TULE Research Programme Progress Report 2005 1 New materials and structures for semiconductor gas sensors (NEWGAS) Pekka Kuivalainen 1 and Vilho Lantto 2 . Abstract The aim of the present research project is to develop new advanced gas sensors with enhanced performance based on new materials such as epitaxial tin dioxide, porous silicon and layered tungsten trioxide together with new micromechanical air bridge structures. The research work utilizes the state of the art measurement systems for gas sensor materials based, e.g., on the Kelvin probe techniques, which allow in situ studies of the gas reactions at the sensor surfaces. The new epitaxial and layered semiconductor materials have been chosen to maximize the sensitivity and the long-term stability. On the other hand, the new micromechanical air bridge structures allow a significant reduction in power consumption, which is vitally important in portable applications. Both the growth of the new materials and the demanding measurement techniques are based on the previous experience of the present research consortium on the epitaxy of semiconductor thin films and the extensive studies of the polycrystalline tin dioxide as a gas sensor material. 1 Partners and Funding 1.1 Electron Physics Laboratory, Helsinki University of Technology The research group consists of the subproject leader professor Pekka Kuivalainen, senior researcher Dr. Sergey Novikov, and a postgraduate student Mikael Kroneld. 1.2 Microelectronics and Materials Physics Laboratories, University of Oulu The research group consists of subproject leader professor Vilho Lantto and postgraduate student Sami Saukko. The research has been done also in a close co- operation with the research group of Professor C.G. Granqvist at the Uppsala University (postgraduate students Luis Reyes and Peter Heszler and Dr. Anders Hoel), and with Professor Janos Mizsei from the Budapest University of Techology. Dr. Johannes Frantti from the Tokyo Institute of Technology has also participated in the research. 1 Electron Physics Laboratory, Helsinki University of Technology, P.O.Box 3500, FIN-02015, HUT- Finland 2 Microelectronics and Materials Physics Laboratories, University of Oulu, P.O.Box 4500, FIN-90014, Universtity of Oulu, Finland TULE Research Programme Progress Report 2005 2 1.3 Funding Table 1. Funding of the project in 1000 EUR in 2003-2006 Partner Funding Organisation 2003 2004 2005 (planned) 2006 (planned) Total HUT Academy 14.860 29.720 28.840 16.580 90 OU Academy 22.238 31.870 21.000 14.892 90 2 Research Work 2.1 Objectives and Work Plan The aim of the present research consortium is to develop and utilize new materials and structures to fabricate semiconductor gas sensors showing enhanced stability and reduced power consumption. They can be realized by using an epitaxial growth of semiconductor thin films and layered materials and by a proper choice of doping and catalyst elements as well as by integrating several sensing elements on one chip, and by reducing the size and layout of a heating element with micromechanical bridge structures allowing a decrease in power consumption. Also the gas sensor characterization techniques based on a Kelvin probe method will be developed and utilized in in situ measurements of the gas reactions at the semiconductor surfaces. 2.2 Progress Report: Common Themes and Collaboration The present research project has been going on since the beginning of July 2003. The main goal, which was to grow high quality gas sensing materials, has been achieved in the case of tin dioxide. By adjusting the growth parameters we are able to grow either polycrystalline nano-particle thin films or monocrystalline thin films. Extensive characterization of the grown thin films has been carried out including, e.g., reflection high energy electron diffraction (RHEED), X-ray diffraction (XRD), atomic force microscope (AFM), Hall-effect, and conductance measurements in close collaboration between the consortium partners, which has produced two joint publications [1 and 7]. Oxygen adsorption-desorption kinetics at the tin oxide surface has been studied experimentally using temperature-programmed desorption method together with the conductance measurements. Also simultaneous Kelvin probe and conductance measurements during heating and cooling in different ambient atmospheres were used to characterize the monocrystalline SnO 2 (101) surface after various surface treatments. Due to a cut in the planned budget the fabrication of the heated substrates was transferred to a TEKES project. However, these substrates are available also for the present project. TULE Research Programme Progress Report 2005 3 Our research results so far are very promising. X-ray diffraction results confirmed that all the sample films grown are highly oriented along the surface as only one peak in addition to the substrate could be seen in the scan. A comparison between samples of different crystalline quality is resented in Fig. 1. (a) (b) 100nm 200nm (c) (d) 0,95 1,15 1,35 1,55 1,75 1,95 2,15 2,35 2,55 2,75 200 300 400 500 600 700 800 900 Time / s G gas /G air 0,99 1,01 1,03 1,05 1,07 1,09 1,11 1,13 1000 1200 1400 1600 1800 2000 2200 2400 Time / s G gas /G air (e) (f) Fig. 1. A comparison between two samples grown at different temperatures; a) RHEED image of a clearly polycrystalline