A new approach to N-polar n-GaN surface treatment for room-temperature transparent ohmic contact formation

In this work, we propose a new approach to obtain as-deposited low-resistivity transparent ZnO:Al (AZO) ohmic contacts to n-GaN N-face by first modifying the GaN surface by depositing a standard Ti/Al/TiN/Au contact, forming it at 750 °C, and then removing the metallization by chemical etching. To identify the mechanisms responsible for the contact’s ohmicity, the GaN interface was examined by (scanning) transmission electron microscopy, as well as time-of-flight secondary ion mass spectrometry. We identified changes formed at the interface in the form of AlN pits growing epitaxially on GaN, a thin inhomogeneous AlN + Ti layer, and argue that this results in the formation of a highly doped subsurface GaN layer due to nitrogen diffusion which modifies the n-GaN surface in a way enabling to obtain an as-deposited low-resistive transparent AZO contact on it with current–voltage characteristics similar to a standard metallic contact formed at 750 °C.