High-Extraction-Efficiency Blue Light-Emitting Diode Using Extended-Pitch Photonic Crystal

Abstract

We have integrated the surface photonic crystal (PhC) on GaN-based blue light-emitting diodes (LEDs) for the first time in order to enhance the extraction efficiency of the LEDs. With the finite-difference time-domain method, we have calculated 3.6-fold enhancement in light output. The theoretical calculations have revealed that the optimum pitch of the PhC is much longer than the emission wavelength when the distance between the PhC and the active layer of LEDs is short. This design enables PhC formation on chemically stable GaN surfaces. In addition, an indium tin oxide (ITO)-based transparent electrode is formed directly on the surface of PhC to realize light emission from the whole area of the LED. The fabricated PhCs have increased the light output of blue LEDs by 1.5 times compared with the LEDs without PhC. We have demonstrated that PhC will realize highly efficient solid-state lighting with GaN-based LEDs.

Lattice-parameter effects on diffracted transmission of GaN two-dimensional square-lattice photonic crystals

“…However, the optimization of PhC is challenging since many parameters of PhC are involved and correlated to each other, such as lattice pitch, fill factor and etch depth. We notice a large variation in PhC lattice pitch employed by different groups in GaN-based blue LEDs corresponding to the Bragg diffraction conditions of the first order [4,5], second order [4] and even higher orders [6][7][8][9]15,17,18], respectively. In particular, David et al [5] thought that the highest extraction efficiency was achieved around the first Bragg diffraction.…”

“…Various approaches have been adopted to extract the light trapped in LEDs. A great number of efforts have been made to use photonic crystals to improve the light extraction or control the radiation pattern of various types of GaN-based LEDs, such as commercial LEDs with GaN on sapphire substrate [1][2][3][4][5][6][7][8][9][10][11][12], thin-film LEDs [4,[13][14][15], flip-chip LEDs [16] and micro-cavity LEDs [17]. Though the coupling between the photonic crystal and guided modes is different due to the different types of LEDs, the strategy of using a PhC grating in most publications mainly relies on Bragg diffraction of the trapped guided modes from the LEDs.…”

Fabrication and optical properties of blue LEDs with silica microsphere coating

“…The light extraction efficiency is a key factor of the external quantum efficiency of LEDs. There have been several reports to improve the light extraction efficiency, for example, a cone-shaped surface texture via laser lift-off [3], a microlens array and photonic crystal structure which are subjected sophisticated process techniques [4,5]. Above those, however, make device fabrication further complicated.…”

Patterning of sapphire/GaN substrates

“…Over the years many novel techniques, such as roughening the upper p-GaN layer [10], forming photonic crystals on the top layer [11,12] and patterning of substrates [13][14][15] have been used to improve the light extraction efficiency of GaN LEDs. In the case of substrate patterning also the material quality of the overgrown films is commonly improved.…”