Sensors and Actuators B 153 (2011) 37–43 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb A route to high sensitivity and rapid response Nb 2 O 5 -based gas sensors: TiO 2 doping, surface embossing, and voltage optimization Hi Gyu Moon a,b , Ho Won Jang a,∗ , Jin-Sang Kim a , Hyung-Ho Park b , Soek-Jin Yoon a,∗∗ a Electronic Materials Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea b Department of Material Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea article info Article history: Received 20 July 2010 Received in revised form 8 September 2010 Accepted 1 October 2010 Available online 14 October 2010 Keywords: Nb2O5 Gas sensors TiO2 doping Embossed Voltage optimization abstract We report a novel route for the fabrication of highly sensitive and rapidly responding Nb 2 O 5 -based thin film gas sensors. TiO 2 doping of Nb 2 O 5 films is carried out by co-sputtering without the formation of secondary phases and the surface area of TiO 2 -doped Nb 2 O 5 films is increased via the use of colloidal templates composed of sacrificial polystyrene beads. The gas sensitivity of Nb 2 O 5 films is enhanced through both the TiO 2 doping and the surface embossing. An additional enhancement on the gas sen- sitivity is obtained by the optimization of the bias voltage applied between interdigitated electrodes beneath Nb 2 O 5 -based film. More excitingly, such a voltage optimization leads to a substantial decrease in response time. Upon exposure to 50 ppm CO at 350 ◦ C, a gas sensor based on TiO 2 -doped Nb 2 O 5 film with embossed surface morphology exhibits a very high sensitivity of 475% change in resistance and a rapid response time of 8 s under 3 V, whereas a sensor based on plain Nb 2 O 5 film shows a 70% resistance change and a response time of 65 s under 1 V. Thermal stability tests of our Nb 2 O 5 -based sensor reveal excellent reliability which is of particular importance for application as resistive sensors for a variety gases. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Recently tremendous efforts have been devoted to develop semiconducting metal oxide gas sensors for applications to toxic gas detection, air quality management, and environmental moni- toring due to the simple operation principle, low cost, small size, and good compatibility with standard microelectronic processing [1–6]. Compared with optical sensors and electrochemical sensors, metal oxide gas sensors have good sensitivity to some relevant gases like CO, NO x ,H 2 and hydrocarbons [7], but possess relatively low selectivity to a specific target gas [8]. Sufficient selectivity of metal oxide gas sensors can be achieved by the use of an array composed of different sensing elements [9]. For the reliable opera- tion of such a sensor array, each sensing element should have high sensitivity, rapid response and recovery time, and good stability [7]. Although various metal oxides such as SnO 2 , ZnO, TiO 2 , WO 3 , In 2 O 3 , Fe 2 O 3 , CuO, NiO, Ga 2 O 3 , and V 2 O 5 have been demonstrated for the sensing elements [10], few reports addressed the feasibil- ity of Nb 2 O 5 for semiconductor gas sensors recently [11–13]. Since Nb 2 O 5 is an n-type semiconductor with a bandgap of ∼3.4 eV [14] and shows good conductivity with high concentration of oxygen ∗ Corresponding author. Tel.: +82 2 958 6728; fax: +82 2 958 6720. ∗∗ Corresponding author. Tel.: +82 2 958 5550; fax: +82 2 958 6720. E-mail addresses: hwjang@kist.re.kr (H.W. Jang), sjyoon@kist.re.kr (S.-J. Yoon). vacancies capable of trapping electrons [15], a reliable gas sensing performance which is comparable or superior to that of sensors based on SnO 2 and TiO 2 can be expected from gas sensors based on Nb 2 O 5 . Thus, to exploit Nb 2 O 5 as a sensing element of the sensor array, an experimental demonstration of high-quality Nb 2 O 5 -based gas sensors is required. In this work, the fabrication of highly sensitive, rapidly respond- ing, and thermal stable gas sensor based on Nb 2 O 5 thin films is presented. Through TiO 2 doping by co-sputtering, surface emboss- ing using polystyrene beads, and the optimization of applied bias voltage, both sensitivity and response time of Nb 2 O 5 thin-film gas sensors are improved remarkably. Furthermore, the TiO 2 -doped Nb 2 O 5 gas sensors exhibit excellent thermal stability at the opera- tion temperature of 350 ◦ C. From these results, it is suggested that our Nb 2 O 5 -based gas sensors are promising elements for the metal oxide sensor array of selective gas detection. 2. Experimental Nb 2 O 5 thin films used in this work were deposited on SiO 2 /Si substrates by room-temperature rf sputtering using a single-phase Nb 2 O 5 target. The base pressure, working pressure, rf power, and gas flow rate were 2 × 10 -6 mTorr, 10 mTorr, 100 W, Ar 30 sccm, respectively. For TiO 2 doping, TiO 2 was co-sputtered using a single- phase TiO 2 target during the deposition. Doping concentrations of 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.10.003