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N IM B
Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B xxx (2005) xxx­xxx www.elsevier.com/locate/nimb

ITO surface smoothing with argon cluster ion beam
C. Heck
a

a,* ,

T. Seki b, T. Oosawa c, M. Chikamatsu c, N. Tanigaki a, T. Hiraga a, J. Matsuo b

c

Laboratory of Purified Materials, National Institute for Advanced Industrial Science and Technology, AIST Kansai, Photonics Research Institute, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan b Kyoto University, Faculty of Engineering, Quantum Science and Engineering Center, Sakyo, Kyoto 606-8501, Japan National Institute for Advanced Industrial Science and Technology, AIST Tsukuba, Photonics Research Institute, Tsukuba, Ibaraki 305-8565, Japan Available online

Abstract Argon gas cluster ion beam (GCIB) was employed to improve the surface smoothness of indium tin oxide (ITO) substrates. The dependence of the smoothness as well as of the sputtered depth on cluster ion energy and dose has been studied. Results show that relatively low-energy (10 keV) clusters with mean size of 2000 Ar atoms are sufficient to reduce the mean roughness of the ITO surface to about one fourth of its initial value. Organic electroluminescent (EL) devices (a-NPD/Alq3/Mg­Ag) were formed on the smoothed surfaces and a considerable improve of the devicesó EL efficiency was observed. ñ 2005 Elsevier B.V. All rights reserved.
PACS: 36.40.þc; 78.60.Fi Keywords: Gas cluster ion beam; Roughness; ITO; Electroluminescent devices

1. Introduction Indium tin oxide (ITO) films are transparent conductors that have been very much researched in the past decades for application as substrates of liquid crystal displays (LCD). Recently, with the demand increase for application of ITO in the electroluminescent (EL) device industry, the characteristics of these substrates have been revised. For example, while for display applications there is no special requirement for ITO with smooth surface, this is not the case for EL device applications [1,2]. First, in EL devices current injection from ITO into the active device layers is of crucial importance, and second, EL devices are composed of much thinner layers, which causes them to be much more susceptible to failure caused by ITO-substrate/device interface imperfections. In the case of com-

mercially available ITO substrates the surface hillocks can be of the order of the coating layer thickness, compromising the function of the devices. Therefore, there is a large demand for smoothed ITO surfaces to improve life expectancy of EL devices, especially in the field of organic EL devices. Gas cluster ion beam (GCIB) technology has been used for improving surface smoothness of various kinds of materials, including very hard materials like SiC and diamond [3­5]. This technology is also suitable for improving the smoothness of ITO films [6]. In this study, ITO films have been sputtered with argon GCIB and the influence of the cluster ionsó energy and dose on the surface smoothness has been analyzed. The effect of the smoothing process on the properties of simple EL devices is also discussed. 2. Experimental procedure

*

Corresponding author. Tel.: +81 72 751 4263; fax: +81 72 751 9637. E-mail address: heck.claire@aist.go.jp (C. Heck).

Argon GCIB was produced by adiabatic expansion, through a Laval nozzle, into a vacuum chamber. The

0168-583X/$ - see front matter ñ 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.062


