- Email: [email protected]
Tungsten-doped ZnO transparent conducting films deposited by direct current magnetron sputtering
Vacuum 85 (2010) 184e186
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Tungsten-doped ZnO transparent conducting films deposited by direct current magnetron sputtering Huafu Zhang*, Hanfa Liu, Chengxin Lei, Changkun Yuan, Aiping Zhou School of Science, Shandong University of Technology, ZiBo 255049, Shandong, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 February 2010 Received in revised form 17 May 2010 Accepted 17 May 2010
Highly conducting and transparent thin films of tungsten-doped ZnO (ZnO:W) were prepared on glass substrates by direct current (DC) magnetron sputtering at low temperature. The effect of film thickness on the structural, electrical and optical properties of ZnO:W films was investigated. All the deposited films are polycrystalline with a hexagonal structure and have a preferred orientation along the c-axis perpendicular to the substrate. The electrical resistivity first decreases with film thickness, and then increases with further increase in film thickness. The lowest resistivity achieved was 6.97 104 U cm for a thickness of 332 nm with a Hall mobility of 6.7 cm2 V1 s1 and a carrier concentration of 1.35 1021 cm3. However, the average transmittance of the films does not change much with an increase in film thickness, and all the deposited films show a high transmittance of approximately 90% in the visible range. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Tungsten-doped ZnO Semiconductors Thin films Magnetron sputtering
1. Introduction Transparent conducting oxides are widely used in microelectronic devices such as liquid crystal displays, organic light-emitting diodes and thin film solar cells [1e3]. It is well known that tindoped indium oxide (ITO) film is the most widely used transparent conducting oxide film due to its high transparency, low resistivity and high work function. However, since indium is a rare and an expensive element, the cost of ITO films is very high. As a result, a stable supply of ITO may be difficult to achieve for the recently expanding market demands. Therefore, it is important to develop alternatives to the ITO thin film transparent electrodes used in liquid crystal displays [4]. Presently, zinc oxide or impurity-doped (such as B, Al, Ga and Zr) zinc oxide films have been actively studied as alternate materials to replace ITO because they are non-toxic, inexpensive and abundant [5]. Moreover, they are also chemically stable under hydrogen plasma processes that are commonly used for the production of solar cells [6]. To dope tungsten into zinc oxide is quite attractive because there is a valence difference of four between Wþ6 and Znþ2, thus each W atom can contribute more than one electron to the electrical conductivity. Furthermore, the ionic radius of Wþ6 and Znþ2 is 0.062 and 0.074 nm, respectively, so it is theoretically possible for Wþ6 to substitute for Znþ2 in ZnO structure. However, only a few authors have studied zinc oxide
* Corresponding author. Tel./fax: þ86 533 2786289. E-mail address: [email protected] (H. Zhang). 0042-207X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2010.05.010
films doped with W in order to improve the conductivity of the films. In this work, highly conducting and transparent ZnO:W thin films were prepared by DC magnetron sputtering at low temperature, and the effects of film thickness on structural, electrical and optical properties of ZnO:W films were investigated. This report may give an added impetus on the applications of this technologically important material and give newer alternatives to ITO in applications.
2. Experimental ZnO:W films were deposited on glass substrates at room temperature by DC magnetron sputtering system with a basic pressure of 6.8 104 Pa. Prior to deposition, the glass substrates were ultrasonically cleaned in acetone for 10 min, immersed in alcohol for 30 min and washed by purified water. A sintered ceramic with a mixture of ZnO (99.99% purity) and WO3 (99.99% purity) was employed as source material. The content of WO3 added to the ZnO target (75 mm diameter, 3 mm thickness) was 7 wt.%. In our experiments, the working gas for DC sputtering was pure Ar (99.999%). During the process of deposition, the DC sputtering power, deposition pressure and target-to-substrate distance were controlled at 120 W, 15 Pa and 75 mm, respectively. The substrate was not heated intentionally during the sputtering. In order to investigate the effects of thickness on the properties of ZnO:W films, deposition time was varied from 20 to 70 min.
