The reliability of Au-free Ohmic contacts has been evaluated under a constant current stress (500 mA/mm) in a high temperature environment (150 °C, 175 °C, and 200 °C). 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 (Rincrease) is accelerated at a higher temperature condition. Moreover, Rincrease 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 (Rc) showed lower degradation with respect to devices with a larger initial Rc. This suggests that obtaining lower Rc is beneficial not only for the performance but also for the reliability.
- AlGaN/GaN HEMTs
- Au-free Ohmic contacts