Epitaxial Low-resistance Contact On GaN
The contact is formed epitaxially on the GaN substrate to provide a very low contact resistance and a very high degree of stability. Kopin says this is the first time an epitaxial (single-crystalline) contact has been formed on GaN. The company believes this should provide connections that are superior to the polycrystalline contacts that are typically used in the GaN industry.
The ohmic contacts are formed by depositing layers consisting of gold, nickel and gold on the p-type GaN surface and annealing in air for 30mins at 470°C. The gold layer in contact with p-GaN grows epitaxially via domain matching epitaxy. This acts as a template for nickel oxide growth via lattice matching epitaxy.
Specific contact resistance in the range of 10µ?.cm2 has been measured. Lower values for contact resistance are expected with further optimisation of layer thickness and annealing conditions. The contact can endure high-temperature environments of 350°C for 30mins and extended harsh operation conditions.
The contacts are used in Kopin's CyberLite blue LED chips. These devices produce as much light as commercially available LEDs, but can be driven by much lower voltages. CyberLites require less than 2.9V and 20mA current to produce 100millicandela brightness. CyberLites with a resistance to electrostatic discharge (ESD) of more than several thousand volts have been obtained.
