**Redox-Active Molecules for Novel Nonvolatile Memory Applications Applications**

**Redox-Active Molecules for Novel Nonvolatile Memory** 

DOI: 10.5772/intechopen.68726

Hao Zhu and Qiliang Li Hao Zhu and Qiliang Li Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.68726

#### **Abstract**

The continuous complementary metal‐oxide‐semiconductor (CMOS) scaling is reaching fundamental limits imposed by the heat dissipation and short‐channel effects, which will finally stop the increase of integration density and the MOSFET performance predicted by Moore's law. Molecular technology has been aggressively pursued for decades due to its potential impact on future micro‐/nanoelectronics. Molecules, especially redox‐ active molecules, have become attractive due to their intrinsic redox behavior, which provides an excellent basis for low‐power, high‐density, and high‐reliability nonvolatile memory applications. This chapter briefly reviews the development of molecular elec‐ tronics in the application of nonvolatile memory. From the mechanical motion of mol‐ ecules in the Langmuir‐Blodgett film to new families of redox‐active molecules, memory devices involving hybrid molecular technology have shown advantageous potential in fast speed, low‐power, and high‐density nonvolatile memory and will lead to promising on‐chip memory as well as future portable electronics applications.

**Keywords:** molecular electronics, redox‐active molecules, self‐assembled monolayer, nonvolatile memory, high‐density, high‐endurance, flash‐like memory

**1. Introduction**

#### **1.1. CMOS scaling challenges and impact on nonvolatile memory**

The complementary metal‐oxide‐semiconductor (CMOS) scaling has deviated from the trends predicted by Moore and the scaling rules set forth by Dennard et al. due to both fun‐ damental physical and technical limitations [1, 2]. This will inevitably slow down the pace of current CMOS scaling when approaching atomic dimension. Scaling limitations such as

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ultrathin gate oxide, channel length modulation, and off state leakage have become growing concern, and there are urgent needs to develop new semiconductor technology and solutions toward CMOS scaling challenges. For example, barriers such as doping, carrier transport, and series resistance scaling have been effectively avoided by innovations such as source/drain process upon silicon‐on‐insulator (SOI) structure, multigate field‐effect transistor (FET), and SiGe BiCMOS technologies [3–6].

Currently, computing architectures and electronic systems built on CMOS components are still pursuing without signs of slowing down of requirements for low‐power, fast speed, and high‐density alternatives [7, 8]. Various scenarios of fundamentally new technology with advanced materials and structures have been proposed and investigated such as quantum computing, DNA computing, single electron device, spin transistor, and molecular electronics for both logic and memory applications. However, these new approaches still need in‐depth studied before they can be considered for application in real‐world device applications.

In modern electronic systems, the main functions lie in the data computing and data stor‐ age, which take up more than half of the semiconductor market. Solid‐state mass storage occupies a large portion of this market; the demand is still growing explosively in areas such as portable electronic devices, due to their compatibility with CMOS scaling, suitability for harsh environment, and the fact that most types of memory are nonvolatile, which means that the data information can be maintained even without power supply. Currently, main types of nonvolatile memory technology that have been investigated include phase‐change mem‐ ory (PCM), ferroelectric random access memory (FeRAM), resistive random access memory (RRAM), magnetic random access memory (MRAM), and flash memory [9–13]. However, these nearer term new technologies have inherent disadvantages that may limit their imple‐ mentations. For instance, even though FeRAM, PCM, and MRAM are currently in commercial production, they are still many years away from competing with dynamic RAM (DRAM) and NAND (Not‐AND) flash for industry adoption [14, 15]. On the other hand, the retention time of floating‐gate DRAM decreases with scaling, and the relatively poor endurance of the flash memory (∼105 ) is the critical limitation for its further application.

#### **1.2. Molecular electronics**

#### *1.2.1. Introduction*

Molecular technology has been aggressively pursued for its potential impact on nanoelec‐ tronics since the early 1970s due to its inherent scalability and intrinsic properties [16–18]. Molecular electronics has been considered to replace the conventional silicon‐based comput‐ ing even before hitting the fundamental limits [19–21]. Molecular electronic devices are typi‐ cally fabricated by forming a self‐assembled monolayer (SAM) or multiple layers on different surfaces using inexpensive and simple processing methods. Such device functions by the controlling of fewer electrons at a molecular scale, and therefore, has potential for fast speed, low‐power, and ultrahigh‐density device and circuit applications.

Molecular electronics are typically achieved through two fundamentally different approaches, which are graphically termed as "top‐down" and "bottom‐up". The top‐down approach includes making nanoscale structures by machining and etching techniques. Molecular electronics rely‐ ing on the bottom‐up approach takes advantage of molecule self‐assembly, building organic or inorganic structure by atom‐by‐atom or molecule‐by‐molecule techniques. In the past decades, the cross‐disciplinary publications in the field of molecular electronics have dramatically increased by chemists, physicists, engineers, and other researchers. Various novel molecular device architectures and electronic systems have been introduced and explored. Nevertheless, most of the molecular electronics has been implemented by top‐down approaches, and recently, the combination of top‐down device fabrication (mainly lithography) with bottom‐up molecule self‐assembly has attracted more and more interest.

