Metal-insulator-semiconductor (MIS) contacts for germanium and its alloys include insulator layers of oxygen-deficient metal oxide deposited by atomic layer deposition (ALD). The oxygen deficiency reduces the tunnel barrier resistance of the insulator layer while maintaining the layer's ability to prevent Fermi-level pinning at the metal/semiconductor interface. The oxygen deficiency is controlled by optimizing one or more ALD parameters such as shortened oxidant pulses, use of less-reactive oxidants such as water, heating the substrate during deposition, TMA “cleaning” of native oxide before deposition, and annealing after deposition. Secondary factors include reduced process-chamber pressure, cooled oxidant, and shortened pulses of the metal precursor.