CHARACTERIZATION AND FIELD EMISSION PROPERTIES OF FIELDS OF NANOTUBES Martin MAGÁT a, Jan PEKÁREK, Radimír VRBA a Department of microelectronics, The Faculty of Electrical Engineeering and Communication, Brno University of Technology, Technická 3058/10, Brno, Czech Republic, magat@feec.vutbr.cz Abstract There is currently considerable interest in the development of field-emitting cathodes, for a variety of potential microelectronic and sensoric applications. In particular, much effort is being devoted to the fabrication of CNTs arrays as field emitters for use in NEMS pressure sensors. We study a possibility of using the cold emission principle in pressure sensors. And we discovered that can be possible. Keywords: field emission, nanotubes 1. MOTIVATION Most of the research in this area has focused on the deposition of metal cones onto silicon substrates using photo-lithographic techniques, although a variety of other possible fabrication methods have been proposed [1] The high conductivity, sharp tips and long, narrow shape of carbon nanotubes suggested to a number of groups that they might make useful field emitters, and some quite promising results have been achieved [2]. In thermionic emission and photoemission electrons are given sufficient energy to overcome the potential barrier at the metal surface. In field emission (also called as cold emission), on the other hand, the barrier is deformed so strongly that unexcited electrons can leak out through it. A physical representation of the field emission is very important. The field emission or Fowler Nordheim tunneling is the process whereby electrons tunnel through a barrier in the presence of a high electric field. [3] The motivation behind these studies is to explore the possibility of using individual nanotube field emitters in cathode ray tubes or as electron emitters in NEMS pressure sensors. In this application is used a two electrodes configuration. A schematic illustration of measuring method is shown on Fig. 1. anode J emited current electrons potential cathode Fig. 1 Schematic illustration of the method to measure the field emission properties. 2. MEASURING INSTRUMENT For measuring the cold emission current a simple measuring instrument was been made. Schematic figure of this instrument is shown on Fig. 2. A final construction of this measurement is shown on Fig. 3. On this instrument are silicon electrodes which are glued on with electronic conductive lacquer. The self measuring of the cold emission must been realized in vacuum chamber on pressure about 10-3 Pa. Because in lower pressures is an ion emission appeared [4].
electrodes adjustable gap isolation nanomanipulator Fig. 2 Schematic illustration of the cold emission measuring instrument. Fig. 3 The cold emission measuring instrument with electrodes. Measuring workplace is shown below. The workplace consists of vacuum chambers containing the measuring instrument with measured samples, turbomolecular pump, resources, and not to tell devices connected via GPIB to the computer and electronics to control nanomanipulator. Fig. 4 The cold emission measuring workplace. 3. EMITING ELECTRODES In our experiment, CNTs were synthesized using plasma enhanced chemical vapor deposition on the silicon wafer with patterned iron catalytic layer. The typical deposition conditions were: flow rates of argon, methane and hydrogen Q Ar = 1000 sccm, Q CH4 = 50 sccm and Q H2 = 200 to 300 sccm, respectively, microwave (mw)
power of 400 W, substrate temperature 900 to 1100 K, deposition time 1 minute. Thin CNTs with a diameter of about 100 nm were standing vertically perpendicular to the substrate due to a crowding effect. A detailed study of the microwave torch for deposition of CNTs and their characterization were published in Ref.[5], [6]. Fig. 5 Carbon nanotubes field edge. 4. RESULTS First, measuring were taken without nanotubes on the electrodes, to determine the value of the voltage off ending occurs without the emission of nanotubes. In this measurement, the potential is set up to 150 V without having to measure any emission current. Measurements were performed for two electrode distances and 100 um and 80 um. When both distance measurement was performed several times with the same result. The measurement consisted in measuring the resulting emission current when changing voltage. Graphs of these dependencies are shown below. Fig. 6 The measured emission current Measurements with carbon nanotubes were performed in voltage 10 V, because there was a higher voltage leads to the degradation of nanotubes field.
Fig. 7 Emitted current from degradated carbon nanotubes (from 15 V to 20 V). The voltage range of 10 V to 15 V there was a nanotube array degradation (Fig. 7), but degradation of emission properties was not permanent. The voltage range of 15 V to 20 V was already observed a significant reduction in emission. A voltage in the range of 20 V to 25 V was observed (Fig. 8) as early as the complete destruction of structures with carbon nanotubes, the samples at voltages above 21 V stopped current emission. Fig. 8 Permanent destruction of carbon nanotubes (Voltage range from 15 V to 20 V). Samples were checker after measurement and if they were permanent loss of emission characteristics, the field of carbon nanotubes loss was observed. 5. CONCLUSION This article describes a preparation of cold emission measuring and construction of cold emission measuring instrument. By measuring has been shown that it is possible to use carbon nanotubes field in NEMS
pressure sensors. There were found working potential value and the measured emission current values for the two electrode distances. It was verified that the chase of distance greatly affects the resulting emission current and the foot of this principle used in combination with a silicon membrane that has been counted and made for another pressure sensor [7]. ACKNOWLEDGEMENT This research has been supported by Czech Ministry of Education within the framework of Research Plan MSM0021630503 MIKROSYN New Trends in Microelectronic Systems and Nanotechnologies, GA ČR project 102/08/1116, MPO MEMS project 2A-1TP1/143 Research of new mechatronic MEMS structures applicable for pressure measurement and by Czech Science Foundation under GA102/09/1601 Intelligent micro and nanostructures for microsensors realized with support of nanotechnology (IMINAS). LITERATURE [1] Brodie I., Spindt C. A.: Vacuum microelectronics, Adv. Eletron. Electron Phys., page 83, 1992 [2] Service R. F.: Nanotubes show image-display talent, Science, page 270, 1119, 1995 [3] Guozhong C.: Nanostructures and nanomaterials, Synthesis, Properties, and Application. London: Imperial Colledge Press, 425 pages, 2005. ISBN 1-86094-415-9 [4] Gomer R.: Field emission and field ionization. New York: American Institute of Physics, 1993, ISBN 1-56396-124-5 [5] Pekarek J., et al.: Electrodes modified by carbon nanotubes for pressure measuring, 32nd International Spring Seminar on Electronics Technology, 2009, pp. 629-633. [6] Zajickova, L. et al.: Synthesis of carbon nanotubes by plasma enhanced chemical vapor deposition in an atmospheric-pressure microwave torch, Pure and Applied Chemistry, Vol. 82, No. 6, pp. 1259-1272. [7] Magát, M.; Vrba, R.; Pekárek, J.; Ficek, R.: Capacitive pressure sensor modelling. In The Second International Conference on Advances in Circuits, Electronics and Micro-electronics, CENICS 2009, 11-16 October 2009, Sliema, Malta. Sliema, Malta: IEEE computer society, 2009. page. 81-85. ISBN: 978-0-7695-3832-7.