NANOWIRE TECHNOLOGY
ASHKA JARIWALA
CHANDUBHAI S PATEL INSTITUTE OF TECHNOLOGY, CHANGA
Nanowire technology is the key to the world of transparent and flexible electronics. Nanowire technology primarily consists of nanometer diameter wires synthesized onto plastic substrates to create electronic devices which are both transparent i.e. technology for next generation of optoelectronic devices which employs wide band-gap semiconductors for the realization of invisible circuits and flexible. This means it’s possible to for example, display information on windshields or windows of cars, or use outer wall surfaces of buildings as solar cells or even design devices flexible enough to bend in any direction. This paper gives a brief discussion of the nanowire structure, synthesis and discusses the possibilities as well as real time applications of nanowire technology to devices like TFT( thin film transistors), Organic LED’s, TRRAM( transparent resistive RAM), and a recent SNAP(superlattice nanowire pattern transfer) technology.
Nanowires, Synthesis and structure, SNAP, TFT, OLED, TRRAM
Nanowires are cylindrical, triangular or trapezoidal shaped (depending on synthesis process) solid structures/wires having diameter of the order of nanometers (10-9m) and having infinite/unconstrained length. It is also referred to as 1-Dimensional since its length to diameter ratio is more than 1000:1. Depending upon their source material, they can behave as an insulator, semiconductor or a conductor. Metal oxides chosen as a synthesizing material decides the behaviour of nanowires. Quantum mechanical properties are very important in case of nanowires and the main principle of nanowire technology is quantum tunnelling. Quantum/electron tunnelling is a phenomenon in quantum mechanics where a particle tunnels through a barrier that it classically could not surmount because its total kinetic energy is lower than the potential energy of the barrier. Quantum tunnelling is a consequence of the wave-particle duality of matter and is often explained using the Heisenberg uncertainty principle. Nanowires have many interesting properties that are not seen in bulk or 3-D materials. This is because electrons in nanowires are quantum confined laterally and thus occupy energy levels that are different from the traditional continuum of energy levels or bands found in bulk materials. Examples of nanowires include inorganic molecular nanowires (Mo6S9-xIx, Li2Mo6Se6), which can have a diameter of 0.9 nm be hundreds of micrometers long. Other important examples are based on semiconductors such as InP, Si, GaN, etc., dielectrics (e.g. SiO2, TiO2), or metals (e.g. Ni, Pt).
Nanowires are not found naturally and must be synthesized/fabricated. There are two basic approaches of synthesizing nanowires: top-down and bottom-up approach.
Top down approach seeks to create nanoscale devices by using larger, externally controlled ones to direct their assembly. It uses traditional workshop or micro fabrication methods where externally controlled tools are used to cut, mill and shape materials into desired shape and order. This technique includes nanolithography via an electron beam and a relatively recent method known as Molecular Beam Epitaxy (MBE).
Molecular Beam Epitaxy:
Molecular beam Epitaxy takes place in high vacuum or ultra high vacuum (10−8 Pa). The most important aspect of MBE is the slow deposition rate (typically less than 1000 nm per hour), which allows the films to grow epitaxially. In solid-source MBE, ultra-pure elements such as gallium and arsenic are heated in separate quasi-Knudsen effusion cells until they begin to slowly sublimate. The gaseous elements then condense on the wafer, where they may react with each other. During operation, reflection high energy electron diffraction (RHEED) is often used for monitoring the growth of the crystal layers. A computer controls shutters in front of each furnace, allowing precise control of the thickness of each layer, down to a single layer of atoms. Intricate structures of layers of different materials may be fabricated this way. Such control has allowed the development of structures where the electrons can be confined in space, giving quantum wells or even quantum dots.
Nanolithography is the branch of nanotechnology concerned with the study and application of fabricating nanometer-scale structures, meaning patterns with at least one lateral dimension between the size of an individual atom and approximately 100 nm. Nanolithography is used during the fabrication of leading-edge semiconductor integrated circuits (nanocircuitry) or nanoelectromechanical systems (NEMS).
Bottom up approach using nanocrystals:
Bottom up approach seek smaller components built into more complex assemblies. They also use chemical molecules to cause single molecule components to: (a) self organize or self assemble into useful conformation (b) rely on positional assembly. The self assembly technique using nanocrystals will be explained below. The main example of bottom up approach is VLS or vapor-liquid-solid process. Other examples are vapor deposition, electrochemical deposition, pressure injection etc.
