Xiaomin Jin

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

The gallium nitride (GaN) light-emitting-diode (LED) top-bottom (or transmission-reflection) grating simulation results with error grating model are presented. The microstructure GaN bottom hole and top pillar gratings are calculated and compared with the non-grating (flat) case. Grating shapes simulated 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 important 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%.

Light extraction improvement of GaN-based light-emitting diodes using patterned undoped GaN bottom reflection gratings

The Gallium Nitride (GaN) Light-Emitting-Diode (LED) bottom refection grating simulation and results are presented. A microstructure GaN bottom grating, either conical holes or cylindrical holes, was calculated and compared with the non-grating (flat) case. A time monitor was also placed just above the top of the LED to measure both time and power output from the top of the LED. Many different scenarios were simulated by sweeping three parameters that affected the structure of the micro-structure grating: unit cell period (Α) from 1 to 6 microns, unit cell width (w) from 1 to 6 microns, and unit cell grating height (d) from 50 to 200nm. The simulation results show that the cylindrical grating case has a 98% light extraction improvement, and the conical grating case has a 109% light extraction improvement compared to the flat plate case.

Top Transmission Grating GaN LED Simulations for Light Extraction Improvement

We study the top transmission gratings improvement on GaN LED light extraction efficiency. We use the finite difference time domain (FDTD) method, a computational electromagnetic solution to Maxwells equations, to measure light extraction efficiency improvements of the various grating structures. Also, since FDTD can freely define materials for any layer or shape, we choose three particular materials to represent our transmission grating: 1) non-lossy p-GaN, 2) lossy indium tin oxide (ITO), and 3) non-lossy ITO (α=0). We define a regular spacing between unit cells in a crystal lattice arrangement by employing the following three parameters: grating cell period (Α), grating cell height (d), and grating cell width (w). The conical grating model and the cylindrical grating model are studied. We also presented in the paper directly comparison with reflection grating results. Both studies show that the top grating has better performance, improving light extraction efficiency by 165%, compared to that of the bottom reflection grating (112%), and top-bottom grating (42%). We also find that when grating cells closely pack together, a transmission grating maximizes light extraction efficiency. This points our research towards a more closely packed structure, such as a 3-fold symmetric photonic crystal structure with triangular symmetry and also smaller feature sizes in the nano-scale, such as the wavelength of light at 460 nm, half-wavelengths, quarter wavelengths, etc.

Study of top ITO nano-gratings on GaN LEDs

This study investigates the effect of nano-scale ITO cone gratings on the light extraction efficiency of GaN LEDs using FDTD analysis. First, we show the existence of a standing wave interference pattern in the region between the MQW layer and the LED surface for the non-grating cases. Then we use this knowledge to determine an ideal ITO material thickness above the MQW region to maximize light extraction at the LED surface. Optimizing the ITO layer thickness allowed us to improve the light extraction efficiency by 22% between the best and worst non-grating cases. We also found that when nano-scale gratings were implemented onto an optimal ITO layer thickness, they improved light extraction up to 27% over the no grating case and up to 30% over the no grating, no ITO LED. When a worst case ITO layer thickness is used, the gratings are shown to improve light extraction efficiency by up to 48% over the no grating case, and up to 70% over the no grating, no ITO LED.

Design Simulation of Top ITO Gratings to Improve Light Transmission for Gallium Nitride LEDs

We present simulation results of the indium tin oxide (ITO) top diffraction grating using a rigorous couple wave analysis (RCWA) for GaN LEDs. We study three different nano-structure patterns: cylindrical pillar grating, conical pillar grating, and cylindrical nano-hole grating. We show the light transmission improvement with nano-grating designs and present design-charts for the nano-hole grating.

Optimization of Top Polymer Gratings to Improve GaN LEDs Light Transmission

We present a grating model of two-dimensional (2D) rigorous coupled wave analysis (RCWA) to study top diffraction gratings on light-emitting diodes (LEDs). We compare the integrated-transmission of the non-grating, rectangular-grating, and triangular-grating cases for the same grating period of 6 µm, and show that the triangular grating has the best performance. For the triangular grating with 6-\mm period, the LED achieves the highest light transmission at 6-m grating bottom width and 2.9-µm grating depth. Compared with the non-grating case, the optimized light transmission improvement is about 74.6%. The simulation agrees with the experimental data of the thin polymer grating encapsulated flip-chip (FC) GaN-based LEDs for the light extraction improvement.

