Seokjae Lim

Monolithic integration of AgTe/TiO 2 based threshold switching device with TiN liner for steep slope field-effect transistors

AgTe/TiN/TiO2/TiN threshold switching (TS) device was monolithically integrated with silicon MOSFET to demonstrate steep subthreshold slope field-effect transistors. The TS device with AgTe top electrode showed the high on-current, since the Te allows an extraction of the Ag out of the filament. The TiN liner was also inserted at the AgTe/TiO2 interface to prevent in-diffusion of Ag into the TiO2 layer during back-end-of-line process. Finally, the transistor with TS device has a sub-5-mV/dec subthreshold slope (SS) and a high on/off current ratio (Ion/Ioff) of >108 with a low drain voltage (0.5 V) even after the 400°C annealing process.

Improved reset breakdown strength in a HfOx-based resistive memory by introducing RuOx oxygen diffusion barrier

We investigated the reset breakdown phenomenon of HfOx-based resistive memory for reliable switching operation in a fully CMOS compatible stack. Through the understanding on the effect of electrode materials and device area, our findings show that observed failure is attributed to additional oxygen vacancies close to the electrode interface, where switching is occurred. Therefore, RuOx serving as an oxygen diffusion barrier was introduced to suppress the generation of unwanted oxygen vacancies by preventing out-diffusion of oxygen through the electrode. As a result, significantly enhanced breakdown strength in HfOx/RuOx stack is achieved and resulting in improved cycle endurance with larger on/off ratio.

Effects of Liner Thickness on the Reliability of AgTe/TiO 2 -Based Threshold Switching Devices

The effects of liner thickness on the reliability of AgTe/TiO2-based threshold switching (TS) devices were investigated. The off-state current of an AgTe/TiO2/Pt TS device was found to be significantly increased by in-diffusion of Ag into the TiO2 layer during the annealing process. Therefore, 3-, 5- and 7-nm TiN liners were introduced and compared to prevent the in-diffusion of Ag. While the 3-nm TiN liner was shown to be incapable of blocking Ag in-diffusion into the TiO2 layer, the 5- and 7-nm liners effectively suppressed in-diffusion and maintained high off-state resistance. However, the TS device with the 7-nm TiN liner exhibited wide threshold voltage distribution and poor endurance characteristics owing to a lack of Ag sources. The TS device with a 5-nm TiN liner, by contrast, was found to have an adequate amount of Ag sources and to demonstrate thermally stable and electrically reliable characteristics. The effects of TiN liner on Ag diffusion were also directly confirmed using energy dispersive spectrometry line profiles, transmission electron microscopy imaging, and mapping analyses.

Field-induced nucleation in threshold switching characteristics of electrochemical metallization devices

In this research, we investigate electrically driven threshold switching (TS) characteristics in electrochemical metallization cells by adopting the field-induced nucleation theory. For this aim, Ag/HfO2 and Ag/TiO2 based TS devices are prepared and examined. First, we carry out the field driven turn-on process to form Ag filaments created as a consequence of sequential nucleation of Ag ions from the bottom electrode. During the filament formation process, it is observed that the prepared devices show switching time exponential in voltage and temperature with different nucleation barrier energies (W0), which confirms the field-induced nucleation theory. Furthermore, we find that the device with higher W0 shows faster dissolution speed. This implies that the slow turn-off speed of the TS device can be improved by finding a material system with a higher W0 value.

Retention modeling for ultra-thin density of Cu-based conductive bridge random access memory (CBRAM)

We investigate the effect of Cu concentration On-state resistance retention characteristics of W/Cu/Ti/HfO2/Pt memory cell. The development of RRAM device for application depends on the understanding of the failure mechanism and the key parameters for device optimization. In this study, we develop analytical expression for cations (Cu+) diffusion model using Gaussian distribution for detailed analysis of data retention time at high temperature. It is found that the improvement of data retention time depends not only on the conductive filament (CF) size but also on Cu atoms concentration density in the CF. Based on the simulation result, better data retention time is observed for electron wave function associated with Cu+ overlap and an extended state formation. This can be verified by analytical calculation of Cu atom defects inside the filament, based on Cu+ diffusion model. The importance of Cu diffusion for the device reliability and the corresponding local temperature of the filament were analyzed by ...

