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Silicon Nanowires finding applications from biological to explosive sensing, photonics Batteries to spintronics

Silicon is predicted to remain as the fundamental material for the micro- and nano-electronics industry and ultra large scale integrated (ULSI) circuits despite the ever-growing research devoted to other materials and their nano structured variants.

 

Silicon Nanowires (SiNW) are one of the members of silicon nanostructure family, which have unique electrical, optical and mechanical properties.  Even more importantly, SiNWs-based nanodevices are compatible with the current Si-based microelectronics industry, and already a number of nanodevices based on SiNWs as building blocks have been demonstrated.

 

Silicon nanowires are quasi one-dimensional (1D) structures with a diameter of less than 100 nm. The very small diameter results in a large surface to volume ratio. This can be exploited in many ways in electronic devices. They have so far shown promising applications in areas ranging from biological sensors, thermoelectric converters, opto-mechanical devices, piezoelectric sensors, and solar cells among others.

 

Silicon is a material with very good thermoelectric properties, with regard to Seebeck coefficient and electrical conductivity. Low thermal conductivities, and hence high thermal to electrical conversion efficiencies, can be achieved in nanostructures, which are smaller than the phonon mean free path but large enough to preserve the electrical conductivity.

Optical Properties

Unlike Bulk Si that has an indirect band gap, SiNWs grown along most of the crystallographic orientations have a direct band gap, meaning that the maximum of the valence band and the minimum of the conduction band occur at the same point in k-space. Not only SiNWs can have a direct band gap, which per se increases the optical efficiency, but its width can in principle be tuned. The possibility of controlling the band gap width is tremendously attractive for optoelectronics applications:

 

This property has allowed to envisage the use of SiNWs as optically active materials for photonics applications.

 

By virtue of the appropriate band-gap and enhanced optical properties, SiNWs-based PDs are able to probe infrared light with high sensitivity and excellent photo-response, which can constitute the core component for large-scale applications in many areas such as military surveillance, target detection and tracking. In order to further boost the device performance (e.g. on/off ratio, response speed) of SiNWs-based PDs, people are resorting to new device structures with suppressed carrier recombination, enhanced light absorption and carrier transportation.

 

Ease of bandgap conversion from indirect to direct one with crystallography, mechanical strain, and alloying bring SiNWs into realm of optical applications e.g. photodetectors and light emitters.

 

Electrical characteristics of silicon nanowire CMOS inverters under illumination

Recently, there have been several studies on the introduction of optoelectronics in industrial and scientific applications for image sensing, surveillance cameras, flame detectors, remote control, chemical and biological sensing, and optical interconnects for inter chip data communication.

 

These technologies commonly use photosensors and photodetectors made of pn- or pin- photodiodes that tend to be easily disrupted due to substrate noise and crosstalk from adjacent diodes. This makes it difficult for the device to perform better than phototransistors. Conventional metal-oxide semiconductor field-effect transistor (MOSFET)-based phototransistors are used in civil and military applications because of their low cost, low-power consumption, high performance, and design flexibility. However, the scaling down of traditional MOSFET-based optoelectronic devices is becoming increasingly difficult due to their fundamental material and process limitations.

 

Silicon nanowire (SiNW) MOSFETs are particularly effective in overcoming these scaling limitations. Top-down fabrication methods enable the controlled assembly of SiNWs into well-ordered arrays at accurate locations, which allows for the implementation of integrated photonic systems. In addition, SiNW MOSFETs have excellent electrical switching characteristics because their 3D gate structures provide immunity from the superior short channel effect and their processes are compatible with current CMOS technologies.

 

In their paper, Korea University researchers report the electrical characteristics of SiNW CMOS inverters under illumination. “Our device is suitable for photosensors and photodetectors because of its high absorption rate and efficient photocurrent generation using transparent substrates. The SiNW CMOS inverters also show variation in the switching threshold voltage (Vinv) under different laser powers and wavelengths. This means that the device can be accurately controlled by sensing the incident light, which enables it to be effectively used in highly sensitive optoelectronic applications within the visible light range.”

 

In addition, breaking the centro-symmetricity in strained nanowires was shown to enhance the second order nonlinear optical susceptibility suitable for second harmonic or frequency difference (THz) generation applications.

 

With the advent of new top-down CMOS compatible fabrication methods for silicon nanowires it is now feasible to build spin-based quantum gates, spintronic devices, MOSFETs with critical dimensions approaching 5 nm or less, logic circuits, memory and memristive devices.

 

Spintronics

Silicon is a strategic semiconductor for quantum spintronics, combining long spin coherence and mature technology. In particular, very long coherence times have been achieved with the introduction of devices based on the nuclear-spin-free 28Si isotope, enabling the suppression of hyperfine coupling, the main source of spin decoherence.2 Single qubits with fidelities exceeding 99% as well as a first demonstration of a two-qubit gate have been reported.

 

Finding a viable pathway towards large-scale integration is the next step. To this aim, access to electric-field-mediated spin control would facilitate device scalability, circumventing the need for more demanding control schemes based on magnetic-field-driven spin resonance. Researchers have reported on an experimental realization of electrically driven electron spin resonance in a silicon-on-insulator (SOI) nanowire quantum dot device.

 

Next Generation Batteries

Researchers have turned to using silicon nanowire and germanium nanowire anodes due to their advantages like efficient electron transport and larger surface area that further increases the battery’s power density, allowing for fast charging and current delivery.

