Since nanotechnology, information technology, material science, and biotechnology have been identified as the four major sciences and technologies of the 21st century, there has been a global wave of research and development in nanotechnology, both internationally and domestically. In particular, in China, "nano" has become one of the hottest buzzwords and a topic of discussion in households nationwide. Nanoscientific promptly proposed the development policy of "a comprehensive understanding of the essence and promotion of healthy development" for nanotechnology, emphasizing that the understanding of nanodevices, especially nanoelectronic devices, and the vigorous development of nanoelectronic devices are crucial issues. For instance, what exactly is a nanodevice? What defines a nanoelectronic device? How should nanoelectronic devices be classified? What role do they play within the broader field of nanotechnology? What considerations should be taken into account in the development of nanotechnology? These questions warrant exploration. This article aims to address these inquiries and introduce two primary types of nanoelectronic devices—solid nanoelectronic devices and molecular electronic devices—so readers can develop a comprehensive and accurate understanding of nanoelectronic devices. Additionally, there are differing perspectives on the classification of nanoelectronic devices both domestically and internationally. This article seeks to act as a catalyst for academic discourse on nanoelectronic devices, which could significantly benefit their development.
Nanodevices can be thought of as devices created using nano-scale processing and fabrication techniques, such as metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), electron beam technology (EB), scanning probe microscopy (SPM), and nano-material preparation methods (such as self-assembly growth and molecular synthesis). These techniques are used to design and fabricate devices with nanoscale dimensions (1-100 nm) and specific functionalities.
Regarding the classification of nanodevices and nanoelectronic devices, nanodevices encompass a range of technologies, including resonant tunneling devices, quantum dot devices, solid nanoelectronic devices, and molecular electronic devices. However, the scope of nanoelectronic devices is limited to two key conditions: (1) the device's operational principle is based on quantum effects, and (2) the device has a typical structure featuring a tunnel barrier surrounding an "island" or "well." Based on these criteria, while devices like nano-CMOS devices, nano-magnetic devices, and nano-electromechanical systems (NEMS) are fabricated using nano-processing technology and are indeed nanoscale, they do not fall under the category of nanoelectronic devices. Instead, nanoelectronic devices can be divided into two main categories: solid nanoelectronic devices, which include resonant tunneling devices (like resonant tunneling diodes RTD and resonant tunneling transistors RTT), quantum dot (QD) devices, and single-electron transistors (SEDs); and molecular electronic devices, primarily consisting of quantum-effect molecular electronic devices and electromechanical molecular electronic devices. This classification is illustrated in Table 1.
Table 1: Classification of Nano Devices
Nano Photonic Devices | Nano Magnetic Devices | Nano CMOS Devices | Nano Electromechanical Systems | Other Nano Devices (Quantum Interference Devices, etc.)
Solid Nanoelectronic Devices
Single Electron Devices | Single Tube Storage | Quantum Effect Molecular Electronic Devices | Electromechanical Molecular Electronic Devices
2.1 Classification of Solid Nanoelectronic Devices
Solid nanoelectronic devices represent a major branch of nanoelectronic devices. Their emergence is tied to advancements in nano-preparation, processing technology, mesoscopic physics, and microelectronic technology, as well as the various "limits" faced globally. These devices feature a quantum effect and a tunneling barrier surrounding an "island" or potential well. Depending on the dimensions of the "island" or potential well, solid nanoelectronic devices can be categorized into three types, as shown in Table 2.
Table 2: Classification of Solid Nanoelectronic Devices
Device Name | Working Mechanism | Number of Terminals | Characteristics
Resonant Tunneling Devices | Quantum resonant tunneling effect | Two or three terminals | Nanoscale dimensions but no quantized Coulomb blocking effect
Quantum Dot Devices | Quantum dot quantization | Three terminals | Potential well quantization and energy distribution within the potential well
The solid nanoelectronic device is surrounded by the aforementioned barrier and injects electrons into the island via the tunneling barrier, then transfers them to the collector through another barrier. For three-terminal devices, the gate voltage controls the current flowing through the device. The characteristics of the device are directly related to the distribution of electron energy within the potential well. The energy distribution in the potential well depends on the quantization dimensions of the potential well and the three-dimensional scale of the potential well.
