. However, the as-deposited ITO films which are commercially used as TCO films deteriorate severely in low-temperature deposition [1]. Recently, our research group reported that amorphous In2O3–ZnO films deposited at substrate temperatures below 150 C by a simultaneous DC sputtering apparatus with a facing-target system showed electrical resistivity as low as 2.3 104 U cm with high transparency in the visible region [4,5]. Inoue et al. also reported amorphous IZO films (In2O3:ZnO ¼ 89.3:10.7 by weight
* Corresponding author. Department of Chemical Science and Technology, Graduate School of Advanced Technology and Science, The University of Tokushima, 2-1 Minami-Josanjima, Tokushima 770-8506, Japan. E-mail address: [email protected] (T. Moriga). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.04.051
percent) deposited by DC sputtering without heating glass substrates showed 4–5 104 U cm with good reproducibility [6,7]. Amorphous indium zinc-oxide films are a good candidate as an alternative to ITO films because of their suitability for lowtemperature deposition. In this paper, we will describe electrical and optical properties of amorphous In–Zn–O films deposited onto PC substrates (In–Zn–O/PC) without heating. 2. Experimental Thin films in the In2O3–ZnO system were deposited by a DC magnetron sputtering apparatus with targets facing each other as shown in Fig. 1. The details of the apparatus were reported in our previous paper [4,5]. This apparatus has two targets for strongly unbalanced planar magnetron sputtering. A zinc-oxide-based target was placed at the 12 o’clock position, an indium-oxide-based target was at the 6 o’clock position and a PC substrate was at the 9 o’clock position. Since the substrate is placed in an off-axis position, film degradation by energetic neutral or charged species could be eliminated. Therefore, film growth can be carried out under damage-free conditions. An In2O3 target and a ZnO target containing either 7.5 wt% Gal2O3 or 4.0 wt% Al2O3 impurities were set at a distance of 10 cm. The amounts by weight of the impurities correspond to ca. 6.4 mol% relative to zinc oxide. Sputtering was performed at a pressure of 0.133 Pa (1 mm Torr) in pure Ar gas for 2 h. A Corning #1737 glass substrate or a polycarbonate (PC) substrate having
558
T. Moriga et al. / Vacuum 83 (2009) 557–560
cooling water mA
heater
M
S
N
S
M
shutter ZnO
target diffusion pump
Plasma In2O3 target substrate M
N
vacuum gauge
M
N
S
gas valve mA cooling water Ar gas
Fig. 1. Schematic drawing of facing-target planar DC magnetron sputtering system.
a dimension of 50 50 1 mm3 was set so as to be perpendicular to the dual targets. The substrate was rotated at a speed of 10 rotations/min during the deposition and the temperature was kept at less than 50 C. Discharge currents applied to both the
: ZnO
: In2O3
(002)
b
: ZnO
Zn2In2O5 (0 0 8)
Intensity (Arb. Units)
Zn2In2O5 (0 0 8)
=0.89
=1.00
Zn2In2O5 (0 0 8)
=0.80
=0.73 =0.67 =0.62
=0.53 =0.50 =0.47 =0.43
=0.57 =0.53 =0.50 =0.47 =0.43
=0.38
=0.38
=0.33
=0.33
=0.27
=0.27
=0.20
=0.20
=0.11 20
30
40
50
2 (CuK ) / deg.
=0.80
=0.67 =0.57
10
=0.89
=0.73 =0.62
(222)
: In2O3
(002)
=1.00
Intensity (Arb. Units)
a
In2O3 and ZnO targets, IIn and IZn, were varied independently from 0 mA to 80 mA, respectively. The current ratio, d ¼ IZn/(IIn þ IZn), was selected as a parameter for deposition to control the film composition.
=0.00 60 70
=0.11 (222) 10
20
30
40
50
=0.00 60 70
2 (CuK ) / deg.
Fig. 2. Variation of X-ray diffraction patterns of In–Zn–O thin films deposited on PC substrate (In–Zn–O/PC) as a function of current ratio d. (a) Impurity free and (b) 7.5 wt% Ga2O3 impurity In–Zn–O.