 

Researchers at the University of California, Riverside (UCR) have developed a silicon anode for lithium-ion batteries that outperforms current materials and gets around previous issues. A research team led by professors Mihri and Cengiz Ozkan now developed an electrode consisting of sponge-like silicon nanofibers having several structural advancements at the nanometer scale that help with the minimization of undesired large volume expansion as observed in other standard Si materials,”

 

The recent advances in the semiconductor nanowire array (NWA) technology provide new opportunities to realize economical and highly efficient photovoltaic (PV) devices because of their distinctive unidimensional geometry with striking electrical and optical features. It has been observed that Si NWA-based solar cells have a higher efficiency limit than planar cells because the restricted aperture of Si NWAs suppresses the optical and entropy losses.The vertically grown  nanowires (NWs) provide an elongated optical path which  enhances the photon absorption

 

Research at University of California, Los Angeles, has shown that growing a SiO2 layer on silicon nanowires (SiNW) can improve cycle life to 400 cycles at a capacity of 2400 mAh/g. Canonical announced on July 22, 2013, that its Ubuntu Edge smartphone would contain a silicon-anode lithium-ion battery.

 

Amprius currently makes silicon nanowires in a small-scale batch process using chemical vapor deposition (CVD), a process borrowed from the semiconductor industry.

 

Silicon Nanowire breakthrough could make key microwave technology much cheaper

Gunn effect, high-frequency oscillation of electrical current flowing through certain semiconducting solids. The effect is used in a solid-state device, the Gunn diode, to produce short radio waves called microwaves. In materials displaying the Gunn effect, such as gallium arsenide or cadmium sulfide, electrons can exist in two states of mobility, or ease of movement. Electrons in the state of higher mobility move through the solid more easily than electrons in the lower mobility state.

 

When no electrical voltage is applied to the material, most of its electrons are in the high mobility state. When an electrical voltage is applied, all its electrons begin to move just as in ordinary conductors. This motion constitutes an electrical current, and in most solids greater voltages cause increased movement of all the electrons and hence greater current flow. In Gunn-effect materials, however, a sufficiently strong electrical voltage may force some of the electrons into the state of lower mobility, causing them to move more slowly and decreasing the electrical conductivity of the material. In electronic circuits incorporating the Gunn diode, this unusual relationship between voltage and current (motion) results in the generation of high-frequency alternating current from a direct-current source.

 

Daryoush Shiri, a former Waterloo doctoral student, showed that if silicon nanowires were stretched as voltage was applied to them, the Gunn effect, and therefore the emission of microwaves, could be induced.

 

Gunn Effect can be induced in silicon nanowires (SiNW) with diameters of 3.1 nm under +3% strain and an electric field of 5000 V/cm, (b) the onset of NDR in the I-V characteristics is reversibly adjustable by strain and (c) strain modulates the resistivity by a factor 2.3 for SiNWs of normal I-V characteristics i.e. those without NDR. These observations are promising for applications of SiNWs in electromechanical sensors and adjustable microwave oscillators.

 

C.R. Selvakumar, an engineering professor at the University of Waterloo said, “Until now, this was considered impossible. The stretching mechanism could also act as a switch to turn the effect on and off, or vary the frequency of microwaves for a host of new applications that haven’t even been imagined yet.”

 

“Although, this hypothetical work is the first step in a development process that could lead to much cheaper, more flexible devices for the generation of microwaves. Now we will see where it goes, how it will ramify.” Their work was recently published in the journal Scientific Reports.

 

Using Silicon Nanowires to Detect Explosives

Scientists at the Naval Research Laboratory (NRL) have developed a technology, using silicon to fabricate a sensor that may revolutionize the way trace chemical detection is conducted. The sensor, called Silicon Nanowires in a Vertical Array with a Porous Electrode, or SiN-VAPOR, is a small, portable, lightweight, low power, low overhead sensor that NRL researchers hope can be distributed to warfighters in the field and to security personnel at airports across the globe.

 

Improvised explosive devices, or IEDs, are homemade bombs that can both injure and kill civilians and service members. For the Department of Defense, one solution to the problem of IEDs is to find them before they explode by detecting the chemicals used in the explosives.

 

The NRL-developed SiN-VAPOR architecture is unique and different from other similar technologies in that it is a 3D architecture, so NRL researchers are maximizing the surface area in order to maximize the sensing capabilities within the sensor. The SiN-VAPOR sensor has demonstrated detection capability on the parts-per-billion, and even parts-per-trillion, level of sensitivity

 

Methods of Fabrication of Nanowires

Many techniques, including both top-down and bottom-up approaches, have been developed and applied for the synthesis of Nanowires including Vapor–Liquid–Solid (VLS) Mechanism, Chemical Vapor Deposition (CVD), Evaporation of SiO, Molecular Beam Epitaxy (MBE), Laser Ablation and Electroless metal deposition and dissolution (EMD) have been discussed here.

 

 

References and Resources also include:

https://pdfs.semanticscholar.org/68c8/bbfd8d4525e98927ca249afdb51e93f7899b.pdf

http://science.dodlive.mil/2013/08/20/using-silicon-nanowires-to-detect-explosives/

https://www.techexplorist.com/silicon-breakthrough-key-microwave-technology-cheaper-better/14399/

About Rajesh Uppal

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