The authors report ultralow specific contact resistivity (rho(c)) in nonalloyed, in situ Ohmic contacts to heavily doped n-type In0.53Ga0.47As:Si layers with 6x10(19) cm(-3) active carrier concentration, lattice matched to InP. The contacts were formed by depositing molybdenum (Mo) immediately after the In0.53Ga0.47As growth without breaking vacuum. Transmission line model measurements showed a contact resistivity of (1.1 +/- 0.6)x10(-8) cm(2) for the Mo/InGaAs interface. The contacts show no observable degradation in resistivity after annealing at 300 and 400 degrees C for 1 min duration.