- Email: [email protected]
ZnO multilayer films
Optics Communications 285 (2012) 1212–1214
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
The influence of Au thickness on the structural, optical and electrical properties of ZnO/Au/ZnO multilayer films Daeil Kim ⁎ School of Materials Science and Engineering, University of Ulsan San 29, Mugeo Dong, Nam Gu, Ulsan 680-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 6 June 2011 Received in revised form 31 August 2011 Accepted 18 October 2011 Available online 4 November 2011 Keywords: ZnO Au XRD Figure of merit Work function
a b s t r a c t Transparent and conductive ZnO/Au/ZnO (ZAZ) multilayer films were deposited on glass substrates by magnetron sputtering without intentional substrate heating. The thickness of Au interlayer was set at 1, 2 and 3 nm. The observed structural, optical and electrical properties were dependent on the thickness of the Au interlayer. For all of the ZAZ films, the diffraction peaks in the XRD pattern were identified as the (002) and (103) planes of a ZnO films and the (111) plane of an Au interlayer. The ZAZ films with a 2 nm thick Au interlayer showed a higher figure of merit than the other ZAZ films prepared in this study, and they also demonstrated the relatively high work function of 5.13 eV. From these results, we concluded that a ZAZ film with a 2 nm thick Au interlayer is an alternative candidate for use as a transparent electrode in OLEDs and various flat panel displays. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The rapidly increasing use of transparent and conductive oxide (TCO) films for large area displays and solar cells has promoted the development of inexpensive materials that exhibit the appropriate optical and electrical properties. Recently, Al-doped ZnO [1] and Gadoped ZnO [2] films have attracted attention because of their high optical transmittances and relatively low electrical resistivity compared to other popular TCO such as tin-doped indium oxide (ITO) films. However, to obtain desirable optical and electrical properties in metal doped ZnO films for display and solar cell applications, high substrate temperatures [3] or annealing [1] are required to obtain the necessary transmittance and electrical resistivity. In this study new transparent and conducting ZnO/Au/ZnO (ZAZ) multilayer films were prepared by reactive RF magnetron sputtering of ZnO and DC sputtering of Au without intentional substrate heating and then the changes in the structural, optical and electrical properties of the ZAZ films with variations in the intermediate Au thickness were investigated.
2. Experimental details Sandwich structured ZAZ films were deposited using RF and DC magnetron sputtering on glass substrates (3× 3 cm2) without intentional substrate heating using ZnO (99.99% purity, 3-inch diameter) and Au targets (99.9% purity, 3-inch diameter), respectively. The
⁎ Fax: + 82 52 259 1688. 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.10.043
substrates were ultrasonically cleaned in an ethanol/acetone solution and then rinsed in deionized water. Substrates were dried in flowing nitrogen gas and the deposition chamber was evacuated to 1 × 10 − 4 Pa prior to placing in the substrate holder. ZnO sputtering was performed in a pure Ar/O2 gas mixture, and Au interlayer sputtering was performed in a pure Ar gas atmosphere. Although the substrate was not heated intentionally, the substrate temperature increased to 70 °C during deposition. By controlling the deposition time, the ZAZ multilayer film thickness was maintained at 50/1/49 nm, 50/2/48 nm, and 50/3/ 47 nm. The various deposition conditions used in this study are shown in Table 1. After deposition, the thickness and crystallinity of the films were measured using a surface profiler (Alpha-step 500, TENCOR, Milpitas, CA) and X-ray diffraction (XRD, Cu-Kα X'pert Pro MRD, Philips), respectively. The variations of surface morphology and root mean square (RMS) roughness as a function of Au thickness were evaluated using an atomic force microscope (AFM, XE-100, Park system). The sheet resistance (Rs) and optical transmittance in a wavelength range of 200–800 nm were measured with four-point proves measurements (HMS-3000, Ecopia) and a UV–vis. spectrophotometer (Cary 100 Cone, Varian), respectively. Bare glass substrates exhibited an optical transmittance of 91% in the visible wavelength range. The performance in an optical and electrical property of ZAZ films was evaluated using figure of merit (FOM) [4]. The FOM is defined as FOM = T 10/Rs where T is the mean optical transmittance in a visible wavelength region (400–800 nm) and Rs is the sheet resistance [5, 6]. Furthermore, in order to substitute ZAZ films as a transparent anode of OLEDs, the work function of the films was considered by UV photoelectron spectroscopy (UPS, AXIS-NOVA, Kratos) analysis at the Korea Basic Science Institute (KBSI) Joenju Center.
