We deposited ZrO2 thin films by atomic-layer deposition (ALD) using zirconium tertiary–butoxide [Zr(t-OC4H9)4, (ZTB)] and H2O source gases on Si substrates at low temperatures. We grew ZrO2 films layer by layer in a temperature range of 175–250 °C to minimize surface roughness. The deposited ZrO2 film thickness had self-limiting properties with the exposure time of ZTB and vapor pressures of ZTB and H2O. The deposition rate per cycle was independent of the vapor pressure of ZTB from 0.01 kPa to 0.04 kPa. Transmission electron microscopy revealed that the formation of an SiOx interfacial layer could be suppressed by using an ALD ZrO2/ALD Si-nitride (~0.5 nm) stack structure. We found the fixed charge, interface trap density, and leakage current density in the ALD ZrO2/ALD Si-nitride stack dielectrics to be less than those in ALD ZrO2 dielectrics. In spite of the same equivalent oxide thickness of 1.6 nm, the relative dielectric constant r (11.5) of the ALD ZrO2/ALD Si-nitride stack capacitor was higher than that (10.5) of the ALD ZrO2 capacitor due to the suppression of formation of the interfacial SiOx layer (1.0–1.5 nm) by an ultrathin ALD Si nitride (~0.5 nm). The current conduction mechanism is identified as direct tunneling of electron except at very low dielectric fields. Comparing structural and electrical properties, ALD ZrO2/ALD Si-nitride stack dielectrics are promising candidates for sub-0.1-µm metal–oxide–semiconductor field-effect transistors.