The so-called semiconductor heterostructure means that semiconductor films of different materials are sequentially
heterojunction
Sequentially deposited on the same substrate. For example, figure 1 describes the basic structure of a laser made of a semiconductor heterostructure. The basic characteristics of semiconductor heterostructures are as follows.
(1) quantum effect: because of the low energy level of the intermediate layer, electrons are easily dropped and confined in the intermediate layer, which can be only a few tens of angstroms (1 angstrom = 10- 10 meter) thick, so in such a small space, the characteristics of electrons will be changed by quantum effect. For example, the quantization of energy level, the increase of ground state energy, the change of energy state density and so on. Among them, the density of energy States and the position of energy levels are very important factors to determine the electronic characteristics.
(2) Mobility increases: Free electrons in semiconductors are mainly due to the contribution of external impurities, so in general semiconductor materials, free electrons will be collided by impurities, and the mobility will be reduced. In the heterostructure, impurities can be added to the intermediate layers on both sides, and the electrons contributed by impurities will fall to the intermediate layer because of low energy (as shown in Figure 3). Therefore, electrons and impurities are separated in space, so the action of electrons will not be limited by the collision of impurities, so their mobility can be greatly increased, which is the basic element of high-speed components.
(3) Singular two-dimensional characteristics: Because electrons are bound in the middle layer and cannot move freely along the direction of the middle layer, electrons have only two degrees of freedom, and semiconductor heterostructure provides a very good physical system for studying low-dimensional physical characteristics. The electronic characteristics in low dimension are quite different from those in three dimension, such as the increase of electron binding energy, the increase of electron-hole recombination rate, quantum Hall effect, fractional Hall effect [1] and so on. Scientists have made various components by using low-dimensional characteristics, including high-speed photoelectric components in optical fiber communication. Quantum and fractional Hall effects won the Nobel Prize in physics respectively.
(4) Artificial material engineering: the intermediate layer or intermediate layer on both sides of the semiconductor heterostructure can be changed according to different needs. For example, in the case of gallium arsenide, gallium can be replaced by aluminum or indium, and arsenic can be replaced by phosphorus, antimony or nitrogen. In this way, the characteristics of the designed materials are varied, so the term artificial material engineering appears. Recently, scientists have replaced gallium with manganese atoms and found that it is ferromagnetic, which has aroused great concern, because future semiconductor components may make use of the characteristics of electron spin. In addition, in semiconductor heterostructure, if the atomic spacing between two adjacent layers is different, the arrangement of atoms will be forced to be the same as that of the lower layer, so stress will occur between atoms, thus changing the energy band structure and behavior of electrons. Now stress can be controlled by crystal growth technology, so scientists have an extra factor to adjust semiconductor materials and produce more novel components, such as silicon-germanium heterostructure high-speed transistors.
2 application role editing
Luminous assembly
Because the semiconductor heterostructure can confine electrons and holes in the middle layer, the recombination rate of electrons and holes increases, so the luminous efficiency is higher; At the same time, changing the width of quantum well can also control the frequency of light emission, so the current semiconductor light-emitting elements are mostly composed of heterostructures. Compared with other light-emitting elements, semiconductor heterostructure light-emitting elements have the advantages of high efficiency, power saving and durability, so they are widely used in brake lights, traffic lights, outdoor display lights and so on. It is worth mentioning that in 1993, Japanese science
heterojunction
Scientists have developed semiconductor elements for blue light, so that the three primary colors of light, red, green and blue, can be made of semiconductors, so all colors can be obtained through semiconductor light-emitting elements. No wonder everyone predicts that household light bulbs and fluorescent lamps will soon be replaced by semiconductor light-emitting elements.
Laser diode
The basic structure of semiconductor laser diode is very similar to the above-mentioned light-emitting module, but the conditions of stimulated emission and * * * vibration must be considered in laser diode. Using semiconductor heterostructure, because electrons and holes are easy to fall into the intermediate layer, it is easy to realize the inversion of particle number, which is a necessary condition for laser exposure, and because electrons and holes are confined in the intermediate layer, their binding rate is high. In addition, the refractive index of the interlayer on both sides is different from that of the intermediate layer, so the light can be confined to the intermediate layer, so that the light will not be lost and the laser intensity will be increased, which is beneficial to the fabrication of lasers with heterogeneous structures and has great advantages. The first semiconductor heterostructure laser emitting continuously at room temperature was made by the research group led by Afalov in 1970, while Clem developed the principle of semiconductor heterostructure laser in 1963. Semiconductor laser diodes are also widely used, such as optical disks (as shown in Figure 4), high-speed optical fiber communication, laser printers, laser pens and so on.
Heterostructure bipolar transistor
In semiconductor heterostructure, the intermediate layer has a lower energy band, so electrons can be easily injected from adjacent layers, so the current from emitter to collector in the transistor can be greatly improved, and the amplification factor of the transistor is also increased. At the same time, the thickness of the base can be reduced and the doping concentration can be increased, so that the reaction rate can be improved, so that the heterostructure can be made into a fast transistor. In 1957, Kerram put forward the suggestion of making transistors with semiconductor heterostructures and analyzing their characteristics. Semiconductor heterojunction bipolar transistor is widely used in satellite communication or mobile phone because of its advantages of high speed and high magnification.
High speed electron mobility transistor
High-speed electron mobility transistor takes advantage of the spatial separation of impurities and electrons in semiconductor heterostructure, so electrons can have high mobility. In this structure, the current from the source to the drain can be controlled by changing the voltage of the gate, so as to achieve the purpose of amplification. Because this module has the advantages of high corresponding frequency (600GHz) and low noise, it is widely used in infinite communication with space (as shown in Figure 5) and astronomical observation.
Other applications
Semiconductor heterostructures are not only used in the above components, but also widely used in other photoelectric components, such as photodetectors, solar cells, standard resistors or photoelectric modulators. Due to the development of crystal growth technology, the thickness of single-layer atomic film can be controlled, so semiconductor heterostructure provides a high-quality low-dimensional system, which enables scientists to meet the requirements of exploring low-dimensional phenomena. In addition to observing quantum and fractional quantum Hall effects in two-dimensional space, scientists are further exploring the electronic behavior in one-dimensional and zero-dimensional heterostructures. It is expected that new phenomena will be discovered one after another in the future, and more novel heterostructure components will appear.