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2017-11-08

A 'chip carrier' (the rectangular white plate) with the dark semiconductor sample at its centre. Nanowires are held on the semiconductor surface, connected by gold threads to the terminals of the chip carrier. The connections are used to measure the nanowire's properties.

Reinventing optical sensors with nanotechnology

Nanotechnology allows us to combine materials with different optical properties in entirely new ways. Components can be made smaller, cheaper and more energy-efficient than those we use today, and new capabilities can be introduced. Researchers from Halmstad and Lund have created unique nanowires that are expected to initiate a new generation of innovative solar cells and light-sensitive detectors, allowing direct integration with conventional silicon-based devices.

English subtitles available.

Håkan Pettersson, Professor of Physics at Halmstad University, explains the recent breakthrough:

– We have succeeded in building extremely high-quality, layered structures at the nanometer scale. The layers are of two materials that have different optical properties, which usually makes them incompatible with each other – it's a bit like trying to build with a mixture of Lego and Duplo bricks.

Catching more rays

The researchers solved the incompatibility by growing the material layers inside needle-like structures called nanowires. The wires have very small diameters, which dramatically reduces the stress built up between the layers, so cracks and other material defects are less likely to occur.

– Heterostructure nanowires with this 'sandwich' structure, make it possible to combine materials with completely different properties. By choosing the right combinations, we can customise nanowires for different applications. Our focus in this project is to develop nanowires for photodetectors. Also, because the diameters of our nanowires are so small, we should be able to grow them directly on a silicon wafer, alongside CMOS* circuitry, which opens the door to new types of integrated optoelectronics, says Håkan Pettersson.

At the most basic level, a photodetector or solar cell converts light into an electrical signal. By using several million heterostructure nanowires working in parallel, these new devices can be made extra-sensitive to light. They also work over a broader range of wavelengths than is possible with today's technology. Both benefits mean more light can be captured, making these kinds of photodetectors and solar cells more energy efficient.

Sandwich structure. a) Nanowires of indium phosphide (InP) seen in an electron microscope. A single photodetector can contain four million of these nanowires, all working in parallel. b) A closeup of a single nanowire, showing three embedded segments of indium arsenide phosphide (InAsP). The segments are only eight nanometers thick. c) An image of a complete InP nanowire with a total of twenty embedded segments.

A leading-edge nanotechnology consortium

The research group at Halmstad University is part of the School of Information Technology, and consists of PhD students Mohammad Karimi, Vishal Jain and Laiq Hussain, and Professor Håkan Pettersson. Håkan Pettersson is also in charge of the Rydberg Core Laboratory (RCL), part of the Rydberg Laboratory of Applied Science (RLAS) research environment, at Halmstad University's School of Business, Engineering and Science.

The group has a longstanding collaboration with NanoLund at Lund University, one of Europe's leading cross-disciplinary consortiums in nanotechnology. Halmstad researchers use the clean-rooms at Lund (Lund NanoLab) to make their devices, before performing electrical and optical measurements in RCL at Halmstad University. The group's research on heterostructure nanowire photodetectors was published in the leading journal Nano Letters in June: 'Room-temperature InP/InAsP Quantum Discs-in-Nanowire Infrared Photodetectors'.

Håkan Pettersson has also recently been selected as Sweden's only representative in the semiconductor physics section of the IUPAP, the International Union of Pure and Applied Physics.

* CMOS (complementary metal-oxide semiconductor) is one of the main techniques for making integrated circuits. CMOS chips are found in almost all modern electronic devices, including camera sensors and computer CPUs.

Image caption: Sandwich structure. a) Nanowires of indium phosphide (InP) seen in an electron microscope. A single photodetector can contain four million of these nanowires, all working in parallel. b) A closeup of a single nanowire, showing three embedded segments of indium arsenide phosphide (InAsP). The segments are only eight nanometers thick. c) An image of a complete InP nanowire with a total of twenty embedded segments.

Text: LOUISE WANDEL
Translation: STRUAN GRAY
Photo and video: IDA FRIDVALL

'We do all our research in-house, so we do a lot of experiments, everything from testing fundamental ideas to applied research', says Håkan Pettersson, shown here with the spectrometer control system.

About nanotechnology and nanowires

Nanowires are typically a few micrometers long, but their diameters are much smaller, varying from a few nanometers to several hundred nanometers (a micrometer is a millionth of a meter, a nanometer is a thousand times smaller yet). Because of the small size of the structures, and because individual atoms can be put in place one-by-one, nanotechnology is often seen as an 'atom-level handicraft'. The properties of the nanowires vary with their length and diameter, and with the materials used to make them.

What makes nanotechnology so special is that materials behave very differently at the nanometer level than they do on larger length scales. Familiar physical laws are no longer applicable. For example, when electrons move through the small embedded discs of InAsP shown in the microscope images above, they behave like waves, and have to be described using quantum mechanics. The segments are called 'quantum discs'.

The electrical and optical properties of the discs can be customised by varying their thickness. In another article recently submitted to a leading journal, the research team has succeeded in creating the first quantum discs whose internal resonances match very long wavelengths of light. This may allow these kinds of nanowires to be used for 'extreme broadband' detectors, that operate over a very wide range of wavelengths. In other work, they have created the first nanowires with a built-in amplifier, using a further segment of material wholly integrated into the nanowire structure.

– As well as having great potential for the smart optoelectronic and materials systems of the future, the research project has revealed new physics at the fundamental level. Doing this kind of basic research and trying to understand it is fun, and is absolutely essential for developing cutting-edge new components, says Håkan Pettersson.

Another useful fundamental property of nanoparticles like nanowires, is that they have a very large surface area to volume ratio. This is important for making things like new kinds of biosensors. Nanotechnology is already used for a wide range of applications in biology, physics and chemistry, including creating new materials, as well as in electronics, medicine, biotechnology, and energy research.

1 nm = 0.000000001 m

A human hair is approximately 50 000 nanometers thick.

Sources: Nobel Museum, Swedish Foundation for Strategic Research and SwedNanoTech.

Researchers use a spectrometer to analyse samples over a range of wavelengths. Their temperature is varied from room temperature down to -268°C, only five degrees above absolute zero. The image shows a cooled sample that has just been taken out of the cryogenic part of the spectrometer.

How to make nanowires

Researchers start by creating a pattern of small gold particles on the surface where they want the nanowires to grow (usually on a chip or wafer). The chip is then heated in a special kind of oven. Gases are introduced to provide the elements needed for building the different layers in the nanowires. For InP and InAsP nanowire segments, indium, arsenic and phosphorus are used. Gold catalyses the growth of the nanowires, and a single nanowire grows up underneath each gold particle. The bright object at the right-hand end of the nanowire in part c) of the figure above the gold particle used to grow it.

Different materials can be grown in sequence by changing the the gases being used. Fast switching permits incredibly thin layers to be grown, creating quantum discs along the axis of the cylindrical wire. Nanowire growth is a relatively simple process because the atoms building the structure self-organise, and automatically find their correct places in the overall structure.

Updated 2017-11-08