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High work function of Al-doped zinc-oxide thin films as transparent conductive anodes in organic light-emitting devices
Applied Surface Science 253 (2006) 1917–1920 www.elsevier.com/locate/apsusc
High work function of Al-doped zinc-oxide thin films as transparent conductive anodes in organic light-emitting devices T.W. Kim a,*, D.C. Choo a, Y.S. No a, W.K. Choi b, E.H. Choi c a
Research Institute of Information Display, Division of Electronics and Computer Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea b Thin Film Material Research Center, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Seongbuk-gu, Seoul 136-701, Republic of Korea c Charged Particle Beam and Plasma Laboratory/PDP Research Center, Department of Electrophysics, Kwangwoon University, Seoul 139-701, Republic of Korea Received 15 November 2005; received in revised form 27 January 2006; accepted 15 March 2006 Available online 5 May 2006
Abstract Deposition of Al-doped ZnO (AZO) films with various film thicknesses on glass substrates was performed to investigate the feasibility of using AZO films as anode electrodes in organic light-emitting devices (OLEDs). The electrical resistivity of the AZO films with a 180-nm thickness was 4.085 10 2 V cm, and the average optical transmittance in the visible range was 80.2%. The surface work function for the AZO films, determined from the secondary electron emission coefficients obtained with a focused ion beam, was as high as 4.62 eV. These results indicate that AZO films grown on glass substrates hold promise for potential applications as anode electrodes in high-efficiency OLEDs. # 2006 Elsevier B.V. All rights reserved. PACS: 68. 55.jk; 72; 15.Eb; 73. 20.At Keywords: Al-doped ZnO; Resistivity; Transmittance; Work function; Organic light-emitting devices
1. Introduction Potential applications of organic light-emitting devices (OLEDs) have driven extensive efforts to fabricate various kinds of OLEDs with high brightness and high efficiency [1–5]. Various device structures, consisting of active layers and electrodes, have been designed to improve the efficiency of the OLEDs [6,7]. Among the several parts of OLEDs, transparent conducting oxide films have become particularly attractive due to their being promising candidates for anodes [8]. The materials of the device anodes for hole injection are particularly important for enhancing device efficiency. tin-doped indium oxides (ITOs) acting as anodes in OLEDs have been extensively used as anodes because of their high conductivity and transparency over the visible range and their high work
* Corresponding author. Tel.: +82 2 2220 0354, fax: +82 2 2292 4135. E-mail address: [email protected] (T.W. Kim). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.03.032
function. However, since the indium in the ITO thin films might diffuse into the organic layers, resulting in the degradation of the OLED efficiencies [9–11], the fabrications of the alternative anodes with thermal stabilities are necessary for improving the efficiency of OLEDs. Among the alternative candidate thin films, Al-doped ZnO (AZO) thin films have been considered as suitable anodes because ZnO thin films are more stable in reducing ambient, more abundant, and less expensive in comparison with the ITO films which make them appropriate for potential use as anodes in OLEDs [12–16]. Since ZnO thin films are large bandgap oxide semiconductors with a large excitonic binding energy and a high chemical stabilization [17,18], their noble physical properties have stimulated applications in many promising optoelectronic devices, such as flat-panel displays. Since the utilization possibility of AZO thin films grown on glass substrates as anodes in OLEDs is strongly affected by the electrical, the optical, and the electronic properties, systematic studies concerning those properties are very important for
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T.W. Kim et al. / Applied Surface Science 253 (2006) 1917–1920