H. Zhang et al. / Vacuum 85 (2010) 184e186
185
The structural properties of the films were analyzed with a D8 ADVANCE XRD system using CuKa1 radiation (l ¼ 0.15406 nm). The thickness of the films was measured using a SGC-10 thin film thickness tester. The electrical properties (resistivity, Hall mobility and carrier concentration) were measured by Hall effect measurements using the van der Pauw technique at a constant magnetic field of 1 T. The optical transmittance measurements were performed with a TU-1901 UVevis spectrophotometers. 3. Results and discussion 3.1. Thickness of ZnO:W films Fig. 1 shows the variation of film thickness with deposition time. It is found that the thickness increases nearly linearly from 114 to 406 nm when the deposition time increases from 20 to 70 min. The growth rate is in range of 5.4e5.8 nm/min, which does not change much with the variation of deposition time. Fig. 2. XRD patterns of ZnO:W film with different thicknesses.
3.2. Structural characterization of ZnO:W films Fig. 2 shows the XRD patterns of ZnO:W films with different thicknesses deposited by DC magnetron sputtering. The qe2q scan data of the ZnO:W film exhibits a strong peak at about 2q ¼ 34.55 , which is close to the preferred orientation of the standard ZnO crystal (34.45 ) and corresponding to the (002) peak. It is notable that when the film thickness is 114 nm, the (200) peak (about 2q ¼ 29.55 ) of the WO3 film was also detected from the XRD pattern, but the intensity of the (200) peak is much weaker than the (002) peak of the ZnO:W film. This indicates that the ZnO:W film is polycrystalline with a hexagonal structure and has a preferred orientation perpendicular to the substrate. No other peaks were detected from the XRD patterns for the thicker ZnO:W films. This may be due to tungsten replacing zinc substitutionally in the hexagonal lattice or tungsten segregating to the non-crystalline region in grain boundary. We find that the position and the fullwidth at half-maximum of the (002) peak do not change much while the intensity increases significantly with an increase in film thickness up to 332 nm, which suggests an improvement in crystallinity. However, with further increase in film thickness, the intensity varies little. Similar results have been reported in Zrdoped ZnO films deposited by RF magnetron sputtering [7]. The
average crystallite dimension of the ZnO:W films is about 21e25 nm estimated from the XRD pattern according to Scherrer’s formula, indicating the grain size of ZnO:W films in this experiment is not sensitive to film thickness. 3.3. Electrical properties of ZnO:W films The effect of film thickness on the electrical properties of ZnO:W films was shown in Fig. 3. The results show that all the films are degenerate doped n-type semiconductors. It is observed that the resistivity, Hall mobility and carrier concentration are sensitive to film thickness. We find that the carrier concentration initially increases with an increase in film thickness up to 332 nm and then it decreases with an increase in film thickness. The similar phenomenon is observed in Hall mobility dependence of film thickness. The increase of carrier concentration and Hall mobility can be attributed to the improvement of the crystallinity [7,8], which is confirmed by the results of XRD analysis mentioned above. The resistivity, however, first decreases with an increase in film thickness due to the increase of both Hall mobility and carrier concentration, and then increases with further increase in film thickness because of the decrease of both Hall mobility and carrier concentration. Similar results have been reported in Zr-doped ZnO films [7] and Al-doped ZnO films [8,9]. When the film thickness is 332 nm, the lowest resistivity is obtained and is 6.97 104 U cm with a Hall mobility of 6.7 cm2 V1 s1 and a carrier concentration of 1.35 1021 cm3. For all the deposited films, the values of Hall mobility are relatively low (from 4.65 to 6.79 cm2 V1 s1), which may be attributed to the relatively low temperature of substrates during sputtering [7]. 3.4. Optical properties of ZnO:W films
Fig. 1. Dependence of film thickness on deposition time for ZnO:W films.
Fig. 4 shows the optical transmittance spectra in the UVevis region of ZnO:W films with different thicknesses. The fluctuation in the spectra is principally due to the interference effect owing to the reflection effect at interfaces. All the ZnO:W films exhibit a high transmittance of approximately 90% in the visible region, regardless of film thickness and all the deposited films have a sharp absorption edge in the UV range of 300e400 nm, which shifts to the longer wavelength side as the thickness increases.
