| With the rapid development of various electronic products, nonvolatile memory in thesemiconductor industry plays a more and more important role. However, the traditionalmemory can’t meet the performance of modern market requirement. In this background, newtype nonvolatile memory arises at the historic moment. According to the features of differentmaterials, the memory presents different storage mechanism. A new type of ferroelectricresistive switching effect attracted many researchers’ great attention. The ferroelectric domaincan realize fast polarization inversion in the external electric field. Form different chargedistribution of polarization in the interface, the device can realize two different kinds of highand low resistance state and the state can be maintained. The great advantage this memory isthat it is compatible with mature complementary metal oxide semiconductor (CMOS)technology, which make it as an potential alternative to the next generation of memory.We adopted the technique of pulsed laser deposition and sputtering, the device wasfabricated in lightly doped Si (111) substrate with Metal-Ferroelectric-Semiconductor (MFS)structure, and ferroelectric thin film capacitor is used to replace the transistor gate structure.At high frequency, the results of the positive and reverse voltage scanning of the capacitancevoltage (C-V) tests is significantly flat band voltage offset (namely C-V curve translationwidth), known as the capacitor “Memory Window”. When applying in the capacitor voltagegradually increased, the memory window of capacitor will increase gradually with voltage,which can be used for storage. And we also analyze the characteristics of C-V in lowfrequency, since the interface properties of lightly doped silicon substrate and barium titanateferroelectric thin film is poorer, which leads to the hysteresis effect is not obvious in lowfrequency response. The I-V curves shows bipolar resistance switching, but write voltage is too large due to the device structure, the next work needs to be improved.In the device of symmetrical MFM structure deposited on platinum substrate, this is thetypical structure of resistance switching memory. The polarization-Voltage (P-V) tests provethat the BTO film has obvious hysteresis loop. As a result of the symmetry structure of thedevice, the C-V tests showed obvious shape of butterfly loop. And the butterfly loops of thedevice in high and low frequency are very obvious, which indicate a good applicationprospect in integrated circuit both in high and low frequency. In the current and voltage (I-V)test, the DC voltage scanning showed strongly rectification effect, which can be used asrectifier device. After a large value of electroforming voltage the devices show a bipolarresistance switching effect, but the ON/OFF ratio is too small, which limits the application inresistance switching device. In the pulse voltage test, the device showed resistance switchingproperties related to ferroelectric polarization with poor stability and quickly fatigue. Thereason may be related to more crystal grain boundary in polycrystalline film, which providemore conductive path for the leakage current. And the interface of top and bottom electrodeswith ferroelectric thin film include more defects, which lead to the large leakage current, theseinterface defects in ferroelectric thin film layer after the pulse voltage polarization will trapcharge. In small voltage sweeping, the current is not really the reflection of the ferroelectricpolarization displacement. So the ratio of high and low resistance state is too small, and thedevices are easy to produce fatigue, but the new test method and the effect of leakage in thedevice laid the foundation for the next work.We adopted niobium doped strontium titanate single-crystal substrate and fabricatedPt/BTO/NSTO heterojunction structure, the P-V test also showed the phenomenon of leakagecurrent, but in a regular pattern of the I-V test procession, the devices not only showed theunidirectional rectification, the bipolar resistance switching effect is also certified. In theconventional voltage scanning mode, it can’t discriminate between ferroelectric polarization displacement and leakage current. So we adopted short pulse voltage with different polarity topolarize the BTO film devices. In small voltage scanning, we found the stable bipolarresistance switching effect. A new type of ferroelectric memory should be equipped with theabove characteristics, which have the advantage of non-destructive resistive readout. Weadopted a series of different magnitude of pulse voltage to polarize the the device, and thecorresponding different resistance states attested the function of memristor. And the bistableresistance switching devices present excellent properties and circularity, when the devices as amultistage storage also process good retention and reproducible performance.We qualitatively analyzed the device, and explained the resistance switching effect andmemristive behavior with energy-band theory. Although the devices showed large leakagecurrent, but with the heterojunction interface effect and Pt/BTO interface and schottky barrierformed at BTO/NSTO interface led to the resistance switching effect. However, the barriermodulation is limited by the short screening length in metals, and NSTO as an n-typesemiconductor can be switched between accumulation and depletion of majority carriers as aresult of a ferroelectric field effect. Thus, the BTO/NSTO interface is more likely to dominatethe transport properties of Pt/BTO/NSTO heterostructure. Based on analysis of our results, theferroelectric polarization is instrictly supposed to be responsible for the above behaviors.The next work that demonstrating the switching ferroelectric domains leads to theresistance switching effect by combining piezoresponse force microscopy (PFM) andconductive atomic force microscopy (CAFM) need to be finished, and we will quantitativelyexplain the phenomena for more detail. |
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