Fine structure of near-band-edge photoluminescence in He+-irradiated GaN grown on SiC

The effect of He ion implantation on the optical properties of epitaxial GaN-on-SiC was studied. We observed that He + irradiation increases the relative intensity of the “blue emission” and resistivity of GaN films and decreases the intensity of the near-band-edge photoluminescence. Because the intensity of the main peak is drastically decreased, the fine structure of the near-band-edge photoluminescence in GaN after He + irradiation was observed. From a comparison of observed sharp lines with photoluminescence peaks of GaNdoped with oxygen, we conclude that oxygen can produce a complex, which is characterized by a strong localization of free carriers and a large lattice distortion. The zero-phonon line of this defect has energy close to the band-gap energy of GaN

Experimental evidence for the preservation of U-Pb isotope ratios in mantle-recycled crustal zircon grains

Zircon of crustal origin found in mantle-derived rocks is of great interest because of the information it may provide about crust recycling and mantle dynamics. Consideration of this requires understanding of how mantle temperatures, notably higher than zircon crystallization temperatures, affected the recycled zircon grains, particularly their isotopic clocks. Since Pb2+ diffuses faster than U4+ and Th+4, it is generally believed that recycled zircon grains lose all radiogenic Pb after a few million years, thus limiting the time range over which they can be detected. Nonetheless, this might not be the case for zircon included in mantle minerals with low Pb2+ diffusivity and partitioning such as olivine and orthopyroxene because these may act as zircon sealants. Annealing experiments with natural zircon embedded in cristobalite (an effective zircon sealant) show that zircon grains do not lose Pb to their surroundings, although they may lose some Pb to molten inclusions. Diffusion tends to homogenize the Pb concentration in each grain changing the U-Pb and Th-Pb isotope ratios proportionally to the initial 206Pb, 207Pb and 208Pb concentration gradients (no gradient-no change) but in most cases the original age is still recognizable. It seems, therefore, that recycled crustal zircon grains can be detected, and even accurately dated, no matter how long they have dwelled in the mantle.This paper has been financed by the Spanish Grants CGL2013-40785-P and CGL2017-84469-P

Synchrotron-excited luminescence of natural zircon

The luminescence properties of two single zircon crystals from kimberlite of Yakutia have been studied, excited by the DORIS HASYLAB synchrotron, Germany, within energy range from the visible to the soft X-ray region (5-25, 50-200, and 500-620 eV) at temperatures of 300 and 10 K. The luminescence spectra in the range of 2.5 to 6.0 eV and excitation spectra of the main bands have been examined, the physical nature of the luminescence centers has been discussed, and the luminescence properties of a crystal containing growth (radiation) structural defects and a crystal with the same impurities but annealed in air at 1200°C are compared. The zoned structure of the mineral has been considered and the value of the energy gap (Eg) in the mineral has been estimated at 7.1 eV. Two groups of luminescence bands caused by impurities of intrinsic (growth, radiation) nature (Emax = 2.1, 2.7-2.8, and 3.2-3.3 eV) and matrix luminescence (Emax = 4.4-4.7 and 5.4 eV) probably with the participation of excitons were distinguished on the basis of selective excitation of zircon with different synchrotron energies relative to the gap value (Eexcit <Eg, Eexcit ~ Eg, and Eexcit > Eg). The short-lived component with a response time of 4 ns has been revealed in the afterglow of zircon in the region of 5.4 eV. © 2010 Pleiades Publishing, Ltd.ACKNOWLEDGMENTS This study was supported by the Presidium of the Russian Academy of Sciences (program 14 “Scientific Principles of Rational Nature Management”) and the Russian Foundation for Basic Research (project no. 09-05-00513)