186
H. Zhang et al. / Vacuum 85 (2010) 184e186
Fig. 3. Dependence of resistivity r, Hall mobility m and carrier concentration n of ZnO:W films on film thickness.
polycrystalline with a hexagonal structure and have a preferred orientation along the c-axis perpendicular to the substrate. The crystallinity, resistivity, Hall mobility and carrier concentration of the deposited films greatly depend on film thickness while the optical transmittance of the films is not sensitive to film thickness. At the optimum thickness of 332 nm, the film has the lowest resistivity of 6.79 104 U cm with a high transmittance of above 92% in the visible range. All experimental results indicate that ZnO:W can be used to fabricate high performance TCO thin films. Acknowledgement This work was supported by Shandong Province Natural Science Foundation No. ZR2009GQ011. We would express sincere thanks to Ms. Fan Xiaoling from the School of Materials Science and Engineering for her experimental assistance. References Fig. 4. Transmittance spectra of ZnO:W films with different thicknesses as a function of wavelength.
4. Conclusions ZnO:W thin films with low resistivity and high transparency were successfully prepared on glass substrates by DC magnetron sputtering at room temperature. The deposited films are
[1] Ma HB, Cho JS, Park CH. Surf Coat Technol 2002;153:131e7. [2] Nisha M, Anusha S, Antony A, Manoj R, Jayaraj MK. Appl Surf Sci 2005;252:1430e5. [3] Zhang HF, Lei CX, Liu HF, Yuan CK. Appl Surf Sci 2009;255:6054e6. [4] Tadatsugu M. Thin Solid Films 2008;516:5822e8. [5] Kim H, Horwitz JS, Kim WH, Mäkinen AJ, Kafafi ZH, Chrisey DB. Thin Solid Films 2002;420/421:539e43. [6] Ellmer K. J Phys D Appl Phys 2001;34:3097. [7] Lv MS, Xiu XW, Pang ZY, Dai Y, Han SH. Appl Surf Sci 2005;252:2006e11. [8] Hao XT, Ma J, Zhang DH, Yang TY, Ma HL, Yang YG. Appl Surf Sci 2001;183:137e42. [9] Teong WJ, Park GC. Sol Energy Mater Sol Cells 2001;65:37e45.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Tungsten-doped ZnO transparent conducting films deposited by direct current magnetron sputtering Huafu Zhang*, Hanfa Liu, Chengxin Lei, Changkun Yuan, Aiping Zhou School of Science, Shandong University of Technology, ZiBo 255049, Shandong, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 February 2010 Received in revised form 17 May 2010 Accepted 17 May 2010
Highly conducting and transparent thin films of tungsten-doped ZnO (ZnO:W) were prepared on glass substrates by direct current (DC) magnetron sputtering at low temperature. The effect of film thickness on the structural, electrical and optical properties of ZnO:W films was investigated. All the deposited films are polycrystalline with a hexagonal structure and have a preferred orientation along the c-axis perpendicular to the substrate. The electrical resistivity first decreases with film thickness, and then increases with further increase in film thickness. The lowest resistivity achieved was 6.97 104 U cm for a thickness of 332 nm with a Hall mobility of 6.7 cm2 V1 s1 and a carrier concentration of 1.35 1021 cm3. However, the average transmittance of the films does not change much with an increase in film thickness, and all the deposited films show a high transmittance of approximately 90% in the visible range. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Tungsten-doped ZnO Semiconductors Thin films Magnetron sputtering
1. Introduction Transparent conducting oxides are widely used in microelectronic devices such as liquid crystal displays, organic light-emitting diodes and thin film solar cells [1e3]. It is well known that tindoped indium oxide (ITO) film is the most widely used transparent conducting oxide film due to its high transparency, low resistivity and high work function. However, since indium is a rare and an expensive element, the cost of ITO films is very high. As a result, a stable supply of ITO may be difficult to achieve for the recently expanding market demands. Therefore, it is important to develop alternatives to the ITO thin film transparent electrodes used in liquid crystal displays [4]. Presently, zinc oxide or impurity-doped (such as B, Al, Ga and Zr) zinc oxide films have been actively studied as alternate materials to replace ITO because they are non-toxic, inexpensive and abundant [5]. Moreover, they are also chemically stable under hydrogen plasma processes that are commonly used for the production of solar cells [6]. To dope tungsten into zinc oxide is quite attractive because there is a valence difference of four between Wþ6 and Znþ2, thus each W atom can contribute more than one electron to the electrical conductivity. Furthermore, the ionic radius of Wþ6 and Znþ2 is 0.062 and 0.074 nm, respectively, so it is theoretically possible for Wþ6 to substitute for Znþ2 in ZnO structure. However, only a few authors have studied zinc oxide
* Corresponding author. Tel./fax: þ86 533 2786289. E-mail address: [email protected] (H. Zhang). 0042-207X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2010.05.010
films doped with W in order to improve the conductivity of the films. In this work, highly conducting and transparent ZnO:W thin films were prepared by DC magnetron sputtering at low temperature, and the effects of film thickness on structural, electrical and optical properties of ZnO:W films were investigated. This report may give an added impetus on the applications of this technologically important material and give newer alternatives to ITO in applications.
