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We utilized the finite-difference time-domain method (FDTD) to investigate the reflectance of an air void photonic crystal (PhC) on the p-electrode of an AlGaN-based deep ultraviolet (DUV) light-emitting diode (LED). Firstly, a transparent p-AlGaN layer with cylindrical air voids on a Ni(1nm)/Al(140nm) metal electrode was optimized to maximize the reflectance at normal incidence. It was shown that by having the optimum AlGaN PhC on the metal electrode, the reflectance as a function of the angle of incidence was advantageously redistributed to increase LEE. The calculated angle-dependent reflectances were converted to average reflectance considering the power distribution of dipole sources in the TE and TM modes. The average reflectances of a reference structure and the PhC structure for the TM mode at wavelengths around 283 nm were 77.1 and 85.2%, respectively. Thus, an incremental increase of 8% in average reflectance for the TM mode was obtained by adopting the optimized PhC. Secondly, we investigated a PhC with air voids in two different layers, a p-GaN layer and a p-AlGaN layer on the Ni/Al. The calculated average reflectances at 283 nm were 38% and 42% for the TE and TM modes, respectively, when the thickness of the p-GaN layer was 70 nm. The average reflectances for the TE and TM modes with a uniform 70nm thick p-GaN layer without a PhC were 4.2 and 3.6%, respectively. This clearly shows that the optimized PhC can reduce light absorption in both the p-GaN layer and the metal electrode. [ABSTRACT FROM AUTHOR]

Nonradiative recombination (NRR) centers in n-AlGaN layers of UV-B AlGaN samples with different numbers of superlattice (SL) periods (SLPs), grown on the c-plane sapphire substrate at 1150 °C by the metalorganic chemical vapor deposition technique, have been studied by using below-gap-excitation (BGE) light in photoluminescence (PL) spectroscopy at 30 K. The SLP affects the lattice relaxation of the SL and n-AlGaN layer. The PL intensity decreased by the superposition of BGE light of energies from 0.93 eV to 1.46 eV over the above-gap-excitation light of energy 4.66 eV, which has been explained by a two-level model based on the Shockley–Read–Hall statistics. The degree of PL quenching from n-AlGaN layers of the sample with SLP 100 is lower than those of other samples with SLP 50, 150, and 200. By a qualitative simulation with the dominant BGE energy of 1.27 eV, the density ratio of NRR centers in n-AlGaN layers of 50:100:150:200 SLP samples is obtained as 1.7:1.0:6.5:3.4. This result implies that the number of SLP changes lattice relaxation and determines the density of NRR centers in the n-AlGaN layer, which affects the performance of LEDs. [ABSTRACT FROM AUTHOR]

The interest in semipolar orientations has been increasing because the reduced piezoelectric field can improve the performance of nitride-based optoelectronic devices. However, the crystalline quality of semipolar AlN on m-plane sapphire is still not good enough to realize light emitters with sufficiently high efficiency. We performed high-temperature annealing on AlN on m-plane sapphire to improve the crystalline quality. For (10-1-3) and (11-22) AlN on m-plane sapphire, the crystalline quality improved as the annealing temperature was increased up to 1700 °C, whereas beyond 1750 °C the AlN layer started to deteriorate and desorb. The crystalline quality was further improved by additional growth of AlN. In addition, X-ray rocking curve measurements and transmission electron microscopy confirmed that the density of stacking faults was reduced after the additional growth of AlN. [ABSTRACT FROM AUTHOR]