Formation of ITO Nanowires Using Conventional Magnetron Sputtering

ITO nanowires could be grown in oxygen-free Ar sputtering gas directly after the formation of ITO ﬁlms with thicknesses of 10–50 nm using conventional magnetron sputtering. Growth of nanorods occurred at substrate temperatures of about 100 ◦ C and higher, whereas nanowires with lengths of 1–10 μ m and diameters of roughly 20–200 nm were formed at about 175 ◦ C and above. The diameter, length and density of the nanowires could be controlled by varying the sputtering time, substrate temperature, and SnO 2 content in the ITO sputtering target.

Nanowires (NWs) composed of metal oxides such as indium tin oxide (ITO) and ZnO have attracted significant attention for use in electrodes in electronic devices such as organic solar cells, light-emitting devices and various sensors. 1 ITO NWs have been synthesized using vapor transport and thermal evaporation methods. [1][2][3] However, from the viewpoint of low-cost production and ease of manufacture, it is desirable that such NWs be grown using magnetron sputtering, because this method is widely used to form ITO films for transparent electrodes in optoelectronic devices. ITO NWs have been produced using thermal evaporation at temperatures 2 that are higher than the maximum allowable temperature (ca. 300 • C) in organic electronic device manufacturing processes. There have also been reports of ITO NW formation by a vapor-liquid-solid (VLS) mechanism using gold catalyst particles as seeds. 1 In the present study, it was found that ITO NWs could be formed using conventional magnetron sputtering at low temperatures (ca. 175 • C), without the need for metal catalyst nanoparticles. The technique involves NW growth directly after deposition of an ITO film on a substrate. This is beneficial because it allows ITO NWs to be formed on a range of different substrates.
A conventional planar magnetron sputtering system (USP-66F, Universal Systems Co.), employing DC and RF (13.56 MHz) power was used in the present study. The sputtering targets with diameters of 4 inches were made from a mixture of In 2 O 3 and SnO 2 . The substrates used were 0.7-mm-thick Corning Eagle XG glass with dimensions of 100 × 100 mm. The common sputtering conditions applied in this paper were a DC power of 300 W and a sputtering gas pressure of 0.666 Pa. The sputtering gas was introduced into the chamber under a vacuum of 5 × 10 −5 Pa or less.
The influence of additional sputtering conditions on the growth of ITO NWs was next investigated. The surface morphology was observed using field-emission scanning electron microscopy (FE-SEM; SU8020, Hitachi High-Technologies Co.) Influence of deposition temperature.- Figure 1 shows the change in the surface morphology with substrate temperature for samples deposited in Ar without O 2 addition using a sputtering target with 7.0 wt% SnO 2 . It can be seen that for temperatures of 100 • C, ITO nanorods (NRs) were produced. On the other hand, for temperatures of 175 • C and above, the growth of NWs became apparent, and their density was much higher than that of the NRs. The temperature of 175 • C is close to the melting point of indium (156.6 • C). In addition, the NWs were composed of trunks and ball-shaped heads. These results suggest that the NWs were formed by a VLS mechanism involving indium or indium-tin droplets. * Electrochemical Society Active Member. z E-mail: yamamoto.naoki@kochi-tech.ac.jp Influence of oxygen in sputtering gas.-The addition of a small amount of O 2 (partial pressure of 0.51%) to the Ar base gas was found to have an inhibitory effect on the growth of NRs and NWs at temperatures of about 100 • C or higher. This is despite the fact that oxygen is present in the sputtering target in the form of In 2 O 3 and SnO 2 powder. Figure 2 shows that an O 2 partial pressure of 0.73% completely suppressed the growth of NWs at a substrate temperature of 300 • C using a sputtering target with 7.0 wt% SnO 2 , regardless of the deposition time. Fig. 2 shows the influence of the deposition time on the growth of NRs or NWs in the absence of O 2 . During the initial stages of the sputtering process, up to about 150 s, a thin ITO layer with a thickness of 10-50 nm was formed. Small ITO NRs began to appear sparsely on the ITO layer after about 30 s. This was followed by the growth of NWs, whose density became notably higher for deposition times greater than 300 s. The fact that a thin ITO layer grew during the initial stages of sputtering suggests that ITO NWs can be formed on any type of surface. It has already been confirmed that this is the case for polyethylene naphthalate (PEN) films with a solid or liquefaction structure, Mo films, Mo 2 O 3 films, Al 2 O 3 films and Si wafers. However, due to space limitations, these results will be reported elsewhere.