2. Experimental ZnO:W films were deposited on glass substrates at room temperature by DC magnetron sputtering system with a basic pressure of 6.8 104 Pa. Prior to deposition, the glass substrates were ultrasonically cleaned in acetone for 10 min, immersed in alcohol for 30 min and washed by purified water. A sintered ceramic with a mixture of ZnO (99.99% purity) and WO3 (99.99% purity) was employed as source material. The content of WO3 added to the ZnO target (75 mm diameter, 3 mm thickness) was 7 wt.%. In our experiments, the working gas for DC sputtering was pure Ar (99.999%). During the process of deposition, the DC sputtering power, deposition pressure and target-to-substrate distance were controlled at 120 W, 15 Pa and 75 mm, respectively. The substrate was not heated intentionally during the sputtering. In order to investigate the effects of thickness on the properties of ZnO:W films, deposition time was varied from 20 to 70 min.
H. Zhang et al. / Vacuum 85 (2010) 184e186
185
The structural properties of the films were analyzed with a D8 ADVANCE XRD system using CuKa1 radiation (l ¼ 0.15406 nm). The thickness of the films was measured using a SGC-10 thin film thickness tester. The electrical properties (resistivity, Hall mobility and carrier concentration) were measured by Hall effect measurements using the van der Pauw technique at a constant magnetic field of 1 T. The optical transmittance measurements were performed with a TU-1901 UVevis spectrophotometers. 3. Results and discussion 3.1. Thickness of ZnO:W films Fig. 1 shows the variation of film thickness with deposition time. It is found that the thickness increases nearly linearly from 114 to 406 nm when the deposition time increases from 20 to 70 min. The growth rate is in range of 5.4e5.8 nm/min, which does not change much with the variation of deposition time. Fig. 2. XRD patterns of ZnO:W film with different thicknesses.
3.2. Structural characterization of ZnO:W films Fig. 2 shows the XRD patterns of ZnO:W films with different thicknesses deposited by DC magnetron sputtering. The qe2q scan data of the ZnO:W film exhibits a strong peak at about 2q ¼ 34.55 , which is close to the preferred orientation of the standard ZnO crystal (34.45 ) and corresponding to the (002) peak. It is notable that when the film thickness is 114 nm, the (200) peak (about 2q ¼ 29.55 ) of the WO3 film was also detected from the XRD pattern, but the intensity of the (200) peak is much weaker than the (002) peak of the ZnO:W film. This indicates that the ZnO:W film is polycrystalline with a hexagonal structure and has a preferred orientation perpendicular to the substrate. No other peaks were detected from the XRD patterns for the thicker ZnO:W films. This may be due to tungsten replacing zinc substitutionally in the hexagonal lattice or tungsten segregating to the non-crystalline region in grain boundary. We find that the position and the fullwidth at half-maximum of the (002) peak do not change much while the intensity increases significantly with an increase in film thickness up to 332 nm, which suggests an improvement in crystallinity. However, with further increase in film thickness, the intensity varies little. Similar results have been reported in Zrdoped ZnO films deposited by RF magnetron sputtering [7]. The
