Recently, the scientific research team led by Academician Liu Ming from the Key Laboratory of the Institute of Microelectronics has made progress in the field of HfO2 based ferroelectric memory. A ferroelectric diode (Fe-diode) based on Hf0.5Z0.5rO2 (HZO) material was proposed, and three-dimensional integration was realized. The result was published on March 13, 2020 in Nature Communications under the title "A highly CMOS compatible hafnia-based ferroelectric diode" (DOI: 10.1038 / s41467-019-13827-6). Associate Professor Luo Qing of the Institute of Microelectronics and Associate Professor Cheng Yan of East China Normal University are the first authors of this article. Academician Liu Ming and Professor Lv Hangbing of the Institute of Microelectronics are co-corresponding authors.
Among various non-volatile memories, ferroelectric random access memories (FeRAM), which achieve non-volatility by switching and sensing the polarization state of a ferroelectric capacitor, were thought as an ideal memory solution due to their outstanding features of low power, high speed, high endurance and good retention. However, the scaling limitation of perovskite ferroelectric materials hindered it from wide applications. The development of FeRAM in semiconductor industry has been stopped at 130 nm node for a long time. The discovery of ferroelectricity in doped HfO2 has renewed the interest in ferroelectric memory by offering a possible solution to bridge the scaling gap between perovskite ferroelectric materials and CMOS technology. The structural origin of the ferroelectricity in HfO2 based materials was hypothesized to be the formation of the noncentrosymmetric orthorhombic phase. There are four different space groups in o-phase, i.e., Pca21, Pbcm, Pmn21 and Pbca. The Pmn21 phase could be easily ruled out by the image of the hafnium sublattice. However, distinguishing between the Pbca, Pbcm, and Pca21 space groups is quite challenging, which could be only realized by visualizing the tiny difference of oxygen sublattice. 
In this work we unambiguously identified the Pca21 phase in HZO film by visualizing both the hafnium/zirconium lattice order and oxygen lattice order with atomic-resolution spherical aberration (Cs)-corrected STEM.
Up to now, ferroelectric HfO2 based memories in forms of 1T1C and FeFET structures have been demonstrated. The 1T1C FeRAM requires large capacitor area and undergoes destructive read out. FeFET can realize non-distructive read and 3D vertical stack. However, it cannot be used as a random access memory. Two terminal devices such as FTJ and Fe-diode have the potential to achieve high density crossbar array. The tunnel electroresistance effect of FTJ was generally formulated by quantum mechanical electron tunneling mechanism. In principle, both the on and off states of FTJ obey linear or quasi-linear I-V relationship, which makes it need extra selector device to diminish the sneaking current in crossbar array. In contrast, the Fe-diode, with the working principle governed by Schottky barrier modulation as polarization reversal, has the potential to gain inherent nonliearity and realize selector-free crosspoint integration. Fe-diode composed by conventional perovskite materials such as PbTiO3, BiFeO3 and Pb(Zr,Ti)O3 were commonly reported. However, low readout currents and low on/off ratio limited their miniaturization. Based on HfO2 ferroelectric material, we further demonstrated a fully CMOS compatible ferroelectric diode with high readout current density (>200 A/cm2) and build-in nonlinearity (>100), which was implemented to three-dimensional integration up to 8 layers with vertical size down to 20 nm。
全文链接：https://www.nature.com/articles/s41467-020-15159-2
Figure a) Atomic structure of HZO orthorhombic phase.b) Cross-section of the 3D vertical structure with Fe-diode devices.c) Typical I-V curve of the TiN/Hf0.5Z0.5rO2/TiN/W device.（d） Voltage dependence of the SET and RESET operation speed for Fe-diode memory.
