Methods for forming a capacitor using an atomic layer deposition process include providing a reactant including an aluminum precursor onto a substrate to chemisorb a portion of the reactant to a surface of the substrate. The substrate has an underlying structure including a lower electrode. An ammonia (NH3) plasma is provided onto the substrate to form a dielectric layer including aluminum nitride on the substrate including the lower electrode. An upper electrode is formed on the dielectric layer. A second dielectric layer may be provided oil the first dielectric layer