Nanosilicon Saves The Day?

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Nanosilicon Saves the Day?  This article is based in part on research from Opportunities for Nanosilicon: 2009 to 2016   Nanocrystalline silicon offers to boost thin-film silicon electronics to higher levels of performance. The benefits of this higher performance are many. Nanosilicon can potentially extend the life of Moore's Law in a way that is much more compatible with the current status quo than are many of the proposed alternatives; it also has the potential to help thin-film silicon PV devices keep up with the performance of other photovoltaic materials. More generally speaking, nanosilicon can provide new options for materials and manufacturing--across several applications--to produce the performance and cost improvements required in the ever-shrinking, perpetually cost-squeezing electronics industry. But is nanosilicon really what is required for these advancements? This article examines this question in the course of evaluating current and emerging nanosilicon technology and markets. Clearly in some respects there is no better material than nanosilicon: silicon is by all counts the best-understood--and most abundant--semiconductor on the planet, not to mention the basis of nearly the entire electronics industry today. Nanosilicon to the Rescue? Maybe. Nanosilicon is not alone among options for boosting electronics to the next level. Other options are available or proposed; some of them are still quite futuristic and not yet viable--CNT transistors, for example--while others are more conservative in nature--tighter-pitch lithography on the currently-used materials, for example. Nanosilicon falls in between these extremes. While nanotechnology is a relatively new industry, using silicon is perhaps the best way to incorporate it without abandoning the significant investments in existing silicon-based fab processes and equipment. Evolution of Silicon Nanomaterials and Nanostructures Nanosilicon materials are a logical extension of microcrystalline silicon, which has been in use for many years in memory, photovoltaic, and other electronics applications. The polysilicon that has been the dominant gate material for MOSFET transistors--so central to the electronics and IC industry--for the past decade or more is in fact made of microcrystalline silicon. Nanocrystalline

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silicon represents at least an order of magnitude--and generally more than one--reduction in crystallite size, typically to the 5-100 nanometer size range. The reduction of particle sizes to such a small scale has two important effects: an increase in the proportion of surface area of the crystallites to their volume, and the onset of quantum confinement. The large amount of surface area of nanocrystalline materials is an advantage because the charges on a semiconductive--or conductiv e--particle generally reside on the surface. This is why nanosilver preparations have enhanced conductiv ity versus conventional silver pastes; the same effect can boost nanocrystalline silicon performance as well. Quantum mechanical effects become signif icant in crystallites a few nanometers in size. It is these effects that are at the core of many of the emerging applications for nanocrystalline silicon; for instance quantum confinement can improve the efficiency of both PV cells and lighting devices by essentially holding charges in place until the desired outcome--conduction to the electrodes in the case of PV cells; recombination to form light in the case of solid-state lighting--can take place. Quantum confinement can also have useful applications for memories and transistors. Nanoparticles that are capable of such quantum confinement are generally called quantum dots. But quantum-confined nanostructures are still evolving and newer versions can exhibit quantum confinement in different ways. For instance, quantum wires only partially confine the charge carriers. These structures are much longer in one dimension than in the other two--forming a "wire" shape--and charge carriers can thus travel the length of the wire but cannot hop off of it. These and other morphologies of quantum-confined structures are discussed further in the main body of the report. Applications for Nanosilicon There are a handful of application areas that present both the potential for improvements due to the use of nanosilicon and the market volumes required to make it profitable. Chief among these, and farthest along on the route to commercialization, are photovoltaic cells and computer memories. TF silicon PV cells are falling behind the other TFPV technologies in terms of conversion efficiency and power output, yet they are still rapidly growing in volume and can thus be a tremendous

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source of revenue, especially if a breakthrough makes them more competitiv e in terms of performance. Current single-junction a-Si PV cells are fairly easy to produce but only yield 9.5 percent conversion efficiency in the lab; significantly lower in actual production. Multijunction TF Si PV cells include either microcrystalline silicon or a Si-Ge alloy to produce the second--and sometimes third--junction; these cells still only achieve about 12 percent efficiency in the lab. a-Si and microcrystalline silicon PV technology are very mature by TFPV standards and there has been little movement in the champion cell efficiency records over the past several years. Nanosilicon, however, has been shown to produce performance improvements and may offer a route to higher conversion efficiencies for TF Si PV cells. Nanosilicon is relativ ely new on the silicon scene and the enhanced properties of nanomaterials yield perhaps the best opportunity to produce breakthroughs in TF Si PV cell performance. The memory industry is continually pressed by the requirement for further miniaturization. There is a strong consensus in the sector that conventional flash memory technology has a limited lifetime because it is nearing the limits of its scalability. There have been various proposals--some of them backed by actual devices--for new memory technologies, including such exotic technologies as magnetic, phase-change, and carbon nanotube memories, but nanocrystalline silicon me mories have been proven to allow a path to tighter packing without loss of performance, and without drastically altering the memory architecture. Other applications are also in the works. Thin-film transistors are widely used for display backplanes, and nanocrystalline silicon can provide a route to less-costly production (by printing with nanosilicon inks) without sacrificing performance. Printed nanocrystalline silicon TFTs can also provide an alternative to organic TFTs for driv ing down the cost of RFID devices, a key step toward bringing RFID to the item level of tagging, and hence dramatically increasing the size of this market. Solid-state lighting is another market where nanocrystalline silicon can provide a choice in addition to organic electronics, in this case OLED lighting devices.

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