improving the efficiencies of OLEDs. Even though some works on the electrical, the optical, and the electronic properties of the AZO thin films have been reported [13,19,20], systematic studies concerning the work functions with high magnitudes of the AZO thin films are still necessary for enhancement of the hole-injection efficiency in OLEDs. The paper reports on the electrical, the optical, and the electronic properties of AZO thin films with various film thicknesses acting as anodes in OLEDs. The AZO thin films were grown on glass substrates by using a reactive radiofrequency sputtering system. The electrical, the optical, and the electronic properties of films were measured to investigate AZO thin films as promising candidates for use as anodes in OLEDs. 2. Experimental details Atomic force microscopy (AFM) measurements were carried out in order to characterize the surface smoothness of the AZO layer, and Van der Pauw Hall effect measurements were performed in order to investigate the electronic parameters [21]. Transmittance measurements were performed in order to investigate the optical properties of the AZO layer, and the secondary electron emission coefficient (g)-focused ion beam (FIB) measurements were performed to determine the g values and the work functions of the AZO thin films. The AZO films were grown on glass substrates at room temperature by using a reactive radio-frequency magnetron sputtering system. A target consisting of ZnO containing 5 wt.% Al was used. The thicknesses of the AZO thin films were 180, 220, and 600 nm, respectively. The film resistivity was determined from the sheet resistance measured by using the four-point probe technique. The Hall mobility and the carrier density measurements were made using the Van der Pauw method at 300 K. The optical transmittance measurements were made using a LAMDA 19 spectrophotometer (300– 700 nm). The g-FIB measurements were performed by using a home made equipment for determining the secondary electron emission coefficients (g) and work functions of the AZO thin films [22]. 3. Results and discussion The as-grown AZO thin films had mirror-like surfaces without any indication of pinholes, which was confirmed by using Normarski optical microscopy and scanning electron microscopy. AFM images of AZO thin films with thicknesses of 180, 220, and 600 nm grown on glass substrates are shown in Fig. 1(a–c), respectively. The root mean squares of the surface roughnesses of the AZO thin films with thicknesses of 180, 220, and 600 nm, as determined from the AFM measurements, were 4.0, 2.8, and 10.5 nm, respectively. The images in Fig. 1 show that the surfaces of all the AZO thin films grown on glass substrates are smooth. Therefore, when the organic layer is grown on the AZO thin films with thicknesses of 180 and 220 nm, the AZO anode/organic heterostructure has an excellent heterointerface, resulting in high-efficiency OLEDs.
Fig. 1. Atomic force microscopy images of AZO films containing 5 wt.% of Al with film thicknesses of (a) 180, (b) 220, and (c) 600 nm.
The resistivities, the Hall mobilities, the carrier types, and the carrier concentrations for AZO films with thicknesses of 180, 220, and 600 nm, as determined from the Hall effect measurements at room temperature, are summarized in Table 1. The carriers for all of the AZO thin film were n-type. While the resistivity of the AZO thin films slightly decreased with increasing film thickness, their mobility significantly increased with increasing thickness. A relative higher resistivity might be caused by an increased in corporation of suboxides of Al–O, either in Al2O3 clusters or some intermediate materials [20]. The increase in the mobility of the AZO thin films with increasing thickness might be attributed to an increase of the grain size along the growth direction for thicker films [20]. Also, the significant higher resistivity values of the deposited Table 1 Resistivities, Hall mobilities, and carrier concentrations of AZO films with thicknesses of 180, 220, and 600 nm, determined from Hall effect measurements at 300 K Film thickness Resistivity (V cm) Hall mobility (cm2/Vs) Carrier concentration (cm 3)
180 nm
220 nm 2
4.085 10 6.507 10 1 2.348 10 20
600 nm 2
2.611 10 1.637 1.460 1019
1.215 10 2 1.605 102 3.201 1018
T.W. Kim et al. / Applied Surface Science 253 (2006) 1917–1920
1919
Fig. 2. Transmittance spectra for the AZO films containing 5 wt.% of Al with film thicknesses of (a) 180, (b) 220, and (c) 600 nm.
