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The semiconductor material is a kind of electronic materials with semiconductor properties and can be used to make semiconductor devices and integrated circuits. Various external factors such as light, heat, magnetism, and electricity will act on semiconductors and arouse some physical effects and phenomena, which can be referred to as the semiconductor properties. The majority of the base materials constituting solid-state electronic devices are semiconductors. Different types of semiconductor devices have different functions and characteristics because of the various semiconductor properties.
Substances and materials in nature can be divided into three categories: conductors, semiconductors, and insulators according to their conductive capability. The resistivity of a semiconductor is between 1 mΩ·cm to 1 GΩ·cm. In general, the conductance of a semiconductor increases with temperature, which is opposite to the metal conductor.
All materials with the above two characteristics can be regarded as semiconductor materials. Various external factors such as light, heat, magnetism, and electricity will act on semiconductors and arouse some physical effects and phenomena, which can be referred to as the semiconductor properties. The majority of the base materials constituting solid-state electronic devices are semiconductors. Different types of semiconductor devices have different functions and characteristics because of the various semiconductor properties.
The basic chemical characteristic of semiconductors is the saturated covalent bonds between atoms. The covalent bond has a tetrahedral lattice structure, so typical semiconductor materials have a diamond or sphalerite (ZnS) structure. Since most minerals on the earth are compounds, the earliest semiconductor materials available were compounds. For example, Galena (PbS) was used for radio detection very early, cuprous oxide (Cu2O) was used as a solid rectifier, sphalerite (ZnS) ) is a well-known solid-state luminescence material, and silicon carbide (SiC) is applied for rectification and detection.
Figure 1. Tetrahedron Structures of Covalent Bond
Selenium (Se) is the first discovered and used elemental semiconductor, and was an important material for solid rectifiers and photovoltaic cells. Electronic devices began to be transistorized after the discovery of elemental semiconductor germanium (Ge). The use of elemental semiconductor silicon (Si) has not only increased the types and improved the performance of transistors, but also brought large-scale and ultra-large-scale integrated circuits into the world. Besides, the discovery of III-V compounds represented by gallium arsenide (GaAs) has promoted the rapid development of microwave and optoelectronic devices.
Semiconductor materials can be divided according to the chemical composition, and the amorphous and liquid semiconductors with special structures and properties are separately classified into a category. Based on this classification method, semiconductor materials can be divided into elements, inorganic, organic, and amorphous, and liquid semiconductor materials.
Eleven types of semiconducting elements are distributed in the IIIA to IVA groups of the periodic table. C, P, Se have two forms of insulator and semiconductor; B, Si, Ge, Te have semiconductivity; Sn, As, Sb have two forms of semiconductor and metal. The melting point and boiling point of P are too low, and the vapor pressure of I is too high, which makes it easy to decompose, so they have little practical value. The stable states of As, Sb, and Sn are metals, and the semiconductors are unstable states. B, C, and Te have not been used because of difficulties in preparation and performance limitations. Therefore, only Ge, Si, and Se have been used among these 11 element semiconductors. And Ge and Si are the most widely used materials in all semiconductor materials.
This kind of semiconductor material can be subdivided into the binary system, ternary system, quaternary system, and so on.
① Groups IV-IV: Both SiC and Ge-Si alloys have a sphalerite structure.
② Group III-V: It is composed of group III elements Al, Ga, In and group V elements P, As, and Sb. The typical representative is GaAs. These elements all have sphalerite structure, and are second only to Ge and Si in applications, which have great development prospects.
Figure 2. Bandgaps of group V and the group III-V binary semiconductor materials as a function of the cubic lattice parameter
③ Group II-VI: They are the compounds formed by group II elements Zn, Cd, Hg, and group VI elements S, Se, Te, which are important optoelectronic materials. ZnS, CdTe, and HgTe have a sphalerite structure.
④ Group I-VII: Compounds formed by group I elements Cu, Ag, Au and group VII elements Cl, Br, I, among which CuBr and CuI have a sphalerite structure.
⑤ Groups V-VI: Compounds formed by group V elements As, Sb, Bi and VI elements S, Se, Te, such as Bi2Te3, Bi2Se3, Bi2S3, As2Te3, etc., which are important thermoelectric materials.
⑥ The oxides of Group B and transition group elements Cu, Zn, Sc, Ti, V, Cr, Mn, Fe, Co, Ni in the fourth cycle are the main thermistor materials.
⑦ Compounds of certain rare earth elements Sc, Y, Sm, Eu, Yb, Tm, and V elements N, As, or group VI elements S, Se, Te.
