Investigation Of Carbon Nanotube Electrodes Based RRAM Devices

In recent years,resistive random access memory?RRAM?devices have developed as one of the most promising candidate for the next-generation nonvolatile memory on account of its excellent scalability,fast switching speed and robust endurance.As the feature size of the device gradually decreases,the nano size of carbon nanotubes?CNTs?provides a way to realize the high-density integration of RRAM.Therefore,this dissertation is focused on the fabrication of CNT electrodes based RRAM devices and the investigation of resistive switching characteristics.The specific research contents are as follows.The growth process of carbon nanotubes?CNTs?was investigated and optimized from the substrate annealing process,catalyst selection,CH4/H2 flow ratio,growth time and growth temperature.The experimental results show that when the substrate is annealed at 900? for 8h with 500 sccm O₂,the root mean square?RMS?roughness of the substrate is the smallest,which is most favorable for the growth of CNTs.The high quality horizontal growth of CNTs can be achieved when the 1nm Ni catalysts grow at 900 ? for 40 min with CH4:H2=500:60.And the HfO₂/CNTs structure-based RRAM was successfully fabricated on the basis of this process conditions.Based on the HfO₂/CNTs structure RRAM device,the effects of top electrode of Ta,Ti,Cu and Al on the performance of device was investigated.It is found that all of the four structures are typical bipolar resistive switching and have the self-compliance characteristics.Al/HfO₂/CNTs structure devices have the lowest power dissipation?Ireset = 130 nA?and the superior performance?self-compliance,forming-free and low resistive state?LRS?nonlinearity,etc?.It's indicated that the Al/HfO₂/CNTs RRAM devices can be applied in high-density storage whit great potential applications.On the basis of Al/Hf O₂/CNTs structure,the thickness of HfO₂ and the catalyst stripes were optimized.In addition,compared with the Al/HfO₂/Ti structure device,it is found that the CNTs electrode plays an important role in the self-compliance,ultra-low power and LRS nonlinearity of the device.The resistance switching mechanism of Al/HfO₂/CNTs devices was further clarified.And the current conduction in high resistive state?HRS?and LRS is controlled by space-charge-limited current?SCLC?and trap-controlled SCLC mechanism,respectively.Besides,the resistive switching mechanism is ascribable to the trapping and detrapping of electrons in HfO₂.The results also show that HfO₂ plays an important role in the resistance mechanism,while CNTs only act as the carrier of the electron injection and release.The physical model of the resistive switching is demonstrated in the end.