Effects of crystallization on the electrical properties of ultrathin HfO2 dielectrics grown by atomic layer deposition

Microstructural evolution and resulting changes in electrical properties of atomic-layerdeposition-grown ${\mathrm{HfO}}_2$ on ${\mathrm{SiO}}_2/\mathrm{Si}$ substrates were studied as a function of annealing temperature in a ${\mathrm{N}}_2$ ambient. As deposited ∼30-Å-thick ${\mathrm{HfO}}_2$ on 15 and 25 Å thermal ${\mathrm{SiO}}_2$ were almost entirely amorphous, although a low density of crystalline seeds were observed and crystallization occurred from these nuclei during furnace anneals at temperatures $\text{>∼500\hspace{0.05em}°}\mathrm{C}.$ The major crystalline phase thus formed was monoclinic, and some fraction of tetragonal phase was observed during crystallization according to transmission electron microscopy electron diffraction analysis. Complete crystallization occurred around 700 °C and, at higher temperatures, significant interfacial silicon dioxide growth was observed due to the presence of a small partial pressure of oxygen in the annealing ambient. No significant increase of leakage current in the trap-assisted tunneling conduction regime was observed during the intermediate and final stage of crystallization process.