As efficient artificial light sources, nitride-based light-emitting diodes (LEDs) have been widely used. However, III nitride white light-emitting diodes (WLEDs) are mostly fabricated by combining a blue LED chip with yellow phosphors, which results in inevitable problems, such as low color-rendering index (CRI) and detrimental effect to human eyes. As a solution for healthy lighting, we report a strategy to produce a WLED with an emission spectrum perfectly matched with natural daylight. By utilizing near-ultraviolet LED chips and a mixture of blue/cyan/amber/red phosphors, a CRI (Ra) of 97.9 is achieved for the WLEDs at a correlated color temperature of around 5000 K. The resemblance ratio of these solar-spectrum WLEDs with the standard normalized daylight spectrum (5000 K) is found to as high as 93.4%. Residual UV light in the normalized spectrum is <5 % . The method will benefit the development of high-efficiency healthy artificial light sources.
We report the regrown Al-rich of n-AlGaN material with improved crystalline quality and reduced stress on nanoporous AlGaN template, which was prepared by the electrochemical etching (EC). First, the EC of Al-rich AlxGa1-xN (x > 50 % ) material was investigated to get suitable nanoporous template. Various anodizing voltage and anodizing time were applied to fabricate the nanoporous AlGaN template. The nanopore size and density were found to increase as the anodizing voltage and the anodizing time increase. Moreover, branching pores and vertical pores were apt to be formed at low and high voltages, respectively. Photoluminescence (PL) measurement and Raman spectra indicate that the nanoporous AlGaN materials exhibit higher PL intensity and dramatical release of stress compared to the as-grown AlGaN films due to the presence of nanopores. Furthermore, the nearly stress-free regrown n-AlGaN with high quality using optimized nanoporous AlGaN material as the template was also obtained, which demonstrates that the nanoporous AlGaN template could potentially be applied to heteroepitaxy of efficient AlGaN-based ultraviolet optoelectronic.
Surface-patterning technologies have enabled the improvement of currently existing light-emitting diodes (LEDs) and can be used to overcome the issue of low quantum efficiency of green GaN-based LEDs. We have applied nanosphere lithography to fabricate nanopillars on InGaN/GaN quantum-well LEDs. By etching through the active region, it is possible to improve both the light extraction efficiency and, in addition, the internal quantum efficiency through the effects of lattice strain relaxation. Nanopillars of different sizes are fabricated and analyzed using Raman spectroscopy. We have shown that nanopillar LEDs can be significantly improved by applying a combination of ion-damage curing techniques, including thermal and acidic treatment, and have analyzed their effects using x-ray photoelectron spectroscopy.
Wide band gap dilute magnetic semiconductors have recently been of interest due to theoretical predictions of room
temperature ferromagnetism in these materials. In this work Ga1-xGdxN thin films were grown by Metalorganic
Chemical Vapor Deposition. These films were found to be ferromagnetic at room temperature and electrically
conducting. However, only GaN:Gd layers and devices grown with a TMHD3Gd precursor that contained oxygen
showed strong ferromagnetism, while materials grown with an oxygen-free Cp3Gd precursor did not show ferromagnetic behavior. This experimental observation was consistent with first-principles calculations based on density functional theory calculations that we completed that showed the ferromagnetism was mediated by interstitial oxygen. The results confirmed the first successful realization of Ga1-xGdxN-based spin-polarized LED with 14.6% degree of polarization at 5000 Gauss is obtained.
Compared with conventional GaN based LEDs structure, vertical electrodes structure can improve the current uniformity
to some extend. However, due to the presence of N-GaN series resistance, current crowing phenomenon still exist at the
electrode edge of vertical structure GaN-based LEDs. The most common solutions to overcome these problems consist
in using transparent current spreading contact layer, generally made of indium tin oxide (ITO). In this paper, the effect of
indium tin oxide (ITO) layer on the current uniformity and the current distribution are analyzed by theoretical
calculation. At last, vertical structure GaN-based LEDs was fabricated and the electrical character was measured.
Experiment result exhibits good agreement with the theoretical calculation.
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