Sensing characteristics of epitaxially-grown tin oxide gas sensor on sapphire substrate Dae-Sik Lee, Gi-Hong Rue, Jeung-Soo Huh, Soon-Don Choi, Duk-Dong Lee * School of Electronic and Electrical Engineering, Kyungpook National University, Taegu 702-701, South Korea Abstract Epitaxial SnO 2 ®lm was grown on a sapphire substrate using a reactive rf magnetron sputter. The microstructure of the thin ®lm was investigated using the 3C 2 beam line from a Pohang Light Source PLS) consisting of a 2 GeV electron accelerator, four circle X-ray diffractometer, atomic force microscopy AFM), and transmission electron microscopy TEM) comparing with the SnO 2 ®lm on the polished alumina. It was con®rmed that the thin ®lm grew epitaxially on the sapphire substrate with a variant crystal structure [Development of microstructure controlled and ultrathin SnO 2 gas sensing ®lm, in: Proceedings of the 5th International Meeting on Chemical Sensors, Rome, Italy, 1994]. No grain boundary was exhibited on the surface of the epitaxial thin ®lm. A large portion of the sorption sites in SnO 2 ®lms appears to react easily with the gas. An epitaxial SnO 2 gas sensor, with a Pt heater and electrodes, exhibited a high sensitivity to combustible gases and a particular sensitivity of 95% to alcohol at 2000 ppm and 3508C. The sensor also showed a good stability with small baseline drift and fast reaction and recovery times of about 5 and 30 s, respectively. This experiment con®rms the good gas-sensing characteristics of an epitaxially-grown SnO 2 gas sensor. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Gas sensor; Sapphire substrate; X-ray scattering; Tin oxide 1. Introduction Since 1962, SnO 2 -based gas sensors have become the predominant solid-state devices for gas alarms used on domestic, commercial and industrial premises [1]. A variety of ceramic, thick-®lm and thin-®lm devices have been reported over the years with mostly an empirical optimiza- tion of their performances. Even though the widespread applications and the success in empirical research, the understanding of chemical sensing and of the in¯uence of crystallization on the response of elementary steps of SnO 2 - based sensors is still immature. This understanding of sen- sing mechanisms, however, is a prerequisite for their further improvement using thin-®lm technology instead of ceramic preparation process. So, the current spectroscopic and microscopic investigations on well-characterized epitaxial ®lms are extremely important, for these have been done to determine their geometric and electronic properties, the interactions with gas molecules in adsorption, the formation of intrinsic point defects or higher dimensional defects at the surface, and their role as `active sites' in chemisorption and catalytic reactions. But there are less reported about the information for the epitaxial growth of SnO 2 ®lm. To date, some epitaxial SnO 2 ®lms which have different orientations and large domains i.e. with negligible concentrations of grain boundaries) have been prepared under ultra-high vacuum conditions [2,3]. These materials, however, have not been used for gas sensing devices. In this study, we report the results of a synchrotron X-ray scattering SXS) and atomic force microscopy AFM). From these studies we can obtain clear information of surface and layer structure, and some gas sensing-responses of epitaxial SnO 2 ®lm. The gas-sensing characteristics of the epitaxial SnO 2 ®lm to combustible gases are investigated. 2. Experimental Samples considered in this work were 30±300 nm-thick ®lms prepared by conventional reactive magnetron sputter- ing at 6008C. Film depositions were carried out in a mixture of pure 99.999%) argon Ar) and oxygen O 2 ) gases 99.999%) the ratio of 1:2 at partial pressure) with an rf Sensors and Actuators B 77 2001) 90±94 * Corresponding author. Tel.: 82-53-939-1073; fax: 82-53-939-1074. E-mail address: ddlee@ee.knu.ac.kr D.-D. Lee). 0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0925-400501)00678-5