Appearance of quantum point contact in Pt/NiO/Pt resistive switching cells
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A resistive switching (RS) phenomenon, namely reversible transitions between the low and high resistance states after forming process, is caused by the formation and rupture of a conductive filament. We confirmed that conductive filaments including a quantum point contact (QPC) in Pt/NiO/Pt RS cells were formed by semiforming, the first step of the forming process. In this study, we examine correlation between microscopic structures in NiO layers and forming characteristics in the Pt/NiO/Pt cells. The appearance condition of the quantized conductance is considered to be associated with the composition ratio of O to Ni of either equivalent to or larger than a critical value. Furthermore, we proposed a RS model based on the forming characteristics especially obtained from the RS cells with different size. Defects which act as the source of a conductive filament including a QPC by semiforming may be randomly distributed in a NiO layer according to Poisson statistics.
I. INTRODUCTION
A Resistive Random Access Memory (ReRAM) possesses a variety of great potential as a next-generation nonvolatile memory.1–4 A key phenomenon behind ReRAMs is a resistive switching (RS). RS mechanism in ReRAM especially based on binary transition metal oxide (TMO) cells has been explained by the formation and rupture of conductive filaments. This reversible resistance transition by applying repeated electrical stress between the low-resistance state (LRS) and the highresistance state (HRS) can occur after an initial operation “forming” process to an initial cell to create conductive filaments in TMO layers between top and bottom metal electrodes. The conductive filaments are believed to be composed of oxygen vacancies3,5,6 or cation interstitials,7–9 which result in the change of microscopic oxygen stoichiometry and the associated formation of point defects in the TMO layers. The oxygen vacancies or cation interstitials typically act as mobile donors because the migration of a negatively charged oxygen ion from the oxygen lattice site creates a positive charge according to charge neutrality condition. In particular, RS memories based on the migration of nanoscale ions and a redox reaction are named as valence change memories (VCMs) after this valence change.6 The voltage polarities between the transition from HRS to LRS (Set) and that from LRS to HRS (Reset) are opposite in VCMs. In other specific Contributing Editor: Ian M. Reaney a) Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2017.254
TMO-based RS cells, RS behaviors are, however, not dominated by the migration of oxygen ions but by thermally controlled diffusion and a redox reaction. These RS memories are categorized into thermochemical memories (TCMs).10 TCMs exhibit uni-polar RS behaviors which means that the voltage polarities between Set and Reset are the same, as opposed to bi-polar RS behaviors in VCMs. On the other hand, RS memories consisting of both one active (oxidizable) electr
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