Investigating charge trapping in ferroelectric thin films through transient measurements

Abstract

A measurement technique is presented to quantify the polarization loss in ferroelectric thin films as a function of delay time during the first 100s after switching. This technique can be used to investigate charge trapping in ferroelectric thin films by analyzing the magnitude and rate of polarization loss.Exemplary measurements have been performed on Hf0.5Zr0.5O2 (HZO) and HZO/Al2O3 films, as a function of pulse width and temperature. It is found that the competing effects of the depolarization field, internal bias field and charge trapping lead to a characteristic Gaussian dependence of the rate of polarization loss on the delay time. From this, a charge trapping and screening model could be identified which describes the dynamics of polarization loss on short timescales. IntroductionCharge trapping at the interfaces of ferroelectric (FE) HfO 2 -based thin films plays an important role in both the dynamics of polarization switching and FE device operation. Charge trapping has a significant impact on device operation in the case of FE field-effect transistors (FeFETs) [22,1] as well as ferroelectric tunnel junctions (FTJs) [9]. In both of these devices, an intentional dielectric (DE) layer is added to a metal-ferroelectric-metal (MFM) stack, which will modulate the impact of charge trapping due to the additional FE-DE interface. Assuming fixed and trapped charge densities is necessary in order to fully model the switching behavior of ferroelectric devices with DE layers [8,14]. Nonetheless, charge trapping is also involved in the switching of HfO 2 ferroelectrics without any intentional interfacial layer (IL) [25,16], due to the unavoidable presence of non-switching 'dead layers' at the FE-electrode interfaces [36]. It has been shown that even after switching, the injection of charges can continue to modify switching behaviour by shifting the coercive voltage through a so-called 'fluid imprint ' effect [4]. As such, a thorough understanding of charge trapping, including its origin, how it can be characterized and how it can be improved or harnessed, is clearly necessary, in order to optimize the operation of FE films in real devices.When a non-switching DE layer is introduced between a FE layer and an electrode, numerous impacts on the FE switching need to be considered. Non-switching layers generate a depolarization field which destabilizes 1