Parameters for ferroelectric phase stabilization of sputtered undoped hafnium oxide thin films

In this work, various stabilization factors for the ferroelectric phase of undoped hafnium oxide prepared by physical vapour deposition were investigated. The capping of the thin films with platinum top electrodes before annealing, as well as the amount of oxygen during sputter deposition and subsequent annealing was shown to have a significant influence on the resulting ferroelectric properties and phase of the HfO (x) layer. When the prepared films were not capped, only one specific set of process parameters was found to lead to the formation of the ferroelectric phase. We conclude that capping is a crucial condition for stabilizing the ferroelectric phase. Furthermore, it is shown that the amount of oxygen supply during all fabrication processes determines the resulting ferroelectric phase fraction for capped samples. Increasing the oxygen flow during sputtering and annealing results in a larger monoclinic phase fraction and thus a decrease of the remanent polarization.