Chemical and electrical characterization of Gd₂O ₃/GaAs interface improved by sulfur passivation

We report the formation of a gate dielectric film on the sulfur passivated GaAs(00l). The Gd₂O₃ films were deposited on the n-GaAs substrates using the electron-beam evaporation. The sulfur passivated GaAs metal-oxide-semiconductor diode exhibited the improved electrical properties compared to a similar GaAs diode that was cleaned with HCl including an enhanced conduction-band barrier and a reduced-leakage current. We investigated the relation between the interfacial bonds and the electrical properties by a systematic characterization using the photoemission spectroscopy. Sulfidation was shown to preserve a stable Gd₂O₃/GaAs interface and to prevent an interfacial GaAs oxide formation. The stable interface and the passivated surface were caused by the bonding transition from As-S to Ga-S. The As-oxide was dissociated to form the interfacial Ga₂O₃ and to volatilize the elemental As in the Gd₂O₃ deposition, conducted without sulfidation. The interfacial oxides and excess As degraded the electrical properties and resulted in the decrease of the conduction-band offset. It was found that the Gd₂O₃ film was composed of two oxidation states: the amorphous Gd-O and the crystalline Gd ₂O₃. The bonding transition with a substrate heating implies that the Gd₂O₃ film on the GaAs surface completely crystallizes above 400°C. Electrical properties were understood in relation to the band alignment and were estimated from the valence band and the core-level spectra analysis. The sulfidation introduced a shift of the Fermi level toward the conduction-band minimum on the GaAs surface and increased the conduction-band offset at the Gd₂O₃/GaAs interface, followed by a saturated accumulation capacitance with a low-leakage current.