FLUORINE AND ALUMINIUM DOPED-ZINC OXIDE FILMS. PHYSICAL
PROPERTIES AND GAS SENSING AND PHOTOCATALYTIC APPLICATION
M. de la L. Olvera, A. Maldonado
Departamento de Ingeniería Eléctrica-SEES, Centro de Investigación y de Estudios
Avanzados del IPN, CINVESTAV-IPN, México D. F. 07000, México.
E-mail:
Fluorine and aluminum-doped zinc oxide thin films, ZnO:F:Al were deposited on glass
substrates by the sol-gel method for testing their sensing and photocatalytic characteristics.
The effect of the film thickness and the solution ageing on the physical characteristics of the
films was analyzed. Two ageing times, namely, two and seven days, and three different
thicknesses, in the order of 220, 330, and 520 nm, were the variables used in this work.
Structural, optical, and morphological characterization was carried out in vacuum-annealed
films.
The X-ray diffraction (XRD) patterns revealed that both as-deposited and vacuum annealed
ZnO:F:Al thin films were polycrystalline with a hexagonal wurtzite-type structure with a
well-defined (002) diffraction peak, irrespective of the aging time of the starting solution.
The (002) peak shows a proportional increase with the thickness magnitude. An average
crystallite size of about 20 nm was estimated using the well-known Scherrer's formula.
On the other hand, as was expected, as-deposited ZnO:F:Al films showed a very high
electrical resistivity, however after a vacuum thermal treatment, it was registered a significant
decrease.
From the surface morphology study it was observed that the grain size is almost independent
of both aging-time of the starting solutions and the film thickness. Films presented an average
optical transmittance in the visible range (400-700 nm) in the order of 90 %, as well as a band
gap of 3.3 eV.
In respect of gas-sensing properties of ZnO:F:Al thin films in an atmosphere carbon
monoxide, CO, atmosphere measured at different operation temperatures were tested. The
highest sensitivity registered in this work, for aged films measured in 500 ppm of CO at
300°C, was of the order of 93 %.
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