D. Kim / Optics Communications 285 (2012) 1212–1214
(a) RMS :5.6nm
Table 1 Deposition conditions of ZnO/Au/ZnO multilayer films.
Working pressure (Pa) Deposition rate (nm / min) Target composition Gas flow rate (sccm)
1213
ZnO
Au
4 × 10− 1 10 99.99% Ar:5, O2:0.2
1 × 10− 1 5 99.9% Ar:5
3. Results and discussions In order to evaluate the performance of the ZAZ films, structural characterization is very important because the optical and electrical properties of the films are depends on the microstructure The XRD patterns of ZAZ films are presented in Fig. 1. For all of the ZAZ films, the diffraction peaks in the XRD pattern were identified as (002) and (103) planes of a ZnO film and (111) plane of an Au interlayer. It is observed that as the Au thickness increased, the peak intensity of ZnO (002) decreased. Surface images of ZAZ films are shown in Fig. 2. As deposited ZnO films showed the rougher RMS roughness of 5.6 nm than that of the ZAZ films. The films with a 1 nm thick Au films showed an RMS roughness of 3.9 nm and the films with 3 nm thick Au films showed the lower RMS roughness of 3.2 nm. The thicker the intermediate Au thickness corresponded with a smoother surface morphology. From the XRD patterns, it was supposed that the surface roughness is affected by the crystallization of the Au interlayer. In a previous study, J. Park also reported that the intermediate Ni layer in ITO/Ni/ITO multilayer films also is effective for the decrement of surface roughness [7]. The optical transmittance of ZnO and ZAZ films is presented in Fig. 3. Pure 100 nm thick ZnO films had an optical transmittance of 85.5% and ZAZ films with a 3 nm thick Au layer show the lowest transmittance of 80% in the visible wave length region. The transmittance was decreased as the thickness of the Au interlayer increased, likely because the metallic Au interlayer may absorb visible light. Thus, although the intermediate noble metal layer may reduce the electrical resistivity of TCO films due to enormous carrier density, its high optical absorption is a serious drawback that limits the optical transmittance of ZAZ films. In order to obtain high quality ZAZ films, the thickness of the Au interlayer should be optimized. The optical transmittance and the sheet resistance (Rs) as a function of the Au thickness are presented in Table 2. In a previous study, the ZnO single layer films exhibited an Rs of 1.22 × 10 6 Ω/□. With a
Fig. 1. XRD pattern of the ZnO and ZAZ films with different thickness of the Au interlayer. (a) ZnO single layer film (100 nm), (b) ZnO/Au/ZnO (50/1/49 nm), (c) ZnO/Au/ ZnO (50/2/48 nm), (d) ZnO/Au/ZnO (50/3/47 nm).
(b) RMS :3.9nm
(c) RMS : 3.2nm
Fig. 2. AFM image of ZnO and ZAZ films with different thickness of the Au interlayer. (a) ZnO single layer film, (b) ZAZ with a 1 nm thick Au interlayer, (c) ZAZ with a 3 nm thick Au interlayer.
1214
D. Kim / Optics Communications 285 (2012) 1212–1214
100
Table 2 Comparison of the optical transmittance (T, %, without substrate), sheet resistance (Rs, Ω/□) and figure of merit (FOM, 10− 4 Ω− 1) of ZAZ films with different thickness of Au interlayer.