For resonant tunneling devices, quantization occurs in only one dimension (the potential well width) at a few nanometers, whereas quantum wire devices exhibit quantization in two dimensions (perpendicular to the "line") at a few nanometers. The scale in the remaining directions is on the order of microns. Thus, significant quantization happens only on scales of a few nanometers, leading to quantization in one dimension for RTD and in two dimensions for quantum wires. Quantum dot devices have a three-dimensional scale of a few nanometers, resulting in quantization in three dimensions. Single-electron devices have a much smaller three-dimensional scale than conventional devices (tens or hundreds of nanometers), but they lack quantization in one dimension (the same direction as electron motion), meaning the quantization dimension is zero. The result of quantization is the appearance of discrete energy levels in "islands" or potential wells. Stronger quantization leads to greater energy gaps between levels. Furthermore, the barrier and potential well structure can be viewed as an isolated electronic system where electrons in the potential well can remain stable without escaping, provided their energy is lower than the barrier height. To move an electron into the trap from the outside, all electrons in the trap must overcome the repelling effect of the electron and have sufficient energy, a phenomenon known as charging energy. Charging energy is related to the three-dimensional scale of the potential well; the smaller the volume of the potential well, the stronger the interaction between charges, and the larger the charging energy. Conversely, the larger the volume of the potential well, the smaller the charging energy.
In summary, for resonant tunneling devices or quantum wire devices, the energy distribution of electrons in the potential well is characterized by large discrete energy levels. When resonance tunneling occurs between electrons and the ground state energy level in the well, a current peak appears in the characteristic. As voltage increases, resonance tunneling occurs between electrons and the first excited state energy level, creating a second current peak on the characteristic. Due to the larger value, the corresponding voltage shift between the two peaks is larger. For single-electron devices, the energy distribution in the potential well is predominantly based on large energy gaps with a quasi-continuous distribution of small energy gaps. Ignoring the effect, electrons must be provided from the emitter with sufficient energy to enter the potential well, resulting in a smooth large step in the /-% characteristic; for D, the energy distribution in the potential well is based on energy gaps and exhibits the characteristic large step set and small step shown in Table 2.
2.3 Resonant Tunneling Device
2.3.1 Working Principle of Resonant Tunneling Device
The resonant tunneling device includes two-terminal resonant diode RTD and three-terminal resonant tunneling transistor RTT. They are among the earliest, most extensively studied, and relatively mature solid nanoelectronic devices. Currently, high-speed digital circuits containing 2000 RTDs have been developed abroad. The basic structure of RTD is a typical two-barrier single-potential well system, as shown in (a). The potential barrier is generally composed of AlAs or AlGaAs, and the potential well is composed of GaAs or InGaAs. The left-side emitter and right-side collector are composed of the same material doped layers as the potential well. The barrier width is 1.5 ~ 3.0nm, and the potential well width is 3.0 ~ 5.0nm, prepared using MBE technology. A discrete energy level appears in the potential well due to quantization, and the ground state energy is "0". Without bias, "0" is higher than the Fermi level in the emitter. After biasing, "0" is relative to the bottom of its conduction band "c" ((b)), when the electron energy in the emitter coincides with "0", satisfying energy conservation and lateral momentum conservation, resonance tunneling occurs, and tunneling current appears. With the increase in the wave vector! The shaded disk in the Fermi disk (Figure ;) in the space represents the electronic state that satisfies the above resonance tunneling conditions. The conversion frequency between the peaks and valleys of the RTD is theoretically expected to reach 1.5 ~ 2.5THz, the actual RTD has reached 650GHz, and the shortest switching time is 1.5ps. Low operating voltage and low power consumption: the operating voltage of a typical RTD is 0.2 ~ 0.5V, and the general operating current is in the order of mA, such as in material growth by adding a pre-barrier layer, the current is on the order of A, which can realize low power consumption. The power consumption of SRAM made by RTD is 50nW / cell.
Negative resistance, bistable and self-locking characteristics: Negative resistance is the basic characteristic of RTD and RTT. The inverter composed of RTD has bistable and self-locking characteristics.
