New challenges on gallium-doped zinc oxide films prepared by r.f. magnetron sputtering
Introduction
Doped zinc oxide films have been extensively studied in the last years, because of their combined electrical and optical properties associated to their low material cost, resource availability and high thermal/mechanical stability [1]. The most common dopants used (elements from Group III) are In, B, F, Ge, Si, Ga and Al. Al has been the most used [2], nevertheless, Al presents a very high reactivity leading to oxidation during the growth of the film, with a deterioration of the electrical properties [3]. Recently, gallium zinc oxide (GZO) has gained great attention because of its superior properties and is suitable for use as transparent conductive oxide (TCO) in several optoelectronic devices. On the other hand, the covalent bond lengths of GaO and ZnO are estimated to be 1.92 Å and 1.97 Å, respectively. The slightly smaller bond length of GaO than that of ZnO 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 [4]. The films were very smooth and essentially free of pinholes. These were the main reasons why gallium was chosen as dopant. Several techniques have been employed and Table 1 presents the best results obtained for GZO films to the best of the knowledge of the authors at the present date: pulsed laser deposition [5], spray pyrolysis [6], MO-CVD [7], d.c. magnetron sputtering [8], arc-discharge ion plating [9] and r.f. magnetron sputtering [10]. The best result achieved in this work is also indicated. Fig. 1 presents a comparison between the lowest values of resistivity obtained for GZO thin films produced by different techniques by different workers. Here, we would like to emphasize that the present work reports the lowest value of resistivity for GZO thin films deposited at room temperature to which is also ascribed the highest transmittance in the visible range.
Section snippets
Experimental procedures
The GZO films were deposited onto Corning 7059 glass substrates by r.f. (13.56 MHz) magnetron sputtering using a ceramic oxide target ZnO/Ga2O3 (95:5 wt.%; 5 cm diameter) with a purity of 99.99% with the target and substrate positioned in a parallel configuration. The sputtering was carried out at room temperature and with the optimum conditions for the system used [11]: argon flux of 20 sccm, deposition pressure of 0.15 Pa and r.f. power of 175 W. The distance between the substrate and the
Results and discussion
Fig. 2 shows the dependence of the growth rate (R) as a function of the target–substrate distance. R decreases as the target-substrate distance increases, from values of 55 nm/min to 15 nm/min for 5 cm and 15 cm, respectively. For the pressure used (0.15 Pa) the mean free path of the species is approximately 5 cm, therefore, the sputtered species have a mean free path comparable to the target-to-substrate distance and as a result, these arriving particles have high surface mobility and, hence,
Conclusions
Highly conducting and transparent GZO films have been deposited by r.f. magnetron sputtering at room temperature as a function of the substrate–target distance (Table 2). A resistivity as low as 2.7×10−4 Ω cm was obtained for a substrate–target distance of 10 cm, a deposition pressure of 0.15 Pa and r.f. power of 175 W. The films produced are polycrystalline presenting a strong crystallographic c-axis orientation (002) perpendicular to the substrate. The average transmittance in the visible
Acknowledgements
The authors would like to acknowledge Augusto Lopes for the SEM analysis and Rui Martins for the XRD measurements. This work was supported by the ‘Fundação para a Ciência e a Tecnologia’ through Pluriannual Contracts with CENIMAT and by the projects: POCTI/1999/ESE/35578, POCTI/1999/CTM/35440 and POCTI/2001/CTM/38924.
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