Conductive and transparent multilayer films for low-temperature TiO2/Ag/SiO2 electrodes by E-beam evaporation with IAD
© Chiu et al.; licensee Springer. 2014
Received: 28 September 2013
Accepted: 3 January 2014
Published: 16 January 2014
Conductive and transparent multilayer thin films consisting of three alternating layers (TiO2/Ag/SiO2, TAS) have been fabricated for applications as transparent conducting oxides. Metal oxide and metal layers were prepared by electron-beam evaporation with ion-assisted deposition, and the optical and electrical properties of the resulting films as well as their energy bounding characteristics and microstructures were carefully investigated. The optical properties of the obtained TAS material were compared with those of well-known transparent metal oxide glasses such as ZnO/Ag/ZnO, TiO2/Ag/TiO2, ZnO/Cu/ZnO, and ZnO/Al/ZnO. The weathering resistance of the TAS film was improved by using a protective SiO2 film as the uppermost layer. The transmittance spectra and sheet resistance of the material were carefully measured and analyzed as a function of the layer thickness. By properly adjusting the thickness of the metal and dielectric films, a low sheet resistance of 6.5 ohm/sq and a high average transmittance of over 89% in the 400 to 700 nm wavelength regions were achieved. We found that the Ag layer played a significant role in determining the optical and electrical properties of this film.
KeywordsTiO2/Ag/SiO2 E-beam Ion-beam-assisted deposition Room temperature Transparent conducting oxides
Transmittances and sheet resistances of various cathodes
transmittance 550 nm
resistance (Ω cm)
A1 (20 nm)
SiO2/Ag/SiO2 (40:10:40 nm)
ZnO/Cu/ZnO (58:10:63 nm)
ZnO/Cu/ZnO (40:10:40 nm)
ZnO/A1/ZnO (40:10:40 nm)
TiO2/Ag/TiO2 (40:10:40 nm)
ZnO/Ag/ZnO (40:10:40 nm)
This assumption is justified if the film boundary effects are negligible [7–9]. Silver was found to perform the best as the middle metal layer in sandwiched DMD structures. A pure Ag metal film has the lowest resistivity of all metals and exhibits relatively low absorption in the visible region. The optical and electrical properties of DMD films can be adjusted to achieve various transmittances with a peak in the spectra by suitably varying the thickness of the Ag layer. TiO2, a dielectric material, is used in the DMD structure because of its high refractive index, good transparency in the visible region, and easy evaporation. SiO2 is very stable and can be used as a protective layer on top of the Ag surface to avoid the deterioration of the properties of the metal during exposure to certain environmental conditions. Ag, SiO2, and TiO2 are also materials that are most frequently used in the fabrication of optical and electrical devices at a relatively low cost. This can be achieved by thin film deposition, applying either evaporation or sputtering methods under normal vacuum conditions.
In the case of SAS material, a minimal current seems to flow into the device because of the low conductivity and charge densities for current flow observed within it. However, Kim and Shin  reported conductivity enhancement achieved by introducing zinc cations into the amorphous silica layer. This means that we can obtain better current injection into the transparent organic light-emitting diodes by properly treating SAS cathodes. Such cathodes exhibit two separate mechanisms for resonant tunneling current injection: one for the low-voltage region and one for transparent conducting oxides (TCOs) currents for the high-voltage region.
In this study, multilayer transparent conductive coatings (DMD) were fabricated for low-temperature-sintered electrodes containing mesoporous TiO2. This compound was chosen as one of the dielectric materials because of its suitable properties as described above. SiO2 was chosen as the other dielectric material, since it also has a low refractive index and exhibits good transparency in the visible region. In addition, this semiconductor is very stable, as mentioned before, and can be easily evaporated. Finally, Ag was chosen as the conductive layer because of its suitable optical properties in the visible region. Hence, TiO2/Ag/SiO2 (TAS) transparent films were fabricated, and their possible application in TCOs was examined.
