Semiconductors are electron vacancy/hole conduction.
Solid-state electronic devices mainly made of amorphous semiconductor materials. Although the overall molecular arrangement of amorphous semiconductor is disordered, it still has a single crystal microstructure, so it has many special properties. 1975, British W.G. Speer successfully doped amorphous silicon thin films prepared by glow discharge decomposition silane method, which changed the resistivity of amorphous silicon thin films by 10 order of magnitude and promoted the development and application of amorphous semiconductor devices. Compared with single crystal materials, amorphous semiconductor materials have simple preparation process, no special requirements for substrate structure, easy to grow in a large area, and the resistivity changes greatly after doping, so they can be made into various devices. Amorphous silicon solar cells have large absorption coefficient, high conversion efficiency and large area, and have been used in calculators, electronic watches and other commodities. Amorphous silicon thin film field effect transistor array can be used as addressing switch for large area liquid crystal panel. Devices for recording and storing photoelectric information by using the structural transformation of some chalcogenide amorphous semiconductor materials have been applied to computers or control systems. Using the charge storage and photoconductivity characteristics of amorphous films, the photoreceptor of electrostatic copier and the target surface of TV camera tube for photoelectric conversion of static images can be made.
Amorphous materials with semiconductor characteristics. Amorphous semiconductor is an important part of semiconductor. In 1950s, B.T. Kolomiyets and others began to study chalcogenide glasses, and few people paid attention at that time. It was not until 1968 that the patent of S.R. Ovschensky on making switching devices with chalcogenide films was published that people became interested in amorphous semiconductors. 1975, W.E. Speer et al. realized doping effect in amorphous silicon prepared by silane glow discharge decomposition method, which made it possible to control conductance and manufacture PN junction, thus opening up a broad prospect for the application of amorphous silicon materials. Theoretically, P.W. Anderson, Mott and N.F. established the electronic theory of amorphous semiconductors, and won the Nobel Prize in Physics from 65438 to 0977. At present, the research of amorphous semiconductors has developed rapidly both in theory and in application.
Classification At present, there are two main types of amorphous semiconductors.
Sulphur glass. Amorphous semiconductors containing sulfur group elements. For example, As-Se and As-S are usually prepared by melt cooling or vapor deposition.
Tetrahedral bond amorphous semiconductor. Such as amorphous silicon, germanium, GaAs, etc. The amorphous state of this material can only be obtained by thin film deposition (such as evaporation, sputtering, glow discharge or chemical vapor deposition). As long as the substrate temperature is low enough, the deposited film is amorphous. The properties of tetrahedral bond amorphous semiconductor materials are closely related to the preparation process and conditions. Figure 1 Optical absorption coefficient of amorphous silicon prepared by different methods gives the optical absorption coefficient spectrum of amorphous silicon prepared by different processes, in which A and B are prepared by silane glow discharge decomposition method, and the substrate temperature is 500K and 300K respectively, C is prepared by sputtering method, and D is prepared by evaporation method. The conductivity and photoconductivity of amorphous silicon are also closely related to the preparation process. In fact, amorphous silicon prepared by silane glow discharge method contains a lot of H, which is sometimes called amorphous Si-H alloy. Different process conditions and different hydrogen content directly affect the properties of materials. On the contrary, the properties of chalcogenide glasses have little to do with the preparation methods. Fig. 2 The optical absorption coefficient spectrum of vapor deposited sputtered films and melt quenched bulk crystals gives a typical example. The optical absorption coefficient spectrum curves of Shi Ying samples prepared by melt cooling method and sputtering method are the same.
Electronic structure of amorphous semiconductors Amorphous semiconductors and crystalline semiconductors have similar basic energy band structures, including conduction band, valence band and forbidden band (see energy band of solids). The basic energy band structure of materials mainly depends on the situation near atoms, which can be explained qualitatively by chemical bond model. Taking amorphous Ge and Si with tetrahedral bonds as examples, four valence electrons in Ge and Si are hybridized by sp, and valence bonds are formed between valence electrons of adjacent atoms, and their bonding states correspond to valence bands. The antibonding state corresponds to the conduction band. Whether Ge and Si are crystalline or amorphous, the basic bonding mode is the same, but the bond angle and bond length are distorted to some extent in amorphous state, so their basic energy band structures are similar. However, in amorphous semiconductors, there are essential differences between electronic states and crystalline states. The structure of crystalline semiconductor is periodic and orderly, or it has translational symmetry. The electron wave function is a Bloch function, and the wave vector is a quantum number related to translational symmetry. Amorphous semiconductors have no periodicity and are no longer good quantum numbers. The movement of electrons in crystalline semiconductors is relatively free, and the average free path of electron movement is much larger than the atomic spacing. The distortion of structural defects in amorphous semiconductors greatly reduces the average free path of electrons. When the average free path is close to the order of magnitude of atomic spacing, the concept of electron drift motion established in crystalline semiconductors becomes meaningless. The change of state density at the edge of the energy band of amorphous semiconductor is not as steep as that of crystalline semiconductor, but there are different degrees of band tails (as shown in the relationship between state density and energy of amorphous semiconductor in Figure 3). The electronic States in the energy band of amorphous semiconductors can be divided into two categories: one is called extended state and the other is called localized state. Every electron in an extended state is occupied by the whole solid and can be found on the whole scale of the solid; Its motion in the external field is similar to that of electrons in crystals. Each electron in a local state is basically confined to a certain region, and its state wave function can only be significantly different from zero in a small scale near a certain point. They need the help of phonons for jump conduction. In the energy band, the central part of the band is an extended state, and the tail part of the band is a local state. There is a boundary between them, such as the sum of the extended state, localized state and mobility edge of the amorphous semiconductor in Figure 4. This boundary is called the mobile edge. In 1960, Mott first put forward the concept of mobile edge. If mobility is regarded as a function of electronic state energy, Mott thinks that there is a sudden change in mobility at the boundary. Electrons in localized state are jump-conducting, and they jump from one localized state to another by exchanging energy with lattice vibration, so when the temperature tends to 0K, the mobility of electrons in localized state tends to zero. The electron conduction in the extended state is similar to that in the crystal, and the mobility tends to a finite value when it approaches 0 K. Mott further thinks that the mobility edge corresponds to the situation that the average free path of electrons approaches the atomic spacing, and defines the conductivity in this case as the minimum metallization conductivity. However, there is still controversy around the mobility edge and the minimum metallization conductivity.
