(Invited) Radiation-Induced Defect Mechanisms in GaN HEMTs
AlGaN/GaN HEMT epitaxial layers were grown on Si substrates by metal organic chemical vapor deposition (MOCVD). The GaN buffer layer is >1 μm and the AlxGa1-xN (x=27%) barrier layer is 17 nm thick. HEMTs were exposed to 2 MeV protons at room temperature in a tandem Van de Graaff accelerator iteratively, up to a fluence of 6 x 1014 H+/cm2. I-V, C-V, and Hall measurements were performed to characterize the radiation-induced changes in HEMT performance, and high-resolution transmission electron microscopy (HRTEM) is used to identify radiation-induced defect mechanisms.
Bombardment of 2 MeV protons causes a uniform distribution of vacancies, interstitials, and Frenkel pairs, with minimal larger damage cascades. The stopping range of 2 MeV is much larger than the epitaxial GaN thickness. The AlGaN/GaN interface of an as-fabricated HEMT has an abrupt junction between the AlGaN and GaN. However, after irradiation, atomic recoil of displaced atoms results in intermixing of the AlGaN and GaN layers. The irradiated HEMT has an increased surface roughness. Since the AlGaN/GaN interface is where the two-dimensional electron gas (2DEG) forms, the surface roughness impacts the 2DEG mobility. The Hall mobility measured on Van der Pauw structures begins to degrade at a fluence of 5 x 1013 H+/cm2. This mobility degradation is attributed to the increase in surface roughness between the AlGaN and GaN layers. In addition, TEM analysis revealed where certain threading dislocations exist, that after proton irradiation, Ni from the Schottky Ni gate diffuses down into the dislocation. This impacts the HEMT performance by changing leakage paths, but also, voids were observed in the Ni gate, where the nickel had diffused into the semiconductor at the gate edges.