Tiangui You

Bipolar electric-field enhanced trapping and detrapping of mobile donors in BiFeO3 memristors.

Pulsed laser deposited Au-BFO-Pt/Ti/Sapphire MIM structures offer excellent bipolar resistive switching performance, including electroforming free, long retention time at 358 K, and highly stable endurance. Here we develop a model on modifiable Schottky barrier heights and elucidate the physical origin underlying resistive switching in BiFeO3 memristors containing mobile oxygen vacancies. Increased switching speed is possible by applying a large amplitude writing pulse as the resistive switching is tunable by both the amplitude and length of the writing pulse. The local resistive switching has been investigated by conductive atomic force microscopy and exhibits the capability of down-scaling the resistive switching cell to the grain size.

Single pairing spike-timing dependent plasticity in BiFeO3 memristors with a time window of 25 ms to 125 μs.

Memristive devices are popular among neuromorphic engineers for their ability to emulate forms of spike-driven synaptic plasticity by applying specific voltage and current waveforms at their two terminals. In this paper, we investigate spike-timing dependent plasticity (STDP) with a single pairing of one presynaptic voltage spike and one post-synaptic voltage spike in a BiFeO3 memristive device. In most memristive materials the learning window is primarily a function of the material characteristics and not of the applied waveform. In contrast, we show that the analog resistive switching of the developed artificial synapses allows to adjust the learning time constant of the STDP function from 25 ms to 125 μs via the duration of applied voltage spikes. Also, as the induced weight change may degrade, we investigate the remanence of the resistance change for several hours after analog resistive switching, thus emulating the processes expected in biological synapses. As the power consumption is a major constraint in neuromorphic circuits, we show methods to reduce the consumed energy per setting pulse to only 4.5 pJ in the developed artificial synapses.

Engineering interface-type resistive switching in BiFeO3 thin film switches by Ti implantation of bottom electrodes.

BiFeO3 based MIM structures with Ti-implanted Pt bottom electrodes and Au top electrodes have been fabricated on Sapphire substrates. The resulting metal-insulator-metal (MIM) structures show bipolar resistive switching without an electroforming process. It is evidenced that during the BiFeO3 thin film growth Ti diffuses into the BiFeO3 layer. The diffused Ti effectively traps and releases oxygen vacancies and consequently stabilizes the resistive switching in BiFeO3 MIM structures. Therefore, using Ti implantation of the bottom electrode, the retention performance can be greatly improved with increasing Ti fluence. For the used raster-scanned Ti implantation the lateral Ti distribution is not homogeneous enough and endurance slightly degrades with Ti fluence. The local resistive switching investigated by current sensing atomic force microscopy suggests the capability of down-scaling the resistive switching cell to one BiFeO3 grain size by local Ti implantation of the bottom electrode.

Resistive switching in thin multiferroic films

Resistive switching properties of multiferroic BiFeO3 and YMnO3 thin films grown by pulsed laser deposition technique have been investigated. Both material systems sandwiched between Au top and Pt/Ti bottom electrodes reveal nonvolatile resistive switching upon application of an electric field. BiFeO3 is switching in bipolar mode when a positive and negative bias is applied. The resistance ratio between high resistance and low resistance state is larger than two orders of magnitude. In contrary to BiFeO3, YMnO3 shows a unipolar resistive switching of abrupt nature with the difference up to five orders of magnitude between both resistance states. This work presents the results on resistive switching in BiFeO3 and YMnO3 and discusses the mechanisms of observed phenomena. Possible applications of our findings are shown on the example of nonvolatile data storage devices.