JOURNAL OF MATERIALS SCIENCE LETTERS 17 (1998) 625±627 A new method of preparing tin dioxide thin ®lm gas sensors A. K. MUKHOPADHYAY, P. MITRA, A. P. CHATTERJEE, H. S. MAITI Central Glass and Ceramic Research Institute, Calcutta 700 032, India In recent times, considerable research activity has been devoted to the development of tin dioxide (SnO 2 ) based gas senors with porous microstructure in the forms of sintered pellets, thick ®lms and, most importantly, thin ®lms [1±5]. SnO 2 thin ®lms have been prepared by sputtering, ion-assisted reactive evaporation, chemical vapour deposition, spray pyrolysis sol±gel, chemical deposition and other techniques [3±9]. Of these, chemical deposition offers the lowest cost of production, easiest control over ®lm thickness and a porous microstructure essential for promotion of chemisorption, which induces gas sensing. A minimum operating tempera- ture of ,200±400 8C is often reported for SnO 2 sensors exposed to toxic and combustible gases [10, 11]. We have recently reported the preparation of zinc oxide (ZnO) thin ®lm gas sensors fabricated by a modi®ed chemical deposition technique [12]. These sensors were found to have high sensitivity at operating temperatures as low as 150 8C [12, 13]. Here, we report for the ®rst time the structural, electrical and gas sensitivity characteristics of phase pure SnO 2 thin ®lms fabricated by a novel, low-cost, modi®ed chemical deposition technique. The tin dioxide thin ®lms were deposited on precleaned substrates. Commercial microscope glass slides were used as substrate. The substrate was successively dipped in a 0.025 M sodium sulphide (Na 2 S) bath kept at room temperature and a 0.025 M stannous chloride (SnCl 2 ) bath kept at ,80 8C. Thus, the reaction SnCl 2  Na 2 S  SnS  2NaCl most likely took place on the substrate to induce isolated nuclei formation. The sequence was repeated suf®- cient times to affect isolated nuclei formation, followed by localized build-up, collapse and ®lm growth. The brown layer of tin sulphide (SnS) formed on the substrate was adherent in nature, as also noted by others [14]. This SnS layer was annealed in air for a suitable period at ,400 8C to induce thermal oxidation, SnS  2O 2  SnO 2  SO 2 . This led to the formation of porous SnO 2 thin ®lms with a matt white appearance. Typically, the rate of ®lm growth was ,0.3 ìm=dip=mol up to 50 dips, where the linear trend in deposition kinetics terminated. Further details of the deposition kinetics are reported elsewhere [15]. Film thickness was measured gravimetrically, assuming a theoretical density of 6.95 g cm 3 for SnO 2 . For a 25-dip ®lm, a typical estimation gave a ®lm thickness of ,0.22 ( 0.04) ìm. X-ray diffraction (XRD) con®rmed the phase purity of these SnO 2 ®lms (Fig. 1). The (1 1 0) plane has the highest intensity, as expected for a cassiterite crystal structure. The other major planes are (1 0 1), (2 0 0) and (2 1 1). Scanning electron microscopy (SEM) revealed that the microstructure consisted of sub-micrometre crystallites (Fig. 2). The conductance (ó) against temperature (T) data of 0.22 ìm thick ®lm (4 cm 3 3 cm) were obtained in a closed quartz tube furnace in air using the conventional two-probe method with Ag contacts (diameter ,1 mm, length ,20 mm, separation ,3 mm) following [12, 13]. The temperature range studied was 27±177 8C (300±450 K) with a control accuracy of 1 8C. The data show a two-stage conductance (Fig. 3): ó  ó 1 exp (E 1 = kT )  ó 2 exp ( E 2 = kT ) (1) with activation energies E 1  0:04 eV in the low (300±370 K) and E 2  0:52 eV in the high (370± 450 K) temperature ranges. In Equation 1, ó 1 and ó 2 are pre-exponential factors for the low and high temperature ranges, respectively, and k is Boltz- mann's constant. The observed values of the activation energy barriers match favourably with 0261-8028 # 1998 Chapman & Hall Intensity (arbitrary units) 10 20 30 40 50 60 70 2θ (degrees) (110) (101) (200) (211) Figure 1 XRD spectra of a 0.22 ìm SnO 2 thin ®lm deposited by the new, modi®ed chemical deposition technique. Figure 2 Typical SEM of a 0.22 ìm SnO 2 thin ®lm deposited by the new, modi®ed chemical deposition technique. 625