1. Improvement on Ti/Al/Ni/Au Electrical Contacts on GaN/AlGaN Superlattices ___________________________________________________________ Tzu-Yung Huang 1,, Yu Song 1, Rajaram Bhat 2, Chung-En Zah 2, and Claire F. Gmachl 1 1 Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA 2 Corning Inc., Corning, NY 14831, USA Work supported by PEI Grand Challenges Program

2. Improvement on Ti/Al/Ni/Au Electrical Contacts on GaN/AlGaN Superlattices Quantum Cascade Lasers are used widely in trace gas sensing applications III-Nitride materials have potential to achieve QCLs with emission wavelengths below 3µm, opening us up to a wider range of chemicals (C-H stretch vibration) ___________________________________________________________

3. Improvement on Ti/Al/Ni/Au Electrical Contacts on GaN/AlGaN Superlattices So far, the overall high electrical resistance causes the device to heat up, precluding any emission We tried several recipes varying Titanium thickness while keeping Al/Ni/Au overlay thickness constant, and found the best results at 6nm (25 Ω) We also optimized the annealing recipe, having the best results at 800°C for 80s ___________________________________________________________

4. Improvement on Ti/Al/Ni/Au Electrical Contacts on GaN/AlGaN Superlattices ___________________________________________________________ -But hold on, are we sure that all of that is from the electrical contacts? A B C D E F G Ti 6nm ----- Ti10nm----- Ti40nm-----

5. Improvement on Ti/Al/Ni/Au Electrical Contacts on GaN/AlGaN Superlattices To answer our question: we varied the gap distance between the outer and inner contact as well as the contact size What we found: the average sheet resistance is 748.5 Ω! While the specific contact resistivity is only 2.87e-4 Ω Our metal contacts weren’t the main culprit! Now we can make a new emitter sample with a new design ___________________________________________________________ Corrected Total Resistance vs. Gap Distance for inner contact size 400 µm R sh : 842.15Ω ρ c : 3.43e-4Ω Different contact sizes with varying gap spacing

6. Improvement on Ti/Al/Ni/Au Electrical Contacts on GaN/AlGaN Superlattices Summary Discover source of high device resistance Measured sheet and specific contact resistance Conclusion Extensive work experience in the clean room Further understanding of Quantum Cascade Lasers and other related semiconductor devices that can be used for environmental applications Results presented at MIRTHE Summer Workshop Co-Author of conference paper to ITQW 2013 Will co-author journal publication Continuation of the project during the school year Provided insights into post graduation plans ___________________________________________________________

7. Improvement on Ti/Al/Ni/Au Electrical Contacts on GaN/AlGaN Superlattices Thank You! ___________________________________________________________