Highly reflective Ag/La bilayer ohmic contacts on p-GaN

• Main Authors: Yuh-dar Chen, 陳昱達
• Other Authors: I-chen Chen
• Format: Others
• Language: zh-TW
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/19167549261936432306

碩士 === 國立中央大學 === 材料科學與工程研究所 === 98 === GaN-based Light-emitting diode has great progress in recent years. In order to achieve the purpose of solid-state lighting, conventional LED which emit most light through the transparent conducting layer on the top of device could not satisfy this demand. Two types of LED device structure, which are vertical structure LED and flip-chip structure LED, have been proposed to fabricate high brightness LED. For these LEDs, it is critical to search for a ohmic contact structure which is highly reflective and low contact resistance to p-GaN. In all materials, silver is a good choice for being used as a p-GaN ohmic contact because it has the highest reflectivity in visible spectrum and forms a good contact to p-GaN by annealing. But Ag film would agglomerate rapidly and be divided into lots of islands when it was rapid-thermal-annealed for obtaining low contact resistance or bonded with another substrate. The phenomena described above resulted in the increase of forward voltage of device and the decrease of light output power. So it is important to avoid the agglomeration of silver thin film. For this reason, we evaporated a thin lanthanum layer on thick Ag layer. After rapid-thermal-annealing at 450℃for 1minute, this structure exhibited the lower specific contact resistivity of 3.449×10-4Ω cm2 and higher reflectivity of 96％ than single Ag contact. The reason why it has better performance is that lanthanum is easy to oxidize into lanthanum oxide. And it may be a good passivation layer for preventing Ag film from exposing in oxygen-containing ambience. As a result, it could inhibit the serious agglomeration of Ag film which was caused by the drastic increase of surface diffusion coefficient of Ag atom. Besides, we tested the thermal stability of Ag/La bilayer structure for investigating the reliability of this structure during the bonding process of two LEDs. Experimental results showed that it still had the good property after long-time heat treatment. Therefore, Ag (200nm) / La (40nm) bilayer structure is suitable for the reflective ohmic contact of p-GaN.