John R. Jameson

Electrochemical metallization memories—fundamentals, applications, prospects

This review focuses on electrochemical metallization memory cells (ECM), highlighting their advantages as the next generation memories. In a brief introduction, the basic switching mechanism of ECM cells is described and the historical development is sketched. In a second part, the full spectra of materials and material combinations used for memory device prototypes and for dedicated studies are presented. In a third part, the specific thermodynamics and kinetics of nanosized electrochemical cells are described. The overlapping of the space charge layers is found to be most relevant for the cell properties at rest. The major factors determining the functionality of the ECM cells are the electrode reaction and the transport kinetics. Depending on electrode and/or electrolyte material electron transfer, electro-crystallization or slow diffusion under strong electric fields can be rate determining. In the fourth part, the major device characteristics of ECM cells are explained. Emphasis is placed on switching speed, forming and SET/RESET voltage, R(ON) to R(OFF) ratio, endurance and retention, and scaling potentials. In the last part, circuit design aspects of ECM arrays are discussed, including the pros and cons of active and passive arrays. In the case of passive arrays, the fundamental sneak path problem is described and as well as a possible solution by two anti-serial (complementary) interconnected resistive switches per cell. Furthermore, the prospects of ECM with regard to further scalability and the ability for multi-bit data storage are addressed.

Bipolar resistive switching in polycrystalline TiO2 films

Bipolar resistive switching was found in thin polycrystalline TiO2 films formed by the thermal oxidation of sputtered Ti films. With a Ag top electrode, TiO2 film, and Pt bottom electrode, bistable resistive switching with a low operating voltage and a good uniformity was observed repeatedly without an initial electrical “forming” process. This switching phenomenon might be described as the formation and rupture of a filamentary conductive path consisting of a chain of Ag atoms. The temperature dependence of the switching voltage is discussed in terms of interstitial ionic diffusion of Ag in the TiO2 matrix.

Field-programmable rectification in rutile TiO2 crystals

The authors report “field-programmable rectification” in crystals of rutile TiO2. A “programming” voltage is applied between two Pt electrodes on the surface of a crystal. Afterwards, current can pass in the direction of the programming voltage, but not in the reverse direction. The polarity of the rectification can be reversed by applying a programming voltage of opposite sign. The effect was observed on the (110) and (100) surfaces, but not the (001) surface. The proposed mechanism is field-induced motion of oxygen vacancies, which pile up under the negative terminal, eliminating a Schottky barrier, but leaving one at the positive terminal intact.

Quantized Conductance in

Ag/GeS2/W conductive-bridge random access memory (CBRAM) cells are shown to program at room temperature to conductance levels near multiples of the fundamental conductance G0 = 2e2/h. This behavior is not accounted for in the traditional view that the conductance of a CBRAM cell is a continuous variable proportional to the maximum current allowed to flow during programming. For on -state resistances on the order of 1/G0 = 12.9 kΩ or less, quantization implies that the Ag “conductive bridge” typically contains a constriction, or even an extended chain, that can be as narrow as a single atom. Implications for device modeling and commercial applications are discussed.

\hbox{Ag/GeS}_{2}/\hbox{W}

A one-dimensional model of filament growth in conductive-bridge memory cells is presented, in which ions are thermally excited from the anode surface into the electrolyte, pulled by the electric field through a periodic series of wells and reduced at the cathode to form a metallic filament. The voltage, temperature, and thickness dependencies of the time required to program a cell are calculated, and material parameters for Ag/GeS2/W cells are obtained by comparison to experiment. The relation of the model to recent observations of quantized conductance is highlighted, as is the need for further study of the Ag/GeS2 interface.

Conductive-Bridge Memory Cells

Current-induced breakdown is investigated for carbon nanofibers (CNF) for potential interconnect applications. The measured maximum current density in the suspended CNF is inversely proportional to the nanofiber length and is independent of diameter. This relationship can be described with a heat transport model that takes into account Joule heating and heat diffusion along the CNF, assuming that breakdown occurs when and where the temperature reaches a threshold or critical value.

One-dimensional model of the programming kinetics of conductive-bridge memory cells

High-temperature data retention is a critical hurdle for the commercialization of emerging nonvolatile memories. For Conductive-Bridge RAM (CBRAM) [1], we discuss high-temperature retention in terms of the physics of quantum point contacts, and we report on a family of CBRAM cells that achieve excellent retention at temperatures exceeding 200°C.

Length dependence of current-induced breakdown in carbon nanofiber interconnects

We show that a straightforward account of dielectric relaxation current in glasses follows from a semiclassical treatment of the double-well model [P. W. Anderson, B. I. Halperin, and C. M. Varma, Philos. Mag. 25, 1 (1972) and W. A. Phillips, J. Low Temp. Phys. 7, 351 (1972)] explaining the linear specific heat of glasses at low temperature. The current is obtained from the field-induced tunneling of the glass between the minima of its potential energy surface, and is found to have the experimentally observed linear dependence on field and inverse dependence on time. The effects of temperature and prior biases are briefly discussed, as well as the relation of the model to the theory of charge trapping. No dielectric relaxation is expected in a perfect insulating crystal, raising the important technological question of how perfect high-k dielectrics like HfO2 and ZrO2 must be in order to serve as gate dielectrics in transistors.

Conductive-bridge memory (CBRAM) with excellent high-temperature retention

Charge trapping at the interface between the two dielectric layers of a high-k gate stack is shown to be caused by Maxwell-Wagner instability, which is the following. The fact that the high-k and interfacial layers have different compositions means that they will also have different conductivities. Then, a gate bias will produce a discontinuity in current at their interface, causing charge to accumulate there until, in steady state, the same current flows through both layers. Maxwell-Wagner instability is shown to be coupled to a second instability, dielectric relaxation of the high-k layer; continuity of current in steady state requires that the electric fields in the two dielectric layers remain fixed, so the change in polarization of the high-k layer due to dielectric relaxation must be compensated for by the conduction of additional charge to the interface. Evidence for this behavior in high-k gate stacks is found in the thickness dependence of their dielectric relaxation current, with the correct dependence being obtained only from a model in which the two instabilities act simultaneously. Uniform dielectrics do not exhibit Maxwell-Wagner instability, and perfect crystals do not exhibit dielectric relaxation, making the ideal high-k gate dielectric a uniform single-layer perfect crystal bonded epitaxially to the Si substrate

Double-well model of dielectric relaxation current

We show that the simplest possible circuit model of high-frequency electrical conduction in carbon nanofibers from 0.1 to 50 GHz is a frequency-independent resistor in parallel with a frequency-independent capacitor. The resistance is experimentally determined and represents the total dc resistance of the nanofiber and its contacts with the electrodes. The capacitance is obtained as a free parameter and has not been previously observed. The experimental method utilizes a ground-signal-ground test structure whose two-port scattering parameters (S-parameters) can be described to within plusmn0.5 dB and plusmn2deg using a simple lumped-element circuit model. The nanostructure is placed in the signal path of the test structure, and its equivalent circuit is deduced by determining what additional elements must be added to the test structure circuit model to reproduce the resulting changes in the S-parameters. This methodology is applicable to nanowires and nanotubes.