JOURNAL OF MATERIALS SCIENCE 31 (1996) 6207-6213 Influence of additives on the morphological, phase and chemical characteristics of gas sensitive Sn02 sprayed films I. STAMBOLOVA, K. KONSTANTINOV BAS, Institute of General and Inorganic Chemistry, Acad. G. Bonchev Blvd., BL 11, 1113 Sofia, Bulgaria The morphology, and also the chemical and phase composition of sprayed SnO2 films doped with Cu, Zr, Fe and P have been investigated. It was found that in general, the film morphology was characterized by a fine grained basic layer with a few large crystallites located on the surface. All the doping elements decreased the grain size in the basic layer. For Fe, Zr and P the grain size depended on the doping level in the spray solution. It was established that the dopant:Sn ratio in the films was much lower than that in the spray solution. The doping element was preferentially accomodated in the large crystallites with only a small quantity of the dopant being introduced into the basic layer. The gas sensing properties of the films have are discussed on the basis of these results. 1. Introduction Thin films of SnO2 find application in gas sensors, photovoltaic and optoelectronic elements and many other electronic devices. It is known that charge trans- port across grain boundaries is responsible for the gas sensitivity of these films [1]. Therefore, the micro- structure of the films is one of the basic factors that determines the gas sensitivity. Currently, the addition of different metal oxides is used to modify the film properties since each individual doping element affects the film microstructure in a different way [2 I. The aim of the present study is to obtain a detailed understanding of the effect of different additives on the microstructure and also the phase and chemical char- acteristics of doped SnO2 thin films. Cu, Fe, Zr and P were chosen as doping elements. These elements increase gas and humidity sensitive properties of SnO2 thin films, which is important for their practical use. 2. Experimental procedures In order to prepare the spraying solutions a 0.2 M solution of SnC14 in ethanol was mixed with the following solutions of the doping elements: 0.2 M ZrCt4 in ethanol + H20 0.2 M Cue12 in ethanol 0.2 M P205 in ethanol 0.2 M Fe(NH4)(SO4)2" 12H20 in ethanol + H20 0.2 M Fe(C204)3 in ethanol + H20 0.025 M PdC12 in ethanol + H20 The sample codes and corresponding dopant:Sn ratios are listed in Table 1. The solutions were sprayed onto substrates, heated to temperatures of 450 500 ~ at regular time intervals of 10 s. The dis- tance between the spray nozzle and the substrate was about 50 cm. The substrates were oxidized silicon wafers and quartz tubes (d = 6 mm, 1 = 20 mm) or plates. The thermal treatment of the films was performed in an oxygen atmosphere. In all cases the samples were rapidly cooled to room temperature. The thermal treatment conditions are presented in Table II. The phase composition of the films was studied by X-ray diffraction (XRD) analysis, using CoK~ radi- ation. The morphology and chemical compositions were investigated by scanning electron microscopy (SEM), energy dispersive X ray spectroscopy (EDX), X ray photoemission (XPS) and Auger electron spec- troscopies (AES). 3. Results 3.1. Phase composition The XRD data show that in all cases the basic phase in the films is a-SnO2 (cassiterite). The investigated dop- ing elements in the SnO2 films can be classified into two groups: - the first group: the doping element (Cu, P) does not lead to the appearance of a secondary phase. The peaks of the cassiterite phase remain unshifted. - the second group: the doping element (Zr, Fe) leads to the appearance of a secondary phase (~-F%O3- hematite or tetragonal ZrOe, respectively). The XRD analysis of SnOa-FeaO3 films shows that SFC1, SFC2, SFS1 and SFS2 are monophase. The analysis indicates the presence of two phases - cas- siterite and hematite for the SFC3-SFC6 and SFS3-SFS6 films. However on the films obtained 002~2461 1996 Chapman & Hall 6207