Porosity characterization of ZnO and ZnO:Al thin films

BOJORGE C. D.; HEREDIA E. A; CASANOVA J. R.; KELLERMANN G.; CÁNEPA H.R.; CRAIEVICH A.; MAROTTI R.E

Buenos Aires

Congreso; 15th International Meeting on Chemical Sensors; 2014

The attracting chemical or physical properties and the multiple ways to obtain nanostructured ZnO for potential technological applications, have stimulated considerable research interests in the past decade. Different methods can be used to obtain pure and doped ZnO films: sol?gel, spray pyrolysis, electrodeposition, chemical deposition, magnetron sputtering, vapour phase epitaxy, etc. [1-4]. This semiconductors is used in a wide variety of technological applications including sensor devices [5-7], piezoelectric materials [8], electroluminescent devices, optical waveguides and as a catalyst [9]. Conductive gas sensors using thin films of n-type zinc oxide are based on the fact that the electrical characteristics of semiconducting metal oxide materials depend on the composition of the surrounding gas atmosphere. Besides, microstructure of the material, modifications in surface geometry and addition of suitable dopant (commonly Al, In, Cu, Sn, etc.) influences the performance of the gas sensor [10]. The microstructural and physical properties of ZnO can be modified by introducing changes into the procedure of its synthesis. Especially for the application of ZnO as gas sensors, porous microstructure of the materials with controlled pore size is sought [11]. The sensitivity and response time of ZnO based sensors strongly depend on the porosity of the material: these pores are considered to play a vital role in the process of gas adsorption as the response of such sensors is based on chemisorption, i.e. exchange of charges between adsorbed gaseous species and the metal oxide surface [12]. The grain size of the polycrystalline zinc oxide material also has remarkable effect on its gas sensing properties: sensitivity decreases with the increase of its mean grain size [13]. In the present work, pure and Al-doped ZnO thin films are synthesized by using sol-gel method and depositing by spin-coating technique upon silica substrates. Samples were characterized by XR a GISAXS techniques in LNLS synchrotron, Campinhas, Brazil. Average density value of the films was determined by direct analysis of the critical angle in XR diagrams. GISAXS diagrams were analyzed using IsGISAXS software, determining the size and shape of the pores and verifying the critical angle by the Yoneda peak position. Porosity values determined from the average density by XR and GISAXS techniques and were compared with those obtained by transmittance measurements.