Abstract: An atomic layer deposition apparatus comprises a reaction chamber, a heater configured to heat a semiconductor wafer positioned on the heater, an oxidant supply configured to deliver oxidant-containing precursors having different oxidant concentrations to the reaction chamber, and a metal supply configured to deliver a metal-containing precursor to the reaction chamber. The present application also discloses a method for preparing a dielectric structure comprising the steps of placing a substrate in a reaction chamber, performing a first atomic layer deposition process including feeding an oxidant-containing precursor having a relatively lower oxidant concentration and a metal-containing precursor to form an thinner interfacial layer on the substrate, and performing a second atomic layer deposition process including feeding the oxidant-containing precursor having an oxidant concentration higher than that used to grow the first metal oxide layer and the metal-containing precursor into the reaction chamber.