AZO films in comparison with those of the AZO films reported in elsewhere might be related to a higher Al concentration of the target used in this work [20]. The resistivities of the unintentionally doped ZnO thin films were very high as with insulator films. Fig. 2 shows the optical transmittance for AZO films grown on glass substrates. The average optical transmittances in the visible ranges between 400 and 700 nm for the samples with thicknesses of 180, 220, and 600 nm were 80.2 0.02, 82.7 0.03, and 74.4 0.04%, respectively, which were relatively high. Because the surface roughness of the AZO film with a thickness of 180 nm was slightly larger than that with a thickness of 220 nm, the average optical transmittance for the AZO film with a thickness of 180 nm was smaller in comparison with that a thickness of 220 nm. The optical transmittance for the AZO films is significantly affected by the film thickness together with the resistivity, the Hall mobility, and the carrier density of the films. The moderate film thickness and carrier concentration of the AZO thin films are necessary for improving their optical transmittance. In order to determine the g values and the work functions of the AZO films, the g values were measured as functions of the acceleration voltages for He, Ne, Ar, and Xe ion sources. The dependences of the values on the acceleration voltages of the FIB were obtained for various sources to determine the work functions of the AZO thin films. When the applied voltage of the collector is negative, the number of secondary electrons emitted is reduced. However, when the applied voltage of the collector is positive, secondary electrons are emitted, and the value of the current appearing at the electrometer is the sum of the incident ion and the secondary electron currents. The g value is the magnitude ratio between the ion current and emitted secondary ion current, and g values were obtained at different acceleration voltages for He, Ne, Ar, and Xe ion sources. While the g values depend on the ion source [23], their shapes show similar behaviors independent of the ion source. Fig. 3 shows
Fig. 3. Secondary electron emission coefficients of the AZO thin films with thicknesses of (a) 180, (b) 220, and (c) 600 nm as functions of the acceleration voltage for He, Ne, Ar, and Xe ions.
the g values of the AZO thin films with thicknesses of (a) 180, (b) 220, and (c) 600 nm as functions of the acceleration voltage for He, Ne, Ar, and Xe ions. The different work functions dependent on the various thicknesses might be attributed to the uniformity of the distribution of the Al atoms in the ZnO host lattices. Fig. 3 shows the g values of the AZO thin film with a thickness of 180 nm as functions of the acceleration voltage for He, Ne, Ar, and Xe ions.
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80.2%, and 4.62 eV, respectively. Even though more systematic investigations on the physical properties of the AZO thin films dependent on deposition parameters and film thicknesses are required, these results indicate that AZO thin films grown on glass substrates hold promise for potential applications as anodes for OLEDs. Acknowledgement This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF2004-005-D00166). References
Fig. 4. Secondary electron emission coefficients of the AZO films with a film thickness of 180 nm as a function of the ionization energy.
