Characterization of Transparent and Conductive ZnO:Ga Thin Films Produced by Rf Sputtering at Room Temperature
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Characterization of transparent and conductive ZnO:Ga thin films produced by rf sputtering at room temperature
E. Fortunato, V. Assunção, A. Marques, I. Ferreira, H. Águas, L. Pereira, R. Martins Materials Science Department/CENIMAT, Faculty of Sciences and Technology of New University of Lisbon, Campus da Caparica, 2829-516 Caparica, Portugal. ABSTRACT Gallium-doped zinc oxide films were prepared by rf magnetron sputtering at room temperature as a function of the substrate-target distance. The best results were obtained for a distance of 10 cm, where a resistivity as low as 2.7×10-4 Ωcm, a Hall mobility of 18 cm2/Vs and a carrier concentration of 1.3×1021 cm-3 were achieved. The films are polycrystalline presenting a strong crystallographic c-axis orientation (002) perpendicular to the substrate. The films present an overall transmittance in the visible part of the spectra of about 85 %, in average. The low resistivity, accomplished with a high growth rate deposited at RT, enables the deposition of these films onto polymeric substrates for flexible applications.
INTRODUCTION Transparent conductive oxides (TCOs) with optical transmission higher than 80% in the visible range and with resistivity lower than 10-3 Ωcm, have been widely used for more than a half-century, in several technological applications as coatings for heat reflective windows, antistatic instrument windows, as well as vehicle window heaters, electrochromic devices and electrochemical cells [1]. More recently, they become the subject of intense academic and industrial investigation for applications as transparent electrodes in amorphous silicon solar cells, flat panel displays and light emitting diodes. Most of the previous research on TCOs has been focused on indium tin oxide (ITO) and fluor tin oxide (FTO). However, TCO films based on zinc oxide are taking a great impact because of the advantages of low cost, resource availability (about a factor of 1000 more abundant than In), non-toxicity and high thermal/chemical stability. Al, In and Ga have been reported as an effective dopant for zinc oxide based films. Most of the works related to zinc oxide use Al as dopant. Nevertheless, Al presents a very high reactivity leading to oxidation during the growth of the film, which may become a problem. Ga is less reactive and more resistant to oxidation compared to Al. On the other hand, the covalent bond lengths of Ga-O and Zn-O are estimated to be 1.92 and 1.97 Å, respectively. The slightly smaller bond length of Ga-O than that of Zn-O is an advantage since it allows minimizing the deformation of the ZnO lattice even in the case of high gallium concentrations. Doping with Ga led to films with the highest quality [2]. The films were very smooth and essentially free of pinholes. These were the main reasons why Ga was chosen as dopant. Several techniques have been employed and Table I presents the best results obtained for GZO films to the best of the knowledge of the authors at the present date: pulsed laser deposition [3], spray pyrolysis [4], Mo
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