High-pressure reactively sputtered HfO2: Composition, morphology, and optical properties

Abstract

Hafnium oxide films were deposited by high pressure reactive sputtering using different deposition pressures and times. The composition, morphology, and optical properties of the films, together with the sputtering process growth kinetics were investigated using heavy ion elastic recoil detection analysis, Fourier transform infrared spectroscopy, ultraviolet-visible-near infrared spectroscopy, x-ray diffraction, and transmission electron microscopy. The films showed a monoclinic polycrystalline structure, with a grain size depending on the deposition pressure. All films were slightly oxygen rich with respect to stoichiometric HfO2 and presented a significant amount of hydrogen (up to 6 at. %), which is attributed to the high affinity for moisture of the HfO2 films. The absorption coefficient was fitted to the Tauc law, obtaining a band gap value of 5.54 eV. It was found that the growth rate of the HfO2 films depends on the deposition pressure (P) as P-1.75. This dependence is explained by a diffusion model of the thermalized atoms in high-pressure sputtering. Additionally, the formation of an interfacial silicon oxide layer when the films were grown on silicon was observed, with a minimum thickness for deposition pressures around 1.2 mbars. This interfacial layer was formed mainly during the initial stages of the deposition process, with only a slight increase in thickness afterwards. These results are explained by the oxidizing action of the oxygen plasma and the diffusion of oxygen radicals and hydroxyl groups through the polycrystalline HfO2 film. Finally, the dielectric properties of the HfO2/SiO2 stacks were studied by means of conductance and capacitance measurements on Al/HfO2/SiO2/Si devices as a function of gate voltage and ac frequency signal.