Fabrication and gas sensing properties of a-Fe 2 O 3 thin ®lm prepared by plasma enhanced chemical vapor deposition PECVD) Eun-Tae Lee a,* , Gun-Eik Jang a , Chang Kyo Kim b , Dae-Ho Yoon c a Department of Materials Engineering, Chungbuk National University, Cheongju, Chungbuk 361-763, South Korea b School of Information Technology Engineering, Soonchunhyang University, Asan, Choongnam 336-745, South Korea c School of Metallurgical and Materials Engineering, Sungkyunkwan University, Suwon, Kyunggi 440-746, South Korea Abstract Pure and Sn-doped a-Fe 2 O 3 thin ®lms were deposited on Al 2 O 3 substrate by plasma enhanced chemical vapor deposition PECVD) process using FeCO) 5 and SnCl 4 as a source materials. The a-Fe 2 O 3 phase was most stable in the deposition temperature range from 80 to 1208C and exhibited a relatively high sensitivity to i-C 4 H 10 and CO. On the other hand, a-Fe 2 O 3 thin ®lm was almost insensitive to CH 4 regardless of heat treatment and gas concentration. This paper describes the fabrication procedure and its sensing characteristics are presented. Particularly, the sensing characteristics of pure a-Fe 2 O 3 and Sn doped a-Fe 2 O 3 have been compared. # 2001 Elsevier Science B.V. All rights reserved. Keywords: a-Fe 2 O 3 thin ®lm; Sensitivity; Sn doped a-Fe 2 O 3 ; Gas; PECVD 1. Introduction Thin ®lm technologies offer several advantages such as low cost production and higher reliability which is well known in microelectronics. Among the widespread use of thin ®lm gas sensors, one of the most promising candidates is thin ®lm SnO 2 based gas sensors [1,2]. However, their instability problems prevent the thin ®lm devices from working as successful sensors. In recent years, a-Fe 2 O 3 draws much attention from the viewpoints of its application as a semiconductor gas sensor. This interest is connected with the properties of a-Fe 2 O 3 which offered a high sensi- tivity to some reducing gases without the application of noble metal catalysts. Pure a-Fe 2 O 3 with the corundum structure is a very stable oxide in nature and it was generally considered to have only a poor gas sensitivity. So far a lot of research works have been carried out to improve the selec- tivity and sensitivity by applying catalysts and promoters [3±6]. The a-Fe 2 O 3 prepared from a solution of iron sulfate by chemical precipitation from mixed solutions containing quadrivalent metal ions such as Si, Zr, Ti and Sn, has been found to be gas sensitive to combustible gases including CH 4 ,H 2 and CO. This sensitivity is considered to be due to ®ne crystallizes and low crystallinity of a-Fe 2 O 3 induced by the existence of residue of these metals. Furthermore, a high sensitivity to ethanol gas was found to nanostructured SnO 2 ± a-Fe 2 O 3 composite prepared by high energy ball milling [7]. Those results suggested that microstructure plays an impor- tant roles in elucidating the basic sensing mechanism of a-Fe 2 O 3 . Based on our previous reports [8], plasma enhanced chemical vapor deposition PECVD) technique has the great advantage to control the microstructure of nanocrystalline at relatively low temperature. In the present study, the thin ®lm types of a-Fe 2 O 3 ±SnO 2 composite were fabricated by PECVD method. The pre- paration of the a-Fe 2 O 3 ±SnO 2 composite and the relations between the microstructure and the gas sensitivities to i-C 4 H 10 , CO and CH 4 gases will be discussed. 2. Experimental 2.1. Film deposition Pure and Sn doped a-Fe 2 O 3 thin ®lms were prepared on Al 2 O 3 substrate by inductive coupling type of PECVD process. The schematic diagram of the PECVD reaction system is shown in Fig. 1. Al 2 O 3 substrates was initially cleaned by a acetone in a supersonic wave cleaner and the dried by N 2 gas in a chamber. Following that, Ar ion etching under the power input 50 W was performed on the substrate Sensors and Actuators B 77 2001) 221±227 * Corresponding author. 0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0925-400501)00716-X