The g values determined from the data in Fig. 3 are shown in Fig. 4 as a function of the ionization energy. The work functions of the AZO thin films can be determined from the dependences of the g values on the type of incident ion and on the acceleration voltage by using Auger neutralization theory [24]. The work function of the AZO films with a thickness of 180 nm, as determined from the half value of the intercept of the straight line in Fig. 4, is approximately 4.62 eV. The work function of the AZO thin film with a thickness of 180 nm, which is comparable with that of the ITO film [25–27], is higher than that of the AZO films, determined from the ultraviolet photoelectron spectroscopy measurements, reported in the other literature [13], indicative of the high efficiency of the hole injection at anode/organic heterointerface. An enhancement of the work function for the AZO thin film might originate from a higher Al content [13], which uniformly distribute into the interstitial sites around the ZnO lattice. This result indicates that the AZO film with a thickness of 180 nm is an alternative candidate for use as an anode in a high-efficiency OLED. 4. Summary and conclusions The electrical, the optical, and the electronic properties of AZO thin films with various film thicknesses grown on glass substrates by using a radio-frequency magnetron sputtering technique were investigated. The optimum thickness of an AZO thin film for use as an anode in an OLED, based on the value of the work function, was approximately 180 nm for the film investigated in this research, and the resistivity, the average optical transmittance, and the work function of the AZO thin film with an optimal thickness of 180 nm were 4.085 10 2 V cm,
[1] J.H. Burroughes, D.D.C. Bradley, A. Brown, R.N. Mackley, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 (1990) 539. [2] J.R. Sheats, H. Antoniadis, M. Hueschen, W. Leonard, J. Miller, R. Moon, D. Roitman, A. Stocking, Science 273 (1996) 884. [3] H. Aziz, Z.D. Popovic, N.-X. Hu, A.-M. Hor, G. Xu, Science 283 (1999) 1900. [4] R.H. Young, C.W. Tang, A.P. Marchetti, Appl. Phys. Lett. 80 (2002) 874. [5] S.R. Forrest, D.D.C. Bradley, M.E. Thompson, Adv. Mater. 15 (2003) 1043. [6] Z.Q. Gao, C.S. Lee, I. Bello, S.T. Lee, R.M. Chen, T.Y. Luh, J. Shi, C.W. Tang, Appl. Phys. Lett. 74 (1999) 865. [7] T. Osada, Th. Kuger, P. Broms, W.R. Salaneck, Synth. Met. 96 (1998) 77. [8] H.T. Cao, C. Sun, Z.L. Pei, A.Y. Wang, L.S. Wen, J. Mater. Sci. Electron. 15 (2004) 169. [9] A.R. Schlatmann, D.W. Floet, A. Hilberer, F. Garten, P.J.M. Smulders, T.M. Klapwijk, G. Hadziioannou, Appl. Phys. Lett. 69 (1996) 1764. [10] C.L. Chao, K.R. Chuang, S.A. Chen, Appl. Phys. Lett. 69 (1996) 2894. [11] S.T. Lee, Z.Q. Gao, L.S. Hung, Appl. Phys. Lett. 75 (1999) 1404. [12] H. Kim, J.S. Horwitz, W.H. Kim, S.B. Qadri, Z.H. Kafafi, Appl. Phys. Lett. 83 (2003) 3809. [13] X. Jiang, F.L. Wong, M.K. Fung, S.T. Lee, Appl. Phys. Lett. 83 (2003) 1875. [14] J. Zhao, S. Xie, S. Han, Z. Yang, L. Ye, T. Yang, Synth. Met. 114 (2000) 251. [15] T. Minami, S. Suzuki, T. Miyata, Thin Solid Films 398–399 (2001) 53. [16] H. Kim, J.S. Horwitz, W.H. Kim, A.J. Makinen, Z.H. Kafafi, D.B. Chrisey, Thin Solid Films 420–421 (2002) 539. [17] R.F. Service, Science 276 (1997) 895. [18] T. Soki, Y. Hatanaka, D.C. Look, Appl. Phys. Lett. 76 (2000) 3257. [19] H. Kim, C.M. Gilmore, J.S. Horwitz, A. Pique´, H. Murata, G.P. Kushto, R. Schlaf, Z.H. Kafafi, D.B. Chrisey, Appl. Phys. Lett. 76 (2000) 259. [20] C. Agashe, O. Kluth, J. Hu¨pkes, U. Zastrow, B. Rech, M. Wuttig, J. Appl. Phys. 95 (2004) 1911. [21] K. Seeger, Semiconductor Physics, Springer-Verlag, New York, 1973. [22] E.H. Choi, H.J. Oh, Y.G. Kim, J.J. Ko, J.Y. Lim, J.G. Kim, D.I. Kim, G.S. Cho, S.O. Kang, Jpn. J. Appl. Phys. 37 (1998) 7015. [23] H.D. Hagstrum, Phys. Rev. 96 (1954) 336. [24] M.P. Seah, Vacuum 34 (1984) 453. [25] M.A. Baldo, D.F. O’Brien, M.E. Thompson, S.R. Forrest, Phys. Rev. B 60 (1999) 14422. [26] H. Kim, A. Pique´, J.S. Horwitz, H. Mattoussi, H. Murata, Z.H. Kafafi, D.B. Chrisey, Appl. Phys. Lett. 74 (1999) 3444. [27] G. Li, J. Shinar, Appl. Phys. Lett. 83 (2003) 5359.