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clusters were then ionized by electron bombardment and size-selected with a retarding technique. The average number of argon particles per cluster was about 2000 (Ar2000). More details about the GCIB equipment are described elsewhere [7]. Commercially available indium tin oxide (ITO) surfaces where sputtered with Ar2000 cluster ions of energies between 5 and 30 keV and dose of 5 · 1015 ions/cm2. In another series of experiments energy was fixed at 20 or 30 keV and the dose was varied from 1 · 1015 to 5 · 1015 ions/cm2. Average surface roughness was determined by dynamic force microscopy (DFM) with scanned area and resolution of 1 · 1 lm2 and 512 · 512 pixels, respectively. The work function of the sputtered samples was determined by means of X-ray photoemission spectroscopy (XPS) and sputtered depth was measured with a commercially available surface profiler. Surface resistivity was measured by a conventional 4-point probe method. Organic EL devices were deposited on the ITO substrates before and after smoothing with 20 keV, 5 · 1015 ions/cm2 Ar2000 clusters. Device structure was composed of [a-NPD (60 nm)]/[Alq3 (60 nm)]/[Mg10­Ag1 (120 nm)], where a-NPD is the hole transport and emitting layer, Alq3 is the electron transport and hole blocking layer and Mg­Ag is the cathode. The films were deposited on the ITO substrates at room temperature under a vacuum better than 2.2 · 10þ4 Pa. 3. Results and discussion Fig. 1 shows the average roughness of the ITO substrates after being sputtered with Ar clusters with different energies (Fig. 1) and doses (Fig. 2). As can be seen in Fig. 1, there is a sharp roughness decrease, from its initial value of about 5 nm, to about 1 nm, after sputtering the surface with 10 keV, 5 · 1015 ions/cm2 Ar2000 clusters. Roughness

Fig. 2. Relation between Ar2000 cluster ion beam dose and ITO average roughness.

Fig. 1. Relation between Ar2000 cluster ion beam energy and ITO average roughness.

is further reduced by increasing clusterós energy up to 30 keV. Also the increase of dose, with energy fixed at 20 keV and 30 keV, causes a sharp roughness decrease around 1 · 1015 ions/cm2, as can be seen in Fig. 2. Similar trends were also observed (not shown) for the values of mean square (RMS) and peak-to-valley (P­V) roughness. The sputtered depth of ITO increased with increasing sputtering energy or dose of Ar2000 clusters. As an example, when the sputtering energy was 20 keV and the dose was 5 · 1015 ions/cm2, the sputtered depth was about 20 nm. The influence of the smoothing process on the surface properties of the ITO surfaces was studied by comparing work function and surface resistivity before and after sputtering with Ar2000 clusters. As a result of sputtering by GCIB, the work function increased from 4.99 eV to 5.15 eV, and surface resistivity increased about 15% with the smoothing process. A simple-structure device (a-NPD/Alq3/Mg­Ag) was deposited on these ITO substrates and measurements of light emission and EL properties were performed for devices produced on the as-received and smoothed substrates. Light emission spectra were similar before and after smoothing, i.e. both spectra had a peak around 526 nm. On the other hand, the EL efficiency increased more than 30% with the smoothing process. One of the possible explanations for this increase is suggested by considering that the sputtering process induces changes on the surface of ITO in such a way as to hinder the holes injection process, i.e. reducing the number of holes released from the ITO surface into the holes transport layer (a-NPD). The reason for this assumption is explained in the following. In this device the electron transport process through the Alq3 layer is less efficient than the holes transported process through the a-NPD layer, leading to an overflow of holes at the light emitting interface. In such a case, the reduction of the number of holes released from the ITO substrate improves the


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electron­hole balance at the a-NPD/Alq3 interface, and consequently its EL efficiency. This idea is not directly supported by the results of work function measurements, which show an increase after smoothing, suggesting that the number of released holes increases after Ar2000 cluster sputtering. On the other hand, the increase of surface resistivity by about 15% after the smoothing process supports our assumption. Considering that the ITO surface is a complex system and small variations of its physical and chemical properties at the surface strongly affect its EL properties [1,2,8], more studies have to be performed to confirm our assumption and to further understand the influence of the smoothing process on the performance of EL devices. 4. Conclusions The surfaces of ITO films were smoothed with Ar2000 cluster ion beams with various energies and doses. Results show a sharp decrease of the mean roughness value after sputtering with 10 keV and 5 · 1015 ions/cm2. Besides of that, there is about 30% increase in EL efficiency of a simple device formed on ITO substrate smoothed by Ar2000 clusters of 20 keV, 5 · 1015 ions/cm2.

Acknowledgements The authors would like to thank Dr. T. Taima from AIST Tsukuba for his kind help with the fabrication of the EL devices and with the XPS measurements, and to Dr. K. Murai from AIST Kansai for his help with the surface resistivity measurements. References
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