(Invited) Microstrucutural Characterizaton of Stressed AlGaN/GaN HEMT Devices

AlGaN/GaN high electron mobility transistors are used in high temperature, high power, and high frequency applications due to their superior combination of high breakdown voltage and high frequency performance. Unfortunately, their reliability in the field has warranted further investigation because of the stochastic nature of the variety of defects that arise during their operation. Many analytical techniques are not well suited to this analysis because they sample regions of the device which are either too small or too large for accurate observation. This talk will give a review of recent studies that employ structural, chemical, and electrical device characterization paired with simulation in order to develop structure-property relationships between defects and device performance. This includes the use of a new approach to chemical deprocessing combined with surface-sensitive analysis techniques such as scanning electron microscopy and scanning probe microscopy in the analysis of large-area defect formation.  Wet etching of the passivation nitride and metal contacts was used to strip the transistor surface features and remove organic contaminants.  This exposed the top surface of the AlGaN epilayer for analysis.  This processing revealed and surface analysis revealed three different defects.  One of these defects is a nanoscale crack which emanates from metal inclusions formed during alloying of the ohmic contacts of the device prior to use in the field, and could impact the yield of production-level manufacturing of these devices. This defect also appears to grow, in some cases, during electrostatic stressing. Another defect, a native oxide at the surface of the semiconductor under the gate, appears to react in the presence of an electric field, which could play a role in the degradation of the gate contact during high-field, off-mode electrostatic stressing.  This defect could also facilitate the formation of the pitting of the AlGaN epilayer beneath the gate contact.  The composition of the as-grown gate interfacial layers were characterized using atom probe tomography and are composed of two distinct oxide layers, NiO_x and AlO_x. Furthermore, using off-state reverse bias electrical stress, Ni-gate metal reactions with AlGaN epilayers emulate the shape and size of the electric field contours between 5 – 6 MV/cm; suggesting a critical field for defect formation.  This talk will summarize these finding and place them in the overall context of failure mechanisms in AlGaN/GaN HEMT structures.