Influence of deposition time.-The upper row of SEM images in
Influence of SnO 2 content in sputtering target.- Figure 3a compares SEM images of NWs grown using sputtering targets with SnO 2 contents of 5-30 wt%. The substrate temperature was 300 • C, and the deposition time was 600 s. Figures 3b-3e show the diameter, length and density of NWs as a function of the SnO 2 content in the sputtering target. The NW dimensions were determined using the built-in FE-SEM measurement function. Each data point in Fig. 3 is the average of 10-15 nanowires in each sample. The density values in Fig. 3e were determined by counting the number of NWs in a 25 μm 2 region. As seen in Fig. 3b, the head diameter decreased with increasing SnO 2 content, and a similar dependence was found for the trunk diameter, as shown in Fig. 3c. The results shown in Figs. 3b-3d indicate that NWs could be grown using an ITO target with a restricted SnO 2 content range of about 5-30 wt%. As shown in Fig. 3e, the NW density increased with SnO 2 content of up to about 30 wt%. The In, Sn and O content at different points in the NWs was analyzed using energy-dispersive X-ray spectroscopy in the FE-SEM system, and the composition of the head and trunk sections was found to be almost the same.
The sputtering power and gas pressures were also investigated as factors influencing the formation of NRs and NWs. The NRs or NWs were grown by sputtering with the wide ranges of 50 W-600 W power and 0.07 Pa -4.0 Pa Ar pressure applied in this work. The length of    NWs reached to about 10 μm by RF sputtering-processing (300 • C, 150 W, 0.666 Pa, 14400 s deposition time, ITO target with 12.0 wt% SnO 2 ).

X-ray diffraction analysis of ITO and NWs.-
The crystal structure and lattice spacing for the ITO NWs were analyzed using X-ray diffraction (XRD; SmartLab, Rigaku). Figure 4a shows an XRD pattern and an SEM image for a sample deposited at 300 • C using a sputtering target with 12 wt% SnO 2 . It can be seen that the intensity of the (400) peak exceeds that of the (222) peak, which indicates that the crystal grains in the sample are oriented in the 400 direction. This was confirmed by comparison with the (400)/(222) intensity ratio for a standard sample of ITO powder (JPDS file No. 01-089-4598). Figure 4b shows that the diffraction angles for the (222) and (400) peaks decrease slightly with increasing SnO 2 content, which indicates an increase in the lattice spacing. As shown in Fig. 4d, the (400)/(222) intensity ratio increases with SnO 2 content. This increase is similar to that observed for the NW density in Fig. 3. Therefore, it is considered that NW growth proceeds along the ITO 400 direction, normal to the substrate.
More detailed elemental and chemical analyzes using electron energy loss spectroscopy and X-ray photoelectron spectroscopy are currently in process. In addition, a crystal structure analysis is being carried out using electron beam nano-diffraction. The results of these analyzes will be described in an upcoming publication.
In summary, a technique was developed for the formation of ITO NWs using a conventional magnetron sputtering system. The key factor is to deposit ITO in Ar sputtering gas in the absence of O 2 .
ITO NRs were grown at substrate temperatures higher than about 100 • C by successive sputtering after the formation of an ITO thin layer with a thickness of 10-50 nm during the initial deposition stage. The growth of NWs became conspicuous at a substrate temperature of about 175 • C or higher. It was found that the diameter, length and density of the NWs could be controlled by varying the sputtering time, substrate temperature, and SnO 2 content in the ITO target. The successful growth of ITO NRs or NWs required a SnO 2 content of about 5-30 wt%. The NWs consisted of trunks and ball-shaped heads, and the XRD analysis results suggested that they grew in the 400 direction.