Project report IMAGING SILICON NANOWIRES PHY564 Submitted by: 1
Abstract: Silicon nanowires can be easily integrated with conventional electronics. Silicon nanowires can be prepared with single-crystal structures, diameters as small as several nanometers and controllable hole and electron doping, and thus represent powerful building blocks for nanoelectronics devices such as field effect transistors. Nanowires can be fabricated in various ways either using a top-down approach with lithography and reactive ion etching or by bottom-up techniques withcvd growth [4,5,6]. Silicon nanowires are made up of 1.5 nanometerdiameter silicon nanodots, nanodots are self assembled into nanowires. The most common length of nanowires is around 10 nanometers. Experimental setup consists of : A laser source to generate light source is used. A single mode fiber optic cable is used for imaging silicon wires. The objective of project is to get the NSOM image of silicon nanowires 100 microns thick and 3-10 microns long spread on a glass substrate. Introduction: Silicon nanowires are perfect at least atomically. Down at the single-atom level, the identical wires have no bumps, bends, or other imperfections. They are perfectly crystalline, even more so than bulk silicon. The full array of nanowires is also highly parallel, and each wire is an excellent metallic conductor [1]. Even though the silicon nanowires are perfectly atomic they are not symmetrical, one side of each nanowire is effectively shorter than other side. Asymmetry is a result of substrate, the substrate consists of two very thin layers of silver each only a single atom thick. The two layers give the substrate a grooved appearance, with the top layer of silver atoms forming regularly spaced 2
lines over the bottom layer. Due to the complex electronic interactions that can occur between silicon and silver atoms, the silicon atoms on silver can selfassemble into ordered structures in this case, nanowires. But those interactions, coupled with the grooves on the silver substrate, also produced the nanowire asymmetry [1]. Silicon nanowires: Silicon nanowires can be easily integrated with conventional electronics. A silicon nano wire can be grown laterally from a vertical surface formed by etching the top silicon layer of a silicon-on-insulator structure into isolated electrodes. Field effect structures can be readily built in silicon nanowires. As the the ratio of surface to volume in a thin nanowire is high, conduction through the nanowire is very sensitive to surface conditions, making it work same as the channel of a field-effect transistor or as the transducing element of a gas or chemical sensor. As the nanowire diameter decreases, a greater fraction of the mobile charge can be modulated by a given external charge, increasing the sensitivity. Having the gate of a nanowire transistor completely surround the nanowire also increases the sensitivity. 3
For a field-effect sensor to be effective, the charge must be physically close to the nanowire so that the majority of the compensating charge is induced in the nanowire and so that ions in solution do not screen the charge. Because only induced charge is being sensed, a coating that selectively binds the target species should be added to the nanowire surface to distinguish between different species in the analyte [2]. Fabrication of silicon nanowires: Silicon nanowires are formed by exposing a catalyst nanoparticle to a semiconductor precursor gas under conditions where the gas does not normally react. A column of the semiconductor (ie, the nanowire) is formed with diameter similar to that of the catalyzing nanoparticle. The growth of the nanowire depends on the size of the catalyzing nanoparticle, as well as the growth conditions. This approach offers the possibility of fabricating nanoscale electronic and sensing devices without costly and slow fine-scale lithography. When the connections to both ends of the nanowire are made during growth, advantages are obtained by combining bottom up fabrication of nanostructures with top down formation of the connecting electrodes using only conventional optical lithography. Self-assembled nanowires and nano-bridges constructed by the methods described here may provide some of the vital building blocks needed to enable the emerging technologies of nano-electronics and nanosensors.[3] 4
Fiber optic: An optical fiber is a glass or plastic fiber that operates on the principle of total internal reflection and allows transmission over long distance and higher data transmission rates.it consists of a core and cladding. However, some of the light signal degrades within the fiber, mostly due to impurities in the glass. The optical fiber acts as a waveguide. There are two major types of fibres viz. single mode fiber and multi mode fiber. Single mode fiber support only a single mode while multiple mode fiber supports multiple modes. Core is the thin glass center of fiber through which light travels, while cladding is the outer optical material surrounding the core that reflects light back in the core and buffer coating is the coating which protects the fiber from damage and moisture. In order to join two ends of a fiber these two ends must be cleaved and spliced together either mechanically or by fusing them together with an electric arc. The fiber used for this project is single mode fiber. Normally the single mode fiber has a core diameter of 8 to 10 μm. The mode structure depends on the wavelength of light used, so actually the fiber supports a small number of additional modes at visible wavelengths. Advantages of fiber optics is: Fiber optics is less expensive. Optical fibers can be drawn to smaller diameters than copper wire. As optical fibers are thinner than copper wires, more fibers can be bundled into a given-diameter cable than copper wires. The signal attenuation in optical fiber is less than in copper wire. Light signals from one fiber do not interfere with those of other fibers in the same cable. As signals in optical fibers degrade less, lower-power transmitters can be used instead of the high-voltage electrical transmitters needed for copper wires. 5
Optical fibers are ideally suited for carrying digital information, which is especially useful in computer networks. As no electricity is passed through optical fibers, there is no fire hazard. An optical cable is light weight as compared to copper wire cable. Future directions: Silicon nanowires can be used for future highly sensitive, label free biosensors [1,2,3] using electrical detection. Silicon nanowires can be embedded in a living cell with no apparent harm to the cell. It may also have the potential to deliver genetic material to specific organelles within a cell [11]. 6
References: [1]http://www.physorg.com/news120313863.html [2]http://meetings.aps.org/Meeting/MAR06/Event/42022 [3] http://www.springerlink.com/content/pw773rgv0u106621/ [4] Cui Y, Wei Q, Park H and Lieber C M 2001 Science 293 1289 [5] Stern E, Klemic J F, Routenberg D A, Wyrembak P N, Turner-Evans D B, Hamilton A D, LaVan D A, Fahmy T M and Reed M A 2007 Nature 445 [6] Juhasz R, Elfström N and Linnros J 2004 Nano Letters 5 (2) 275-280 [7] http://www-g.eng.cam.ac.uk/edm/research/silicon/silicon_nanowires.html [8] Li Z, Chen Y, Li X, Kamins T I, Nauka K and Williams R S 2004 Nano Letters 4 (2) 245-247 [9] Hsu J-F, Huang B-R, Huang C-S and Chen H-L 2005 J. J. Appl. Phys. 44 (4B) 2626-2629 [10] Patolsky F, Zheng G, Hayden O, Lakadamyali M, Zhuang X and Lieber C M 2004 PNAS 101 (39) 14017-14022 [11] http://www.lbl.gov/science-articles/archive/sabl/2007/jul/embedded.html 7