2, U.S. Naval Research Laboratory, Washington, District of Columbia, United States
AlGaN/GaN-based high electron mobility transistors (HEMTs) are model candidates for next generation radio-frequency and optoelectronic devices. Unfortunately, further understanding about device reliability and the physics of degradation mechanisms in GaN technology is still required to push GaN devices to their theoretical operating limits. This lapse in collective knowledge has led to the creation of a large research community devoted to GaN reliability physics. Directly connecting device electrical degradation with physical defects is challenging due to the complex electro-thermo-mechanical mechanisms active in these devices, but pitting, cracks, and extended dislocation structures have been observed in degraded devices.
There have been many experimental and theoretical studies on pre-catastrophic degradation of AlGaN/GaN HEMTs. Several proposed theories are based on degradation of the device active layers, including TD-based and inverse/converse piezoelectric effect-based failure mechanisms; newer theories involve time-dependent degradation based on defect percolation, electrochemical degradation, and metal-metal/semiconductor reactions. Unlike the discussion of pre-catastrophic degradation mechanisms, there is relatively little looking into the forensic material science of catastrophic device failure. Catastrophic device failure is physically realized as crater formation and electrically realized as large increases in leakage current or shorting of multiple electrodes and the cessation of device operation.
Here we present the characterization of a catastrophically degraded AlGaN/GaN HEMT in and around the breakdown area of crater formation. Specifically, we employ scanning electron microscopy (SEM) and transmission electron microscopy (TEM) based techniques to determine the microstructural and chemical changes the device underwent during catastrophic failure. We find a Ni-Ga alloy defect within the gate stack, show that the Ni/Au gate stack experienced severe morphological and diffusional changes throughout the device, and lastly we see curved and inclined dislocations emanating throughout the GaN layer near the failure region, indicating dislocation movement during device operation.