Compared with crystalline semiconductor, defective amorphous semiconductor has many defects. These defects introduce a series of local energy levels into the band gap, which have an important influence on the electrical and optical properties of amorphous semiconductors. Tetrahedral bond amorphous semiconductor and chalcogenide glass have significant differences in defects.
The defects in amorphous silicon are mainly vacancies and microcavities. There are four valence electrons in the outer layer of silicon atom, which should normally form four valence bonds with four adjacent silicon atoms. There are vacancies and micro-voids, which make four adjacent atoms around some silicon atoms insufficient, resulting in some hanging bonds, and there is an unbound electron on the neutral hanging bond. There are two possible charged states of dangling bonds: releasing unbound electrons to become positive charge centers, which are donor states; Accepting the second electron as the negative center is the acceptor state. Their corresponding energy levels are within the forbidden band, which are called donor energy levels and acceptor energy levels respectively. Because the acceptor state indicates that there are two electrons in the dangling bond, the Coulomb repulsion between the two electrons makes the acceptor level higher than the donor level, which is called positive correlation energy. Therefore, in general, the dangling bond is kept in a neutral state occupied by only one electron, and the spin * * * vibration of unpaired electrons on the dangling bond is observed in the experiment. In 1975, Speer et al. first realized the doping effect of amorphous silicon by silane glow discharge, because the amorphous silicon prepared by this method contains a lot of hydrogen, and the combination of hydrogen and dangling bonds greatly reduces the number of defect states. These defects are also effective recombination centers. In order to improve the lifetime of unbalanced carriers, the density of defect States must also be reduced. Therefore, controlling defects in amorphous silicon has become one of the key problems in material preparation.
The form of defects in chalcogenide glasses is not a simple dangling bond, but a valence pair. At first, it was found that chalcogenide glasses were different from amorphous silicon, and the spin vibration of defective electrons could not be observed. Aiming at this seemingly abnormal phenomenon, Mott and others put forward MDS model according to Anderson's negative correlation energy hypothesis. When the defect state occupies two electrons, it will cause lattice distortion. If the energy reduced by twisting exceeds the Coulomb repulsion energy between electrons, it will show negative correlation energy, that is, the acceptor level is below the donor level. D, D and D respectively represent the states of non-possession, one-possession and two-possession of electrons on the defect. Negative correlation may mean:
2D——→D+D
It gives off heat. Therefore, defects mainly exist in the form of D and D, and there are no unpaired electrons, so there is no spin vibration of electrons. Many people have analyzed the structure of D, D and D defects. Take amorphous selenium as an example. Selenium has six valence electrons, which can form two valence bonds, usually in a chain structure, and the other two unbound P electrons are called lone pair. There is a neutral dangling bond at the end of the chain, which is likely to be twisted, and it will combine with the adjacent lone pair bond to release an electron (D type), and the released electron will combine with another dangling bond to form a pair of lone pairs (D type), as shown in Figure 5. So this D and D are also called commutative pairs. Due to Coulomb gravity, D and D usually come together in pairs to form a close valence pair. As long as the bonding mode in chalcogenide glasses changes slightly, a group of close valence pairs can be formed, as shown by the valence pair self-reinforcing effect in Figure 6. This self-reinforcing effect requires little energy and has a self-reinforcing effect, so the concentration of such defects is usually high. The valence pair model can be used to explain a series of experimental phenomena such as photoluminescence spectrum and photoelectron spin * * * vibration of chalcogenide amorphous semiconductors.
The application of amorphous semiconductor has great potential in the technical field. Amorphous sulfur has been widely used in copying technology for a long time. The As-Te-Ge-Si glass semiconductor pioneered by S.R. Ovsinski has been produced commercially, and the optical memory made by using the characteristics of light pulse vitrified tellurium microcrystalline film is under development. At present, the most researched application of amorphous silicon is solar cells. Compared with crystalline silicon, the preparation process of amorphous silicon is simpler and easy to make in large area. Amorphous silicon has a high efficiency of absorbing sunlight, and the device only needs a thin film material with a thickness of about 1 micron. Therefore, it is expected to be made into cheap solar cells, which has attracted the attention of energy experts. Recently, some people tried to use amorphous silicon field effect transistors in liquid crystal displays and integrated circuits.