Solid materials can be divided into conductors, semiconductors and insulators according to their electrical conductivity. Among them, the conductivity of semiconductor materials is between that of conductors and insulators, and is easily affected by temperature, light, magnetic fields and impurity atoms. In fact, the conductivity of semiconductor materials can be adjusted by doping different concentrations of impurity atoms, and the adjustment range can be as high as 1010. In addition, the effect of temperature on the conductivity of semiconductor materials is also very different with the difference of metal materials. Generally speaking, the conductivity of metal materials is not greatly affected by temperature changes, and basically the higher the temperature, the smaller the conductivity; while the conductivity of semiconductor materials is closely related to temperature, and as the temperature increases, its conductivity increases. However, due to its high conductivity and sensitivity, semiconductors have become the most important materials in electronic device applications.
Most semiconductor solar cell products are mainly made of silicon semiconductors, and some are made of semiconductor materials such as gallium arsenide (GaAs), indium gallium phosphide (GalnP), copper indium gallium selenium (CuInGase) and cadmium telluride (CdTe). Among them, semiconductors such as silicon and germanium are composed of one element, called element semiconductor. Other semiconductors composed of two elements (group III and group V, group II and group VI), three elements or even four elements are called compound semiconductors. Compared with elemental semiconductors, the synthesis of compound semiconductors often requires more complicated processes. When selecting materials, they usually choose the absorption characteristics that conform to the solar spectrum, and consider the material and preparation costs.
1.1 Crystal structure
Solid materials can also be classified according to their atomic arrangement, valence bond type, and crystal geometric structure (Figure 1.1). One type of solid material lacks long-range order or obvious short-range order. Order structure (short range order), called amorphous (amorphous) material, another type of solid material, atoms or groups of atoms arranged in a regular and orderly manner to form a periodic three-dimensional space array, called crystalline (crystalline) material. Crystal materials can be further divided into single-crystalline solids and polycrystalline solids. As the name implies, the single crystal structure is that the atoms inside the material are regularly arranged throughout the crystal, while the polycrystalline structure is that there are many hundreds of angtrom to hundreds of micrometers (micrometer) in the entire bulk. Grain, although the atoms inside each grain are arranged in a regular and orderly manner like a single crystal, there is no regular orientation and spacing between the grains, so there are grain boundaries.
Figure 1.1 Three types of structures of solid materials
There are often many defects, dangling bonds and impurities (impurity) in the grain boundary, which adversely affect the physical and chemical properties of the material, especially for the carrier transport characteristics, the trapping caused by the grain boundary (trap) and scattering (scattering) effects seriously affect the mobility of carriers in the material.
Through X-ray and electron beam diffraction (diffraction) technology, single crystal, polycrystalline or amorphous structures can be accurately distinguished. However, because the analysis of the behavior of electrons in amorphous materials is much more complicated than the behavior of electrons in single crystal materials, the single crystal structure becomes the basis for solving the physical properties of solid-state materials. Although the analysis of the properties of amorphous and polycrystalline semiconductor materials is complicated and difficult to understand, in fact, the concept extended from the solid-state theory of crystal structure combined with defect theory can still be applied to these materials. Therefore, the basic concepts of semiconductors introduced in this chapter also focus on single crystal semiconductor materials.
Single crystal structure has periodic three-dimensional spatial atomic arrangement, and some building blocks can be found according to its arrangement rules and symmetry. If these constituent units are repeatedly stacked together and extend continuously in all directions, the entire crystal structure can be produced. Therefore, this constituent unit is called a unit cell. For a specific crystal structure, there are many possibilities for the choice of a unit cell. Bravais found from symmetry analysis that crystals can be divided into 14 structures, including triclinic, monoclinic, orthorhombic, tetragonal, cubic, and triangular. There are seven types of crystals (trigonal) or rhombohedral and hexagonal crystals. Of which cubic
The body can be divided into simple cubic (simple cubic) body-centered cubic (body-centered cubic) and face-centered cubic (ace-centered cubic.) three structures. For semiconductor materials, face-centered cubic crystals are one of the most important structures, and both diamond structure and sphalerite structure belong to face-centered cubic crystals.
Figure 1.2 is the periodic table of the elements. The elements marked with color are the main members of the semiconductor material. Among them, silicon belongs to the group IV element, which means that it has 4 valence electrons, and these 4 valence electrons can form covalent bonds with neighboring atoms. . In single crystal silicon, atoms are arranged in a diamond lattice with tetrahedral valence bonds (diamond lattice Figure 1.3(a)), and the angle between each valence bond is 109.5°. In particular, this arrangement can use two The unit cells of a face-centered cube are stacked through each other. The lattice constant a is the side length of the unit cell. A complete lattice can be formed by stacking several unit cells. The other is similar to the diamond structure is the zinc blende lattice (zincblende lattice) Figure 1.3(b)], it is different from the diamond structure in that the constituent atoms in the two interpenetrating face-centered cubic sub-lattices are different, one is group III or group II, and the other is group V or group VI .Most of III-V group semiconductors
And II-VI group semiconductors have this structure, such as gallium arsenide (III-V group) and cadmium telluride (II-VI group).
Figure 1.2 Periodic Table of Elements
(a) Diamond lattice: elemental semiconductor (such as silicon, germanium, carbon)
(b) Sphalerite lattice: III-V compound semiconductors (such as gallium arsenide, gallium phosphide, indium phosphide…)
Figure 1.3 Two common structures of semiconductor materials
For a single crystal material, the arrangement of atoms is a three-dimensional space, arranged along each plane or direction, the arrangement period of the atoms and the distribution of the valence bond electron cloud are different. It is conceivable that the physical properties will also be different. Therefore, in crystals, the so-called Miller indices (Miller indices) are used to define different planes in a crystal. The Miller index determination law is the intercept of the plane on the three orthogonal axes of the orthogonal coordinate system (using the lattice The constant is the unit of measurement) take the reciprocal and reduce it to the simplest ratio of integers, and finally express the ratio of integers in (hkl), which is the Miller index of a single plane. Figure 1.4 shows the method of determining the Miller index and the cube The Miller index of important planes in the crystal.
Figure 1.4 Miller index determination method and Miller index of important plane in cubic crystal