Observing the Electrical Switching Mechanism in ZnO-Based ReRAM Devices

Resistive random access memory (ReRAM) is an advanced, non-volatile memory technology. It is a front-runner to replace existing non-volatile memory technologies such as flash. According to researchers, ReRAM is capable of less than 10 ns switching time, is faster, and more energy efficient compared to traditional non-volatile memories.

This technology can be scaled down to dimensions smaller than 30 nm, making it more compatible with future semiconductor processing nodes. Also, ReRAM can be integrated into three-dimensional device structures, which significantly increases its versatility and density.

The basic structure of a ReRAM device consists of a thin semiconductor or insulator layer sandwiched between two metal layers. This basic structure is generally known as a metal-insulator-metal or MIM device. Scientists have investigated different types of insulator materials, and discovered that specific oxides work well for ReRAM devices, including NiO, TiO2, TaO2, ZnO, and SiOx

Modern electronic systems rely on devices that work in two different states - “on” and “off”. The on-off functionality forms the basis of binary code, which is comprised of “1”s (on) and “0”s (off), and is the language of electronics. ReRAM devices also work in two distinct on and off states. They achieve this on-off functionality from the insulating layer, which functions as a variable resistor.

When the ReRAM device is turned on, a low resistance pathway is formed allowing electrical current to easily pass between the metal electrodes. When it is turned off, the insulator has a large resistance, which hinders the flow of electrical current.

Figure 1.

Scientists have postulated that the “turn on” mechanism is caused by the formation of metallic filament, where electrical current can easily flow.

The Protochips Fusion heating and electrical biasing system is ideal for analyzing electrical devices through in situ electron microscopy. The custom Fusion electrical biasing software can be used to easily obtain voltage and current measurements on devices.

Users can simultaneously image a sample and compare the electrical measurements with SEM and TEM observations, such as diffraction, structural changes, energy dispersive x-ray (EDS) spectra, and chemical changes in electron energy loss (EELS).

Experiment

In the experiments discussed in this article, scientists from National Chiao Tung University in Hsinchu, Taiwan developed a thin sample from a ReRAM device, which was composed of a ZnO layer sandwiched between two Pt electrodes. The ZnO layer was 100 nm thick and deposited through RF magnetron sputtering.

A focused ion beam (FIB) system was used to cut out a small section and thin it to ~50 nm, in order to create a sample suitable for TEM . FIB induced metal deposition was used to make electrical connections from the metal leads on an E-chip to the device.

Once the device was inserted into the TEM, the scientists used the electrical biasing tools integrated into the system to apply a voltage to the device and at the same time measured the current in situ. They simultaneously observed the device behavior in real-time using JEOL 2100F TEM that operates in bright-field mode, and finally examined the ZnO structure changes using diffraction, dark-field imaging, EDS, and EELS.

Discussion

The scientists imaged the formation of Zn filament in several areas of a device, and the behavior that was observed corresponded well with the earlier reports. In addition, given the ability to directly observe the behavior of the Zn filament in real time and control the electrical stimuli, the team was able to elucidate the switching mechanism more clearly and ultimately proposed a model to describe the behavior.

The researchers also described the chemical and physical behavior of filament formation, by combining diffraction data, information from high-resolution images, and EELS spectra to support their proposed model. A redox process results in the formation of Zn filament, where oxygen atoms migrate, leaving oxygen depleted regions of Zn and ZnO1-x.

This observation is supported by previous reports, which demonstrated that when compared to Zn, oxygen species are more mobile in an electric field.

The schematic above visually describes the formation of filament and the redox process. When the filament forms, it often starts with a conical shape and changes to a dendritic shape (Figure 2). This is the outcome of the electric field enhancement at the tip of the cone, causing the filament to branch out. When an appropriate voltage is applied, it is possible to reset the device to its original state and the process can be observed multiple times.

Applications

With feature sizes of electronic devices becoming increasingly smaller, the TEM becomes a more powerful and useful tool to investigate the behavior and operation of these devices, as it is capable of resolving features down to the atomic scale. The resolving power, combined with the in situ heating and electrical biasing capabilities of the Fusion system, make new and existing TEMs a more valuable analysis tool.

This information has been sourced, reviewed and adapted from materials provided by Protochips.

For more information on this source, please visit Protochips.

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