A highly CMOS compatible hafnia-based ferroelectric diode

Memory devices with high speed and high density are highly desired to address the ‘memory wall’ issue. Here we demonstrated a highly scalable, three-dimensional stackable ferroelectric diode, with its rectifying polarity modulated by the polarization reversal of Hf0.5Zr0.5O2 films. By visualizing the hafnium/zirconium lattice order and oxygen lattice order with atomic-resolution spherical aberration-corrected STEM, we revealed the correlation between the spontaneous polarization of Hf0.5Zr0.5O2 film and the displacement of oxygen atom, thus unambiguously identified the non-centrosymmetric Pca21 orthorhombic phase in Hf0.5Zr0.5O2 film. We further implemented this ferroelectric diode in an 8 layers 3D array. Operation speed as high as 20 ns and robust endurance of more than 109 were demonstrated. The built-in nonlinearity of more than 100 guarantees its self-selective property that eliminates the need for external selectors to suppress the leakage current in large array. This work opens up new opportunities for future memory hierarchy evolution.

The total energy is the energy difference relative to 0% displacement.   The reference current can be expressed as (1) and the current of the Fe-diode cell can be expressed as Icell=VREF/R_cell (2) The current flowing through the transistors M2 to M5 is the reference current Iref, and the current flowing through the transistors M6 to M9 is the current Icell. According to Kirchhoff's current law, the currents flowing to the two input parasitic capacitances Cr and Cm of the comparator are and Icell-Iref (4) respectively. When the resistance value R_cell of the Fe-diode is smaller than the reference resistance R_ref, the current Icell flowing through the Fe-diode cell is greater than the reference current Iref, the parasitic capacitance Cr is discharged on the comparator refside, the voltage at the point A is lowered, and the parasitic capacitance Cm is charged on the matside of the comparator. The voltage at point B rises and the comparator outputs a high level "1". On the contrary, when the resistance value R_cell of the Fe-diode cell is greater than the reference resistance R_ref, the current Icell flowing through the Fe-diode is smaller than the reference current Iref, the parasitic capacitance Cr is charged on the comparator refside, the voltage at the point A is increased, and the parasitic capacitance of the comparator matside is opposite. As a result, Cm is discharged, and the voltage at point B decreases, with the comparator outputting a low level "0".
Thanks to the symmetric structure, the CSA has tolerance for process variation and 15 residual current, which reduces the time required for BL precharge. Compared to other SAs, the CSA achieves faster read speeds. Note: 10 nm device shows the property with high on-off ratio, high nonlineairty, while the thinner films (i.e. 5nm) exhibit higher current density and lower swtiching voltages.

Supplementary
The inset show the equivalent circuit diagram of the Fe-diode. The interface effect is equivalent to a diode and the bulk resistance is equivalent to a resistor. By further decreasing the HZO film thickness, lower switching voltages and higher operation current could be obtained due to the decreasing of the bulk resistance.

GND (c)
Crossbar can ideally offer the smallest footprint of cell size -4F 2 (F is the feature size) 11 . However, the crossbar structure has a serious problem called the sneaking current issue 12 . Taking a 2×2 array as an example, if the three neighbor cells are at lowresistance state (LRS), whatever the state of designated cell is, its readout state will always be LRS, resulting in reading error or cross talk (Supplementary Figure 15a).
The larger array size is, the more serious cross talk will be. To inhibit the sneaking current from the unselected cells, high nonlinearity is necessary for the memory cell, by integrating separate selector device as a common solution. 13 .
The Fe-diode demonstrated here exhibits spontaneous high nonlinearity (more than

Supplementary Notes
Supplementary Note 1: According to the model of metal-ferroelectric interface 15,16 , the variation in the built-in potential with ferroelectric polarization can be given by , where Ф is the built-in potential without contribution from polarization, Ф is the built-in potential with contribution from polarization, the apparent is the permittivity of free space, is the static dielectric constant, P is ferroelectric polarization, and δ is the thickness of a interface layer between the surface polarization charge and the metallic electrode. The value of δ is estimated to be the order of a unit cell 17 . For this work the remanent polarization is ~17 uC/cm 2 , and dielectric constant is about ~30 18 .
Then the variation in the built-in potential ∆Ф can be as high as 0.32V for δ value of 0.5 nm. Considering that the Schottky emission equations is: By purely using the Schottky emission equation, the theoretical on/off ratio is about exp (q∆Ф /kT)=exp(0.32/0.025)=3.6×10 5 , which is larger than the measured on/off ratio (>10 4 ). This result implies that other conduction mechanism may be dominant in OFF state, such as trap-assisted-tunneling, that is shown in majority of transition metal oxides 19, 20 .