Influence of moisture on the ferroelectric properties of undoped sputtered hafnium oxide thin films

The influence of different fabrication parameters during deposition and annealing on the ferroelectric behaviour of hafnium oxide has already been extensively studied. However, the influence of different atmospheres during the actual switching event of ferroelectric thin films has not yet been examined. In our work, we studied the ferroelectric switching behaviour in vacuum, dry oxygen, dry nitrogen and in different humid nitrogen atmospheres. In Figure 1, the resulting current measurements are shown exemplarily. Whereas the thin film shows an actual polarisation of 18 µC/cm^2 for ambient atmosphere, the ferroelectric switching is nearly completely suppressed in vacuum conditions (a). For dry nitrogen and oxygen atmosphere, a small increase of the polarisation compared to vacuum is recognizable, however the ferroelectric switching is still not very distinctive (b). In humid nitrogen atmosphere on the contrary, we observe a dependence of the resulting polarisation on the moisture content. By closer investigation of the wake up behaviour, an initial bias field is recognized which is induced by the initial oxygen vacancies concentration. In vacuum conditions, the bias field shifts the coercive voltage to higher voltages compared to ambient atmosphere resulting in a suppressed wake up effect. In humid atmospheres, hydroxides are incorporated at the grain boundaries of the hafnium oxide layer and occupy the oxygen vacancies which leads to a reduction of the bias field. Consequently, the coercive voltage is lowered which allows ferroelectric switching to take place.

Figure 1: a) Measured current in ambient atmosphere and vacuum. b) Measured current in dry oxygen and nitrogen atmosphere.