Atomic Force and Electron Microscopy Studies of Tin Dioxide Films Prepared from Solutions with High Fluorine Content
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ATOMIC FORCE AND ELECTRON MICROSCOPY STUDIES OF TIN DIOXIDE FILMS PREPARED FROM SOLUTIONS WITH HIGH FLUORINE CONTENT Dwight R. Acosta(1 ) , Rebeca Castanedo(2 ) Walter Estrada(3 ) , Rosario Avila-Godoy(4 ) (1) Instituto de Física, UNAM, A.P. 20-364; 01000 México D.F.,MEXICO (2) Inst. Politécnico Nacional; Lab. de Materiales, Querétaro, MEXICO (3) Univ. Nacional de higeniería, Fac. de Ciencias, Lima, PERU (4) Univ. de Los Andes, Fac. de Ciencias, Mérida, VENEZUELA
ABSTRACT Atomic force microscopy and electron microscopy were used to study surface and structural properties in SnO2:F thin films deposited by spray pyrolysis on glass at a constant substrate temperature. The influence of high levels of fluorine in the starting solution on the surface topology and morphology of thin films and correlations with electrical properties are presented in this work.
INTRODUCTION Tin oxide films (SnO2) are used as transparent conducting films for many applications because they have a high transmission coefficient, low electrical resistivity and chemical stability [1-3]. Also porous tin oxide exhibits semiconductor properties which make it suitable for applications in gas sensor dispositives [4]. Gas detection in thin-film based dispositives has to do principally with electrical conductivity changes in air and in other gas atmospheres. Among the many factors that affect the sensing properties of semiconductor gas sensors, the electrical conductance is directly related to the surface morphology and structural details, which in turn are related to more fundamental phenomena like surface-band bending caused by chemisorption-induced surface states. Other parameters, for example, the number, size and morphology of grains forming the surface material, influences electrical response due to atomic diffusion and surface space charges. Several models have been proposed to explain the sensing properties in dispositives based in polycrystalline materials [5-6]. The evolution of surface roughness has been studied as a function of film thickness from theoretical [7] and experimental considerations [8]. The incorporation of fluorine in SnO2 films (SnO2:F) has been proposed by several authors for decreasing the electrical resistivity of the oxide; the effect of fluorine is readily noticed in the sheet resistance but for films prepared using the spray pyrolysis technique, the fluorine concentration in the films is not always proportional to the initial concentration in the starting solution. In order to gain more insight into the influence of the fluorine on the electrical properties the resonant nuclear reaction (RNR) method has been used and reported previously [9, 10]. In this work we report results of complementary studies of samples studied in reference [10] carried out using atomic force and electron microscopy methods. AA3.6.1
EXPERIMENTAL SnO2:F films were deposited by pneumatic spray pyrolysis on glass substrates at 300˚C for 15 minutes. The spray solution was SnCl4 dissolved in ethanol at a concentration of 0.2 M. Doping with fluorine
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