Studies Of Two Kinds Of Memory Materials For Cognition Devices

The concept of memristor was put forward 45 years ago in 1971.Since 2008,memristor mimicking biological synapse has made considerable progresses,which mainly focused on stimulation of human brains and functions of nerve synapse.Encoding,training,memory solidification and retrieval were the main processes in cognitive neuroscience.To accomplish these processes,the artificial recognition memory should be realized.That was the future mainstream technology of memory storage development.Because of its own unique mechanism,ferroelectric and organic polymer were two important materials to be used to fulfill memristor.Ferroelectric tunnel junctions?FTJs?were quantum nanostructures that had great potential in the hardware basis for future neuromorphic applications.Among recently proposed possibilities,the artificial cognition had high hopes,where encoding,training,memory solidification and retrievalconstitute a whole chain that was inseparable.However,it was yet envisioned but experimentally unconfirmed.The poor retention or short-term store of tunneling electroresistance,in particular the intermediate states,was still a key challenge in FTJs.Here we reported the encoding,training and retrieval in BaTiO₃ FTJs,emulating the key features of information processing in terms of cognitive neuroscience.This was implemented and exemplified through processing characters.Using training inputs that were validated by the evolution of both barrier profile and domain configuration,accurate recalling of encoded characters in the retrieval stage was demonstrated.It provided viable solutions for cognitive neuroscience and artificial cognition of processing and memory information.In addition,we tried to produce key materials of cognition devices of organic material.Moreover we produced flexible substrate PDMS and fabricated TIPS pentacene films via spin-coating.We hoped to replace traditional vacuum evaporation films with spin-coating polymer films.Following were major conclusions of the thesis.1.We developed BaTiO₃ thin film by pulsed laser deposition and observed the structure of the cross section of the film using high resolution transmission electron microscope.By using atomic force microscope,piezoelectric force microscope and conductive atomic force microscope,we made a series of research on the characterization of the pattern of film,ferroelectric domain polarization switching and effect of tunneling electroresistanceresistance?TER?.2.We developed Pt top electrode on heterostuctures of BaTiO₃/La0.67Sr0.33MnO₃,and observe TER through atomic force microscope.Four TER bands were ruled to discriminate one code from another,taking into account both the device-to-device and the cycle-to-cycle variations of TER.The inputs were grouped into 4 voltage intervals.The kernel of each interval respectively,produced TER with differences of one order of magnitude.Given these 4 inputs,the outputs were accordingly labeled as four codes based on the TER values in descending order.We used the code to encode ASC? of the characters of "NJU".3.We loaded a single pulse and training pulses to make comparison.We discovered that after encoding a single pulse the TER decayed according to time period and caused retrieval mistakes as time passed by.Whereas after encoding training pulses,TER kept stable holding performance and was able to make precise retrieval and further recalling.4.Through piezoelectric force microscope,we made studies on domain structure of device.We discovered that the cause of TER decay contributed to the reducing of barrier height cased by relaxation of ferroelectric domain.After a comparison between a single pulse and training pulses,we found that applying continuous impulses could change domain structure and restrain the relaxation of ferroelectric domain and recover barrier height to control the decay of TER and improve hold performance and temporary storage of TER effect.Retrieval and recall had the potential of simulating functions of human brains.The enhanced stability of TER by fine control of training inputs,offered a promising solution to the long sought improved retention in FTJ applications.5.We produced flexibility PDMS substrates.We fabricated TIPS pentacene films via spin-coating.The surface morphologies of the TIPS pentacene thin film as deposited were investigated by using SEM and AFM.We wanted to produce an organic three terminal Artificial Cognition device on flexibility subatrate in the future.