Study of a sandwich structure of transparent conducting oxide films prepared by electron beam evaporation at room temperature
© Chiu et al.; licensee Springer. 2012
Received: 29 November 2011
Accepted: 1 May 2012
Published: 14 June 2012
Transparent conducting ZnO/Ag/ZnO multilayer electrodes having electrical resistance much lower than that of widely used transparent electrodes were prepared by ion-beam-assisted deposition (IAD) under oxygen atmosphere. The optical parameters were optimized by admittance loci analysis to show that the transparent conducting oxide (TCO) film can achieve an average transmittance of 93%. The optimum thickness for high optical transmittance and good electrical conductivity was found to be 11 nm for Ag thin films and 40 nm for ZnO films, based on the admittance diagram. By designing the optical thickness of each ZnO layer and controlling process parameters such as IAD power when fabricating dielectric-metal-dielectric films at room temperature, we can obtain an average transmittance of 90% in the visible region and a bulk resistivity of 5 × 10−5 Ω-cm. These values suggest that the transparent ZnO/Ag/ZnO electrodes are suitable for use in dye-sensitized solar cells.
A dielectric-metal-dielectric (DMD) layer structure is a low-energy film structure. It can effectively decrease the transmitted light in the near-infrared (NIR) region, usually by reflection and without affecting visible-light transmission properties . DMD transparent electrodes, where a thin metal layer is embedded between two dielectric layers, have been used recently [2–6]. Compared to single-layered transparent conducting oxide (TCO) film electrodes, DMD electrodes are thinner [3–6]. They are also more durable than single-layered metal films as the top oxide layer protects the metal layer.
ZnO can be doped with a wide variety of ions to ensure its applicability in several fields. Typical dopants that have been used to produce conducting films of ZnO belong to the group IIIa elements of the periodic table (B, Al, Ga, In). Thin films of doped ZnO have been prepared using many techniques such as sputtering [7–9], metal organic chemical vapor deposition (MOCVD) , vapor transport , pulsed laser deposition , spray pyrolysis, and pyrosol processes.
It is well known that the optical and electrical properties of very thin metal films vary according to their structures . To realize bulk-like properties, metal films should form a continuous structure, although they must be thin to ensure high transmittance. Among metal films, Ag films have the highest transmittance for visible light and good conductivity at room temperature, and they are already being used as transparent conducting electrodes in indium tin oxide (ITO)-based  multilayer devices. However, there are few reports related to the preparation of Ag- and ZnO-based multilayers at room temperature for application in low-resistance transparent electrodes. To compensate for such deficiency, we prepared Ag- and ZnO-based multilayers at room temperature using electron beam (e-beam) evaporation with the aid of collocated ion beams.
The thin-film characteristics of e-beam-evaporated films can be enhanced with ion-beam-assisted deposition (IAD). Another significant advantage of this process is that thin films prepared by ion-assisted e-beam evaporation can be used both with and without low-temperature post-deposition annealing.
In this study, the Macleod software was used to design structures with optimal optical properties. We then investigated the influence of preparation process variables on film properties.
ZnO and Ag films were deposited using an e-beam evaporator and an IAD system. The substrates used in these experiments included a 2-in square glass, a 2-in square plastic (PMMA), and a 1-in Si (100) wafer. The glass substrate and the wafer were cleaned in ultrasonic baths of acetone and ethanol sequentially and blow dried with dry N2. The background pressure in the deposition chamber was reduced to 2 × 10−6 Torr. The substrates were at room temperature before the start of deposition. Samples were obtained by depositing ZnO and Ag by e-beam evaporation with IAD (sample 1) as well as without IAD (sample 2). The working pressure for the deposition of the first layer (ZnO) was maintained at 4 × 10−4 Torr O2. The working pressure for the deposition of the third layer (ZnO) was maintained at 6 × 10−6 Torr without O2 in the 0–10 nm thickness range and at 4 × 10−4 Torr O2 in the 10–40 nm thickness range. The ZnO deposition rate was 0.2 nm/s. The working pressure for the deposition of the second layer (Ag) was maintained at 6 × 10−6 Torr without O2. The Ag deposition rate was 0.5 nm/s. The ZnO film was simultaneously bombarded by oxygen ions with ion beam energies of 400–500 W. The Ag film was simultaneously bombarded by argon ions with ion beam energies of 400–500 W.
The crystal orientation of the deposited films was examined by X-ray diffraction (XRD) with Cu Ká radiation. Cross-sectional morphology investigation and electron energy loss spectroscopy (EELS) were carried out using high-resolution transmission electron microscopy (HRTEM). The optical transmission spectra of the films were measured with a Lambda950 spectrometer, and their electrical characteristics were evaluated using an HL5500 system.
Results and discussion
Value of y E for different thicknesses of ZnO in the multilayer structure
value of yEZnO/Ag/ZnO
20 nm/11 nm/20 nm
30 nm/11 nm/30 nm
40 nm/11 nm/40 nm
50 nm/11 nm/50 nm
60 nm/11 nm/60 nm
Electrical behavior of multilayer ZnO/Ag/ZnO samples manufactured with or without IAD
of the metal
The thickness and structure of the Ag layer were the main factors that determined the electrical and optical properties of these multilayer structures. Through optical design and simulation, the average transmittance of visible light can be maximized.
E-beam evaporation with the IAD process can enhance the optical and crystal properties of ZnO and Ag films. Using this process, it is possible to fabricate flexible TCO films that have an average transmittance of 90% in the visible region and a bulk resistivity of 5 × 10−5 Ω-cm.
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