In addition to these binary system compounds, there are solid-solution semiconductor materials such as Si-AlP, Ge-GaAs, InAs-InSb, AlSb-GaSb, InAs-InP, GaAs-GaP, and the like. The study of these solid solutions can play a significant role in improving certain properties of a single material or opening up new applications.
Figure 3. A binary phase diagram displaying solid solutions over the full range of relative concentrations
Group ①: It is composed of a group II and a group IV atom to replace two group III atoms in group III-V, such as ZnSiP2, ZnGeP2, ZnGeAs2, CdGeAs2, CdSnSe2, and the like.
Group ②: One group I atom and one group III atom are in the place of two group II atoms in group II-VI like CuGaSe2, AgInTe2, AgTlTe2, CuInSe2, CuAlS2, etc.
Group ③: This is composed of one group I atom and one group V atom instead of two group III atoms, such as Cu3AsSe4, Ag3AsTe4, Cu3SbS4, Ag3SbSe4, etc.
In addition, there are quaternary compound materials such as Cu2FeSnS4 and inorganic compounds with more complex structures.
Naphthalene, anthracene, polyacrylonitrile, phthalocyanine, and some aromatic compounds are all well-known organic semiconductors, but they have not yet been used as semiconductor materials.
The amorphous and liquid semiconductor material does not have a crystalline structure with a strictly periodic arrangement, which is greatly different from the crystalline semiconductor is that
Figure 4. Structural Models of Silicons
Although there are many types of semiconductor materials, they have some inherent properties, which are called characteristic parameters of semiconductor materials. These parameters can reflect the differences between semiconductor materials and other non-semiconductor materials, but more importantly, reflect the quantitative differences in the characteristics of various semiconductor materials and even the same material in different situations.
The characteristic parameters of commonly used semiconductor materials are:
It is determined by the electronic state and atomic configuration of the semiconductor, reflecting the energy that makes the valence electrons in the atoms be excited from a bound state to a free state.
They represent the conductivity of the material. Carriers are the electrons and holes participating in the conduction of semiconductors.
It denotes the relaxation property of the internal carriers trans from the non-equilibrium state to the equilibrium state under the external action (such as light or electric field).
Dislocations are the most common types of crystal defects. Dislocation density can be used to measure the degree of lattice integrity of semiconductor single-crystal materials. Of course, there is no such characteristic parameter for amorphous semiconductors.
Figure 5. Crystal Defects
According to the working principle of transistors, materials are required to have a large non-equilibrium carrier life and carrier mobility. Transistors made of materials with high carrier mobility can work at higher frequencies with better frequency response. Crystal defects can affect the properties of a transistor or even cause it to fail. The operating temperature limit of the transistor is determined by the size of the bandgap. The larger the forbidden bandwidth, the higher the temperature limit for the normal operation of the transistor.
The radiation frequency range applicable to radiation detectors that use the photoconduction (increased conductance after illumination) of semiconductors is related to the bandgap of the material. The longer the non-equilibrium carrier lifetime of the material, the higher the sensitivity of the detector, and the longer the relaxation time of the detector. Therefore, it is difficult to balance high sensitivity and short relaxation time.
For solar cells, in order to obtain high conversion efficiency, the material with a large non-equilibrium carrier lifetime and a moderate band gap (between 1.1 and 1.6 electron volts) is required. Crystal defects can greatly reduce the luminous efficiency of the semiconductor light-emitting diodes and semiconductor laser diodes.
Figure 6. Bandgap& Efficiency of Solar Cells
In order to improve the conversion efficiency of thermoelectric devices, there should be a large temperature difference between the two ends of the device. When the temperature at the low temperature (usually the ambient temperature) is fixed, the temperature difference is determined by the high temperature, which is the operating temperature of the thermoelectric device. Also, to adapt to the high operating temperature, the forbidden bandwidth of the material should not be too small, and large electromotive power, a small resistivity, and a small thermal conductivity are required.
The size of the characteristic parameters of the semiconductor material has a great relationship with the impurity atoms and crystal defects in the material. For instance, resistivity may vary widely in different types and numbers of impurity atoms, while carrier mobility and non-equilibrium carrier life generally decrease with the increase of impurity atoms and crystal defects.
On the other hand, the various semiconductor properties of semiconductor materials are inseparable from the role of various impurity atoms. Generally, we should reduce and eliminate crystal defects as much as possible, but in some cases, it is also desirable to control them to a certain level, and even when defects already exist, they can be used after appropriate treatment.