Optical transmittance (%)
80
60
Au thickness
1 nm
2 nm
3 nm
Rs T FOM
297 77 2.7
21 81 55
20 80 53
40 Glass substrate ZnO thin film ZnO/Au/ZnO (50/1/49 nm) ZnO/Au/ZnO (50/2/48 nm) ZnO/Au/ZnO (50/3/47 nm)
20
0 300
400
500
600
700
800
Wave length (nm) Fig. 3. The optical transmittance of ZnO and ZAZ films with different thickness of the Au interlayer (without substrate).
1 nm thick Au interlayer in the ZnO film, the Rs decreased effectively to 297 Ω/□ but when the Au thickness was increased to 3 nm, the Rs decreased to as low as 20 Ω/□. Although ZAZ films with a 3 nm thick Au layer have the lowest Rs, ZAZ film with a 2 nm thick Au interlayer shows the highest FOM of 5.5 × 10 − 3 Ω − 1 due to the relatively lower optical transmittance of the ZAZ films with a 3 nm thick Au layer (Table 2). The high work function of ITO films, which is close to the value of the highest occupied molecular orbital (HOMO) of the organic layer, allows hole injection from the ITO to the organic layer of OLED resulting in a decrease in the turn-on voltage of the OLED. However, the work function of conventional ITO films is lower than the HOMO of the organic layer of OLEDs. Thus, several techniques have been developed to increase the work function of ITO [7–9]. Using a ZAZ films with a 2 nm thick Au interlayer allowed us to determine the work function values directly from the spectra by fitting straight lines to their kinetic energy cut-offs and determining the intersects with the base lines of the spectra (Fig. 4). The variation of work function as a function of the Au thickness is presented in Table 3. The work function of the ZAZ films with a 2 nm thick Au interlayer had the highest value of 5.13 eV, which is the higher than the Ar ion beam irradiated ITO films [9]. These results indicate that the ZAZ film with a 2 nm thick Au interlayer is an alternative candidate for use as a transparent anode in OLEDs and flat panel displays. 4. Conclusions Transparent and conducting ZAZ films with various Au interlayer thicknesses were prepared on glass substrates using magnetron sputtering. The effects of intermediate Au thickness on the structural, optical and electrical properties were investigated. From XRD measurements, ZAZ films exhibited ZnO (002) diffraction peaks indicating synthesis of a crystallized ZAZ film at the low temperature of 70 °C. ZAZ films with a 2 nm thick Au interlayer had the highest figure of merit of 5.5 × 10− 3 Ω− 1 and a work function of 5.13 eV.
Fig. 4. UPS measurement of ZAZ films with 2 nm thick Au interlayer.
Table 3 Variation of the work function of ZAZ films. TCO films
Work function
Reference
ITO ZnO 49 nm/Au 1 nm/ZnO 50 nm ZnO 48 nm/Au 2 nm/ZnO 50 nm ZnO 47 nm /Au 3 nm/ZnO 50 nm
3.90 eV 4.81 eV 5.13 eV 4.89 eV
[7] This study This study This study
These results indicate that ZAZ films with a 2 nm thick Au interlayer are an alternative for use as a transparent conducting anode in OLEDs and various flat panel displays.
References [1] [2] [3] [4] [5] [6] [7] [8]
G. Fang, D. Li, B. Yao, Vacuum 68 (2002) 363. L. Gong, J. Lu, Z. Ye, Solar Energy Materials and Solar Cells 94 (2010) 1282. S. Suzuki, T. Miyata, M. Ishii, T. Minami, Thin Solid Films 434 (2003) 14. G. Haacke, Journal of Applied Physics 47 (1976) 4086. Y.S. Kim, J.H. Park, Daeil Kim, Vacuum 82 (2008) 574. Daeil Kim, Journal of Alloys and Compounds 493 (2010) 208. J.H. Park, J.H. Chae, D. Kim, Journal of Alloys and Compounds 478 (2009) 330. Y.S. Kim, Y.J. Lee, S.B. Heo, H.M. Lee, J.H. Kim, S.K. Kim, J.H. Chae, J.I. Choi, Daeil Kim, Optics Communication 284 (2011) 2303. [9] Daeil Kim, Applied Surface Science 257 (2010) 704.
