Application Of Laser Technique In Improving Light Extraction Efficiency Of LEDs

In the past few decades, light emitting diodes (LEDs) have undertaken many breakthroughs with reported very high internal quantum efficiency. As there are several advantages of LEDs, such as high efficiency and solid packaging, LEDs are widely recognized as key components in next generation lighting and display devices. However, due to the high refractive index of the compound semiconductors, the light extraction efficiency of LEDs devices is extremely low～1/4n2(GaN, n=2.5@460nm, ηext=4%; GaP, n=3.3@650nm, ηext=2.5%). Most photons are trapped into guided modes and finally absorbed by semiconductors. As a result, how to improve the light extraction efficiency becomes a hot spot of this area with a variety of methods developed, which include LED shape design, surface texturing, photonic crystal and PSS substrate. Among these, surface texturing is the most widely adopted due to its advantages such as large area, low cost, high light extraction efficiency and excellent controllability.Dry etching and wet etching processes are two main surface texturing methods. Dry etching process is a universal technique, but it could create point defects into the semiconductors and deteriorate electric properties. In comparison, wet etching process needs more understanding on materials properties, especially defect properties of the semiconductors. Most materials cannot be etched in a controlled manner by wet chemical etching.Laser fabrication technique is of low cost, highly controllable and can be performed on large scale production. This dissertation introduces laser fabrication into LEDs chip process and provides a new idea for LEDs fabrication. At the same time, some new phenomenon discovered during the laser fabrication process can provide theoretical and practical inspiration.Seven chapters are included in this dissertation. They are Chapter1introductions, Chapter2Optimization of LED light extraction efficiency using ray-tracing simulation, Chapter3Stress states of standard and flip-chip GaN epilayer after wafer bonding and laser lift-off process, Chapter4Improve the light extraction efficiency of LEDs using laser direct fabrication technique, Chapter5Improve the light extraction efficiency of AlGaInP based LEDs using laser enhanced wet etching technique, Chapter6Application of local coupling effect in laser enhanced wet etching technique, Chapter7Conclusions and Prospects. Detailed research contents are as follows:Chapter2:Simulate different structures of GaN and AlGaInP based LEDs using ray-tracing method in order to study the light extraction, with the aim of getting useful conclusions to make optimization of LEDs chips.1) Using ray-tracing method to optimize the top angle of pyramidal structures in GaN and GaP material systems. This research had been carried out in non-absorption and absorption models. The conclusion of this simulation is the best angle of the pyramid is28.1°for GaN and18.9°for GaP.2) Simulate the light extraction patterns of conventional, vertical GaN based LEDs and conventional, wafer bonding AlGaInP LED chips.3) Simulate the light extraction efficiency of Ag mirror and Au mirror LEDs, the results reveal that Ag mirror is better than Au mirror.4) Simulate the light extraction efficiency of conventional GaN based and AlGaInP based LEDs versus the chip size.5) From simulation results, we find that SiC substrate GaN LEDs has higher light extraction efficiency than sapphire, the difference value is about50.7%.6) Run simulations of surface texturing and mirror texturing, the results reveal that surface texturing can improve light extraction efficiency for86%, and the value of mirror texturing is94%. Both site texturing can improve the extraction efficiency for97%.7) Run a simulation to study the thickness of GaP layer and the results reveal that best efficiency can be obtained when the thickness of GaP layer is near8μm.Chapter3:We studied the in-plane biaxial strain states of standard GaN epilayer and flip-chip GaN epilayer bonded on Si wafer by XRD, Raman spectrpscopy and PL spectroscopy. We conclude that the flip-chip process will affect the in-plane strain states of GaN epilayers. Chapter4:Set up equipment to perform the laser direct fabrication process and applied this technique in different structure GaN and AlGalnP based LEDs. Then perform the optical and electrical test of the textured chips.1) Design and set up equipment to carry out the nano-second solid laser direct fabrication process.2) Study the relationship between output power of the laser and the ablation deepness.3) Analyze the high temperature damage of the multi quantum well introduced by laser fabrication using a1D thermal diffusion equation, the result reveal that the damage to MQW is neglectable.4) Perform optical and electrical test of conventional GaN based LEDs, the series resistance is decreased and34.9%enhancement of light extraction efficiency has been obtained over180°.5) Perform optical and electrical test of vertical GaN based LEDs, and58.0%enhancement of light extraction efficiency has been obtained over180°.6) Perform optical and electrical test of conventional AlGaInP based LEDs, and53.2%enhancement of light extraction efficiency has been obtained over180°.7) Perform optical and electrical test of wafer bonding AlGaInP based LEDs, and51.9%enhancement of light extraction efficiency has been obtained over180°.Chapter5:Applied the laser enhanced wet etching technique to GaP window layer etching process. Optical and electrical test were performed and morphology modulation of laser wavelength and polarization had been studied.1) Studiy the mechanism and recipe of laser enhanced wet etching of [100] off axis GaP, obtain high light extraction efficiency structures.2) Study the mechanism of morphology modulation of laser wavelength and obtain different etching structures using different lasers.3) Study the mechanism of morphology modulation of laser polarization and obtain different etching structures using lasers with different polarization.4) Perform optical and electrical test of conventional AlGaInP based LEDs, and 66.2%enhancement of light extraction efficiency has been obtained over180°.5) Packaged textured conventional AlGalnP LEDs properties:198.3mcd, Vf=1.992V, flux=1.3481m, output power5.919mW, flux efficiency32.1lm/W, dominant wavelength618nm, peak wavelength627.7nm.Chapter6:Study the modulation mechanism of LSP in laser enhanced wet etching.1) Fabricate Au nano-particles using sputter and annealing method with different diameters and duty ratios.2) Perform UV absorption spectra test of Au nano-particles before and after annealing process.3) Au nano-particle can promote the532nm laser enhanced wet etching of GaP.4) Ag nano-particle can restrain the450nm laser enhanced wet etching of GaP.5) Analyze the mechanism of phenomenon in3and4using band structure of GaP.Innovations of this dissertation contains:(1) demonstrate the laser direct fabrication process and applied this technique in different structure GaN and AlGaInP based LEDs;(2) applied the laser enhanced wet etching technique to [100] GaP window layer etching process and obtained66.2%extraction enhancement;(3) discovered the phenomenon of morphology modulation through laser wavelength;(4) disscovered the phenomenon of morphology modulation through laser polarization;(5) introduction of LSP can modulate the speed and morphology of laser enhanced wet etching process.In conclusion, firstly we use ray-tracing simulation to optimize the original design. Then study the laser direct fabrication and laser enhanced wet etching technique, finally obtained outstanding light extraction efficiency. During the experiment morphology modulation of laser wavelength and polarization had been studied. Finally, think of the LSP absorption, GaP band gap and laser photon energy we performed the LSP laser enhanced wet etching and discover several new phenomena. Currently, our country is making grate efforts to cultivate core competitiveness and thinking highly of intellectual properties. However, few intellectual property has been obtained in LEDs field, this phenomenon restrict the development of LEDs industry seriously. In this dissertation, the study starting from the basic research of new phenomena, and finally ending at the light extraction efficiency of LEDs, and provide two techniques easy to be integrated into LED industry. We would be grateful if the research work could provide new ideas for research and production of high efficiency LED devices.