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Department of Applied PhysicsFinal Year Project ReportLED reliability and failure analysisStudent Name: Lui Ming YeungStudent Number: 17021178DProgramme code: 11439-OPT2018 ” 2019AbstractAcknowledgementI would like to express my gratitude to the support of the Department of Applied Physics, The Hong Kong Polytechnic University, especially my supervisor Professor Dai. Professor Dai has been very supportive throughout my final year project. He never stops teaching me knowledge about LED, practically or theoretically. The insightful instruction always benefits my project. Assistance provided by Dennis and Mr Lam were greatly appreciated.
I would also take this chance to thanks my colleague by giving me mental and technical support throughout the year. Table of ContentsList of figures and/or tables TOC h z c “Figure” Figure 1: Method of generation of white LED light implemented as di-, tri-, tetrachromatic sources  PAGEREF _Toc4940351 h 3Figure 2: Basic structure of a white colour LED  PAGEREF _Toc4940352 h 4Figure 3: Schematic diagram of blue light emission from a GaN-based LED PAGEREF _Toc4940353 h 4Figure 4: Grinding LED sample in water on smooth surface PAGEREF _Toc4940354 h 9Figure 5:Attach the LED sample on hot plate PAGEREF _Toc4940355 h 9Figure 6: Obtaining blue LED chip area under optical microscope with width=124.
09m and length= 223.33m. PAGEREF _Toc4940356 h 10Figure 7: Optical power decrease measured at different current for two set of samples PAGEREF _Toc4940357 h 13Figure 8: Comparison of an unstressed blue LED (top) and four temperature + current stress LEDs (below) for 500 hours PAGEREF _Toc4940358 h 13Figure 9: C-V characteristic measured during stress at 60mA and 90oC on one of the blue LED samples. Measurement frequency is 100kHz PAGEREF _Toc4940359 h 15Figure 10: IV characteristic of white LED before and after temperature stress at 120oC PAGEREF _Toc4940360 h 16 Chapter 1 Introduction 1.1 Background of LEDOver the past decades, with technology become mature for light emitting diode (LED), owing to the endeavour to multiple research groups. LED starts to replace the older generation lighting device such as incandescent lamp in recent years, as their excellent robustness, high efficiency, environmentally friendly and long-lifetime. Hence, LEDs are currently available in the market for the public and also integral to human’s daily life. For example, LED used as the backlight of television, traffic light, data transmission and general lighting. Base on people more likely to use the LED, reliability must be well considered when producing it. 1.2 Definition of reliability and failure analysisThe term reliability generally refers to the probability of operating a product for a given time period under specified conditions without failure. According to Reliability and Failure of Electronic Materials and Devices , reliability also reflect the physical performance of products over time and is taken as a measure of their future dependability and trustworthiness. Failure analysis used in all phase of product development cycle. It targets to identify the cause of failure . Failure analysis test conducted in every step from manufacture of the product. Malfunction of an electronic device in manufacture can divide into four stages: design, prototype, fabrication and field operation. Those four stages failure analysis can provide useful information such as failure mechanism and possible failure risks. Generally, failure analysis tests undergo in a more extreme condition than the normal using situation. The products usually push to their maximum as to ensure it function well in any circumstance. 1.3 ObjectivesThe main objective in this report including:Investigate the impact of temperature and current stress to LED lifetimeThe aim is to find the how LED lifetime would change under abnormal working conditions. We can estimate the working lifetime by speeding up the aging of LED.Investigate the causes of reduction of optical power in LEDIn order to know why the reduction of optical power after stress test conducted, study the LED chip at microscopic level give a better understanding of the how to make the LED more reliable1.4 MotivationChapter 2 Literature Review2.1 Review on structure of LEDs2.1.1 White LEDThere are multiple ways to generate white light with LED. Refer to figure 1, generation of white LED light can be the combination of two, three or four monochromatic light sources. Di-chromatic white light source has the poorest colour rending index and highest luminous efficacy. Tetrachromatic white source have the greatest colour rending index and lowest luminous efficacy. Besides mixture of light source, different phosphors are coated on the surface LED. Light fully or partially excited one or several phosphors. As shown in Figure 2, the basic structure of white LED mainly consists of blue LED chip, yellow phosphors, bond wire, leak and bond wire. These are the component determine the colour, brightness and working principle of an LED. Besides from those components mentioned above, the copper frame and aluminium plate responsible to support elements inside the LED. Also, there is a transparent encapsulate seal all the elements to be a