English
In the semiconductor field, silicon materials have firmly occupied the C-suite, both in the past decades and in the foreseeable future. It can be processed into wafers, silicon electrodes for etching, silicon boats and other components, silicon targets for sputtering coating, silicon window sheets and other silicon products.
Silicon in the semiconductor field can obtain today's position, thanks to some of its own unique advantages: abundant raw materials, accounting for 26% of the content of the earth's crust; environmentally friendly, completely non-toxic; low cost; the surface of the formation of SiO2 is easy to form such a highly structurally stable insulating layer; the device operating temperature is higher, up to 250 ℃; critical shear stress is large, easy to grow dislocation-free single crystals and so on.
In nature, mainly in the form of oxides mainly in the state of compounds exist. The chemical properties of these compounds at room temperature is very stable. At high temperatures, silicon can be almost all substances undergo chemical reactions. Such as Si and oxygen react at high temperatures to generate SiO2, Si and water (H2O) at high temperatures to generate SiO2 and hydrogen.
At room temperature, the forbidden band width of silicon is 1.12eV, theoretically does not absorb infrared light. Monocrystalline silicon in the infrared band refractive index of 3.5. high-purity silicon in the near-infrared band (1.1-1.5 μm) is almost transparent, and therefore can be used to make near-infrared lenses. And when doped in silicon, the optical properties change as the doping concentration changes. As shown in the figure below for N-type silicon, the absorption of light by carriers increases as the doping concentration increases.
However, silicon is an indirect bandgap semiconductor and therefore cannot be used for lasers and light emitting tubes. In addition, silicon has no linear photoelectric effect and therefore cannot be used as a modulator or photoelectric switch.
Silicon crystal hardness, tensile stress, but no ductility at room temperature, is a typical brittle material, very fragile. Therefore, the additivity is poor. As the temperature increases, the yield stress of silicon gradually decreases. Its brittle to plastic transition temperature of about 780 ℃.
High-purity silicon single crystal, the vast majority of its atoms are covalent bonds with four neighboring atoms. There is almost no excess electrons and holes within the crystal, there is no electrical conductivity. However, once other impurity elements are present within the crystal, it is likely that additional electrons and holes (collectively known as carriers) will be introduced, leading to an increase in electrical conductivity. With this in mind, researchers have devised processes to control the electrical properties of silicon by doping it in single crystals. According to the differences in doping elements, two typical types of doped silicon are formed, as shown in the figure below, namely P-type Si and N-type Si. Among them, doping boron (B) elements in silicon single crystals introduces holes and obtains P-type Si, while doping phosphorus (P) elements in silicon single crystals introduces electrons and obtains N-type Si. Undoped high-purity silicon is called intrinsic silicon, and its electrical conductivity is very poor. The theoretical value of resistivity at room temperature is about 230 KΩ-cm. and after proper doping, the resistivity of silicon single crystal will be significantly reduced. If one part per million of phosphorus is doped into a silicon single crystal, its resistivity will drop from about 230KΩ-cm to about 0.2KΩ-cm.
According to the doping concentration, doped silicon can be categorized as lightly doped and heavily doped. Among them, the former doping concentration <1×10^16 (/cm3), room temperature dopant can be considered all ionized, resistivity and doping concentration is a simple inverse relationship; the latter doping concentration >1×10^16 (/cm3), and when the doping concentration is higher, resistivity and doping concentration is no longer a simple linear relationship. This is mainly because the heavily doped dopant can not be fully ionized at room temperature, and the carrier mobility decreases significantly with the increase in dopant concentration.
At low temperatures, the coefficient of thermal expansion of silicon decreases with increasing temperature, and continues to increase when the temperature reaches a certain critical value.
