Read an Excerpt
Electrical Memory Materials and Devices
By Wen-Chang Chen
The Royal Society of ChemistryCopyright © 2016 The Royal Society of Chemistry
All rights reserved.
Organic Electronic Memory Devices
BIN ZHANG, YU CHEN, KOON-GEE NEOH, AND EN-TANG KANG
As the performance of digital gadgets for information technology advances, the complexity of data storage devices increases correspondingly. Conventional memory devices are implemented on semiconductor-based integrated circuits, such as transistors and capacitors. In order to achieve greater density of data storage and faster access to information, more components are deliberately packed onto a single chip. The feature size of transistors has decreased from 130 nm in the year 2000 to 32 nm at present. Silicon-based semiconductor devices become less stable below 22 nm, and the reliability to store and read individual bits of information will be substantially reduced by severe "cross-talk" issues. Moreover, power consumption and unwanted heat generation are also of increasing concern, and the fidelity of addressing the memory units diminishes correspondingly. Therefore, the current state-of-the-art memory technologies are no longer capable of fulfilling the requirements for information storage of the near future.
Regarding the aspiration for new data storage technologies, ferroelectric random access memory (FeRAM), magnetoresistive random access memory (MRAM), phase change memory (PCM), and organic/polymer memory have appeared on the scene of the information technology industry. Instead of information storage and retrieval by encoding "0" and "1" as the amount of stored charge in the current silicon-based memory devices, the new technologies are based on electrical bistability of materials arising from changes in certain intrinsic properties, such as magnetism, polarity, phase, conformation and conductivity, in response to the applied electric field. The advantages of organic and polymer electronic memory include good processability, molecular design through chemical synthesis, simplicity of device structure, miniaturized dimensions, good scalability, low-cost potential, low-power operation, multiple state properties, 3D stacking capability and large capacity for data storage.
Extensive studies toward new organic/polymeric materials and device structures have been carried out to demonstrate their unique memory performances. This chapter provides an introduction to the basic concepts and history of electronic memory, followed by a brief description of the structures and switching mechanisms of electrical memory devices classified as transistors, capacitors and resistors. Then, the progress of organic-based memory materials and devices is systematically summarized and discussed. Lastly, the challenges posed to the development of novel organic electrical memory devices are summarized.
1.2 Basic Concepts of Electronic Memory
The basic goal of a memory device is to provide a means for storing and accessing binary digital data sequences of "1's" and "0's", as one of the core functions (primary storage) of modern computers. An electronic memory device is a form of semiconductor storage which is fast in response and compact in size, and can be read and written when coupled with a central processing unit (CPU, a processor). In conventional silicon-based electronic memory, data are stored based on the amount of charge stored in the memory cells. Organic/polymer electronic memory stores data in an entirely different way, for instance, based on different electrical conductivity states (ON and OFF states) in response to an applied electric field. Organic/polymer electronic memory is likely to be an alternative or at least a supplementary technology to conventional semiconductor electronic memory.
According to the storage type of the device, electronic memory can be divided into two primary categories: volatile and non-volatile memory. Volatile memory eventually loses the stored information unless it is provided with a constant power supply or refreshed periodically with a pulse. The most widely used form of primary storage today is volatile memory. As shown in Figure 1.1, electronic memory can be further divided into sub-categories, as read only memory (ROM), hybrid memory, and random access memory (RAM). ROM is factory programmable only; data is physically encoded in the circuit and cannot be programmed after fabrication. Hybrid memory allows data to be read and re-written at any time. RAM requires the stored information to be periodically read and re-written, or refreshed, otherwise the data will be lost. Among these types of electronic memory, write-once read-many-times (WORM) memory, hybrid non-volatile and rewritable (flash) memory, static random access memory (SRAM) and dynamic random access memory (DRAM) are the most widely reported polymer memory devices.
