Correlation between Oxygen Composition and Electrical Properties in NiO Thin Films for Resistive Random Access Memory
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1250-G05-05
Correlation between Oxygen Composition and Electrical Properties in NiO Thin Films for Resistive Random Access Memory Yusuke Nishi, Tatsuya Iwata, and Tsunenobu Kimoto Department of Electronic Science and Engineering, Kyoto University, Kyotodaigaku-katsura, Nishikyo, Kyoto, 615-8510, Japan ABSTRACT Admittance spectroscopy measurement has been performed on NiOx thin films with various oxygen compositions (x=1.0-1.2) in order to characterize localized defect levels. The activation energy and concentration of localized defect levels in NiOx films with low oxygen composition (x≤1.07) are 120-170 meV and lower than 2×1019 cm-3, respectively. From I-V measurement of Pt/NiOx/Pt structures, samples with high oxygen composition (x≥1.10) did not show resistance switching operation, while samples with low oxygen composition (x≤1.07) did. The best oxygen composition of NiOx thin films turned out to be 1.07 in order to realize repeatable and stable resistance switching operation. INTRODUCTION In recent years, a variety of nonvolatile memories have emerged and been developed as the next-generation nonvolatile random access memories (RAMs), which may replace the Flash memories. Resistive RAMs (ReRAMs) using binary transition metal oxides such as NiO [1,2], CoO [3], CuxO [4] or TiO2 [5,6] have attracted extensive interest owing to many advantages of low-power, high-speed operation, high on/off ratio and compatibility with complementary metaloxide-semiconductor (CMOS) technologies. Despite its promising properties, the lack of fundamental understanding of resistance switching mechanism in metal oxide thin films has hindered the industrial application of ReRAMs. In the present study, admittance spectroscopy measurement has been performed on NiOx thin films with various oxygen compositions deposited on n-Si substrates. The oxygen composition x was intentionally changed in the wide range from 1.0 to 1.2. Current-voltage (I-V) measurement of Pt/NiOx/Pt structures on p-Si substrates has also been conducted. The correlation between the oxygen composition and these electrical properties is discussed. EXPERIMENT NiO thin films were deposited on n-Si substrates by a reactive radio-frequency (RF) sputtering method using an Ni target of 99.99% (4N) purity. The ratio of O2 flow rate in the Ar + O2 gas mixture was varied in the range from 3% to 10% to change the oxygen composition. The total pressure, substrate temperature and RF power during sputtering were kept at 1.0 Pa, 300ºC and 100W, respectively. The thickness of NiO films was 200 to 300 nm. From the cross-
sectional high-resolution transmission electron microscopy (HRTEM), the deposited films are a mixture of both polycrystalline and amorphous regions. The oxygen composition x of NiOx films was determined as 1.0 to 1.2 (metal deficient p-type semiconductor [7]) by using Rutherford backscattering (RBS) and energy dispersive X-ray spectroscopy (EDX). Pt electrodes with a typically 300 µm diameter were deposited by electron beam evaporation through a metal mask. Admittance sp
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