Periodic Nanoindentation Patterns Fabricated On AlGaInP Leds And Their Effects On Light Extraction

The past decade has witnessed extensive applications of light emitting diodes (LEDs) as lighting sources for traffic signals, outdoor displays, liquid crystal display (LCD) back lighting, and recently, general lighting applications. Currently, high brightness red LEDs are mainly manufactured based on aluminum gallium indium phosphide (AlGaInP) alloy. Varying the molar fraction of AlGaInP alloy enables direct band-gap emission of red, orange, yellow and green colors. However the Shell’s law indicates that the LEE (η) of a planar LED is related to the reflective index n by η≈1/(4n2). As a result merely ～2.3% of total electroluminescent photons can escape from the surface of a planar AlGaInP LED.A variety of techniques have been developed to improve the LEE of LEDs, including surface roughening, surface patterning by using optical, e-beam or ion-beam lithography, and plasmonic nanostructures. The essential design principle is to create additional optical channels for photons to escape from LED surface. Among these, photonic crystal (PC) can significantly increase the LEE of a LED via scattering guided modes into radiation modes, and inhibiting particular guided modes by photonic band gap. Patterned sapphire substrates (PSSs) are another effective methods to increase the LEE of a LED. They can reduce the density of threading dislocations by interrupting and/or bending them, thus increasing the internal quantum efficiency (IQE).On the other hand, they can intercept the guided optical modes in LEDs and force them to escape from the chips; thus greatly increasing the LEE.However, either expensive facilities for precise fabrication or complex chemical reaction are involved for current effort to increase the LEE of LEDs, which greatly hinders their applications. Indentation method has not been applied to fabricate surface pattern on LEDs so far. Indentation mechanics has a long history which can be traced back to Herz’s investigation on contact mechanics of two axisymmetric subjects in 1880s. Nowadays, indentation mechanics has been extensively used in the analysis and characterization of the mechanical properties of materials, such as elastic constant, hardness and surface stiffness. However, its potentials on fabricating surface patterns have not been explored.Four chapters are included in this thesis to explore the feasibility of fabricating patterns on the GaP window layer of AlGaInP LEDs via nano indentation techniques for improving the LEE. Further improvement of LEE by chemical etching of the indented regions is also investigated. They are Chapter 1 Introduction, Chapter 2 Periodic Indentation Patterns Fabricated on AlGaInP Light Emitting Diodes and Their Effects on Light Extraction, Chapter 3 Chemical Etching Assisted Nano indentation Pattern Improve AlGaInP LED Light Extraction Efficiency and Chapter 4 Conclusions and Prospects. The main topics of this thesis are as follow:(1) We first performed a systematic review on the classic indentation theory with emphasis on the theoretical basis of modern nano indentation techniques. We paid specific attentions on the generation of cracks, and indent damages, and the stress field of the elastic-plastic areas. With the help of nano indentation tests, the elastic modu of the GaP window layer of AlGaInP LEDs have been determined. According to the tests, patterned sapphire is chosen as arrayed indenter for fabricating periodic indentation patterns on AlGaInP LEDs.(2) We then use patterned sapphire as arrayed indenter to fabricate indentation patterns on AlGaInP LEDs in a self-designed steel mold, powered by a hydrostatic machine. Periodic pits with desired indentation depth were created on the wafer after pressure and duration optimization. The microstructural and morpholocial features of the GaP layer indicate that the indentation patterns agree with the indentation theory of brittle materials, and elastic-plastic areas are under control.(3) The electrical properties of the indented AlGaInP LED are examined. Indentation effect can slightly increase the threshold voltage and the dynamic resistance before reaching the working voltage. However when the applied voltage reaches the working voltage, the differences between samples with/without indents vanish. According to the angle-resolved electroluminescent (EL) tests, the EL intensities of indented AlGaInP LED have beenenhanced to 255% of the original one, without change of peak position and the full width at half maximum of the EL peak, which suggests the multiple quantum wells (MQWs) stay intact.(4) Chemical etchings are carried out on indented LEDs to further increase the LEE by a mixture of hydrofluoric acid (HF) and hydrogen peroxide (H2O2). We further studied the effect of etching time on the surface morphologies of indented AlGaInP LEDs by varying the etching time from 30 s to 10 min. It turns out that the LEDs cannot properly function when the etching time is longer than 2 min due possibly to damages on MQWs.(5) Nanoindented AlGaInP LEDs are etched for 10 s,30 s and 1 min, respectively. After etching, the electrical and optical properties of nanoindented AlGaInP LEDs are tested. The results suggest that a longer etching time may play a negative role on enhancing LEE of nanoindented AlGaInP LED compared a relatively short etching time. As a result, when the nanoindented AlGaInP LEDs are etched for 30s, an enhancement ratio of 21.2% is obtained in comparison with the indented AlGaInP LEDs.Innovations of this thesis can be summarized as:1) we creatively adopted the modern nano indentation techniques into fabricating light extraction patterns on AlGaInP LEDs; 2) we fabricated a large area periodic pattern on the GaP window layer of AlGaInP LED and realized a great enhancement of LEE; 3) we applied chemical etching on indented LEDs and realized a further light extraction enhancement.In summary, periodic indentation patterns have been successfully fabricated on AlGaInP LEDs by mechanical indentation using patterned sapphire. It is confirmed that this technique can be utilized to increase the light emission intensity. We further studied the effects of chemical etching on indented AlGaInP LEDs. We find that a short time etching (～30 s) has a positive effect on the light extraction of indented AlGaInP LEDs.