A WORM memory device can be used to store archival standards, databases and other massive data where information has to be reliably preserved for a long period of time. Conventional CD-Rs, DVD [+ or -] Rs or programmable-read-only-memory (PROM) devices are examples of WORM memory. Flash memory is another type of non-volatile electronic memory. Different from WORM memory, its stored state can be electrically reprogrammed and it has the ability to write, read, erase and retain the stored state. Thus it is mutable or rewritable in nature. Due to its non-volatility, no power is needed to maintain the information stored in flash memory. DRAM is a type of volatile random access memory that stores each bit of data in a separate capacitor within an integrated circuit. Since real-world capacitors have charge-leaking tendencies, the stored data eventually fade unless the device is refreshed periodically. Because of this periodical refresh requirement, it is a volatile and dynamic memory. The volatility, ultrafast data access time and structural simplicity hold great promise for high density and fast responding performance, making DRAM memory the main memory for most computers. SRAM is another type of volatile memory. The term "static" differentiates it from "dynamic" RAM (DRAM) which must be periodically refreshed. SRAM exhibits data remanence, but it is still volatile and the stored data are eventually lost when the memory remains in the power-off state. SRAM is faster and more reliable than the more common DRAM. Due to its high cost, SRAM is often used only as a memory cache.
Parameters of importance to the performance of a memory cell include switching (write and erase) time, ON/OFF current ratio (or memory window), read cycles, and retention ability. The switching time influences the rate of writing and accessing the stored information, the ON/OFF current ratio defines the control of the misreading rate during device operation, with a higher value being essential for the device to function with minimal misreading error, while the number of read cycles and retention ability are related to the stability and reliability of the memory devices. For practical applications, other factors, such as power consumption and cost, structural simplicity and packing density, as well as mechanical stiffness and flexibility, are of equal importance when designing and fabricating new memory devices.
1.3 History of Organic/Polymer Electronic Memory Devices
Different forms of storage, based on various natural phenomena, have been reported since the 1940s. A computer system usually contains several kinds of storage, each with an individual purpose. In the 1960s, there was a great interest in the electrical properties of amorphous semiconductors and disordered structures, arising from their unusual electrical properties which also make them promising materials for device applications.
In 1968, Gregor observed bistable negative resistance in polymer materials and noted that a Pb/polydivinylbenzene/Pb bistable electrical switching device is capable of acting as an information storage device. In 1969, Szymansk et al. reported bistable electrical conductivity phenomena in thin tetracene films sandwiched between metal electrodes. In 1970, Sliva et al. reported that devices based on Saran(r) wrap, phthalocyanines and polystyrene all exhibited bistable switching behavior. Subsequently, Segui et al. demonstrated reproducible bistable switching in polymer thin films prepared by glow-discharge polymerization. Inspired by these pioneering studies, a wide variety of organic and polymer materials have been explored for threshold and memory switching effects. Many of the observed electrical memory effects were due to the formation of filamentary conduction paths, and the performance was not satisfactory for practical applications. Memory switching effects in polymethylmethacrylate, polystyrene, polyethylmethacrylate and polybutylmethacrylate films were ascribed to field-controlled polymer chain ordering and disordering. Memory switching effects in poly(N-vinylcarbazole) (PVK) thin films were attributed to trapping–detrapping processes associated with impurities in PVK.
Studies of the transition behavior of some ferroelectric polymers began in the 1980s. Thin films of ferroelectric materials can be repeatedly switched between two stable ferroelectric polarization states, and are capable of exhibiting non-volatile memory effects. Polymer films obtained by solution processing techniques were so thick that some devices required operating voltages of at least 30 V. Bune et al.reported a major breakthrough in the fabrication of ferroelectric films by the Langmuir–Blodgett (LB) technique in 1995.39 The resulting ferroelectric films are as thin as 1 nm and can be switched using a voltage as low as 1 V.40 Rapid progress in polymer ferroelectric random access memory (FeRAM) as a promising memory technology has since been achieved.
An organic transistor memory device using a sexithiophene oligomer as the conductor and an inorganic ferroelectric material as the gate insulator was demonstrated in 2001 by Velu et al. Subsequently, ferroelectric organic and polymer materials have also been utilized as gate insulators in field-effect transistors (OFETs). High performance all-organic or polymer transistor memory devices have been demonstrated by Naber et al. Transistor memory devices can be faster and more readily integrated with traditional electronics. However, they are not able to meet the high density and low-cost requirements since an additional terminal is required between the gate and the semiconducting channel. A WORM type memory device based on polymer fuses was demonstrated by Forrest and coworkers in 2003. The memory element consists of a thin film p–i–n silicon diode and a conductive polymer fuse, composed of poly(ethylene dioxythiophene) (PEDOT) oxidatively p-doped by poly(styrene sulfonic acid) (PSS).