In order to limit and utilize impurity atoms and crystal defects, it is necessary to develop a set of methods for preparing satisfactory semiconductor materials, which is the so-called semiconductor material technology. These processes can be roughly summarized as purification, single crystal preparation, and thin-film epitaxial growth. We'll mainly discuss the process of purification and thin-film expitaxial growth.
The purification of semiconductor materials is mainly to remove impurities from materials. Purification methods can be divided into chemical and physical purifications.
Chemical purification is to make materials into intermediate compounds to systematically remove certain impurities, and finally remove the materials (elements) from a compound that is easily decomposed.
Figure 7. Traditional Chemical Route for Silicon Purification.
Physical purification is commonly used in the area of melting technology. The semiconductor material is cast into an ingot, and the melting region with a certain length is formed from one end of the ingot. Because of the segregation of impurities during the solidification process, after the melting zone moves repeatedly from one end to the other, impurities are concentrated at both ends of the ingot. When the two ends are removed, the rest is a material with higher purity. Besides, there are physical methods such as vacuum evaporation and vacuum distillation. Germanium and silicon are the highest-purity semiconductor materials that can be obtained, and the proportion of major impurity atoms can be less than one of ten billion.
Most semiconductor devices are made on a single wafer or an epitaxial wafer with a single wafer as the substrate. Semiconductor single crystals are made by the melt growth method. The czochralski method is the most widely used. 80% of the silicon single crystal, most of the germanium single crystal, and indium antimonide single crystal are produced by this method, among which the maximum diameter of the silicon single crystal has reached 300 mm. The czochralski method with a magnetic field in the melt is called a magnetron czochralski method, by which a silicon single crystal with high uniformity can be produced. Adding a liquid covering agent to the surface of the crucible melt is called the liquid-sealed czochralski method, which is applied for a single crystal with a high dissociation pressure such as gallium arsenide, gallium phosphide, and indium phosphide.
Figure 8. Czochralski Process Czochralski Process
The melt in the floating zone melting method does not contact the container, which can produce a high-purity silicon single crystal. Horizontal zone melting is used to produce germanium single crystals. The horizontal oriented crystallization method is mainly used for preparing gallium arsenide single crystal, and the vertical oriented crystallization method is used for cadmium telluride and gallium arsenide.
Figure 9. Schematic of a float zone refining system
After the single crystals are produced, they should be sent for crystal orientation, barrel grinding, reference surface, slicing, grinding, chamfering, polishing, etching, cleaning, inspection, packaging, etc. to be made into corresponding wafers.
The growth of a single crystal thin film on a single crystal substrate is called epitaxy. Epitaxial methods include gas phase, liquid phase, solid phase, and molecular beam epitaxy. Industrial production mainly uses chemical vapor phase epitaxy, followed by liquid phase epitaxy. Vapor phase epitaxy and molecular beam epitaxy of metallorganic compounds are used to prepare microstructures such as quantum wells and superlattices. Amorphous, microcrystalline, and polycrystalline thin films are mostly made on glass, ceramic, metal, and other substrates by different chemical vapor deposition, magnetron sputtering, and other methods.
The semiconductor material industry has four characteristics:
According to SEMI (Semiconductor Equipment and Materials Association) statistics, the global semiconductor material industry market size in 2016 reached $ 44.3 billion, making up nearly 15% of the global semiconductor industry size in 2016, which is approximately $ 300 billion.
Semiconductor materials have the most subdivided fields in the semiconductor industry chain. Among them, wafer materials include silicon wafers, photoresists, photoresist supporting reagents, wet electronic chemicals, electronic gases, CMP polishing materials, and target materials. Chip packaging materials include package substrates, lead frames, resins, bonding wires, solder balls, and plating solutions. At the same time, wet electronic chemicals also include various reagents such as acids and alkalis, producing hundreds of subdivided industries.
Figure 10. Silicon Wafer Discs
Generally, the technical threshold of semiconductor materials is higher than that of other materials in the electronics and manufacturing fields. It has high purity requirements and complex processes. And in the R&D process, batch testing is required for the downstream production lines. Also, for different chip manufacturing processes, the downstream manufacturers have different material requirements, resulting in different parameters of the materials.
Although the overall industry scale of semiconductor materials is huge, due to the numerous sub-industries of sub-materials, single sub-materials often account for a relatively low proportion in the cost of semiconductor production. Taking the target material as an example, the proportion of semiconductor targets in semiconductor materials is about 3%, and the production cost is only 3 ‰ to 5 ‰ of the semiconductor materials.
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