T. Moriga et al. / Vacuum 83 (2009) 557–560
Cationic composition ratio ([Zn]/([In] þ [Zn])) of the films was determined by X-ray fluorescence analysis using a Shimadzu Rayny EDX-800. Phase identification was carried out by X-ray diffraction apparatus (Rigaku RINT-2500VHF). Film thickness was estimated from the optical interference observed between the film and substrate, presuming the refractive indices of the amorphous In–Zn–O and PC to be 2.0 [8] and 1.6 [3], respectively. Electrical resistivity of the films was measured with a four-point probe. UV–vis transmittance spectra were collected using a spectrophotometer (JASCO V550DS).
3. Results and discussion Fig. 2(a) shows variation of the X-ray diffraction pattern of impurity-free In–Zn–O thin films of deposited on PC substrates (In–Zn–O/PC films) as a function of current ratio d. The films were deposited from the targets of In2O3 and ZnO. The amorphous phase appeared in a wide range of d-values from 0.20 to 0.67. The d-range where the amorphous phase appeared was the same as that observed for the films on Corning #1737 glass substrates (In–Zn–O/ glass). For the addition of Ga2O3 impurities, X-ray diffraction patterns of In–Zn–O/PC films are shown in Fig. 2(b) as a function of d. The films were deposited from the targets of In2O3 and ZnO containing 7.5 wt% Ga2O3 impurities. The d-range where the amorphous phase was observed was widened up to 0.2 d 0.73, just larger than the value 0.67 for the impurity-free In–Zn–O/PC films in Fig. 2(a). For the addition of Al2O3 impurities, the d-range where the amorphous phase was observed at 0.2 d 0.73 was the same as that of Ga2O3. The contribution of the Ga2O3 or Al2O3 to the structural and physical properties would be larger in the films with the higher zinc content, because the impurity levels of Ga2O3 and Al2O3 in the ZnO targets were fixed to 7.5 wt% and 4 wt%, respectively. Therefore, a fresh amorphous region appeared not in the lower-d but in the higher-d films. Fig. 3(a) indicates d-value dependence of the resistivities of the In–Zn–O/glass and In–Zn–O/PC films. Resistivity in the amorphous region increased with increasing d-value, that is, with increasing zinc content in the film. Cationic composition ratios ([Zn]/ ([In] þ [Zn])) by X-ray fluorescence analysis for the In–Zn–O/PC and In–Zn–O/glass films with no impurities are shown in Fig. 3(b). The ratios of [Zn]/([In] þ [Zn]) in the amorphous films on the In–Zn–O/ glass films contained less zinc than that anticipated from the current ratio d ¼ IZn/(IIn þ IZn). The composition ratio in the amorphous In–Zn–O/PC films increased to be almost equal to the current ratio
a
100
559
d. These results indicate that more zinc may adhere to a PC substrate than to the glass substrate. In Fig. 3(a), the resistivities of impurity-free In–Zn–O/PC films were higher by about one order of magnitude than those of impurity-free In–Zn–O/glass films over all the d-values in the amorphous region. This result will be related to the data in Fig. 3(b). Carriers of the amorphous In–Zn–O films were reported to be mainly the oxygen vacancies [4,5,9,10]. It is also reported that carrier density decreased with an increase of the composition of Zn in the amorphous In–Zn–O films [4,5]. Since the Zn content in the amorphous In–Zn–O/PC films increases even though the current ratio is identical, the number of oxygen vacancies would be reduced so that resistivity of the amorphous In–Zn–O/PC films increased. For the addition of Ga2O3 and Al2O3 impurities in ZnO targets, the data of the resistivities of In–Zn–O/PC and In–Zn–O/glass films were also indicated in Fig. 3(a). Open circles and triangles in Fig. 3(a) indicate d-value dependence of resistivity of the In–Zn–O/ PC films with Ga2O3 