Study of GaN LED ITO Nano-Gratings With Standing Wave Analysis

This study reveals the effect of nanoscale ITO transmission gratings on light emission from the top, sides, and bottom of a GaN light-emitting diode (LED), based on the substrate standing wave analysis. First, we show that sapphire substrate thickness affects the standing wave pattern in the LED and find the best- and worst-case sapphire thicknesses. Second, we find that adding nanoscale ITO transmission gratings can improve light extraction by 222% or 253%, depending on the reference chosen. Third, we observe that maximizing top light emission with the nano-grating can significantly reduce bottom and side light emissions. Finally, we study grating performance over different wavelengths and generate the LED spectrum.

Light extraction improvement of GaN LEDs using nano-scale top transmission gratings

In this paper, we use a Finite-Difference Time-Domain GaN LED model to study constant wave (CW) average power of extracted light. The structure simulated comprises of a 200nm-thick p-GaN substrate, 50nm-thick MQW, 400nm-thick n-GaN substrate, and a 200nm n-GaN two-dimensional Photonic Crystal(2PhC) grating. We focus on optimizing three design parameters: grating period (A), grating height (d), and fill factor (FF). In the primary set of simulations, we fix the fill factor at 50% and simulate ten different grating periods (100 to 1000nm in steps of 100nm) and four different grating heights (50 to 200nm in steps of 50nm), and calculate the average power output of the device. The results from these simulations show that for both conical and cylindrical gratings, the maxmium light extraction improvement occurs when A =100nm. In the second set of simulations, we maintain a constant grating period A = 100nm and sweep the fill factor from 25 to 75%. The results of these simulations show that the fill factor affects clyindrical and conical gratings differently. As a whole, we see that the nano-structure grating mostly depends on period, but also depends on height and fill factor. The grating structure improves light extraction in some cases, but not all.

Study of Nano-scale ITO Top Grating of GaN LED

We study nano-scale ITO top transmission gratings to improve light extraction efficiency (LEE). We use the finite difference time domain (FDTD) method to measure light extraction from a device with various grating structures and layer thicknesses. We simulate our device using a twodimensional model with top triangular-gratings in a crystal lattice arrangement described by grating cell period (Α), grating cell height (d), and grating cell width (w). We also define ITO layer thickness (L) as the layer between the p-type GaN and the ITO surface layers. Simulation models vary in grating period, grating width, and ITO layer thickness. Our simulations monitor the amount of light emitted from the top, bottom, and sides of the LED model. We calculate the total light extraction and determine which grating duty cycle maximizes LEE. We found that adding a nano-scale grating with optimum duty cycle can achieve 165.67% and 136.77% LEE improvement, respectively, for ITO layer thickness of 230nm and 78nm.

Study of grating layer location of a GaN nano-grated LED for improvement of transmission efficiency

We study nano-grated surface GaN LED to improve light extraction efficiency by optimizing the device parameters. Our study is based on rigorous coupled wave analysis (RCWA) to obtain total transmission across a device. Our simulation results allow us to optimize the device parameters to maximize light extraction efficiency. We simulate our device using a two-dimensional model with square-grating cells in a crystal lattice arrangement whose parameters we define as follows: grating cell period (Λ), grating cell height (d), and grating cell width (ω). We also define grating layer location (L) as the distance between the multi-quantum wells (MQW) source and the grating surface layer. Each simulation varies in grating cell period, grating cell width, and grating layer location and provides a result of total transmission across the device. These results are used to calculate improvement over the non-grated surface GaN LED. Our preliminary study focused on 50% fill factor and showed that location of the grating as well as the grating period both strongly effect the total transmission across the device. In addition, we noticed that optimizing the surface grating location might affect the total transmission. Our study allowed us to improve the light extraction efficiency of nano-grated GaN LED by an average of 133% when fill factor is 50%. We also present our study in detail which includes fill factors ranging between 0 to 100%.