Solidification processes of Fe-B and Fe-C eutectic alloys have been investigated by a time-resolved synchrotron x-ray diffraction under containerless cooling conditions using a conical nozzle levitation technique. To observe relative variations of structure from the undercooled liquid to crystalline phase, we have conducted millisecond order time-resolved x-ray diffraction experiments with a two-dimensional detector. The structural variations observed during the solidification of the Fe83C17alloy were identified as the phase transformation process expected from the Fe-C phase diagram. As for the Fe83B17alloy, it was revealed that a metastable phase composed of Fe23B6compound was precipitated as a primary crystalline phase from the undercooled liquid. In addition, decomposition of the metastable Fe23B6phase showed dependence on the cooling rate of the sample. At the cooling rate of 30 K/s, the Fe23B6phase decomposed to bcc-Fe and Fe2B phases with decreasing temperature. On the contrary, at the cooling rate of 180 K/s, the metastable Fe23B6phase remained in spite of an appearance of the bcc-Fe phase. By comparing the primary crystalline phase between the Fe83C17and the Fe83B17alloys, we suggest that the formability of the metastable Cr23C6-type compound is closely related with the glass-forming ability of Fe-metalloid binary alloys.

We have found for the first time that blue emission from Mg-doped GaN was greatly enhanced by remote plasma treatment (RPT) with plasma containing atomic hydrogen, in particular, water vapor plasma, at low temperatures of 300-400 C. The highest enhancing factor was twenty, achieved by water vapor RPT at 400 C for 30 min. The enhanced blue emission was stable up to 500 C, similarly to blue emission from as-grown samples, suggesting the same origin and mechanism. We have confirmed that the emission mechanism is donor-acceptor pair (DAP) recombination, and have concluded that RPT produces a hydrogen-related donor level at E c À 0:37 eV involved in the DAP emission.

A comparative study on structural variations during the solidification of Fe-C and Fe-B eutectic alloys was conducted by synchrotron x-ray diffraction under containerless cooling conditions using a conical nozzle levitation technique. It was revealed that a metastable phase composed of Fe23B6 precipitated as the primary crystalline phase from the undercooled liquid Fe83B17 alloy, whereas solidification of the Fe83C17 alloy occurred via a normal phase transformation process according to the Fe-C phase diagram. In addition, decomposition of the metastable Fe23B6 phase to the equilibrium phases shows a dependence on the cooling rate of the sample. By comparing the primary crystalline phase in the Fe83C17 and Fe83B17 alloys, we suggest that the formability of the metastable Cr23C6-type structure is closely related to the glass-forming ability of Fe-metalloid binary alloys.