Memristive devices are electrical resistance switches that can retain a state of internal resistance based on the history of applied voltage and current. These devices can store and process information, and offer several key performance characteristics that exceed conventional integrated circuit technology. An important class of memristive devices are two-terminal resistance switches based on ionic motion, which are built from a simple conductor/insulator/conductor thin-film stack. These devices were originally conceived in the late 1960s and recent progress has led to fast, low-energy, high-endurance devices that can be scaled down to less than 10 nm and stacked in three dimensions. However, the underlying device mechanisms remain unclear, which is a significant barrier to their widespread application. Here, we review recent progress in the development and understanding of memristive devices. We also examine the performance requirements for computing with memristive devices and detail how the outstanding challenges could be met.

Memristive devices for computing

https://cyberleninka.org/article/n/247805.pdf

A brief review of atomic layer deposition: from fundamentals to applications

Atomic layer deposition (ALD) is gaining attention as a thin film deposition method, uniquely suitable for depositing uniform and conformal films on complex three-dimensional topographies. The deposition of a film of a given material by ALD relies on the successive, separated, and self-terminating gas–solid reactions of typically two gaseous reactants. Hundreds of ALD chemistries have been found for depositing a variety of materials during the past decades, mostly for inorganic materials but lately also for organic and inorganic–organic hybrid compounds. One factor that often dictates the properties of ALD films in actual applications is the crystallinity of the grown film: Is the material amorphous or, if it is crystalline, which phase(s) is (are) present. In this thematic review, we first describe the basics of ALD, summarize the two-reactant ALD processes to grow inorganic materials developed to-date, updating the information of an earlier review on ALD [R. L. Puurunen, J. Appl. Phys. 97, 121301 (2005)], and give an overview of the status of processing ternary compounds by ALD. We then proceed to analyze the published experimental data for information on the crystallinity and phase of inorganic materials deposited by ALD from different reactants at different temperatures. The data are collected for films in their as-deposited state and tabulated for easy reference. Case studies are presented to illustrate the effect of different process parameters on crystallinity for representative materials: aluminium oxide, zirconium oxide, zinc oxide, titanium nitride, zinc zulfide, and ruthenium. Finally, we discuss the general trends in the development of film crystallinity as function of ALD process parameters. The authors hope that this review will help newcomers to ALD to familiarize themselves with the complex world of crystalline ALD films and, at the same time, serve for the expert as a handbook-type reference source on ALD processes and film crystallinity.

Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends

Two-dimensional (2D) nanosheets, which possess atomic or molecular thickness and infinite planar lengths, are regarded as the thinnest functional nanomaterials. The recent development of methods for manipulating graphene (carbon nanosheet) has provided new possibilities and applications for 2D systems; many amazing functionalities such as high electron mobility and quantum Hall effects have been discovered. However, graphene is a conductor, and electronic technology also requires insulators, which are essential for many devices such as memories, capacitors, and gate dielectrics. Along with graphene, inorganic nanosheets have thus increasingly attracted fundamental research interest because they have the potential to be used as dielectric alternatives in next-generation nanoelectronics. Here, we review the progress made in the properties of dielectric nanosheets, highlighting emerging functionalities in electronic applications. We also present a perspective on the advantages offered by this class of materials for future nanoelectronics.

Two‐Dimensional Dielectric Nanosheets: Novel Nanoelectronics From Nanocrystal Building Blocks

The resistance switching behaviour of several materials has recently attracted considerable attention for its application in non-volatile memory (NVM) devices, popularly described as resistive random access memories (RRAMs). RRAM is a type of NVM that uses a material(s) that changes the resistance when a voltage is applied. Resistive switching phenomena have been observed in many oxides: (i) binary transition metal oxides (TMOs), e.g. TiO(2), Cr(2)O(3), FeO(x) and NiO; (ii) perovskite-type complex TMOs that are variously functional, paraelectric, ferroelectric, multiferroic and magnetic, e.g. (Ba,Sr)TiO(3), Pb(Zr(x) Ti(1-x))O(3), BiFeO(3) and Pr(x)Ca(1-x)MnO(3); (iii) large band gap high-k dielectrics, e.g. Al(2)O(3) and Gd(2)O(3); (iv) graphene oxides. In the non-oxide category, higher chalcogenides are front runners, e.g. In(2)Se(3) and In(2)Te(3). Hence, the number of materials showing this technologically interesting behaviour for information storage is enormous. Resistive switching in these materials can form the basis for the next generation of NVM, i.e. RRAM, when current semiconductor memory technology reaches its limit in terms of density. RRAMs may be the high-density and low-cost NVMs of the future. A review on this topic is of importance to focus concentration on the most promising materials to accelerate application into the semiconductor industry. This review is a small effort to realize the ambitious goal of RRAMs. Its basic focus is on resistive switching in various materials with particular emphasis on binary TMOs. It also addresses the current understanding of resistive switching behaviour. Moreover, a brief comparison between RRAMs and memristors is included. The review ends with the current status of RRAMs in terms of stability, scalability and switching speed, which are three important aspects of integration onto semiconductors.

https://iopscience.iop.org/article/10.1088/0034-4885/75/7/076502/pdf

Emerging memories: resistive switching mechanisms and current status.

