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news digest ♦ Novel Devices


detection of a trace amount of light elements, thus expanding the range of the impurity concentrations covered by SC-XAFS


GaAs quantum dots


assemble themselves Quantum dots can self-assemble at the apex of a GaAs/ AlGaAs (gallium arsenide/aluminium gallium arsenide) core/ shell nanowire interface. This breakthrough could bolster quantum photonics and solar cell efficiency


Scientists from the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) and other labs have demonstrated a process where quantum dots can self- assemble at optimal locations in nanowires.


Figure 2 : (a) The XAFS spectrum of the SiC wafer without heat treatment immediately after the N ion plantation at 500 °C, and those of the SiC wafer heat-treated at high-temperatures after the ion implantation


This breakthrough could improve solar cells, quantum computing, and lighting devices.


Quantum dots are tiny crystals of semiconductor a few billionths of a metre in diameter. At that size they exhibit beneficial behaviours of quantum physics such as forming electron-hole pairs and harvesting excess energy.


The researchers demonstrated how quantum dots can self- assemble at the apex of the GaAs/AlGaAs core/shell nanowire interface.


Crucially, the quantum dots, besides being highly stable, can be positioned precisely relative to the nanowire’s centre. That precision, combined with the materials’ ability to provide quantum confinement for both the electrons and the holes, makes the approach a potential game-changer.


Electrons and holes typically locate in the lowest energy position within the confines of high-energy materials in the nanostructures. But in the new demonstration, the electron and hole, overlapping in a near-ideal way, are confined in the quantum dot itself at high energy rather than located at the lowest energy states. In this case, that’s the GaAs core. It’s like hitting the bulls-eye rather than the periphery.


Figure 2 : (b) The XAFS spectra assumed from the first- principle calculations with the Si site replaced by N and with the C site replaced by N


The experiment data agrees with the result of the calculation on the assumption that the C sites were replaced in the comparison of (a) the measured spectra and (b) the calculated spectra for the 3C and 4H polytypes, which were two typical crystal structure SiC.


The developed technology is expected to contribute to the optimisation of the doping process of SiC semiconductors. Besides SiC, SC-XAFS will be applied to the analysis of other wide-gap semiconductors, magnetic materials, etc.; their functions depend on trace light elements.


What’s more, improvement will be attempted in the resolution of the superconducting X-ray detector and the capability of the


138 www.compoundsemiconductor.net March 2013


The quantum dots, as a result, are very bright, spectrally narrow and highly anti-bunched, displaying excellent optical properties even when they are located just a few nanometres from the surface - a feature that even surprised the scientists.


“Some Swiss scientists announced that they had achieved this, but scientists at the conference had a hard time believing it,” says NREL senior scientist Jun-Wei Luo, one of the co-authors of the study.


Luo got to work constructing a quantum-dot-in-nanowire system using NREL’s supercomputer and was able to demonstrate that despite the fact that the overall band edges are formed by the gallium arsenide core, the thin aluminium-rich barriers provide quantum confinement both for the electrons and the holes inside the aluminium-poor quantum dot. That explains the origin of the highly unusual optical transitions.


Several practical applications are possible. The fact that stable


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