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A new step towards an accurate definition of the ampere - will the quantum metrological triangle be closed with the SINIS turnstile?

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Single-atom transistor discovered

Researchers from Helsinki University of Technology (Finland), University of New South Wales (Australia), and University of Melbourne (Australia) have succeeded in building a working transistor, whose active region composes only of a single phosphorus atom in silicon. The results have just been published in Nano Letters.
The working principles of the device are based on sequential tunneling of single electrons between the phosphorus atom and the source and drain leads of the transistor. The tunneling can be suppressed or allowed by controlling the voltage on a nearby metal electrode with a width of a few tens of nanometers.

The rapid development of computers, which created the present information society, has been mainly based on the reduction of the size of transistors. We have known for a long time that this development has to slow down critically during the future decades when the even tighter inexpensive packing of transistors would require them to shrink down to the atomic length scales. In the recently developed transistor, all the electric current passes through the same single atom. This allows us to study the effects arising in the extreme limit of the transistor size.

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Copper the reason for the decrease in efficiency in solar cells?

TKK researchers at Micronova have approached the stability issues present in silicon wafers from a new angle and focused their research on copper. The researchers have found that copper causes strong degradation of lifetime in charge carriers in silicon wafers as a result of a few hours of exposure to light. The phenomenon is seen both in p- and n-type silicon. The oxygen present in silicon wafers manufactured using the Czochralski method enhances the effect of copper.

Copper can also have a significant part in so called light degradation, which is seen in typical silicon solar cells. Because of the light degradation the efficiency of the solar cell decreases as a result of the light during a few hours, after which it stabilises. The decrease in efficiency is due to the decrease in the lifetime of the charge carriers and it has traditionally been minimised by optimising the oxygen and boron concentrations in the wafer. As a new solution the researchers at TKK present removing copper from the silicon wafer.

With support from Tekes the researchers have started working on the possibilities for commercialisation of their results. Their goal is to licence the technical implementation as a part of manufacturers' solar cell processes.

The results of the research was published in Applied Physics Letters in October 2009 "Role of copper in light induced minority-carrier lifetime degradation of silicon", H. Savin, M. Yli-Koski and A. Haarahiltunen:

http://link.aip.org/link/?APPLAB/95/152111/1


Additional information:
Antti Haarahiltunen, D.Sc. (Tech.)
Aalto University School of Science and Technology
P.O. BOX 13500, FI-00076 Aalto University, FINLAND
Tel. +358 9 471 24987
antti. haarahiltunen@tkk.fi


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Researcher's view: A photodegradation mechanism in high-power fiber lasers - Mikko Söderlund

I work as an research scientist in Prof. Seppo Honkanen's Photonics group at the Department of Micro- and Nanosciences. After completing my MSc in Electrical engineering in 1999, I worked as a research scientist at VTT Photonics group for 5 years. I then joined a start-up company Liekki (currently nLIGHT/Lohja), and worked there for 3 years as a Product manager. In 2007, I switched to TKK to pursue the doctoral degree full time, and to lead a small research team focused on active optical fibers. The Photonics group's other research fields are planar nanophotonics on glass and silicon nanophotonics. Last december, I defended my PhD dissertation titled "Characterization and analysis of photodarkening in double cladding ytterbium-doped silica fibers".

Ytterbium-doped fibers (YDF) are used in high-power fiber lasers operating in the 1.06 µm wavelength region. Ytterbium-ions typically exhibits much higher optical-to-optical conversion efficiency (i.e. lower waste heat generation) in comparison to erbium (known for the erbium-doped fiber amplifier EDFA), and has therefore become the dopant of choice for many high-power fiber laser applications. Rapid development of ytterbium-doped fiber lasers (YDFL) during the last 10 years has transformed YDFLs from laboratory devices which, in the year 2000, operated with a meager ~1 W output power, into commercial devices with more than 3 kW of singlemode output power today. This incredible brightness enhancement has enabled new laser applications for example in materials processing (e.g. marking, cutting and welding) and facilitated the development of novel aerospace and military applications. In many ways, the YDF has revolutionized the field of high-brightness lasers, similarly to the enabling role that the erbium-doped fiber had in development of modern optical communication networks.

In light of the tremendous progress in power scaling of ytterbium-doped fiber lasers, it was rather surprising that as late as 2006, we (the Product development team at Liekki) were the first to observe and report on a serious photodegradation mechanism present in Yb-doped fibers [1]. This effect is today referred to as photodarkening and it manifests as progressive growth of broadband loss centered at the visible wavelengths. The tail of the induced loss extends to the Yb-doped fiber operating wavelengths between 0.9-1.1 µm, and therefore degrades the fiber efficiency and reduces the output power over time. As illustrated by the measured absorption coefficient change spectrum shown in Fig. 1a), the induced loss at visible wavelengths can be very high (e.g. in the 102 dB/m range). The two images of an Yb-doped fiber before/after irradiation in Fig 1b) illustrate that the change in material transparency can be observed even with a naked eye as "darkening" of the core material.


