Photoelectric Properties Of Silicon-based Memristive Materials And Their Device Applications

This dissertation focuses mainly on silicon-based memristive materials(films)and their device applications as artificial synapses.First,the crystallization behavior,localized structure and photoelectric properties of amorphous silicon(a-Si)thin films irradiated by either femtosecond or continuous lasers have been studied systematically.Second,two electrical memristors based on silicon-based films have been designed and successfully fabricated,and their resistive switching(RS)behavior,working mechanisms and biomimetic synaptic plasticities have been investigated thoroughly.Finally,based on the previous and well-known physics fundamentals of the interaction between light and a-Si thin film,a new a-Si-based photonic synapse has been proposed and demonstrated for the first time,making handy use of light induced band engineering of the a-Si film.The operating mechanism,synaptic plasticity and other morphological behavior similar to the human visual system have been systematically studied.The main contents and obtained results are summarized as follows:1.The inhomogeneous crystallization and localized structure of a-Si films irradiated by femtosecond laser or continuous laser have been systematically studied,respectively.(1)For a-Si film irradiated by a femtosecond laser,the research results show that when laser energy is applied beyond the etching threshold of a-Si film,the surface of the a-Si film is liquefied instantly by the laser and flies from the center to the edge of the irradiation area in the form of droplets,forming an inhomogeneous nanocrystalline Si layer with a thin center and a thick edge on the upper surface of the irradiation area.The introduction of Ag nanoparticles into the a-Si film during the fabrication process can improve the crystallization quality of the film.Nanocrystalline Si induced by using femtosecond laser has a significant effect on the optical and electrical properties of both a-Si and a-Si:Ag films,which is expected to be applied in optical and electrical memristive synapses.Moreover,the crystallization method through femtosecond laser also provides a new way for the fabrication of Si-based optical and electrical synapses.(2)For a-Si film irradiated by a continuous laser,the research results demonstrate that different wavelengths of light will induce defects with diverse energy levels in the a-Si film,modulating the band structure inside the a-Si film.This research result can be used for guiding the design of a-Si-based photonic synapse.2.The Raman spectra of nanocrystalline Si and amorphous Si irradiated by femtosecond laser or continuous laser have been studied intensively by employing the phonon confinement model,the modified phonon confinement and the Fano models,respectively.(1)For the nanocrystalline Si,the research results show that when an a-Si film is crystallized with a femtosecond laser energy higher than the etching threshold of a-Si,the a-Si film undergoes an instant liquefaction-flight-condensation and other complex dynamic processes.This dynamic process causes small-sized nanocrystalline Si gathering at the edge of the irradiation spot,resulting in a significant Fano interference effect in this area.This research result not only demonstrates a deep understanding of the interaction between light and nanocrystalline Si,but also provides an essential reference when using nanocrystalline Si in photonic synapses.(2)For the Raman spectroscopy of the a-Si film,the research results show that under continuous laser irradiation,the stress-induced modification of the 3-membered ring structure inside the a-Si network would cause the"modified phonon confinement model"(MPC model)to fail.Considered the stress effect,the MPC model can be improved,and an amendatory MPC model(named as IMPC model)has been hypothesized.The IMPC model could well describe the Raman spectrum of the a-Si film irradiated by continuous laser and obtain the short-program size which is consistent with the empirical formula.This model paves the way for the quantitative study of the resistance changing mechanism of Si-based photoelectric memristors using Raman spectroscopy.3.Two electrical memristors based on a-SiO_x film and a-SiN_x film have been designed and successfully fabricated,and their memristive behaviors,working mechanisms and bionic synaptic plasticities have been studied,respectively.(1)Taking TiO_xthin film as an insert layer,a double-resistive-layered Pt-Ag/a-SiO_x:Ag/TiO_x/p⁺⁺-Si memristor has been designed and investigated.The research results show that with the assist of an applied electric field,the Ag in the top electrode enters the a-SiO_x:Ag layer,combined with the embedded Ag particles,forming a conductive filament channel and acting as a new electrode,while the TiO_x layer plays the role of a real resistive layer to make the device realizing a robust and gradual resistance switching behavior.Furthermore,the Pt-Ag/a-SiO_x:Ag/TiO_x/p⁺⁺-Si memristor can behave as a bionic synapse,and several synaptic plasticities such as PPF,STP,LTP and more complicated hippocampal synaptic STDP behaviors have also been realized.Therefore,it is hoped that the device could have broad potential applications in neuromorphic computing.(2)A novel memristor structured as Ag/a-SiN_x/p⁺⁺-Si has been designed and fabricated by using Ag as the top electrode,and its resistive switching behavior and memristive mechanism have been deeply investigated.The research results indicate that the co-migration of N and Ag ions makes the memristor exhibit unique memristive characteristics such as formation-free,self-rectification,adaptive current limiting,and gradual adjustment of resistance states.These features enable the device to have better stability and could be used for suppressing the crosstalk and overshoot.More importantly,the memristor has application potential in biomimetic synapses,because its resistance states can be gradually adjusted.4.Based on the well-known physics fundamentals of the interaction between light and a-Si thin film,a new photonic synapse of ITO/a-Si/p⁺⁺-Si has been proposed and demonstrated for the first time,and its synaptic performance and working mechanism have been studied systematically.It is shown that the ITO/a-Si/p⁺⁺-Si photonic synapse exhibits two different kinds of photoconductivity behaviors under 635 nm or 450 nm light stimulations,being of volatile or non-volatile photoconductivity.These two unique photoconductive behaviors enable the device to achieve dual plasticity functions,namely STP for the 635 nm light and STP,LTP,STP-to-LTP transition for the 450nm light.The photonic synapse also presents other neuromorphic behaviors such as learning experience and associative learning behavior when stimulated by the 450 nm light.The dual photoconductivity also makes the ITO/a-Si/p⁺⁺-Si photonic synapse hold a unique feature,that is,it can respond to both light intensity and different colors.Based on this feature,the photonic synapse could imitate the human visual system without additional circuit modules,realizing the functions of light signal detection,grayscale and color signal extraction,color pattern recognition and memory.More importantly,the photocurrent response could be adjusted by setting different reading voltages,allowing the device to imitate the human visual system to realize the functions of environmental adaptability and selective memory.The ITO/a-Si/p⁺⁺-Si photonic synapse has a simple structure and is fully compatible with the existing Si-CMOS technology,having a broad application prospect in neuromorphic computing and advanced robot vision systems.