Memristive Behaviors In Te-Based Chalcogenide Materials

Memristor has been extensively interested by researchers around the world since it was implemented in2008. Since memristor can remember the charge that passed through the device, there is broad prospects for applications such as non-volatile memory, logic operation, neuromorphic circuits and programmable circuits. However, the research on memristor is still in the primary stage and further research on memristive materials, devices, mechanism and resistance tuning is necessary.In this study, I focus on three Te-based chalcogenide materials:Ge2Sb2Te5, Ag5In5Sb60Te3and Sb2Te3, and fabricate memristive devices. I investigate the characteristics of different materials, propose the memristive mechanisms, and then discuss the improving method, resistance tuning method and prospective applications for Te-based chalcogenide memristors. The main content and achievements are summarized as follows:Firstly, I investigate the memristive behaviors in amorphous Ge2Sb2Te5film without excess Sb atoms and fabricate two kinds of Ge2Sb2Te5memristors. The first one is Ge2Sb2Te5memristor based on inactive electrode with TiW/Ge2Sb2Te5/TiW structure. The ratio of RoFF/Ron is less than one order of magnitude, but the resistance can be tuned gradually. A memristor model based on intrinsic space charge limited conduction (SCLC) is proposed to explan this phenomenon. The other kind is Ge2Sb2Te5memristor based on active Ag electrode with Ta/Ge2Sb2Te5/Ag and Ag/Ge2Sb2Te5/Ag structures. The resistive switching is owing to the formation and crack of Ag filaments in Ge2Sb2Te5film. The devices can be repeatedly switched with the ratio of RoFF/RoN beyond50. Thus I propose a memristor model based on electrochemical metallization effect (EME).Secondly, I find that amorphous Ag5ln5Sb6oTe3o film behaves excellent memristive behaviors, and propose two memristive mechanisms in Ag5In5Sb60Te30film: intrinsic SCLC and extrinsic EME. I demonstrate repeatable gradual resistance tuning in Ag/Ag5ln5Sb6oTe3o/Ag memristor based on the coexistence of SCLC and EME, showing that the resistance can be tuned precisely. Otherwise, reproducible gradual resistance tuning in both bipolar and unipolar modes are demonstrated in Ag/Ag5In5Sb60Te30/Ag memristor, while the bipolar gradual resistance tuning results in a lower energy consumption. Finally, I investigate the memristive behaviors in crystalline Sb2Te3film and propose that the change in resistance is related to charged defects in the film. The conduction mechanism of high resistance state is Poole-Frenkel emission, and thus the device resistance decreases during increasing the applied voltage. When the applied voltage is large enough, the charged defects in Sb2Te3film can migrate, which results in abruptly change in resistance. If Ag is used as an active electrode, the Ag atoms would diffuse into Sb2Te3film and bond with Te after annealed. Consequently, more charged defects are created at the interface of Ag/Sb2Te3. By applying suitable pulses, both of resistive switching and gradual resistance tuning can be achieved in Ag/Sb2Te3/Ta memristor.In this study I demonstrate the memristive behaviors in above mentioned Te-based chalcogenide materials. However, they have different memristive characteristics owing to different mechanisms. In conclusion, the Ge2Sb2Te5memristor with Ag electrode has the best switching behavior, which is suitable for digital applications such as non-volatile memory and logic operation. The Ag5In5Sb60Te30memristor behaves best in gradual resistance tuning, which provides feasible options for multi-level storage and future analog applications such as electronic synapses. Sb2Te3memristor has potential in both applications above mentioned.