Ferroelectric Si-doped HfO₂ is a promising candidate for future generation memory devices. However, such devices are vulnerable to significant threshold voltage shifts resulting from charge trapping in oxide defects. We use complementary characterization and modeling techniques to reveal significant electron trapping/de-trapping behavior, together with a strong temperature dependence of the electron emission kinetics in ferroelectric layers, which results from the onset of polarization of the ferroelectric layer. This can lead to an apparent difference in the defect characteristics in ferroelectric-HfO₂ compared to the paraelectric-HfO₂ structures they are shown to closely resemble when this contribution is decoupled. The results demonstrate the presence of a defect band closely aligned to the silicon conduction band, which can easily be accessed during device operation.

Density functional theory predicts an abrupt drop in the stability, in the kinetic barrier for polarization reversal and spontaneous polarization of a Sc_xAl_1−xN wurtzite phase, when the Sc:Al ratio approaches 50:50. The same effect is obtained by the application of a tensile strain. The resulting polarization reversal barrier correlates with experimental coercive fields, and highly textured film measurements exhibit polarization values close to the theoretically predicted ones. Film thickness below 5 nm has a significant impact on the elastic properties.

The ferroelectric (FE)-HfO₂-based field-effect transistor (FEFET) is a promising candidate for emerging memory. However, data retention (DR) loss has been flagged as a key issue. Although two models of DR loss in FEFET have been proposed--the charge trapping model and the depolarization model--it is difficult to separate the dominant cause. In this article, simultaneous P-V and I-V measurements are performed which enable separation of these mechanisms. Different behavior of polarization and threshold voltage loss and recovery in a ``set'' state between two types of FEFET structures, metal (TiN)/FE-HfO₂/SiO₂/Si substrate (MFIS) and metal (TiN)/Si/FE-HfO₂/SiO₂/Si-substrate (MSFIS), clarify that the dominant mechanism of the DR loss in both structures is different. In addition, the impact of external bias and interfacial layer (IL) on the DR loss is revealed: the polarization loss is strongly affected by the external bias in both types of FEFET, indicating effective work function (eWF) has a strong impact on depolarization. On the other hand, the thickness and processing of IL strongly impact the charge trapping characteristics. Based on our understanding, excellent improvement of the DR is achieved: memory window ~0.8 V at 85 °C, extrapolated to ten years.

The ability to develop ferroelectric materials using binary oxides is critical to enable novel low-power, high-density non-volatile memory and fast switching logic. The discovery of ferroelectricity in hafnia-based thin films, has focused the hopes of the community on this class of materials to overcome the existing problems of perovskite-based integrated ferroelectrics. However, both the control of ferroelectricity in doped-HfO₂ and the direct characterization at the nanoscale of ferroelectric phenomena, are increasingly difficult to achieve. The main limitations are imposed by the inherent intertwining of ferroelectric and dielectric properties, the role of strain, interfaces and electric field-mediated phase, and polarization changes. In this work, using Si-doped HfO₂ as a material system, we performed a correlative study with four scanning probe techniques for the local sensing of intrinsic ferroelectricity on the oxide surface. Putting each technique in perspective, we demonstrated that different origins of spatially resolved contrast can be obtained, thus highlighting possible crosstalk not originated by a genuine ferroelectric response. By leveraging the strength of each method, we showed how intrinsic processes in ultrathin dielectrics, i.e., electronic leakage, existence and generation of energy states, charge trapping (de-trapping) phenomena, and electrochemical effects, can influence the sensed response. We then proceeded to initiate hysteresis loops by means of tip-induced spectroscopic cycling (i.e., “wake-up”), thus observing the onset of oxide degradation processes associated with this step. Finally, direct piezoelectric effects were studied using the high pressure resulting from the probe’s confinement, noticing the absence of a net time-invariant piezo-generated charge. Our results are critical in providing a general framework of interpretation for multiple nanoscale processes impacting ferroelectricity in doped-hafnia and strategies for sensing it.

The wake-up in doped hafnia ferroelectric devices is an extremely important process to understand in order to integrate these materials successfully into working ferroelectric memory devices. The crystallographic origins of this process are clarified with three main mechanisms. Strain relaxation in the ferroelectric orthorhombic phase led to an adjustment of the unit cell volume toward a “bulk-like” value. The undistorted cell allowed for easier polarizability within the unit cell, allowing higher polarization. Reversible phase transformations between the tetragonal and orthorhombic phases depend on the nature of the strain. Finally, a model is developed describing grain reorientation, inducing a 90° rotation of the orthorhombic unit cell and allowing the phase to respond to the E-field more readily under cycling.