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.
Metal nanowire networks hold a great promise, which have been supposed the only alternative to ITO as transparent electrodes for their excellent performance in touch screen, LED and solar cell. It is well known that the difficulty in making transparent ohmic electrode to p-type high-Al-content AlGaN conducting layer has highly constrained the further development of UV LEDs. On the IWN-2014, we reported the ohmic contact to n, p-GaN with direct graphene 3D-coated Cu nanosilk network and the fabrication of complete blue LED. On the ICNS-2015, we reported the ohmic contact to n-type AlGaN conducting layer with Cu@alloy nanosilk network.
Here, we further demonstrate the latest results that a novel technique is proposed for fabricating transparent ohmic electrode to high-Al-content AlGaN p-type conducting layer in UV LEDs using Cu@alloy core-shell nanosilk network. The superfine copper nanowires (16 nm) was synthesized for coating various metals such as Ni, Zn, V or Ti with different work functions. The transmittance showed a high transparency (> 90%) over a broad wavelength range from 200 to 3000 nm. By thermal annealing, ohmic contact was achieved on p-type Al0.5Ga0.5N layer with Cu@Ni nanosilk network, showing clearly linear I-V curve. By skipping the p-type GaN cladding layer, complete UV LED chip was fabricated and successfully lit with bright emission at 276 nm.
KEYWORDS: Gallium nitride, Sapphire, Crystals, Atomic force microscopy, Scanning electron microscopy, Etching, Luminescence, Metalorganic chemical vapor deposition, Chemical species, Reflection
Thick GaN films were grown on sapphire in a home-made vertical HVPE reactor. Effect of nucleation
treatments on the properties of GaN films was investigated, including the nitridation of sapphire, low
temperature GaN buffer and MOCVD-template. Various material characterization techniques,
including AFM, SEM, XRD, CL and PL have been used to assess these GaN epitaxial films. It was
found that the surface of sapphire after high temperature nitridation was flat and showed high density
nucleation centers. In addition, smooth Ga-polarity surface of epitaxial layer can be obtained on the
nitridation sapphire placed in air for several days due to polarity inversion. This may be caused by the
atoms re-arrangement because of oxidation. The roughness of N-polarity film was caused by the huge
inverted taper domains, which can penetrate up to the surface. The low temperature GaN buffer grown
at 650 °C is favorable for subsequent epitaxial film, which had narrow FWHM of 307 arcsec. The
epitaxial growth on MOCVD-template directly came into quasi-2D growth mode due to enough
nucleation centers, and high quality GaN films were acquired with the values of the FWHM of 141
arcsec for (002) reflections. After etching in boiled KOH, that the total etch-pit density was only 5×106
cm-2 illustrated high quality of the thick film on template. The photoluminescence spectrum of GaN
film on the MOCVD-template showed the narrowest line-width of the band edge emission in
comparison with other two growth modes.
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