What will come after V-NAND – Vertical resistive switching memory?

The NAND flash memory serves as the key enabler of the flourishing of portable handheld information devices, such as the cellular phone. The recent upsurge in the sales of vertical NAND flash memory (V-NAND) entails a further increase in the available information capacity at the edge devices and the servers with higher performance and lower power consumption compared with the magnetic hard-disc drives. Nonetheless, there will certainly be an upper limit for the number of stacked layers, which will be the point at which further memory density increase will stop. While V-NAND is a supreme outcome of semiconductor memory technologies, it still relies on conventional Si-based materials. The newly explored memory materials and concepts, such as the resistance-based memories, can, therefore, be an appealing contender to or successor of V-NAND. In this talk, the current state of V-NAND is first briefly looked into, and then the eventual limitation of memory density increase and performance boost are discussed. Most importantly, the possible strategies of integrating the resistance-based memories into the vertical architecture are then discussed. Among them, V-NAND-like vertical resistance-switching random access memory will be focused. The figure below shows the schematic diagram for this type of vertical device (a), and its equivalent circuit diagram (b). For this application, higher performance of channel material, other than the current amorphous-like Si, is necessary. For such purpose, the programming characteristics of charge trap flash memory device adopting amorphous In2Ga2ZnO7 (a-IGZO) oxide semiconductors as channel layer were evaluated, where the a-IGZO thin film was grown by either metal-organic chemical vapor deposition (MOCVD) or RF-sputtering processes. The MOCVD film showed superior performance to the sputtered film, perhaps due to the involvement of the appropriate level of hydrogen. Please click Additional Files below to see the full abstract

Invited; An overview of the three-dimensionally stacked dynamic random access memory

As the DRAM scaling proceeds, challenges related to device integration are becoming overwhelming. The challenges include securing the cell transistor performance, cell capacitance, and low wire resistance. The current two-dimensional integration will meet a significant barrier for further device integration near the design rule of ~ 10nm, which will happen within the next ten years. The die-stacking technology, making the highbandwidth memory, is an alternative approach but lacks cost-competitiveness. Therefore, cell-stacking technology will be needed for the higher-density DRAM fabrication up to tera-bit, which has been implemented in the vertical-NAND flash. However, DRAM requires a much higher cell transistor performance than the NAND flash, where the polycrystalline Si could meet cell transistor requirements. Please click Download on the upper right corner to see the full abstract

Oxide semiconductor based charge trap device for vertically integrated NAND flash memory

Vertically integrated NAND flash memory (V-NAND) is the data storage component of modern hand-held electronic devices, which will also critically contribute to the futuristic devices for internet of things. The present V-NAND adopts thin poorly crystallized Si as the channel layer to minimize the cell-to-cell and device-to-device variability. However, the very low carrier (electron) mobility of such channel (only ~0.03 cm2/V·sec) deteriorates the device performance (write and read speed), which will eventually limit the maximum stackable number of device layers even if the various process-related issues for V-NAND fabrication are solved. An alternative channel material with amorphous structure and higher carrier mobility, therefore, is necessary for further development of V-NAND, and amorphous oxide semiconductor (AOS), such as In2Ga2ZnO7 (IGZO) or ZnSnO3 (ZTO), is an appealing contender for such application. Figure 1 shows a schematic diagram (left panel) and achieved memory performance (middle and right panels) of a double-layer stacked integrated charge trap flash (CTF) where the ZTO channel was grown by metal-organic chemical vapor deposition (MOCVD), and other Sicontaining materials were grown by standard Si processes. Both top and bottom CTF devices showed feasible memory performances in the drain current – gate voltage sweep mode with sufficiently high memory window, which were also stable at 85oC guaranteeing the 10-year-retention time. However, the program time, estimated in pulse program/erase mode, was impractically long (order of sec.) suggesting that there must be significant improvements in material stack or process conditions. The relatively thick tunneling oxide (4nm SiO2) may seriously limit the electron tunneling, but thinner SiO2 could not guarantee sufficient retention time. In the presentation, the material and integration strategies to improve such problem will be discussed. These strategies include AOS material variation (IGZO vs. ZTO), process method (sputtering vs. MOCVD), and integration schemes (tunneling oxide thickness and thermal treatments), of which details are dependent on the AOS material and process methods Please click Additional Files below to see the full abstract