and Al2O3 impurities, respectively. We see the decrease of the resistivity in the In–Zn–O/PC films with Ga2O3 and Al2O3 impurities, compared with the impurity-free In–Zn–O/PC film. The addition of Al2O3 impurity decreased the resistivity at 0.4 d 0.6, whereas the addition of Ga2O3 impurity decreased at 0.2 d 0.6. The In–Zn–O/PC films with 7.5 wt% Ga2O3 impurities were reduced to the level of the impurity-free In–Zn–O/glass films. Especially in the amorphous In–Zn–O/PC films with Ga2O3 impurities, resistivities of some 104 U cm could be attained in a wide d-range. Though it is reported that dopings of cation do not work well on the resistivity in the amorphous In–Zn–O/glass, the addition of impurities in the amorphous In–Zn–O/PC was very effective in improving resistivity. A minimum resistivity as low as 3.56 104 U cm was observed for the indium-richest amorphous In–Zn–O/PC film with the 7.5 wt% Ga2O3 impurity. Fig. 4(a) and (b) shows optical transmittance spectra of the amorphous thin films with d ¼ 0.27 and 0.50, respectively. Compared with the films on the Corning glass substrates, the absorption edge of the films on PC substrates is somewhat shifted to the higher wavelength side. Larger blue shifts were observed in the In–Zn–O/PC with the addition of Ga2O3 or Al2O3 impurities for all current ratios d. The magnitude of shifts from the impurity-free In–Zn–O/PC film seemed to be larger with an increase of d-value, because the amount of Ga2O3 or Al2O3 impurities should increase. Blue shifts seen in heavily Al-doped amorphous and homologous In–Zn–O/glass films [11] or Al- or Ga-doped amorphous Sn–Zn–O/ glass films [12] have been reported in several papers, and the origin of these shifts could be ascribed to a lattice distortion due to
b
/PC:no impurity /PC:Al2O3=4.0 wt
10–1
/PC:Ga2O3=7.5 wt
Resistivity
(
cm)
/glass:no Impurity
10–2
10–3
10–4
0
value
1
Fig. 3. (a) d-Value dependence of resistivity of the films on the glass and PC substrates. (b) Cationic composition ratios ([Zn]/([In] þ [Zn])) in In–Zn–O/PC and In–Zn–O/glass thin films as a function of current ratio d.
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T. Moriga et al. / Vacuum 83 (2009) 557–560
a
b
100
100 δ =0.50
Transmittanse (%)
Transmittanse (%)
δ =0.27
10
50 5
0 200
0 300
400
350
600
400
800
10
50 5
0 200
Wavelength (nm)
0 300
400
400
800
Wavelength (nm)
Fig. 4. Optical transmittance spectra of the amorphous indium-zinc-oxide thin films for (a) d ¼ 0.27 and (b) d ¼ 0.50. , In–Zn–O/PC Ga2O3 ¼ 7.5 wt% and , In–Zn–O/PC Al2O3 ¼ 4.0 wt%. impurity free;
the substitution of constituent metal ions by Al3þ. The distortion effect by the Al impurity on the shift seen in the amorphous In– Zn–O/PC films would be larger than that by the Ga impurity.
350
600
, In–Zn–O/glass impurity free;
, In–Zn–O/PC
Grant-in-Aid for Scientific Research (C) (Approval No. 19560677) from Japan Society for Promotion of Science. References
4. Conclusion The resistivities of In–Zn–O/PC films with no impurities were higher than those of In–Zn–O/glass with no impurities. With the addition of Al2O3 and Ga2O3 impurities, we could obtain the amorphous In–Zn–O/PC films whose resistivities were reduced to the level of the non-doped In–Zn–O/glass films. The Ga2O3 addition was effective in decreasing resistivity in the amorphous In–Zn–O/ PC films. By the addition of either Ga2O3 or Al2O3 impurities, the optical band gap energies significantly increased. Acknowledgements The authors would like to thank Prof. J.B. Metson of University of Auckland, New Zealand for editing the manuscript. This work was supported by the research grant from Iketani Science and Technology Foundation (Approval No. 0181090A) and by the
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