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In this work, we investigated the influence of N-polar wet etching on the properties of nitride-based hexagonal pyramids array (HPA) vertical-injection light emitting diodes (V-LEDs). The cathodeluminescence images showed the randomly distribution of hexagonal pyramids with isolated active regions. The transmission electron microscopy images demonstrated the reduced density of threading dislocations. The IQE was estimated by temperature dependence of photoluminescence, which showed 30% increase for HPA V-LEDs compared with broad area (BA) V-LEDs. The improved extraction efficiency was verified by finite difference time domain simulation, which was 20% higher than that of roughened BA V-LEDs. The electrical properties of HPA V-LEDs were measured by conductive atomic force microscopy (CAFM) measurements. HPA V-LEDs exhibited much lower leakage current due to the improved crystal quality.
The nanophase ionic conductors Ca1-xLaxF2+x with x=0 and 0.25 were synthesized by an inert gas condensation and in situ compacting technique. The samples with average grain size of 16 na for nanophase CaF2 and 11 nm for nanophase Ca0.75 La0.25F2.25 were prepared under the compacting pressure of 0.5 GPa. The alternating ionic conductivity was deduced from the temperature dependence of the complex impedance.
The results indicated that the logarithm of ionic conductivity obeys Arrhenius relation in the temperature range from 300 °C to 530 °C both for nanophase CaF2 and for nanophase Ca0.75La0.25F2.25. Their activation energies are 1.14 eV and 1.00 eV, respectively. The ionic conductivity of nanophase CaF2 is about one and two orders of magnitude higher than that of polycrystalline and single crystal CaF2, respectively. While the ionic conductivity of nanophase Ca0.75La0.25F2.25 is about one order of magnitude higher than that of nanophase CaF2. Further analysis indicated that the enhanced ionic conductivity of nanophase Ca1-xLaxF2+x is related to the large volume fraction of interfaces.
With the existing GPS, the replenishment of GLONASS and the launching of Galileo there will be three satellite navigation systems in the future, with a total of more than 80 satellites. So it can be expected that the performance of the global navigation satellite system (GNSS) will be greatly improved, especially in urban environments. This paper studies the potential benefits of GPS/GLONASS/Galileo integration in an urban canyon – Hong Kong. The navigation performances of four choices (GPS alone, GPS+GLONASS, GPS+Galileo and GPS+GLONASS+Galileo) are evaluated in terms of availability, coverage, and continuity based on simulation. The results show that there are significant improvements in availability, coverage and continuity, by using GPS+GLONASS+Galileo compared with the other choices. But the performance is still not good enough for most navigation applications in urban environments.
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