High work function of Al-doped zinc-oxide thin films as transparent conductive anodes in organic light-emitting devices T.W. Kim a,*, D.C. Choo a, Y.S. No a, W.K. Choi b, E.H. Choi c a
Research Institute of Information Display, Division of Electronics and Computer Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea b Thin Film Material Research Center, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Seongbuk-gu, Seoul 136-701, Republic of Korea c Charged Particle Beam and Plasma Laboratory/PDP Research Center, Department of Electrophysics, Kwangwoon University, Seoul 139-701, Republic of Korea Received 15 November 2005; received in revised form 27 January 2006; accepted 15 March 2006 Available online 5 May 2006
Abstract Deposition of Al-doped ZnO (AZO) films with various film thicknesses on glass substrates was performed to investigate the feasibility of using AZO films as anode electrodes in organic light-emitting devices (OLEDs). The electrical resistivity of the AZO films with a 180-nm thickness was 4.085 10 2 V cm, and the average optical transmittance in the visible range was 80.2%. The surface work function for the AZO films, determined from the secondary electron emission coefficients obtained with a focused ion beam, was as high as 4.62 eV. These results indicate that AZO films grown on glass substrates hold promise for potential applications as anode electrodes in high-efficiency OLEDs. # 2006 Elsevier B.V. All rights reserved. PACS: 68. 55.jk; 72; 15.Eb; 73. 20.At Keywords: Al-doped ZnO; Resistivity; Transmittance; Work function; Organic light-emitting devices
1. Introduction Potential applications of organic light-emitting devices (OLEDs) have driven extensive efforts to fabricate various kinds of OLEDs with high brightness and high efficiency [1–5]. Various device structures, consisting of active layers and electrodes, have been designed to improve the efficiency of the OLEDs [6,7]. Among the several parts of OLEDs, transparent conducting oxide films have become particularly attractive due to their being promising candidates for anodes [8]. The materials of the device anodes for hole injection are particularly important for enhancing device efficiency. tin-doped indium oxides (ITOs) acting as anodes in OLEDs have been extensively used as anodes because of their high conductivity and transparency over the visible range and their high work
* Corresponding author. Tel.: +82 2 2220 0354, fax: +82 2 2292 4135. E-mail address: [email protected] (T.W. Kim). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.03.032
function. However, since the indium in the ITO thin films might diffuse into the organic layers, resulting in the degradation of the OLED efficiencies [9–11], the fabrications of the alternative anodes with thermal stabilities are necessary for improving the efficiency of OLEDs. Among the alternative candidate thin films, Al-doped ZnO (AZO) thin films have been considered as suitable anodes because ZnO thin films are more stable in reducing ambient, more abundant, and less expensive in comparison with the ITO films which make them appropriate for potential use as anodes in OLEDs [12–16]. Since ZnO thin films are large bandgap oxide semiconductors with a large excitonic binding energy and a high chemical stabilization [17,18], their noble physical properties have stimulated applications in many promising optoelectronic devices, such as flat-panel displays. Since the utilization possibility of AZO thin films grown on glass substrates as anodes in OLEDs is strongly affected by the electrical, the optical, and the electronic properties, systematic studies concerning those properties are very important for
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T.W. Kim et al. / Applied Surface Science 253 (2006) 1917–1920
