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 (Dit) 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 Dit 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.