Solid nanoelectronic devices and molecular devices

Since nanotechnology and information technology, material science and technology, and biotechnology are listed as the four major science and technology in the 21st century, there has been a wave of nanotechnology research and development both internationally and domestically.

Especially in China, "nano" has become one of the hottest terms and one of the hot topics that every household talks about. However, Nanoscientific put forward the development policy of "comprehensive understanding of the connotation and promote healthy development" for nanotechnology in a timely manner, pointing out that the understanding of nanodevices, especially nanoelectronic devices, and the vigorous development of nanoelectronic devices are a very important issue. For example, what is a nanodevice, what is a nanoelectronic device, how to classify a nanoelectronic device, what position does a nanoelectronic device play in nanotechnology, what issues should be paid attention to in the development of nanotechnology, and so on. The purpose of this article is to try to answer the above questions and introduce two main types of nanoelectronic devices-solid nanoelectronic devices and molecular electronic devices, so that readers have a comprehensive and correct understanding of nanoelectronic devices. On the other hand, there are different opinions on the classification methods of nanoelectronic devices in the world and domestic academic circles. This article is willing to serve as a "introducer" to provoke academic discussions on nanoelectronic devices. Development is very beneficial.

Nano-devices can be considered as nano-scale processing and preparation techniques (such as metal organic compound deposition technology (M0CVD), molecular beam epitaxy (MBE), electron beam technology (EB), scanning probe microscope (SPM), nano-material preparation Methods (self-assembly growth, molecular synthesis), etc., design and prepare devices with nanoscale (1-100 nm) scale and certain functions.

About how to divide nano devices and nano electronic devices, nano devices! Resonant tunneling device, quantum dot device, solid nanoelectronic device, nanoelectronic device, "molecular electronic device. The scope of nanoelectronic devices is limited to the following two conditions: (1) The working principle of the device is based on quantum effects; (2) Both have a similar typical device structure-a tunnel barrier surrounding an "island (or well)) structure. According to these two conditions, like nano-CMOS devices, nano-magnetic devices, nano-electromechanical systems (NEMS), etc. Although the device itself is on the order of nanometers, it has also been developed using nano-processing technology, but they can only be attributed to nano-devices. It is not in the category of nanoelectronic devices in the general category. According to these two conditions, nanoelectronic devices can be divided into two categories: one is solid nanoelectronic devices, which mainly include resonant tunneling devices (resonant tunneling diode RTD and resonant tunneling transistor RTT), quantum dot (QD) devices and Single electronic device (SED); the other is molecular electronic device, mainly including quantum effect molecular electronic device and electromechanical molecular electronic device. The above classification method is shown 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.) p Single Electron Crystal Single Electronic Devices "Single Tube Storage] Quantum Effect Molecular Electronic Devices Electromechanical Molecular Electronics Device 2 Solid Nanoelectronic Devices 2.1 Classification of Solid Nanoelectronic Devices Solid nanoelectronic devices are a major branch of nanoelectronic devices. Its emergence is related to the development of nano-preparation, processing technology, mesoscopic physics and microelectronic technology The various "limits" facing in the world are inseparable. It has a quantum effect and tunneling barrier surrounding the "island" (potential well) device structure. In this device structure, due to the specific dimensions of the "island" or potential well Unlike the quantized "dimension", solid nanoelectronic devices can be divided into the three types of nanoelectronic devices shown in Table 2.