single LED bulb which most likely to be silicone.Figure SEQ Figure * ARABIC 1: Method of generation of white LED light implemented as di-, tri-, tetrachromatic sources Figure SEQ Figure * ARABIC 2: Basic structure of a white colour LED 2.1.2 Blue LEDBlue LED is a very important invention to the development of LED technology. It set the foundation of developing the white LED in later years. At 1993, Shuji Nakamura develops the first blue LED . The basic structure of Blue LED is shown in figure 3. Charged carrier pumped from p-type to n-type region respectively. Radioactive recombination of electrons and holes occur in the InGaN multi quantum well (active region) which produce blue light. Figure SEQ Figure * ARABIC 3: Schematic diagram of blue light emission from a GaN-based LED2.1.3 Surface roughening of GaN-Based LEDsCommonly, the conventional GaN-based LEDs have an internal quantum efficiency of nearly 100%. However, the external quantum efficiency only has 5 to 10 percent in most of the commercial LEDs . The loss of light extraction efficiency mostly due to total internal reflection of light in the active region of LED. Surface roughening of LED is one of the approaches to improve the light extraction efficiency . It reduces the scatter of light and total internal reflection of the light outward. The PEC etching process produce roughened surface of LED without damage any properties of LED. T. Fujii demonstrate the optical power have hugely improved with surface roughened. Figure SEQ Figure * ARABIC 4: SEM image of N-face GaN surface etched by Potassium hydroxide based PEC method for 2 minutes(left) and 10 minutes(right)Chapter 3 Experimental Method 3.1 Experimental MethodTwo groups of blue, red, green and white LED are used to conduct the temperature and current stress test. Six blue LED are used to conduct the temperature + current stress test. For temperature stress test, LED were placed into the oven for 120oC. For current stress, the LED were placed into 30mA stress. 3.2 Procedure3.2.1 LED light bulb temperature stressBlue, red, green and white colour LED prepared for the temperature stress. First of all, the LED conducted the test for voltage-current characteristic and measure the optical power. Next, placed the LED inside the oven for 120oC. The LEDs were taken out weekly for measuring the voltage-current characteristic and aged optical power. 3.2.2 LED light bulb current stressLEDs were tested for voltage-current characteristic and optical power initially. Then the LEDs were placed on the LED aging tester. Set the current to 30mA and frequency be 50Hz. The duty ratio was set to 10:90 to minimize the difference between junction temperature and oven temperature. The timer can monitor the stress time of LED. The LEDs were taken out weekly for measuring the current stressed voltage-current characteristic and the aged optical power.3.2.3 LED light bulb temperature + current stressLEDs were tested for voltage -current characteristic and optical power at first. Place the LED in the oven with 90oC and connect it to the current stress. The aging tester monitors the aging process. The LEDs were taken out weekly for measuring the current stress voltage current characteristic and aged optical power.3.2.4 Optical Power MeasurementConnect the power supply to the LED with the correct polarity. The polarity determines whether the LED light up or not. Place the LED into the aperture of the integrating sphere. Input the wavelength of the LED ensure the optical power measurement correct. Before the measurement start, set the optical power meter to zero. Then joint the Thorlabs PM100D power energy meter to the integrating sphere. Calibrate the power supply gradually and record the optical power. 3.2.5 Capacitance-Voltage characteristic of LEDsPlace the LED in the hole represented by positive and negative charge in Stanford Research SR 720 LCR. Calibrate the LCR meter and connect the DC power supply. Select the capacitance measurement to 100kHz. Gradually increase the power supply to the LCR meter from 0V to 5V. Polarity change and do the measurement from -5V to 0V. 3.2.6 Examine Side-view structure of LEDs by MicroscopeHeat the polishing tool on the hot plate by around 150oC. Attach the LED sample on the heated polishing tool (see figure 4) by melted wax. As the sample fully attached, prepare different grade of sand paper to scrape the LED sample. The grind should perform on distill water (see figure 5) to avoid residue of LED reside on sand paper. Polish the LED surface with polishing paper. Repeat the steps for the other side of the LED. The flat sized LED can be observed under optical microscope.Figure SEQ Figure * ARABIC 5: Grinding LED sample in water on smooth surfaceFigure SEQ Figure * ARABIC 6:Attach the LED sample on hot plate3.2.7 Observation surface area of the LED chipAs the LED sample scrape into thin layer, cutter used for divide epoxy near to the LED chip. The LED chip should reside at one of the fragments. Carefully obtain the chip from the fragment by forceps. All the process needs to perform on a container since the LED may lost while cutting from the epoxy. The chip can be observed under optical microscope. With the assistance of the program installed in microscope, the length and