Physical properties of ZnO:F obtained from a fresh and aged solution of zinc acetate and zinc acetylacetonate

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Abstract

Fluorine-doped zinc oxide thin films, ZnO:F, were deposited by the spray pyrolysis technique on sodocalcic glass substrates. Two different zinc precursors were used separately, namely, zinc acetate and zinc pentanedionate. The effect of the zinc precursor type, the aging of the starting solution, the substrate temperature and a vacuum-annealing treatment on the electrical, morphological, structural and optical properties was studied, in order to obtain conductive and transparent ZnO:F thin films. The resistivity values of ZnO:F thin films deposited from aged solutions were lower than those films obtained from fresh solutions. The lowest resistivity values of as-grown films deposited at 500 °C, using a two-day aged starting solution of zinc acetate and zinc pentanedionate, were 1.4×10−2 and 1.8×10−2 Ω cm, respectively. After a vacuum annealing treatment performed at 400 °C for 30 min a decrease in the resistivity was obtained, reaching a minimum value of 6.5×10−3 Ω cm for films deposited from an aged solution of zinc acetate. The films were polycrystalline, with a (0 0 2) preferential growth orientation in all the cases. Micrographs obtained by SEM show a uniform surface covers by rounded grains. No evident change in the surface morphology was observed with the different precursors used. The transmittance of films in the visible region was higher than 80%.

Introduction

In recent years, the zinc oxide (ZnO) has received much attention due to its many electronic-device applications, such as light-emitting diodes [1], transparent conductors [2], electroacustic transducer [3], compact disk memories [4], gas sensors [5], ultraviolet blocking [6] and varistors [7], among others. Some of these applications are based in the simultaneous occurrence of low resistivity and high transmittance in the visible spectrum when ZnO is manufactured in thin solid film form.

Several chemical and physical deposition techniques such as sputtering [8], reactive evaporation [9], chemical-vapor deposition (CVD) [10], spray pyrolysis [11] and screen-printing [12], among others, have been used to obtain ZnO thin films. Highly conductive and transparent ZnO thin films have been obtained by the chemical-spray pyrolysis technique. Due to the low cost of the compound sources and the simplicity of the deposition system required, we have selected this technique in our laboratory. Henceforth we are interested particularly in ZnO thin films obtained by chemical spray.

Undoped ZnO thin films contain an excess of zinc atoms, accommodated in interstitial sites, which leads to an n type conductivity, however, their resistivity values measured are still very high for transparent electrode applications [13]. Then doping with cationic elements such as In [14], Al [15] and Ga [16], the electrical properties have been improved, decreasing the resistivity of chemically sprayed ZnO thin films, and of these In impurity is the most successful until now.

The doping of chemically sprayed ZnO thin films with anionic impurities such as F has not been as successful as In, as should be expected [6]. Lack of detailed studies concerning the increase of the fluorine, F, doping efficiency, is a rather surprising fact and so on the lowering in the resistivity, in the chemical-spray technique. With respect to this point, a detailed study of thin films deposited from different zinc compounds can elucidate the way to decrease the resistivity of ZnO:F thin films. On the other hand, the use of low cost fluorine compounds is attractive as compared to more expensive and hazardous indium salts.

In this work, we report on the electrical, structural and optical characteristics of chemically sprayed ZnO:F thin films deposited from two different zinc precursors (zinc acetate and zinc pentanedionate). A comparison between the different deposition conditions was made in order to obtain conductive and transparent thin films.

Section snippets

Experimental procedure

ZnO thin films were deposited on glass substrates using two distinct starting solutions. These were prepared by dissolving the zinc precursor, at a 0.25 M concentration (zinc acetate-Merk, zinc pentanedionate-Alpha Aesar) in a mixture of deionized water, ethanol and acetic acid (25:70:5 volume proportion, respectively). Doping of the films was achieved by adding the corresponding quantity of ammonium fluoride (dissolved in deionized water at 1.6 M concentration) to the starting solution. Both

Electrical properties

Fig. 1 shows the electrical resistivity variation as a function of the substrate temperature for as-grown ZnO:F thin films deposited from both fresh and aged solutions and the two different zinc precursors used (zinc acetate and zinc pentanedionate). From this figure we can observe that the resistivity values for films obtained from zinc acetate are slightly lower than those obtained with zinc pentanedionate. In films deposited at low substrate temperatures, 450 and 475 °C, from two-day aged

Conclusions

ZnO:F thin films were deposited from two distinct zinc compounds, namely, zinc acetate and zinc pentanedionate. As-grown thin films deposited from a fresh solution were more resistive than films deposited from two-day aged solutions, for both zinc precursors. The resistivity difference reached was near one order of magnitude. After a vacuum heat treatment, a further decrease in the resistivity was observed, more pronounced in thin films deposited from fresh solutions and low substrate

Acknowledgements

The technical support of M.A. Luna-Arias, G. López-Fabián, A. Palafox, M. Guerrero and A. Soto is acknowledged. This work was partially supported by CONACyT—México under the Contract 39487.

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