Memristors Based On Amorphous Sr-doped LaMnO₃ Thin Films

Nowadays, conventional memory technology based on CMOS processes is approaching its physical limit. Meanwhile, the memory-wall problem caused by speed gap between the processor and the memory also becomes more and more serious. These both hinder the development of the computer technology greatly. The memristor is the fourth fundamental element of electrical circuits, which has the ability to integrate logic operations with memory. The memristor, which is worthy of being studied, is a hopeful technology to relieve and even solve the memory-wall problem, as well as a novel technology for next-generation memory.The Sr-doped La Mn O3(La1-x Srx Mn O3, LSMO) thin film is a typical kind of perovskite functional materials. Under external voltages or currents, crystalline LSMO thin films could exhibit resistance-change effect, which, however, has some serious problems, such as low resistance ratio(ROFF/RON), high operational voltages, and high deposition temperature. In this thesis, amorphous LSMO(a-LSMO) thin films were deposited by radio frequency(RF) magnetron sputtering at relatively low temperature, and the Ag/a-LSMO/Pt memristor was prepared. The Ag/a-LSMO/Pt device exhibited excellent resistive switching(RS) and analog resistance-change properties. Compared to the crystalline LSMO, a-LSMO has much better RS performances and is easier to be deposited. Analysis indicates that the RS originates from the Ag nanofilament, growing from Pt electrode by electrochemical deposition in the solid-electrolyte-like a-LSMO thin film.The deposition techniques of a-LSMO thin films by RF magnetron sputtering were studied. The insulative a-LSMO films with resistivity of 107~108 Ω·cm were deposited at 50℃. Sputtering pressure and oxygen partial pressure had great effects on the packing density of a-LSMO thin films. Increasing sputtering pressure was advantageous to lower packing density, and the packing density without oxygen was smaller than that with oxygen. The refractive index of a-LSMO thin films increased with the increasing of Sr doping level. The thickness of a-LSMO thin films could be controlled by the sputtering power and time. Then the Ag/a-LSMO/Pt memristor was prepared on the Pt/Ti/Si O2/Si substrate. Using Rutherford backscattering spectrometry, atomic force microscopy(AFM) and transmission electron microscopy, the middle LSMO layer was confirmed to be amorphous and the chemical composition of the a-LSMO thin film was determined to be La0.79Sr0.21 Mn O3.The resistance-change properties of Ag/a-LSMO/Pt memristor were studied by quasi-static voltage sweeping and high-frequency voltage excitations. Under quasi-static voltage sweeping, the device exhibits nonvolatile bipolar RS with a large ROFF/RON ratio(>104), stable endurance(>102), and long retention(>104 s). All the threshold voltages are relatively low. The average values of SET voltage(VSET) and RESET voltage were 0.26 V and-0.19 V, respectively, and the electroforming voltage was about 0.9 V. The RS processes were fast, and the SET and RESET time were 50 ns and 200 ns, respectively. At the low resistance state(ON state), the resistance could be controlled by the compliance current(CC). Under some circumstances, the device can exhibit unipolar, nonpolar and volatile RS properties. The memristor could exhibit excellent pinched hysteresis loops under high-frequency excitation. The hysteresis loop is zero-crossing and pinched, and is confined to the first and third quadrants of the current-voltage(I-V) plane. In the range of 500 Hz~2.5 MHz, the area enclosed by the pinched hysteresis loops shrank with increasing frequency of the forcing voltage, and the pinched hysteresis loops degenerated into almost a straight line as the frequency was increased to 2.5 MHz. These are characteristics typical of memristors and consequently, the Ag/a-LSMO/Pt cells we fabricated were indeed memristors. The memristor also showed continuously tunable synapse-like resistance under continuous voltage excitations. The memristive performances show no obvious degradation after 100 repeating-pulse I-V cycles under 250 k Hz voltage excitation, and the ROFF/RON was about 10 and varied insignificantly during the 103 SET-RESET pulse tests, indicating that the Ag/a-LSMO/Pt memristors are highly reliable and reproducible.On the basis of impedance spectra, conductive AFM images, variable-temperature resistances, the conductance quantization during SET processes, and the I-V curves, the mechanism of resistance-change was studied. The RS behavior of Ag/a-LSMO/Pt memristor is based on the migration of Ag+, and originates from the redox-controlled formation and dissolution of Ag nanofilaments in the a-LSMO thin film. The RESET processes are based on the Joule-heating-assisted electrochemical dissolution of the filament. When the resistance of ON state(RON) is above 12.9 kΩ, the Ag nanofilament is at tunneling state; when RON<12.9 kΩ, the Ag nanofilament is at bridge state. When the CC increases, the Ag-nanofilament dimension and RESET current increase; thus RON decreases. The electrocrystallization process of Ag on the Pt electrode is the possible rate-limiting step of the SET process. The migration speed of Ag+ and the formation rate of Ag nuclei are both increase at higher temperature, so the VSET decreases. The RESET processes in the I-V curve of bipolar RS can be divided into three types: electrochemical type, instable type and Joule-heating type. The analog resistance-change properties under high-frequency voltage excitations originate from the thickening/thinning of Ag nanofilaments, and the nanovoids and open channels in the a-LSMO thin film are large enough for the diameter change of the Ag nanofilament. The relationship between the microstructure of a-LSMO thin film and the resistance-change property of Ag/a-LSMO/Pt memristor was built by the spectroscopic ellipsometry, and it was found that with the packing density of the a-LSMO thin film increasing the electroforming voltage and its distribution range increases.Finally, three other memristor devices were studied, including the memristor using transparent indium-tin oxide as the bottom electrode, the nanoscale memristor prepared by ultrathin porous alumina masks, and the micro-scale crossbar memristor prepared by stainless steel masks. The excellent RS properties and analog resistance-change properties of memristors based on a-LSMO thin films have great application prospects in the fields of nonvolatile memory, artificial neuromorphic networks and beyond von Neumann computers.