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
The influence of Au thickness on the structural, optical and electrical properties of ZnO/Au/ZnO multilayer films Daeil Kim ⁎ School of Materials Science and Engineering, University of Ulsan San 29, Mugeo Dong, Nam Gu, Ulsan 680-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 6 June 2011 Received in revised form 31 August 2011 Accepted 18 October 2011 Available online 4 November 2011 Keywords: ZnO Au XRD Figure of merit Work function
a b s t r a c t Transparent and conductive ZnO/Au/ZnO (ZAZ) multilayer films were deposited on glass substrates by magnetron sputtering without intentional substrate heating. The thickness of Au interlayer was set at 1, 2 and 3 nm. The observed structural, optical and electrical properties were dependent on the thickness of the Au interlayer. For all of the ZAZ films, the diffraction peaks in the XRD pattern were identified as the (002) and (103) planes of a ZnO films and the (111) plane of an Au interlayer. The ZAZ films with a 2 nm thick Au interlayer showed a higher figure of merit than the other ZAZ films prepared in this study, and they also demonstrated the relatively high work function of 5.13 eV. From these results, we concluded that a ZAZ film with a 2 nm thick Au interlayer is an alternative candidate for use as a transparent electrode in OLEDs and various flat panel displays. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The rapidly increasing use of transparent and conductive oxide (TCO) films for large area displays and solar cells has promoted the development of inexpensive materials that exhibit the appropriate optical and electrical properties. Recently, Al-doped ZnO [1] and Gadoped ZnO [2] films have attracted attention because of their high optical transmittances and relatively low electrical resistivity compared to other popular TCO such as tin-doped indium oxide (ITO) films. However, to obtain desirable optical and electrical properties in metal doped ZnO films for display and solar cell applications, high substrate temperatures [3] or annealing [1] are required to obtain the necessary transmittance and electrical resistivity. In this study new transparent and conducting ZnO/Au/ZnO (ZAZ) multilayer films were prepared by reactive RF magnetron sputtering of ZnO and DC sputtering of Au without intentional substrate heating and then the changes in the structural, optical and electrical properties of the ZAZ films with variations in the intermediate Au thickness were investigated.
2. Experimental details Sandwich structured ZAZ films were deposited using RF and DC magnetron sputtering on glass substrates (3× 3 cm2) without intentional substrate heating using ZnO (99.99% purity, 3-inch diameter) and Au targets (99.9% purity, 3-inch diameter), respectively. The
⁎ Fax: + 82 52 259 1688. 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.10.043
substrates were ultrasonically cleaned in an ethanol/acetone solution and then rinsed in deionized water. Substrates were dried in flowing nitrogen gas and the deposition chamber was evacuated to 1 × 10 − 4 Pa prior to placing in the substrate holder. ZnO sputtering was performed in a pure Ar/O2 gas mixture, and Au interlayer sputtering was performed in a pure Ar gas atmosphere. Although the substrate was not heated intentionally, the substrate temperature increased to 70 °C during deposition. By controlling the deposition time, the ZAZ multilayer film thickness was maintained at 50/1/49 nm, 50/2/48 nm, and 50/3/ 47 nm. The various deposition conditions used in this study are shown in Table 1. After deposition, the thickness and crystallinity of the films were measured using a surface profiler (Alpha-step 500, TENCOR, Milpitas, CA) and X-ray diffraction (XRD, Cu-Kα X'pert Pro MRD, Philips), respectively. The variations of surface morphology and root mean square (RMS) roughness as a function of Au thickness were evaluated using an atomic force microscope (AFM, XE-100, Park system). The sheet resistance (Rs) and optical transmittance in a wavelength range of 200–800 nm were measured with four-point proves measurements (HMS-3000, Ecopia) and a UV–vis. spectrophotometer (Cary 100 Cone, Varian), respectively. Bare glass substrates exhibited an optical transmittance of 91% in the visible wavelength range. The performance in an optical and electrical property of ZAZ films was evaluated using figure of merit (FOM) [4]. The FOM is defined as FOM = T 10/Rs where T is the mean optical transmittance in a visible wavelength region (400–800 nm) and Rs is the sheet resistance [5, 6]. Furthermore, in order to substitute ZAZ films as a transparent anode of OLEDs, the work function of the films was considered by UV photoelectron spectroscopy (UPS, AXIS-NOVA, Kratos) analysis at the Korea Basic Science Institute (KBSI) Joenju Center.
