Researchers X-ray Next Data Memory Innovation

DORIS sheds light on instability of ReRAM memory cells

Memories for data storage are crucial for a variety of modern-day technologies. Widespread consumer electronics including tablet computers and cell phones, for instance, develop rapidly and demand memory that is smaller, faster, and more energy-efficient than ever. Researchers from RWTH Aachen University and Forschungszentrum Jülich in Germany used DESY’s X-ray source DORIS to examine a promising new type of memory called ReRAM, or resistive switching memory. ReRAM is over a thousand times faster than current state-of-the-art memory types and may replace them in the near future.

Switching mechanism in an electrochemical metallization memory (ECM) cell, which is a type of resistive switching memory (ReRAM): Two metal electrodes (blue and gray spheres) are separated by a solid, non-metallic electrolyte (yellow). Under normal conditions, the electrolyte acts as an insulator for the two electrodes and the electrical resistance of the memory cell is large (upper panel, OFF state). When an external voltage is applied, the metal of the active electrode (blue) dissolves into the electrolyte material, forming a nanowire that bridges the two electrodes (lower panel, ON state). The memory cell’s electrical resistance is now small and it can conduct an electric current. High- and low-resistance states of the ECM are the two possible values of a binary digit, or bit. (Credit: Ilia Valov, RWTH Aachen/Forschungszentrum Jülich)

Memory cells must show high levels of endurance as well as store data in a stable and reproducible manner over large periods of time. However, current ReRAM devices become unstable at some point, resulting in a loss of information. In their study, the scientists investigated the instability of ReRAM memory cells and described the underlying mechanisms leading to memory loss in the journal Scientific Reports. These results are an important step towards improving ReRAM materials and developing them into industrial standards.

A thousand times faster than flash memory

Computer memories store data as a sequence of binary digits, or bits, which can each only take one of two values, ON or OFF. Depending on the memory type, these two states are realized differently. A personal computer’s dynamic RAM (random-access memory), for example, stores information in capacitors, which are either charged or discharged. Flash memory, used in solid-state drives, USB flash drives, memory cards, and other devices, stores information in transistors which can be switched between a conducting and a non-conducting state. In ReRAM, a new type of memory currently under development, the electrical resistance of an insulating thin film is altered to conduct an electric current. High- and low-resistance states represent the two values of the stored bit. Unlike dynamic RAM, whose stored data becomes erased when the device is powered off, flash memory and ReRAM are non-volatile, i.e. they “remember” stored information even without external power supply.

“In many ways, ReRAM is better than flash memory,” says scientist Ilia Valov from the Aachen and Jülich institutes. “The speed of ReRAM memory cells is less than ten nanoseconds [ten billionth of a second], which is at least a thousand times faster than flash memory.” In addition to speeding up read/write processes, ReRAM consumes less power and therefore enhances battery life and reduces energy costs in electronics applications.

The researchers studied a type of ReRAM known as electrochemical metallization memory (ECM). ECM memory cells consist of two metal electrodes with a non-metallic electrolyte film between them. Since the electrolyte insulates the two electrodes, the cell has a high electrical resistance and poorly conducts electric currents. However, when a voltage is applied, one of the electrodes, termed the active electrode, partially dissolves into the electrolyte and forms a metallic nanowire that bridges the two electrodes. The cell can now conduct an electric current and has been switched from a high- to a low-resistance state. The switching process is reversible: When the voltage is inverted, the nanowire disappears and the cell returns to its initial state.

Instability of the electrode-electrolyte interface

A little controlled problem of ECM is that the active-electrode material can diffuse into the electrolyte even without an applied voltage. At some point, this diffusion results in a loss of electric contact between the active electrode and the electrolyte. When this happens, the ECM can no longer be switched into its low-resistance state at normal voltages and has lost its functionality. “In our study, we investigated the underlying degradation mechanism at the electrode-electrolyte interface,” Valov says. “We wanted to precisely determine how the electric contact is lost.”

The researchers investigated 50-nanometer thin films of the electrolyte silver iodide, onto which they deposited silver layers of increasing thicknesses as an active electrode material. Using a method called X-ray diffraction, the researchers determined that the inner structure of the silver iodide film becomes increasingly disordered as the silver layer becomes thicker. “This result strongly suggests a substantial dissolution of silver into silver iodide,” explains scientist Deok-Yong Cho from Seoul National University, South Korea, who was a Humboldt postdoctoral fellow at RWTH Aachen University at the time of the study.

Formation of silver atom clusters

However, what happens exactly when silver penetrates the silver iodide? To address this question, the research team X-rayed their samples at DESY's light source DORIS using the X-ray absorption spectroscopy (XAS) technique. XAS allows scientists to “zoom in” on specific atom types and precisely determine their average local structure within a radius of a few tenths of a nanometer. If there were chemical interactions between the dissolved silver and the silver iodide, associated changes in the silver and iodine environments would manifest themselves as characteristic changes in the XAS data.

Surprisingly, the researchers found the silver and iodine environments to be unchanged despite the silver’s diffusion into the electrolyte. “The XAS results showed that there is neither a chemical interaction nor any local structure distortion at the silver/silver iodide interface,” Cho says. “Moreover, theoretical simulations of the XAS data [for the metallic silver environments] reproduce the experimental data only if we consider at least 43 silver atoms in the calculation.” The researchers therefore conclude that the silver penetrates the silver iodide in the form of silver atom clusters, i.e. silver aggregates whose diameters exceed half a nanometer.

Improving ReRAM materials

Unlike silver iodide, other electrolytes have previously been shown to dissolve silver chemically. Germanium sulfide, for instance, has a flexible germanium-to-sulfide ratio and can incorporate silver from the electrode into its unstructured matrix. This alteration changes the electrochemical properties of the electrolyte and may lead to the observed instability in the ECM material. “An important finding of our study is that, even if the silver is not chemically dissolved, it can diffuse [into the electrolyte] as a cluster and cause a loss of electric contact,” Valov points out.

Knowledge of the exact diffusion mechanism at the electrode-electrolyte interface ought to be of great interest for future ECM developments. Specifically designed barrier films, for example, could improve the stability of silver/silver iodide memory cells by preventing silver atom clusters from diffusing into the electrolyte.  

“However, one should keep in mind that there is no clear winner yet when it comes to choosing materials for ReRAM devices,” Cho emphasizes. ”We plan to extend our XAS analyses to other ECM materials and focus on the interfaces, which seem to control the behavior of the systems.”


Cho, D., Tappertzhofen, S., Waser, R., and Valov, I.: "Chemically-inactive interfaces in thin film Ag/AgI systems for resistive switching memories"; Sci. Rep. 3, 1169; DOI:10.1038/srep01169 (2013).