| 其他摘要 | The basic properties of transparent conductive oxides (TCOs) of metals include large band gap (>3eV), high transmittance in the visible region (>80%), high reflectance in the IR region (>60%), and strong attenuation to the microwave. At present, TCO films, such as SnO2, In2O3, ZnO, and their doped forms, are extensively used in electronics and photonics applications, such as transparent electrodes in solar cells, flat display devices, gas sensors, waveguide devices, infrared reflective windows, etc. Although In2O3:Sn(ITO) films are extensively applied, their high cost, toxicity, and instability restrict its further applications. On the contrary, ZnO:Al(ZAO) films have not only comparable optical and electrical properties as those of ITO, but also feature low cost, nontoxic, and high stability. Therefore, ZAO films are going to substitute for ITO and have an extensive application prospect.
This paper focused on the preparation, characterization, properties and applications of transparent conductive oxides (TCOs) films based on ZnO. Transparent conducting films of both ZnO:Al and ZnO:(Al,Mn) have been successfully fabricated by using DC reactive magnetron sputtering alloy targets of Zn/Al and Zn/(Al, Mn) in Ar+O2 atmosphere, respectively. The dependence of electrical and optical properties of both films on deposition parameters, such as oxygen partial pressure, substrate temperature and working pressure have been investigated in detail, as well as the structure and morphology of the films with the aid of XRD, SEM, TEM and XPS, etc. The conduction mechanism and optical properties were discussed.
Both ZnO: Al and ZnO:(Al, Mn) films exhibit (002) textured with a c-axis preferred orientation due to the fact that the most packed (002) planes have the lowest surface free energy under the thermodynamic equilibrium. The microstructure indicated that the conically shaped columnar grains started perpendicular to the substrate surface and grew through the entire film thickness. The films contained intrinsic compressive stress parallel to the film surface, which is the cumulative results of chemical and micro-structural defects induced during deposition. XPS analysis revealed that Zn、O and Al elements well-distributed through the films and no Al monaxial existed which denies the standpoint of Al’s existence in grain boundary. The diffuse layer length on glass for ZAO is small comparable to ITO. No doubting this provided experimental prospect for the substitution of ZAO for ITO.
The film optical properties were in competition with the film electrical properties. Increasing carrier density led to increase in the visible absorption and it was a better strategy to increase conductivity without compromising the optical properties by increasing Hall mobility rather than the carrier concentration. The films prepared at a oxygen partial pressure of 2.6×10-2 Pa and substrate temperature of 150℃ exhibited columnar structure, having high carrier concentration(6 ）, high hall mobility(24 ）and the lowest resistivity(4.6×10－4 ). Furthermore ZAO films show favorable stability.
The carrier concentration of ZAO films was more than 1020cm-3 and highly degenerated with direct allowed transition band structure. It was observed that the band gap increased with increasing electron density. Based on BM effect, the fundamental band gap and the fundamental absorption edge of ZAO films were determined to be 3.7 eV and 320 nm, respectively. The band gap shift of the studied films was found to be nearly proportional to N2/3, indicating that the band gap widening effect dominated over the band gap narrowing performance (Many-body effect). There was a closed relationship between dispersion and band gap transition. The experimental results showed that the optical function was between 1.8~2.00 for ZAO films. Both refractive n and extinction coefficient k deeply decreased with the increase of wavelength and finally became constant.
we report different methods to reduce the sheet resistance of ZnO: Al (ZAO) films on flexible substrates without degrading the optical transmittance in the visible range. Under proper bias, ZAO films deposited on Al2O3-buffered flexible substrates showed a significant decrease of sheet resistance when compared with those deposited on bare polymer. The films with resistivity as low as 8.4×10-4 and the optical transmittance about 80 % have been obtained by improved methods. By calculating the Al doping efficiency and the mean free path of electrons, ionized impurity scattering was considered to be the dominant factor for the transport mechanism of carriers.
ZnO:Al (ZAO) films were employed as the anode to fabricate organic light-emitting diode (OLED) with ZAO/NPB/Alq3/Al configuration. The I-V curve reveals that the turn-on voltage is roughly 8 V for the device. A maximum luminance of 5000 cd/m2 was observed at 20 V for the device based on ZAO films. The maximum luminance efficiency measured from the device was 3.0 cd/A at ~100 A/m2. The measured results demonstrate that ZAO is a potential alternative anode material to indium tin oxide (ITO).
On the whole, ITO is still predominant for high-grade applications among TCOs films, Al-doped ZnO films, which have low manufacturing cost and comparable electrical and optical properties, are becoming the most potential alternative materials for some transparent conducting applications. |
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