VLS Method:
The vapor-liquid-solid method (VLS) is a mechanism for the growth of one-dimensional structures, such as nanowires, from chemical vapor deposition. Growth of a crystal through direct adsorption of a gas phase on to a solid surface is generally very slow. The VLS mechanism circumvents this by introducing a catalytic liquid alloy phase which can rapidly adsorb a vapor to supersaturation levels, and from which crystal growth can subsequently occur from nucleated seeds at the liquid-solid interface. The physical characteristics of nanowires grown in this manner depend, in a controllable way, upon the size and physical properties of the liquid alloy. The VLS mechanism is typically described in three stages:
- Preparation of a liquid alloy droplet upon the substrate from which a wire is to be grown
- Introduction of the substance to be grown as a vapor, which adsorbs on to the liquid surface, and diffuses in to the droplet
- Supersaturation and nucleation at the liquid/solid interface leading to axial crystal growth
STRUCTURE:
Transmission electron microscopy (TEM) is often used to examine the
crystallinity and structural morphology of Nanowires. Nanowires are primarily classified into 3 types of structures:
- Crystalline- Nanowires are made with structured alignments of polymer chains.
- Polycrystalline- Nanowires are made with repeating chemical units for molecule
- Amorphous- Nanowires are made with random alignment of polymer chains.
Conductivity and working of nanowires:
The conductivity of a nanowire is expected to be much less than that of the corresponding bulk/substrate. Also the nanowires exhibit different mechanical, electrical and magnetic properties than the bulk/substrate due to their nanometer scale diameter like increased surface area, very high density of electronic states and joint density of quantum energy states. The conductivity of nanowires mainly depends on diameter as well as a phenomenon called ‘edge effect’ which also depends on the size of the diameter. If the diameter of the nanowire decreases, conductivity proportionally decreases since the free electrons do not get enough channel area for conduction. The edge effects come from atoms that lay at the nanowire surface and are not fully bonded to neighboring atoms like the atoms within the bulk of the nanowire. The unbonded atoms are often a source of defects within the nanowire, and may cause the nanowire to conduct electricity more poorly than the bulk material. As a nanowire shrinks in size, the surface atoms become more numerous compared to the atoms within the nanowire, and edge effects become more important.
Another interesting property is that some nanowires are ballistic conductors. In normal conductors, electrons collide with the atoms in the conductor material. This slows down the electrons as they travel and creates heat as a byproduct. In ballistic conductors, the electrons can travel through the conductor without collisions. Nanowires could conduct electricity efficiently without the byproduct of intense heat.
Techniques for mass production of nanowires haven’t yet been invented. However, if we use nanocrystals as a material for nanowire synthesis, this is possible. Once the nanowires are synthesized by any of the above methods, using the self assembly method, they are, at low temperatures integrated/etched upon the plastic substrate. The main advantage of nanowires here is that it is possible to etch them on plastic substrate at low temperature unlike other inorganic materials which require etching at high temperatures which resulted in damage/imperfections in the substrate. This technique physically separates the synthesis of the transparent nanomaterials from the subsequent transfer onto the substrate, which occurs under mild condition such as room temperature. As a result, the harsh conditions associated with the material synthesis, such as high temperatures, are separated from the device fabrication. Nanocrystals separate each layer of oxides like SnO2, SiO2, and finally an electrode layer of ITO. This technique ensures high uniformity and fabrication over large areas.
SNAP(superlattice nanowire pattern transfer):
This technique is based on translating vertical thickness control in thin-film growth into lateral spatial patterns. First, molecular beam epitaxy is used to grow a GaAs/AlGaAs superlattice consisting of alternating layers of GaAs and AlGaAs. Then the AlGaAs layers are selectively etched away to a depth of roughly 20-30 nm, tilting the superlattice and evaporated metal onto its end. Metal is only deposited onto the GaAs layers, since the sample tilt meant that the etched AlGaAs layers were not accessible. To transfer the resulting wires to a substrate, the metal-coated superlattice is placed face down onto a 10 nm thick epoxy film on top of a silicon wafer. The epoxy layer is cured using a heat treatment, “gluing” the wires to the substrate, and then the layer of GaAs oxide between the nanowires and the GaAs layers is etched away to free the wires from the superlattice
Its applications in electronics include novel demultiplexing architectures; large-scale, ultrahigh-density memory circuits and complementary symmetry nanowire logic circuits.
Nanowires have immense potential applications in the electronics industry. As different materials in the form of nanowire have different new properties, thus opening up many new applications, the potential applications of nanowires is therefore unlimited. Some of the important potential applications include:
- · manufacturing TFT’s
- · Organic LED’s
- · TRRAM
- · Vapor and Temperature Sensors for accuracy and atomic precision
- · As photon ballistic waveguides as interconnects in quantum dot/quantum effect well photon logic arrays
- · Photovoltaic solar cells
- · Flexible electronic devices using nanowire arrays integrated into plastic substrates
- · As interconnect wires in field-effect transistors, resonators, nanomagnets, and spintronic systems
- · Nanoelectrochemical systems (NEMS)
We will discuss the applications of nanowires in following applications in detail:
- TFT (Thin film transistors) :
TFT ‘s or Thin Film Transistors have many applications in displays, printed electronics like Display screens, RFID’s( Radio Frequency Identification Tag) etc. However, when it comes to transparent and flexible applications, because of low electron mobility within the material, speed of operation is reduced. TFT’s designed using nanowires demonstrate considerably higher speeds TFT’s designed with nanowires demonstrate high electron mobility because of quantum tunnelling and because of near lack of imperfections in their crystalline structure which means less electron scattering.
