Electrical and optical properties of amorphous indium zinc oxide films
Introduction
Transparent conductive oxide (TCO) films have been widely used in the field of optoelectronic devices, such as transparent electrodes for various kinds of flat panel displays (FPDs) or photovoltaic devices [1], [2], [3]. With recent demands for the large-area and high-quality FPDs, amorphous transparent conductive films including amorphous Sn-doped In2O3 (ITO) films have attracted significant attention to their very flat surface, low internal stress and good etchability for micro-patterning. However, crystallinity of sputter deposited ITO films are reported to depend heavily on total gas pressure during the deposition and fully amorphous films could be deposited only at rather high total gas pressure of 3–5 Pa, when the substrate temperature (Ts) was around RT [4], [5]. Inoue et al. reported that IZO films (In2O3/ZnO = 89.3:10.7 by wt.%) with entirely amorphous structure could be obtained with high reproducibility under the wide range of the deposition conditions such as total gas pressure or substrate temperature up to 300 °C, where the resistivity of the films was about 4–5 × 10− 4 Ω cm [6], [7]. These amorphous IZO films were reported to have the smaller compressive stress and much smoother surfaces than the polycrystalline ITO films deposited under the same sputtering conditions [8]. Wet etching properties of the amorphous IZO films with optimized electrical properties were also reported to be superior to those of the polycrystalline ITO films since they could be etched easily with weak acid solution such as oxalic acid and formatted to the taper-shaped fine pitch pattern [7]. Therefore, the IZO is expected to be a new candidate for transparent electrode especially for thin film transistor (TFT)-LCDs or organic ELDs.
In this study the IZO films were deposited by dc magnetron sputtering which should be one of the most promising deposition techniques for the commercial uniform coatings in large area with high packing density and strong adhesion. The valence electron control in IZO was attempted and electron transport mechanisms were investigated.
Section snippets
Experimental details
The films were deposited on unheated non-alkali glass (AN100, Asahi Glass) or Si wafer substrates by dc magnetron sputtering using the oxide ceramic In2O3–ZnO target (89.3 wt.% In2O3 and 10.7 wt.% ZnO, Idemitsu Kosan Co., Ltd.). Substrate temperature during the deposition was confirmed to be lower than 50 °C by the thermo-label. Total gas pressure (Ptot) of Ar or Ar + H2 was kept at 0.5 Pa for all the depositions. Water partial pressure of the residual gas was maintained to be less than 8 × 10− 4
Results and discussion
Fig. 1 shows XRD patterns of the IZO films deposited under 100% Ar gas without substrate heating and post-annealed in air at 100–500 °C for 1 h. The XRD profiles of the as-deposited film and post-annealed films at l00–400 °C showed halo pattern at around 2θ = 30–35°, implying that all these films were entirely amorphous. The film post-annealed at 500 °C showed In2O3 polycrystalline structure, indicating that crystallization temperature of the IZO films was between 400 and 500 °C in air. The
Conclusion
Amorphous IZO films were deposited on unheated substrates by dc magnetron sputtering using the oxide ceramic In2O3–ZnO target. Valence electron control on the IZO films was investigated in order to increase carrier density. The carrier density was successfully increased from 3.08 × 1020 to 7.65 × 1020 cm− 3 by introducing H2 gas (H2/(Ar + H2) = 30%) into the sputter deposition process. The IZO film with the lowest resistivity of 3.39 × 10− 4 Ω cm was obtained for H2 gas flow ratio of 10%. Decrease in the
Acknowledgements
This work was partially supported by a Grant-in-Aid for the 21st COE Program from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government.
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