| dc.description.abstract | The morphological effect of the patterned sapphire substrate on the light radiation pattern of LED is investigated. Three patterned sapphire substrates are fabricated by the wet-etching process. The GaN-LEDs were processed on the three-patterned sapphire wafers (NPSS I, NPSS II, and NPSS III) by using MOCVD. The LED chips were packaged as white-light LEDs with YAG phosphors. The light radiation pattern of LEDs is measured and shows a dependence on patterned sapphire morphology. The divergence angle of the NPSS I, NPSS II, and NPSS III LED are estimated to be 42.5º, 38º, 32º, respectively. In the white-light package of NPSS I LED, the average CCT of white-light LED with 1.5 mm, 1.7 mm, 2.0 mm phosphor dome is 6732 K, 6283 K, and 6971 K, respectively. The ACCTD of white-light LED with 1.5 mm, 1.7 mm, 2.0 mm phosphor dome is 710 K, 2131 K, and 4222 K, respectively. The light path length in the phosphor layer and the light radiation pattern are the keys to affect the local CCT of the white-light LED.
In the 2-D analysis, an incident angle change model is built to predict the change of incident angle of light from any direction, and also the range of incident angle in which the total internal reflection will happen. The model of the characteristic of light transmitting the patterned sapphire is also built to describe the incident angle change of the light which travels into the blue angle region which guides the light back into the GaN layer and travels to the GaN/air interface. The incident angle to the GaN/air interface is predictable, and also shows that the light travel into the blue angle region can neither transmit the LED top surface nor transmit into the sapphire cavity. This results in a decrease of the sidewall emission.
By the Monte Carlo ray-tracing method, the light radiation pattern can be simulated. The simulation result shows that the high-intensity region of the light radiation pattern becomes more centralized as the dihedral angle and pattern coverage increases. The output power of the LED chip in top surface emission and sidewall emission are also simulated. The sidewall emission power ratio decreases as the pattern dihedral angle and the pattern coverage increases. As the sidewall emission power decreases, the light radiation pattern becomes more centralized and makes the divergence of the light radiation pattern decrease. The simulation result can correspond to the 2-D analysis and the experiment results. | en_US |