Fabrication of TiO2/Ag/SiO2 transparent films
TAS multilayers were fabricated by electron-beam (E-beam) evaporation with ion-assisted deposition ion-beam-assisted deposition (IAD) under a base pressure of 5 × 10−7 Torr. The substrates were kept at room temperature before starting deposition. The working pressure for the deposition of the first layer (TiO2) was maintained at 4 × 10−4 Torr with O2, whereas the deposition of the third layer (TiO2) was maintained at 6 × 10−6 Torr (without O2) in the 0- to 10-nm thickness range and at 4 × 10−4 Torr (O2) in the 10- to 70-nm thickness range. The working pressure for the deposition of the second layer (Ag) was maintained at 6 × 10−6 Torr (without O2). The deposition rate of TiO2 was 0.3 nm/s and that of Ag was 0.5 nm/s. The ZnO film was bombarded by oxygen ions with ion beam energies of 400 to 500 W, whereas the Ag film was bombarded by argon ions with ion beam energies of 400 to 500 W. The film thickness was determined using an optical thickness monitoring system, and the evaporation rate was deduced from the measurements of a quartz oscillator placed in the deposition chamber. The thicknesses of the glass-attached TiO2 layer, Ag layer, and protective layer SiO2 were determined using the Macleod simulation software.
Optical properties, electrical properties, and microstructure analysis
Optical transmittance measurements were performed on the TAS multilayers using an ultraviolet–visible-near-infrared (UV–vis-NIR) dual-beam spectrometer in 400 to 700 nm wavelength range. Optical polarization was applied to the single films by ellipsometric measurements to increase the refraction index. The crystal orientation of the deposited films was examined by x-ray diffraction (XRD) with Cu Kα radiation. A transmission electron microscope (JEOL 2000 EX H; JEOL Ltd., Akishima, Tokyo, Japan), operated at 200 kV, and a field-emission gun transmission electron microscope, operated at 300 kV, were used for cross-sectional microstructure examination. Energy-dispersive spectra (EDS) and electron diffraction patterns obtained using this equipment enabled detailed sample characterization. The sheet resistance of the samples was measured by a Hall system. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Thermo Scientific K-Alpha spectrometer (Thermo Fisher Scientific, Hudson, NH, USA). An Al x-ray at 1,487 eV was used as the light source, and the peak positions were internally referenced to the C 1s peak (arising from the methylene groups of dodecanethiolate) at 284.9 eV . All the binding energies are referenced to the clean Ag 3ds/2 peak at 368.22 eV.
Results and discussion
Optical spectra of a substrate TiO 2 /Ag/SiO 2 /air structure simulated using the Macleod software
Value of yE (Tio2/Ag/SiO2)
Optical spectroscopy of the conductive and transparent films
Microstructure of the TAS multilayers
AFM topographic analysis
The ideal work function of Ag is 4.4 eV, which is smaller than that of TiO2 (4 to 6 eV)  and higher than that of SKh (3.03 to 3.41 eV) . When two layers are in contact with each other, the Fermi levels align in equilibrium by the transfer of electrons from Ag to SiO2 and TiO2. The electrical properties of the system improve under these conditions. In this case, there is no barrier for the electron flow between Ag and SiO2, which means that the electrons can easily move from the Ag layer to the SiO2 layer. According to Schottky’s theory, we expect high carrier concentrations in multilayer TAS films.
X-ray photoelectron spectroscopy
E-beam evaporation with IAD has been applied to produce TAS layers with favorable properties: the sheet resistivity of the obtained material was 6.5 Ω/sq and its average transmittance (400 to 700 nm) was 89%. Environmental testing under high temperature and humidity conditions demonstrated that the amorphous SiO2 layer was stable and could avoid silver oxidation and vulcanization. The resulting thickness and structure of the Ag layer were the main factors determining the electrical and optical properties of the multilayer structures. According to the results of both optical design and simulations, the first layer was fabricated using a high-reflection-index material, whereas the last layer was fabricated using a low-reflection-index material. This structure was introduced to maximize the average transmittance of visible light.
The authors would like to thank the National Science Council of the ROC, Taiwan (contract no. 102-2622-E-492 -018 -CC3) for financially supporting this research.
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