||With the advent of advanced technologies such as the Internet of Things (IoT), artificial intelligence (AI), and cloud computing, huge amounts of data are continuously produced. The demand for memory, therefore, is dramatically increasing. Traditional flash memory currently faces severe challenges when device size shrinks. A thin tunneling oxide layer may cause severe leakage path issues, resulting in degradation of data retention and component reliability. Therefore, it is imperative to develop a new generation of memory. Among next generation memories, the resistive random access memory (RRAM) has the advantages of being simple metal-insulator-metal structure, as well as having low power consumption and high operation performance, factors which are important in becoming a building block for the new generation of memory devices.|
In this thesis, we applied an indium tin oxide (ITO) thin film as the electrode in a HfO2-based RRAM to investigate its resistance switching (RS) characteristics. The experimental results show that, compared to a traditional inert metal electrode, the ITO electrode has robust RS characteristics and better reliability. Both lower operating current and faster operating speed can be achieved simultaneously. On the basis of electrical measurements, we conclude that the oxygen ions can enter the ITO electrode by the given electric field, thus forming a semiconductor-like ITO region. This also causes the effects of a self-compliance current and low power consumption. In addition, due to the bulk oxygen-ion storage in the ITO electrode, the device exhibits better RS performance.
Since the transition element has a semi-full electron orbital region, it easily forms bonds with oxygen ions. We, therefore, doped gadolinium (Gd) into the ITO electrode as a ITO:Gd combination to examine the RS mechanism in the HO2-based RRAM. The experimental results show that the ITO:Gd device can produce a high memory window at a low operating current. By means of the current fitting method and a temperature effect experiment, we are able to confirm that a modification in the conducting mechanism has occurred, from the Ohmic conducting mechanism in the pure ITO electrode to Schottky emission in the ITO:Gd electrode. We conclude that the ITO:Gd electrode has better bonding ability with oxygen ions due to the Gd doping, which in turn allows the ITO:Gd device to have better resistance switching characteristics.
Since the ITO material also possesses semiconductor characteristics, we have also attempted co-sputtering the ITO thin film with nitrogen gas as the ITON thin film to act as the insulator layer of RRAM, thereby further simplifying the device fabrication process. Experimental results shown that the ITON thin film can also induce RS characteristics. In addition, a lower operating voltage can be achieved with only ±3 volts to complete the forming, set, and reset operations.
Apart from this gas co-sputtering method, we also introduced an oxygen plasma treatment to oxidize the ITO thin film as the insulator in the RRAM. The experimental results show that the oxidized ITO thin film exhibits an insulator-like property and exhibits good RS characteristics. Both DC and AC electrical measurements were conducted to verify the continuous RS characteristics. Finally, a conducting model is proposed to explain the possible RS mechanism on the basis of the current fitting method and a temperature experiment.