amorphous materials with semiconductor properties. Amorphous semiconductor is an important part of semiconductor. In the 195s, B.T. Kolomiyets and others began to study chalcogenide glasses, and few people paid attention to it at that time. It was not until the publication of the patent of S.R. Ovschensky on making switching devices with chalcogenide films in 1968 that people became interested in amorphous semiconductors. In 1975, W.E. Speer and others realized the doping effect in amorphous silicon prepared by silane glow discharge decomposition, which made it possible to control conductance and manufacture PN junction, thus opening up a broad prospect for the application of amorphous silicon materials. In theory, P.W. Anderson, Mott and N.F. established the electronic theory of amorphous semiconductors, and won the Nobel Prize in Physics in 1977. At present, the research of amorphous semiconductors is developing rapidly both in theory and in application.
classification there are two main types of amorphous semiconductors at present.
chalcogenide glass. Amorphous semiconductors containing sulfur group elements. For example, As-Se and As-S, the usual preparation method is melt cooling or vapor deposition.
tetrahedral bond amorphous semiconductor. Such as amorphous Si, Ge, GaAs, etc., the amorphous state of such materials can only be obtained by thin film deposition (such as evaporation, sputtering, glow discharge or chemical vapor deposition, etc.). As long as the substrate temperature is low enough, the deposited thin film is amorphous. The properties of tetrahedral bond amorphous semiconductor materials are closely related to the preparation process and conditions. Fig. 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, the substrate temperature is 5K and 3K respectively, C is prepared by sputtering, and D is prepared by evaporation. 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 spectra of vapor-deposited sputtered thin films and melt-quenched block Assite give a typical example. The optical absorption coefficient spectra of Assite samples prepared by melt cooling and sputtering have the same curve.
electronic structure of amorphous semiconductors amorphous 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 neighboring atoms, and their bonding state corresponds 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 the amorphous state, so their basic energy band structures are similar. However, there are essential differences between the electronic states and the crystalline states in amorphous semiconductors. The structure of crystalline semiconductor is periodic and orderly, or it has translational symmetry. The electron wave function is Bloch function, and the wave vector is the quantum number associated with translational symmetry. Amorphous semiconductor does not have periodicity and is no longer a good quantum number. 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 approaches 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 has different degrees of band tail (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 localized state. Every electron in the extended state is possessed by the whole solid and can be found in the whole scale of the solid; Its motion in an external field is similar to that in a crystal.