#### *1.2.2. Advantages and challenges of molecular electronics*

ultrathin gate oxide, channel length modulation, and off state leakage have become growing concern, and there are urgent needs to develop new semiconductor technology and solutions toward CMOS scaling challenges. For example, barriers such as doping, carrier transport, and series resistance scaling have been effectively avoided by innovations such as source/drain process upon silicon‐on‐insulator (SOI) structure, multigate field‐effect transistor (FET), and

Currently, computing architectures and electronic systems built on CMOS components are still pursuing without signs of slowing down of requirements for low‐power, fast speed, and high‐density alternatives [7, 8]. Various scenarios of fundamentally new technology with advanced materials and structures have been proposed and investigated such as quantum computing, DNA computing, single electron device, spin transistor, and molecular electronics for both logic and memory applications. However, these new approaches still need in‐depth studied before they can be considered for application in real‐world device applications.

In modern electronic systems, the main functions lie in the data computing and data stor‐ age, which take up more than half of the semiconductor market. Solid‐state mass storage occupies a large portion of this market; the demand is still growing explosively in areas such as portable electronic devices, due to their compatibility with CMOS scaling, suitability for harsh environment, and the fact that most types of memory are nonvolatile, which means that the data information can be maintained even without power supply. Currently, main types of nonvolatile memory technology that have been investigated include phase‐change mem‐ ory (PCM), ferroelectric random access memory (FeRAM), resistive random access memory (RRAM), magnetic random access memory (MRAM), and flash memory [9–13]. However, these nearer term new technologies have inherent disadvantages that may limit their imple‐ mentations. For instance, even though FeRAM, PCM, and MRAM are currently in commercial production, they are still many years away from competing with dynamic RAM (DRAM) and NAND (Not‐AND) flash for industry adoption [14, 15]. On the other hand, the retention time of floating‐gate DRAM decreases with scaling, and the relatively poor endurance of the flash

) is the critical limitation for its further application.

low‐power, and ultrahigh‐density device and circuit applications.

Molecular technology has been aggressively pursued for its potential impact on nanoelec‐ tronics since the early 1970s due to its inherent scalability and intrinsic properties [16–18]. Molecular electronics has been considered to replace the conventional silicon‐based comput‐ ing even before hitting the fundamental limits [19–21]. Molecular electronic devices are typi‐ cally fabricated by forming a self‐assembled monolayer (SAM) or multiple layers on different surfaces using inexpensive and simple processing methods. Such device functions by the controlling of fewer electrons at a molecular scale, and therefore, has potential for fast speed,

Molecular electronics are typically achieved through two fundamentally different approaches, which are graphically termed as "top‐down" and "bottom‐up". The top‐down approach includes

SiGe BiCMOS technologies [3–6].

58 Redox - Principles and Advanced Applications

memory (∼105

*1.2.1. Introduction*

**1.2. Molecular electronics**

Molecular electronics competes to a large extent with conventional microelectronics based on traditional metal‐oxide‐semiconductor (MOS) structures. Molecular electronics has been expected to possess the following advantages [22]. First, the inherent scalability of molecules enables functional structures and ultrahigh device density with atomic control over a diver‐ sity of physical properties. Second, molecules can be self‐assembled through intermolecular interaction to form nanostructures, and further for desired molecular devices and circuits. Third, molecular properties can be tailored with choice of composition and geometry, includ‐ ing electrical transport, binding, and structural properties. Fourth, a variety of molecules have multiple distinct stable electrical and geometric structures, therefore, molecular switching devices and circuits can be achieved through the transitions between different structures under electrical or chemical stimulus.

The major challenges molecular electronics are facing lie in the unknown reliability at high temperature, volatile environments, and electrical stress. The instability at high temperature makes most molecules incompatible with current CMOS process integration. The retention time is the biggest challenges for most of the molecular memory devices as at least >10‐year retention time is necessary in order to be considered as a candidate technology for universal memory applications. Some specific condition and environment need to be taken care of when integrating molecular electronics in conventional CMOS devices and circuits. Furthermore, molecular technology development requires advancement in both molecular properties and device integration processes.

#### *1.2.3. Redox‐active molecules*

Redox‐active molecules have attracted more and more interest recently, due to their intrin‐ sic and reliable redox behavior, which can be readily utilized for charge‐storage memory applications. Physically, a redox‐active molecule contains a redox component acting as the charge‐storage center surrounded by insulators/barriers formed by the linkage and the sur‐ face group. The electrons tunnel through the barrier during the oxidation and reduction processes. Typically, the application of an oxidation voltage will cause electron loss in the redox molecules; reversely, the electrons will be driven back to the molecules by applying a reduction voltage. Generally, the redox molecules have multiple stable states. The switching between these states is dynamically reversible through the loss or capture of a charge, that is, oxidation and reduction of the redox centers. Distinct charged or discharge states can be deemed as logic on and off states, at different voltage with very fast write and erase speeds. It has been demonstrated that the redox molecules attached on silicon structures are stable and can endure more than 1012 program/erase cycles [23]. Such advantageous properties of redox molecules make them very attractive for future applications of fast speed, low‐power, high‐endurance, and high‐density nonvolatile memory.

There have been great efforts to effectively integrate molecules as the active component for future micro‐/nanoelectronic devices. The following sections will review the attachment and characterization of redox molecules on active surfaces, and the strategies to involve such attractive medium in low‐power and high‐density nonvolatile memory applications.