Al‐Doped TiO2 Films with Ultralow Leakage Currents for Next Generation DRAM Capacitors

The recent progress in the metal-insulator-metal (MIM) capacitor technology is reviewed in terms of the materials and processes mostly for dynamic random access memory (DRAM) applications. As TiN/ZrO 2 -Al 2 O 3 -ZrO 2 /TiN (ZAZ) type DRAM capacitors approach their technical limits, there has been renewed interest in the perovskite SrTiO 3 , which has a dielectric constant of > 100, even at a thickness ∼ 10 nm. However, there are many technical challenges to overcome before this type of MIM capacitor can be used in mass-production compatible processes despite the large advancements in atomic layer deposition (ALD) technology over the past decade. In the mean time, rutile structure TiO 2 and Al-doped TiO 2 fi lms might fi nd space to fi the gap between ZAZ and SrTiO 3 MIM capacitors due to their exceptionally high dielectric constant among binary oxides. Achieving a uniform and dense rutile structure is the key technology for the TiO 2 -based dielectrics, which depends on having a dense, uniform and smooth RuO 2 layer as bottom electrode. Although the Ru (and RuO 2 ) layers grown by ALD using metal-organic precursors are promising, recent technological breakthroughs using the RuO 4 precursor made a thin, uniform, and denser Ru and RuO 2 layer on a TiN electrode. A minimum equivalent oxide thickness as small as 0.45 nm with a low enough leakage current was confi rmed, even in laboratory scale experiments. The bulk dielectric constant of ALD SrTiO 3 fi lms, grown at 370 ° C, was ∼ 150 even with thicknesses ≤15 nm. The recent development of novel group II precursors made it possible to increase the growth rate largely while leaving the electrical properties of the ALD SrTiO 3 fi lm intact. This is an important advancement toward the commercial applications of these MIM capacitors to DRAM as well as to other fi elds, where an extremely high capacitor density and three-dimensional structures are necessary.

Capacitors with an Equivalent Oxide Thickness of <0.5 nm for Nanoscale Electronic Semiconductor Memory

Limiting the location where electron injection occurs at the cathode interface to a narrower region is the key factor for achieving a highly improved RS performance, which can be achieved by including Ru Nanodots. The development of a memory cell structure truly at the nanoscale with such a limiting factor for the electric-field distribution can solve the non-uniformity issue of future ReRAM.

Highly improved uniformity in the resistive switching parameters of TiO2 thin films by inserting Ru nanodots.

SrTiO3 (STO) thin films were deposited at 370°C by atomic layer deposition using H2O as the oxidant, and Ti(O–iPr)2(thd)2 and Sr(thd)2 as Ti, and Sr precursors, respectively. Denser STO films were produced at this deposition temperature. The saturated growth rate was 0.15A∕cycle. The adoption of a thin crystallized seed layer resulted in crystallized perovskite STO films at the as-deposited state without higher temperature post-annealing. A tox of 0.72nm (dielectric constant of 108) and a low leakage current density (∼10−7A∕cm2 at 0.8V) were obtained from a planar capacitor structure consisting of Pt∕20-nm-thick STO/Ru (bottom).

Enhanced electrical properties of SrTiO3 thin films grown by atomic layer deposition at high temperature for dynamic random access memory applications

The ever-shrinking dimensions of dynamic random access memory (DRAM) require a high quality dielectric film for capacitors with a sufficiently high growth-per-cycle (GPC) by atomic layer deposition (ALD). SrTiO3 (STO) films are considered to be the appropriate dielectric films for DRAMs with the design rule of ∼20 nm, and previous studies showed that STO films grown by ALD have promising electrical performance. However, the ALD of STO films still suffers from much too slow GPC to be used in mass-production. Here, we accomplished a mass-production compatible ALD process of STO films using Ti(O-iPr)2(tmhd)2 as a Ti-precursor for TiO2 layers and Sr(iPr3Cp)2 as a Sr-precursor for SrO layers. O3 and H2O were used as the oxygen sources for the TiO2 and SrO layers, respectively. A highly improved GPC of 0.107 nm/unit-cycle (0.428 nm/supercycle) for stoichiometric STO films was obtained at a deposition temperature of 370 °C, which is ∼7 times higher than previously reported. The origin of such high GPC values in ...

Jeong Hwan Han

Papers

Al‐Doped TiO2 Films with Ultralow Leakage Currents for Next Generation DRAM Capacitors

Capacitors with an Equivalent Oxide Thickness of <0.5 nm for Nanoscale Electronic Semiconductor Memory

Highly improved uniformity in the resistive switching parameters of TiO2 thin films by inserting Ru nanodots.

Enhanced electrical properties of SrTiO3 thin films grown by atomic layer deposition at high temperature for dynamic random access memory applications

Atomic Layer Deposition of SrTiO3 Thin Films with Highly Enhanced Growth Rate for Ultrahigh Density Capacitors