H$_3$PO$_4$-based wet chemical etching for recovery of dry-etched GaN surfaces

The impact of several wet etchants commonly encountered in the microelectronic industry on the surface chemistry of GaN on silicon was explored. In order to get closer to fully recessed gate HEMT fabrication processes, we investigated different kinds of GaN surfaces. This study was conducted on as-grown GaN and dry etched GaN, with etching consisting of inductive coupled plasma reactive ion etching (ICP-RIE), followed by atomic layer etching (ALE) and O2 plasma stripping. The impact of each wet treatment was evaluated by parallel Angle Resolved X-ray Photoelectron Spectroscopy (pAR-XPS). Treatment with phosphoric acid (H3PO4) showed a significant modification of the surface and further studies were performed using this treatment. The impact of H3PO4 on GaN surface chemistry and morphology was assessed by pAR-XPS and atomic force microscopy (AFM) respectively. A delayed effect was observed for dry etched samples compared to as-grown samples, with a successful recovery of the surface after 60 minutes of treatment. We also proposed a mechanism explaining the progressive formation on steps on the surface over time. Further research was performed on dry etched samples without ALE which also modified the delay time of the H3PO4 treatment, but still enabled a recovery of the surface morphology. In contrast to other studies, we showed that, with the appropriate choice of parameters for the H3PO4 treatment, it was possible to successfully recover the GaN surface after dry etching without significantly opening dislocation holes. This is therefore a promising treatment to be used during GaN HEMT processing to recover good quality surfaces after etching.