Reduction Scheme of the GaN Interface State on a Metal–Oxide Semiconductor Device by Superacid

Abstract

The performance and reliability of gallium nitride (GaN) metal–oxide semiconductor high-electron-mobility transistors (MOS-HEMTs) are critically limited by high interface state densities (D_it) originating from unstable native oxides and surface reoxidation during conventional subtractive cleaning processes. In this study, we propose an interface engineering scheme using perfluorobutanesulfonic acid (PFBS), a superacid, as a short-chain self-assembled monolayer (SAM) prior to the atomic layer deposition (ALD) of Al₂O₃. X-ray photoelectron spectroscopy (XPS) and electrical characterizations reveal that PFBS effectively suppresses Ga–O bond formation. We investigate how the bulky perfluoroalkyl chains provide a steric “umbrella effect,” effectively shielding the underlying Ga surface atoms from atmospheric oxygen and moisture, despite steric hindrance preventing a one-to-one bonding ratio. Furthermore, unlike conventional long-chain SAMs, the short-chain nature and high vapor pressure of PFBS enable clean thermal desorption without carbon contamination during the 350 °C ALD vacuum heating process. This allows for an unhindered, high-quality initial reaction with trimethylaluminum. Consequently, the D_it evaluated by the conductance method was significantly reduced to the lower 10¹¹ eV⁻¹ cm^–2 range, representing a 50% reduction compared to conventionally treated samples. This functional surface modification, combining robust ex situ protection with clean in situ desorption, provides a key technology not only for enhancing the performance of MOS-HEMTs but also for the practical realization of GaN MOSFETs, which have yet to be commercialized.