Kopin announces novel contacts for GaN LEDs
The new patent-pending ohmic contacts, together with the previously announced NanoPockets technology and other improvements, have enabled Kopin to produce blue LED chips that it has called CyberLite LEDs (see related story). These are as bright as those commercially available and yet can be driven by much lower voltage. CyberLites require less than 2.9 V at 20 mA - significantly lower than 3.3 volts for commercially available LEDs - and yet have 100 mcd brightness. CyberLites with resistance to electrostatic discharge (ESD) over several thousand volts are also obtained.
“The initial reaction from our potential customers is excellent, and we are making very good progress towards the goal of mass production," said John Fan, president and CEO of Kopin. “We didn t start the coming LED lighting revolution, but we intend to accelerate it significantly.”
Kopin’s new ohmic contacts are formed by depositing layers consisting of gold, nickel and gold on the p-type GaN surface and annealing in air for 30 minutes at 470 degrees C. The gold layer in contact with p-GaN grows epitaxially via domain matching epitaxy, which acts as a template for nickel oxide growth via lattice matching epitaxy.
Specific contact resistance in the range of 10 µohm cm2 has been measured. Lower values for contact resistance are expected with further optimization of layer thickness and annealing conditions. The contact is stable enough to endure high-temperature environments (350 degrees C for 30 minutes) and extended harsh operating conditions.
The CyberLite contacts utilize the domain epitaxy technique invented by Professor Jagdish Narayan, Director of NSF Center for Advanced Materials and Smart Structures at North Carolina State University. The domain epitaxy technique is licensed by Kopin. “Domain epitaxy is a new paradigm for thin film growth where films with large lattice mismatch can be grown via matching of integral multiples of lattice planes across the film-substrate interface,” said Narayan who is a co-author of the Applied Physics Letters paper.
“The formation of epitaxial Au layer on GaN at room temperature was exciting and it can be explained by our domain epitaxy. This unique epitaxial composite structure of gold and nickel oxide is envisaged to be important in achieving low-resistivity ohmic contacts in p-GaN. Not only is this ohmic contact technology important for blue LED manufacturing, but also very exciting scientifically.”