GaN基紫光LED的高反射率p型欧姆接触

High-reflectance Ohmic Contacts to p-GaN of GaN-based Violet LEDs

研究用于GaN基大功率倒装焊(Flip-chip)紫光LED(UV-LED)的高反射率p型欧姆接触的电学和光学性能。用磁控溅射的方法在GaN基LED外延片表面沉积了不同厚度Ag,Al,Au和Pd四种金属,测量了样品的反射率和透射率。结合同步辐射高强度X射线衍射和AFM对金属薄膜的晶体结构进行分析,并对表面形貌进行了观测,对由金属薄膜构成的多层膜结构及其对光反射率的作用机理进行了研究。测量结果表明,在入射光波长为400nm时,Ni/Au/Ag和Ni/Au/Al电极的反射率比Ni/Au的反射率提高了三倍。同时与p-GaN有良好的欧姆接触特性。

High-reflectance ohmic contacts to p-GaN are the key for Flip-chip ultra-violet light emitting diode （UV-LED） to improve the external quantum efficiency （ EQE）, so the kinds of metal or alloy film both ohmic contact and the reflector of light are important to choose the appropriate high-reflectance and low-resistance ohmic contact to p-type GaN. The GaN-based LED samples used were grown by Metalorganic Chemical Vapor Deposition （MOCVD）. Four different metal films were deposited on the surfaces of p-GaN and annealed Ni/Au alloy. The thickness of these metal films varied from 30 nm to 840 nm. The reflectance and transmittance of the samples were measured by WGS-9 system. The 2θ scans XRD and surface AFM images of the samples were also observed. As results, when the films＇ thickness under the 130 nm the reflectance of Ag-deposited samples increased with the increasing thickness and the transmittance decreased at creasing of reflectance is the reduction of the light transmitted wavelength of 400 nm. The reason of the inthrough the films. When the films＇ thickness over 130 nm, the reflectance of Ag-deposited samples decreasing. From the results of XRD and AFM, the metal film formed multi-layers film and the roughness Ra and Rq increased with the increasing thickness. It is suggested that the diffusion of the multi-layers films is the main reason of reduction of reflectance with the films thickness increasing. Other metal films show the same characteristics as the Ag-deposited films. From the result of reflectance, Al and Ag are the suitable candidates for the reflector, but it is reported that the single Al or Ag films don＇t make good ohmic contacts on p-GaN. So a thin annealed Ni/Au is adopted to interspersed between the Al or Ag reflector and the p-GaN in order to provide efficient hole injection into the device. Fromthe results of reflectance and I-V curves of Ni/Au/Ag and Ni/Au/A1, the 1-V characteristics of N^Au/Ag and Ni/Au/Al contacts are similar to this of Ni/Au annealed. However the reflectance of Ni/Au/Ag and Ni/Au/ Al contacts is three times higher than that of Ni/Au at wavelength of 400 nm. These results demonstrated that Ni/Au/Ag and Ni/Au/Al can be used as high-reflectance and low-resistance p-GaN ohmic contacts for flip-chip UV-LEDs