To complete a certain logic function, only a few RTD devices are needed: due to its self-locking characteristics, a small number of devices can be used to complete the logic functions that can be completed by multiple conventional devices. For example, to construct an XOR gate, 33 devices are required for TTL, 16 devices are required for CMOS, 11 devices are required for ECL, and only 4 devices are required for RTD.
Apply RTD to microwave oscillation: RTD can be made into microwave oscillators and mixers, but it is limited in power.
Use RTD and other high-speed digital circuits: RTD and RTT constitute the following logic units, such as RTD + HBT bistable logic unit, RTD + MODFET bistable logic unit, RTBT (bipolar RTT with RTD as emitter) + HBT XOR gate logic, monostable-bistable conversion logic unit (MOBILE), Schottky / RITD pipeline logic gate, etc., these basic logic units can be used to further form different basic logic gates, flip-flops, SRAM, frequency dividers, A / D and D / A converters, shift registers, adders, etc.
RTD photoelectric integrated circuit: UCT-PD and RTD form a high-speed photoelectric bistable logic unit, which is used in 80Gb / s time division multiplexing (TDM) system. The structure of the RTD is also used to make a new photoelectric negative resistance RTD device.
2.4 Single-electron Devices
Single-electron devices include single-electron transistors (SET) and single-electron memory (SEM). SET is more common. This article only introduces SET.
2.4.1 Working Principle of Single Electron Transistor
The upper part is a schematic diagram of single electron transistor. The source, drain and island are composed of semiconductor or metal. The tunnel junction between the source and the island, and between the island and the drain forms a barrier around the island, which is composed of an insulating layer or a broadband semiconductor and a potential barrier formed by a negative voltage. Applying voltage to the grid! g can control the potential energy of the electrons in the island. The corresponding energy band diagram is given in the lower part, and the island corresponds to the potential well between the two barriers. As mentioned above, for SET, the charging energy " dominates, that is, when an electron is moved from the source into the island, it must have " = 2 / 2C (where is the electron charge, C is the total capacitance of the island to the surrounding part), overcome the charge on the island is only possible for it. If the electron does not have this energy, then the conduction process cannot occur. This phenomenon is called the Coulomb blocking effect. If it passes! G or provides energy for the electron, then the Coulomb blocking disappears and the conductive phenomenon resumes. So with! As d (or! g) increases, a stepped d-! d characteristics and oscillation waveform-! d Characteristic curve, as shown. Since the electrons in the emitter (whose energy is (f) obtain the energy of " = 2 / 2C before entering the potential well, the energy of the electrons in the potential well must be higher than (f) the energy of 2 / 2C. Because the electrons in the emitter and the holes in the well have an attractive effect, so the hole energy level in the well is lower than (f) 2/2, so in the potential well, the difference between the electron energy level and the hole energy level is 2 / C. Here, the electron energy in the well is higher than (f, which also reflects the Coulomb blocking effect of 2.4.2 Single-electron Transistor Characteristics High-frequency high-speed operation: because the tunneling mechanism is a high-speed process, and SET has a very small capacitance, so the working speed is very fast.
Very low power consumption: because the transportation process is single-electronic, the current and power consumption are very low.
High degree of integration: Because the SET device scale is very small, the degree of integration is high.
2.4.3 Application of Single Electron Transistor
Using SET to Make Next Generation High Speed ​​and High Density 1C: Because of the features described above, SET is one of the best candidate devices for the next generation of high speed and high density 1C.
Ultra-high sensitivity electrometer: Because SET can realize single electron conduction, it is suitable for ultra-high sensitivity electrometer. It is expected to increase the sensitivity by 1000 times than the existing electrometer.
Single photon device can be realized: Since SET can realize single electron transport, if a single hole device (made with + type material) is used in conjunction with it, single electron and single hole recombination can be controlled to produce single photon generation device.
High-sensitivity infrared radiation detector: It has been found that the SET is near the Coulomb blocking threshold voltage, and the tunneling current is very sensitive to infrared radiation induction, which may also be called the "photoinduced tunneling" effect.
