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What is a Nanowire? • A nanowire is a connecting structure that has a diameter of 10-9 meters (10-11) feet), which is extremely small. Also known as quantum wires, these connectors are used to connect tiny components together into very small circuits. These structures are no larger than a tenth of a nanometer wide. There is no restriction on how wide they can grow, but cannot grow more than a few nanometers in height.
Four Different types of Nanowires • Metallic • Semi-conducting • Insulating and Molecular.
Metal nanowires are made from nickel, platinum or gold. • Semi-conducting wires are comprised of silicon, indium phosphide or gallium nitride and insulating are made of silicon dioxide or titanium dioxide • To create a molecular nanowire, the process involves repeating organic or inorganic molecular units in a particular format.
Nanowires are experimental and are not available in commercial or industrial applications. The conductivity and tiny size make them ideal for future computer processors and connectors. • The use of nanowire in a transistor is an ideal method of producing smaller and faster microprocessor components for the computer and electronic industry. Although nanowire transistors function better than the current transistors, the high costs required to create them is a barrier to wider manufacturing.
Silicon Nanowires • the most promising building blocks for future nanoscale electronic devices. 15um Silicon Nanowires (SiNW) grown on Si wafer with gold thin film catalyst @ 550°C from Silane (SiH4) precursor gas.
Atomic configuration of a silicon wire created through etching of crystalline silicon. The wire is 3 nanometers long and one nanometer wide, attached to aluminum electrodes. The aluminum atoms are depicted as large blue balls; the silicon atoms as yellow balls, and the passivating hydrogen atoms as small blue balls.
Electronic density of states near the Fermi level for undoped (left) and doped (right) silicon nanowires created through an assembly of silicon clusters, and connected to aluminum electrodes. In both cases, the density of states iso-surfaces (depicted in red) is superimposed on the atomic structure of the nanowire. Note the aluminum doping atoms inside each of the silicon clusters shown on the right. The electronic density of states extends through the entire doped nanowire, bridging the two connected aluminum electrodes, resulting in a low threshold bias voltage for the current flow through the doped nanowire. On the other hand, for the undoped nanowire (left), the density of states penetrates only a short distance into the silicon, and a relatively large bias voltage is required for current flow. Aluminum atoms are shown as large blue balls; silicon atoms as yellow balls and passivating hydrogen atoms as small bluballs.
Electronic and Electrical properties of Silicon Nanowires • The small sizes of SiNWs make their electronic and electrical properties strongly dependent on growth direction, size, morphology and surface reconstruction. • A well-known example is the size dependence of the electronic band gap width of SiNWs irrespective of wire direction. As the wire diameter decreases, the band gap of the nanowire widens and deviates from that of bulk silicon gradually.
The orientation of the wire axis and the surface have a great effect on the electronic properties of SiNWs.
Applications • Silicon nanowires for solar photovoltaic applications • Silicon nanowire solar cells consist of arrays of radial p-n junction nanowires (see Figure 1) where the darker outer shell is composed of n-type silicon, to which the electron acceptor phosphorous has been added, and the lighter inner core from p-type silicon, to which the electron donor boron has been added. Each individual nanowire in the array has a p-n junction and acts as a tiny photovoltaic cell.
Figure 1. Diagram of the silicon nanowire solar cell. Each individual nanowire is a tiny p-n junction. The darker outer shell is n-type silicon. The lighter inner core is p-type silicon.
Silicon nanowires functionalized with palladium nanoparticles in hydrogen sensors • SiNWs were fabricated via a thermal evaporation method using tin (Sn) as the catalyst. The as-grown SiNWs were chemically treated to remove surface oxide and then coated with a thin layer of Pd nanoparticles. • A gas sensor device was fabricated with the Pd-functionalized SiNWs. The sensor showed better sensitivity to hydrogen and faster responding time than the macroscopic Pd wire hydrogen sensor.
Silicon Nanowires to Reduce Size of Microchips • Silicon nanowires can help to further reduce the size of microchips. In its never-ending quest to develop more efficient and more powerful microchips, the semiconductor industry is constantly advancing the miniaturization of circuits. Currently, the transistors lie on the surface of the substrate. Vertical silicon nanowires would reduce the space requirement considerably. Such nanowires are suitable for applications in the micro-chip industry, unlike nanowires which form on gold, the material that has mostly been used as a catalyst material up to now. Gold reduces the quality of microelectronic components drastically, and must not even come close to the production machines.
Silicon nanowires upgrade data-storage technology • Scientists have fabricated a memory device that combines silicon nanowires with a more traditional type of data-storage. Their hybrid structure may be more reliable than other nanowire-based memory devices recently built and more easily integrated into commercial applications. • the device is a type of “non-volatile” memory, meaning stored information is not lost when the device is without power. In this new device, nanowires are integrated with a higher-end type of non-volatile memory that is similar to flash, a layered structure known as semiconductor-oxide-nitride-oxide-semiconductor (SONOS) technology.
Summary • Silicon nanowire (SiNW) is a very attractive one-dimensional material for future nanoelectronic applications. Reliable control of key field effect transistor (FET) parameters such as conductance, mobility, threshold voltage and on/off ratio is crucial to the applications of SiNW to working logic devices and integrated circuits. • Single crystalline SiNWs were synthesized by thermal evaporation without gold catalysts. FETs were fabricated using both as-grown SiNWs and post-growth n-doped SiNWs. Promising device structures to implement ambipolar devices into large scale integrated circuits were proposed. The contributions of this study are to further understanding of the electrical transport properties of SiNWs and to provide optimized processes to fabricate emerging high performance nanoelectronic devices using SiNWs for future generation beyond bulk silicon.