Effect of Conductance Linearity and Multi-level Cell Characteristics of TaO x -based Synapse Device on Pattern Recognition Accuracy of Neuromorphic System

To improve the classification accuracy of an image data set (CIFAR-10) by using analog input voltage, synapse devices with excellent conductance linearity (CL) and multi-level cell (MLC) characteristics are required. We analyze the CL and MLC characteristics of TaOx-based filamentary resistive random access memory (RRAM) to implement the synapse device in neural network hardware. Our findings show that the number of oxygen vacancies in the filament constriction region of the RRAM directly controls the CL and MLC characteristics. By adopting a Ta electrode (instead of Ti) and the hot-forming step, we could form a dense conductive filament. As a result, a wide range of conductance levels with CL is achieved and significantly improved image classification accuracy is confirmed.

Controllable quantized conductance for multilevel data storage applications using conductive bridge random access memory

In this paper, we investigate the quantized conduction behavior of conductive bridge random access memory (CBRAM) with varied materials and ramping rates. We report stable and reproducible quantized conductance states with integer multiples of fundamental conductance obtained by optimizing the voltage ramping rate and the Ti-diffusion barrier (DB) at the Cu/HfO2 interface. Owing to controlled diffusion of Cu ions by the Ti-DB and the optimized ramping rate, through which it was possible to control the time delay of Cu ion reduction, more than seven levels of discrete conductance states were clearly observed. Analytical modeling was performed to determine the rate-limiting step in filament growth based on an electrochemical redox reaction. Our understanding of the fundamental mechanisms of quantized conductance behaviors provide a promising future for the multi-bit CBRAM device.

CMOS compatible low-power volatile atomic switch for steep-slope FET devices

In this paper, we demonstrate a volatile atomic switch that can be utilized for obtaining steep subthreshold swing (SS) ( 60 mV/dec). The result shows an improvement in the SS, which results from the transition of the atomic switch between the ON and OFF states, which is caused by the formation and rupture of a conductive filament. As a result, excellent switching characteristics are obtained for the FETs, such as low IOFF (∼10−5 μA/μm), high ION/IOFF ratio (∼105), low VDD (∼0.25 V), and steep SS ( 60 mV/dec). The result shows an improvement in the SS, which results from the transition of the atomic switch between the ON and OFF states, which is caused by the formation and rupture of a conductive filament. As a result, excellent switching characteristics are obtained for the FETs, such as low IOFF (∼10−5 μA/μm), high ION/IOFF ratio (∼105), low VDD (∼0.25 V), and steep SS (<5 mV/dec).

Effect of nitrogen-doped GST buffer layer on switching characteristics of conductive-bridging RAM

We demonstrate the device characteristics of W/Cu/N-GST/Al2O3/Pt conductive-bridging RAM, focusing on the nitrogen-doped Ge2Sb2Te5 buffer layer to realize non-volatile memory applications. The on/off ratio of typical Cu/Al2O3-based CBRAM was improved from 102 to 105 with the N-GST buffer layer. The switching uniformity also improved compared to that of a non-buffer layer device. The improved properties that were realized are attributed to the effects of buffer layer such as controlled Cu-ion injection, internal resistor, and Joule heating confinement during the reset process. Furthermore, to verify the effect of nitrogen on the switching properties, we compared the GST and N-GST buffer layers. We believe that doped nitrogen helped to control Cu-ion injection into the resistive switching layer, and to confine the joule heating during the reset process, enabling a high on/off ratio and improved switching uniformity.

Effect of TiO x -based tunnel barrier on non-linearity and switching reliability of resistive random access memory

In this paper, the effect of the titanium oxide-based tunnel barrier on the non-linearity and switching uniformity of resistive random access memory has been investigated with the object of achieving excellent device non-linearity and reliability for cross-point array applications. To form the tunnel barrier of titanium oxide, its thickness was engineered using the deposition time. The tunnel barrier effectively controls the current flow in the devices with a tunneling mechanism that modifies the tunnel barrier thickness for non-linearity and switching reliability of devices. The tunnel barrier controls the current behavior of the device because most of the bias is applied to the tunnel barrier owing to its dominant resistance state. In addition, the tunnel barrier can exhibit uniform resistive switching during the set operation with the controlled current flow.