2.5 Quantum Dot Device
Here, a concept must be clarified. The quantum dot here refers to a few nanometers in each dimension in three-dimensional space, and a significant quantization occurs. For dots and particles with a size of micrometers or even 102 nanometers, which have no significant quantization effect, they cannot be included in this QD category. In terms of energy, it must be, and both are very large. If the three dimensions are all in the order of micrometers, "4e", it belongs to the category of SET. Therefore, some of SET and D are indistinguishable. Real, D devices, there are few reports, please refer to.
3 Molecular Electronic Devices
3.1 Problems with Solid Nanoelectronic Devices
Although solid nanoelectronic devices currently dominate nanoelectronic devices, some devices (such as RTD) have entered the application stage, but in general, they still face the following problems.
3.1.1 Problems with RTD
RTD is one of the fastest devices currently available, but it is a device at both ends, and the input and output cannot be isolated and there is no current gain. If it is made into GRTD or parallel with HEMT, the working speed and frequency will be greatly reduced.
3.1.2 Low Temperature Operation
At present, RTD can work at room temperature, but most devices of SET can only work at low temperature. SET working at room temperature has very high requirements on the manufacturing process. It is only possible to reduce the capacitance of the island and the tunnel junction to a few aF (10-18F), and the process is very difficult.
3.1.3 Material Problems
The performance of RTD and SET made with compounds is relatively better than those with Si-based materials. Research on RTD and SET of Si-based materials should be vigorously carried out and combined with the mature Si process.
3.1.4 Background Charge Problems
Randomly distributed background charges often accumulate in the vicinity of quantum effect devices and single electron devices, and affect the normal operation of the device through electrostatic induction.
3.1.5 The Control of the Accuracy and Consistency of the Manufacturing Process
The tunneling current is very sensitive to the barrier width, and the control of the accuracy and consistency of the barrier width and the "island" scale is a difficult problem in the process.
Due to the existence of the above problems, the research of solid nanoelectronic devices has entered a gentle stage in the recent period, while molecular electronics has entered a relatively rapid development period.
3.2 The Concept of Molecular Electronics
The concept of molecular electronics is different from the organic micro-transistors or organic devices produced in the "bulk" material and the "bulk" effect that appeared in the previous period. Molecular electronics, also known as "intramolecular electronics," is composed of covalently bonded molecular structures that are electrically isolated from the "bulk" substrate, or molecular wires and molecules that consist of discrete molecules and nanomolecular supramolecular structures. The switch is connected. From the aspect of preparation technology, molecular electronics is easier to produce hundreds of millions of almost identical nanoscale structures at a lower cost than solid nanoelectronic devices. This is mainly due to the emergence of new methods of nano-processing and nano-manipulation, namely mechanical synthesis and chemical synthesis technology of nano-scale structure. Mechanical synthesis refers to the use of scanning tunneling microscope (STM), atomic force microscope (AFM) and a new micro-electromechanical system to control the 5 micro-nanoelectronic technology to give the I-ness of the molecular RTD measured by Reed, its current peak The valley ratio is about 1.3: 1. The simulation results of a research team at the company are shown below. Two atomic wires are connected by a movable switching atom to form a relay. If the switching atoms are in situ, the entire device can conduct electricity; if the switching atoms are detached from the in situ, the resulting gap suddenly reduces the current flowing through the atomic wire, making the entire device into a control and operating molecule for synthesis. Chemical synthesis includes chemical self-assembly growth of nanostructures, methods borrowed from biochemistry and molecular genetics, etc. Chemical electronic methods can be used to synthesize molecular electronic devices in organic molecules.
