Zhoutong He

GaAs-based room-temperature continuous-wave 1.59 {mu}m GaInNAsSb single-quantum-well laser diode grown by molecular-beam epitaxy

Starting from the growth of high-quality 1.3μmGaInNAs∕GaAs quantum well (QW), the QW emission wavelength has been extended up to 1.55μm by a combination of lowering growth rate, using GaNAs barriers and incorporating some amount of Sb. The photoluminescence properties of 1.5μm range GaInNAsSb∕GaNAs QWs are quite comparable to the 1.3μm QWs, revealing positive effect of Sb on improving the optical quality of the QWs. A 1.59μm lasing of a GaInNAsSb∕GaNAs single-QW laser diode is obtained under continuous current injection at room temperature. The threshold current density is 2.6kA∕cm2 with as-cleaved facet mirrors.

High-indium-content InxGa1−xAs/GaAs quantum wells with emission wavelengths above 1.25 μm at room temperature

High-indium-content InxGa1-xAs/GaAs single/multi-quantum well (SQW/MQW) structures have been systematically investigated. By optimizing the molecular-beam epitaxy growth conditions, the critical thickness of the strained In0.475Ga0.525As/GaAs QWs is raised to 7 nm, which is much higher than the value given by the Matthews and Blakeslee model. The good crystalline quality of the strained InGaAs/GaAs MQWs is proved by x-ray rocking curves. Photoluminescence measurements show that an emission wavelength of 1.25 mum at room temperatures with narrower full width at half maximum less than 30 meV can be obtained. The strain relaxation mechanism is discussed using the Matthews-Blakeslee model

Raman study of correlation between defects and ferromagnetism in graphite

The variation of ferromagnetism induced by 12C+ ion implantation in highly oriented pyrolytic graphite was systematically studied by using Raman spectroscopy in conjunction with magnetic moment measurements and annealing treatments. It was found that the magnetization of the implanted sample was closely correlated with the density of the defects, which was characterized by the Raman spectra, produced by the implantation. It is clear that by using consecutive implantation steps at different energies to increase the vacancy defects in the implanted layer, the magnetization of the sample increases with the number of the implantation steps until the fourth step of implantation, which causes the near surface layer to be highly disordered or amorphous, weakening the magnetic coupling and thus resulting in the decrease in magnetization. The annealing treatments of the sample indicate that the ferromagnetism induced by the implantations is stable at room temperature. However, when the sample is annealed at 473 K (the Wigner energy release temperature), the density of vacancies and interstitials is abruptly decreased and the magnetism induced by the implantations is extinguished. This finding gives a clear indication of the key role of the defects produced by C+ ion implantation in graphite.

Irradiation-induced magnetic ordering in SiC: Experimental results and a density functional study

Magnetism of 6H-SiC single crystals implanted with 3 MeV protons is studied both experimentally and theoretically. We found that proton irradiation can induce stable ferromagnetism in 6H-SiC with a Curie temperature above 300 K. There is a dose window available for tuning the magnetization of the samples. The maximum saturation magnetizations (0.17 emu/g) are three orders of magnitude larger than that reported in neutron-irradiated SiC crystals (1 × 10−4 emu/g). First-principles calculations indicate that the ferromagnetism is related to the divacancy-related defects (VSiVC + nH, (n = 1–3)) generated under proton irradiation. This offers a promising route for the development of metal-free SiC magnets.

Effects of FLiNaK infiltration on thermal expansion behavior of graphite

The changes in both the crystallographic coefficient of thermal expansion (CCTE) and the coefficient of thermal expansion (CTE) of FLiNaK-infiltrated nuclear graphite were studied using in situ-heating X-ray diffraction analysis and horizontal push-rod dilatometry. It was found that, at temperatures lower than the melting point of FLiNaK, the CCTE of the d(002) spacing of graphite decreases with an increase in the weight of the graphite sample because of FLiNaK infiltration. On the other hand, the CTE of bulk graphite increases after molten salt infiltration. The CCTEs of the as-prepared FLiNaK salt, and the salt in the graphite pores were compared. The decrease in the CCTE of the FLiNaK salt after it had infiltrated into the graphite pores confirmed that interactions occur between the graphite and the salt. These interactions are probably induced by the difference in the CTEs of graphite and the solidified salt. Further, it is likely that the crystallization pressure also plays an important role here. Thus, both the causes need to be considered when using nuclear graphite in molten salt reactors.

Microstructure and molten salt impregnation characteristics of a micro-fine grain graphite for use in molten salt reactors

Abstract The microstructure and molten salt impregnation characteristics of a micro-fine grain isotropic graphite ZXF-5Q from Poco Inc. was investigated. The microstructural characteristics of the pores caused by gas evolution, calcination cracks, Mrozowski cracks, and the crystal structure were characterized by optical microscopy, mercury porosimetry, helium pycnometry, transmission electron microscopy, X-ray diffraction and Raman spectroscopy. Results show that the ZXF-5Q has uniformly-distributed pores caused by gas evolution with very small entrance diameters (∼0.4 μm), and numerous lenticular Mrozowski cracks. Molten salt impregnation with a molten eutectic fluoride salt at 650 °C and 1, 3 and 5 atm, indicate that ZXF-5Q could not be infiltrated even at 5 atm due to its very small pore entrance diameter. Some scattered global salt particles found inside the ZXF-5Q are possibly formed by condensation of the fluoride salt steam during cooling.