Au-free Ohmic Contacts to Gallium Nitride and Graphene
This work deals with Au-free contact metallization schemes for gallium nitride (GaN) and graphene semiconductors. Graphene and gallium nitride are promising materials that can potentially be integrated together in the near future for high frequency high power applications. Realizing and optimizing Au-free technology to GaN and graphene can reduce manufacturing costs and allow the processing of these materials to be compatible with the existing Si-CMOS technology.
For GaN, as a first step, equilibrium energy band diagrams were simulated by solving the Poisson equation in 1-D. Moreover, since tunneling is an important factor in ohmic contacts, tunneling probabilities were simulated for the different potential barriers obtained from solving the Poisson equation. A computationally simple model for calculating contact resistivity as a function of doping concentration and metal work function has been developed. The model was solved numerically to obtain the contact resistivity at each doping concentration and barrier height.
Schottky diodes of TiN and TaN were fabricated and analyzed after rapid thermal anneals (RTA) where performed in the range of 300 ? to 1100 ?. J-V characteristics were analyzed extensively both in forward and reverse bias to extract the doping concentration at the metal-GaN interface and the barrier heights. The change in current between different temperature anneals has been attributed primarily due to the change in barrier height and not to the change in doping concentration at the interface.
For graphene, Au-based contacts were fabricated as a baseline and studied at anneal temperatures ranging from 400 ? to 800 ?. Experiments with different metallization schemes on graphene proved that stress in the metal films werean important parameter in the realization of Au-free contacts. Stress measurements indicated that Au had a highly compressive contribution to the overall stress in the metal film stack and with this information,Au-free contacts have been engineered using a Al/Ta layer. The Au-free contact scheme was devoid of stress issues and had better ohmic contact parameters than the Au-based contacts.