width of the chip can be obtained. By multiply the length and width of the chip. Compare the stressed and unstressed LED chip, defects should be observed on the stressed LED chip.Figure SEQ Figure * ARABIC 7: Obtaining blue LED chip area under optical microscope with width=124.09m and length= 223.33m.3.3 Chapter 4 Results and Discussions4.1 Result: Blue LED Temperature + Current stress:Total of six LED sample was involved in temperature + current stress test. Three LEDs were carried out at 60mA current stress and 90oC temperature stress simultaneously. The other three LEDs were at 30mA current stress and 90oC temperature stress at the same time. In figure 7, blue colour LEDs was under temperature and current stress for 1000 hours. As can be notice, temperature and current stress induce drastically decrease in optical power emitted by the LED. For under 60mA current stressed LED, the optical power reduced more compare to the 30mA. The 60mA stressed LED have completely lost is worth notice. This result suggests current and temperature stress induce defect to LED chip. According to A Review on the physical mechanism that limit the reliability of GaN-Based LEDs , optical degradation of LEDs due to direct current stress can induce increase the defectiveness of the active layer. The crystal defect reduces the lifetime of LED due to non-equilibrium electron-hole pairs  and increase the density of state in the forbidden gap in the active region of LED. The result also shows the greater current stress induce more defect on the nonradiative centre of LED chip. After the 1000 hours temperature + current stress, the epoxy also changes into yellowish (see figure 8). Colour change of the epoxy lens also contribute reduction of light transmission . Figure SEQ Figure * ARABIC 8: Optical power decrease measured at different current for two set of samplesFigure SEQ Figure * ARABIC 9: Comparison of an unstressed blue LED (top) and four temperature + current stress LEDs (below) for 500 hoursStudies Charged carrier in the active region of LEDs can enhance the understanding of LEDs degradation process better. Therefore, the C-V characteristic was measured during the stress 60mA at 90oC. The grey curve is the unstressed measurement. P-type region doped more heavily than n-side , figure 9 show the capacitance in the undoped region and to the n-side of the diode. As the figure 9 shown stress test cause increase in capacitance in the region of 0 V < V < 3 V. The region is the barrier of space charge region is in the active layer of the LED. As a result, figure 9 show the stress induced change in charge density in the active region of LED. The change in charge density in the active region can be calculate by the following formula:€QT=1qAVIVfCTV-CoV dVwhere A is the junction area of LED chip, q is the charge of an electron (=1.602176E-19) , CT(V) is the junction capacitance at time T as the measured voltage V. Vf and Vo represent the final and initial voltage in the C-V characteristic graph. As the LED chip area was calculate in figure 6, A=124.09m — 223.33m= 2.771E-8 m2 Therefore:€QT=11.602176—10-19—2.771—10-8—199.8648 pF=4.502—1016 C€™m-2Figure SEQ Figure * ARABIC 10: C-V characteristic measured during stress at 60mA and 90oC on one of the blue LED samples. Measurement frequency is 100kHz4.2 Result: LED Temperature stress4.2.1 White LEDAs figure 10 shows, the black curve is the unstressed IV characteristic. The rest of the curve are lag behind. The arrow represents the trend of the IV characteristic. The stressed LED required more current to achieve the same level of voltage. It also reflects the power decrease for longer stress time. The result provide evidence that temperature stress can induce defects in the active region of stressed LED and reduce lifetime.Figure SEQ Figure * ARABIC 11: IV characteristic of white LED before and after temperature stress at 120oC4.3 Result: LED Temperature stress4.3.1 White LEDChapter 5 Conclusions and Future RecommendationConclusions5.2 Future recommendationsReferencesB. Marius and B. Titu, Failure analysis – When?, (Chichester, UK: John Wiley & Sons, Ltd, 2011), pp. 37-39M. Ohring, J. R. Lloyd and L. Kasprzak, Reliability And Failure Of Electronic Materials And Devices, 2nd ed. (Elsevier Science & Technology, 2014.), pp. 15-16.E.F. Schubert, Solid-state light sources getting smart. 308.5726 (2005), doi: 1095-9203Z.C. Feng, III-Nitride Devices and Nanoengineering (World Scientific Publishing Company, Singapore, 2008).Fujii, T., Gao, Y., Sharma, R., Hu, E., DenBaars, S. and Nakamura, S. (2004). Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening. Applied Physics Letters, 84(6), pp.855-857.M. Meneghini, A. Tazzoli, G. Mura, G. Meneghesso, and E. Zanoni, IEEE Transactions on Electron Devices 57, 108 (2010)P.V. Dollen, S. Pimputkar and J.S. Speck, Let There Be Light- With Gallium Nitride: The 2014 Nobel Prize in Physics, 53, 51(2014), doi: 10.1002/anie.201410693A. Uddin, A.C. Wei, T.G. Andersson, Study of degradation mechanism of blue light emitting diodes, (Elsevier, Thin Solid Films ,2005), pp.378″381S. Buso, G. Spiazzi, M. Meneghini, and G. Meneghesso, IEEE Transactions on Device and Materials Reliability 8, 312 (2008).
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