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news digest ♦ Novel Devices


the future, we’d like to scale up this concept to include multiple qubits.”


This work is further described in the paper, “Electrical control over single hole spins in nanowire quantum dots,” by V. S. Pribiag et al in Nature Nanotechnology, (2013). DOI:10.1038/ nnano.2013.5


The research was supported by the Dutch Organisation for Fundamental Research on Matter, the Netherlands Organisation for Scientific Research, and the European Research Council.


Frolov and his Netherlands colleagues were recent winners of the 2012 Newcomb Cleveland Prize, an annual honour awarded to the author/s of the best research article/report appearing in Science, which is published weekly by the American Association for the Advancement of Science (AAAS).


Lattice location determines


trace nitrogen dopants in SiC A new technology is expected to contribute to the optimisation of the doping process of silicon carbide. The SC-XAFS techniques could also be applied to the analysis of other wide- gap semiconductors such as GaN (gallium nitride)


Researchers have developed an instrument for X-ray absorption fine structure (XAFS) spectroscopy equipped with a superconducting detector.


The study was conducted by Masataka Okubo and others from the Research Institute of Instrumentation Frontier of the National Institute of Advanced Industrial Science and Technology, in collaboration with the Institute of Materials Structure Science of the High Energy Accelerator Research Organisation and Ion Technology Centre Co., Ltd.


With the instrument, the researchers say they have realised, for the first time, local structure analysis of nitrogen dopants (impurity atoms at a very low concentration), which were introduced by ion plantation in SiC, a wide-gap semiconductor, and are necessary for SiC to be an-type semiconductor.


Wide-gap semiconductor power devices, which enable reduction of power loss, are expected to contribute to the suppression of CO2 emissions. To produce devices using SiC, one of the typical wide-gap semiconductor materials, introduction of dopants by ion plantation is necessary for the control of electrical properties.


The dopant atoms need to be located in the particular lattice site in a crystal. However, there has not been a microstructural analysis method.


SC-XAFS was used to measure the XAFS spectra of the nitrogen dopants at a very low concentration in the SiC crystal, and the substitution site of the nitrgoen dopants was determined by comparison with a first-principle calculation. In addition to SiC, SC-XAFS can be applied to wide-gap semiconductors such as GaN and diamond, magnets for low-


136 www.compoundsemiconductor.net March 2013


The superconducting X-ray detector developed by AIST, used to identify N dopants at a very low concentration in SiC (left) and SC-XAFS installed at a beam line of Photon Factory, KEK (right)


SiC has a band gap larger than that of general semiconductors and possesses excellent properties including chemical stability, hardness, and heat resistance. Therefore, it is expected to be a next-generation energy-saving semiconductor which can function in a high-temperature environment.


In recent years, large single-crystal SiC substrates have become available and devices such as diodes and transistors appeared on the market; however, doping, which is necessary to produce devices with the semiconductor, is still imperfect, preventing SiC from fully utilising its intrinsic energy-saving properties.


Doping is a process in which a small amount of impurity is introduced (for substitution) into a crystal lattice site to form a semiconductor with electrons playing a major role in electrical conduction (n-type semiconductor) or with holes playing a major role in electrical conduction (p-type semiconductor).


SiC is a compound, and thus has a complex crystal structure, which means that doping SiC is far more difficult than doping silicon .


Since dopants should be light elements such as boron, nitrogen, aluminium, or phosphorus, there was no measurement method to study at which site in the SiC crystal they are located, namely the silicon site or the carbon site. Although transmission electron microscopy can visualise atoms, it is difficult to distinguish a trace light element from light elements constituting the matrix material.


To determine dopant lattice sites, XAFS spectroscopy is effective. X-ray fluorescence analysis allows to measure XAFS spectra of a specific element in matrices, and reveals the atomic arrangement and the chemical state around the element.


So far, however, it has been impossible to distinguish the characteristic X-ray of a light element at a very low concentration from those of the matrix elements, silicon and carbon. The lack of the analysis method has hindered the development of wide-gap semiconductors.


AIST has been developing advanced measurement technologies for industrial research and scientific studies, making them available for public use, and standardising them. As a part of these efforts, SC-XAFS using a superconducting measurement technology was completed in 2011.


Nitrogen has an atomic number larger than carbon by one. The energy of its characteristic X-ray is 392 electron volts (eV); the


loss motors, spintronics devices, solar cells, etc.


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