Performance and reliability of InGaN-GaN light-emitting diodes with mirror wafer bonding technology

High-brightness GaN-based light-emitting diodes (LEDs) have attracted great attention for their versatile applications in mobile phones, full-color display and lighting. Although the development of these GaN-based LEDs is very successful, the poor conductivity of p-GaN still limits the performance of LEDs because of the current crowding problem. Furthermore, due to the poor electrical property (Electrical resistivity=10¹¹-10¹⁶ ohmcm) of the insulating sapphire substrate, the electrodes of the p- and n-metal are both on the top surface of the devices. The heat dissipation of sapphire substrate is also poor, thus the GaN blue LEDs are typically operated under low injection current. To solve these problems, we have developed a novel Vertical Omni-directional Mirror Adhesion (V-OMA) technique to bond GaN-based light-emitting diodes to a Si substrate with vertical electrodes. A high-temperature stable organic film, instead of a solder metal, is used as the bonding agent in order to avoid any possible reaction with the metal reflector. The thermal conductivity of Si (168 Wm^-1K^-1) is 4.8 times higher than that of sapphire (35 Wm^-1K^-1). The V-OMA InGaN LED devices could be operated in a higher injection current than that of used in sapphire-substrate LEDs. The devices are also designed to improve the extraction efficiency by reflecting the downward light using a metal mirror. In comparison to the process of conventional GaN-based LEDs, that of the V-OMA InGaN LED devices is easier and with greater advantages. Another advantage of the VOMA technique is that the light emitting area increases because there is only single electrode on the topside of LED devices. Figures 1(a) and (b) show the top view and side view of a V-OMA InGaN LED chip driven with a low current. Since the device structure was p-side down and n-side up with an n-metal electrode, the current can spread uniformly without a current spreading layer. The V-OMA InGaN LED, thus, does not have the current crowding problem on the top emitting area as the n-GaN LED. The device exhibited normal p-n diode behavior with a forward voltage of 3.2 V at 20 mA, which was similar to that of the conventional LEDs. This indicated that the wafer-bonding process did not degrade the performance of LEDs. Figure 2 shows the effects of injection current on the luminous intensity of the V-OMA InGaN LED chip and a standard sapphire-substrate device with a size of 12 × 12mil². The light output of the V-OMA InGaN LED chip is 2.8 times brighter than that of the conventional InGaN LED. The life test of V-OMA InGaN LED was performed at a forward current of 20 mA at 55°C as shown in Figure 3. It was found that no degradation of luminescence-output intensity was observed after 720 h of life test.