sample, b) RHEED image of a monocrystalline sample, c) and d) AFM images of the corresponding samples, e) and f) conductivity responses towards an exposure of 1000 ppm hydrogen at 400 °C. Three-dimensional diffraction spots and rings on the RHEED image obtained from sample in Fig. 1. a) indicate a rough surface and a polycrystalline nature of the film. TULE Research Programme Progress Report 2005 4 Sample grown at higher temperature (Fig. 1. b)) gives an image with elongated spots, which indicates that the electron beam hits a monocrystalline region. The AFM inspection confirms that the crystalline quality in the case of higher growth temperature is clearly better, see Fig. 1 c) and Fig. 1 d). As is evident from Fig. 1. e) and Fig. 1. f), the film of better crystalline quality shows lower response towards the test gas, which may be expected, due to the reduced number of grain boundaries. The rise time for the film of smaller grains is shorter, but the decrease in the response during the gas exposure is evident in the case of smaller grains. Results allow and encourage us to further optimize the growth parameters and thus the structural characteristics of the sensing films. So far the sample films have been grown on an r-cut sapphire substrate, with very encouraging results. To further optimise the growth, we have started growth experiments with sapphire substrates with a particular off-cut from the r-plane, which are to provide us even better film qualities due to the optimisation of three-dimensional growth. 2.3 Progress Report: Progress by the Electron Physics Laboratory, HUT We have succeeded in the growth of the single crystal tin dioxide thin films on sapphire substrates. This was one of the main goals in the project plan. The samples have been grown by using molecular beam epitaxy, MBE. For comparison also polycrystalline tin dioxide thin films have been grown. Altogether the number of succesful sample growths is 16 at the present. For these samples the following measurements have been carried out: Resistivity vs. temperature, Hall-effect (carrier concentration) vs. temperature, AFM-measurements to determine the grain size in the thin films, and X-ray diffraction measurements to check the crystallinity of the samples. At Oulu University we have carried out gas response measurements for hydrogen, sulphur hydrogen and carbon monoxide. Based on the results of these measurements we can state that we have succeeded in the growth of the single crystal tin dioxide, and the samples’ resistivity reacts strongly to the gas exposure. The measured sensitivity is higher in polycrystalline samples, but the stability seems to be better in single crystal thin films. This remains to be verified during the project. Recently we have fabricated sensor material including catalyst such as Pt. A new gas measurement system has been built at HUT, which includes gas sources for over ten different gases and a new scanning Kelvin probe to measure the gas responses of the sensor materials. This system will be utilized during the latter part of the project. 2.4 Progress Report: Progress by the Microelectronics Laboratory, OU Traditionally, semiconductor gas sensors, and in particular gas sensors using SnO 2 as a sensing material, are based on polycrystalline thin or thick films. Sensing mechanisms of these types of sensors have been widely studied and are quite well understood. The TULE Research Programme Progress Report 2005 5 novel approach is to use monocrystalline SnO 2 thin film as the sensing layer. The intergrain potential barriers that are mostly responsible for the sensing mechanism in polycrystalline films are missing in monocrystalline films. With monocrystalline structure, however, it may be possible to overcome some of the problems that are typical for polycrystalline sensors such as long term stability, response and recovery rates and variation in electrical properties from sensor to sensor, especially in the case of thick film sensors. We have studied different surface phenomena on SnO 2 surfaces using both monocrystalline and polycrystalline samples using various experimental techniques, such as TPD, surface potential (Kelvin probe) and conductance measurements in different ambient conditions in order to understand better the sensing mechanism in monocrystalline films. So far, the results show that the monocrystalline film behaves very differently compared to the polycrystalline samples. Stability with both time and temperature is better and overall characteristics of the different sensors are more alike. Even though the premilinary testing shows that the sensitivity is lower, as expected, in the case of monocrystalline films as compared to the polycrystalline films, the increased overall stability of the new structure makes it, as a whole, a promising structure for gas sensors. In the last part of the project we will do more characterizations for the films and try to improve the sensitivity and selectivity using different catalyst materials and film structures. Monocrystalline film makes it also easier to study the possible influence of electrodes on the sensitivity and selectivity. In addition, we have been able to improve the excellent sensitivity of WO 3 -based sensors towards H 2 S at room-temperature operation by modifying the microstructure of the sensing