CMOS-compatible Metal-oxide Based Memristive Devices

With the emergence of technologies such as big data,cloud computing,internet of things and artificial intelligence,human society has gradually moved toward the information age.Faced with the massive data challenges of the information society,the development of new storage devices and new computing architectures is considered to be the key to solve data storage and data processing problems.In terms of non-volatile memory applications,memristor has the advantages of high speed,low power consumption,high density and easy preparation,and are expected to replace flash memory as the main technology of nextgeneration memory.In terms of logic operation and neuromorphic computing applications,memristor combines the characteristics of information storage and processing to help break the traditional von Neumann architecture bottleneck and realize the high-speed,parallelization and intelligence of information processing under the new architecture.This thesis focuses on the theme of memristor,following the general steps of "material analysis-process preparation-device characterization-mechanism analysis-application examples" to study several CMOS-compatible metal-oxide materials.The specific content is as follows:(1)The first principle calculation of 2×2×2 Cu2 O supercells was carried out by LDA-1/2 pseudopotential method,and the formation energies of various point defects under Curich/O-poor and Cu-poor/O-rich conditions were obtained.In accord with the material analysis of X-ray diffraction,X-ray photoelectron spectroscopy and laser confocal Raman spectroscopy,the Cu2 O material prepared by sputtering is proved to have the Cu vacancy as the main point defect,and the presence of Cu vacancy defects is responsible for the material exhibiting P-type conductivity.The electrical properties of the crossbar Cu/Cu2O/Ti W memristor in high and low resistance states were studied by I-V curve fitting,temperaturedependent measurement and AC impedance test.A conduction model based on the Schottky barrier at the Cu/Cu2 O interface and conductive filaments composed by Cu vacancies was proposed.(2)A variety of test methods were used to investigate the effect of Ru doping on the properties of Al2O3 films.It is found that Ru element is doped in Al2O3 in the form of ions,and the high conductance regions of the films are randomly distributed,which is helpful for the formation of multiple conductive channels.The experiment about forming process of Pt/Al2O3/Ru planar devices demonstrates that Ru can act as active electrode,participating in the resistive switching process.The experimentally prepared Pt/Al2O3:Ru/Ru device and Pt/Al2O3/Ru device in cross-point structure have stable resistive switching behaviors,but the former has better gradual resistance states than the latter.Furthermore,the former device can achieve linear and symmetric conductance change under two pulse sequences formed by identical pulses of 3 V/500 ?s and-3 V/1.5 ms,respectively.By analyzing the conduction mechanisms of the doped device under different operating currents,we can conclude that the linear and symmetric conductance change is due to the multiple conductive channels composed of Ru atoms.In addition,when the operating current is relatively large,the strong conductive path formed in the forming process plays a leading role,and its size change is the reason why the device has multilevel cell characteristic.(3)It is found that the simple three-layer structure Ti/Hf O2/Pt memristor based on amorphous Hf O2 has good device performance and multilevel cell ability.The resistive switching behavior can be derived from the space-charge-limited conduction mechanism determined by the exponential distribution of charge traps.The scaling effects on the switching parameters are discussed from the aspects of functional layer thickness and device area.It is found that the reduction of the Hf O2 film thickness is beneficial to the reduction of operating voltages and the increase of switching window,whereas the reduction of device area has the opposite trend.Therefore,the two size factors need to be compromised when considering the scaling issue.(4)The Cross-point Pt/Hf O2/Ta memristor prepared by the two-step photolithography process has stable memristive characteristics and its switching window is adjustable.The small window mode has good DC switching parameters and good pulse characteristics.It can achieve 106 stable switching cycles under the pulse of 1.9 V/200 ns and-2.8 V/200 ns,with a window about 10 and good retention characteristics of high and low resistance states.In addition,the pulse width required for device operation can be further reduced,achieving stable resistive switching under 20 ns pulses.(5)Taking the enhanced NMOS transistor as an example,the performance of 1T1 R memristor in two different switching modes was analyzed,then the integration mode of the 1T1 R device in this study was determined.Using the 6-inch wafer supplied by a commercial foundry,a BOEL process for integrating memristors on the transistor arrays was developed.Different sizes of 1T1 R arrays were fabricated and the Pt/Hf O2/Ta device in 1T1 R structure was used to perform structure and material analysis.The Pt/Hf O2/Ta device has better uniformity than the cross-point device.Additionally,the 1T1 R device has low resistance states with high linearity,which can be regarded as fixed resistors in the low voltage region.This property is beneficial to the implementation of some applications based on vector matrix multiplication operation.The use of Ir and Pd to replace CMOS-incompatible Pt is proved to be feasible to obtain good device performance.(6)Based on the new array test system which can achieve synchronous operations,an array test method for high precision conductance regulation was designed.With this universal programming test method,not only basic electrical operations such as SET,RESET and READ but also the writing of any conductance matrix can be realized.Taking the twodimensional discrete cosine transform as an example,the image compression in the ratio of 20:3 was completed by the hardware system,and the compressed image reconstructed by software still maintained a good picture quality.