We had demonstrated several novel methods to improve the luminescence efficiency of the GaN-based light emitting
diodes (LEDs). The high-aspect-ratio GaN nanorods, formed by spun nano-spheres and inductively coupled plasma (ICP)
etching, contributed to an enhancement in light output power and better light directionality. Nevertheless, the etching
process would affect the electrical properties. We then attempt to rough the surface by synthesizing ZnO nanorods in
liquid solution at room temperature. The LEDs with ZnO nanorods enjoyed high extraction efficiency and comparable
electric performance than that without nanorods. At third part, we fabricate a high efficiency GaN-based LED by
regrowth on SiO2 nanorod patterned sapphire substrate. It could improve the light extraction and internal efficiencies
simultaneously.
A free-standing nanopillar with a diameter of 300 nm, and a height of 2 μm is successfully demonstrated by focused ion
beam milling. The measured micro-photoluminescence (μ-PL) from the embedded InGaN/GaN multiple quantum wells
shows a blue shift of 68 meV in energy with a broadened full-width at half maximum, ~200meV. Calculations based on
the valence force field method suggest that the spatial variation of the strain tensors in the nanopillar results in the
observed energy shift and spectrum broadening. Moreover, the power-dependent µ-PL measurement confirms that the
strain-relaxed emission region of the nanopillar exhibits a higher radiative recombination rate than that of the as-grown
structure, indicating great potential for realizing high-efficiency nano devices in the UV/blue wavelength range.
High efficiency GaN-based light-emitting diodes (LEDs) are demonstrated by a nanoscale epitaxial lateral
overgrowth (NELO) method on a SiO2 nanorod-array patterned sapphire substrate (NAPSS). The SiO2 NAPSS was
fabricated by a self-assembled Ni nano clusters and reactive ion etching. The average diameter and density of the formed
SiO2 nanorod-array was about 100 to 150 nm and 3 x 109 cm-2. The transmission electron microscopy images suggest
that the voids between SiO2 nanorods and the stacking faults introduced during the NELO of GaN can effectively
suppress the threading dislocation density. The output power and external quantum efficiency of the fabricated LED by
NELO method on NAPSS were enhanced by 52% and 56% respectively, compared to those of a conventional LED. The
improvements originated from both the enhanced light extraction assisted by the NAPSS, and the reduced dislocation
densities using the NELO method.
We have made a GaN-based single nanopillar with a diameter of 300nm using the focused ion beam (FIB)
technique. The micro-photoluminescence (μ-PL) from the embedded GaN/InGaN multi-quantum wells reveals
a blue shift of 68.3 meV in energy. In order to explain the spectrum shift, we have developed a valence force
field model to study the strain relaxation mechanism in a single
GaN-based nanopillar structure. The strain
distribution and strain induced polarization effect inside the multiple quantum wells is added to our self-consistent
Poisson, drift-diffusion, and Schrodinger solver to study the spectrum shift of μ-PL.
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