Transparent conducting oxide (TCO) films have been widely used as transparent electrodes in various displays, thin film solar cells and other optoelectronic devices. Up to now several transparent conducting films of oxide series have been developed, among which Sn-doped In2O3 (ITO) films have been most frequently used due to their superior electrical and optical properties. However, the exhausting of resources has resulted in an increase in the price, leading to the limitation of use of ITO films. Hence, there is an increasing interest in developing transparent conducting films by using more cost-effective materials.
In recent years, doped ZnO thin films have attracted great attention due to their high conductivity, good transmission in the visible light range, abundant resource of raw materials, low cost of fabrication, non-toxicity, etc. and they are regarded as potential substitutes for ITO. Among them, Al-doped ZnO (AZO) films are the most studied due to the rich resources of Al and good electrical and optical properties.
In general, the electrical and optical properties of AZO films prepared by r. f. magnetron sputtering are greatly influenced by process parameters such as sputtering power, working pressure, substrate temperature and target-substrate distance, and many studies have been focused on analyzing the effect of these process parameters on the properties of the films and finding optimal process parameters.
The electrical properties of AZO films, in general, are greatly related to the doping state of Al into ZnO crystal grains, which could be qualitatively evaluated by considering the binding state of Al and compositional elements through XPS measurements. However, previous studies that have considered the effect of film thickness on the properties of AZO films, failed to analyze the doping state of Al in connection with XPS measurements. In addition, there is no literature regarding the effect of film thickness on the properties of AZO films in relation to deposition temperature.
Ri Kang Hyon, a researcher at the Faculty of Materials Science and Technology, has observed the structural and electrical properties of AZO films prepared at two different substrate temperatures (100℃ and 350℃) by r. f. magnetron sputtering for film thickness, and analyzed them in combination with the results of X-ray photoelectron spectroscopy (XPS) measurements.
The results showed the following.
First, the resistivity of films deposited at 100℃ or 350℃ decreased significantly by about one order with increasing film thickness from about 80nm to 600nm, i.e. from 34.16×10-3Ω·cm to 3.14×10-3Ω·cm and from 6.71×10-3Ω·cm to 0.75×10-3Ω·cm, respectively. Second, the resistivity differs greatly with deposition temperature even for the same film thickness, which means the average resistivity at 100℃ is 4-5 times greater than at 350℃. Third, Al2O3 segregates and the amount of chemisorbed oxygen at the grain boundary decrease with increase to 600nm of film thickness and to 350℃ of deposition temperature, resulting in the increase in carrier concentration.
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