improving the efficiencies of OLEDs. Even though some works on the electrical, the optical, and the electronic properties of the AZO thin films have been reported [13,19,20], systematic studies concerning the work functions with high magnitudes of the AZO thin films are still necessary for enhancement of the hole-injection efficiency in OLEDs. The paper reports on the electrical, the optical, and the electronic properties of AZO thin films with various film thicknesses acting as anodes in OLEDs. The AZO thin films were grown on glass substrates by using a reactive radiofrequency sputtering system. The electrical, the optical, and the electronic properties of films were measured to investigate AZO thin films as promising candidates for use as anodes in OLEDs. 2. Experimental details Atomic force microscopy (AFM) measurements were carried out in order to characterize the surface smoothness of the AZO layer, and Van der Pauw Hall effect measurements were performed in order to investigate the electronic parameters [21]. Transmittance measurements were performed in order to investigate the optical properties of the AZO layer, and the secondary electron emission coefficient (g)-focused ion beam (FIB) measurements were performed to determine the g values and the work functions of the AZO thin films. The AZO films were grown on glass substrates at room temperature by using a reactive radio-frequency magnetron sputtering system. A target consisting of ZnO containing 5 wt.% Al was used. The thicknesses of the AZO thin films were 180, 220, and 600 nm, respectively. The film resistivity was determined from the sheet resistance measured by using the four-point probe technique. The Hall mobility and the carrier density measurements were made using the Van der Pauw method at 300 K. The optical transmittance measurements were made using a LAMDA 19 spectrophotometer (300– 700 nm). The g-FIB measurements were performed by using a home made equipment for determining the secondary electron emission coefficients (g) and work functions of the AZO thin films [22]. 3. Results and discussion The as-grown AZO thin films had mirror-like surfaces without any indication of pinholes, which was confirmed by using Normarski optical microscopy and scanning electron microscopy. AFM images of AZO thin films with thicknesses of 180, 220, and 600 nm grown on glass substrates are shown in Fig. 1(a–c), respectively. The root mean squares of the surface roughnesses of the AZO thin films with thicknesses of 180, 220, and 600 nm, as determined from the AFM measurements, were 4.0, 2.8, and 10.5 nm, respectively. The images in Fig. 1 show that the surfaces of all the AZO thin films grown on glass substrates are smooth. Therefore, when the organic layer is grown on the AZO thin films with thicknesses of 180 and 220 nm, the AZO anode/organic heterostructure has an excellent heterointerface, resulting in high-efficiency OLEDs.
Fig. 1. Atomic force microscopy images of AZO films containing 5 wt.% of Al with film thicknesses of (a) 180, (b) 220, and (c) 600 nm.
The resistivities, the Hall mobilities, the carrier types, and the carrier concentrations for AZO films with thicknesses of 180, 220, and 600 nm, as determined from the Hall effect measurements at room temperature, are summarized in Table 1. The carriers for all of the AZO thin film were n-type. While the resistivity of the AZO thin films slightly decreased with increasing film thickness, their mobility significantly increased with increasing thickness. A relative higher resistivity might be caused by an increased in corporation of suboxides of Al–O, either in Al2O3 clusters or some intermediate materials [20]. The increase in the mobility of the AZO thin films with increasing thickness might be attributed to an increase of the grain size along the growth direction for thicker films [20]. Also, the significant higher resistivity values of the deposited Table 1 Resistivities, Hall mobilities, and carrier concentrations of AZO films with thicknesses of 180, 220, and 600 nm, determined from Hall effect measurements at 300 K Film thickness Resistivity (V cm) Hall mobility (cm2/Vs) Carrier concentration (cm 3)
180 nm
220 nm 2
4.085 10 6.507 10 1 2.348 10 20
600 nm 2
2.611 10 1.637 1.460 1019
1.215 10 2 1.605 102 3.201 1018
T.W. Kim et al. / Applied Surface Science 253 (2006) 1917–1920
1919
Fig. 2. Transmittance spectra for the AZO films containing 5 wt.% of Al with film thicknesses of (a) 180, (b) 220, and (c) 600 nm.
