A single-exposure holographic technique for fabricating 2-dimensional photonic crystals (PCs) with hexagonal lattices is
presented. A specially made holographic optical element (HOE), which consists of three holographic gratings, is used to
generate three interference beams simultaneously under the illumination of a single laser beam. Theoretical analysis
indicates that by selecting appropriate grating period and illuminating wavelength, the hexagonal lattice patterns formed
by the interference of the three beams can have the lattice constants in the range from micron to sub-micron. Moreover,
to obtain uniform lattice pattern with high contrast, the gratings in the HOE must have proper grating shape and depth. In
the experiment, hexagonal lattices with lattice constants from 5 μm to 620 nm were fabricated in photoresist using the
method. The patterned photoresist layers can be used as the masks to generate PCs in semiconductor materials through
dry or wet etchings.
Fabricating 2-dimensional (2D) photonic crystals (PCs) on InSnO2 (ITO) layers of LEDs can largely enhance light
extraction efficiency. The paper presents the theoretical analysis for determining the optimal lattice constants for
PC-LEDs to acquire more efficient light extraction. The equations for obtaining optimal hexagonal lattice constants
corresponding to different LED wavelengths were derived. Calculation shows that for 450 nm GaN-LEDs, the optimal
lattice constant is about 500 nm, and for the lattice constants in the range from a few micron to 500 nm, light extraction
increases with the reduction of lattice constant. In the experiment, hexagonal PCs with 1.8 micron, 1.5micron and 1.2
micron lattice constants were fabricated in ITO layers of GaN-LEDs using holographic 3-beam interference approach
and wet etching, resulting in 6%, 12% and 22.5% output increase respectively under 20 mA current injection. The
experimental result demonstrates that higher light extraction is acquired with the lattice constants closer to the optimum,
which is consistent with the theoretical analysis.
Light-emitting diodes (LEDs) have the potential to become the main light source for the advantages of low energy
consumption, being environmental friendly and long lifetime. However so far the brightness of LEDs is not sufficient for
many applications due to low light extraction efficiency resulted from total internal reflection of the emitted light at
semiconductor/air interface. To overcome this problem, fabricating 2-dimensional photonic crystals (PhCs) on the
surfaces of LEDs is considered as one of the most effective ways. At present, the method of fabricating PhCs is primarily
through e-beam lithography and ion-beam etching. Such processes are complicated and unsuitable for low cost mass
production. In this paper we propose a novel method of using holographic and wet etching processes to fabricate PhCs in
p-contact ITO layers of LEDs. The PhC patterns are firstly fabricated in photoresist layers coated on LEDs, using
holographic approach, then acid solution are used to transfer the patterns into ITO layers. The experiments of adopting
different etching solutions and etching rates were carried out and the results were compared, in order to find out the best
etching condition. Experimental results demonstrate that solution HCl : HNO3 : H2O made less damage to photoresist
mask and can better control the etching depth in ITO. PhCs with different lattice constant and depth were fabricated to
obtain the best PhC parameters for higher light extraction. 30% increase of external quantum efficiency has been
achieved in the experiment.
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