8 mil×10 mil InGaN/GaN blue LEDs with indium tin oxide（ITO） emitting at 460 nm were fabricated. A vacuum evaporation technique was adopted to deposit ITO on P-GaN with thickness of 240 nm.The electrical and optical properties of ITO films on P-GaN wafers,as well as rapid thermal annealing（RTA） effects at different temperatures（100 to 550℃） were analyzed and compared.It was found that resistivity of 450℃RTA was as low as 1.19×10^（-4）Ω·cm,along with a high transparency of 94.17%at 460 nm.AES analysis indicated the variation of oxygen content after 450℃annealing,and ITO contact resistance showed a minimized value of 3.9×10^（-3）Ω·cm^2.With 20 mA current injection,it was found that forward voltage and output power were 3.14 V and 12.57 mW.Furthermore,maximum luminous flux of 0.49 lm of ITO RTA at 550℃was measured,which is the consequence of a higher transparency.
The path of photons in the thin film （TF） light emitting diode （LED） was analyzed. The reflectivity of reflector in AlGaInP TF LED with and without the AlGaInP layer was contrasted. The absorption of the AlGaInP layer was analyzed and then the light extraction was calculated and shown in figure. The TF AlGaInP LED with 8μm and 0.6μm GaP was fabricated. At the driving current of 20 mA, the light output power of the latter is 33% higher. For the 0.6μm GaP LED, the etching of heavily doped GaP except the ohmic contact dot area is advised. The design and optimizing of current spreading between the n-type electrode and the p-type ohmic contact dot need further research.
In order to connect several independent LEDs in series, inductively coupled plasma （ICP） deep etching of GaN is required for isolation. The GaN-based high-voltage （HV） LEDs with a 5 μm deep isolation groove and an acceptable mesa sidewall angle of 79.2° are fabricated and presented. The surface morphology and construction profile of the etched groove are characterized by laser microscopy and scanning electron microscopy. After contact metal formation and annealing, the electrical properties are evaluated by I-V characteristics. The trend of the I-V curve has good accordance with conventional LEDs. The contact resistance of HV LEDs is also tested and was reduced by 4.6 Ω compared to conventional LEDs, while the output power increased by 5 W. The results show that this technique can be applied to practical fabrication.
Wire bonding is one of the main processes of the LED packaging which provides electrical interconnec- tion between the LED chip and lead frame. The gold wire bonding process has been widely used in LED packaging industry currently. However, due to the high cost of gold wire, copper wire bonding is a good substitute for the gold wire bonding which can lead to significant cost saving. In this paper, the copper and gold wire bonding processes on the high power LED chip are compared and analyzed with finite element simulation. This modeling work may provide guidelines for the parameter optimization of copper wire bonding process on the high power LED packaging.
In order to determine the environmental effects on the luminescence properties of a phosphor layer for high-power light emitting diodes, a high humidity and temperature test （85℃/85%RH） and a thermal aging test （85℃） were performed on silicone/YAG phosphor composites. The luminescence properties of silicone/phosphor composites are monitored by a fluorescence spectrometer. The results show that high temperature could result in an increase in conversion efficiency of composites during the early aging stage and red shift of YAG phosphor; and high humidity could result in a significant decrease in conversion efficiency of composites while having a small influence upon the optimal excitation wavelength of the YAG phosphor.
A new type application specific light emitting diode （LED） package （ASLP） with freeform polycarbonate lens for street lighting is developed, whose manufacturing processes are compatible with a typical LED packaging process. The reliability test methods and failure criterions from different vendors are reviewed and compared. It is found that test methods and failure criterions are quite different. The rapid reliability assessment standards are urgently needed for the LED industry. 85℃/85 RH with 700 mA is used to test our LED modules with three other vendors for 1000 h, showing no visible degradation in optical performance for our modules, with two other vendors showing significant degradation. Some failure analysis methods such as C-SAM, Nano X-ray CT and optical microscope are used for LED packages. Some failure mechanisms such as delaminations and cracks are detected in the LED packages after the accelerated reliability testing. The finite element simulation method is helpful for the failure analysis and design of the reliability of the LED packaging. One example is used to show one currently used module in industry is vulnerable and may not easily pass the harsh thermal cycle testing.