layer using different growth parameters. Further, by adding small amounts of noble metal nanocatalysts into the sensing layer we were able to improve the sensitivity down to the sub ppm level at H 2 S exposure [3]. Research with porous silicon has also continued in the Microelectronics Laboratory at OU and some test measurements on the gas response of these samples have been also done. 3 International Aspects The new scanning Kelvin probe applicable in a gas atmosphere was planned together with a Hungarian group headed by Dr. Tibor Pavelka at Semilab Ltd. During the project Professor Janos Mizsei from the Budapest University of Technology has visited both the Microelectronics Laboratory at OU and the Electron Physics Laboratory at HUT. As a result a joint publication has been written ([1]; see the list below). There has been also a close research co-operation in the research with tetragonal nanocrystalline WO 3 samples between researchers in the Microelectronics Laboratory at OU and in the Ångström Laboratory at the Uppsala University (research group of Professor Granqvist). Dr. Johannes Frantti from the Tokyo Institute of Technology has participated also in the experimental characterization of the structure of the WO 3 sensing films using Raman spectroscopy. TULE Research Programme Progress Report 2005 6 4 Publications and Academic Degrees Table 2. Publications and academic degrees produced in the project. Partner Type of publication 2003 2004 2005 Total Publication numbers HUT Ref. journal art. - - 1 1 1 Ref. conf. papers - - 1 1 7 Monographs - - - - - Doctoral dissert. - - - - - Licentiate degrees - - 1 1 9 Master degrees - - - - OU Ref. journal art. - 4 2 6 1,2,3,4,5,6 Ref. conf. papers - 1 1 2 7,8 Monographs - - - - Doctoral dissert. - 1 - 1 10 Licentiate degrees - - - - Master degrees - - - - 5 Other Activities The results of the project have been presented to Dr. Mikko Utriainen at Environics Ltd. in several meetings. 6 Publications 6.1 Refereed Journal Articles [1] S. Saukko, U. Lassi, V. Lantto, M. Kroneld, S. Novikov, P. Kuivalainen, T.T. Rantala, and J. Mizsei, Experimental studies of O 2 -SnO 2 surface interaction using powder, thick film and monocrystalline thin films, Thin Solid Films 2005. In press. [2] L.F. Reyes, S. Saukko, H. Hoel, V. Lantto, and C.G. Granqvist. Structure engineering of WO 3 nanoparticles for porous film applications by advanced reactive gas deposition. Journal of European Ceramic Society, 24, Issue 6:1415–1419, 2004. TULE Research Programme Progress Report 2005 7 [3] A. Hoel, L. F. Reyes, S. Saukko, P. Heszler, V. Lantto, and C. G. Granqvist. Gas sensing with films of nanocrystalline WO 3 and Pd made by advanced reactive gas deposition. Sensors and Actuators B: Chemical, 105(2):283–289, 2005. [4] V. Lantto, S. Saukko, N.N. Toan, L.F. Reyes, and C.G. Granqvist. Gas sensing with perovskitelike oxides having ABO 3 and BO 3 structures. Journal of Electroceramics, 13:721– 726, 2004. [5] A. Hoel, L. F. Reyes, P. Heszler, V. Lantto, and C. G. Granqvist. Nanomaterials for environmental applications: novel WO 3 -based gas sensors made by advanced gas deposition. Current Applied Physics, 4(5):547–553, 2004. [6] L. Reyes, S. Saukko, A. Hoel, V. Lantto, and C.G. Granqvist. Improved gas response at room temperature of activated nanocrystalline WO 3 films. Physica Scripta, T114:240–243, 2004. 6.2 Refereed Conference Papers [7] M. Kroneld, S. Novikov, S. Saukko, P. Kuivalainen and V. Lantto: “Gas sensing properties of SnO 2 thin films grown by MBE”. Abstract submitted to Eurosensors'05 (a full length manuscript also ready). [8] L.F. Reyes, S. Saukko, A. Hoel, V. Lantto and C.G. Granqvist, H 2 S sensing with nano- structured WO 3 films in different oxygen atmospheres, Proc. (CD-ROM) 17 th European Conference on Solid-State Transducers (Eurosensors XVII), Sept. 21-24, 2003, Guimaraes, Portugal, pp 859-860. 6.3 Monographs 6.4 Doctoral, Licentiate, and Master Theses [9] M.Kroneld, Gas sensing properties of mono- and polycrystalline tin dioxide thin films grown by MBE, HUT 2005, 90 pages. (Licentiate thesis) [10] A. Hoel, Electrical Properties of Nanocrystalline WO 3 for Gas Sensing Applications, Acta Universitatis Upsaliensis. Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 948. Uppsala (2004) 86 pp. (Doctoral thesis) 7 Other Outputs  An invited lecture with the title “On some structural effects of transition metal oxides with relevance to gas sensing: WO 3 as an example oxide” was given by V. Lantto 12.2.2004 in the Yamazoe Laboratory at the Kyushu University, Fukuoka, Japan.  An oral lecture with the title “Gas sensing with nanocrystalline tetragonal WO 3 films” was given by V. Lantto in the 9 th International Conference on Electroceramics & their Applications, 31.5 3.6.2004, Cherbourg, France.  An invited plenary lecture with the title “Some structural and electronic aspects on gas sensing with transition metal oxides” was given by V. Lantto in the Opening Session of the Semiconductor Gas Sensors (SGS´2004) Seminar, 19 22.9.2004, Ustron, Poland. . Progress Report 2005 1 New materials and structures for semiconductor gas sensors (NEWGAS) Pekka Kuivalainen 1 and Vilho Lantto 2 . . Objectives and Work Plan The aim of the present research consortium is to develop and utilize new materials and structures to fabricate semiconductor gas sensors