We observed two types of unipolar resistance switching (RS) in NiO film: memory RS at low temperature and threshold RS at high temperature. We explain these phenomena using a bond percolation model that describes the forming and rupturing of conducting filaments. Assuming Joule heating and thermal dissipation processes in the bonds, we explain how both RS types could occur and be controlled by temperature. We show that these unipolar RS are closely related and can be explained by a simple unified percolation picture.

We fabricated Pt/NiO/Pt capacitor structures with various bottom electrode thicknesses, t BE , and investigated their resistance switching behaviors. The capacitors with t BE ≥ 50 nm exhibited typical unipolar resistance memory switching, while those with t BE ≤ 30 nm showed threshold switching. This interesting phenomenon can be explained in terms of the temperature-dependent stability of conducting filaments. In particular, the thinner t BE makes dissipation of Joule heat less efficient, so the filaments will be at a higher temperature and become less stable. This study demonstrates the importance of heat dissipation in resistance random access memory.Resistance switching phenomena, observed in numerous materials, 1,2 have regained a great deal of attention recently due to their potential application in nonvolatile memory devices called resistance random access memory (RRAM). 3-9 Achieving good scalability is an important issue for meeting the current demands for device miniaturization. 10-12 One of the important scalability issues is reducing the bottom electrode thickness, t BE , to make the etching process easier. 13 Reducing t BE also lowers the device fabrication cost, especially when using expensive electrode materials such as Pt or other noble metals.Many binary transition oxides such as NiO, TiO 2 , and Fe 2 O 3 exhibit unipolar resistance switching. 5-8 Although the mechanism is still somewhat ambiguous, it is widely accepted that unipolar resistance switching is due to the formation and rupture of conducting filamentary paths under external bias. 1,6,[14][15][16][17][18][19] It is also generally assumed that the rupturing process of the conducting filaments may be closely related to Joule heating. [17][18][19] If this is true, we could control Joule heating effects in unipolar resistance switching by changing the thermal properties of the RRAM device structure, which is typically made with metal/oxide/metal. In particular, t BE could significantly affect the thermal heat dissipation process through the bottom electrode.In this letter, we investigated resistance switching behaviors of Pt/NiO/Pt capacitor structures as a function of t BE . We found that capacitors with t BE ≤ 30 nm exhibited volatile resistance switching behaviors called threshold switching. On the other hand, all capacitors with t BE ≥ 50 nm exhibited typical unipolar memory switching behaviors. We explained this interesting t BE dependence in terms of the thermal stability of conducting filamentary paths, which are closely related to heat dissipation through the bottom electrode. This result indicates that thermal heat dissipation through the electrodes is crucial for RRAM, just as it is for phase change random access memory. 20

We report scanning tunneling microscopy of semiconductor-semiconductor carbon nanotube junctions with different band gaps. Characteristic features of the wave functions at different energy levels, such as a localized defect state, are clearly exhibited in the atomically resolved scanning tunneling spectroscopy. The peaks of the Van Hove singularity on each side penetrate and decay into the opposite side across the junction over a distance of 2 nm. These experimental features are accounted for, with the help of tight-binding calculation, by the presence of pentagon-heptagon pair defects at the junction.

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We observed reversible-type changes between bipolar (BRS) and unipolar resistance switching (URS) in one Pt/SrTiOx/Pt capacitor. To explain both BRS and URS in a unified scheme, we introduce the “interface-modified random circuit breaker network model,” in which the bulk medium is represented by a percolating network of circuit breakers. To consider interface effects in BRS, we introduce circuit breakers to investigate resistance states near the interface. This percolation model explains the reversible-type changes in terms of connectivity changes in the circuit breakers and provides insights into many experimental observations of BRS which are under debate by earlier theoretical models.