Kopin Corp. announced a new ohmic contact technology on gallium nitride (GaN) that has very low contact resistance and a very high degree of stability. Details of this new contact, which is formed epitaxially on GaN, are described in the Nov. 18 edition of the prestigious Applied Physics Letters. This is the first time an epitaxial (single-crystalline) contact has been formed on GaN, and should be superior to polycrystalline contacts that are typically used in the industry.
The new patent-pending ohmic contacts, together with the previously announced NanoPockets technology and other improvements, have enabled Kopin to produce blue LED chips (CyberLites) as bright as those commercially available and yet can be driven by much lower voltage. CyberLites require less than 2.9 volts of electricity (for 20 milliamperes current) - significantly lower than 3.3 volts for commercially available LEDs - and yet have 100 millicandela brightness. CyberLites with resistance to electrostatic discharge (ESD) over several thousand volts are also obtained.
"There's a lot of innovative science behind our bright little light," said Dr. John C. C. Fan, president and chief executive officer of Kopin. "The initial reaction from our potential customers is excellent, and we are making very good progress towards the goal of mass production. 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 microohm cm squared has been measured. Lower values for contact resistance are expected with further optimization of layer thickness and annealing conditions. The contact is very stable to endure high-temperature environments (350 degrees C for 30 minutes) and extended harsh operation conditions.
The CyberLite contacts utilize the domain epitaxy technique invented by Dr. Jagdish Narayan and licensed by Kopin Corporation. "The 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 Dr. Narayan, Distinguished University Professor and Director of NSF Center for Advanced Materials and Smart Structures at North Carolina State University, and 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."
According to Professor Paul Holloway, Distinguished Professor of Materials Science and Engineering, University of Florida, "Domain epitaxy to fabricate epitaxial contact layers with large lattice misfit represents a very significant milestone in thin film growth, not only for ohmic contacts but for thin film heterostructures, in general. In addition, a contact resistance of 10 microohm cm squared for p-type GaN is excellent and results in the better performance of the LED."