Synchrotron radiation photoelectron spectroscopy study of ITO surface

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Abstract

Synchrotron radiation photoelectron spectroscopy (SRPES) has been applied to surface analysis of indium tin oxide (ITO) thin films. Several different components of In and Sn were observed at the clean ITO surface. By comparing the chemical compositions of the film before and after vacuum annealing, the contents of In2O3−x and Sn3O4 were found to be the major factors influencing the electrical conductivity and optical transparency of the film.

Introduction

Because of its high conductivity and transparency, indium tin oxide (ITO) is widely used in many kinds of optoelectronic devices, including organic light-emitting devices (OLEDs) [1], [2], solar cells [3], [4], and panel liquid-crystal displays. Due to its efficient hole-injection capability, ITO is often used as the anode contact in OLEDs. Particular treatments of the substrates, which modify the surface potential and reduce the potential barrier at the interface, might affect the luminance and lifetime of the devices. Degradation studies and failure analyses of OLEDs have revealed that the operational instability of the devices is mainly caused by damage of ITO/organic-film interface, which results partly from the Joule effect [5], [6]. It has also been found that after some kinds of treatment to the ITO surface, device performance can be enhanced, but how the ITO surface properties affect device performance has yet to be fully understood [7], [8], [9], [10], [11], [12], [13], [14]. The surface compositions and the chemical shifts related to the In and Sn oxides are important parameters in governing device performance. However, it is difficult to accurately distinguish the different chemical states of various In and Sn oxides using conventional X-ray photoelectron spectroscopy (XPS) [15], [16] and Auger electron spectroscopy (AES) [9], [17] due to the small chemical shifts that occur. In contrast, synchrotron radiation photoelectron spectroscopy (SRPES) can provide high surface sensitivity and energy resolution and should be a better method for studying the chemical states of the ITO surface. To the best of our knowledge, there has been no SRPES analysis in the literature concerning the chemical states of the ITO surfaces thus far.

In this work, we have analyzed the chemical states of In and Sn at the ITO surface and found the existence of three chemical states for In and Sn each. The samples were annealed in a vacuum at 450°C, after which In was found to be further oxidized and more Sn3O4 appeared. Accompanying the changes of the components, the resistance and transparency of ITO were also changed after annealing, indicating that the contents of In2O3−x and Sn3O4 were the major factors affecting the ITO surface properties.

Section snippets

Experiments

The samples used in the experiment were commercially available ITO/glass substrates. The thickness and sheet resistance of the ITO layer were about 1 μm and 200 Ω/□, respectively. The samples were ultrasonically cleaned by sequential rinses in acetone, ethanol and de-ionized water (DIW) for 10 min each, followed by drying in flowing nitrogen. Then they were loaded into the ultra-high vacuum system of SRPES.

The SRPES measurements were performed at the National Synchrotron Radiation Laboratory,

Results and discussion

SRPES spectra of In 4d and Sn 4d core levels for the as treated and annealed ITO surfaces are shown in Fig. 1, Fig. 2, respectively. All peaks are curve-fitted with fixed spin-orbit splitting shifts and d5/2/d3/2 branching ratios. They are, respectively, 0.85 eV and 1.5 for In 4d, 1.00 eV and 1.5 for Sn 4d [18]. The binding energies referred to below are the values of the d5/2 levels with respect to the valence-band maximum of the clean ITO surface.

In 4d spectra of the samples as treated,

Conclusion

The study of chemical states of ITO surfaces by SRPES shows that the forms of the In and Sn oxides in an ITO film will complementarily change after vacuum annealing. The increase of ITO resistivity results mainly from further oxidation of In2O3−x and reduction SnO2. The appearance of more Sn3O4 after annealing at 450°C can cause lower transparency of the ITO film.

Acknowledgements

This work was supported by the National Natural Science Foundation of China with Grants No. 69776034 and No.59832100.

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