D. Kim / Optics Communications 285 (2012) 1212–1214
(a) RMS :5.6nm
Table 1 Deposition conditions of ZnO/Au/ZnO multilayer films.
Working pressure (Pa) Deposition rate (nm / min) Target composition Gas flow rate (sccm)
1213
ZnO
Au
4 × 10− 1 10 99.99% Ar:5, O2:0.2
1 × 10− 1 5 99.9% Ar:5
3. Results and discussions In order to evaluate the performance of the ZAZ films, structural characterization is very important because the optical and electrical properties of the films are depends on the microstructure The XRD patterns of ZAZ films are presented in Fig. 1. For all of the ZAZ films, the diffraction peaks in the XRD pattern were identified as (002) and (103) planes of a ZnO film and (111) plane of an Au interlayer. It is observed that as the Au thickness increased, the peak intensity of ZnO (002) decreased. Surface images of ZAZ films are shown in Fig. 2. As deposited ZnO films showed the rougher RMS roughness of 5.6 nm than that of the ZAZ films. The films with a 1 nm thick Au films showed an RMS roughness of 3.9 nm and the films with 3 nm thick Au films showed the lower RMS roughness of 3.2 nm. The thicker the intermediate Au thickness corresponded with a smoother surface morphology. From the XRD patterns, it was supposed that the surface roughness is affected by the crystallization of the Au interlayer. In a previous study, J. Park also reported that the intermediate Ni layer in ITO/Ni/ITO multilayer films also is effective for the decrement of surface roughness [7]. The optical transmittance of ZnO and ZAZ films is presented in Fig. 3. Pure 100 nm thick ZnO films had an optical transmittance of 85.5% and ZAZ films with a 3 nm thick Au layer show the lowest transmittance of 80% in the visible wave length region. The transmittance was decreased as the thickness of the Au interlayer increased, likely because the metallic Au interlayer may absorb visible light. Thus, although the intermediate noble metal layer may reduce the electrical resistivity of TCO films due to enormous carrier density, its high optical absorption is a serious drawback that limits the optical transmittance of ZAZ films. In order to obtain high quality ZAZ films, the thickness of the Au interlayer should be optimized. The optical transmittance and the sheet resistance (Rs) as a function of the Au thickness are presented in Table 2. In a previous study, the ZnO single layer films exhibited an Rs of 1.22 × 10 6 Ω/□. With a
Fig. 1. XRD pattern of the ZnO and ZAZ films with different thickness of the Au interlayer. (a) ZnO single layer film (100 nm), (b) ZnO/Au/ZnO (50/1/49 nm), (c) ZnO/Au/ ZnO (50/2/48 nm), (d) ZnO/Au/ZnO (50/3/47 nm).
(b) RMS :3.9nm
(c) RMS : 3.2nm
Fig. 2. AFM image of ZnO and ZAZ films with different thickness of the Au interlayer. (a) ZnO single layer film, (b) ZAZ with a 1 nm thick Au interlayer, (c) ZAZ with a 3 nm thick Au interlayer.
1214
D. Kim / Optics Communications 285 (2012) 1212–1214
100
Table 2 Comparison of the optical transmittance (T, %, without substrate), sheet resistance (Rs, Ω/□) and figure of merit (FOM, 10− 4 Ω− 1) of ZAZ films with different thickness of Au interlayer.