Broad Phase Transition of Fluorite-Structured Ferroelectrics for Large Electrocaloric Effect

Field-induced ferroelectricity in (doped) hafnia and zirconia has attracted increasing interest in energy-related applications, including energy harvesting and solid-state cooling. It shows a larger isothermal entropy change in a much wider temperature range compared with those of other promising candidates. The field-induced phase transition occurs in an extremely wide temperature range, which contributes to the giant electrocaloric effect. This article examines the possible origins of a large isothermal entropy change, which can be related to the extremely broad phase transitions in fluorite-structured ferroelectrics. While the materials possess a high entropy change associated with the polar–nonpolar phase transition, which can contribute to the high energy performance, the higher breakdown field compared with perovskites practically determines the available temperature range

Interfacial chemical bonding-mediated ionic resistive switching.

In this paper, we present a unique resistive switching (RS) mechanism study of Pt/TiO2/Pt cell, one of the most widely studied RS system, by focusing on the role of interfacial bonding at the active TiO2-Pt interface, as opposed to a physico-chemical change within the RS film. This study was enabled by the use of a non-conventional scanning probe-based setup. The nanoscale cell is formed by bringing a Pt/TiO2-coated atomic force microscope tip into contact with a flat substrate coated with Pt. The study reveals that electrical resistance and interfacial bonding status are highly coupled together. An oxygen-mediated chemical bonding at the active interface between TiO2 and Pt is a necessary condition for a non-polar low-resistance state, and a reset switching process disconnects the chemical bonding. Bipolar switching mode did not involve the chemical bonding. The nature of chemical bonding at the TiO2-metal interface is further studied by density functional theory calculations

Review and perspective on ferroelectric HfO₂-based thin films for memory applications

The ferroelectricity in fluorite-structure oxides such as hafnia and zirconia has attracted increasing interest since 2011. They have various advantages such as Si-based complementary metal oxide semiconductor-compatibility, matured deposition techniques, a low dielectric constant and the resulting decreased depolarization field, and stronger resistance to hydrogen annealing. However, the wake-up effect, imprint, and insufficient endurance are remaining reliability issues. Therefore, this paper reviews two major aspects: the advantages of fluorite-structure ferroelectrics for memory applications are reviewed from a material’s point of view, and the critical issues of wake-up effect and insufficient endurance are examined, and potential solutions are subsequently discussed

Unusual transport characteristics of nitrogen-doped single-walled carbon nanotubes

Electrical transport characteristics of nitrogen-doped single-walled carbon nanotubes (N-SWCNTs), in which the nitrogen dopant is believed to form a pyridinelike bonding configuration, are studied with the field effect transistor operations. Contrary to the expectation that the nitrogen atoms may induce a n -type doping, the electrical transports through our N-SWCNTs are either ambipolar in vacuum or p -type in air. Through the first-principles electronic structure calculations, we show that the nitrogen dopant indeed favors the pyridinelike configuration and the Fermi level of the pyridinelike N-SWCNT is almost at the intrinsic level.open01

Prognostic Determinants in Patients with Traumatic Pancreatic Injuries

The aim of this study was to identify factors that predict morbidity and mortality in patients with traumatic pancreatic injuries. A retrospective review was performed on 75 consecutive patients with traumatic pancreatic injuries admitted to the Emergency Medical Center at Masan Samsung Hospital and subsequently underwent laparotomy during the period January 2000 to December 2005. Overall mortality and morbidity rates were 13.3% and 49.3%, respectively. A multivariate regression analysis revealed that greater than 12 blood transfusions and an initial base deficit of less than -11 mM/L were the most important predictors of mortality (p<0.05). On the other hand, the most important predictors of morbidity were surgical complexity and an initial base deficit of less than -5.8 mM/L (p<0.01). These data suggests that early efforts to prevent shock and rapidly control of bleeding are most likely to improve the outcome in patients with traumatic pancreatic injuries. The severity of pancreatic injury per se influenced only morbidity