低接触电阻率Ni／Ag／Ti／Au反射镜电极的研究

Reflective Ni/Ag/Ti/Au electrode with low specific contact resistivity

研究了Ag的厚度、退火时间、沉积温度对于Ni/Ag/Ti/Au电极的反射率及与p-Ga N欧姆接触性能的影响.利用分光光度计测量反射率,采用圆形传输线模型计算比接触电阻率.结果表明:随着Ag厚度的增加,Ni/Ag/Ti/Au电极的反射率逐渐增大;在氧气氛围中,随着退火时间从1 min增至10 min,300°C退火时,比接触电阻率持续下降,而对于400—600°C退火,比接触电阻率先减小后增大;在300和400°C氧气中进行1—10 min的退火后,Ni/Ag/Ti/Au的反射率变化较小,退火温度高于400°C时,随着退火时间的增加,反射率急剧下降;在400°C氧气中3 min退火后,比接触电阻率可以达到3.6×10 3Ω·cm2.此外,适当提高沉积温度可以增加Ni/Ag/Ti/Au的反射率并降低比接触电阻率,沉积温度为120°C条件下的Ni/Ag/Ti/Au电极在450 nm处反射率达到90.1%,比接触电阻率为6.4×10 3Ω·cm2.综合考虑电学和光学性能,在沉积温度为120°C下蒸镀Ni/Ag/Ti/Au(1/200/100/100 nm)并在400°C氧气中进行3 min退火可以得到较优化的电极.利用此电极制作的垂直结构发光二极管在350 m A电流下的工作电压为2.95 V,输出光功率为387.1 m W,电光转换效率达到37.5%.

The specific contact resistivity and reflectivity of Ni/Ag/Ti/Au contact with p-GaN are studied. It is found that the thickness of Ag, anneal time and deposition temperature have a great effect on the performance of Ni/Ag/Ti/Au electrode. Its optical refiectivity is measured by refiectivity spectrophotometer, and its specific contact resistivity is calculated by circular transmission line model. It is observed that the contact reflectivity values of Ni （1 nm）/Ag/Ti （100 nm）/Au （100 nm）, when the thickness values of Ag are 25 nm and 50 nm, are low： their values are 68.5% and 82.1% at 450 nm, respectively, and they start to increase with increasing the Ag thickness, then reach their saturated values when Ag thickness is beyond 200 nm. When the anneal time changes from 1 min to 10 min in oxygen atmosphere, the specific contact resistivity decreases at 300 ℃, decreases further and then increases at 400-600℃. After annealing at temperatures at 300 ℃ and 400℃ in oxygen atmosphere, the contact reflectivity value of Ni/Ag/Ti/Au remains almost unchanged, even when anneal time increases from 1 min to 10 min. However, The contact reflectivity of Ni/Ag/Ti/Au decreases significantly after annealing at a temperature higher than 400℃ and it becomes smaller with longer annealing time. After 400 ℃ annealing in oxygen atmosphere for 3 min, the specific contact resistivity reaches 3.6 ×10^-3Ω·cm^2. Additionally, the deposition temperature of Ni/Ag is investigated. It is noticed that the specific contact resistivity decreases and the refiectivity increases with increasing the deposition temperature from room temperature to 120℃. The reflectivity rises to 90.1% at 450 nm and the specific contact resistivity reaches 6.4×10^Ω·cm^2 for the Ni/Ag/Ti/Au electrode at a deposition temperature of 120℃. However, the effects of improving the electrical and optical characteristics weaken when deposition temperature changes from 120℃ to 140℃. With a overall consideration, Ni （1 nm）/Ag （200 nm）/Ti （100 nm）/Au （100 nm） is made at a deposition temperature of 120℃, and then anneals at 400 ℃ for 3 min in oxygen atmosphere to achieve the optimized electrode. The vertical light emitting diode with this Ni/Ag/Ti/Au electrode is fabricated. Its working voltage is 2.95 V and the light output power is 387.1 mW at 350 mA. The electro- optical conversion efficiency reaches 37.5%.