Electrical Memory Materials and Devices
Foreword: Introduction to Organic Memory Technologies
Published:16 Oct 2015
Special Collection: 2015 ebook collection , 2011-2015 materials and nanoscience subject collectionSeries: Polymer Chemistry Series
The discovery of how to store information in the form of numbers and letters is generally considered as one of the most important steps of social evolution in human history as it allowed passing of information from generation to generation. The simplest forms appeared at least forty thousand years ago in the form of unary numeral systems (tally marks). Real alphabets developed, however, much later around 4000 BC in Mesopotamia. The invention of a moveable metal type printing press by Johannes Gutenberg (1450) is considered as another big leap since it enabled a relatively cheap, large scale duplication of information. Nowadays, information is stored digitally encoded in a binary language consisting of zeros and ones, i.e. binary logical states. The origin of the rapid development in this field dates back to the 1940s, starting from the first computer prototypes with an implemented memory – Zuse3 (1942, Germany) and ENIAC (1946, USA) – via the launch of the first personal computers (1970s) to the present state where digital memory is found in nearly every electronic device. Recent calculations showed that approximately 300 exabytes (1018 bytes) of information is stored worldwide.1
As predicted 50 years ago memory capacity develops according to Moore’s law,2 meaning that roughly every 18 months a doubling of the number of transistors per chip at approximately equal cost is found. Although this observation has been doubtlessly valid for the past decades, a strong convergence to an upper integration density is predicted. It is expected that within the next 10 to 20 years it will not be possible to further downsize the traditional complementary metal-oxide-semiconductor (CMOS) architecture due to physical restrictions at such small dimensions. In order to overcome this fact, two parallel strategies are usually presented. First, more Moore, dealing with the continuous miniaturization of classical CMOS building blocks as well as post-CMOS materials and devices. Second, the more-than-Moore approach which targets system integration and functional diversification rather than scaling issues: the inclusion of non-digital functionalities like sensors and actuators, bio-chips, photonic applications, etc. interfaces the integrated circuit to the outside world. Materials and devices presented in this book actually address both approaches by investigating organic and hybrid material memory technologies based on capacitors, transistors and resistive switching elements and their advantages (more Moore) and secondly demonstrating how organic and hybrid memory devices may be integrated into flexible electronic applications (more-than-Moore).
However, there are numerous prerequisites that novel materials, device concepts and memory elements based on these materials should fulfil to outperform established technologies. The most important ones are listed in the following:
Non-volatility: state should be preserved at least for several years even if the supply voltage is turned off
Fast read, write and erase times in the range of the CPU clock (∼GHz): this prevents a bottleneck effect in the communication between the individual components in an electronic application
Cyclability: 1012–1015 write–read–erase–read cycles should be possible
Operation temperature not higher than 85 °C
ON/OFF ratio: clear difference between ‘0’ (OFF) and ‘1’ (ON) state which enables for simple peripheral electronics and a reliable read-out
High integration density (down to 4F2, where F denotes the smallest lithographically obtainable feature size)
Low power consumption
Compatibility with existing semiconductor fabrication plants
3D-integration on multiple layers, multiple bits per cell
Random Access: each bit cell should be accessible directly and sequential read-out is necessary.
As discussed in Chapter 1 in detail, today’s dominant memory technologies do not meet all of these criteria simultaneously and the decision about which memory or storage technology is used strongly depends on its actual application case. Figure 1 depicts the memory-storage hierarchy. Usually storage and memory are considered separately, where the first term includes non-volatile, high capacity technologies and the latter term is used for fast, lower capacity and volatile technologies. Along these lines, the main task in the long run is the development of a technology which unifies memory and storage technology into a ‘storage class memory’ with the advantages of both. This would allow, for example, the design of computers where the operating systems and other data do not need to be read out from the hard disk drive to the main memory when it is switched on.
Today three technologies dominate the market – each of them highly specialized in its field of application, dynamic random access memory (DRAM), flash memory, and magnetic hard disk drives. Emerging technologies such as ferroelectric RAM (FeRAM), magnetoresistive RAM, phase-change RAM, and resistive memory have or are about to enter the market. Over the past decades a wide variety of organic and hybrid materials concepts have also been utilized as hysteretic elements in memory cells, including conjugated polymers, small molecules, organic/hybrid materials, chalcogenides, thin-film perovskites, electromechanical switches, ferroelectric organic polymers, etc. Depending on the individual functional principle, these materials are integrated into memory elements utilizing a transistor, diode, capacitor or resistor device structure rendering an individual memory cell. Yet, according to the International Technology Roadmap for Semiconductors, in an overall comparison and benchmarking of the specific type of hysteretic materials used in memory cells, it is not only important to judge the material’s electrical characteristics (with respect to scalability, read-out time, write time, switching power, retention time and other parameters) but also if the material is compatible with fabrication technology and if the device element fulfils the design constraints for high density integration. Amongst the here-presented candidates, which cover ferroelectric and floating gate organic transistor memory and different resistive memory technologies, the latter one is doubtless the most promising candidate not only to replace the existing technology with an organic counterpart but also to act as storage class memory.
The contributions to this book address and summarize the ongoing development in organic memory technology based on resistive switching and transistor-based memory from the material development, processing as well as from the device operation point of view. The classes of materials presented range from conjugated polymers, to donor–acceptor structures to hybrid material composite concepts to name just a few of them. For all material concepts one finds a comprehensive discussion of the observed electrical switching in the device, reflecting on the fact that despite considerable research effort in this field, the true nature of resistive switching is still uncertain.
Chapter 1 gives a basic and historic overview on existing and emerging memory device concepts based on organic, polymeric and hybrid materials. Existing transistor-based, capacitor-based and resistor-based device concepts are discussed together with the observed operation principles. Chapter 2 focuses on the recent development of resistive memory technologies based on organic and polymeric materials. The chapter covers basic materials design principles to be applied, followed by a comprehensive discussion on the switching mechanism in organic resistor memory elements, followed by a number of application-related examples of organic memory devices. Chapter 3 reports on the use of donor–acceptor structures in resistive memory devices. The chapter highlights the importance of structure to property relationships in understanding the switching mechanism and also discusses multilevel resistive memory devices. Chapter 4 focuses on describing the use of polyimide and functionalized polyimides for resistive memory devices. Different applications of such devices are reviewed together with an in detail discussion on the switching mechanism. Chapter 5 reports on the use of non-conjugated polymers with electroactive chromophore pendants in resistive memory devices. The design of molecules is discussed together with their performance in devices and the observation of switching. Chapter 6 focuses on the use of polymer composites for resistive switching devices. In addition to switching observations in hybrid polymer–nanoparticle systems and multilayer structures, polymer–ionic liquid composites are also discussed. Chapter 7 discusses the design and development of different conjugated polymers for resistive switching elements, including a discussion of the different observed switching mechanisms. Chapter 8 describes the use of a variety of homo- and copolymers bearing donor–acceptor moieties for their use in resistive memory devices. Chapter 9 reviews the use of ferroelectrics, polymer electrets, polymer–molecular hybrids and self-assembled monolayers in non-volatile transistor based memory devices. Chapter 10 discusses the physics and fabrication of floating gate charge storage devices based on organic transistors and possible implications for their use in emerging flexible digital memory electronics. Chapter 11 discusses the use of ferroelectric materials in capacitor-, transistor- or diode-based storage devices elucidating the interplay of polymer orientation, interfacial engineering and device configuration, and memory performance.