Proton irradiation-induced metal voids in gallium nitride high electron mobility transistors

Abstract

Gallium nitride/aluminum gallium nitride high electron mobility transistors with nickel/gold (Ni/Au) and platinum/gold (Pt/Au) gating are irradiated with 2 MeV protons. Destructive physical analysis revealed material voids underneath the gate finger of the device. These voids, which are not found in fresh devices, cause a decrease in effective gate contact area, ultimately degrading the device performance and reliability. Scanning transmission electron microscopy images and respective line scans of the gate-foot area-of-concern confirm that voids were present. The gate-finger’s silicon nitride passivation layer and Au metallization layer were removed via focused ion beam stripping in order to analyze the gate electrode Ni or Pt layers for defects. Scanning electron microscopy images revealed voids along the gate finger edge and circular voids in the gate finger’s length that contribute to the device’s reduction in parameter performance. The current hypothesis is that the Ni or Pt voiding is a result of the Kirkendall effect. The effect is seen in the energy dispersive spectroscopy analysis, where Ni has migrated into the Au layer. A model was developed to predict void sizes versus dose and validated using data on dislocation density.