New progress in research on flip-chip light-emitting diode chips

Prof. Sheng Liu from the School of Science and Technology Dynamics and Mechanics made new progress in the field of high-efficiency flip-chip light-emitting diodes (LEDs). Research results were published in Optics Express, a leading journal in the field of optics.

The paper entitled Numerical and experimental investigation of GaN-based flip-chip light-emitting diodes with highly reflective Ag/TiW and ITO/DBR Ohmic Contacts (Numerical Simulation and Experimental Study of High-Efficiency Gallium Nitride Based Flip Chip LED Chips The first author is Zhou Shengjun, an associate professor of the School of Power and Mechanical Engineering, and the author of the communication is Liu Sheng.

The high-power flip-chip LED chip has broad application prospects in the high-end LED lighting market characterized by point light sources, extremely high current density, and optical density, such as automotive headlights, stage lights, projectors, and searchlights. Flip-chip technology is a core technology in the field of three-dimensional packaging. Forming a p-type electrode with high reflectivity and low-resistance ohmic contact is a key technology for high-efficiency flip-chip light emitting diode (LED) chips. Due to the low bonding strength between the electron beam evaporated Ag film and p-GaN, and the high ohmic contact resistance, currently academics and industry have increased the adhesion strength by lowering the ohmic contact resistance by inserting a Ni film under the Ag film. Ni has a strong absorption of visible light, resulting in a sharp decline in the reflectivity of Ni/Ag, thereby reducing the luminous efficiency of flip-chip LED chips.

In order to solve this problem that plagued the academic community and industry, Liu Sheng team used ion beam sputtering simulation to prepare Ag film to improve the adhesion strength of Ag and p-GaN, rapid thermal annealing at 600 °C in a nitrogen atmosphere to make pure Ag film Forms a low resistance ohmic contact with p-GaN. Although the bonding strength and ohmic contact between pure Ag and p-GaN are solved by ion beam sputtering and high temperature thermal annealing, high temperature annealing causes clustering of the pure Ag film, resulting in a decrease in reflectivity of the Ag film. Through a large number of experiments, they found that sputtering a thin film of TiW on the Ag film can effectively suppress the clustering phenomenon of the Ag film during the high-temperature annealing process, thereby obtaining a p-type electrode Ag with high reflectivity and low resistance ohmic contact. TiW, and successfully applied to flip-chip LED chip.

Based on the breakthrough of high-current-density LED chip special lighting technology, it is expected to significantly improve the international competitiveness of China's semiconductor lighting industry. Based on the drift-diffusion equation, carrier continuity equation and heat transfer theory, Liu Sheng’s team established an electro-optical-thermal-reliability analysis model for LED chips with large current density, and optimized the design of flip-chip LED chips. A high-efficiency, high-current-density flip-chip LED with Ag/TiW as p-type ohmic contact was successfully fabricated, which increased the light output saturation current of the chip to twice the original.

This research topic has been funded by the National Natural Science Foundation of China (U1501241). Research results are expected to be applied in the fields of automotive lighting and special lighting.