Proton implant isolation in GaN

Proton implantation into GaN epitaxial layers was performed to test its suitability for electrical isolation of devices in monolithic integrated circuits. The experimental samples were unintentionally doped GaN epitaxial layers, 2 μm thick, grown on a sapphire substrate. The electron concentration was 1016 cm-3 resulting in the layer sheet resistance value of 2×103 Ω. The test structures were circular transmission line model (CTLM) patterns of Ti/Al/Ni/Au ohmic contacts that allowed for measurements of the GaN layer resistivity before and after implantation processes. The metallization provided also a mask for the implanted ions. The implantation processes were performed using multiple energy implants of (270/150/80 keV) steps with doses of (2-5)×1012 cm−2 and (2-5)×1013 cm−2. As a result increase of the GaN layer sheet resistance value by three to four orders of magnitude, to the value of (6-34)×106 Ω was obtained. The post-implanted layer temperature dependence of the resistance measured up to 200 °C showed exponential decrease of the sheet resistance with the activation energy, Ea from 350 to 800 meV. After annealing, the sheet resistance measured at room temperature was above the level of 106 Ω, which appeared to be sufficient electrical isolation of the device.