高反射率p-GaN欧姆接触电极

根据光学薄膜原理计算了GaN/Ti/Ag、GaN/Al和GaN/Ni/Au/Ti/Ag、GaN/Ni/Au/Al多层电极结构的反射率,得出Ag基和Al基反射电极均能在全角范围内提供较高的反射率。实验测量结果表明,反射率能高于80%的Ag基反射电极,具有低欧姆接触的电学特性。并将GaN/Ni/Au/Ti/Ag多层反射电极应用在上下电极结构的GaN基LED中。实验上采用两步合金法获得了低接触电阻、高反射率的电极结构,并引入Ni/Au覆盖层克服了Ag高温时的团聚和氧化现象。解决了Ag电极的稳定性问题,显著地提高了LED的出光效率,成功制备了具有上下电极结构的GaN基LED管芯。

Ohmic Contact of High Reflectivity on p-Type GaN

GaN-based light-emitting diodes provide higher performance in the short-wavelength part of the visible and ultraviolet spectrum than any other material system. However, there is still a great need for improvement of the extraction efficiency. Because many parts of emitting light of LED suffer absorption of the metal contacts and bond pads, also by active layer. So the employment of highly reflective metal ohmic contacts with a low contact resistance could substantially improve the GaN extraction efficiency of GaN-based LED, especially for vertical structure LED fabricated by laser lift-off process, which have more light output surface and high thermal conductivity than normal structure LED. The high reflectivity of omni-direction reflector is reported, that is, incorporation of Ag and Al onto the contact layers acting as a reflecting layer. Firstly, the reflectivity of the layers Ti/Ag, Al, Ni/Au/Ti/Ag and Ni/Au/Al were calculated through the theory of optical films. Then the reflectivity and contact characteristic of the GaN-based LED samples with these multi-layers on p-type GaN were measured. The result showed high omni-directional reflectors can be achieved over Ag-based films structure, and Al-based also, but the Ag-based films can provide higher reflectivity of 80%, which is better than that of Al-based film. The current-voltage curves showed low resistance on these contact layers except the Al direct on p-GaN. And the resistance of Ni/Au/Ti/Ag and Ni/Au/Al just depends on the annealed Ni/Au layers. Here the Ni/Au layers not only supply the ohmic contact but also the spreading the current. The conglomerate and oxidation of Ag film under high temperature are eliminated by introducing other Ni/Au cladding layers. Because the thickness of Ag is enough for high reflectivity so the additional layers after Ag also have no effect on the reflectivity. The good stability, low resistance and high reflectivity were obtained by two-step annealing methods. Then we applied the multi-layer Ni/Au/Ti/Ag/NiAu to vertical structure light emitting diodes (LED) by laser lift-off technology. The light output efficiency was improved significantly. The vertical structure LED with high reflective contact layer was fabricated successfully.