Nanowire based TFT’s use mostly metal oxides like ZnO, In2O3, and SnO2 rather than Si because of advantages such as optical transparency, high mobility, and mechanical flexibility. Due to proper utilization of wide band gaps (<3 eV) of these metal oxides, nanowire TFT’s can be optically transparent. Also, nanowires synthesized using metal oxides allow higher density since larger area can be covered during fabrication.
In this case, the fabrication process consists of synthesizing the nanowires under optimized conditions (usually high temperatures), followed by transferto the substrate in a separate step, using solution- or dry-based transfer/alignment methods to complete device fabrication at low temperatures. Fig. shows nanowire based TFT’s consisting of single nanowire and multi nanowire network. Nanowire Network films can be deposited on the substrate/gate/dielectric by spin-coating, dropcasting, and thermal transfer. The TFT structure is completed by physical vapor deposition or printing of the source/drain contacts. The field effect mobilities in these TFTs can be extracted by conventional equations used for amorphous silicon.
OLED’s or Organic LED’s are a very recent technology used primarily in display devices. Compared with well-established liquid crystal displays (LCDs) and plasma screens, displays based on organic LED’s offer more brilliant images with high levels of contrast. Typically, OLED’s emit their light through the glass substrate which comprises an electrode of a transparent conducting oxide (TCO), e.g. indium tin oxide (ITO). The top electrode (typically the cathode) is usually an opaque low-work-function metal layer. For a transparent OLED, the top electrode needs to be see-through as well.
Nanowire based OLED’s have the main advantage of being used in active matrix OLED displays. An active-matrix display is able to precisely direct the flow of electricity to produce video because each picture element, or pixel, possesses its own control circuitry.
OLED’s are now used in cell phones and MP3 displays and prototype television sets, but their production requires a complex process, and it is difficult to manufacture OLED’s that are small enough for high-resolution displays. Nanowire based OLED’s offer a solution here. Nanowire fabrication method is scalable, providing a low-cost way to produce high-resolution displays for many applications. Unlike conventional CMOS computer chips, the nanowire thin-film transistors could be produced less expensively under low temperatures, making them ideal to incorporate into flexible plastics that would melt under high-temperature processing. Nanowire TFT’s can be used in Active Matrix OLED displays as active switching and driving transistors as they increase aperture radio efficiency and decrease power consumption.
The main disadvantage of using nanowire TFT’s in OLED’s is the cost factor as cost of OLED’s becomes double that of LCD screens due to usage of ITO as indium is a rare element.
- TRRAM (Transparent Resistive RAM):
TRRAM or Transparent Resistive RAM is the world’s first transparent computer chip similar to existing CMOS chips but with the difference that it is totally transparent. This technology has been developed by the Korean scientists and is non volatile in nature. It allows a device to store digital information in the same way as a memory card. By integrating TRRAM with other electronic devices like OLEDs, we can create totally transparent embedded systems and displays.
TRRAM’s could turn the concepts like Nokia ‘Morph’ into reality through see-through computer chips. And this is possible through the use of nanowires alongside TRRAMs. Though TRRAM is rigid in initial stages, it can be made flexible with the use of nanowire technology. Nanowires synthesized onto plastic substrate using bottom-up approach at low temperature result offer great flexibility since plastic is rugged, flexible, light and of course, transparent. This makes flexible electronic devices possible which when integrated with devices like TRRAM, can create devices both transparent and flexible.
The main possible applications of TRRAMs can be in transparent computer chips, concept mobile phones like nokia’s morph. Also, it is possible to manufacture TRRAM’s without using rare and expensive metals like iridium which would lower the cost considerably.
- CONCLUSION:
After analyzing the nanowire technology and its feasible applications, it can be concluded that nanowire technology is indeed the answer to many of the practical difficulties faced in designing transparent and flexible electronics and transforming them from science fiction to reality. Nanowire technology has immense applications and great future potential in nanoscale applications and devices and since last decade there has been continuous research on this technology for finding its potential in various applications. The major drawbacks it seems of this technology is the scarcity and cost of the elements used in the nanowire manufacturing and synthesis which results in increased cost of devices utilizing nanowire technology and the fact that no reliable technique has been developed to this date for mass production of nanowires and it remains confined to laboratory scale production.
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[9] Nanoelectronics- Nanowires, Molecular electronics and Nanodevices- Krzysztof Iniewski
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