The term “memory” is very much associated with none other than the brain, a very special part of our living body with receiving, storing, and recalling abilities. Memory allows us to memorize the past events of life. Similar to our body, the memory element is a fundamental need of the storage-class devices and is divided into two broad groups on the basis of the requirement of power to memorize the stored data. One type is volatile memory (VM), which needs constant power supply to remember the state, and the other
is nonvolatile memory (NVM), which is capable of remembering the data even when no power supply is available. In practice, dynamic random access memory (DRAM) and static random access memory (SRAM) are VM type and flash is NVM type. Due to fundamental limitations associated with the shrinking device size and increased process complexity with miniaturization of baseline NVM devices, emerging NVM devices with exciting architectures are being explored. Because of the simple structure, faster operation, reliable storage capacity, superior scalability, and cost-effective design, resistive random access memory (RRAM) is receiving a huge amount of attention among emerging technologies. Defects like cations or anions dominate the resistive switching (RS) events in the RRAM devices. The performance of the RRAM devices can be manipulated and improved by incorporation of nanocrystals (NCs) in the RS structure. The attractive features of the NCs, especially the ability to enhance the electric field, can effectively enhance the performance of the RRAM devices. In this chapter, we give an overview of the fabrication processes, properties, and improvements in electrical characteristics of NC-based RRAM devices. This chapter will convey a basic understanding of RRAM technology as well as the future scope. 8.1 Introduction
8.1.1 BackgroundThe present generation is depending more and more heavily on advanced electronic equipment, in which different memory technologies-including volatile memory (VM), for example, static random access memory (SRAM) and dynamic random access memory (DRAM), and nonvolatile memory (NVM), that is, flash memory-have been used for specific works. A single type of memory is not enough to do all things together. In general, SRAM is the fastest memory, where the write/erase speed is about 100 ps, but the design of each SRAM cell requires six transistors, which costs a lot of space on a wafer. Therefore, one-transistor-one-capacitor-
based compacted DRAM with moderate efficiency is a point of focus. However, its data storage capacity is very limited due to leaky capacitors. One-transistor-based flash memory is an obvious choice because of its nonvolatility and cost effectiveness. The advantages of flash memory are very useful for mass storage applications . Silicon-based flash memory devices hold the biggest share of the semiconductor memory market. But unfortunately, the shrinking of the cell size is creating a lot of issues in SRAM, DRAM, and flash technologies. The miniaturization is increasing the operation power of the VMs. Along with the physical limitation of flash memory device, it has other disadvantages, such as high-voltage and low-speed operation and poor endurance as compared to DRAM. According to a news report from Imec, Leuven, Belgium, , it is very clear thatthe scaling of cell size leads to gradual reduction of the number of
electrons stored on the floating gate, with a projected number of less than 30 electrons for memorizing a (multilevel) cell state, for an assumed 15-nm feature size. To meet the requirements of next-generation information technology memory, we are looking for such nonvolatile, scalable, cheap memory technology with ultrafast, low-power, ultrahigh endurance and retention capacities in a single cell. This kind of memory is known as “universal memory” . The need of the time has enhanced the research area for identifying a suitable alternative NVM . For regular industrial adoption, the expectations are really high from any alternative NVM. Compared to the existing memory, a new technology is expected to be a highly scalable one. It should be operable at low power with high speed, have higher endurance and be highly reliable, and be obviously cheaper in price . Some of those promising prototype NVM technologies are ferroelectric random access memory (FeRAM), phase change memory (PCM), and spin-transfer torque magnetic random access memory (STTRAM or STTMRAM). All of those baseline and prototype memory technologies, with their advantages and disadvantages, are summarized in Table 8.1 .