Chemically Sprayed Fluorine-Doped Zinc Oxide Thin Films Deposited from Zn(C5H7O2)2: Effect of the Molarity and Substrate Temperature on the Physical Properties

Abstract

Fluorine-doped zinc oxide thin films, ZnO : F, were deposited by the chemical spray pyrolysis technique using zinc acetylacetonate, Zn(C 5 H 7 O 2 ) 2 , and ammonium fluoride as precursors. The molar concentration in the starting solution and the substrate temperature, T s , were varied in order to obtain transparent and conductive films. For molarities higher than 0.3 M, growth rate values on the order of 3.5 nm/s were obtained. As the molarity increases, the resistivity r decreases up to a minimum value in the order of 10 À2 W cm for films deposited from a 0.4 M starting solution and T s ¼ 475 C; for higher molarities an increase in r is observed. The preferential growth direction is affected by the molarity and T s , showing a switching of the (101) planes obtained for the lowest molarities (0.1 and 0.2 M) and T s < 475 C to the (002) preferential growth for molarities higher than 0.1 M and all the substrate temperatures used. The mobility and the carrier concentration were varied from 1 to 3 cm 2 /Vs and 4 Â 10 20 to 5 Â 10 20 cm À3 , respectively. From scanning electron micrographs, small grain size is evidenced, which can give rise to a high density of trapping states in the grain boundaries. Films were uniform, pinholes-free, with a transmittance in the visible in the order of 85%.

Spray pyrolysis for highTcsuperconductors films

“…The growth rate of the oxide is linked with the concentration of the precursor: it increases linearly with the concentration up to a limit where it saturates. This has been verified at least for In 2 O 3 and ZnO for different precursors [4,5]. This suggests that at low concentration, mass transport governs the growth rate while for higher concentration the diffusion of the reactants on the surface become the limiting mechanism.…”

“…This parameter is especially important when highly organized and crystallized films are expected, as is the case for HTS films. As was recalled for In 2 O 3 and ZnO [4,5], a high concentration of precursor makes the limiting step of the deposition rate to be surface diffusion. This is propitious for the formation of highly textured or epitaxial films and applies very well to the case of YBCO films.…”

“…Many papers (more than 70 at the date of this article) have been published on oxide films formation by spray pyrolysis over the last two years, however very few concentrate on the process itself. Most works aim to synthesize transparent conducting films based on In 2 O 3 [4], SnO 2 [5] and ZnO [6] for various applications such as gas sensors, transparent electrodes, piezoelectric devices, varistors, surface acoustic waves, etc. They have the advantage of having being made on simpler oxides than YBCO and then it is useful to review their content.…”

“…This suggests that at low concentration, mass transport governs the growth rate while for higher concentration the diffusion of the reactants on the surface become the limiting mechanism. Concentration also affects the preferential growth direction [5].…”