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We demonstrated visible color tunable three-dimensional (3D) pyramidal light emitting diodes by depositing the MgO on and near the tip of the pyramid as an insulating layer. Here, we show that the degradation of the materials (i.e., p-GaN) crystallinity and the built-in electric field due to the nanoscale geometry of the tip region is responsible for the large leakage current observed in LEDs. Confocal scanning electroluminescence microscopy images clearly showed that the intensity of the light emitted out of the side facet of the pyramid is much higher than that of the light extracted out of the tip surface, indicating that the MgO layer prohibited the carrier injection to the MQWs layer, suppressing the leakage occurring at or near the tip region of the pyramids. The color range of the LEDs can be also tuned by using the MgO layer, a blue-shift by 10.3 nm in the wavelength. This technique is simple and scalable, providing a promising solution for developing 3D pyramidal LEDs with low leakage current and controllable light emission.

A chemical approach was applied to decrease the work function of few-layer graphene. Li2CO3, K2CO3, Rb2CO3, and Cs2CO3 were used as n-doping materials. The sheet resistance of graphene doped with carbonate salt slightly increased from 1100 to 1700–2500 Ω/sq, and the transmittance of doped graphene with 0.1 M alkali metal at 550 nm decreased from 96.7 to 96.1–94% due to the formation of metal particles on the surface of graphene. A higher sheet resistance and lower transmittance were obtained at a higher concentration of alkali metal carbonate. The G peak in the Raman spectra was shifted to a lower wavenumber after alkali metal carbonate doping and the intensity ratio of the carbon double bond to the carbon single bond decreased with doping in the X-ray photoemission spectroscopy spectra, suggesting the charge transfer from metal ions to graphene sheets. Ultraviolet photoemission spectroscopy data showed that the work function of the graphene sheets decreased from 4.25 eV to 3.8, 3.7, 3.5, and 3.4 eV for g...

We demonstrated a novel contact structure of a silver (Ag) reflector that had an enhanced thermal stability at an elevated temperature. The Ag film was epitaxially grown on GaN by the insertion of an ultrathin Ni contact layer (∼10 A). The Ag film in the Ni/Ag structure, which was tensilely strained due to the expansion of the lattice constant by oxidation of Ni (3.532 A) to NiO (4.176 A), could compensate for the thermal compressive stress of 0.83 GPa that developed at the interface of Ag with GaN at 500 °C. The tensile stress, as determined by high resolution X-ray diffraction, was 0.24 GPa for a single period of the Ni/Ag contact, but it increased to 0.65 GPa for triple periods of the Ni/Ag contact, resulting in further suppression of Ag agglomeration and improved long-term thermal stability.