Stability evaluation of Au-free Ohmic contacts on AlGaN/GaN HEMTs under a constant current stress

摘要

The reliability of Au-free Ohmic contacts has been evaluated under a constant current stress (500 mA/ mm) in a high temperature environment (150 ℃, 175 ℃, and 200 ℃). Two Ohmic contact schemes with different Ti/Al thicknesses (5 nm/100 nm and 10 nm/100 nm) have been tested. The results showed that the degradation of the resistance (R_(increase)) is accelerated at a higher temperature condition. Moreover, R_(increase) has a square root dependence with the time. This indicates that a diffusion process could be responsible for the observed degradation. This has been confirmed by means of physical failure analyses (FIB/TEM) performed on a highly degraded device: an apparent roughness of the interface between TiN and the Ohmic metal layer was observed. On top of this. Nitrogen out-diffusion into the Ohmic metal was observed in the EDS/EELS analyses, suggesting that a material diffusion phenomenon might play a role. In addition, we observed that the devices with a lower initial contact resistance (R_c) showed lower degradation with respect to devices with a larger initial R_c. This suggests that obtaining lower R_c is beneficial not only for the performance but also for the reliability.