비휘발성 메모리 응용을 위한 산화물 박막의 전기적 특성

Abstract

As portable mobile communications is developed rapidly, the demands of memory devices have been heavily increased. Up to now, DRAM has accounted for a large portion of memory components in the electric appliances because of high-density, low-power consumption, and low-product cost. As the DRAM cell size is continuously decreased, however, DRAM processing is now facing the limits for scalability of capacitors. Therefore, the development of new memory device with high performances, such as high-speed, low-power consumption, highdensity, and non-volatility, are strongly required. Among various memory devices developed as the new memory with high performances, the resistive switching memory showing electric-field-induced resistance change is highly expected to be a next generation memory device due to its non-volatility, simple device structure, and the possibility of easy scale-down. In this thesis, the electrical characteristics of resistive switching memory device were studied. First, in order to confirm that the resistive switching phenomena are related to the formation of conducting paths composed of various defects, the influence of variation of defects in NiO film on the resistive switching properties was investigated. As the growth temperature was increased, the resistive switching voltages were increased due to the decrease of defects in oxide. Because the conducting path is composed of various defects, the decrease in defects caused by formation of dense structure makes the formation of conducting path difficult. In addition, with increasing the compliance SET current, the RESET voltage as well as the low-R state current was increased. It was attributed to the increase in density of conducting path with increasing external electric field. From the low-temperature current-voltage characteristic, as-deposited NiO film exhibited typical oxidic behavior. After forming process, however, the NiO film showed different conduction behaviors. Low-R state showed metallic behavior which the current was increased with decreasing temperature, while high-R state showed metallic behavior below ~ 250 K and thermionic emission behavior above ~250 K. This result suggested that the mechanism of resistive switching is related to the formation of conducting path after forming process. For the commercial memory device applications, poly-SrZrO₃:Cr-based metal-oxide-metal (MOM) structures were fabricated on Si substrates, instead of SrTiO₃ single crystal substrates, by sputtering and their resistive switching characteristics were investigated. The MOM structures showed reproducible resistive switching behaviors with ON/OFF ratio higher than 20. Particularly, lowswitching voltage of ±2.5 V could be obtained through the reducing the plasma damage by off-axis sputtering. In addition, the switching voltage of poly- SrZrO₃:Cr-based MOM structure was much lower than that of epi-SrZrO₃:Cr-based MOM structure because of easy formation of conducting path through grain boundary where various defects can exist. These results suggest that poly- SrZrO₃:Cr film grown by sputtering is good candidate material for the application of commercial resistive switching memory device. To investigate the energy dependency of resistive switching phenomena, the influences of SET power and sweep delay time on resistive switching characteristics were observed. With increasing the SET power, the low-R state current was increased and the linear low-R state was observed. In addition, as the sweep delay time was increased from 1 to 100 msec, the resistive switching properties were improved. These results are attributed to the increase of conducting paths by sufficient SET energy, and imply the energy dependency of resistive switching behaviors. From above results, it is suggested that the resistive switching mechanism is related to the formation of local conducting paths inside oxide films and that the resistive switching memory is a promising candidate for the next generation nonvolatile memory applications.