3.3 Quantum Effect Molecular Electronic Device
The representative of quantum effect molecular electronic device is a molecular resonance tunneling diode, referred to as molecular RTD. It has a device structure similar to a solid RTD surrounding a potential well and the same working principle. The structure and working principle of the molecular RTDs recently synthesized by Tour and confirmed by Reed are given in. It can be seen from the figure that the molecular RTD consists of four parts: (1) The emitter and collector of the backbone molecular wire molecular RTD are composed of polyphenylene-based molecular chains. This aromatic organic molecule has a conjugated v electron orbital. More than one such long unfilled or partially filled Gan track can provide a channel. When a bias exists at both ends of the molecule, electrons can move from one end of the molecule to the other. It is estimated that 2-1011 electrons per second can pass through each molecule. This kind of molecular wire is usually called Tour molecular wire; (2) "Islands" or potential wells composed of a single fatty ring have lower energy, which The size is about 1 nanometer, which is smaller than the scale of the solid RTD potential well, namely! Bigger or bigger! > 4e; (3) Two potential barriers are formed by two fatty methylene molecules, that is, two methylene molecules with insulating properties (one CH, ―) are inserted on both sides of the "island" and form between the left and right molecular wires Two potential barriers for molecular RTD; (4) Terminal leads of molecular electronic devices. Both ends of molecular devices are often pasted on gold (Au) electrodes through thiol (-SH) as their leading ends. The one (SH) next to the metal is often called the "crocodile clip" of the molecular device. The working principle of a molecular RTD is basically the same as that of a solid RTD. When electrons are confined in a narrow potential well, their energy is quantized to form discrete energy levels. When the energy levels in the potential well and the emitter are not filled with electrons When the energy of the molecular orbital is misaligned, resonance tunneling does not occur and the device does not conduct. When the bias voltage is applied, the energy level in the well is aligned with the energy of the orbital filled with electrons, and the energy level in the well and the empty energy state of the collector are also aligned, the state of resonance tunneling occurs.
Bias voltage / V molecular RTD / -F characteristics Electromechanical molecular electronic devices There are many types of electromechanical molecular electronic devices, and two examples will now be given for explanation.
Atomic Relay
Atomic relay is similar to a molecular gate switch. In the original appliance, a movable atom is not fixedly attached to the liner, but moves forward or backward between the two electrodes. Hitachi is broken. The third atomic wire very close to the switch atom constitutes the gate of the atomic relay. A small negative charge is placed on the gate wire to move the switch atom away from its original position and turn off the device. To reset the gate, the switch atoms are pulled back to their original positions and the device is turned on again. In actuality of the atomic relay, the "switch" atom can be attached to a "rotor", which can make the "switch" atom fill the gap of the atomic wire by swinging, and make the atomic relay conduct (see (a)); Or make the "switch" atoms swing away from the atomic wires to turn off the current (see (b)). The direction of the rotor is controlled by adjusting the polarity of the charge on the gate molecules (located in the upper part of the figure). There are many types of molecular switching devices, limited to space, only to introduce these.
4 Several Suggestions on the Development of Nanoelectronic Devices must pay attention to and vigorously carry out research on nanodevices, especially nanoelectronic devices. Academician Bai Chunli once proposed that “the level of nanodevice development and application is an important indicator of whether we are entering the nano era†and pointed out that “China must pay attention to the research work of nanodevice development and nanoscale detection and characterizationâ€. According to the current status of nanotechnology development in China, we must vigorously advocate the research, development and application of nanodevices, especially nanoelectronic devices. Because the research of nanoelectronic devices is the fulcrum of nanotechnology and information technology, it plays a vital role in the economy and the entire science and technology.
In the research and development of nanoelectronic devices, in addition to strengthening the research of solid nanoelectronic devices such as RTD and SED, it is also necessary to vigorously carry out research work on molecular electronic devices in a timely manner. Internationally, the United States and Japan attach great importance to the research of molecular electronics.
The world’s top ten scientific and technological progress reported the development of molecular transistors in the United States, saying that Bell Labs used a single organic molecule to make the world’s smallest transistor, which is a molecular electronic device. This kind of chemical organic synthesis method for manufacturing electronic devices is much lower than EB, MBE and other technologies for manufacturing RTD and SED, and it is suitable for large-scale production. Chemists and electronics scientists should be called on to work closely together to conduct research on molecular electronic devices.
Effectively organize the domestic related departments of nanotechnology, especially nanodevice research units, concentrate technical force, key and key issues in cat quasi-nanodevices, avoid duplication of research content, and obtain the results of source innovation as soon as possible. It is hoped that the Nanotechnology Guidance and Coordination Committee can fully and specifically understand the actual situation of domestic nanodevice research units, mobilize the research enthusiasm of each unit, and contribute to the development of nanotechnology.
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