Microstructure and optical properties of Ag/ITO/Ag multilayer films
© Sun et al.; licensee Springer. 2013
Received: 18 July 2013
Accepted: 29 September 2013
Published: 17 October 2013
Transflective and highly conductive Ag/ITO/Ag multilayer films were prepared by magnetron sputtering on glass substrates. The microstructure and optical properties of Ag/ITO/Ag multilayer films were systematically investigated by X-ray diffraction, scanning electron microscopy, and ultraviolet-visible spectroscopy. The optical properties of the multilayer films were significantly influenced by the thickness of the Ag surface layer from 3.0 to 12.6 nm. The multilayer film of Ag9.3nm/ITO142nm/Ag9.3nm shows the best comprehensive property. It could satisfy the requirement for transflective LCD.
KeywordsAg/ITO/Ag multilayer films Microstructure Optical properties
Indium tin oxide (ITO) thin films have low electrical resistance and high transmittance in the visible spectrum. Because of their unique photoelectrical properties, they play an important role in optoelectronic devices, such as flat displays, thin-film transistors, solar cells, and so on [1–6].
It is well known that transmissive LCD has low contrast ratio in bright light and high power consumption. Reflective LCD has low contrast ratio in weak light, and most of them belong to monochromatic LCD. However, transflective LCD possesses high contrast ratio in bright and weak light as well as low power consumption.
Ag is a noble metal with excellent photoelectrical properties. In addition to good conductivity, it has high reflectivity in the visible range and good chemical stability. Thus, Ag/ITO composite material is the optimizing material to make new transflective LCD. Miedziński reported the electrical properties of Ag/ITO composite films . Choi fabricated ITO/Ag/ITO multilayer films and obtained a high-quality transparent electrode which has a resistance as low as 4 Ω/ϒ and a high optical transmittance of 90% at 550 nm . Bertran prepared Ag/ITO films with a high transmittance (near 80%) in the visible range by RF sputtering and studied their application as transparent electrodes in large-area electrochromic devices . Guillén prepared ITO/Ag/ITO multilayer films with visible transmittance above 90% by sputtering at room temperature and investigated the optical and electrical characteristics of single-layer and multilayer structures. Besides, the transmittance is found to be mainly dependent on the thickness of Ag film . Although much work has paid more attention on the investigation of Ag/ITO/Ag multilayer films, few studies have been carried out to study their photoelectrical properties.
In this study, Ag/ITO/Ag multilayer films with various surface layer thicknesses have been prepared on a glass substrate by direct current (DC) magnetron sputtering. The microstructure and optoelectronic properties of the Ag/ITO/Ag films were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and ultraviolet-visible spectroscopy (UV-vis).
The multilayer films were prepared by an ultrahigh vacuum multifunctional magnetron sputtering equipment (JGP560I, SKY Technology Development Co., Ltd, Shenyang, China). The multilayer films with a sandwich structure were deposited on glass substrates. The Ag layers were deposited by DC magnetron sputtering with a power density of 1.73 W/cm2, while the ITO coatings were deposited by radio frequency magnetron sputtering with a power density of 2.12 W/cm2. Ceramic ITO targets of In2O3:SnO2 disk (90:10 wt.%, 4N) and an Ag metal target (4N) were used for ITO and Ag layer deposition separately. The target-to-substrate distance was 60 mm. The base vacuum was 6.0×10-4 Pa, and the deposition pressure was 1.0 Pa with an argon (4N) flow rate of 45 sccm. During the deposition, the substrates were kept in room temperature.
The thicknesses of Ag/ITO/Ag multilayer films were controlled by sputtering time. Ag films and ITO films were firstly prepared respectively at a fixed time, and their thicknesses were tested using a surface profiler (XP-1, Ambios Technology, Santa Cruz, CA, USA). The deposition rate of Ag films and ITO films was calculated according to the sputtering time and film thicknesses. Then Ag/ITO/Ag multilayer films could be prepared with different sputtering time.
Microstructure parameters and the average reflectance with 300 to 900 nm of Ag/ITO/Ag multilayer films
Ag(111) grain size (nm)
Average reflectance R(%)
Ag 3.0 nm/ITO 142 nm/Ag 9.3 nm
Ag 6.4 nm/ITO 142 nm/Ag 9.3 nm
Ag 9.3 nm/ITO 142 nm/Ag 9.3 nm
Ag 12.6 nm/ITO 142 nm/Ag 9.3 nm
The microstructure was analyzed with a MXP 18AHF X-ray diffractometer (MAC Science, Yokohama, Japan). The X-ray source was CuKα, with an accelerating voltage of 40 kV, a current of 100 mA, scanning range from 20° to 80°, glancing angle of 2°, scanning step of 0.02°, and scanning speed of 8°/min. The surface morphology of the films was studied with a Hitachi S-4800 type SEM (Hitachi, Tokyo, Japan). The optical properties were tested with a Shimadzu UV-2550 type ultraviolet-visible spectroscope (Shimadzu, Kyoto, Japan). The scanning range was from 300 to 900 nm, scanning step was 1 nm, and slit width was 2 nm.
Results and discussion
where K is 0.9, λ is 1.54056 Å, β is the full width at half maximum of the diffraction peak, and θ is the diffraction angle. The value of Ag grain size D(111) was calculated, and the results were listed in Table 1. With the increase of Ag surface layer thickness from 3.0 to 12.6 nm, the Ag grain size of all films increases.
Ag/ITO/Ag multilayer films with different thicknesses of the surface Ag layer were prepared by magnetron sputtering on a glass substrate. Microstructural analysis shows that the multilayer films have a polycrystalline structure. As the thickness of the Ag surface layer increases, the preferred orientation of Ag (111) intensified. With increasing thickness of Ag surface layer, the transmittance spectra and reflectance spectra of Ag/ITO/Ag multilayer films decrease and increase, respectively. Ag3/ITO/Ag multilayer film shows the best comprehensive property and can be used as a potential transflective candidate in future LCD.
This work is supported by the National Natural Science Foundation of China (nos. 51072001 and 51272001), National Key Basic Research Program (973 Project) (2013CB632705), and the National Science Research Foundation for Scholars Return from Overseas, Ministry of Education, China. The authors would like to thank Yonglong Zhuang and Zhongqing Lin of the Experimental Technology Center of Anhui University for the electron microscope test and discussion.
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