Development of semiconductor light sources for spotlight luminaire

In contrary to conventional lighting sources featured a mature manufacturing technique, the progressive improvements of the LED’s luminous efficiency encourages the replacement of conventional lamps with highly efficient LED luminaires. From a practical point of view, part of the input electrical power will be converted into heat energy during LED operation; thereby, degrading their optical properties in terms of the light emission efficiency and the device lifetime provided the heat generated from the pn junction of the LEDs does not easily conduct to the attached heat sink. In this project, we shall fabricate a thin-film InGaN light-emitting diode (Thin-film InGaN LED) by using laser lift-off technology to remove the poor thermal conductivity sapphire substrate. The use of such LEDs with a substrate-free geometry not only helps to facilitate heat dissipation through the shorter way (p-GaN) to a heat sink but also effectively enhances the light output power of the LEDs at high current injection levels. It is known that the amount of light emitted from the multiple-quantum-well (MQW) active layer and directly escape out of the top surface of the LEDs mainly depends on total internal reflections. In addition to engineer the surface topography of the LEDs, a particular interesting is focused on micro-cavity light-emitting diode (MCLED) to get enhanced extraction efficiency and achieve the improvement of the emission directionality. For the LEDs with a wavelength-scale cavity, the resonator will modify the spectral density of optical modes in the direction parallel to the cavity axis. This will cause the preferential propagation direction of the photons being forced towards the light extraction cone. However, the presence of the guided waves in the cavity will result in the limited efficiency of the MCLEDs. One possible way to tailor the guided waves and further improving the device performance can be done by combining the MCLEDs with a diffraction grating/photonic crystal. As a result of diffracting the light of a guided wave, the resultant waves could fall into the extraction cone. The program schedule is organized as: 1. An asymmetric InGaN MQW will be performed to increase hole distribution uniform among the wells, which is beneficial to improve the optical properties of the fabricated LEDs. 2. Development of an omni-directional reflector with high reflectivity and low specific contact resistance on top of the p-GaN. 3. To fabricate a thin-film MCLED by electrodeposition and laser lift-off technologies. 4. To fabricate a thin-film MCLED with diffraction grating/photonic crystal by using electron beam lithography technique. Finally, we anticipate that highly directional semiconductor light sources (beam emission angle ＜ 50o) made up of thin-film MCLEDs have a light intensity and luminous efficiency respectively greater than 400 lumens and 90 lm/W and suitable for use in spotlight luminaires.

Project ID:PB10106-0248
External Project ID:NSC101-2622-E182-001-CC2