Recently, ferroelectric tunnel junctions have attracted much attention due to their potential applications in non-destructive readout non-volatile memories. Using a semiconductor electrode has been proven effective to enhance the tunnelling electroresistance in ferroelectric tunnel junctions. Here we report a systematic investigation on electroresistance of Pt/BaTiO3/Nb:SrTiO3 metal/ferroelectric/semiconductor tunnel junctions by engineering the Schottky barrier on Nb:SrTiO3 surface via varying BaTiO3 thickness and Nb doping concentration. The optimum ON/OFF ratio as great as 6.0 × 106, comparable to that of commercial Flash memories, is achieved in a device with 0.1 wt% Nb concentration and a 4-unit-cell-thick BaTiO3 barrier. With this thinnest BaTiO3 barrier, which shows a negligible resistance to the tunnelling current but is still ferroelectric, the device is reduced to a polarization-modulated metal/semiconductor Schottky junction that exhibits a more efficient control on the tunnelling resistance to produce the giant electroresistance observed. These results may facilitate the design of high performance non-volatile resistive memories.
Ferroelectric Hf0.5Zr0.5O2 films, 5.8 nm in thickness, were deposited on Nb:SrTiO3 semiconductor substrates to form a Pt/Hf0.5Zr0.5O2/Nb:SrTiO3 metal/ferroelectric/semiconductor ferroelectric tunnel junction (FTJ). A high tunneling electroresistance ratio of 800 was achieved at room-temperature. It is observed that in the low resistance state, the transport characteristic obeys direct tunneling, while in the high resistance state, it is dominated by thermal emission. It implies that the Schottky barrier on the surface of the semiconductive electrode is modulated by the polarization in the ferroelectric Hf0.5Zr0.5O2 barrier, generating the high electroresistance ratio. The FTJ also exhibits excellent retention for more than 10 000 s and good switching endurance for more than 1500 cycles. The results suggest the potential of this HfO2-based FTJ for next generation nonvolatile memories.
As nanoelectronic synapses, memristive ferroelectric tunnel junctions (FTJs) have triggered great interest due to the potential applications in neuromorphic computing for emulating biological brains. Here, we demonstrate multiferroic FTJ synapses based on the ferroelectric modulation of spin-filtering BaTiO3/CoFe2O4 composite barriers. Continuous conductance change with an ON/OFF current ratio of ∼54 400% and long-term memory with the spike-timing-dependent plasticity (STDP) of synaptic weight for Hebbian learning are achieved by controlling the polarization switching of BaTiO3. Supervised learning simulations adopting the STDP results as database for weight training are performed on a crossbar neural network and exhibit a high accuracy rate above 97% for recognition. The polarization switching also alters the band alignment of CoFe2O4 barrier relative to the electrodes, giving rise to the change of tunneling magnetoresistance ratio by about 10 times and even the reversal of its sign depending upon the resistance states. These results, especially the electrically switchable spin polarization, provide a new approach toward multiferroic neuromorphic devices with energy-efficient electrical manipulations through potential barrier design. In addition, the availability of spinel ferrite barriers epitaxially grown with ferroelectric oxides also expends the playground of FTJ devices for a broad scope of applications.
Recently, ferroelectric tunnel junctions (FTJs) have attracted great attention due to promising applications in non-volatile memories. In this study, we report high-temperature tunneling electroresistance (TER) of metal/ferroelectric/semiconductor FTJs. Hysteretic resistance-voltage loops are observed in the Pt/BaTiO 3 /Nb:SrTiO 3 tunnel junction from 300 to 513 K due to the modulation of interfacial Schottky barrier by polarization switching in the 4 u.c.-thick BaTiO 3 barrier via a ferroelectric field effect. The Pt/BaTiO 3 /Nb:SrTiO 3 device exhibits a giant R OFF /R ON resistance ratio of $3 Â 10 5 at 383 K and maintains bipolar resistance switching up to 513 K, suggesting excellent thermal endurance of the FTJs. The temperature-dependent TER behaviors are discussed in terms of the decrease of polarization in the BaTiO 3 barrier, and the associated junction barrier profiles are deduced by transport and capacitance analyses. In addition, by extrapolating the retention time at elevated temperature in an Arrhenius-type relation, activation energy of $0.93 eV and room-temperature retention time of $70 years can be extracted.
Recently, complementary resistive switches (CRSs) have attracted considerable attention because of the effective suppression of the sneak leakage that is an inherent problem of crossbar memory arrays. In this work, we propose a new CRS device enabling nondestructive readout based on back-to-back in-series Pt/BaTiO/Nb:SrTiO ferroelectric tunnel junctions (FTJs). The FTJ elements exhibit not only a nonvolatile resistance switching but also a typical diode-like transport in the high-resistance state (HRS) because of the ferroelectric enhancement on the Schottky barrier of the BaTiO/Nb:SrTiO interface. With the rectifying characteristic, the complementary HRS + LRS (low-resistance state) and LRS + HRS states can be well-distinguished and nondestructively read out by a subthreshold voltage. In addition, the sneak current is significantly suppressed in the Pt/BaTiO/Nb:SrTiO CRS crossbar array, and the maximum scaling size is increased by about 50 times, in comparison to the array constituted by only the single-FTJ devices. These results facilitate the design of high-performance resistive memories based on the crossbar architecture.
Strain engineering plays a critical role in ferroelectric memories. In this work, we demonstrate dynamic strain modulation on tunneling electroresistance in a four-unit-cell ultrathin BaTiO 3 metal/ferroelectric/semiconductor tunnel junction by applying mechanical stress to the device. With an extra compressive strain induced by mechanical stress, which is dynamically applied beyond the lattice mismatch between the BaTiO 3 layer and the Nb : SrTiO 3 substrate, the ON/OFF current ratio increases significantly up to a record high value of 10 7 , whereas a mechanical erasing effect can be observed when a tensile stress is applied. This dynamic strain engineering gives rise to an efficient modulation of ON/OFF ratio due to the variation of BaTiO 3 polarization. This result sheds light on the mechanism of electroresistance in the ferroelectric tunnel junctions by providing direct evidence for polarization-induced resistive switching, and also provides another stimulus for memory state operation.
Hf0.5Zr0.5O2 (HZO) thin films have been deposited on (110)-oriented SrTiO3 (STO) substrates buffered with epitaxial La0.7Sr0.3MnO3 (LSMO) by pulsed laser deposition. The HZO/LSMO/STO heterostructures show smooth surface and clear interface. It is observed that ferroelectric orthorhombic HZO is enhanced, as non-polar tetragonal HZO is suppressed with the increasing LSMO thickness or decreasing HZO thickness. Completely orthorhombic HZO films are achieved with desired LSMO and HZO thickness. These HZO films are (111)-oriented with in-plane [2¯11] and [01¯1] directions along LSMO [11¯0] and [001], respectively, and exhibit ferroelectric properties at room temperature with an optimized remanent polarization around 26 μC/cm2 without the need of a wake-up process, a long retention up to 104 s and a fatigue endurance up to 109 cycles. Epitaxial HfO2-based films with robust ferroelectric properties deposited on (110)-oriented STO substrates provide additional opportunities to understand the profound effects of orientation, strain, and interface microstructures on the metastable polar phases and ferroelectric properties of HfO2 thin films.
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