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.
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.
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.
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