The short-time helicon-wave excited N-2-Ar plasma treatments of n-type GaAs (100) substrates were performed in order to investigate the effect of these treatments on the interface characteristics of GaAs and to explore a possibility of a robust surface passivation of GaAs. X-ray photoelectron spectroscopic measurements indicated formation of GaN at the insulative-layer GaAs interface. The current-density-voltage (J-V) characteristics of the Schottky or tunnel metal-insulator-semiconductor diodes were measured before and after the plasma treatment. The analysis of these data suggested that the "intrinsic" surface Fermi level pinning near the midgap of the GaAs forbidden band was partially removed and the Fermi level was newly pinned at E-c- 0.2-0.3 eV (Ec: the conduction band edge) after the plasma treatment. This is probably due to generation of high-density plasma-induced donor-like interface states having the energy level (or levels) in this energy region. The reverse leakage current increased with this plasma treatment. However, it decreased after the post-thermal annealing at moderate temperatures (200-500 degrees C) in N-2 ambient. The thermal and air-exposure stability were substantially improved with the plasma treatment. The J-V characteristics did not show any deterioration after air exposure for three months for the plasma-treated samples, whereas these greatly degraded with air exposure for three weeks for the untreated ones.