The deep levels in GaN associated with yellow luminescence transitions have been investigated using photoluminescence, Hall measurements, and deep level transient spectroscopy (DLTS). Hall measurements on Si-doped GaN show the presence of donor levels at ∼18, ∼35, and ∼70 meV, which are respectively associated with the Si shallow donors, O impurities, and the nitrogen vacancies (VN). DLTS measurements, on the other hand, reveal trap levels at Ec−0.1 eV, Ec−(0.2–0.24) eV, and Ev+0.87 eV. The trap level at Ec−0.1 eV obtained from DLTS can be correlated to the 70 meV deep donor (VN) obtained from Hall measurements. The deep donor band at Ec−(0.2–0.24) eV is attributed to the ON related defect complex decorated along dislocation sites while the hole level at Ev+0.87 eV is attributed to the Ga vacancy (VGa). Thermal annealing at 750 °C in nitrogen ambient results in reduction of yellow luminescence, which could be due to decrease in the concentration of VN and ON-related defect complexes. From these observations, we propose that yellow luminescence in GaN arises from the transitions from the Ec−(0.2–0.24) eV levels to the deep level at Ev+0.87 eV.

Growth of gallium nitride (GaN) on strain relaxed nanoporous GaN template by metal-organic chemical vapor deposition has produced GaN layer with 60% reduction in threading dislocation density (TDD). The porous GaN was annealed at $850°\mathrm{C}$ for $3\min$ in a mixed of nitrogen and ammonia ambient, which annihilated most TDs within the porous region via air-gap formation coupled with surface edge step pinning of dislocations. Enhancement of optical quality was indicated by doubled Raman intensity of $E_2$ phonon peak of annealed porous as compared to as-fabricated porous GaN. Besides, a redshift of $0.7{\mathrm{cm}}^{-1}$ in $E_2$ phonon peak of porous GaN with respect to as-grown GaN corresponds to a relaxation of compressive stress by $0.17\pm 0.05\mathrm{GPa}$ . Further overgrowth of GaN on annealed porous GaN template gives high quality GaN with reduction in TDD.

This article proposes a method for optimal placement of phasor measurement units (PMUs) to improve measurement redundancy distribution in a system under normal operating condition or in presence of contingency. The minimum number of PMUs is determined using integer linear programming method, after which the measurement redundancy is optimized using a novel model to represent the redundancy level of each bus. Weight parameters are assigned to auxiliary variables, which enables control of priority of different redundancy level. The impact of zero injection bus is integrated into the model, as well as the channel limitation of PMUs and existing measurements. Besides, contingency is modeled in a generalized form, which enables N-1 or N-2 contingency analysis. The proposed method is tested on various IEEE systems, and the results show that the number of PMUs obtained by the proposed method is further reduced comparing with other methods, while with a highly improved measurement redundancy.

Phosphor-free cool white emitting light emitting diodes (LEDs) have been fabricated using a dual stacked $\mathrm{In}\mathrm{Ga}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ multiple quantum wells (MQWs) comprising of a lower set of MQWs emitting yellow and an upper set of MQWs emitting blue. The lower set of MQWs incorporates indium-rich InGaN connected-dot nanostructures with a height of $\sim 1.0\mathrm{nm}$ in the well. The well is first grown with an InGaN layer serving as the wetting layer, then treated with trimethylindium (TMIn) to initiate nanostructure growth of another InGaN layer to complete the well layer. This gives a broadened yellow emission peak. With the combination of emission from the upper blue emitting $\mathrm{In}\mathrm{Ga}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ MQWs subsequently grown, cool white light emission is achieved. The In-rich nanostructures formed during TMIn treatment enhance indium incorporation in InGaN well and also act as effective radiative recombination sites for carriers at the lower set of MQWs.