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Researcher's view: Solar cells

Dr. Kati Miettunen is a research associate in the New Energy Technologies (NEW) Group lead by prof. Peter Lund at the Department of Applied Physics. She received her D.Sc. degree in Applied Physics in 2009 and her M.Sc. degree in Engineering Physics and Mathematics in 2006. In her post doctoral period she leads one of the teams in the NEW group that is focused on experimental dye solar cell research. New challenges are ahead as a one year post-doc position at Imperial College London in the group of prof. James Durrant is starting from the beginning of 2011.

Nanostructured dye solar cells (DSC) are photovoltaic (PV) devices which can be produced from cheap materials using easy manufacturing methods. These third generation solar cells are the hot topic in PV. A key issue in the research of the NEW group is flexible DSCs which are suitable for industrial roll to roll manufacturing. An important part of this study was Miettunen's doctoral thesis in which she studied the deposition of the DSC on stainless steel sheets. The DSCs deposited on these alternative substrates are interesting also from the application point of view. For instance, one long term vision regarding metal based DSCs is that they could be used directly as functional roofing material.


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New on the research facility environment

Nanomicroscopy Center

Nanomicroscopy Center is now in full operation. The Center is a concentration of research equipment of atomic resolution, needed for the research of both nanomaterials and biomaterials and it is one of the largest of its kind in Europe
The most important new pieces of equipment of the Center include Finland's first next-generation transmission electron microscopes (TEM). One of them is a high-resolution TEM for hard materials, and the other a liquid-helium cryo-TEM intended for soft/biomaterials. These microscopes take the imaging and manipulation of nanostructures and biological samples to an entirely new level: the transmission electron microscopes allow materials to be characterized and manipulated even at the atomic level.



Professor Janne Ruokolainen who is the director of the Center I very satisfied now that the equipments are running after such a long planning and installation period.
"Nanomicroscopy Center is an open access facility for academic research, research institutes and also companies, says Janne Ruokolainen.

The nanomicroscopy equipment cannot be used in traditional laboratories because the level of external disturbances must be notably lower than usually. The new building is designed to minimize all external disturbances such as electromagnetic fields, mechanical and acoustic vibration and any disturbances caused by changes in temperature.

Please contact:
Professor Janne Ruokolainen
janne.ruokolainen@tkk.fi
tel. +358 9 470 22167

http://nmc.tkk.fi/en/


Micronova Nanofabrication Centre offers cleanroom services for academic and company users

Micronova is the joint research centre for micro- and nanotechnology of VTT Technical Research Centre of Finland and Aalto University. Micronova Nanofabrication Centre - Nanofab - is one of Finland's National Research Infrastructures.

Nanofab operates Micronova's 2600 m2 cleanrooms and the extensive and versatile base of process equipment. Micronova has a complete set of processes for the fabrication of microsystems. Micronova's researchers also have the expertise to solve the most complex device and processing problems.

We offer open access to researchers from universities, research institutes and companies.

Micronova offers the following services:

• Access to the cleanroom and equipment
• Support for collaborative projects with Micronova's R&D teams
• Cleanroom and safety training
• Equipment/process training
• Processing services and process support
• Prototyping and small-volume production
• Renting of cleanroom space and equipment use agreements for
universities, research institutes and companies

Some of Micronova's fabrication processes

• Lithography
• Nanostructuring
• Dry etching
• Wet processes
• Furnace processes
• Physical vapour deposition
• PECVD & ALD
• Ion Implantation
• Annealing
• Electrochemical deposition
• Epitaxial growth
• Wafer bonding
• Back-end processes
• Characterization

Please contact:
Tommy Holmqvist
Coordinator
Micronova Nanofabrication Centre
tommy.holmqvist@tkk.fi
Tel: +358 50 5663497

Ulrika Gyllenberg
Technology Manager, VTT
ulrika.gyllenberg@vtt.fi
Phone 020 722 6664 or 040 541 6184

Veli-Matti Airaksinen
Director, Aalto University, School of Science and Technology
veli-matti.airaksinen@tkk.fi
Phone (09) 470 26075 or 050 341 4766

www.micronova.fi


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Equipment grouped by departments Department of Applied Physics Department of Chemistry Department of Biotechnology and Chemical Technology Department of Materials Science and Engineering Department of Forest Products Technology Department of Electrical and Communications Engineering Department of Micro- and Nanosciences Equipment grouped by application areas Spectrometry / spectroscopy Microscopy X-ray Growth and sample preparation Electrical measurements Mechanical properties Magnetometer Thermal analysis Vapor generation system