| dc.description.abstract | In order to further push the performance/cost ratio of solid state lighting, LED manufacturing favors the products with high operation power and large chip sizes, which can greatly save material cost in device fabrication and packaging. However, the devices with increased input power and emitting area are constantly haunted by severe thermal degradation and current crowding, which are the contributing factors to the undesired efficiency droop. The problem is particularly difficult for sapphire-based devices considering the poor thermal conductivity of the substrate. In this dissertation, III–nitride blue LEDs were successfully fabricated on ceramic substrates (thermal conductivity: 230 W/m•K) using thin-film and high-voltage processes.
In middle size aspect (508*1066 um2), we demonstrated ceramic-based high-voltage thin-film GaN LEDs comprising serially connected 31 sub-cells has 26.7% improvement in wall-plug efficiency at the current density of 200 A/cm2 comparing to 1*1 sub-cell owing to its superior current spreading.
In large size aspect (1143*1143 um2), we designed ceramic-based high-voltage thin-film GaN LEDs comprising serially connected 2*2, 3*3 and 4*4 sub-cells, which are fabricated with different n-electrode areas to optimize the wall-plug efficiency. We found that although current spreading can be improved by increasing the cell numbers, leading to lower voltage/cell characteristic and enlarge emitting area. However, owing to the loss of emitting area caused by the increased area of isolation trenches, the output power and the wall-plug efficiency decrease when the cell number exceeds 9 (3*3). Furthermore, 2*2 and 3*3 sub-cells with 4.6% and 2.7% n-electrode/mesa respectively show the optimized design, the wall-plug efficiency are improved by 6.4% comparing to 1*1 sub-cell.
We compared electrical-optical characteristics with increasing current density for large size (1143*1143 um2) light emitting diodes with three different kinds of structures. It include Si-based vertical thin-film LED, sapphire-based LED comprising serially connected 4*4 sub-cells and ceramic-based thin-film LED comprising serially connected 4*4 sub-cells. We found that although the current spreading of Si-based vertical thin-film LED worse than that of sapphire-based LED comprising serially connected 4*4 sub-cells, however, owing to better thermal conductivity of Si (thermal conductivity: 150 W/m•K) than sapphire (thermal conductivity: 36 W/m•K), the saturation current of Si-based vertical thin-film LED is superior to sapphire-based LED comprising serially connected 4*4 sub-cells. For the ceramic-based thin-film LED comprising serially connected 4*4 sub-cells, it simultaneously holds advantages on heat sinking and current spreading, leading to the saturation current density larger than 450 A/cm2, which is better than those of the devices fabricated with identical epitaxial structure on Si or sapphire substrates. | en_US |