Optical transmittance (%)
80
60
Au thickness
1 nm
2 nm
3 nm
Rs T FOM
297 77 2.7
21 81 55
20 80 53
40 Glass substrate ZnO thin film ZnO/Au/ZnO (50/1/49 nm) ZnO/Au/ZnO (50/2/48 nm) ZnO/Au/ZnO (50/3/47 nm)
20
0 300
400
500
600
700
800
Wave length (nm) Fig. 3. The optical transmittance of ZnO and ZAZ films with different thickness of the Au interlayer (without substrate).
1 nm thick Au interlayer in the ZnO film, the Rs decreased effectively to 297 Ω/□ but when the Au thickness was increased to 3 nm, the Rs decreased to as low as 20 Ω/□. Although ZAZ films with a 3 nm thick Au layer have the lowest Rs, ZAZ film with a 2 nm thick Au interlayer shows the highest FOM of 5.5 × 10 − 3 Ω − 1 due to the relatively lower optical transmittance of the ZAZ films with a 3 nm thick Au layer (Table 2). The high work function of ITO films, which is close to the value of the highest occupied molecular orbital (HOMO) of the organic layer, allows hole injection from the ITO to the organic layer of OLED resulting in a decrease in the turn-on voltage of the OLED. However, the work function of conventional ITO films is lower than the HOMO of the organic layer of OLEDs. Thus, several techniques have been developed to increase the work function of ITO [7–9]. Using a ZAZ films with a 2 nm thick Au interlayer allowed us to determine the work function values directly from the spectra by fitting straight lines to their kinetic energy cut-offs and determining the intersects with the base lines of the spectra (Fig. 4). The variation of work function as a function of the Au thickness is presented in Table 3. The work function of the ZAZ films with a 2 nm thick Au interlayer had the highest value of 5.13 eV, which is the higher than the Ar ion beam irradiated ITO films [9]. These results indicate that the ZAZ film with a 2 nm thick Au interlayer is an alternative candidate for use as a transparent anode in OLEDs and flat panel displays. 4. Conclusions Transparent and conducting ZAZ films with various Au interlayer thicknesses were prepared on glass substrates using magnetron sputtering. The effects of intermediate Au thickness on the structural, optical and electrical properties were investigated. From XRD measurements, ZAZ films exhibited ZnO (002) diffraction peaks indicating synthesis of a crystallized ZAZ film at the low temperature of 70 °C. ZAZ films with a 2 nm thick Au interlayer had the highest figure of merit of 5.5 × 10− 3 Ω− 1 and a work function of 5.13 eV.
Fig. 4. UPS measurement of ZAZ films with 2 nm thick Au interlayer.
Table 3 Variation of the work function of ZAZ films. TCO films
Work function
Reference
ITO ZnO 49 nm/Au 1 nm/ZnO 50 nm ZnO 48 nm/Au 2 nm/ZnO 50 nm ZnO 47 nm /Au 3 nm/ZnO 50 nm
3.90 eV 4.81 eV 5.13 eV 4.89 eV
[7] This study This study This study
These results indicate that ZAZ films with a 2 nm thick Au interlayer are an alternative for use as a transparent conducting anode in OLEDs and various flat panel displays.
References [1] [2] [3] [4] [5] [6] [7] [8]
G. Fang, D. Li, B. Yao, Vacuum 68 (2002) 363. L. Gong, J. Lu, Z. Ye, Solar Energy Materials and Solar Cells 94 (2010) 1282. S. Suzuki, T. Miyata, M. Ishii, T. Minami, Thin Solid Films 434 (2003) 14. G. Haacke, Journal of Applied Physics 47 (1976) 4086. Y.S. Kim, J.H. Park, Daeil Kim, Vacuum 82 (2008) 574. Daeil Kim, Journal of Alloys and Compounds 493 (2010) 208. J.H. Park, J.H. Chae, D. Kim, Journal of Alloys and Compounds 478 (2009) 330. Y.S. Kim, Y.J. Lee, S.B. Heo, H.M. Lee, J.H. Kim, S.K. Kim, J.H. Chae, J.I. Choi, Daeil Kim, Optics Communication 284 (2011) 2303. [9] Daeil Kim, Applied Surface Science 257 (2010) 704.