We report on nonalloyed ohmic contact ton+-GaAs(≈2×1018 cm−3)with a Ti-based metal/ultrathin GaS (≈15 nm)/GaAs quasi-metal-insulator-semiconductor (QMIS) structure. The GaS film was grown by molecular beam epitaxy employing the precursor, tertiarybutyl-galliumsulfide-cubane([(t-Bu)GaS]4).ForAu/Pt/Ti/GaS/n+-GaAsQMIS structure, the as-deposited contact resistivity of low10−2 &OHgr; cm2was improved to4.1×10−6 &OHgr; cm2by low-temperature annealing at 300 °C for 10 min. With annealing, we observed the formation of TiGaS on GaAs, but did not see any alloying reaction between GaAs and metals and sulfur diffusion into GaAs. ©1998 American Institute of Physics.