Plasma-enhanced atomic layer deposition was explored to produce thin HfO"2 films, where oxygen plasma acted as oxidant. The interfacial layer (IL) was controlled by in situ pre-oxygen plasma treatment (PRO) and pre-ammonia plasma treatment (PRN). Post oxygen plasma treatment (POP) to HfO"2 film was in situ executed. The IL thickness was 1.1nm, which was detected to be HfSiON by X-ray photoelectron spectroscopy (XPS). With 4nm thick amorphous HfO"2 film, an equivalent oxide thickness (EOT) of total gate dielectric stacks of 0.87nm was obtained. Small leakage current density of 0.02mA/cm^2 was measured at a gate bias of |V"g-V"f"b|=1V.