Improvement of light extraction from GaN-based thin-film light-emitting diodes by patterning undoped GaN using modified laser lift-off

Abstract

To improve light extraction from GaN-based light-emitting diodes (LEDs), we demonstrated an approach of modified laser lift-off (M-LLO) technique for patterning undoped GaN (u-GaN). The M-LLO consists in sequentially forming a two-dimensional triangular lattice pattern with a 4μm period on a polymer layer over a sapphire substrate backplane by UV imprint and delivering the pattern onto u-GaN accompanied with the removal of the sapphire substrate. The enhancement of light extraction from GaN-based M-LLO LED with a reflective Ag film on the 120nm deep u-GaN pattern was about 31% and 100% compared to that of a LLO-LED with a reflective film and a conventional LED with a sapphire substrate, respectively.

Enhanced Light Extraction Efficiency from GaN Light Emitting Diodes using Photonic Crystal Grating Structures

“…The experimental data published in Ref. [18] also supports the strong dependence of light extraction efficiency on the grating depth. Sequential single variable optimization simply obtains a local maximum.…”

“…The technique etches the nanostructure onto u-GaN using a sapphire backplane, UV light, and a high power KrF laser. Peking University's (PKU) experiment varied the grating depths from 75nm to 120nm [18]. Imprint lithography, that can also produce similar air holes that measure 180nm in diameter, with a depth of 67nm, and a period of 295nm [19].…”

“…We define the parameter limits in the following way: 50 ≤ d ≤ 200 nm, 1 ≤ A ≤ 6µm, 1 ≤ w ≤ 6 µm, w ≤ A to prevent overlap, and the smallest element must be larger than the smallest grid size. The fabricated grating height parameters range from 75 nm to 120 nm [18]. This determines our simulation range.…”

“…Ref. [9] and [18] show the published experimental data on our simulated LED structure. Ref [9] only has electroluminescence (EL) data.…”

“…Ref. [18] presents the experimental data of the modified laser lift-off (M-LLO) LEDs for the light extraction improvement. The experiment shows about 120% improvement for a 120nm-depth grating and 66% for a 75nm-depth grating [18], which agrees with our simulation.…”