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Unformatted text preview: I NSTITUTE OF PHYSICS PUBLISHING N ANOTECHNOLOGY Nanotechnology 17 (2006) S291S297 doi:10.1088/0957-4484/17/11/S11 Metal-catalysed, bridging nanowires as vapour sensors and concept for their use in a sensor system T I Kamins, S Sharma, A A Yasseri, Z Li and J Straznicky Quantum Science Research, Hewlett-Packard Laboratories, Palo Alto, CA 94304, USA E-mail: kamins@hp.com Received 19 December 2005, in final form 6 February 2006 Published 19 May 2006 Online at stacks.iop.org/Nano/17/S291 Abstract Metal-catalysed silicon nanowires were grown between silicon electrodes and exposed to vapours containing HCl or NH 3 at reduced pressure. Charge from adsorbed vapour modulated the conductance of the nanowires by changing the number of mobile carriers. Exposing the nanowires to HCl vapour increased the conductance while exposure to NH 3 vapour decreased the conductance. The observed results suggest the use of an array of nanowire sensors integrated with silicon electronics. Preliminary area estimates indicate that integrated amplification and signal processing is feasible for an array of 1000 sensors. (Some figures in this article are in colour only in the electronic version) 1. Introduction Detecting small quantities of gases and chemicals is becoming increasingly important for consumer, health, and security applications. Nanowires are especially attractive as sensors. The high surface area and small volume of the nanowires allows their use as highly effective field-effect sensing elements [ 15 ]. In a field-effect sensor, charge from an adsorbed or nearby analyte induces compensating charge in the nanowire, modulating its conductance and, consequently, the current flowing between two electrodes. The device, therefore, acts as a chemically sensitive field-effect transistor, with adsorbed charged species acting as the gate. Nanowire sensors can be very sensitive because a significant fraction of the carriers in the nanowire can be depleted so that a small number of carriers induced by the analyte changes the conductance significantly. However, using the nanowires requires that they be connected to electrodes that can interface with other circuitry. The size mismatch between the nanoscale wires and conventional electrodes has impeded their adoption. Searching for and making connection to individual nanowires is not cost effective; a massively parallel connection technique is needed. In addition, an intimate, low-resistance connection is essential, so that small changes in conductance of the nanowires can be accurately sensed, and contact resistance does not limit the accessible sensitivity. Previous reports [ 6 , 7 ] have demonstrated a platform in which metal-catalysed, self-assembled silicon nanowires are grown between two electrically isolated electrodes so that intimate contact is made between the nanowires and microscale electrodes during growth. Because the connection is between Si nanowires and heavily doped Si electrodes of the same conductivity type, good electrical connection...
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