Bistable electrical switching and memory effects involving charge transfer (CT) complexes were first reported in an electronic device based on a copper (electron donor) and 7,7,8,8-tetracyanoquinodimethane (TCNQ, electron acceptor) complex. Subsequently, a wide variety of organometallic and all-organic CT complexes have also been explored for non-volatile electronic memory applications. Polymer memory devices based on CT effects from doping of a polymer matrix by electron donors, such as 8-hydroxyquinoline (8HQ), tetrathiafulvalene (TTF), polyaniline (PANI), poly-3-hexylthiophene (P3HT), or electron acceptors such as gold nanoparticles, copper metallic filaments and phenyl C61-butyric acid methyl ester (PCBM), have been reported. Carbon nanotubes (CNTs) possess intense pconjugation and strong electron-withdrawing ability. The CT complexes of CNTs and P3HT, a conjugated copolymer or poly(N vinylcarbazole) (PVK) have been reported to exhibit a bistable electrical memory effect. By utilizing copolymers containing both donor (D) and acceptor (A) moieties in the basic unit, phase separation and ion aggregation could be effectively avoided in a single-component polymer film, resulting in uniform film morphology and improved device performance. D–A polymer-based electrically bistable memory devices have received considerable attention. The molecular design cum-synthesis approach has allowed several polymer electronic memory devices, including flash memory, WORM memory and DRAM to be realized.
In order to achieve ultrahigh density memory devices, organic materials with multilevel stable states are highly desirable. In 2004, Pal et al. reported multilevel conductivity and conductance switching in supramolecular structures of Rose Bengal. Subsequently, they observed one low- and three high-conducting states in ultra-thin film devices, and all four accessible states have associated memory effects for data-storage applications. Multilevel conductance switching in poly[2-methoxy-5-(2'-ethyl-hexyloxy)1,4-phenylene vinylene] (MEH-PPV) films was first reported by Lauters et al. in 2005. They observed that the ITO/MEH-PPV/Al device had the ability to store a continuum of conductance states. These states were non-volatile and could be switched reproducibly by applying appropriate programing biases above a certain threshold voltage. Devices demonstrating multistability where more than two conducting states can be programmed into a single switching element will dramatically increase the amount of data stored per area or volume. Further progress in the development of multilevel organic/polymer memory has been made in recent years.
In 1971, Chua proposed a new circuit element, a memristor, which is the fourth passive circuit element beyond the fundamental resistor, capacitor and inductor. A memristor is capable of processing information in the same way as biological systems, mimicking the function of a mammalian synapse, with the ability to learn and memorize new information. According to the redefinition of Chua in 2011, all two-terminal non-volatile resistive switching memory devices are memristors, regardless of the device material or the physical operating mechanism. A polymer memristor was first reported in cobalt(iii)-containing conjugated (CP) and non-conjugated (NCP) polymers with an azo-aromatic backbone by Higuchi et al. in 2011. Single crystals of a cyclodextrin-based metal–organic framework (MOF) infused with an ionic electrolyte and flanked by silver electrodes can act as memristors. The metal/single-crystal MOF/ metal heterostructure can be switched between high and low conductivity states due to the self-limiting oxidative reactions of the metal anode.
The International Technology Roadmap for Semiconductors (ITRS) has identified polymer memory as an emerging memory technology since the year 2005. Figure 1.2 indicates the number of related publications each year worldwide since the year 2000. Research work on polymer memory before 2008 was summarized in a comprehensive review by Ling et al. Liu and Chen highlighted recent developments in the field of D–A polymers for resistive switching memory device applications. Chen et al. reviewed the application of electrically, thermally and chemically modified graphene and polymer-functionalized graphene derivatives for switching and information storage applications. Most recently, Huang and coworkers summarized recent progress concerning the use of polymers or polymer composites as active materials for resistive memory devices. The impetus for the research effort in this area arises from the fact that organic/polymer electronic memory devices have been a promising alternative or supplementary device to conventional memory technologies facing the problem of miniaturization from microscale to nanoscale.
1.4 Classification of Electrical Memory Devices
According to the device structure, electronic memory devices can be divided into three primary categories: transistors, capacitors and resistors. With their respective ability to amplify electronic signals, to store charges, and to produce proportional electric currents, electronic memory devices can be constructed from transistors, capacitors and resistors.
Excerpted from Electrical Memory Materials and Devices by Wen-Chang Chen. Copyright © 2016 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.