Cerium oxide capping on Y-doped HfO2 films for ferroelectric phase stabilization with endurance improvement

The effects of 1-nm-thick CeOx capping on 7.5-nm-thick Y-doped HfO2 films on the ferroelectric characteristics are investigated. From the ferroelectric characteristics of the samples annealed at different temperatures from 450 to 600oC and annealing durations, the time (τ) required to stabilize the ferroelectric phase at each temperature was shortened by the capping. The identical activation energy (Ea) of 2.65 eV for ferroelectric stabilization without and with capping suggests the same kinetics for phase transformation. However, an increase in the remnant polarization (Pr) was obtained. Only a few Ce atoms diffused into the underlying HfO2 film even after 600oC annealing. Ferroelectric switching tests revealed an improvement in endurance from 107 to 1010 by the capping, presumably owing to the suppression of conductive filament formation. Therefore, CeOx capping is effective in promoting the ferroelectric phase in HfO2 with high switching endurance.

Positive-to-negative subthreshold swing of a MOSFET tuned by the ferroelectric switching dynamics of BiFeO3

Ferroelectricity can reduce the subthreshold swing (SS) of metal-oxide-semiconductor field-effect transistors (MOSFETs) to below the room-temperature Boltzmann limit of ~60 mV/dec and provides an important strategy to achieve a steeper SS. Surprisingly, by carefully tuning the polarization switching dynamics of BiFeO3 ferroelectric capacitors the SS of a commercial power MOSFET can even be tuned to zero or a negative value, i.e., the drain current increases with a constant or decreasing gate voltage. In particular, in addition to the positive SS of lower than 60 mV/dec, the zero and negative SS can be established with a drain current spanning for over seven orders of magnitude. These intriguing phenomena are explained by the ferroelectric polarization switching dynamics, which change the charge redistributions and accordingly affect the voltage drops across the ferroelectric capacitor and MOSFET. This study provides deep insights into understanding the steep SS in ferroelectric MOSFETs, which could be promising for designing advanced MOSFETs with an ultralow and tunable SS.

Direct measurement of ferroelectric polarization in a tunable semimetal

Ferroelectricity, the electrostatic counterpart to ferromagnetism, has long been thought to be incompatible with metallicity due to screening of electric dipoles and external electric fields by itinerant charges. Recent measurements, however, demonstrated signatures of ferroelectric switching in the electrical conductance of bilayers and trilayers of WTe2, a semimetallic transition metal dichalcogenide with broken inversion symmetry. An especially promising aspect of this system is that the density of electrons and holes can be continuously tuned by an external gate voltage. This degree of freedom enables measurement of the spontaneous polarization as free carriers are added to the system. Here we employ capacitive sensing in dual-gated mesoscopic devices of bilayer WTe2 to directly measure the spontaneous polarization in the metallic state and quantify the effect of free carriers on the polarization in the conduction and valence bands, separately. We compare our results to a low-energy model for the electronic bands and identify the layer-polarized states that contribute to transport and polarization simultaneously. Bilayer WTe2 is thus shown to be a fully tunable ferroelectric metal and an ideal platform for exploring polar ordering, ferroelectric transitions, and applications in the presence of free carriers.