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news digest ♦ Power Electronics difference from that of carbon, 277 eV, is only 115 eV.


Although the energy resolution of the latest semiconductor X-ray detectors is 50 eV or so, which is smaller than the difference, at this resolution, while light elements can be distinguished if they exist in a large mount, it is not possible to distinguish a light element at a very low concentration, such as dopants.


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


116 www.compoundsemiconductor.net March 2013


Figure 1 : (a) The energy resolution of the superconducting X-ray detector with respect to the characteristic X-ray of oxygen (b) An example of the detection of the N dopant in a very low concentration in SiC


In contrast, the superconducting X-ray detector developed by AIST has the resolution that exceeds the theoretical limitation of semiconductor X-ray detectors. Therefore, it is possible to measure the XAFS spectrum of the nitrogen dopant in SiC using the superconducting detector.


This SC-XAFS is installed in the BL-11A beam line of Photon Factory, KEK and has been available to the public since 2012 in the projects such as the AIST advanced equipment sharing innovation platform and the microstructural analysis platform in the nanotechnology platform project.


AIST says itself and only the Advanced Light Source in the USA have this kind of advanced measurement instrument; and only AIST has developed a superconducting detector, the key of the analytical instrument. ITC developed the ion injection technology and the heat treatment technology applicable to SiC and supplies samples to users.


Figure 1 (a) shows a histogram of the energy resolution of each element of the superconducting array detector. At a maximum resolution of 10 eV, which exceeds the limit of 50 eV of semiconductor detectors, the detector can distinguish a trace amount of nitrogen (N) from the matrix carbon (C) in a large quantity (Fig. 1 (b)), thus enabling the acquisition of XAFS spectra with accuracy usable for comparison with first-principle calculation (Fig. 2 (b)).


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