Stabilization of Tetragonal HfO2 under Low Active Oxygen Source Environment in Atomic Layer Deposition

The structural and electronic properties of HfO2 thin films grown by atomic layer deposition (ALD) under various ozone concentrations were investigated using X-ray diffraction (XRD), photoemission (XPS), reflectometry (XRR), and absorption spectroscopy (XAS) and their fine structure (XAFS) analysis. It was found that in the as-grown states, the oxygen stoichiometry and local atomic structure in amorphous HfO2 domains are maintained nearly as constants even when the film is grown without external ozone supply, while some C-O bonds remain between the almost stoichiometric HfO2 domains due to incomplete oxidation of the precursors. After a postdeposition annealing (PDA), the films crystallize with a monoclinic structure (P2(1)/c), except for the case of the no-ozone supply in which the film possesses a tetragonal crystal structure (P4(2)/nmc). It is demonstrated that the carbonate bonds play a major role in stabilizing the tetragonal structure through nanoscale separation of the HfO2 domains. Accordingly, the roles of the oxygen source and the PDA are also newly addressed as being related to the carbonate bonds. The ozone gas acts as an oxygen supplier, and more importantly, it reduces the residual carbonates to stabilize the bulk crystal structure in the thin films. The PDA not only delivers the thermal energy to induce the crystallization but also eliminates C atoms to increase the size of the HfO2 domains leading to the densification of the films.