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Novel Devices ♦ news digest 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.


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)).


Figure 1 : (b) The strong peak of abundant C in SiC and the weak peak of N are distinguishable. In the insertion in (b), the vertical axis is in a linear scale. It is clear that N exists in a very low concentration


The SiC wafer into which the nitrogen dopant was introduced by ion plantation at a temperature of 500 °C and the wafers heat-treated at 1400 °C or 1800 °C after the ion plantation were subjected to the measurement of XAFS spectra (Fig. 2 (a)). The result of this experiment agreed with the first-principle calculation with FEFF, in which it was assumed that nitrogen atoms were located in the C sites (Fig. 2 (b)).


Thus, it was confirmed that most of N atoms were located in the C sites immediately after the ion plantation. It was empirical knowledge that ion plantation at a temperature as high as 500 °C was necessary for the doping to SiC, the reason for which, however, was unknown. The reason revealed in the present study is that it is necessary to locate N in the C sites before heat treatment at high temperature.


What’s more, according to the spectrum in the region lower than 400 eV, it is presumed that a chemical bond is formed between carbon and nitrogen in a disordered crystal state immediately after the ion plantation. As the crystal disorder resolves as a result of the heat treatment at high temperature, this chemical bond breaks, leaving only the chemical bond of nitrogen and silicon, which is preferable for the doping.


As described here, it is revealed that the doping to SiC is complex and requires a completely different method from that for the doping to Si, in which the lattice site substitution can be realised by heat treatment after ion implantation at room temperature.


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


It is now possible to determine the lattice site of the trace N dopant introduced in SiC; no such measurement was possible hitherto. What’s more, the state of the chemical bonds of the N dopant with the base materials, Si and C, is revealed. By combining SC-XAFS and the first-principle calculation, it is proved that the detection and microstructural analysis of a trace amount of the light elements in a crystal is possible, both of which were impossible until now.


March 2013 www.compoundsemiconductor.net 137


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