The ohmic reflectivity of Ni/Ag/Ti/Au in contact with p-GaN is studied. It is found that under different thickness values of Ni, different annealing temperatures and different annealing atmospheres, the performances of Ni/Ag/Ti/Au electrode are greatly changed. The contact resistivity is measured using the transmission line model. The reflectivity of the electrode is investigated by using a spectrophotometer. The results reveal that the thinner the Ni metal layer, the higher its reflectivity is, in addition, the thickness value of Ni metal has a little influence on contact resistivity. There appears an abrupt decrease in reflectivity of electrode after annealing at a temperature higher than 400 ℃. It is noticed that the reflectivity decreases more sharply after annealing in oxygen atmosphere than in nitrogen atmosphere. However, annealing in oxygen atmosphere is more helpful to reduce the contact resistivity. The comprehensive evaluations of the contact resistivity and reflectivity indicate that the better performances of Ni (1 nm)/Ag/Ti/Au electrode after rapid annealing in oxygen atmosphere at 400 ℃ are achieved: its contact resistance reaches 5.5× 10-3 Ω·cm2 and reflectivity rises up to 85% at 450 nm. Light emitting diode (LED) of vertical structure is fabricated with an optimal electrode. The LED under an injection current of 350 mA can achieve the following working parameters: the working voltage is 3.2 V, the optical output power is 270 mW, and the electro-optical conversion efficiency is 24%.
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 reflectivity is measured by reflectivity 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 3.6×10-3 Ω·cm2. Additionally, the deposition temperature of Ni/Ag is investigated. It is noticed that the specific contact resistivity decreases and the reflectivity 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 6.4×10-3Ω·cm2 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%.
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