1. Time response and recovery characteristics of tin dioxide thin film to humidity H. Mustafa Çakmak*, H. Ali Çetinkara, Süleyman Kahraman and Samed Çetinkaya Department of Physics, Mustafa Kemal University, Hatay 31034, Turkey Abstract Thin film was deposited on quartz substrates by vacuum evaporation of tin metal. For the purpose of converting tin film to tin oxide (SnO 2 ), heat treatment procedure under O 2 flow was achieved at the temperature of 800 o C, for 2h. Morphological, structural and gas sensing properties of the film were investigated by using Scanning Electron Microscopy (SEM), X-Ray diffractometer (XRD). SEM measurements have revealed that deposited tin dioxide film is composed of micro/nanoparticles. The heat-treated film indicated two strong reflection peaks which can be indexed to a tetragonal SnO 2 crystal structure. Gas sensing characteristics of the film was investigated for different concentrations of humidity. It was found that the sensor is highly sensitive to humidity. Conclusion In summary, Tin dioxide film has been successfully deposited on quartz substrates by vacuum evaporation and subsequent heat treatment was performed under constant oxygen flow to obtain tin dioxide film. From the SEM images, the film has porous clustered structure which is an important parameter for sensing applications. It is revealed from the XRD patterns of the film that the film indicates two strong reflection peaks of tetragonal structure. It is shown from the gas sensing measurements; the film was highly sensitive to humidity. Thus, the film investigated is featured with reversible, reproducible response to humidity. Introduction Thin film structures for gas sensing applications have found wide usage area in industrial production, environmental monitoring and protection, etc. [1-3]. Among the investigated all gas sensors, tin dioxide based sensors attracted much interest since they can detect a wide variety of gases with high sensitivity, good stability and also low production cost [4,5]. SnO 2 is an n-type transparent semiconducting oxide with a wide band gap (E g =3.6 eV at 300 K) well known for its potential applications as optical filters, covering layers for fiber optical systems, catalysts, detecting elements of ecological monitoring sensors measuring the concentration of toxic and explosive gases in the atmosphere, photovoltaic devices [6,7],etc. In this study, we used physical vapor deposition technique to fabricate thin films of nano-sized tin dioxides. Experimental details Quartz glass was used as substrate which was cleaned. At first, to improve sensing properties of the film, inter-digitated gold electrodes were obtained on the substrate by using photolithography technique. Tin metallic film was achieved on the inter-digitated substrate in a vacuum evaporator (Handy 3S from Vaksis Company) at the pressure of 10 -5 Torr. Oxygen enrichment of the film was carried out at 800 o C for 2h under constant O 2 flow in a PROTHERM PTF 12/50/450 tube furnace. Results and discussion Morphological, structural and gas sensing properties of the film were investigated by using Scanning Electron Microscopy (SEM), X-Ray diffractometer (XRD) and gas sensing system. Fig. 1a. shows the SEM images that the coating is uniformly deposited on the surface and the film has porous clustered and aggregated structure. It is an important parameter for sensing applications that having porous structure offers very high surface to volume ratio, thus affects the sensitivity and selectivity of a sensor. As shown in Fig. 1b XRD patterns of the film indicates two strong reflection peaks of tetragonal structure in the orientations of (101) and (200) at 2θ = 33.89 o and 37.95 o. In our case both peak seen in XRD pattern can be indexed to a tetragonal SnO 2 crystal structure. (cassiterite, Joint Committee on Powder Diffraction Standards (JCPDS) card No. 41-1445, space group: P42/mnm, a 0 = 4.738 Å, c 0 = 3.1865 Å). No other peaks were detected in XRD pattern which ensured that the film under investigation is completely converted from metallic Sn film to a single phase SnO 2 film.Schematic diagram of the gas sensing set-up is shown in Fig. 2a. By using gas sensing system temperature and concentration dependant responses of the film to humidity was investigated. Gas sensing characteristics of the film was investigated for different concentrations of humidity. it was seen Fig. 2b that the sensor was highly sensitive to humidity. Referances [1] J.J. Ho, Solid- State Electron. 47 (2003) 827–830. [2] D.L. Cheng, L. Gao, J. Colloid Interface Sci. 279 (2004) 137-142. [3] S.K. Song, J.S. Cho,W.K. Choi, H.J. Jung, D.S. Choi, J.Y. Lee, H.K. Baik, S.K. Koh, Sens. Actuators, B, Chem. 46 (1998) 42-49. [4] H.C. Wang, Y. Li, M.J. Yang, Sens. Actuators B 119 (2006) 380-383. [5] M. Ivanovskaya, P. Bogdanov, G. Faglia, P. Nelli, G. Sberveglieri, A. Taroni, Sens. Actuators B 77 (2001) 268-274. [6] B.A. Akimov, A.V. Albul, A.M. Gaskov, V.Yu. Ilin, M. Labo, M.N. Rumyantseva, L.I. Ryabova, Phys. Tech. Semiconductors 31 (1997) 400. [7] Morgado, N. Barbagallo, A. Charas, M. Matos, L. Alcacer, F. Cacialli, J. Phys. D., Appl. Phys. 36 (2003) 434. Fig.1a.Fig.1b. Fig.2a. Gas sensing setup Fig.2a. Gas sensing characteristic *Corresponding author: mustafamku@gmail.commustafamku@gmail.com