저항변화 메모리 적용을 위한 바나듐 도핑에 따른 TiO2막의 전기적 특성

Abstract

The resistance switching behavior of several oxide and chalcogenide thin-film materials recently attracts great interest for the application in nonvolatile memory elements in semiconductor memory devices. Apart from random access memories using phase change material(PRAM) where a large reset current hinders high-density integration, the resistive switching RAM (ReRAM) without a phase change of the material offers the possibility of very high density integration and lower power operation. Of the various materials with resistive switching characteristics, binary metal oxide, such as TiO2, has a clear advantage over perovskite oxides in terms of their excellent process compatibility with the standard CMOS logic. However, several problems still remain, such as the large reset current and the large variations in set/reset voltage. In addition, the large variations in resistance in both the low resistance state(LRS) and the high resistance state(HRS) are another concern. For solving these issues, we proposed a vanadium doped TiO2 thin film. We could dope vanadium through placing the vanadium chips on the TiO2 target during RF magnetron sputtering and adjust dopant element content by changing a number of vanadium chips from 1ea to 3ea. Vanadium was chosen as a dopant since it has an atomic radius similar to Ti. In addition, it shows metal-insulator transition(MIC) characteristics and vanadium oxide is known to be a common catalyst of TiO2. The basic properties were analyzed through AFM, XPS, ICP. As a result, the surface roughness was 0.064nm and the concentration of vanadium increased with the numbmer of vanadium chips. The electrical properties were measured for checking switching characteristics of TiO2. It revealed an unipolar switching and uniform I-V curve through cycling test. For the confirming of vanadium doping effect, additional current-voltage characteristics were measured. The best result was obtained in the case of 0.7% vanadium doped TiO2 layer(2-chips vanadium doped TiO2). It showed large resistance gap between LRS and HRS and the more uniform reset current resulted in. The uniform set/reset voltage were obtained although there were no large difference between TiO2 film and vanadium doped TiO2 film. Reliability test for ReRAM device application was carried out to check vanadium doping effect. As a result, small variation of HRS was obtained at 0.03ppm vanadium doped TiO2 by endurance test. Furthermore, switching characteristic of both samples was checked during retention test and the vanadium doped TiO2 turned out to be more stable than the undoped one.