Study of Top and Bottom Photonic Gratings on GaN LED With Error Grating Models

Abstract

The gallium nitride (GaN) light-emitting-diode (LED) top-bottom (or transmission-reflection) grating simulation results with error grating model are presented. The microstruc ture GaN bottom hole and top pillar gratings are calculated and compared with the non-grating (flat) case. Grating shapes simu lated are either conical or cylindrical. A direct comparison of 181 different combined transmission-reflection grating cases using the finite difference time domain method is presented. The simulation results show that simple or direct combinations of the optimized top grating with the optimized bottom grating only produce a 42% light extraction improvement compared to the non-grating case, which is much lower than that of an optimized single grating case. This is due to the mismatch of grating parameters with the direct addition of the second grating structure, which changes the optical field distribution in the LEDs. Therefore, it is very im portant to optimize both top and bottom gratings simultaneously for the double-grating design. We also show the optimization of a double grating structure can achieve better performance than a single grating. Finally, transmission-reflection error gratings are also presented. It is also the first time to present randomization in GaN LED grating design and its effects in fabrication. Our data shows that the favorable light extraction improvement is at approximately 10-15% randomization. The randomization can achieve 230% improvement over the original grating at a randomization intensity factor of 12.8%.

Virtual International Research/Education Center: Energy Saving LEDs

“…2) With the virtual center, we are able to further expand our research topic from GaN LEDs [3]- [5] to OLEDs (Organic LEDs) and collaborate with both Prof. Zhang from PKU and Prof. Dong from Tsinghua University who are experts in those fields. Cal Poly EE department also has a Polymer LED teaching Lab and photonic research/teaching lab; we invited Prof. Dong to visit us to improve our Polymer lab and will send our students to work in her lab in summer 2013.…”

Comparison of triangular and squared ITO nano-grating of GaN LEDs

“…However, with these advantages come challenging solutions towards highly efficient LEDs [1,4,5]. There is tremendous need to reduce power consumption, minimize heat generation, and increase luminosity [6,7]. To achieve an industry desirable LED, optimization of internal quantum efficiency and external quantum efficiencies, also known as light extraction efficiency, are required.…”

“…As defined by Snell's Law, light can only radiate into the ambient medium when the incident angle is smaller than the critical angle. The low critical angle of GaN results in total internal reflection (TIR), which causes large portions of the photons generated in the device to be trapped in the multiple quantum well (MQW) of the device and to never escape [2,3,6,8,9]. Inefficiencies of GaN LEDs also include the absorptions of light with the device due to dislocation and defection within the GaN crystal.…”

“…Light trapped and absorbed in the semiconductor is a crucial aspect of light extraction efficiency because they generate heat within the device. The generation of heat within the device limits the lifetime of the LED and causes it to consume more energy [6,9]. Reducing the absorption within the LED is crucial to achieve high performance.…”

Light Extraction Enhancement of GaN Based LEDs Using Top Gratings, Patterned Sapphire Substrates, and Reflective Surfaces

“…18 Two specific methods include: electron-beam lithography and nano-imprint lithography. 19 Previous studies using simulations 20,21,22 and experimental data 23 show that the period, shape, and height of the grating all affect light extraction efficiency. In this first set of simulations we adjust all three of these parameters and observe light extraction enhancement.…”