Light-emitting diodes (LEDs) are a kind of solid-state light-emitting device made of semiconductor materials. They own the advantages of small size, low power consumption, and high reliability and are combinations of III-V chemical elements such as gallium phosphide and gallium arsenide. By applying a voltage to the semiconductor, holes and electrons will recombine under the action of different electrode voltages. At this time, electrons will fall to lower energy levels and release photons simultaneously. Thereby, light is emitted by converting electrical energy to photo energy.
In LED applications, we expect the metal-semiconductor interface in LED structures is an Ohmic contact. In other words, the contact resistance between the metal-semiconductor interface is extremely small, enabling LEDs to have linear and symmetric current-voltage characteristic curves. The formation of an Ohmic contact is related to the work function between the metal and the semiconductor. Take gallium nitride for example. For n-type gallium nitride, if the barrier height is to be eliminated and forming an Ohmic contact, a metal having a low work function should be used. Contrarily, for p-type gallium nitride, a metal having a high work function should be used.
Please refer to FIG. 1, which shows a thin-GaN LED. The structure thereof comprises a semiconductor layer 2, which comprises an n-type semiconductor layer 21, a multiple quantum well layer 23, and a p-type semiconductor layer 22. The p-type semiconductor layer 22 is just p-type gallium nitride. Below the p-type semiconductor layer 22, it comprises an Ohmic contact layer 8, a metal reflection layer 4, a bonding layer 5, and a substrate 6 sequentially. In addition, a first electrode 71 and a second electrode 72 are disposed on both ends of the LED.
The material of the Ohmic contact layer 8 is usually the combination of the metals nickel/gold/nickel/aluminum, which contains high-work-function metals such as gold and nickel for being used as the Ohmic contact. Although the contact resistance between the p-type semiconductor layer 22 and the metal reflection layer 4 can be reduced, its high absorptivity lowers significantly the light emitting efficiency of GaN LEDs.
In addition to high absorption coefficient and low thermal stability, the high-work-function metals adopted in the Ohmic contact layer 8 cannot prevent the metals in the metal reflection layer 4 from diffusing to the p-type GaN in the p-type semiconductor layer 22. Consequently, the electrical characteristics of the p-type GaN are affected.
Accordingly, for forming LEDs with high light-emitting efficiency, the technical problems to be solved include forming Ohmic contacts effectively, prevention of substantial light absorption by the Ohmic contact layer on the path of light reflection towards the metal reflection layer, and avoidance of metals in the metal reflection layer towards the p-type GaN.