average crystallite dimension of the ZnO:W films is about 21e25 nm estimated from the XRD pattern according to Scherrer’s formula, indicating the grain size of ZnO:W films in this experiment is not sensitive to film thickness. 3.3. Electrical properties of ZnO:W films The effect of film thickness on the electrical properties of ZnO:W films was shown in Fig. 3. The results show that all the films are degenerate doped n-type semiconductors. It is observed that the resistivity, Hall mobility and carrier concentration are sensitive to film thickness. We find that the carrier concentration initially increases with an increase in film thickness up to 332 nm and then it decreases with an increase in film thickness. The similar phenomenon is observed in Hall mobility dependence of film thickness. The increase of carrier concentration and Hall mobility can be attributed to the improvement of the crystallinity [7,8], which is confirmed by the results of XRD analysis mentioned above. The resistivity, however, first decreases with an increase in film thickness due to the increase of both Hall mobility and carrier concentration, and then increases with further increase in film thickness because of the decrease of both Hall mobility and carrier concentration. Similar results have been reported in Zr-doped ZnO films [7] and Al-doped ZnO films [8,9]. When the film thickness is 332 nm, the lowest resistivity is obtained and is 6.97 104 U cm with a Hall mobility of 6.7 cm2 V1 s1 and a carrier concentration of 1.35 1021 cm3. For all the deposited films, the values of Hall mobility are relatively low (from 4.65 to 6.79 cm2 V1 s1), which may be attributed to the relatively low temperature of substrates during sputtering [7]. 3.4. Optical properties of ZnO:W films
Fig. 1. Dependence of film thickness on deposition time for ZnO:W films.
Fig. 4 shows the optical transmittance spectra in the UVevis region of ZnO:W films with different thicknesses. The fluctuation in the spectra is principally due to the interference effect owing to the reflection effect at interfaces. All the ZnO:W films exhibit a high transmittance of approximately 90% in the visible region, regardless of film thickness and all the deposited films have a sharp absorption edge in the UV range of 300e400 nm, which shifts to the longer wavelength side as the thickness increases.
186
H. Zhang et al. / Vacuum 85 (2010) 184e186
Fig. 3. Dependence of resistivity r, Hall mobility m and carrier concentration n of ZnO:W films on film thickness.
polycrystalline with a hexagonal structure and have a preferred orientation along the c-axis perpendicular to the substrate. The crystallinity, resistivity, Hall mobility and carrier concentration of the deposited films greatly depend on film thickness while the optical transmittance of the films is not sensitive to film thickness. At the optimum thickness of 332 nm, the film has the lowest resistivity of 6.79 104 U cm with a high transmittance of above 92% in the visible range. All experimental results indicate that ZnO:W can be used to fabricate high performance TCO thin films. Acknowledgement This work was supported by Shandong Province Natural Science Foundation No. ZR2009GQ011. We would express sincere thanks to Ms. Fan Xiaoling from the School of Materials Science and Engineering for her experimental assistance. References Fig. 4. Transmittance spectra of ZnO:W films with different thicknesses as a function of wavelength.
4. Conclusions ZnO:W thin films with low resistivity and high transparency were successfully prepared on glass substrates by DC magnetron sputtering at room temperature. The deposited films are
[1] Ma HB, Cho JS, Park CH. Surf Coat Technol 2002;153:131e7. [2] Nisha M, Anusha S, Antony A, Manoj R, Jayaraj MK. Appl Surf Sci 2005;252:1430e5. [3] Zhang HF, Lei CX, Liu HF, Yuan CK. Appl Surf Sci 2009;255:6054e6. [4] Tadatsugu M. Thin Solid Films 2008;516:5822e8. [5] Kim H, Horwitz JS, Kim WH, Mäkinen AJ, Kafafi ZH, Chrisey DB. Thin Solid Films 2002;420/421:539e43. [6] Ellmer K. J Phys D Appl Phys 2001;34:3097. [7] Lv MS, Xiu XW, Pang ZY, Dai Y, Han SH. Appl Surf Sci 2005;252:2006e11. [8] Hao XT, Ma J, Zhang DH, Yang TY, Ma HL, Yang YG. Appl Surf Sci 2001;183:137e42. [9] Teong WJ, Park GC. Sol Energy Mater Sol Cells 2001;65:37e45.