AZO films in comparison with those of the AZO films reported in elsewhere might be related to a higher Al concentration of the target used in this work [20]. The resistivities of the unintentionally doped ZnO thin films were very high as with insulator films. Fig. 2 shows the optical transmittance for AZO films grown on glass substrates. The average optical transmittances in the visible ranges between 400 and 700 nm for the samples with thicknesses of 180, 220, and 600 nm were 80.2 0.02, 82.7 0.03, and 74.4 0.04%, respectively, which were relatively high. Because the surface roughness of the AZO film with a thickness of 180 nm was slightly larger than that with a thickness of 220 nm, the average optical transmittance for the AZO film with a thickness of 180 nm was smaller in comparison with that a thickness of 220 nm. The optical transmittance for the AZO films is significantly affected by the film thickness together with the resistivity, the Hall mobility, and the carrier density of the films. The moderate film thickness and carrier concentration of the AZO thin films are necessary for improving their optical transmittance. In order to determine the g values and the work functions of the AZO films, the g values were measured as functions of the acceleration voltages for He, Ne, Ar, and Xe ion sources. The dependences of the values on the acceleration voltages of the FIB were obtained for various sources to determine the work functions of the AZO thin films. When the applied voltage of the collector is negative, the number of secondary electrons emitted is reduced. However, when the applied voltage of the collector is positive, secondary electrons are emitted, and the value of the current appearing at the electrometer is the sum of the incident ion and the secondary electron currents. The g value is the magnitude ratio between the ion current and emitted secondary ion current, and g values were obtained at different acceleration voltages for He, Ne, Ar, and Xe ion sources. While the g values depend on the ion source [23], their shapes show similar behaviors independent of the ion source. Fig. 3 shows
Fig. 3. Secondary electron emission coefficients of the AZO thin films with thicknesses of (a) 180, (b) 220, and (c) 600 nm as functions of the acceleration voltage for He, Ne, Ar, and Xe ions.
the g values of the AZO thin films with thicknesses of (a) 180, (b) 220, and (c) 600 nm as functions of the acceleration voltage for He, Ne, Ar, and Xe ions. The different work functions dependent on the various thicknesses might be attributed to the uniformity of the distribution of the Al atoms in the ZnO host lattices. Fig. 3 shows the g values of the AZO thin film with a thickness of 180 nm as functions of the acceleration voltage for He, Ne, Ar, and Xe ions.
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T.W. Kim et al. / Applied Surface Science 253 (2006) 1917–1920
80.2%, and 4.62 eV, respectively. Even though more systematic investigations on the physical properties of the AZO thin films dependent on deposition parameters and film thicknesses are required, these results indicate that AZO thin films grown on glass substrates hold promise for potential applications as anodes for OLEDs. Acknowledgement This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF2004-005-D00166). References
Fig. 4. Secondary electron emission coefficients of the AZO films with a film thickness of 180 nm as a function of the ionization energy.
The g values determined from the data in Fig. 3 are shown in Fig. 4 as a function of the ionization energy. The work functions of the AZO thin films can be determined from the dependences of the g values on the type of incident ion and on the acceleration voltage by using Auger neutralization theory [24]. The work function of the AZO films with a thickness of 180 nm, as determined from the half value of the intercept of the straight line in Fig. 4, is approximately 4.62 eV. The work function of the AZO thin film with a thickness of 180 nm, which is comparable with that of the ITO film [25–27], is higher than that of the AZO films, determined from the ultraviolet photoelectron spectroscopy measurements, reported in the other literature [13], indicative of the high efficiency of the hole injection at anode/organic heterointerface. An enhancement of the work function for the AZO thin film might originate from a higher Al content [13], which uniformly distribute into the interstitial sites around the ZnO lattice. This result indicates that the AZO film with a thickness of 180 nm is an alternative candidate for use as an anode in a high-efficiency OLED. 4. Summary and conclusions The electrical, the optical, and the electronic properties of AZO thin films with various film thicknesses grown on glass substrates by using a radio-frequency magnetron sputtering technique were investigated. The optimum thickness of an AZO thin film for use as an anode in an OLED, based on the value of the work function, was approximately 180 nm for the film investigated in this research, and the resistivity, the average optical transmittance, and the work function of the AZO thin film with an optimal thickness of 180 nm were 4.085 10 2 V cm,
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