Fabrication and vacuum annealing of transparent conductive AZO thin films prepared by DC magnetron sputtering
Introduction
ZnO is a II–VI group semiconductor material with wide direct bandgap and wurtzite structure. The high stability, melting point and excitation energy make it a promising ultraviolet (UV) and blue optoelectronic material. In addition, ZnO thin films offer a variety of applications in integrated optic, piezoelectronic, and gas sensitive devices.
A transparent conducting electrode is a necessary component of most types of flat panel displays (FPDs). The commercially most important material for a transparent conducting film nowadays is Sn-doped In2O3 (ITO), owing to its unique characteristics of high visible transmittance (90%), low DC resistivity, high infra-red reflectance and absorbance in the microwave region. The high quality of ITO films deposited by sputtering of oxide targets has already been successfully achieved on a commercialized production scale. On the other hand, ZnO films have attracted interest as a transparent conductive coating material, because the materials (1) consist of cheap and abundant element, (2) are readily produced for large-scale coating, (3) allow tailing of ultraviolet absorption, (4) have a high stability in hydrogen plasma, and (5) have low growth temperature. Besides, it is nontoxic and easy to fabricate. The electrical resistivity of ZnO thin film is readily modified by the addition of impurity or post-deposition annealing [1], [2], [3]. Aluminum-doped zinc oxide (AZO) thin films have been prepared by thermal evaporation [4], [5], chemical vapour deposition (CVD) [6], plasma enhanced chemical vapour deposition (PECVD) [7], sol–gel [8], [9], spray pyrolysis [10], [11], [12], pulsed laser deposition (PLD) [13] and magnetron sputtering [14], [15], [16], [17], [18], [19], etc. Magnetron reactive sputtering, using a Zn–Al alloy target, has advantages in target fabrication and film deposition rate [18]. Its drawback is that the chemical composition of AZO thin films is easily modified by the Zn oxidation on the target surface and is hard to control. A relative high-density (94%) AZO conductive ceramic target has been used for AZO film fabrication in this work. The results of fabrication, post-deposition annealing and their effects on the optical and electronic properties are reported in this paper. Influence of substrate temperature, sputtering power, sputtering pressure (argon pressure), target to substrate distance, and post-deposition annealing in air, argon or reducing gas on AZO film quality has been reported [11], [12], [18], [19], [20], [21]. The structural and physical properties of AZO films synthesized by thermal evaporation, sol–gel, spray pyrolysis, and RF sputtering following by annealing in vacuum have been studied [4], [9], [10], [20]. However, post-deposition annealing of DC sputtered AZO thin film in vacuum has been less studied. This area is our main concern in this work.
Section snippets
Experiment
AZO thin films have been prepared by DC planar magnetron sputtering with AZO oxide target. The schematic diagram of the magnetron sputtering system is described in Ref. [22]. The target was prepared by pressing the mixture of ZnO powder (purity of 99.9%) and Al2O3 powder (purity of 99.99%) under a pressure of 800 MPa in air at room temperature into a pellet of 110 mm diameter and 5 mm thickness. The doping amount of Al2O3 in ZnO powder is about 2 wt%. Then the pellet was sintered at 1300°C for 6 h
Structural characteristics
It is well known that sputtered ZnO films are highly textured with the c-axis perpendicular to the substrate surface. XRD was employed to determine the crystal structure. The changes in crystal structure of the films under different deposition temperatures with or without vacuum annealing were investigated.
Table 1 displays XRD data of AZO target powders. It shows typical polycrystalline hexagonal ZnO structures without obvious preferential growth. The (1 0 1) peak other than (0 0 2) peak has the
Conclusions
In conclusion, transparent and conductive AZO thin films have been deposited on glass sheet by DC planar magnetron sputtering without introducing any oxygen at different temperatures and post-deposition annealing in vacuum. AZO thin films deposited at 320°C and subsequently annealed at 400°C for 2 h under 10−3 Pa possess a resistivity of 1.5×10−4 Ω cm with a carrier concentration of 1.26×1021 cm−3 and mobility of 33.3 cm2/V s and the highest transparency. With the increase of substrate temperature or
Acknowledgements
This work is partially supported by the 985 research fund of Tsinghua University, by China Postdoctoral Science Foundation, and by the Education Department of Hubei Province under grant no. 2002z06003.
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2022, VacuumCitation Excerpt :Transparent conductive oxide films (TCO's) have attracted increasing interest due to the distinctively transparent and electrical properties and have been extensively used as electrodes in various opto-electrical devices, including photovoltaic solar cell [1,2], smart window [3], flat panel display [4,5], electrochromic device [6], thermocouples [7] and sensor [8]. Various semiconductor TCO films have been studied sufficiently to reach large-scale production and multifunctional application, especially In2O3-based film (ITO [9,10], IZO [11–14]), SnO2 based film (ATO [15], FTO [16]), ZnO based film (AZO, FZO, BZO) [11,17,18] and multi-compounds [19]. Among the aforementioned TCO films, ITO is the most prospective candidate due to a much higher electrical conductivity as well as higher visible transparency than those of SnO2 and ZnO-based films [1,20].