Table 2 Classification Table of Solid Nanoelectronic Devices Name Quantum Dimensional Device Working Mechanism The relationship between device terminal numbers! -Characteristic resonant tunneling devices, quantum wire one-dimensional or two-dimensional quantized quantum resonant tunneling effect Both ends or three-terminal single-electron transistor three-dimensional are nanometers, but none of them have quantized Coulomb blocking effect Three-terminal quantum dot devices Quantization, the island is composed of quantum dots. The quantization effect at both ends of the 2.2 potential well. The dimension of the quantization of the potential well and the energy distribution in the potential well. The solid nanoelectronic device is surrounded by the above barrier and injected into the island through the tunneling barrier Then move to the collector through another barrier. For three-terminal devices, the gate voltage controls the amount of current flowing through the device. In this case, the characteristics of the device are directly related to the distribution of electron energy in the potential well. The energy distribution in the potential well depends on the dimension of the potential well quantization and the three-dimensional scale of the potential well. The resonant tunneling device in Table 2 has only one-dimensional (potential well width) scale of several nanometers, and quantum wire devices have two-dimensional (two directions perpendicular to the "line") scale of several nanometers. The scale in the rest of their directions is on the order of microns. Because significant quantization occurs only on the scale of a few nanometers, RTD quantization occurs in only one dimension, and quantum wires quantize in two dimensions. Quantum dot devices have a three-dimensional scale of several nanometers, so quantization occurs in three dimensions. Although single-electron devices have a much smaller three-dimensional scale than conventional devices (tens or hundreds of nanometers), there is no one-dimensional device that can reach the order of a few nanometers (the same direction as the electronic motion), so the quantization dimension is zero . The result of quantization is that discrete energy levels appear in "islands" or potential wells. The stronger the quantization, the greater the energy between the energy gaps. On the other hand, the barrier and potential well structure can be regarded as an isolated electronic system. The electrons in the potential well can stably stay in the well without escaping, and their energy must be 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 it has! Energy is possible, this! Called charging energy. ! Related to the three-dimensional scale of the potential well, the smaller the volume of the potential well, the closer the charges are in the well, and the stronger the interaction, the larger the potential. Conversely, the larger the volume of the trap, the smaller the value. RTD or quantum wire devices are quantized in one or two dimensions, which is larger, but larger and smaller, so 4e! ! , Play a leading role. The three-dimensional dimensions of D are all on the order of a few nanometers, and all quantization occurs, so be reconciled! Both are larger and higher ", and the three-dimensional SED can not reach the quantized scale, so it is smaller, but its total volume is much smaller than conventional devices, the larger, 4e, plays a leading role.

In summary, for RTD or quantum wire devices, the energy distribution of electrons in the potential well is a large discrete energy level. When resonance tunneling occurs at the electron "below the electron and the ground state energy level in the well", a current peak appears in the characteristic. When the voltage increases, the electron and the first excited state energy level undergo resonance resonance tunneling, /- The second current peak appears on the% characteristic. Due to the larger value, the corresponding voltage shift between the two peaks is larger. For the SED case, 4e, the energy distribution in the potential well is present! On a large basis, the quasi-continuous distribution with small energy gaps is very small. Ignoring the effect, electrons must be provided from the emitter! The energy can only enter the potential well, so the /-% characteristic is a smooth large step (see Table 2); for the case of D, "4e, the energy distribution in the potential well is based on the energy gap and! For the energy level distribution with the same magnitude, the-% characteristic of the large step set and small step shown in Table 2 appears accordingly.

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 the earliest, most studied, most in-depth and relatively mature Solid nanoelectronic devices. At present, high-speed digital circuits containing 2000 RTDs have been developed abroad. Its basic structure 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 layer as the potential well. The barrier width is 1.5 ~ 3.0nm, and the potential well width is 3.0 ~ 5.0nm, which is prepared by 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 It falls between the bottom of its conduction band "c" ((b)), when "F's electron energy coincides with", satisfies energy conservation and lateral momentum conservation, resonance tunneling occurs, and tunneling current appears. With the 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 oscillator and mixer, 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-electronic devices Single-electronic devices include single-electron transistors (SET) and single-electron memory (SEM). SET is more common. This article only introduces SET. 2.4.1 The 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. Apply 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) obtains 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 of 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.

The power consumption is very small: 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 occupy a dominant position in nanoelectronic devices, some devices (such as RTD) have entered the stage of application, but in general they exist The following questions.

3.1.1 Problems with RTD RTD is one of the fastest devices at present, 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 characterization”. 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.